Focusing on the inverse synthetic aperture radar (ISAR) imaging of maneuvering targets, this paper presents a new imaging method which works well when the target’s maneuvering is not too severe. After translational motion compensation, we describe the equivalent rotation of maneuvering targets by two variables—the relative chirp rate of the linear frequency modulated (LFM) signal and the Doppler focus shift. The first variable indicates the target’s motion status, and the second one represents the possible residual error of the translational motion compensation. With them, a modified Fourier transform matrix is constructed and then used for cross-range compression. Consequently, the imaging of maneuvering is converted into a two-dimensional parameter optimization problem in which a stable and clear ISAR image is guaranteed. A gradient descent optimization scheme is employed to obtain the accurate relative chirp rate and Doppler focus shift. Moreover, we designed an efficient and robust initialization process for the gradient descent method, thus, the well-focused ISAR images of maneuvering targets can be achieved adaptively. Human intervention is not needed, and it is quite convenient for practical ISAR imaging systems. Compared to precedent imaging methods, the new method achieves better imaging quality under reasonable computational cost. Simulation results are provided to validate the effectiveness and advantages of the proposed method. Inverse synthetic aperture radar (ISAR) imaging plays important roles in civil and military fields, and has attracted the attention of many researchers in the past decades ,5,6,7,8.After translational motion compensation, the range-Doppler (RD) algorithm ,16 is usThe RID algorithm can be divided into two categories. One is based on the time-frequency representations ,20. The In this paper, we study the ISAR imaging of targets with not too severe maneuvering motion, and propose an adaptive imaging method based on modified Fourier transform. After translational motion compensation, the maneuvering motion can be regarded as uniform accelerated rotation and can be described by the rotational angular velocity and acceleration. The jerk of the target is very small and can be neglected. For targets with rigid body, the relative chirp rate of LFM signal is the same for all scatterers and only depends on target’s motion status. Additionally, considering the possible residual error of translational motion compensation, a new variable called Doppler focus shift is defined to describe the shift of the equivalent rotation axis in the azimuth direction. With the relative chirp rate and the Doppler focus shift, both the quadratic phase terms of LFM signals and the rotation axis shift could be compensated in the modified Fourier transform matrix. Then the blurring in ISAR images is effectively eliminated. On this basic, ISAR imaging of maneuvering targets is converted into a parameter optimization problem. The optimal relative chirp rate and Doppler focus shift are obtained through a two-dimensional gradient descent method according to the optimal imaging quality. Moreover, we design an efficient and robust initialization process for the gradient descent method, thus, optimal imaging result could be obtained adaptively without human intervention. This is very helpful and convenient for practical ISAR imaging system. Except for the advantages of clear imaging result, robustness, and automation, the proposed method does not require heavy computation.The rest of the paper is organized as follows: In O-XY defines a Cartesian coordinate system, where the origin O is located at the rotating axis of the target. The radar is located on the target plane along the Y-axis. Assuming that The ISAR imaging geometry is shown in Supposing the transmitted LFM signal is:ensation  is adoptAfter matched filtering, the obtained range profile is expressed as:After translation motion compensation, the range profile can be written as:Next, we concentrate on the pulse compression along the cross-range direction, so the sinc function related to fast time is neglected in the following discussion. The range profile can be simplified as:For targets with smooth motion, the equivalent angular velocity can be denoted as Substituting Equation (6) into Equation (5) yields:ith scatterer. The ISAR image can be obtained by performing Fourier transform with respect to m represents the pulse number related to slow time, n represents range cell related to fast time. There are M pulses and N range cells in total. We denote the discrete ISAR image as P. The process of RD algorithm can be expressed as:Obviously, time dependent phase component in Equation (7) corresponds to the cross-range position of the However, for maneuvering targets, the equivalent angular velocity is time-varying and the rotation angle is The mathematic definition of relative chirp rate is:T is the pulse repetition interval. The features of (1)For all scatterers on the target, the relative chirp rate (2)For scatterers located in the same Doppler cell, their compensation components for the quadratic phase terms are approximately identical.(3)The values of Obviously, P. After getting the exact value of P. We denote P can be expressed as:Each Doppler cell in ISAR image is related to a row vector in Fourier transform matrix Mth row, the value of P can be expressed as:From the first row to the We denote the Fourier transform matrix containing quadratic phase compensation as P. That is to say, our proposed algorithm is a generalized version of the classical RD algorithm.It should be noted that for targets with smooth motion, the relative chirp rate The analysis above builds upon an implicit assumption that the coefficients of quadratic phase terms are proportional to the scatterer’s azimuth coordinate and Doppler frequency. This assumption requires that the equivalent rotation axis is located at the center of the azimuth direction, and it is reasonable when there is no error in the translational motion compensation process. However, when the entropy-minimization-based autofocusing technique  is adoptThe cross-range cell in which the equivalent rotation axis locates is denoted as Doppler focus. The shifted cell number of Doppler focus caused by an inaccurate translational motion compensation is denoted as Doppler focus shift. When the Doppler focus shift is considered, the compensation term in Equation (14) turns into:Equation (9) indicates that the relative chirp rate can be got by estimating the centroid frequency and the chirp rate of LFM component in the cross-range signal. Several methods, such as FrFT, Lv’s distribution (LVD)  work welA well-focused image results in small entropy. The accurate relative chirp rate and Doppler focus shift can be obtained via minimizing the entropy of the ISAR image.However, the parameter searching process above is a brute force solution. Though feasible, it is costly in computation. In general, the change of the image entropy is smooth, the minimum entropy corresponding to the optimal ISAR image is unique, thus, the parameter searching process is a convex optimization problem. The gradient descent method is promising in obtaining the global optimization of the convex object function ,35. The Step 1: Step 2: Determine the minimum image entropy and the corresponding relative chirp rate, denoted as Step 3: Let Step 4: Search for the minimum entropy and the corresponding Doppler focus shift Step 5: Carry out the two-dimensional gradient descent process based on the initial value and step length of Step 6: When the image entropy change between two adjacent iterations is smaller than preset threshold, the iteration stops. The relative chirp rate and Doppler focus shift in last iteration are output as optimal values.Steps 1–4 is the initialization process of the two-dimensional gradient descent method. This process not only considers the wide value ranges of In our new method, the modified Fourier transform, instead of the conventional Fourier transform, is used to perform the azimuth compression. When the relative chirp rate and Doppler focus shift are zero, the modified Fourier transform degrades to the conventional Fourier transform. Our new method unifies the imaging problem of smooth targets and maneuvering targets into the same imaging model. Meanwhile, with the efficient initialization process and two-dimensional gradient descent optimization, our method can achieve the well-focused images of maneuvering targets adaptively.To summarize, the flowchart of the proposed ISAR imaging method is shown in In this section, the new imaging method is verified using simulated data. Imaging results of precedent methods are provided as a comparison.v =  m/s, and the target’s acceleration is a =  m/s2. Modified envelope correlation [The parameters of the radar are given in relation  and the v = 225, 0, 0 m/s,Then, we compute the time-frequency representations of the cross-range signals using RSPWVD, as shown in In the following, the proposed algorithm is used to generate the ISAR images. To demonstrate the coupling between Image contrast is an alternative criterion to evaluate the image quality. Additionally, the mathematical definition of image contrast  is:(18)CThe entropies and contrasts of the ISAR images based on different methods are listed in To more clearly compare the image resolution obtained by the new method, STFT and SPWVD, the azimuth profiles in the 91st range cell obtained by these algorithms are given in Simulation times of these methods are listed in Now, the proposed method is tested on the simulated data of a Boeing B727 airplane provided by the U.S. Naval Research Laboratory . The datNext, the new method is applied to obtain a well-focused image. Simulation results are shown in Firstly, the Doppler focus shift is not considered, only the relative chirp rate is considered. Simulation results indicate that the optimal relative chirp rate is 107.13. The optimal imaging result is shown in E is a convex function. The point corresponding to the minimum image entropy is unique. Therefore, the parameters The image entropy versus the relative chirp rate and Doppler focus shift is shown in In To more clearly compare the image resolution obtained by the new method, STFT and SPWVD, the azimuth profiles in the 34th range cell obtained by these algorithms are given in In this paper, an adaptive ISAR imaging method based on a modified Fourier transform is proposed for targets with not too severe maneuvering. In our new method, the relative chirp rate and Doppler focus shift are used to describe the target’s motion after translational motion compensation. The relative chirp rate depends on the maneuvering status of the target. Additionally, the Doppler focus shift presents the possible residual error of translational motion compensation. With them, compensations are carried out in a modified Fourier transform matrix, and the imaging problem is converted into a two-dimensional parameter optimization. Through an efficient and robust initialization, and the consequent two-dimensional gradient descent process, the optimal imaging result could be obtained adaptively. Compared with precedent ISAR imaging methods for maneuvering targets, the new method has the following features: tolerance for translational motion compensation error, clear and stable imaging result, little computational complexity, and excellent convenience for practical application. The results of the simulated data are provided to validate the effectiveness and efficiency of the proposed method.


