Ti-CFRP-Ti laminated stacks have been widely used in aviation, aerospace, shipbuilding and other industries, owing to its excellent physical and electrochemical properties. However, chip blockages occur easily when drilling into Ti-CFRP-Ti laminated stacks, resulting in a rapid rise of drilling temperature and an increase of axial drilling force, which may lead to the intensification of tool wear and a decline of drilling quality. Cutting force signals can effectively reflect the drilling process and tool condition, however, the traditional plate dynamometer is typically difficult in realizing the follow-up online measurement. Therefore, an intelligent tool holder system for real-time sensing of the cutting force is developed and constructed in this paper, and the variable parameter drilling method of Ti-CFRP-Ti laminated stacks is studied on this basis. Firstly, an intelligent tool holder system with high flexibility and adaptability is designed; Secondly, a cutting force signal processing method based on compressed sensing (CS) theory is proposed to solve the problem of high-frequency signal transmission; Lastly, the drilling experiment of Ti-CFRP-Ti laminated stacks is carried out based on the intelligent tool holder system, and the drilling parameters are optimized using a compromise programming approach and analytic hierarchy process (AHP). The comparison of results show that the optimized drilling parameters can effectively reduce the hole wall surface roughness and improve the drilling efficiency while ensuring a small axial force. In recent years, the requirements for strength, stiffness, fatigue resistance and lightweight properties of materials are becoming more urgent, with the continuous progression of aerospace technology. Under this context, lightweight high-strength materials such as Carbon Fiber Reinforced Plastics (CFRP) and titanium alloy (Ti) have found widespread use in the field of aerospace ,2. In prLaminated stacks are generally assembled by using bolts or riveting, and consequently, a large number of installation holes need to be processed. In actual production, the laminated stacks are usually clamped as a whole and drilled at one time to ensure assembly accuracy and to improve the efficiency of processing ,7. HowevDue to the differences in physical and cutting properties between the CFRP and titanium alloy, the optimal cutting parameter range remains inconsistent. For example, CFRP is suitable for a high cutting speed and low feed rate, while titanium alloy recommends a low cutting speed and high feed rate ,12,13. TTherefore, from the perspective of developing an intelligent hardware with a cutting force real-time sensing function, the overall integration scheme of a cutting force measurement system based on a tool holder is firstly studied. Secondly, a cutting force signal processing method based on compressive sensing is proposed. Lastly, the performance of a verification experiment on the intelligent tool holder system and a drilling experiment of Ti-CFRP-Ti laminated stacks are designed, and the multi-objective optimization of drilling parameters is realized based on the experimental results.The remainder of this paper is organized as follow: in Generally, a higher spindle speed and lower feed rate are used for drilling CFRP, while a lower spindle speed and higher feed rate are suitable for drilling titanium alloy. Therefore, it is necessary to select appropriate cutting parameters according to the mechanical properties of each material, which means that three groups of cutting parameters should be used for drilling three layers of materials, respectively. Variable parameter technology refers to the change of appropriate cutting parameters when drilling into the interface of two materials. According to the contact between the cutting tool and a workpiece material, the drilling process can be divided into seven stages, as shown in xF, yF, zF and T, as shown in zF and T in drilling process, therefore, the system designed in this paper provides data for drilling Ti-CFRP-Ti laminated stacks by sensing the axial force zF and torque T. In order to realize the real-time perception of cutting force, it is necessary to study the high-precision force sensor, the optimization design of tool holder matrix structure and the signal processing transmission mode.During the cutting process, the cutting system is composed of a spindle, tool holder, cutting tool and workpiece, as shown in It was found from the author’s previous research  that the(1) Universality. The basic structure of the improved tool holder enables the clamping of the tool holder system on the machine tool spindle and the clamping of the tool as well as the handling of the manipulator without changing the existing device of the cutting system.(2) Dynamic balance. The centroid of the improved tool holder should be on the axis of rotation to avoid eccentricity and accidents.(3) High sensitivity. As the elastic element, the improved tool holder should have good sensitivity to realize precise sensing of the micro cutting force.(4) High stiffness. In order to ensure the machining accuracy in the cutting process, the improved tool holder should have good rigidity to reduce the cutting deformation.(5) Manufacturability. The structure of the improved tool holder should be designed and fabricated on the premise of easy processing compared with the standard tool holder.Following the above design criteria, it was determined that an annular groove is processed on the standard tool holder for attachment of the strain gauge to form a resistance sensor, and the circuit carrier is mounted outside the cylindrical surface of the tool holder, where two sets of signal processing modules and power modules are embedded on. The force data are collected and transmitted to the host computer via Wi-Fi. The system model and the assembled prototype are shown in The intelligent tool holder system has a high cutting frequency in the case of high-speed and multi-edge cutting. According to the classical Nyquist–Shannon sampling theorem, the sampling rate must be greater than twice the signal bandwidth to ensure accurate signal reconstruction. The maximum frequency is equal to the cutting frequency during the cutting process. This method generates redundant data and causes data congestion problems, due to the limitation of data bandwidth by the standard transmission protocols and hardware devices. By using prior knowledge of signal sparsity, compressive sensing (CS) theory indicates that the signal can be compressed while sampling, which achieves a sampling frequency far lower than Nyquist rate and sparse signals are reconstructed with high precision. Therefore, in order to realize the processing of cutting force signals based on CS, the sparse representation of cutting force signals, the acquisition of sampling signals, and the high-precision reconstruction of sampling signals are studied in this paper.x can be represented by a linear combination of a set of N standard bases that is arbitrary and the length is N. As follows in Equation (1):N × N representation basis with N × 1 sparse vector with The sparse signal is defined when only a few elements are non-zero. However, few signals in nature are sparse in the time domain, and most signals may be sparse in the transform domain. According to the theory of harmonic analysis, the one-dimensional discrete signal x can be expressed as sparsely represented under the basis K  constructs the signals relationship between time domain and frequency domain. However, the cutting force signal obtained by the intelligent tool holder system is a continuous analog signal and cannot be recognized by the computing device directly, thus it should be converted to a discrete digital signal to process. Discrete Fourier Transform (DFT) is an FT method that can realize the spectrum analysis of discrete signals with a finite length in both the time domain and frequency domain. Its form is as follows in Equation (2), and is rewritten into matrix form as follows in Equation (3):Which isK = 103.The data as shown in y is the compressed signal, M × 1 and M is the measuring value, M × N and The sparse signal needs to be mapped to a low-dimensional space, as follows in Equation (5), and the compression ratio is defined as e matrix .(5)y=Φ·x is N and the sparsity is K, the measuring value M only needs to satisfy the relationship M is no less than 235, the high-precision reconstruction of the cutting force signal shown in In this paper, a Gaussian random measuring matrix is used to construct the sampling matrix. When the length of the original signal In order to further improve the compression performance of the measurement matrix and the accuracy of signal reconstruction, a measuring matrix optimization method combining approximate  decomposition and minimum average mutual coherence coefficient is proposed.Q and an upper triangular matrix R for any Q satisfies relationship According to the matrix decomposition theory, the correlation of the matrix decreases with the increase of the minimum singular value . As a spR unchanged and setting other position elements to zero. Subsequently, Q and v and R and In approximate QR decomposition, the diagonal matrix G and t is a non-negative parameter.In order to further reduce the correlation of matrix, the minimum average mutual coherence coefficient method is introduced, which calculates the t-average mutual coherence coefficient defined in Equation (8), as proposed by Elad .