Closed-loop Temperature Control Research Articles

Closed-Loop Control of Melt Pool Temperature during Laser Metal Deposition

Laser metal deposition (LMD) is a technology for the production of near-net-shape components. It is necessary to control the manufacturing process to obtain good geometrical accuracy and metallurgical properties. In the present study, a closed-loop control method of melt pool temperature for the deposition of small Ti6Al4V blocks in open environment was proposed. Based on the developed melt pool temperature sensor and deposition height sensor, a closed-loop control system and proportional-integral (PI) controller were developed and tested. The results show that with a PI temperature controller, the melt pool temperature tends to the desired value and remains stable. Compared to the deposition block without the controller, a flatter surface and no oxidation phenomenon are obtained with the controller.

Peltier-based temperature regulation: A method for performance optimization in solid-state lasers

This article presents a direct method for temperature control in solid-state lasers, where temperature stability is crucial for optimizing the performance and reliability of such lasers. The proposed method utilizes Peltier chips for both cooling and heating the laser crystal to achieve precise temperature regulation.The system design is based on the step response of the open-loop thermal system and employs a proportional-integral (PI) controller for closed-loop temperature control. Comprehensive testing on a femtosecond Titanium-Sapphire Laser (Ti:Sapphire laser) demonstrated that the system is capable of maintaining the desired operating temperature with remarkable stability and efficiency, highlighting its practicality for real-world applications.Method Outline:•Utilization of Peltier chips for precise temperature control.•Estimation of first-order transfer function based on step response.•Implementation of a proportional-integral (PI) controller for closed-loop temperature regulation.

Model predictive control for real-time microstructure control in laser heat treatment

Metals’ surface microstructure can be altered through controlled heating and cooling of the surface using the laser heat treatment (LHT) method. Lasers have the advantage in this process of being able to heat treat specific regions of the workpiece without affecting the whole product. However, because a laser is being used, this process is extremely sensitive to changes in the process inputs and disturbances, which results in changes in the thermal dynamics and alterations in the peak temperature and cooling rate, in particular. These alterations have a direct effect on the material and its mechanical characteristics, such as hardness and hardening depth. Since these variations are caused by complex metallurgical phenomena, it is important to control the thermal dynamics in real-time. Real-time control systems are usually error-based and do not have the ability to predict the effect of the control action on the system, deal with constraints, or correlate the mechanical and material properties of the processed materials to LHT process parameters. Hence, the development of accurate model-based controllers is required. Therefore, a Model Predictive Control (MPC) algorithm is developed in this paper to control the thermal dynamics during the LHT process, e.g., the peak temperature, in real-time. This controller uses a finite difference thermal model to find the optimal process speed that results in the desired peak temperature. Moreover, consistent hardness and hardening depth will be obtained by closed-loop peak temperature control.

Investigation of directional discrimination in the nociceptive system using temperature-controlled laser stimuli.

Cutaneous laser stimulation has commonly been employed to investigate the thermal properties of the nociceptive system. The aim of this study was to investigate how a temperature-controlled laser system improves the assessment of directional discrimination in the nociceptive system. In total, twenty healthy volunteers participated in this study. To determine the directional discrimination threshold (stimulation length 50% correct, expressed in mm), thermal stimuli were delivered using a diode laser and the laser beam was perpendicularly displaced across the skin to give a linear stimulation in four different directions (distal, proximal, lateral and medial) and displacement lengths (3 for lateral-medial and 5 for distal-proximal). Two temperature control modes were used in the stimulation system, open-loop and closed-loop control. The subjects had to report the perceived stimulus direction, the degree of certainty regarding the perceived direction and the intensity of the perceived stimulus (0-10 numerical rating scale, 3: pain threshold). During closed-loop control, the orientation of stimuli was discriminated significantly more accurately than during open-loop control. During closed-loop control, the directional discrimination threshold was 31.9 and 26.1 mm for distal-proximal and lateral-medial directed stimuli, respectively. A numerical rating scale was significantly higher for the lateral/medial directions. Moreover, the variability of the discrimination threshold is reduced in the closed-loop control system. The findings show that discrimination ability is better in the lateral-medial directions compared to the distal-proximal directions. This study indicates that using a system enabling closed-loop temperature control, allows more robust probing of the temporo-spatial mechanisms in the nociceptive system. This study shows that a newly developed temperature-controlled laser stimulation system enhances the possibilities to investigate the nociceptive temporo-spatial integration, as shown by a less variable directional discrimination threshold. The results also show that different orthogonal directions are discriminated differently. This new method allows a better investigation of the combined temporal and spatial mechanisms in the nociceptive system.