The linear array push broom imaging mode is widely used for high resolution optical satellites (HROS). Using double-cameras attached by a high-rigidity support along with push broom imaging is one method to enlarge the field of view while ensuring high resolution. High accuracy image mosaicking is the key factor of the geometrical quality of complete stitched satellite imagery. This paper proposes a high accuracy image mosaicking approach based on the big virtual camera (BVC) in the double-camera system on the GaoFen2 optical remote sensing satellite (GF2). A big virtual camera can be built according to the rigorous imaging model of a single camera; then, each single image strip obtained by each TDI-CCD detector can be re-projected to the virtual detector of the big virtual camera coordinate system using forward-projection and backward-projection to obtain the corresponding single virtual image. After an on-orbit calibration and relative orientation, the complete final virtual image can be obtained by stitching the single virtual images together based on their coordinate information on the big virtual detector image plane. The paper subtly uses the concept of the big virtual camera to obtain a stitched image and the corresponding high accuracy rational function model (RFM) for concurrent post processing. Experiments verified that the proposed method can achieve seamless mosaicking while maintaining the geometric accuracy. Linear array push broom is a widely used imaging mode for the high resolution optical satellite (HROS). To increase the resolution, the camera is typically designed with a long focal length to achieve a narrow field angle. To increase the field of view (FOV), it is necessary to assemble additional TDI-CCD detectors together because the existing TDI-CCD detector cannot achieve the large field of view requirement, such as IKONOS, Pleiades-HR, Worldview series, ZiYuan3, etc. ,2,3. AnoResearchers have focused on investigations involving stitching several satellite images together ,5; thereIn this paper, we propose an image mosaicking approach for double-camera systems on the GF2 satellite based on the big virtual camera (BVC) in the satellite body coordinate system, which is built with no internal distortion and its FOV covers all the camera’s FOV. According to the original rigorous imaging model and the updated rational function model (RFM) by relative orientation of each camera, images obtained at the same time by the double-camera system are re-projected to the same big virtual camera coordinate system to produce the entire central projection stitched image by feather processing alone. Moreover, the high accuracy rational function model (RFM) of the stitched image can be obtained concurrently, which will provide the geometric information required for post processing. The proposed approach has been validated for precision and robustness, and it works under varying topographical conditions and achieves the goal of fully automated image data pretreatment on the ground.In the double-camera push-broom imaging system on the GF2 satellite, the principal optic axes of the cameras are installed in the same plane as the greatest extent and the angle between them is 2.01°, as shown in The focal plane of the single camera is composed of five-collinear TDI-CCD detectors within the field of view and there are many overlapping pixels between the adjacent TDI-CCD detectors for mosaicking. The overlapping regions of the double-camera imaging system can be divided into the camera overlapping region and the detector overlapping region as shown in The imaging parameters of the rigorous geometric imaging model can be divided into the satellite auxiliary data and the camera parameters. The satellite auxiliary data of time, attitude and orbit help to convert the satellite body coordinates into object coordinates, which determines the ray of light from the projection center to the ground points in the satellite body coordinate system. The interior LOS parameters help to convert the image coordinates into the camera coordinates, which determines the accurate LOS of each detector in the camera coordinate system ,15. For k angles ,17 betweThe interior LOS of the virtual single detector can be easily determined by: ocessing ,19 to geThe single camera auxiliary data can also be applied to the BVC using steady-state processing. The BVC parameters consist of the exterior installation matrix and the interior LOS parameters. Based on the symmetrical installation relationship in the double-camera system shown in Based on the above analysis, the LOS The serial number of virtual detectors Subsequently, the LOS of The projected BVC coordinates of the two endpoints The BVC can be designed with no internal distortion by dividing the interior LOS equally according to the number of virtual detectors, and we can subsequently determine the LOS Then, we can determine the BVC rigorous imaging model as follows:The advantage of placing the virtual detector in the center line of the double-camera’s detectors along the track direction is to reduce the mosaic and virtualization errors. Based on the determined BVC rigorous imaging model, the corresponding RFM can be determined to generate the complete stitched images.One straightforward method to produce the complete stitched image is to generate the stitched image from each single camera first, subsequently, stitch the two images to generate the complete stitched image from the double-cameras. However, it can be time-consuming to resample all the original single images twice. The image mosaic method workflow is based on the virtual single detector and the big virtual camera is designed with six primary steps as shown in Step 1: Determine the rigorous imaging model (RIM) for the original single camera. The n method ,21,22. TStep 2: Determine the RFM for the virtual single detector (VSD). The VSD shares the same installation angles with RIM and the interior VSD LOS can be designed with no distortion according to the RIM field of view. Based on the new integration time from averaging the original integration times, the interpolated projection center and the smoothed attitude data can be obtained to determine recision .Step 3: Generate the virtual single images (VSI)s of the camera overlap region. Although the consistency of the image positioning accuracy between the adjacent TDI-CCD detectors can be ensured after an on-orbit high accuracy calibration, the consistency of the image positioning accuracy between the adjacent double cameras is hard to maintain due to the inevitable changes of their relative installation relationship caused by the external environmental temperature, which will lead to the dislocation of the camera overlap region. Therefore, to ensure seamless mosaicking solely based on the geometrical information, the relative orientation of the Step 4: The relative orientation of the two virtual single images (VSIs). Many homonymy points for the two adjacent VSIs can be matched automatically using the SFIT algorithm, and the image space coordinates Step 5: Determine the RFM for the big virtual camera (BVC). Based on the rigorous imaging model of the single camera, the rigorous imaging model of the big virtual camera can be determined, and combined with the steady-state processed auxiliary data, the high accuracy Step 6: Generate the complete stitched images. To generate the virtual single images in the BVC coordinate system, twice forward and backward intersections are performed.(1) First, for the image point (2) The image point (3) Then, the updated object space coordinate (4) The final image point (5) Thus, point (6) Repeat the above steps until every pixel for all the single virtual images on the big virtual detector is complete; then, ten single virtual images can be produced.(7) Finally, the single virtual images produced will be stitched based on the same initial pixel coordinates for the big virtual detector in the big virtual camera, and feather processing without performing the local correction in the overlapping region can produce the seamless mosaic image.G is the object space point of the image space point P in the overlapping region of the stitched image. The image space points P1 and P2 in the adjacent images are determined using backward-projection from G to achieve the conjugate relationship; this means that P1 and P2 are homonymy points. In other words, the corresponding object space points G1 and G2 of P1 and P2 based on their own geometric imaging model should be as close as possible and the distance of the two object space points, which reflects the consistency of the positioning accuracy between the adjacent single images.Because the geometric imaging model is important in the proposed mosaicking approach, the consistency of the TDI-CCD detectors’ positioning accuracy is the geometric foundation for the mosaic accuracy. As shown in The positioning accuracy is determined by the accuracy of the satellite auxiliary data and the camera parameters. Using an on-orbit high accuracy geometric calibration ,17 for tThe difference between the interpolated elevation and the actual elevation that is caused by the limit of the absolute positioning accuracy and the elevation data precision is another key factor for the mosaic accuracy.G is the intersection point of the homonymy points P1 and P2 for the actual elevation, and ΔH is the elevation error. Δl represents the object space projection distance of the mosaic error caused by ΔH, as:α1 and α2 of the homonymy points P1 and P2 are the bias FOV angle of the single CCD detector in the satellite body coordinate system along the track.As shown in v represents the virtualization error caused by the elevation error as follows:α and β represent the bias FOV angles of the image homonymy point Q1 on the original single image and Q2 on the corresponding single virtual image.For the non-overlapping area, ΔThe larger the intersection angle of the homonymy point’s LOS, the greater the influence of the elevation error will be, therefore, to ensure the mosaic and virtualization accuracy, the required interpolated elevation accuracy should be achieved. The elevation error will rarely affect the mosaic accuracy of the detector overlap region due to collinear installation, but has a significant effect on the mosaic accuracy of the camera overlap region. Placing the virtual detector in the center line of the double-camera’s TDI-CCD detectors along the track direction can divide the elevation error equally to all the detectors to ensure the mosaic and virtualization accuracy. After the on-orbit high accuracy calibration and relative orientation, the accuracy of the image positioning and the interpolated elevation from the global DEM can be ensured. Based on the analysis above, the goal is to achieve a seamless mosaic image using image feather processing when stitching the single BVC virtual images directly based on their image coordinate for the same big virtual detector coordinate system.Four sets of double-panchromatic-images from the GF2 satellite were used in the experiments to verify the reliability of the proposed approach. Visual and geometric accuracy evaluation of the complete stitched images were performed to comprehensively validate the effectiveness of our approach.Based on the high accuracy and stable interior calibrated parameters, seamless mosaicking of the detector overlapping region can be achieved directly based on the geometric relationship. Then feather processing is performed during the local processing, therefore, a significant dislocation of the seamline will occur if there is a significant image positioning deviation for the homonymy points on the adjacent single CCD detector, which can be effectively detected