(8)μt algorithm  could piteration . The recc = 0.7, the reconstruction error is 2.10% and the reconstruction time is 0.016 s for milling cutting force signal as shown in c = 0.6, the reconstruction error is 1.10% and the reconstruction time is 0.052 s for drilling cutting force signal with the sparsity K = 201.When the DFT basis is used to sparsely represent the cutting force signal and the optimized Gaussian random measuring matrix is used to construct sampling signal, the reconstruction effect of the force signal during the drilling process by CoSaMP algorithm is shown in To verify the performance of the cutting force measurement in a real-time machining process, several cutting tests were carried out. For the actual cutting process, there are many factors affecting the cutting force, thus it is hard to calculate the cutting force precisely through the theoretical formulas. The actual measuring effect can only be assessed by comparing the measurement results of the high-precision reference dynamometer. In this paper, a Kistler 9119AA2 plate dynamometer was selected as the reference dynamometer, a 10 mm straight handle twist drill made of HSS with helix angle of 30 and apex angle of 135  was used and assembled in the intelligent tool holder. The experiments were conducted by drilling 45 steel under dry cutting conditions using a XK714D three-axis vertical machining center . The cutting experiment platform was built as shown in 3 and 60 × 40 × 6 mm3, respectively. The properties of these two materials are shown in The laminated workpiece consists of two titanium alloy layers (TC4) and one CFRP layer (T300) with inner filaments laid in directions of 45°, −45°, 0°, 90° with the size of 60 × 40 × 5 mmThe experiment was carried out on a DV800 three-axis vertical machining center. The intelligent tool holder was installed on the spindle of the machine tool. Based on the specially designed fixture, titanium alloy and CFRP were stacked layer by layer to form a laminated workpiece where the number of layers can be adjusted randomly. Moreover, the titanium alloy and CFRP were cut along the axial direction after drilling, and the Form Talysurf i60 desktop roughness profiler was utilized to measure the surface roughness of the hole wall. The experimental platform is shown in A cemented carbide twist drill was selected as the cutting tool according to the previous research results  with a 6There are six drilling parameters in the process of variable parameter peck drilling: spindle speed of upper titanium alloy layer n = 500 r/min and feed speed vf = 50 mm/min are shown in n = 500 r/min and feed speed vf = 50 mm/min. It can be seen that the mean value of the tool holder system and reference dynamometer measurement in the axial force direction was 1113.0 N and 1114.3 N with a deviation of 0.12%, and that in the torque direction was 7.9064 N·m and 7.4122 N·m with a deviation of 1.02%. A difference between the measurement method and sensing position will lead to the deviation of the measurement results between the intelligent tool holder system and the dynamometer. Overall, the deviation of torque is slightly larger than the axial force. The reason is that the semiconductor sensor adopted for the measurement of axial force comes with a higher sensitivity and resolution. Considering the bonding process, the resistance sensor is used for torque measurement with slightly lower sensitivity. Generally, the maximum deviations do not exceed 2%, which can meet the actual machining requirements.The comparisons of torque and axial force measurement results between the intelligent tool holder and reference dynamometer at a spindle speed The results of the drilling test show that the intelligent tool holder system and reference dynamometer bear good consistency in measuring performance, which means that the system has high measuring accuracy and can be used to measure cutting force in actual production and processing.The orthogonal experiments of drilling parameters were carried out by changing the parameters at the interface region of the two materials. The main effect plot was obtained by range analysis, so as to analyze the axial force and surface roughness of each layer. The mathematical regression of axial force and surface roughness was carried out by statistical method.As the most important physical quantity in drilling process, the axial force has an important impact on tool wear and hole quality. The range analysis of the axial force in the experiment results of Ti-CFRP-Ti peck drilling with variable parameters was carried out to study the influence of drilling parameters on the axial force. It can be observed that the variation trend of axial force of the titanium alloy and CFRP with drilling parameters is essentially the same, but the specific values differ. On one hand, the axial force fluctuates with an increase of spindle speed, but tends to decrease as a whole. The reason may be that an increase of cutting speed changes the interface friction coefficient, while the strength and hardness of machining materials become reduced due to heat accumulation, thus reducing the cutting load. On the other hand, the axial force increases linearly with feed rate, as the increase of feed rate makes the drilling area larger, and the friction in drilling deformation area and the resistance of drilling material will lead to an increase of the axial force. In addition, the variation range of the axial force caused by the spindle speed is relatively small, but the variation of the axial force caused by the feed rate is relatively large during the whole drilling process of Ti-CFRP-Ti laminated stacks. Therefore, the influence of feed rate on the axial force in Ti-CFRP-Ti peck drilling process is much greater than that of spindle speed.The mathematical model of the axial force and drilling parameters was established by regression analysis of the data of the axial force in the peck drilling of Ti-CFRP-Ti laminated stacks with variable parameters in order to further study and predict the relationship between the axial force and drilling parameters. The exponential relation was used as a regression model of the axial force, as follows in Equation (10).For drilling of upper titanium alloy:For drilling of CFRP:For drilling of lower titanium alloy:Goodness of fit is generally used to measure the fitting degree of the regression model to experimental value to verify the accuracy of the regression model. The statistical measure of goodness of fit is the coefficient of solution (determinate coefficient), which is expressed by It is believed that the regression model has high reliability and can accurately predict the test values when Surface roughness of the hole wall is an important index to measure drilling quality. Small surface roughness can not only bring reliable assembly accuracy, but also improves the reliability and service life of parts. The surface roughness of Ti-CFRP-Ti laminated stacks was measured after variable parameter peck drilling and the results were obtained by main effect analysis, as shown in It can be observed that the variation trend of the hole wall surface roughness with drilling parameters is similar to that of cutting force. On one hand, the hole wall surface roughness of CFRP decreases with an increase of spindle speed, while the hole wall surface roughness of titanium alloy shows a local fluctuation with spindle speed, however, it also shows a negative correlation. The reason may be that lower spindle speed is more likely to produce built-up edges and scales, which will scrape the machined surface. On the other hand, the hole wall surface roughness of each layer shows a nearly linear growth trend with the feed rate, which is due to the increase of chips which strengthen the scribing and friction effects on the surface of the hole wall. In addition, it was found that the surface roughness of CFRP layer was significantly greater than that of titanium alloy layer, The reason may be that CFRP is made of high-strength carbon fiber with a rough section, and the chip removal process of the lower titanium alloy layer will scratch the hole wall of CFRP layer.A, B, C, D, E, F are regression constants, n and f are spindle speed and feed rate, respectively.Regression analysis was used to analyze the experimental results and a function model of the hole wall surface roughness on drilling parameters was established in order to obtain the relationship between the hole wall roughness and drilling parameters. The researches in references ,3 show tThe surface roughness of each hole wall was measured separately, and a mathematical model of the hole wall surface roughness and drilling parameters of Ti-CFRP-Ti laminated stacks was obtained.For drilling of upper titanium alloy layer:For drilling of CFRP layer:For drilling of lower titanium alloy layer:The coefficients of the regression model for the roughness of each layer were iP, The multi-objective optimization model includes three elements: design variables, objective functions