Closed-loop temperature controlled solid-state additive manufacturing of Ti-6Al-4V with forging standard out-of-plane tensile properties

Here, we present the first demonstration of additive friction stir deposition of Ti-6Al-4V with closed-loop temperature control, wherein the print head rotation rate is adjusted in real-time to maintain a constant tool temperature during deposition. The as-printed Ti-6Al-4V is fully dense, achieving forging standard tensile properties along both the in-plane and out-of-plane directions. The measured yield strength, ultimate tensile strength, and elongation are 903 MPa, 1000 MPa, and 17% along the in-plane direction, and 948 MPa, 1024 MPa, and 16% along the out-of-plane direction. With closed-loop temperature control, the microstructure is found to be comparable along the thickness direction, contrasting the previous work performed under parameter-controlled conditions. For all the deposition conditions explored in this work, the deposited Ti-6Al-4V exhibits a lamellar microstructure with alpha laths forming inside prior beta grains, indicating higher peak temperatures than the beta transus. The increase of a pseudo heat index leads to an increase in the prior beta grain size and a decrease of the out-of-plane yield strength. Deposition of Ti-6Al-4V is seen to modify the microstructure of the substrate near the interface: the original equiaxed microstructure is replaced by lamellar and bimodal microstructures. We finally discuss the possibility of forming equiaxed microstructures in the as-deposited Ti-6Al-4V and provide evidence for different types of microstructures forming in the same deposition layer, which may be caused by the thermal gradient during reheating.

Variable-Structure Proportional-Integral-Derivative Laser Solder Joint Temperature Intelligent Control Method with Adjustable Power Upper Limit.

Laser soldering is a crucial soldering technique in the realm of electronic assembly. The temperature of the solder joint is intimately connected with the quality of the solder. This paper introduces an adjustable power upper limit variable-structure Proportional-Integral-Derivative (PID) intelligent control method for regulating the temperature of the solder joint during laser soldering. Distinct laser power limits are employed for workpieces with varying heat capacities. The solder joint temperature is monitored through an infrared thermometer, which enables closed-loop temperature control via a variable-structure PID algorithm. Residual neural network (ResNet) models are utilized to predict key soldering process parameters. This method has been executed and validated on a practical testing platform. Compared to other laser soldering control techniques, the proposed method demonstrates a low overshoot, rapid dynamic response, and swift adjustment capabilities, effectively enhancing the soldering quality and production efficiency.

The use of the general thermal sensation discriminant model based on CNN for room temperature regulation by online brain-computer interface

Brain-computer interface (BCI) technology can realize dynamic room temperature adjustment based on individual real-time thermal sensation, which can provide the basis for future intelligent buildings. However, the generalization ability of previous thermal sensation discrimination model (TSDM) is limited, which is a serious obstacle to the application. In this paper, a general TSDM was developed by using convolutional neural network (CNN), which can be well applied to new subjects.In the study, the CNN-TSDM was established and evaluated based on the offline experimental data, and then the BCI closed-loop online room temperature control experiment was carried out based on this CNN-TSDM to further verify. The offline analysis results show that the recognition performance of CNN-TSDM in new subjects is significantly higher than that of typical shallow learning algorithms, and its area under the ROC curve (AUC) value reaches 0.789. In the online experiments of the two simulated environments, BCI using the CNN-TSDM dynamically controlled the air conditioning to improve the room temperature to the comfortable level according to the subjects' thermal sensation. The subjective score of subjects decreased from 3.1 to 3.0 for the hot uncomfortable to 1.1 and 1.2 for the cool comfortable (p < 0.001, p < 0.001). Moreover, in a hotter simulated experimental environment, BCI automatically controlled the air conditioner for longer cooling to obtain a same degree of thermal comfort. The total cooling time (p < 0.05) and the single cooling time (p < 0.05) of the air conditioner were significantly increased. This further confirmed the effectiveness and robustness of the general CNN-TSDM.