by visual evaluation. Due to the unstable small changes in the relative installation relationship of the double-cameras, relative orientation is required for the seamless mosaicking of the camera overlap region. Areas 4 and 5 could achieve seamless mosaicking when feather processing is performed for the detector overlapping regions. That is due to the high accuracy and stable on-orbit calibrated interior parameters, which are the most important factors for the consistency of the positioning accuracy in the single camera. We can also see that the stitched areas of the camera overlap regions are also seamless and smooth without a local correction after relative orientation with translation transformation. The stitching result will not be affected by the overlapping region terrain, which is benefited from the geometric characteristics we talk about in the following part.The RFM fitting precision is one key index of the geometry quality for the complete stitched image, and it can be calculated using check points evenly distributed among the virtual control points when fitting the RFM based on the rigorous imaging model as shown in −4 in the sample and line directions, which may be ignored because it fully achieves the accuracy requirement. This is due to the virtual detector of the big virtual camera being designed without internal distortion and external attitude data in the steady-state processing, which are interpolated using the polynomial model that can filter the observation noise using the least square method (LSM). The high RFM accuracy for the complete stitched image obtained using the big virtual camera provides the necessary geometric information for post processing.The maximum and minimum error absolute values for the rational function coefficients (RPCs) fit using the original attitude reaches approximately 0.65 pixels in the sample and 0.93 pixels in the line directions, respectively. However, the RMSE of the RPC fit using the smooth attitude becomes better than 1.00 × 10The mosaic accuracy we focus on can be divided into the mosaic accuracy of the detector overlap region and the camera overlap region. Due to the collinear installation of the multi-TDI-CCD detectors, the same scanning line in the detector overlap region is concurrently imaged; then, the auxiliary data are concurrently interpolated by time. Based on the high-accuracy camera interior LOS parameters, the mosaic accuracy of the detector overlap region can stably reach approximately zero pixels. Therefore, feather processing is needed for seamless mosaicking. The mosaic accuracy of the camera overlap region is difficult to ensure using an on-orbit calibration due to the inconsistent relative installation relationship of the double-cameras caused by the changing thermal environment. Therefore, the relative orientation should be performed for the double-images captured by the double-cameras. Theoretically speaking, affine transformation should be performed in the relative orientation considering the rigorous installation relationship. However, the small imaging overlap region of the double-cameras and the complicated imaging terrain features cause the homonymy points auto-matching to be difficult and not completely reliable in most cases. Therefore affine transformation will probably cause a more unreliable geometric accuracy for the double images, especially at the edge of the double-images away from the overlapping region. Considering these problems, translation transformation may be an alternative choice if it can also achieve the ideal mosaic accuracy.s,l)left is the image coordinate of the matched point in the left single virtual image, and right is the image coordinate of the matched corresponding homonymy point in the adjacent right single virtual image. Because the homonymy points image coordinates of the left and right single virtual image are in the same big virtual detector plane coordinate system, the homonymy points image coordinates should be the same under ideal conditions. The deviation of the homonymy points’ image coordinates will directly affect and reflect the mosaic accuracy. The mosaic errors (pixels) relationship with the line number are shown in Based on the original RFM of virtual single detector, the adjacent single images can be projected to the big virtual detector according to Step 3 in the workflow. Subsequently, the mosaic accuracy can be evaluated using the consistency of the homonymy points in the overlapping regions. More than ten thousand evenly distributed dense homonymy points were matched automatically using the SIFT algorithm. Based on the rigorous imaging model for the single camera, the rigorous imaging model for the big virtual camera was established, which would exactly apply to the complete stitched image. Additionally, the image mosaic workflow based on the virtual single detector and the big virtual camera were presented in detail.2)High accuracy camera parameters on the satellite are necessary in the proposed mosaicking approach. Therefore, high accuracy on-orbit geometric calibration is the precondition to guarantee the effectiveness of our approach.3)Benefiting from the platform stability and the small relative installation error, the mosaic error of the camera overlap region could be controlled in one pixel after an on-orbit high accuracy geometric calibration and the relative orientation, otherwise, a local correction may be required to achieve seamless mosaicking.4)Cloud coverage and complex terrain in the camera overlap region may influence the homonymy point matching. Although we verified the ability of the translation transformation in the relative orientation to reduce the dependence on the homonymy point quantity and quality, it still cannot be applied to all situations.5)The relative installation instability of the double-cameras and the high-frequency platform vibration are key factors in achieving the seamless mosaic from the double-cameras, therefore, if they can be securely attached in the future satellite platform, a simpler workflow without relative orientation can achieve a more ideal mosaic result.In this paper, we have proposed an automatic image mosaicking approach for the double-camera system on the optical remote sensing satellite GF2, and subtly used the concept of the big virtual camera to obtain the stitched image and the corresponding high accuracy RFM for post concurrent processing. The following conclusion may be drawn from our results:


Responses of ecosystems to human-induced stress vary in space and time, because both stressors and ecosystem vulnerabilities vary in space and time. Presently, ecosystem impact assessments mainly take into account variation in stressors, without considering variation in ecosystem vulnerability. We developed a method to address ecosystem vulnerability variation by quantifying ecosystem vulnerability distributions (EVDs) based on monitoring data of local species compositions and environmental conditions. The method incorporates spatial variation of both abiotic and biotic variables to quantify variation in responses among species and ecosystems. We show that EVDs can be derived based on a selection of locations, existing monitoring data and a selected impact boundary, and can be used in stressor identification and ranking for a region. A case study on Ohio’s freshwater ecosystems, with freshwater fish as target species group, showed that physical habitat impairment and nutrient loads ranked highest as current stressors, with species losses higher than 5% for at least 6% of the locations. EVDs complement existing approaches of stressor assessment and management, which typically account only for variability in stressors, by accounting for variation in the vulnerability of the responding ecosystems. Some authors even speak of an ‘anthropogenic biosphere’2. The diagnosis of environmental impacts, including identification and ranking of stressors, is of key importance for protective and/or restorative management. The influence of human stressors on ecosystems depends not only on the types and magnitudes of the stressors, but also on the vulnerability of the impacted ecosystems. Hence, to reach policy goals regarding sustainable management of our environment, such as the UN development goals at the global scale3 or good ecological status goals for ecosystems at the regional scale  boundary, which is derived from the responses of a small selection of individual species exposed to single compounds under laboratory conditions12. Neglecting variation of ecosystem vulnerability to stressors leads, however, to either under-protection14 or over-protection15.Ecosystems are under influence of human-induced stress all over the world16. For chemicals, the overlay of the SSD with the stressor exposure distribution for a region has been proposed as a measure of ecological risk, and has – via groundbreaking studies on atrazine risks across sites in North America17– had global influences on environmental protection, assessment and management regarding plant protection products18. EVDs can be applied likewise, but now for stressor identification and ranking across multiple potentially relevant stressors based on vulnerability distributions described on the basis of actual monitoring data on species assemblages. Thereby, the approach can assist in bridging the current gap between assessments of the impacts of chemical mixtures and those of other stressors19.In this study we designed a method to characterize the variability of ecosystem vulnerabilities over a large region, and we refer to the resulting distribution as the Ecosystem Vulnerability Distribution (EVD). The EVD quantifies the range of ecosystem vulnerabilities in a region for a stressor based on the environmental responses of a set of species assemblages as observed in a selection of reference sites. This is similar to the derivation and use of species sensitivity distribution (SSD) models for describing across-species variation in sensitivity for a toxic compound20. For the species observed at those reference sites species distribution models (SDMs) are established using monitoring data of the whole region. The SDMs enable quantification of the probability of occurrence of each of the species in relation to the set of environmental variables measured at any site. Next, the SDMs are used to quantify the stressor-response relationships of the species assemblages of each of the reference sites to a selection of the environmental variables, i.e., the stressors. This is done by varying the stressor of interest across its range while fixing all other environmental variables at their site-specific values  and ranking . Freshwater ecosystems are amongst the most intense human-impacted habitats22) and the derivation of species-specific SDMs for the occurrence of fish species at those sites. The environmental variables taken into account in the fitting of the SDMs were drainage area, water acidity (pH), mixture toxic pressure, nutrient loads, hardness, conductivity and habitat quality. The latter was quantified by the Qualitative Habitat Evaluation Index (QHEI23), a measure for the physical-habitat quality for fish, developed for running waters in Ohio that ranges from 0  to 100 . Mixture toxic pressure was expressed as the multi-substance Potentially Affected Fraction of species (msPAFEC50) derived from the 50% effect concentrations of metals, ammonia and nitrite25. Further methodological details are explained in the method section.The case study started with a collection of statewide biomonitoring data for Ohio freshwater systems , the selection of a subset of 18 sites that are in good and stable ecological condition while representing the region under consideration  rare species. Yet, this