and constraints. Surface roughness, axial force and material removal rate are taken as objective functions of the optimization problem. Spindle speed and feed rate are taken as design variables of optimization problem in order to investigate the influence of spindle speed and feed rate on the objective function. In addition, the constraints refer to the range of drilling parameters. The multi-objective optimization models can be listed according to the actual situations as follows in Equation (19).On one hand, the objectives are often mutually constrained, and generally unable to achieve several optimal objective functions at the same time in multi-objective optimization. The improvement of one objective often leads to the reduction of other objectives, so only coordination and compromise between them can be carried out to optimize each objective as far as possible. Compared with the single objective optimization problem, the solution of the multi-objective optimization problem is not unique, but a group of optimal solutions verified by experiments. On the other hand, the regularity between the variable parameters selected by each layer and the target parameters obtained is relatively independent.Firstly, a unified evaluation function for the optimization of each layer was established by transforming multi-objective optimization into single-objective optimization based on compromise programming approach. Then, the weight factor of each layer material objective function was determined by AHP, so as to construct the evaluation function. Finally, the genetic algorithm was used to optimize the parameters of the evaluation function, and the unique solution was obtained.It is often desirable that the drilling efficiency can be improved on the premise of guaranteeing the processing quality. Therefore, an optimization model with the objective function of the hole surface roughness, axial force and material removal rate was established. The function of the axial force and the surface roughness of the hole wall is the mathematical model obtained by regression analysis in the previous paper. The material removal rate function is as follows in Equation (20).d = 6 mm is a fixed value and the minimum value of the function is required in the optimization process, thus the negative number of material removal rate is taken as the objective function, which is shown in the Equation (21).The drill diameter The linear weighting method is generally used to transform a multi-objective into a single-objective, but the evaluation function constructed is only applicable to the problem assuming that each objective function is convex. Moreover, the order of magnitude of the objective function in this paper is quite different. Therefore, the compromise programming approach was selected to keep each objective function in the same order of magnitude. The mathematical expression of the compromise programming approach is as follows in Equation (22).The optimization is divided into a decision layer , criterion layer  and scheme layer (spindle speed and feed rate), as shown in The relative importance of three objects is determined to construct the mutual judgment matrix according to the scale table . The judFor the optimization model of drilling parameters of the upper titanium alloy, it was found that the axial force in drilling the titanium alloy layer is large and the processing system vibrated easily. However, the surface roughness of the hole wall is almost the same value needed to meet accuracy requirements. Therefore, the importance of the upper titanium alloy is There are several methods to calculate the ranking weight of each index from the judgment matrix. The most widely used method is the eigenvalue method (EM). It is used to calculate the weight of each index as follows.Let The consistency test and random consistency test of the judgment matrix are carried out in order to ensure the accuracy and reliability of the judgment matrix and avoid the influence of subjective factors on the judgment matrix. The weight factors pass the test by calculation.It was observed that the surface roughness of CFRP layer was large, the quality of the drilling was poor, and the axial force was stable. There should be It was found that the value of the axial force was large and the fluctuation was large when drilling lower titanium alloy. In addition, the spindle speed was low considering drilling temperature and other factors, which lead to a low processing efficiency. The surface roughness of the hole wall of lower titanium alloy was larger compared with the upper titanium alloy, but essentially satisfies the manufacturing requirements. Thus, there is The optimization functions for each layer are established as follows.Compared with traditional optimization methods, genetic algorithm has the advantages of good convergence, strong global search ability, high robustness and scalability, etc. The genetic algorithm in MATLAB was used to solve the above optimization model. Setting population size, crossover probability, crossover function, mutation function, maximum generation and deviation value of fitness function to solve the optimization model, the subsequent calculation results are shown in A variable parameter peck drilling experiment was carried out using the optimized drilling parameters. The experimental results and the calculated results of the regression model with the optimized parameters are shown in In summary, multi-objective optimization based on AHP and a compromise programming approach can provide a sound consideration to axial force, surface roughness of the hole wall and drilling efficiency, which shows that the optimization results are excellent.In this paper, the variable parameter drilling method of Ti-CFRP-Ti laminated stacks based on real-time sensing of drilling axial force was studied. An intelligent tool holder system with the function of real-time cutting force measurements was developed, the cutting force signal processing method based on compressive sensing was explored, and an experimental platform was built based on the intelligent tool holder system to optimize the drilling parameters of Ti-CFRP-Ti laminated stacks. The conclusions are summarized as follows:(1) An intelligent tool holder system with a real-time sensing function of the axial force and torque for milling or drilling processes is designed. The resistance sensor, data acquisition, transmission module, and power module are integrated into the tool holder as a complete system. Compared with the plate dynamometer, the equipment is not restricted by the size of the workpiece and avoids damage to the structure of the workpiece. In addition, it can provide a key data source for the analysis and optimization of the machining process of aerospace parts.(2) A cutting force signal processing method based on compressive sensing is proposed. DFT orthogonal basis is used for sparse representation of the cutting force signal. Approximate QR decomposition and the minimum average mutual coherence coefficient method are combined to improve the Gaussian random measurement matrix, and the compression measurement of the original signal is realized. Subsequently, the CoSaMP algorithm is introduced to complete the high-precision reconstruction of measurement signal. The test results show that the number of data samples can be significantly compressed, and the high-precision reconstruction of the cutting force signal under the premise of ensuring the reconstruction efficiency is realized. This method may provide reference for the construction of a sensor network and data perception of workshops.(3) The measurement performance of the intelligent tool holder system is verified by experimentation, and the results show that the intelligent tool holder system and reference dynamometer bear good consistency with a deviation less than 2%. The orthogonal experiment of Ti-CFRP-Ti laminated material variable parameter drilling is designed. According to the measurement results of axial force and surface roughness of the hole wall, the drilling parameters are optimized using a compromise programming approach and AHP. The maximum error between the results of experiment with the optimized parameters and the calculation results of the regression model is 12.7%, which verifies the accuracy of the model from an experimental point of view. In addition, it was found that the overall excellent rate of CFRP layer and titanium alloy layers could reach 80% and 90%, respectively, by comparing the experimental results of optimal parameters with the orthogonal experimental results, which shows that the optimization results can take into account the drilling axial force, surface roughness of hole wall and processing efficiency. The proposed method can be used to select suitable machining parameters for laminated stacks, and will be beneficial for improving tool life and processing efficiency.In future work, the temperature measurement system will be deployed, and the delamination of CFRP, burr of titanium alloy and tool life in the drilling process of laminated stacks will be studied. In addition, we will further explore the application of an intelligent tool holder system in the field of tool condition monitoring and chatter recognition.