A TEC Cooling Soft Robot Driven by Twisted String Actuators.

Similar to biological muscles in nature, artificial muscles have unique advantages for driving bionic robots. However, there is still a large gap between the performance of existing artificial muscles and biological muscles. Twisted polymer actuators (TPAs) convert rotary motion from torsional to linear motion. TPAs are known for their high energy efficiency and large linear strain and stress outputs. A simple, lightweight, low-cost, self-sensing robot powered using a TPA and cooled using a thermoelectric cooler (TEC) was proposed in this study. Because TPA burns easily at high temperatures, traditional soft robots driven by TPAs have low movement frequencies. In this study, a temperature sensor and TEC were combined to develop a closed-loop temperature control system to ensure that the internal temperature of the robot was 5 °C to cool the TPAs quickly. The robot could move at a frequency of 1 Hz. Moreover, a self-sensing soft robot was proposed based on the TPA contraction length and resistance. When the motion frequency was 0.01 Hz, the TPA had good self-sensing ability and the root-mean-square error of the angle of the soft robot was less than 3.89% of the measurement amplitude. This study not only proposed a new cooling method for improving the motion frequency of soft robots but also verified the autokinetic performance of the TPAs.

Design and Analysis of EAST Predictive Control System for Hot Nitrogen Baking Process

To realize the automatic temperature closed-loop control of EAST (Experimental advanced super-conducting tokamak) baking system, the transfer function of EAST baking system is obtained by the step response method after being familiar with the mechanism structure of the baking system. According to the transfer function of the system, two control methods, conventional PI control and PI control based on Smith prediction are novelty proposed. MATLAB simulation analysis shows that PI control based on Smith prediction does not produce an overshoot compared with conventional PI control, and the final error given by tracking slope can be reduced by selecting appropriate PI parameters. Generally, PI control based on Smith prediction has better stability and dynamic performance. The novelty here lies in the introduction of PI control based on Smith prediction to solve the problem of large overshoot in conventional PI control.

The Non-Uniformity Control Strategy of a Marine High-Speed Diesel Engine Based on Each Cylinder’s Exhaust Temperature

To improve the non-uniformity of a multi-cylinder marine diesel engine caused by manufacturing assembly errors and performance degradation of the fuel injection system, with the instantaneous speed applied as the control target, the feedback variable of each cylinder’s exhaust temperature was used to obtain the non-uniformity information and the injection quantity of each cylinder was applied as the control variable; the inhomogeneity control was accomplished by modifying the injection pulse spectrum. The model of AVL Cruise M was established and validated by bench test data. The non-uniformity control strategy based on the instantaneous speed and the exhaust temperature of each cylinder was developed in SIMULINK, and the control effect was compared with the closed-loop control of cylinder pressure by software in-loop simulation. The results showed that the non-uniformity control strategy based on exhaust temperature could significantly improve the uniformity of each cylinder; although the improvement effect was not as great as the non-uniformity control strategy based on cylinder pressure, the cost was significantly reduced, and the practicality and reliability were better. With the closed-loop control of exhaust temperature and instantaneous speed, the CV (Coefficient of Variation) of IMEP (indicated effective pressure) was close to the closed-loop control of cylinder pressure; the maximum occurred at 25% load when it was 0.199%. This co-simulation provided a theoretical basis for the subsequent hardware-in-the-loop simulation and actual engine tests.