implies that other complementary indicators may be needed to complement the stressor analyses from a species conservation perspective33. Further, we utilized fish species observed at the reference sites for the derivation of the EVDs. Species that are not currently observed at a reference site but are observed at other sites in the region, may, however, still be able to reach and survive there. To check the possible dependency of the EVD-approach on species selection, we also derived EVDs from an alternative viewpoint, that is, the calculated RSR for all fish species in Ohio for which an SDM could be derived. This provides an EVDall_species per stressor, as if all species are present at all reference sites. Able to include 98 species in this exploration, we found that the EVDs for physical habitat quality, total phosphorus and pH became more steep  should be considered in the derivation and use of EVDs, and although our stressor identification and ranking results are in line with literature and expert assessments, the effects of methodological issues deserves further investigation. Fourthly, in the absence of observed natural tipping points, we selected T = 5% reduction of the species richness as an impact threshold needed to define the vulnerability distribution. We explored whether this operational choice matters for the key conclusion, and found that a higher impact threshold only resulted in right-shifted EVDs  species deletion is predicted to occur36. Again, the vulnerability of ecosystems was found to be a distributed characteristic. Finally, the method proposed does not yet account for interactions among stressors and among species. The EVD is derived by increasing one stressor in the model while keeping the other stressors at their current value, while biogeochemical processes can be such that a change in one stressor results in change in other stressors. Inclusion of stressor interactions might alter the SDMs, and subsequently the derived EVDs. In addition to abiotic interactions across the stressors, also biotic interactions among the species may occur, in the form of predator-prey relationships or resource competition. The method presented in this paper did not include this. Quantifying inter-species interactions across many fish species is an evolving field with substantial challenges37, particularly because trophic relationships among fish species may alter during the natural life history. This would be different for some terrestrial systems, where species may exhibit clear and strong mutual interactions, like between pollinating species and host plant38. Yet, the addition of location-specific data on trophic interactions among fish species can be a pathway to improve the SDMs39 and may also allow for identification of thresholds based on secondary effects36.The shape and position of EVDs depend on various methodological characteristics. Firstly, the EVDs may change with species selection. SDMs can be established only for species with enough monitoring observations, so that the method may not represent situations where there are rare or poorly detectable species. Theoretically, common species are usually considered more relevant from an ecosystem functioning perspective SI Fig. . As a co4. One of the difficulties in the implementation of these regulations is the meaningful evaluation of variation in stressor levels in relation to the variation in vulnerabilities. The use of pre-set, generic decision criteria in the evaluation of monitoring data may easily lead to Type-1 errors (stressor impacts diagnosed when absent)40. The EVD takes this variation into account and could be used as first-tier method for the screening and ranking of potential stressor influences across multiple locations in a larger region. When EVDs are based on sites representing reference conditions, stressor levels in the region lower than the EVD range can be interpreted as identifying sites at which the stressor is not limiting for the good ecological status of a site; levels higher than the EVD range may imply an affected status; and locations with stressor levels within the EVD range are at risk and should be subject to further assessments. The EVD-method thereby also allows for a dynamic assessment of implications of future changes in stressor levels caused by societal developments and/or stressor abatement strategies. This links ideally with the recent call for solution-focused approaches, which are characterized by the early definition of alternative solutions in the assessment process, and the associated need to explore the expected responses of alternative management scenarios43. Therefore, EVDs can also be used in regional planning activities, deciding on the prohibition or allowance of new activities in the region such as a change in land use or the installation of new wastewater treatment techniques. These types of change in activities have impact on levels of different stressors at the same time  and the approach in this paper can be used to assess site-specific assemblage responses to changing levels of different stressors simultaneously.We showed that ecosystem vulnerabilities for stressors in a region are distributed, irrespective of methodological choices. This implies, amongst others, that exceedance of a protective criterion, such as a regional concentration criterion for a chemical used as protection threshold, will result in dissimilar responses across ecosystems. Furthermore, we showed how EVD models operationalize this implication for the purpose of stressor identification and ranking. This approach can, as example, be used to assess whether the goals of the (European) Water Framework Directive or the (American) Clean Water Act are reached, and if not, which stressors are probably responsible for observed impactsfurther affect the species assemblages at such sites, beyond the impacts already present.As explained above, the choice of the set of reference sites is important, and commonly the ‘anchor point’ to judge impacts will relate to the policy goals of a good ecological status of all water bodies in a region. However, the method can also be applied when it is based with purpose on deriving the EVDs using a selection of impacted sites. In such an approach, the vulnerability distribution  is based on a set of sites influenced by human impacts. Overlaying such a ‘stressed-sites’ EVD with the distribution of the stressor levels would provide insights whether stressor levels would be able to 44 and the chemical footprint method45.Next to stressor identification and ranking, the method allows for defining region-specific impact boundaries, e.g. at the ‘safe side’ (left) of the EVD range or at the stressor level at which a selected minimal percentage of the locations exceeds a selected impact threshold. Such region-specific boundaries could be applied in risk management and when assessing ambient stress via sustainability metrics such as the nitrogen footprint methodet al.7 pose that various stressors primarily operate at the regional scale, and thus also cross boundaries at local and regional levels. In those cases aggregation to the planetary level is needed to judge global impacts46. Application in this context is a feasible next step in EVD development and use. The EVD approach can be used to explore the fraction of sites that are at risk of exceeding local ecosystem vulnerability for a suite of stressors, given a selected impact threshold or a chosen ‘safe’ boundary below that.Finally, in the context of the planetary boundaries concept, Steffen 47. When multiple measurements were available per location the median value was used. The ecosystem vulnerability distribution approach requires a subset of sites for which the vulnerability distribution is derived as ‘point of departure’20. In this study, the EVD-derivation and use is illustrated by starting with a subset of 18 sites in stable ecological conditions as identified in an earlier study22. In that study reference sites were selected that are ecologically in balance according to a judgment of food-web allometry and network metrics [Ref. 50. Environmental variables selected for the SDMs were pH, concentrations of N  and P , hardness, conductivity, mixture toxic pressure, drainage area and the Qualitative Habitat Evaluation Index (QHEI). These variables were selected because of ecological relevance51 while exhibiting low multicollinearity52. We operationalized the latter by selecting only predictors with a variance inflation factor (VIF) less than 553. We did this by sequentially removing predictors with VIF > 5 until all remaining predictors had VIF below the threshold. Although some environmental variables contained a few outliers , the data were distributed according to the patterns expected for a region characterized by a variety of natural background conditions and forms and intensities of human activities . Per species, the weighted average regression coefficients were calculated with the MuMin package, based on the set of models within two AIC units from the model with the lowest AIC. Finally, accuracy of the resulting SDMs was evaluated based on the Area Under the Curve (AUC) of the receiver-operating characteristics (ROC) plots using the R package PresenceAbsence55.We obtained the SDMs with logistic regression  to link the species observations (presence-absence) to the environmental variables, thereby including quadratic terms to account for non-linear responsesThe fitted SDMs were used to derive stressor-specific response relationships for each reference site, based on the species occurring and the observed environmental conditions at the reference sites. This was done by changing the level of the stressor of concern within the range of observed values in the region–leaving out extreme outliers–while maintaining the other stressors at their observed location-specific values, quantifying the corresponding probability of occurrence of each fish species occurring at that site with the fitted SDM, and then aggregating the probabilities of occurrence over the species. This yields relationships between the value of a stressor and the change in species richness per reference site. Non-disturbed reference conditions were thereby normalized, defining the relative species richness via RSR = Y = 1.35. When the stressor-response patterns do not show these sudden responses, a user-defined impact judgment threshold T needs to be defined. Here we defined this impact threshold as the stressor level at which the predicted species richness, as derived from the stacked SDMs, is 5% lower than the species richness in reference conditions. This yields a range of the critical stress levels across the reference sites that define the distribution of vulnerabilities across reference sites. Subsequently, the EVD–when based on a selection of sites that represents the variability of the natural, minimally disturbed conditions of a region–is assumed to represent the quantitative distribution of ecosystem vulnerabilities of all sites in the region. The overlays of the distributions of stressor values for the 1,826 sites and their EVDs provide insights in the identities and the ranking of the stressors potentially affecting the ecosystems in the region.EVDs were derived per stressor by combining the relative species richness stressor-response relationships with a selected impact threshold (T). This was done for all environmental variables except drainage area, which was not considered a stressor, but as a predictor that describes natural variation amongst the reference sites. The threshold would ideally be based on scientifically derived ‘tipping points’Supplementary information