In the present research work, an electrical resistance tomography (ERT) system is utilized as a means for real-time fault detection and diagnosis (FDD) during a reactive crystallization process. The calcium carbonate crystallization is part of the carbon capture and utilization scheme where process monitoring and malfunction diagnostics strategies are presented. The graphical logic representation of the fault tree analysis methodology is used to develop the system failure states. The measurement consistency due to the use of a single electrode from a set of ERT electrodes for malfunction identification is experimentally and quantitatively investigated based on the sensor sensitivity and standard deviation criteria. Electrical current measurements are employed to develop a LabVIEW-based process automation program by using the process-specific knowledge and historical process data. Averaged electrical current is correlated to the mechanical failure of the stirrer through standard deviation evaluation, and slopes of the measured data are used to monitor the pump and concentrations status. The performance of the implemented methodology for detecting the induced faults and abnormalities is tested at different operating conditions, and a basic signal-based alarming technique is developed. Crystallization has a significant impact on the final characteristics of particulate systems and plays an essential role in the manufacturing stream of various industrial processes such as agricultural chemicals, cosmetics, pigments, food ingredients, and the highly-regulated active pharmaceutical ingredients (APIs). Real-time anomaly detection and monitoring the functioning stability of the physical–chemical components during crystallization processes are crucial to ensure a reliable operation, minimize performance variations, and, therefore, improve product quality and production volumes.In particular, there has been recent growth in need of chemical industries for precipitation processes, which lies in the extreme requirement of energy-efficient operation, process intensification, and sustainability . In compConsidering the extensive applications of crystallization, a wide range of in situ and online instrumentation and distributed sensing technologies have been developed for process monitoring and control, which noticeably improve the quality of the final crystals. For instance, the following sensor technologies have been extensively used: focused beam reflectance measurement , particlCrystallization monitoring tools provide abundant information from the system that augments the understanding of the unit operation and enables a framework to identify potential problems and abnormal behaviors. Among such distributed sensing measurement systems, process tomography is an emerging technology that delivers the capability of measuring spatio-temporal field information , visualization through image reconstruction, and cross-sectional images (tomographs) of particular property distribution .Tomographic measurements in chemical engineering processes can generate a large amount of data from process parameters and facilitate the route towards developing: (i) pattern recognition for fault detection via machine learning algorithms and data fusion strategies , (ii) daElectrical resistance tomography (ERT) is one of the fast-evolving tomographic modalities used to characterize a process through quantifying the electrical field of the medium. The fast growth of ERT is mostly due to the economic aspects, the relative simplicity of implementation, and the potential for sub-millisecond temporal resolution . On the Electrical resistance tomography is a subcategory of electrical impedance tomography in which the real component of the impedance is measured. Most applications of ERT in chemical engineering are dedicated to the extent of agitation performance in stirred tank reactors: liquid/liquid mixing , solid/lConsidering the nature of the ERT, spatially averaged conductivity values allow the measurement to be obtained across the planar region of interest. ERT has the potential for species transformation measurement and reaction progress monitoring through quantifying the topology-based visualizations of reactive crystallization processes. Successful implementation and interpretation of the physical phenomena improves the understanding of the mixing and feed addition in a stirred tank reactor and facilitates process optimization. Additionally, due to the characteristics of the ionic solutions, rapid changes can be recorded and correlated to specific events during the process, for instance, identifying the feed addition time and location, the start of mixing, etc. To the best of our knowledge, utilizing electrical resistance tomography sensor information as a means for real-time fault detection and diagnosis (FDD) during (reactive) crystallization processes has not been investigated.Common methodologies for fault detection, malfunction identification, and abnormal events characterization in chemical processes are carried out by failure assessment techniques such as fault tree analysis (FTA), Bayesian network (BN), and principal component analysis (PCA) . Even th3 is investigated. A basic qualitative fault tree analysis was carried out to identify the key events during the process, which includes stirrer states, pump, and feed concentration. The utilized voltage exciting-current measurement ERT system comprises a single plane of 16 stainless steel electrodes around the perimeter of the crystallizer. The measured electrical current of a single electrode was used as input to an in-house developed LabVIEW program where dynamic statistical analysis was carried out for process automation, decision making, and alerting system. The strategy for selecting a single ERT sensor was experimentally and quantitatively investigated based on the relative sensitivity criterion of the individual electrode. Theoretical calculations, experimental repetitions, and process-specific knowledge were used to analyze the statistical patterns in the measured electrical current and to ensure the selection of the suitable sensor (electrode). The investigated crystallization process is part of the carbon capture and utilization scheme where process malfunction and monitoring are presented in this research work.In this paper, a real-time ERT-based fault detection and diagnosis approach for the reactive crystallization process of CaCOThe chemical reaction governing the liquid–liquid reactive crystallization of calcium carbonate is presented in Equation (1):In the process under investigation, the semi-batch feed contains dissociated The feed solution is a result of absorbing A gas-liquid hollow fiber membrane contactor  was used for olutions . FirstlySecondly, in the subsequent chemical reactions, which are very fast at higher pH values ,32, aqueHydrogen carbonate formation occurs immediately after carbon dioxide dissolution (Equation (2)); hence, the aqueous form of the given in ,34, respThe circular electrodes with a diameter of 12 mm were made of stainless steel and were configured inside the wall of the reactor. There was no visible oxidation or rust on the electrodes, and they are not interfering with the chemical reaction. Experiments were conducted at a temperature of (20 ± 2) °C. A plastic-made Rushton impeller was used for agitation; in comparison to the metal impeller, the plastic-made impeller reduces the induced noise intensity during the data acquisition process, which is favorable in terms of image reconstruction and statistical signal analysis. The impeller tip speed was kept constant at The fault tree analysis methodology was used to develop the system failure states irrespective of their severity. The FTA is a graphical logic representation of combinations of failures or events that may occur to a functional system and shows a map of failure paths. Depending upon the criticality of the process, each branch can be developed further. The logic gates in the FTA are commonly represented by OR and AND gates. In OR gate, system components interactions are in series: failure of any single element leads to failure of the entire process. In AND gate, system components interact in parallel; thus, simultaneous failure of all