A Reconfigurable AC Power Cycling Test Setup for Comprehensive Reliability Evaluation of GaN HEMTs

To address the challenges in long-term reliability testing of GaN power devices, this paper proposes a reconfigurable AC power cycling test setup. While addressing the limitations of existing aging tests, the proposed setup enables accelerated aging of GaN devices under a wide range of realistic stress conditions. Specifically, various hard and soft switching modes can be tested to realistically emulate different application scenarios. In addition to the ability to control test temperature, the setup also features adjustable power loop inductance that enables precise control over voltage overshoot during hard switching transients. Further, an on-state resistance based junction temperature estimation technique is used for closed loop junction temperature control. For this, a fast drain-source voltage measurement circuit along with an algorithm for closed-loop control of the mean junction temperature is presented in detail. Furthermore, the magnitudes of the temperature swing under different operation conditions are discussed in depth which provides a good insight to control temperature swings. Finally, a prototype is built to validate the proposed test design.

Spatio-temporal LPV model of 2D workpiece temperature for Direct Laser Deposition

Additive manufacturing of metallic components presents challenges regarding quality assurance and consistency, owing to the workpiece's complex spatially-dependent temperature history and a lack of suitable models for closed-loop temperature control of the fabrication process. The selected case study is the laying of cladding material (workpiece) through Direct Laser Deposition. The paper proposes identifying a linear spatio-temporal parameter-varying model for the workpiece temperature for control-oriented applications. For this, the temperature distribution is reconstructed from data measured by an infrared camera. The model structure is derived from the inhomogeneous heat equation. Finally, the model parameters are identified using a nonlinear least-squares solver. The relationships between theoretical and estimated model parameters are presented.

Toward defect-free components in laser metal deposition with coaxial wire feeding through closed-loop control of the melt pool temperature

Laser metal deposition (LMD) is an additive manufacturing process in which a metal powder or wire is added to a laser-induced molten pool. This localized deposition of material is used for the manufacturing, modification, and repair of a wide range of metal components. The use of wire as feedstock offers various advantages over the use of powder in terms of the contamination of the process environment, the material utilization rate, the ease of handling, and the material price. However, to achieve a stable process as well as defined geometrical and microstructural properties over many layers, precise knowledge on the effects of the input variables of the process on the resulting deposition characteristics is required. In this work, the melt pool temperature was used as an input parameter in LMD with coaxial wire feeding of stainless steel, which was made possible through the use of a dedicated closed-loop control system based on pyrometry. Initially, a temperature range was determined for different process conditions in which a stable deposition was obtained. Within this range, the cause-effect relationships between the melt pool temperature and the resulting geometry as well as the material properties were investigated for individual weld beads. It was found that the melt pool temperature is positively correlated with the width of the weld bead as well as the dilution. In addition, a dependence of the microhardness distribution over the cross section of a weld bead on the melt pool temperature was demonstrated, with an increased temperature negatively affecting the hardness.

Real-time regulation of room temperature based on individual thermal sensation using an online brain-computer interface.

Regulation of indoor temperature based on neurophysiological and psychological signals is one of the most promising technologies for intelligent buildings. In this study, we developed a system for closed-loop control of indoor temperature based on brain-computer interface (BCI) technology for the first time. Electroencephalogram (EEG) signals were collected from subjects for two room temperature categories (cool comfortable and hot uncomfortable) and used to build a thermal-sensation discrimination model (TSDM) with an ensemble learning method. Then, an online BCI system was developed based on the TSDM. In the online room temperature control experiment, when the TSDM detected that the subjects felt hot and uncomfortable, BCI would automatically turn on the air conditioner, and when the TSDM detected that the subjects felt cool and comfortable, BCI would automatically turn off the air conditioner. The results of online experiments in a hot environment showed that a BCI could significantly improve the thermal comfort of subjects (the subjective thermal comfort score decreased from 2.45 (hot uncomfortable) to 0.55 (cool comfortable), p < 0.001). A parallel experiment further showed that if the subjects wore thicker clothes during the experiment, the BCI would turn on the air conditioner for a longer time to ensure the thermal comfort of the subjects. This has further confirmed the effectiveness of TSDM model in evaluating thermal sensation under the dynamic change of room temperature and showed the model's good robustness. This study proposed a new paradigm of human-building interaction, which is expected to play a promising role in the development of human-centered intelligent buildings.