They took five tests that measured knowledge from their previous studies: Hungarian language and literature, mathematics, history, science and English as a Foreign Language (EFL). A further, sixth test based on the MicroDYN approach, assessed students' DPS skills. A brief questionnaire explored learning strategies and collected data on students' background. The testing took place at the beginning of the first semester in three 2-h sessions. Problem-solving showed relatively strong correlations with mathematics (r = 0.492) and science (r = 0.401), and moderate correlations with EFL (r = 0.227), history (r = 0.192), and Hungarian (r = 0.125). Weak but still significant correlations were found with certain learning strategies, positive correlations with elaboration strategies, and a negative correlation with memorization strategies. Significant differences were observed between male and female students; men performed significantly better in DPS than women. Results indicated the dominant role of the first phase of solving dynamic problems, as knowledge acquisition correlated more strongly with any other variable than knowledge utilization.There is a growing demand for assessment instruments which can be used in higher education, which cover a broader area of competencies than the traditional tests for disciplinary knowledge and domain-specific skills, and which measure students' most important general cognitive capabilities. Around the age of the transition from secondary to tertiary education, such assessments may serve several functions, including selecting the best-prepared candidates for certain fields of study. Dynamic problem-solving (DPS) is a good candidate for such a role, as tasks that assess it involve knowledge acquisition and knowledge utilization as well. The purpose of this study is to validate an online DPS test and to explore its potential for assessing students' DPS skills at the beginning of their higher education studies. Participants in the study were first-year students at a major Hungarian university ( The social and economic developments of the past decades have re-launched the debate on the mission of schooling, more specifically, on the types of skills schools are expected to develop in their students in order to prepare them for an unknown future. One of the most characteristic features of these debates is a search for a new conception of the knowledge and skills students are expected to master  developing new assessment methods for higher education, including innovative and efficient selection processes for choosing students for higher education studies, and assessing university outcomes beyond disciplinary knowledge and domain-specific skills. These demands have directed the attention of researchers to (2) the twenty-first-century skills as desired outcomes of higher education. Rapidly developing technology-based assessment has made it possible to measure several twenty-first-century skills and to include them in large-scale assessments. (3) Dynamic problem-solving (DPS) is one of those skills which by now has an established research background and may satisfy the needs of higher education. Solving problems in the process of being assessed in DPS based on computer-simulated scenarios involves the component skills of scientific reasoning, knowledge acquisition and knowledge utilization, all necessary for successful higher education studies . The AHELO program included assessment of domain-specific competencies as well as of generic cognitive skills, for which the test tasks were adapted from the Collegiate Learning Assessment instrument  indicates, admissions processes have always been based on assessing generic cognitive skills  Hungarian language and literature, (2) mathematics, and (3) history. Beyond these, students must choose a further subject out of a large number of electives. An examination can be taken at two levels in any subject; there is an intermediate and an advanced exam. There is no exact (measurable) definition for the differences between the two levels. Intermediate exams are taken at students' schools before committees formed from teachers in their own schools, while the advanced exams are centralized and are taken before (independent) committees formed from teachers in other schools. The admissions scores are computed by complex formulas; for advanced exams, extra scores are awarded, and other factors may also be taken into account.The inadequacy of such a selection criterion is widely discussed, but few research results are available to make evidence-based judgments about the validity of the current practice and about potential alternative solutions. It seems possible that a reformed matriculation examination could serve to certify completion of secondary studies and at the same time could act as a major component of the admissions process Csapó, . Such a The new admissions processes are expected to provide a better prediction of students' success in a changed world of higher education than those of the traditional methods introduced so many decades ago. Assessment of generic cognitive skills, possibly a representative member of the twenty-first-century skills, could also be a component of a new admissions process. To explore the feasibility and validity of such an admissions model, we have measured five domain-specific competencies plus dynamic problem-solving, and we report the results in the present study.A number of studies have analyzed the requirements of the knowledge-based economy and concluded that science, technology, engineering, and mathematics (STEM) education should be strengthened and that skills relevant to a dynamically changing technology-rich environment should be developed. In this context, societies today and in the foreseeable future are characterized by a new group of skills, which are often called the twenty-first-century skills, or, in other contexts, transversal skills  project, among other studies. The first phase of the ATC21S project dealt with definitions and psychometric, technological and policy issues , while in other cases it would not be possible to implement the assessment process without technology , after which students depict the results of their observations on a concept map. Then, they have to manipulate the variables so that they reach a goal state; this is the knowledge utilization phase (or rule application phase). The results from the two phases are scored separately, and as previous research  correlates with problem-solving, but the strengths of the relationships with the different domains is still open.RQ2: What are the relationships between subject matter tests and problem-solving performance measured at the beginning of higher education studies? We assume that although the disciplinary knowledge tests correlate well with problem-solving, problem-solving measures other aspects of knowledge; therefore, it has considerable added value over subject matter tests.RQ3: Are there differences between students in different disciplines? We assume that those who study within different divisions at the university also differ in their problem-solving skills and in the relationships between problem-solving and other tests.RQ4:How do students' characteristics and background variables influence their problem-solving performance? We may assume that students' family background, mother's level of education, students' intention to learn and students' learning strategies influence how their problem-solving skills develop.We expect that the results from these analyses may contribute to improving matriculation examinations as well as to devising better admissions processes.Participants in the study were students admitted to a large Hungarian university and starting their studies. The university has 12 divisions , but they vary in size (number of students). All of the divisions participated in the study, but because of the differences between them, not all analyses are equally relevant for every division.The population for the study was formed exclusively of students who had just finished their high school studies and immediately applied for admission to the university. They took their matriculation examinations in May, and the assessment for this study was carried out in September of the same year.The target population was 2,319 students, of whom 1,468 (63.3%) participated in the assessment; 57.7% of them were female. The participation rate by division varied from 28.18 to 74.16%.Student participation was voluntary; students were notified of their option to take part in the assessment prior to commencing their studies. As an incentive, they received credits for successful completion of the tests.Students completed a DPS test based on the MicroDYN approach. Several tests composed of similar tasks based on this model have already been used in other studies in Hungary  impacted the value of one dependent (target) variable (sweetness of the lemonade). They moved sliders on the screen to set the current value for the blue and for the green syrup. The system responded by indicating the resultant sweetness level. Students observed what happened and attempted a new setting, observing the sweetness level with such a setting. They had 180 s for the knowledge acquisition phase in each task. In the knowledge utilization phase, they had to reach the required value of the dependent variable (sweetness) by setting the proper values of the independent variables in no more than 180 s. In a difficult item, students had to comprehend more complex relationships between three independent variables  and three dependent variables . . The reliability  of the entire test was 0.88. The reliabilities of the two problem-solving phases were also high .Five disciplinary knowledge tests were prepared for the assessment: Hungarian language and literature , mathematics, history, science, and English as a Foreign Language (EFL). Test content was based on the students' high school studies. The tests covered the major topics of the particular disciplines. Difficulty levels for the tests were adjusted approximately to the intermediate-level standards of the matriculation examination. These tests were prepared by experts practiced in preparing matriculation examination tests. The tests made use of the options made available by computer-based testing; using a variety of stimuli  and response capture . The descriptive statistics for the entire sample and the reliability of the instruments are summarized in Table A background questionnaire was administered to participating students via the same platform as the tests. Data were collected in this way about their matriculation examination results and their learning strategies and SES. To minimize the time devoted to administering the questionnaire, only the most relevant variables were explored, where strong relationships were expected. Family background was represented by mothers' level of education (from primary school to master's degree). Students' commitment to study (intention to learn) was measured with the highest degree they intend to earn . Two scales for learning strategies that use self-reported Likert scales were adapted from the PISA 2000 assessment  were offered to the students in the first 2 weeks of the semester. The tests and the questionnaire were administered using the eDia online platform.Students received detailed feedback on their performance a week after the testing period ended. The feedback contained detailed analyses of their performance in the context of normative comparative data.Data from the achievement tests were analyzed with IRT models. Plausible values were computed to compare the achievements of the age groups, and Weighted Likelihood Estimates (WLE) were used to compute person parameters. The analyses were performed with the ACER ConQuest program package ; therefore, it