the system components is required to fail the whole process.The fault tree comprises a top-level event, intermediate events, and potential root causes for each scenario. Three primary causes induce the failure of the topmost event: stirrer status, initial concentration in the receiving tank (reactor status), or pump status (feed addition). Reagent distribution in the crystallizer is not efficient when the mixer is in switched-off mode and results in extensive aggregation and agglomeration of particles. Moreover, the lack of Single- and multi-electrode current measurements and real-time statistical analysis of the ERT electrodes can be exploited for real-time fault detection and malfunction diagnosis when out-of-specification events occur throughout the entire process. In the present study, the graphical user interface for data acquisition and malfunction identification was developed in the LabVIEW software environment. A first-in-first-out (FIFO) buffer memory was deployed to store the electrical current measurements from all the electrodes where the newly available measurements continuously substitute the oldest data points in the local history. Moving average and a user-controlled sample length are used for sampling the measurements of a pre-selected electrode. Real-time tuning of the sampling length and intervals provides a rational approximation of the events from inside the suspension and the precipitation process.Averaged single-electrode data from the ERT system were correlated to the mechanical failure of the stirrer through standard deviation (SD) evaluation. The measured electrical signals were averaged every 5 s, and then, a moving average with a time difference of 20 s was used for dynamic evaluation of the standard deviation, which provides satisfactory performance for basic applications. According to the relation in Equation (5), when the real-time calculation of the SD ratio criterion is greater than a pre-determined parameter, it is an indication of an abrupt change in the status of the mixer . The mixer standard deviation ratio criterion ) during the calcium carbonate crystallization process results in the variation of the electrical current flow in the opaque solution. To enhance the accuracy of the process automation in terminating the crystallization , a minimum runtime criterion was defined. The theoretical minimum runtime was determined by the total ion balance modeling of the entire process—that is, the minimum amount of the feed solution required to consume a known calcium chloride concentration in the receiving tank. A final command to terminate the crystallization process is applied when both the experimentally measured slope factor criterion is satisfied, and the minimum runtime has elapsed.The data presented in impeller , N is thAs presented in The operating architecture of the utilized ERT system is voltage exciting and current measurement via a system of equidistant electrodes fixed around the boundary of the stirred tank reactor. Gauss–Newton reconstruction algorithm was used for post-processing the ERT tomographs. Even though tomographic data can provide cross-sectional representation from within a stirred tank reactor, in the current investigation, the ERT reconstructions could not be utilized due to non-deterministic and insensitive data from the crystal suspensions. Employing them for fault identification does not efficiently characterize malfunction situations. Moreover, the complex nature of the topology-based visualization renders the real-time interpretation a challenging task. The transient correlation of the reconstructions to physico-chemical phenomena of the fast-kinetic crystallization process becomes extremely limited due to a relatively slow reconstruction time and the global resolution. In summary, more research efforts are needed for image reconstructions studies in detail. On the other hand, statistical analysis of the single- and multi-electrode current measurement of the ERT provides satisfactory information from the suspension that contains a conductive liquid phase and non-conductive solid phase. Additional statistical analysis of the electrical currents from ERT electrodes demonstrates a continuous pattern in the acquired signals that can be linked to physio-chemical changes of the precipitation process. As discussed in The rise in signal amplitude due to a change in the mixer status is robustly observed over electrode no. 2 b—a quantAlthough employing a single electrode to analyze the ERT measurements provides partial knowledge of the suspension and the precipitation process, it demonstrates as a promising tool to investigate faults and malfunction throughout the process. In the present study, the measurement of the electrical current from electrode no. 2 was used to develop the FDD methodology. The selected electrode was located close to the feed point and demonstrated a relatively higher sensitivity to the process under investigation.A good qualitative agreement was identified between the theoretical analysis of the total ion balance a and theTo investigate the dynamical effects of solid calcium carbonate on the electrical field of the ERT system, two different amounts of solid particles were added into the reactor containing 3 L water. Solid samples of commercial calcite  were used for the measurement. Particles were added from the top of the reactor by hand.Different scenarios of the ERT-based malfunction investigation are listed in In all the cases, the initial concentration of calcium chloride was 1.6 , the slope of the measured electrical current has a descending trend because of the continuous reagent addition and formation of solids. The decreasing slope of the fitted linear line between 5–12 min, which equals ca. Furthermore, as shown in Figure 9To ensure a satisfactory single-sensor operation during the precipitation process, the performance and sensitivity of different electrodes of the ERT system to pump failure were analyzed. As displayed in In addition, According to the FTA analysis, the content of the feed solution can lead to reactive crystallization failure. Issues in the feed solution could be due to prior faults in the system, for instance, inefficient In this paper, real-time malfunction diagnosis and fault detection in the reactive crystallization process of The fault detection and diagnostics of the investigated process were developed based on the fault tree analysis approach. It has been shown that failures in the critical process parameters such as mixing, pump, and concentrations can be effectively identified by the existing patterns of a single-electrode measurement. Stirrer operational condition was developed based on standard deviation analysis of the incoming signals, while the electrical current slope assessment was used for reporting the pump (feed addition) situation. To ensure a reliable single-electrode operation, experimental repetitions and quantitative analysis of the standard deviation and slope factors are thoroughly investigated. The process automation program was developed in the LabVIEW software environment, and the threshold values to trigger the alert system were experimentally investigated.Although the tomograph reconstructions failed for the reasons mentioned in this paper, it was possible to develop a fully functional fault detection system using ERT sensor data. The most sensitive measurement point based on the experimental measurements was selected as sensor data transmitter to fault diagnosis.The current fault detection approach may be studied by varying the input parameter values broadly to check system sensitivity and robustness. In this context, investigation of different signal processing schemes  could be tested in malfunction identification. Adding more automation to the associated program, such as the capabilities to dynamically identify the most sensitive electrode, can be the subject of further investigations. Additionally, a key area of further research concerns the development of ERT reconstructions and investigates its potential to be used in the process under investigation. Subsequently, the integrated approach  provides a means to simultaneously study the particle suspension densities at the outlet section and inside the crystallization reactor.


