Study On Paraffin Thermomechanical Characteristics And Its Wireless Actuator

Along with the development of microelectromechanical systems, it stares us in the face that to enhance the research on power and energy. For a microrobot used in medical endoscope able to enter into the cavities of human to diagnose with minimal invasive or non-invasive, or able to enter into industrial pipes to do inspection and maintenance in the dangerous field of flammability or explosibility, its microactuator must satisfy the specifications: high output force, large stroke and small temperature change. The thermal actuation is a new way to solve the problem, in which paraffin is used as an expansive material whose coefficient of volume expansion in the melting point temperature range is very high. Moreover, the mode of energy supply for the paraffin actuator is a key technology of the above problem. The energy supply with a wire limits the movement and use environment of the actuator, and the wire is a big burden on the microactuator. Therefore, the wireless actuation is an important breakthrough in the practical application of paraffin actuator. The main works and originalities of this paper are as follows:An experimental apparatus of thermal expansion characteristics of paraffin with water is established. The thermomechanical characteristics, such as the united relationship of volume expansion, temperature and pressure, the linearity of melting curve, and the influences of cooling means and spring pressure on thermal hysteresis phenomenon, have been achieved by the use of static and dynamic method, and lay the foundation for the development of paraffin actuator. The volume expansion of paraffin is around 10% under a pressure of 0.36MPa during the solid-to-liquid phase change. The output pressure and the fractional stroke are enough to manipulate the medical endoscope. A low cooling speed of cooling means and small elasticity coefficient of the spring can decrease the degree of paraffin thermal hysteresis. For ensuring the enough rapid cooling speed, the degree of thermal hysteresis paraffin should be reduced as far as possible.When the actuator displacement is controlled by the paraffin temperature, the thermal hysteresis of paraffin makes it be complex. In order to master the rule of the thermal hysteresis, its influence factors are studied for the first time.The paraffin actuator is driven wirelessly by the mechanism of electromagnetic induction heating. An experimental apparatus of paraffin with electromagnetic induction heating is established. The realization of high power factor of the experimental apparatus, the parameter matching of coils, and the induction heating characteristics are obtained. The electromagnetic induction heating only needs 5~6s in the temperature range of 25 to 60℃of paraffin, and can provide adequate energy and heating rate for the actuator.In comparison with the experimental results, a mathematical model and simulation procedure is developed by ANSYS software with symmetrical induction heating for the paraffin actuator. The coupling analysis between electromagnetic and thermal physical phenomena, the paraffin phase change from solid to liquid, the process of the thermal conductivity of the paraffin mixture, and the effective thermal conductivity of glass, as well as the temperature-dependence and non-linear properties of most materials have been completed. The relationship of the material and size of wireless actuator, the actual induction heating parameter versus the temperature field of paraffin is achieved, and establishes the theory basis to optimize the structure of the wireless actuator. It is shown that the comparison between numerical and experimental results shows an excellent agreement.The configuration parameter of the coils is optimally designed, and the structure of paraffin actuator is improved to completely avoid the adhesion. An experimental apparatus of unilateral movement characteristics of wireless paraffin actuator is established, and the movement characteristics have been obtained. The unilateral average speed of the actuator is related with the cooling means. Although it is not fast, it can meet the requirement of the medical endoscope. In addition, a wireless bidirectional combined model of paraffin actuator is put forward. If it comes true, the challenge of wireless bidirectional movement of paraffin actuator will be solved.