is worth examining the correlations between the matriculation examination results and the phases of problem-solving separately. Here, we only deal systematically with the three mandatory matriculation examination subjects, as these data are available for all participants, while only a small proportion of students took the exams in a science discipline or EFL as an elective. As few students took the matriculation examinations at the advanced level, this analysis involves the results from the intermediate exams. For a comparison, we have computed the correlations between matriculation examination results and those from the knowledge tests (see Table Performance in two phases of problem-solving (knowledge acquisition and knowledge utilization) correlated at the moderate level . When we note differences between correlations, they are statistically significant.Two major observations stand out from Table Correlations for the six tests are summarized in Table These correlations confirm once again that knowledge acquisition is a more decisive component of problem-solving items than knowledge utilization. To examine the details of this relationship, we performed regression analyses with problem-solving and its two phases as dependent variables, using the disciplinary knowledge test results as independent variables Table .The differences between these analyses confirm previous observations on the role of knowledge acquisition and indicate that it is only mathematics and science whose contribution to the variance explained is significant and positive. Furthermore, even in the cases of knowledge acquisition, ~70% of the variance remained unexplained.As can be expected, there are large differences between the divisions at the university, both in performance on knowledge tests and on problem-solving. Therefore, it is anticipated that problem-solving has different relationships with disciplinary knowledge. To examine these differences, we have chosen two divisions with a large number of students participating in the assessments and with different study profiles. The division that deals with the humanities, known as the Faculty of Arts , participated with 212 students , and the division that deals mainly with the natural sciences, known as the Faculty of Science and Informatics , was represented with 380 students . They performed differently on each test  and 0.217 (Science) for the entire problem-solving test when the same analyses were performed.To examine the details of the relations between disciplinary knowledge and problem-solving, we performed the regression analyses separately for the two divisions. Taking into account the decisive role of knowledge acquisition, we present only the results for this phase of problem-solving in Table Although the same amount of variance of knowledge acquisition was explained by the same set of independent variables, the contributions of the individual variables are different. Mathematics and science play an important role at both divisions, and the contribution of EFL is also significant at the Faculty of Science.Previous studies ; on all other tests, men performed better. The largest difference was found in favor of men in problem-solving. Here again, knowledge acquisition shows a much larger difference, indicating that this is the more sensitive phase of problem-solving.p < 0.001) correlation between these two variables. The correlations of test results with mother's educational level and intention to learn are summarized in Table The relationship of test performance with students' socio-economic status is a well-known phenomenon, although there are large differences in this respect between countries and also between domains of assessment. International assessment programs  usually involve complex indices for this purpose, but we have only one variable to represent students' family background, mothers' education. A further variable that may be interesting in this context is what degree students want to earn (intention to learn). We have found a small . Taking into account the observations reported in the previous sections, here we deal only with the knowledge acquisition phase. As the main aim of the present study is to validate the DPS test and to explore its usefulness at the beginning of university studies, we conceived a model by using variables with significant correlations. We assume that students' gender and learning strategies influence their disciplinary test results, while these results  influence achievement in DPS.A model that adequately fits the data  is presented in Figure In this model, positive impacts of elaboration strategies on mathematics and science were found, while success in the knowledge acquisition phase of DPS was positively influenced by science and mathematics. Gender and memorization strategy as well as history have a negative relationship.Our results confirmed or extended several findings from previous research  and identified some new relationships as well .Previous research has already identified several characteristics of DPS at different ages, including primary and secondary school students  has been found between problem-solving and inductive reasoning  has a stronger relationship with any other observed variable than the second phase (knowledge utilization or rule application). Other studies have found similar differences, although the dominance of knowledge acquisition was not so obvious , mathematics being the second largest one with a much lower difference (49 points). The difference in knowledge acquisition is especially high (93 points). In PISA 2012, gender differences in problem-solving varied from country to country. The OECD mean was 7 points, and in Hungary it was below average, though it was not significant, only 3 points OECD, . To inteAlthough there are large differences between students according to the socio-economic status (SES) of their family—and Hungary belongs to a group of countries where the impact of SES is especially strong—there was no large effect found in problem-solving in the PISA 2012 survey. In our study, we also found a modest impact of mothers' level of education on students' problem-solving performance. The fact that problem-solving is less determined by social background than domain-specific competencies indicates a potential opportunity for disadvantaged students as they may show their strengths on these kinds of assessments.Previous studies have indicated a strong relationship between low- and high-achieving high school students and the different learning strategies they use Yip, . Our resThe results of the SEM indicate the complex nature of the relationships between the variables being explored. The DPS tasks are constructed so that completing them requires no preliminary knowledge within any discipline. Therefore, we may assume that if there are relationships between disciplinary knowledge tests and DPS tests, these relationships are established by factors other than the factual knowledge represented in the knowledge tests. Such factors may be learning strategies (we have variables for representing them) and certain cognitive skills needed both for completing the disciplinary tests and the DPS tests . In this model, gender as a variable (most probably) mediates women's better reading and poorer mathematics achievement . In sum, this model indicates that men outperform women, and this impact is mediated by the higher mathematics performance among men. The negative impact of memorization is transmitted via mathematics and science.As the PISA 2012 assessments also indicated, there are large differences between countries not only on the level of problem-solving performance, but also in the strengths of the relationships between several relevant variables as well; therefore, some particular results found in one country cannot be generalized over countries and cultures. Although some general tendencies were found, we have seen that the strength of the relationships we have examined in this study differs by division. Therefore, the generalizability of the strengths of these relationships is limited; nevertheless, the method we applied in this study is generalizable and may be useful to explore the actual relationships in any higher educational context. Participation was voluntary in the study; the actual samples are thus not representative of the divisions. Nevertheless, the analyses revealed some generalizable tendencies as well.The results from the present study have raised several further questions worth researching. The dominant role of knowledge acquisition indicates a promising line of inquiry to explore this phase in more detail. One promising direction is to identify students' knowledge acquisition strategies, e.g., the way they manipulate the independent variables when they attempt to discover how these manipulations impact changes in the dependent variable. Students' activities are logged, and their strategies may be ascertained with log file analyses. Latent class analysis may be an effective method to identify students' exploring strategies.The knowledge acquisition phase also deserves further study from the perspective of its relationships to learning strategies as well, for example, examining if poor problem-solving performance can be an indicator of inadequate learning strategies. If such a connection can be proven, problem-solving assessment could be a diagnostic tool for identifying poor leaning strategies, possibly more reliable than self-reported questionnaires. Further insights into the nature of cognition in the knowledge acquisition phase could be expected from studying it in relation to the learning to learn assessments  would provide foundations for the development of problem-solving by strengthening its component skills as well.The results from this study indicated that technology-based assessment of problem-solving may be a useful instrument to moderate the secondary–tertiary education transition. To improve its usefulness, the scoring system may be further developed, extending it with an automated log file analysis. Such an instrument would be especially helpful in selection processes (admissions tests) for the STEM disciplines. More detailed analyses of the relationship between problem-solving and the study profile would be needed to improve the test. In the present study, we compared divisions of study within the university, but a division is still not homogeneous; for example, students in biology training may be different from those in mathematics.We have found significant positive relationships with the questions on elaboration learning strategies and negative relationships with the questions on memorization strategies. In the present study, there were not enough questions to use sophisticated scales for representing these learning strategies, but the findings indicate the relevance of exploring the role learning strategies play in the development of problem-solving. This seems a promising area both for research and practice not only for higher education but also for earlier phases of school education.The predictive power of DPS can be explored later when data is available on the university achievement of the students participating in the present assessment. The test may have a diagnostic value (indicating poor study strategies or insufficient problem-solving skills) and can also be used to aid students in selecting a study track better suited to their cognitive skills.BC and GM have made an equal contribution to the study, including the design, data collection, analyses, and writing of the manuscript, and have both approved it for publication.The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. The reviewer, CL, and handling Editor declared their shared affiliation.