The acceptance of coronary CT angiogram (CCTA) scans in the management of stable angina has led to an exponential increase in studies performed and reported incidental findings, including pulmonary nodules (PN). Using low-dose CT scans, volumetry tools are used in growth assessment and risk stratification of PN between 5 and 8 mm in diameter. Volumetry of PN could also benefit from the increased temporal resolution of CCTA scans, potentially expediting clinical decisions when an incidental PN is first detected on a CCTA scan, and allow for better resource management and planning in a Radiology department. This study aims to investigate how cardiopulmonary hemodynamic factors impact the volumetry of PN using CCTA scans. These factors include the cardiac phase, vascular distance from the main pulmonary artery (MPA) to the nodule, difference of the MPA diameter between systole and diastole, nodule location, and cardiomegaly presence.Two readers reviewed all CCTA scans performed from 2016 to 2019 in a tertiary hospital and detected PN measuring between 5 and 8 mm in diameter. Each observer measured each nodule using two different software packages and in systole and diastole. A multiple linear regression model was applied, and inter-observer and inter-software agreement were assessed using intraclass correlation.A total of 195 nodules from 107 patients were included in this retrospective, cross-sectional and observational study. The regression model identified the vascular distance (p < 0.001), the difference of the MPA diameter between systole and diastole (p < 0.001), and the location within the lower or posterior thirds of the field of view (p < 0.001 each) as affecting the volume measurement. The cardiac phase was not significant in the model. There was a very high inter-observer agreement but no reasonable inter-software agreement between measurements.PN volumetry using CCTA scans seems to be sensitive to cardiopulmonary hemodynamic changes independently of the cardiac phase. These might also be relevant to non-gated scans, such as during PN follow-up. The cardiopulmonary hemodynamic changes are a new limiting factor to PN volumetry. In addition, when a patient experiences an acute or deteriorating cardiopulmonary disease during PN follow-up, these hemodynamic changes could affect the PN growth estimation. Cardiopulmonary hemodynamic factors affect volumetry for pulmonary nodule management.The impact of hemodynamic changes in volumetry measurements is not related to the cardiac cycle phase, making them relevant even in non-gated scans.During follow-up of a pulmonary nodule, hemodynamic changes (such as decompensated heart failure) could potentially impact growth estimation.The introduction of coronary CT angiograms (CCTA) in the diagnosis and management of stable coronary artery disease (CAD) has improved clinical outcomes and reduced the need for invasive coronary angiography . As a rePulmonary nodules (PN) are among the most common incidental extracardiac findings on CCTA (up to 28% of scans) and require follow-up, even if only rarely malignant –5.Several scientific societies have published guidelines and recommendations for PN management . Small sIn solid non-calcified PN between 5 and 8 mm in size, the growth rate is a better discriminator between benign and malignant pathology than size or morphological features . The tim3), or 3–6 months if there are multiple solid nodules with a diameter greater the 6 mm (> 100 mm3) , p < 0.001); with the increasing difference of the MPA diameter between systole and diastole ; when it is in the lower third  compared to the upper (reference) in the coronal plane; and when it is in the posterior third  compared to the anterior (reference) in the axial plane.The results show that the volume measurement increases with the average diameter , and with increasing vascular distance .Volume measurement decreases when segmentation is considered appropriate , followed by the appropriateness of segmentation , the location of the PN in the lower third  or the posterior third  of the FOV, the difference of the MPA diameter between systole and diastole  and for the distance between the MPA and the PN .The effect size is larger for the average diameter  and somewhat smaller in women , but with a negligible effect size.The results also show a tendency for the nodule to be slightly larger in the presence of cardiomegaly  = 0.998, partial η2 = 0.002).The cardiac cycle phase is not statistically significant in our regression model (p = 0.348), and there was no statistically significant difference in volume measurements based on cardiac phase using multivariate tests  with no statistically significant difference between them . There is no reasonable reliability between the two software packages , and there is a statistically significant difference between the measurement made with the two software tools .Intraclass correlation analysis Table  shows veThe true volume of in vivo pulmonary nodules is unknown, and even when those nodules are surgically excised, their volume changes because of the sudden stop in blood flow . TherefoOur study identified factors related to cardiopulmonary circulation, namely the location of the PN, the difference of the MPA diameter between systole and diastole, and vascular distance between the MPA and the PN, which were statistically associated with the measurements of volumetry tools.These results could be explained by the propagation of vascular pressure from the heart to the nodule through the complex arterial and capillary network. Assuming the pressure difference at the MPA between systole and diastole, one might expect a similar pressure difference at the capillaries surrounding the PN and that the cardiac cycle phase during acquisition would also be important. However, our results do not support this intuition nor corroborate earlier results from Boll et al. . This isA physiological model of cardiopulmonary hemodynamics should consider that the total time required for the pressure wave to reach the PN (transit time) depends on the complex interaction between vascular and hydrostatic pressures, vascular resistance, the distance the blood travels between leaving the right ventricle and reaching the nodule (vascular distance), and the cross-sectional area of the vessels along the way. All these factors may affect the transit time for every PN differently so that the pressure peak reaches each PN at a different cardiac cycle phase.The dynamic change of the intravascular pressure felt around or inside the PN could change the blood volume included in the segmentation result and consequently the volume measurement. If the change to the segmentation volume is not consistent between follow-up scans, the VDT will be either under- or overestimated.During PN follow-up, events such as the onset of pleural effusion related to congestive heart failure could alter the PN position, vascular distance , and intravascular pressure, potentially impacting subsequent volume measurements.Chronic thromboembolism and vasculitis are associated with increased resistance and tortuosity of the distal pulmonary vessels, affecting the propagation of the intra-vascular pressure wave, and the pressure magnitude. Clinically, these hemodynamic changes can lead to pulmonary hypertension and ultimately right ventricular dysfunction. This phenomenon increases the diameter of the MPA in both systole and diastole, but predominantly in the later, as can be seen on CT, reducing the diameter difference between both cardiac phases. In contrast, pulmonary valvular regurgitation would increase this difference by predominantly reducing the diastolic pressure .Changes in hydrostatic pressure (anteroposterior gradient