Closed-Loop Temperature and Force Control of Additive Friction Stir Deposition

Additive Friction Stir Deposition (AFSD) is a recent innovation in non-beam-based metal additive manufacturing that achieves layer-by-layer deposition while avoiding the solid-to-liquid phase transformation. AFSD presents numerous benefits over other forms of fusion-based additive manufacturing, such as high-strength mechanical bonding, joining of dissimilar alloys, and high deposition rates. To improve, automate, and ensure the quality, uniformity, and consistency of the AFSD process, it is necessary to control the temperature at the interaction zone and the force applied to the consumable feedstock during deposition. In this paper, real-time temperature and force feedback are achieved by embedding thermocouples into the nonconsumable machine tool-shoulder and estimating the applied force from the motor current of the linear actuator driving the feedstock. Subsequently, temperature and force controllers are developed for the AFSD process, ensuring that the temperature at the interaction zone and the force applied to the feedstock track desired command values. The temperature and force controllers were evaluated separately and together on setpoints and time-varying trajectories. For combined temperature and force control with setpoints selected at a temperature of 420 °C and a force of 2669 N, the average temperature and force tracking errors are 5.4 ± 6.5 °C (1.4 ± 1.6%) and 140.1 ± 213.5 N (5.2 ± 8.0%), respectively.

Mild-temperature photothermal treatment method and system based on photoacoustic temperature measurement and control

As a non-invasive technology for tumor treatment, photothermal therapy (PTT) is increasingly applied during clinical treatment. However, due to the lack of non-invasive temperature measurement methods, excessive therapeutic light dose will damage the healthy tissue around the lesion. Monitoring and control the therapy temperature during PTT application is necessary to ensure effective therapy while minimizing thermal damage to adjacent tissue. In order to ensure that the therapy temperature in PTT is stable in the mild range (below 43 °C), it is necessary to design a mild-temperature PTT method and system that can detect and control the temperature in real-time. A mild-temperature photothermal treatment method and system based on photoacoustic temperature measurement and control is proposed in this paper. On the basis of reference temperature, the relative temperature change is measured based on the photoacoustic information, and the treatment temperature of the target area is precisely controlled in real time using the closed-loop temperature control algorithm. Based on the experimental results of agar-based phantoms and pig liver tissue, the proposed system can achieve temperature imaging in real time, and the control steady-state error and overshoot are 0.62 °C and 0.3 % respectively. The results show that our method has the potential for accurate temperature monitoring and closed-loop temperature control during PTT.

Closed-loop control of meltpool temperature in directed energy deposition

The objective of this work is to mitigate flaw formation in powder and laser-based directed energy deposition (DED) additive manufacturing process through close-loop control of the meltpool temperature. In this work, the meltpool temperature was controlled by modulating the laser power based on feedback signals from a coaxial two-wavelength imaging pyrometer. The utility of closed-loop control in DED is demonstrated in the context of practically inspired trapezoid-shaped stainlesssteel parts (SS 316L). We demonstrate that parts built under closed-loop control have reduced variation in porosity and uniform microstructure compared to parts built under open-loop conditions. For example, post-process characterization showed that closed-loop processed parts had a volume percent porosity ranging from 0.036% to 0.043%. In comparison, open-loop processed parts had a larger variation in volume percent porosity ranging from 0.032% to 0.068%. Further, parts built with closed-loop processing depicted consistent dendritic microstructure. By contrast, parts built with open-loop processing showed microstructure heterogeneity with the presence of both dendritic and planar grains, which in turn translated to large variation in microhardness.