This paper proposes a non-contact nondestructive evaluation (NDE) technique that uses laser-induced ultrasonic waves to visualize corrosion damage in aluminum alloy plate structures. The non-contact, pulsed-laser ultrasonic measurement system generates ultrasonic waves using a galvanometer-based Q-switched Nd:YAG laser and measures the ultrasonic waves using a piezoelectric (PZT) sensor. During scanning, a wavefield can be acquired by changing the excitation location of the laser point and measuring waves using the PZT sensor. The corrosion damage can be detected in the wavefield snapshots using the scattering characteristics of the waves that encounter corrosion. The structural damage is visualized by calculating the logarithmic values of the root mean square (RMS), with a weighting parameter to compensate for the attenuation caused by geometrical spreading and dispersion of the waves. An intact specimen is used to conduct a comparison with corrosion at different depths and sizes in other specimens. Both sides of the plate are scanned with the same scanning area to observe the effect of the location where corrosion has formed. The results show that the damage can be successfully visualized for almost all cases using the RMS-based functions, whether it formed on the front or back side. Also, the system is confirmed to have distinguished corroded areas at different depths. Nondestructive evaluation (NDE) techniques have been successfully applied for the past several decades in various fields, including civil, mechanical, and aerospace engineering . HoweverEffective corrosion detection methods in SHM should be intuitive, and the results should be easily understandable. To this end, some researchers have developed acoustic and ultrasonic wave technologies, including laser vibrometry, laser interferometry, and pulsed lasers. In a previous study , a Lamb A PZT sensor can also be used in LDV scanning, and the biggest advantage of using an LDV-PZT system is that any arbitrary waveform can be applied at the excitation location of the laser point with the highest energy level. However, even though some aspects of LDV systems have improved, such as automatic focusing and laser spot visibility when measuring very small objects , there aOther researchers have proposed using LDV with an air-coupled transducer (ACT) for NDE when scanning large surfaces. This method has effectively detected various types of damage . HoweverMany techniques have been developed using LDV, Nd:YAG pulsed laser, ACT, and PZT systems for damage detection by implementing filtering methods and wavenumber-based methods. Ultrasonic wave imaging has been achieved using methods based on wave propagations, and the results have shown good detection abilities for various kinds of damage ,25,26,27Structural damage can be visualized by calculating the root mean square (RMS) values of the measured wavefield-time signals at every laser excitation point ,35. In oFew studies have used this technology to detect corrosion on internal structures or on the backs of structures, so the authors scan both sides of the plate using the same scanning area. In addition, few research has previously considered the effects of both the corrosion depth and corrosion size. The corrosion conditions for different depths and sizes are also discussed to verify the abilities of this method, including fast scanning of a large area, sensitive locating for corrosion detection, and effective classification of the damage degree. The major focus is on the experimental analysis.2. The laser mirror scanner can adjust the pulse laser scans for a specified location. The objective of this design is to operate two galvanometers at a wavelength of 532 nm. To ensure that the laser beam can efficiently scan the 2-D area of the target structure, the operating angles of the galvanometer are orthogonal to each other. The f-theta lens is installed at the end of the laser scanner system to reflect the laser beam, which focuses on the target detection area. As shown in As shown in During detection, the laser beams scan the target structure, and the ultrasonic waves are generated by a thermoelastic mechanism and are then propagated. An ultrasonic transducer installed on the front or back side of the structure measures the responses of the ultrasonic wave. In this study, the ultrasonic transducer is an amplifier-integrated acoustic emission (AE) sensor made of lead zirconate titanate piezoelectric ceramics. As shown in The specimens were corroded by applying concentrated hydrochloric acid every day. When the concentrated hydrochloric acid was no longer applied, the damage area ceased increasing as soon as the reaction of the aluminum plate had finished. To ensure safety, the specimens were cleaned after each reaction had finished. The damage depths were measured and recorded upon completion of each corrosion reaction, and the specimens were scanned after each corrosion reaction. Since the thickness of the specimens was small, the authors scanned the back side to compare the results to those of the front side. In fact, damage is not visible on the back side, so it is more important to be able to detect damage on the back side than on the front side.The specimen was fixed on a metal support, as shown in T denotes the total simulation time, and w are the signals of the reflected waves.The filtered UWPI data can be inversely transformed to effectively obtain the damage visualization image. The figures can be used to describe this process according to the RMS distribution of a reflected signal. The equation is given in :(1)wRMSIn addition, the energy distributions of standing waves are not uniform, so the color distribution in the RMS snapshots will be not uniform. To address that problem, the authors used the logarithmic values of the RMS function to ease the uniform distribution of the standing wave energy.RMSw and log) function snapshots of RMSw function, as shown in k is the weighting parameter and RMSw_kW is the proposed weighted RMS function. Therefore, the RMS snapshots can be improved by using the logarithmic values of the RMSw_kW function.Since the accumulations of standing wave energy in the vicinity of the sensor are more frequent, larger values are produced near the location of the sensor in the RMS function. Therefore, the farther the damage is from the sensor, the harder it is to detect. However, the RMS value of the entire area can be equalized during the subsequent time period by multiplying it with the following weighting parameter:RMSw_kW) function for Specimen 1 , the reflected wave becomes a source of new waves in the UWPI snapshots when the propagating waves encounter the corroded area, whether the damage was on the front or on the back side. To detect damage on the front side, the color becomes deeper as the damage becomes more severe because the RMS value of the corroded areas decreases. However, to detect damage on the back side, the color becomes lighter as the corrosion becomes more severe because the RMS value of the corroded areas increases. Therefore, the depth of the corrosion can be distinguished. Since all damaged areas were located on the front sides of the specimens, the results for the front side were clearer than those for the back side. When the corrosion depth for Specimens 1 and 2 was 0.5 mm, the corroded areas could not be clearly detected from the back side of Specimen 2, possibly because the damaged areas were too small. It is worth noting that in This method can be applied in SHM systems with potential for high detection rates and greatly reduced operator effort and setup time. Further studies will improve the accuracy of the damage level classification as well as the visual effects for damage detection. In addition, the study will be extended for more complex structures to further verify the usability of this technique.


