| Thermal control system is a crucial component to guarantee the normal operation of instrument or equipment. With the advancement and development of science and technology, some sophisticated equipments put forward higher requirements for the thermal control accuracy of working temperature and the weight of thermal control subsystems. However, the existing thermal control methods have become increasingly difficult to meet this demand. Therefore, to explore new means of thermal control has a very important practical significance. Active electric heating method is one of the common means to achieve high-precision temperature control for thermal control systems, using PTC (Positive Temperature Coefficient) thermosensitive material to replace the current widely used ordinary resistor as a heating element to conduct active thermal control is likely to achieve adaptive temperature control so as to reduce the weight of thermal control system, and also to further improve the temperature control accuracy. However, the Curie temperatures of existing PTC materials are generally high and the researches or applications of PTC thermal control materials are mainly limited to areas of high temperature, which is inappropriate for most of the equipments that usually work at room temperature segment. Additionally, since the characteristic of PTC resistor and ordinary resistor are greatly differed, so the active temperature control method of the PTC material is also different from that of conventional electric heating material. Therefore, in this study, the preparation methods and characteristics of the novel PTC materials which are suitable for room temperature thermal control are investigated, based on this, the adaptive temperature control characteristics as well as high-precision temperature control method and theory of the PTC materials are studied profoundly.In this paper, the preparation methods of novel polymer-based PTC material and ceramic-based PTC material with room temperature Curie point are studied firstly. Through massive comparative analysis, it is found that the low-temperature resistivity of polymer-based PTC material can be greatly reduced by adding a small amount of carbon black. Accordingly, the preparation scheme of polymer-based PTC materials which use paraffin/LDPE/carbon black as main component is determined, and the best component ratio and preparation method of polymer-based room temperature PTC material are found. The test results show that the low-temperature resistivity has been effectively reduced while the PTC intensity has no great increase. In addition, the barium titanate-based ceramic room temperature PTC material has also been prepared, the PTC intensity of material is about 1.4, but the low-temperature resistivity is relatively high. It is found that some experimental conditions haven’t be controlled strictly during preparation, which may be the main reason that restrict the characteristics of ceramic-based room temperature PTC material from further improving. However, the outcomes of the study also lay foundation for preparing room temperature PTC materials with better properties in the future. According to the above analysis, since the prepared polymer-based room temperature PTC material has higher PTC intensity and lower low-temperature resistivity compared to the ceramic-based room temperature PTC material, this paper will use the novel polymer-based room temperature PTC materials as the basis for further research.For several kinds of prepared typical polymer-based room temperature PTC material, their resistance-temperature properties are measured and characterized, then their stabilities are analyzed. The results show that the purity of paraffin in blend substrate have a major impact on the resistance-temperature property of the PTC material, while using n-octadecane and n-eicosane as blend substrates, the temperature coefficients are about one magnitude higher than industrial paraffin (1.57/℃,2.06/℃, 0.21/℃). It can be found through thermal cycling test that the PTC intensities of materials decrease slightly after about 15 cycles, then will reach to a stable value, indicating that the PTC materials have favorable property and stability.Based on the novel room temperature PTC material, its adaptive temperature control characteristics and mechanism are investigated systematically by theoretical and experimental studies. The so-called PTC adaptive temperature control refers that when the controlled temperature exceeds the Curie point, the resistance of PTC materials increases sharply, then the heating power declines sharp, making it possible to achieve adaptive temperature control without any control methods. The PTC adaptive temperature control experimental platform and theoretical model are established in this paper, the study shows that the PTC material has better adaptive temperature control ability compared to ordinary resistor; while the temperature coefficient of PTC material are larger, the equilibrium temperature fluctuations of controlled element will be smaller and the adaptive temperature control ability will be stronger. Under condition that the initial heating power does not exceed the critical heating power, the initial heating power also has an important influence on the performance of adaptive temperature control, the greater the initial heating power is, the better stability of adaptive temperature control will be. The research results lay theoretical and experimental foundations for adaptive temperature control of room temperature PTC material.Based on the novel room temperature PTC material, an optimization method is proposed for the proportional, integral and differential coefficient of PID algorithm and the new PID algorithm which suits for the high-precision temperature control of PTC material is derived, then its high-precision temperature control characteristics and mechanism are investigated systematically by theoretical and experimental studies. Through comparison with high-precision temperature control characteristics of ordinary resistor, it is found that the PTC material can achieve higher temperature control accuracy than ordinary resistor when using on-off control method, the highest control accuracy reaches to ±0.01℃; when using the proposed PID algorithm, the temperature control overshoot of PTC material can be effectively reduced, but the temperature regulating process of PTC material becomes more sluggish; when three parameters of PID controller(Kp,TI,TD) are within ranges of 5-10,15-30 and 0.1-10 respectively, the PID temperature control overshoot of PTC material is smaller and its PID temperature control accuracy is higher, the highest control accuracy reaches to ±0.001K and the temperature overshoot is substantially eliminated; the higher temperature coefficient of PTC material is, the more obvious of its PID temperature control performance enhancement will be. The study results have significant importance to high-precision temperature control in room temperature field.This paper further studies the possibility to improve the temperature control performance of PTC material by external means, an optimized circuit by using PTC resistor for temperature control is designed, it uses the amplifying circuit to equivalently amplify the temperature coefficient of PTC material. The preliminary theoretical analysis of the method is conducted firstly, based on this, the optimization circuit is designed、simulated and produced, and it is used for actual temperature control experiment. Theoretical analysis shows that the program can effectively improve the temperature coefficient of PTC material; during the experiment, it is found that due to issues of rapid heating of PTC resistor and small gain of multiplier chip, the load resistance can not be effectively heated with a too small voltage. For solving the above problems, the corresponding improvement measures are put forward. The study provides experience and possible solution for PTC material temperature control using external optimized circuit method.In order to measure the effect of thermal control accuracy on thermal deformation, a set of temperature-thermal deformation test platform with 40 temperature and 60 deformation measurement channels is designed and established in this paper finally. Based on the platform, the actual measurement has been carried out for a test component, the results show that the platform is able to realize precision measurement of temperature, thermal deformation and stress with multi-point simultaneously; the static full-channel temperature measurement uniformity (difference between the maximum and the minimum value) is less than 2℃; the deformation measurement accuracy can reach to ±0.0 1mm, the deformation test results are consistent with the theoretical value, which means the system also has high precision deformation measurement. This platform provides an experimental foundation for conducting related studies of high-precision temperature control and thermo-solid coupling in the future. |
PDF Full Text Request