Pyrometry-based closed-loop control of the melt pool temperature in Laser Metal Deposition with coaxial wire feeding

Laser metal deposition with coaxial wire feeding is a directed energy deposition process that enables near-net-shape and direction-independent additive manufacturing of metal parts. However, the high sensitivity of the process to disturbances poses a major challenge, as this frequently leads to defective components and rejects. To counteract this, a pyrometer was coaxially coupled into the beam path of the laser processing head to accurately determine the melt pool temperature without limiting accessibility. Applying deterministic test signals, a system model describing the dynamic relationship between the temperature and the laser power was identified. Based on this model, a closed-loop process control was designed. The control system was validated by introducing various disturbances, for which the reference temperature could be tracked with good approximation. The designed temperature control results in reduced effort for tuning the deposition process as well as a lower probability of defects, which saves manufacturing time and cost.

Temperature control of vibrating heat-generating hardware using spray evaporative cooling in the nucleate boiling region

A temperature control approach using evaporative spray cooling of vibrating surfaces in the nucleate boiling region is proposed and verified experimentally. This is relevant to temperature control of heat-generating automotive vehicle components. By exploiting an experimentally calibrated dynamic correlation model to represent evaporative spray cooling of a flat test-piece, a PID controller has been adopted with emphasis focused on the choice of gain parameters to ensure both stability of temperature control, and favourable responses in terms of relevant performance measures. Optimum linearisation of the correlation model has been achieved by solving an appropriate Wiener-Hopf equation, mainly to undertake a practical stability assessment of the closed-loop temperature control system. To verify the predicted control system performance, experimental measurements have been obtained from an instrumented, and spray-evaporatively-cooled, flat test-piece exposed to displacement vibration from a shaker. Experimental testing, appropriate to automotive vehicle component applications, includes large-amplitude, low frequency vibration at 12 mm and 1.9 Hz, and low amplitude, high-frequency vibration at 0.02 mm and 400 Hz. To assess the effects of different PID controller gains on the thermal performance of the thermal management system, a coefficient of performance (COP) is used, defined as the ratio of heat power removal to the required pumping power. To achieve a reduction in the settling time, and an increase in the rise time of stable control, a PID controller with a negative proportional gain showed most promising results. A 10.5% increase in COP was achieved in comparison to a PID controller with positive gains. This information is useful for the design and optimization of thermal management systems using evaporative spray cooling.

Prefabricated electrically conductive concrete (ECC) slabs with optimized electrode configuration and integrated sensor system

Conventional de-icing methods with salts and snow removal causes considerable maintenance cost and outstanding indirect costs due to corrosion of the steel reinforcement of concrete infrastructures. Alternatively, heating systems based on Electrically Conductive Concrete (ECC) have been lately developed to reduce operational and reparation cost especially for highly frequented urban areas. The aim of this work is to develop an optimized prefabricated ECC slab with a safe level of electrical current, integrated snow sensors, and a satisfactory energy consumption.In this work we employed economic ECC mix designs which were recently developed with a reduced amount of conductive inclusions (graphite powder and steel fibers) and with a satisfactory electrical conductivity of 270 Ω-cm. Firstly, small-scale ECC slab tests were carried out in an environmental controlled chamber to optimize the heating rate, power consumption and electrical safety by varying the electrical configuration and slab thickness. The optimal configuration consists of an ECC system with an optimal zig-zag electrode and an applied voltage of 30 V, which assures user safety and heats from −9 °C to 5 °C (within an environmental chamber at fixed temperature of −9 °C) in less than 60 min with an average consumption of about 700 W/m2. Finally, the developed ECC system was tested in a real field with an integrated snow/ice sensor connected to an electrical closed-loop temperature control system. As a result, the energy consumption was always under 400 W/m2 under different case studies, such as snowstorm, ice-rain formation and imposition of 9 cm of compacted snow as a simulation of a black-out situation.