Soft actuators made from elastomeric active materials can find widespread potential implementation in a variety of applications ranging from assistive wearable technologies targeted at biomedical rehabilitation or assistance with activities of daily living, bioinspired and biomimetic systems, to gripping and manipulating fragile objects, and adaptable locomotion. In this manuscript, we propose a novel two-component soft actuator design and design tool that produces actuators targeted towards these applications with enhanced mechanical performance and manufacturability. Our numerical models developed using the finite element method can predict the actuator behavior at large mechanical strains to allow efficient design iterations for system optimization. Based on two distinctive actuator prototypes’ (linear and bending actuators) experimental results that include free displacement and blocked-forces, we have validated the efficacy of the numerical models. The presented extensive investigation of mechanical performance for soft actuators with varying geometric parameters demonstrates the practical application of the design tool, and the robustness of the actuator hardware design, towards diverse soft robotic systems for a wide set of assistive wearable technologies, including replicating the motion of several parts of the human body. Due to multiple inherent advantages offered by most soft materials such as light weight, low manufacturing cost, large number of degrees of freedom and high adaptability, soft robotics12345678910111213143451Soft actuators are a critical component of soft robotic systems and can achieve actuation by employing a variety of methods, including actuation with the aid of electrical charges5105131314Depending on the design and structure of the actuator, SPAs can be used to generate a variety of different motion profiles, such as linear extension, contraction, bending and rotary motion, and apply mechanical forces or moments within the desired range. This makes them potentially very useful for practical implementation in human-assistive, wearable devices, targeted at either restoring or replicating the motion of parts of the body, or assisting with activities of daily living. There are several such examples of assistive, rehabilitative and wearable devices in literature which utilize SPAs as the principal actuator to drive them. Despite the established potential of SPAs and their diverse implementation towards robotic systems for meeting desired functional requirements in crucial applications such as the ones described above, the lack of repeatability in currently existing SPA design and fabrication procedures greatly limits their potential and performance in such systems. Discussed in further detail below, existing SPAs are largely confined to laboratory prototypes and their reported results are difficult to reproduce due to multiple, and in several cases, complex manufacturing steps requiring high manual precision or intervention, and unpredictable, uncontrolled mechanical failure. Because mechanical performance of SPAs depends greatly on the geometric parameters, small changes in the fabrication timing, sequence, or environment can contribute significantly to the actuator performance, making the production of repeatable results very challenging.In an effort to circumvent some of these fabrication and repeatability issues, newer actuator designs have been developed where the actuators are made in a single molding step16181920916221In parallel with the development of newer designs and manufacturing techniques for SPAs, there has also been an increased interest in the development of predictive models for these actuators in the recent past, since predicting the performance of the actuators is non-trivial due to the non-linear behavior of the materials used, the complex interaction between multiple materials employed and the sophisticated geometries involved. Several studies have employed analytical models to describe the mechanical deformations in SPAs25261622232829231629http://rrl.epfl.ch) where it is possible to use the models developed in the present work as a starting point to modify and create different geometries and materials for any robotic application.In the present study, we present two major findings from the development of a novel actuator design and its experimental results compared to predictions from the FEA based design tool. The presented two-part, shell-reinforced, SPA design allows “fool-proof” prototyping of both bending and linear actuators and produces results in the desired performance range. The presented numerical models using FEA accurately predict the complex mechanical response and the performance obtained with the designed actuators while allowing rapid design iterations to optimize the design parameters. The design tool and models used in the current study are also available open-source on the Reconfigurable Robotic Laboratory website . The un-stretchable shell, made from much stiffer polyethylene terephthalate (PET), (Q-ConnectTM), is mounted onto the actuator body and constrains its motion and mechanical performance in the desired range. A wide variety of materials were tested to select the best combination of materials for forming the actuator. For forming the actuator core, which is required to simultaneously exhibit high flexibility as well as mechanical strength at relatively low input pressure values (up to 50 kPa), highly stretchable materials such as EcoflexTM 00–10, EcoflexTM 00–20, ElastosilTM and DragonskinTM were tested. Actuators formed from EcoflexTM 00–10 and EcoflexTM 00–20 were observed to be not as robust (unable to withstand pressures up to 50 kPa without rupture or significant leakage) as those formed from EcoflexTM 00–30, due to the lower shore hardness for these materials as compared to EcoflexTM 00–30. On the other hand, actuators formed from ElastosilTM and DragonskinTM were found to be too stiff and required application of much higher pressures (>80 kPa) to achieve the same deflection as that achieved with EcoflexTM 00–30. Based on these results, EcoflexTM 00–30 was selected as the material to form the actuator core for further study and characterization. For forming the actuator shell reinforcement, a variety of materials were tested as well. These included PET, KaptonTM, fabric and nylon. Due to fabrication complexities and lack of repeatability issues incurred with some of these materials considered such as fabric and nylon, where the contained elastomer core was not adhering well enough to the reinforcement material to maintain a strong, robust contact and suffered from leakage during elastomer curing, and KaptonTM, in which case the shell was more difficult to cut precisely and glue onto the core, PET was chosen as the preferred material to form the shell and considered for further study. Thus, the combination of soft materials used here delivers the desired range of high flexibility as well as adequate robustness in actuation, at the same time. A schematic view of the actuator is shown in The technique used to fabricate the SPA presented in this work ensures robustness and repeatability in performance, since it leaves minimal manual intervention and margin for manual error. The two main parts of the actuators developed here are the actuator body and the un-stretchable shell. The actuator body is made out of highly elastomeric EcoflexThe actuator core comprises of a single air chamber, created in a single-step molding process. The un-stretchable shell mounted on top of the actuator body surface constrains the actuator body to inflate in only the desired configuration. The pattern created on the shell surface governs the pattern of displacement obtained with the actuator. Although it is possible to achieve other motion profiles as well by varying the patterns on the shell surface, bending and linear actuators were selected for further study in this work, patterns corresponding to which are shown in To determine the material properties and characterize the complex mechanical behavior for the stretchable materials used, we conducted thorough mechanical testing . To model the mechanical behavior of the shell material (PET), a linear elastic model was used, with a Young’s modulus of 2 GPa and a Poisson’s ratio of 0.35. The actuators reach a steady state and exhibit stable mechanical response within a time frame of 1–3 s after the application of internal pressure loading. Since the current study was aimed primarily at characterizing and comparing the performance obtained with different SPA designs in the steady state, the dynamics of airflow into the actuator core was not taken into consideration in the model here. The response of the SPAs to different levels of pressure loading was tested to find quasi-static solutions using the dynamic, implicit solving scheme. As compared to a fully static solution, the quasi-static solution improves the convergence of the model at higher strains. The air pressure load was modeled as an applied boundary condition on the entire internal surface of the core, and ramps up linearly from zero to the desired value in pseudo-time, to study the effect of varying levels of input pressure on the mechanical performance obtained with the SPA. Surface-to-surface contact between the shell elements and the actuator body elements was maintained using the slip interaction property, with finite sliding along the tangential direction. A penalty friction formulation was used to incorporate a finite coefficient of friction with isotropic directionality, with value in the range of 0.1 to 0.6. To model the highly non-linear mechanical behavior of the actuator body, an appropriate hyperelastic model was used. Uniaxial and planar tests in tension were performed. Further details on material testing are described in the We experimentally validated the performance of the SPA prototypes in terms of free displacement for both the bending and linear actuators, blocked force for linear actuators, and blocked torque for bending actuators. For free-displacement experiments, the proximal end-cap with the air inlet was clamped in a rigid fixture. The other end of the actuator was permitted to move freely while inflating, thereby generating a curved or straight trajectory, depending on whether a bending or linear actuator was tested. The experiments were performed three times with each geometric variant to analyze the positioning repeatability as well as the pressure-to-angle repeatability. To test the maximum blocked force delivered by the linear actuators, each end-cap of the actuator was rigidly clamped, with the actuator in an un-pressurized state. On the distal clamped end of the actuator , a six-axis force/torque sensor was used to measure the force produced by the tip of the actuator as the input pressure was ramped up from zero. The blocked torque testing for bending actuators was done in two steps. First, the actuator was clamped only at the end containing the air inlet, and the unconstrained end was allowed to inflate up to a fixed angle. In the second step, while maintaining the final orientation of the actuator achieved in the first step , the free distal end-cap of the actuator was clamped rigidly. The actuator was then subsequently inflated to higher pressures, starting from the level of pressure attained in the course of step 1, and the force/torque sensor was used to measure torque generated at the tip of the actuator.How to cite this article: Agarwal, G. et al. Stretchable Materials for Robust Soft Actuators towards Assistive Wearable Devices. Sci. Rep.6, 34224; doi: 10.1038/srep34224 (2016).