in a patient in decubitus position on the CT scanner), changes in the vessels' cross-sectional luminal area , and the vascular distance further explain the significance of the location of the nodule in our model.Cardiomegaly was also significant in our model but had negligible effect size. One possible explanation is the increased pressure in the left atria with retrograde capillary recruitment around the PN . The potThe volume measurement is unsurprisingly strongly related to the average diameter, as both relate to a different aspect  of the same characteristic . Likewise, the volume is also related to the appropriateness of the nodule segmentation with a moderate effect size. In clinical practice, cases considered inadequate will tend to overestimate the volume  and should be manually corrected. By including this variable in the model, we can minimize inter-observer variability by avoiding manual correction of the semiautomatic segmentation results but still be able to distinguish its effect from the effect attributable to cardiopulmonary hemodynamic factors.As expected, the inter-observer agreement is very high globally but even higher in diastole, which could be related to minimizing cardiac motion using ECG-gating, as suggested by Boll et al. .No reasonable inter-software agreement can be assumed, which exposes the differences in segmentation performance across different software packages, and reinforces the current recommendations that the same software package should be used throughout the follow-up period .CCTA scans allow multiple independent measurements of a PN (at different phases of the cardiac cycle) in a single acquisition, controlling for most other factors (such as the absence of true growth). In this way, it is similar to the coffee-break study design, which also assumes a lack of true growth (zero-change datasets). However, the latter is more useful in phantom studies because of the increased radiation exposure . NeverthWe propose future research into the variability of the volume measurement in CCTA scans since growth estimation is the most critical application of volumetry in PN between 5 and 8 mm.A limitation of the present study is the low percentage of cases included in the study from the large study sample, which is due to the large number of CCTA scans with missing systolic or diastolic phases in PACS. This is related to the department approach of sending only the most diagnostically relevant phases for archiving. Nevertheless, to our knowledge, this study represents the largest published series on PN volumetry in ECG-gated CT scans and using current and updated volumetry tools in clinical use. Another limitation of the study is the lack of information on risk factors such as smoking. Given the substantial overlap between lung cancer and CAD risk factors, we assume that their prevalence is high in the study population. We realize that the model simplifies the complex mechanisms of cardiopulmonary hemodynamics and does not attempt to address the systemic bronchial arteries or the regulatory mechanisms of ventilation and perfusion. Other non-hemodynamic factors may also influence the measurement, like the distribution of the nodules along the airways, which may also be confounded with the vascular distance.Our data show that the studied factors related to cardiopulmonary hemodynamics influence the results of volumetry tools applied to PN between 5 and 8 mm. Because the cardiac cycle phase is not statistically significant in our model, there is no optimal phase that could control this effect, but more importantly, this raises the possibility of the effect also being relevant to non-gated CT scans.The vascular distance between the MPA and the PN and the diameter difference of the MPA between systole and diastole are related to cardiac function and resistance in the pulmonary circulation. Therefore, these factors may change between the baseline and the follow-up evaluations  and affect the growth estimation of a PN.The inter-observer reliability of the volumetry tools is very high, which is the motivation for automatic and semiautomatic volumetry software packages.The study design based on ECG-gated scans seems suited to study the impact of momentary changes in the cardiopulmonary circulation on PN volumetry and is likely a good model for the potentially more extreme hemodynamic changes that a patient can experience between scans.


Fatigue is considered a brain-derived emotion that could impact performance during the execution of physical exercises. Infrared thermography is a valuable technique able to measure the psychophysiological state associated with emotions in a contactless manner. The aim of the study is to test the capability of infrared thermography to evaluate the central and peripheral physiological effect of fatigue through facial skin and muscle temperature modulations collected during the execution of a unilateral resistance exercise of the lower limb. Both time- and frequency-domain analyses were performed on the temperature time course of the face and limbs. Particularly, significant correlations between features extracted from the thermal signals and the perceived exertion were found. These findings confirmed the ability of thermal imaging to detect both peripheral and central effects of fatigue in response to physical exercises. These results could foster the employment of infrared thermography to monitor the psychophysiological state of the athletes during training. The possibility to calibrate the training load in accordance with the psychophysiological conditions could improve the performance of the athletes during the training process and competitions.ROI: pre–post exercise temperature variation) and frequency-domain  analyses were performed to extract features from regions of interest (ROIs) positioned on the exercised and nonexercised leg, nose tip, and corrugator. The ANOVA-RM revealed a significant difference between ∆TROI  = 15.14; p = 0.0018), and between ∆PSD of myogenic  = 15.20; p = 0.0021) and neurogenic bands  = 9.96; p = 0.0034) of different ROIs. Moreover, significant correlations between thermal features and RPE were found. These findings suggest that IRT could assess both peripheral and central responses to physical exercise. Its applicability in monitoring the psychophysiological responses to exercise should be further exploredInfrared thermography (IRT) allows to evaluate the psychophysiological state associated with emotions from facial temperature modulations. As fatigue is a brain-derived emotion, it is possible to hypothesize that facial temperature could provide information regarding the fatigue related to exercise. The aim of this study was to investigate the capability of IRT to assess the central and peripheral physiological effect of fatigue by measuring facial skin and muscle temperature modulations in response to a unilateral knee extension exercise until exhaustion. Rate of perceived exertion (RPE) was recorded at the end of the exercise. Both time- (∆T Infrared thermography (IRT) is a technology able to measure superficial temperatures in a contactless manner . This teThanks to its contactless features, IRT may be highly suited in applications where the ecological setting has to be preserved, such as clinical applications and physical exercise ,10,11,12Intense exercise induces also physical fatigue, which is predominantly considered a brain-derived emotion, part of a complex regulation system that aims to protect the whole-body homeostasis ,17. SincThe unilateral exercise is well-known to induce strength gains on the contralateral limb ,24,25 viTherefore, this study intended to investigate the capability of IRT to assess the central and peripheral physiological effect of physical fatigue by measuring facial and muscle temperature modulations in response to a unilateral resistance exercise of the lower limb . To this aim, skin temperature was measured over four regions of interest (ROIs) placed on the Exercised Leg, Nonexercised Leg, Nose Tip and Corrugator. The ROI located over the exercised muscle was indicative of the variation in microcirculation induced by the exercise in the active muscles. The one placed over the contralateral leg was used to study the cross-over effect, whereas the facial regions were considered indicative of brain-derived fatigue associated with exercise . The theThe study sample was composed of 8 young healthy female subjects , recruited from a subelite futsal team. Inclusion criteria were no history of musculoskeletal injury in the lower limbs within the year before the study and a training routine based on resistance exercise  of at least 1 session per week. Exclusion criteria consisted of altered fitness status, which refers to disease or illness presence. None of the participants reported recent lower limb injuries at the time of data collection.Participants were instructed to not perform strenuous physical activity in the two days before the trials, and to avoid alcoholic or caffeine-containing products and creams or cosmetics for four hours before the start of the experiment . All theTwo digital thermal infrared cameras FLIR SC660  were used in this study. The cameras were placed at 60 cm from the participant and pointed one towards the face and the other towards the legs of the subject . The samROI) in order to obtain the temperature dynamic for the duration of the experiment. To this aim, a tracking algorithm was used to track each ROI across the images of the video to properly consider the temperature from each thermogram. The tracking software has been developed and validated in Perpetuini et al. (2021) [ROI) between the last recovery and the initial baseline temperature was considered for each ROI. Specifically, TROI was averaged over a temporal window of 10 s . Regarding the frequency-domain analysis, the power spectral density (PSD) of the thermal time course was computed. The area under the curve of the PSD was evaluated for different frequency bands [The mean value of the pixels of each ROI was extracted for each frame (T. (2021) . Both ticy bands : metabolp-value lower than 0.05 was considered statistically significant.Data were expressed as mean ± standard deviation (SD). The normality of the data distribution was checked by the Shapiro–Wilk’s normality test. All the data met the assumption of normality. A one-way analysis of variance with repeated measures (ANOVA RM) was used to compare the effect of the exercise on the thermal features between the ROIs . Least sROI between the different ROIs  = 15.14; p = 0.0018). ROI of each ROI, together with the post hoc results.The temperature time course of the different ROIs of a representative participant is shown in  = 15.20; p = 0.0021) and neurogenic bands  = 9.96; p = 0.0034). The boxplots showing ∆PSD for all the frequency bands are shown in Concerning the frequency-domain analysis, the ANOVA RM did not reveal significant differences between the different ROIs in ∆PSD of the metabolic, cardiac and respiratory bands. Conversely, there was a significant difference between ROIs in ∆PSD of myogenic (FROI and ∆PSD) are reported in ROI of the Exercised Leg and ∆PSD of the myogenic band computed for the Nose Tip and the Corrugator. Moreover, a positive significant correlation was found between RPE and ∆PSD of the neurogenic band calculated for the Nose Tip.The r Pearson’s correlation coefficients between RPE and the thermal features evaluated  and muscles  skin temperature time courses. To the best of the authors’ knowledge, this is the first study combining a time- and a frequency-domain analysis on thermographic data to assess the central and peripheral effects of fatiguing exercise.ROI of the Exercised Leg. Moreover, the significant variation of ∆TROI on the Nose Tip found in the present study could be suggestive of the effect of exercise-induced fatigue on facial temperature. In fact, it is well-known that the facial temperature variations and distribution could be indicative of physical fatigue [ROI was calculated based on the difference between the end of the recovery phase and the previous seconds before the first resistance exercise set. Hence, such a metric is sensitive to a cumulative effect of both exercise and the recovery phase, characterized by an initial vasoconstriction followed by a vasodilation, respectively [Time-domain analysis showed an increment in the skin temperature of the Exercised Leg and Nose Tip, whereas a slight decrement in the other ROIs  was found . These f fatigue . Notably fatigue . As fati fatigue , the fac fatigue . It is wectively . Howeverectively .The results of the frequency-domain analysis showed a significant higher power content in the neurogenic band for the Corrugator and Nose Tip with respect to the muscle ROIs (Exercised and Nonexercised Legs), as shown in Another aspect that is worth discussing is the effect of the unilateral knee extension exercise on the contralateral leg. Specifically, higher myogenic frequency contents were found in the Exercised Leg with respect to the Nonexercised Leg . The higThe present study has some limitations that should be acknowledged. First, the relatively low sample size might have contributed to impair statistical power to highlight possible differences between the ROIs. Although the reduced sample size would not allow a generalization of these results, it should be highlighted that the adopted design along with the homogeneous sample  would partially compensate for that limitation by also reducing data variability. However, increasing the number of participants, and extending measurements on male individuals, may provide more generalizable results . A largeOverall, although preliminary, these results demonstrated the capability of IRT to assess the state of physical fatigue in response to a resistance training session. It is worth noticing that the resistance training session considered in the present study involved three sets of knee extension exercise, as commonly employed within a field-based training routine. However, further studies should investigate the capability of IRT to assess exercise-induced fatigue also in other kind of resistance training involving different numbers of sets and repetitions. From a practical viewpoint, thanks to the contactless features of this technique, IRT could be a powerful tool for the evaluation of the psychophysiological state of the subjects and further monitoring resistance exercises within a strength training programme . For insROI of the Exercised Leg) and RPE confirmed the ability of IRT based on both time- and frequency-domain analyses to assess both peripheral and central effects of exercise to fatigue. Although preliminary, these findings could foster the application of this technology within the field of training load monitoring process.The findings of the present study demonstrate the capabilities of IRT to assess physical fatigue during a unilateral resistance exercise. Specifically, differences in temperature variations were found between different ROIs, revealing the potentiality of thermal imaging to discriminate physiological processes underlying the perception of fatigue. These results were also corroborated by the frequency-domain analysis, showing different behaviours of ROIs in the neurogenic and myogenic bands. Finally, the significant correlations between thermal features (i.e., ∆PSD of myogenic and neurogenic bands of the facial areas, and ∆T