Giant magnetostrictive effect of the new high-speed hydraulic switching valve

Abstract: This paper introduces a new type of hydraulic high-speed on-off valve that employs a giant magnetostrictive actuator and a cone-type spool structure. The valve has a high switching speed and frequency, can be used as a high-flow high-speed on-off valve pilot control valve, can also be used directly in the small flow circuit control valve. Keywords: Giant Magnetostrictive Actuator Hydraulic High-speed On-off Valve 1 Introduction In the construction machinery hydraulic system using PWM pulse modulation digital control has many significant advantages, such as easy connection with the computer, the system is low cost, anti-pollution and anti-interference ability Strong and so on. However, this control system requires a high-speed on-off valve. At present, some high-speed on-off valves developed at home and abroad mostly use electromagnets as a drive, so the switching speed of the valve is slow. Although the piezoelectric crystal type driver improves the switching speed and the frequency, the required voltage is high and the power consumption is large. Therefore, it is necessary to develop a new type of high-speed on-off valve. In recent years, the application research of giant magnetostrictive effect of rare earth ferrite is paid more attention at home and abroad. The so-called magnetostrictive effect, refers to the ferromagnetic materials and ferrimagnetic materials under the action of the magnetic field will lead to small changes in the volume and length of the phenomenon. Some compounds and amorphous alloys formed by adding Fe and Ni to Fe and rare earths are called giant magnetostrictive material (GMM) due to their large magnetostriction coefficient, and are typically represented by Terfenol-D. Linear actuators made of this material have many uses, among them electro-mechanical switching devices as hydraulic control valves are characterized by fast switching speed, high frequency and low energy consumption. 2 on-off valve structure and working principle of 2.1 Giant magnetostrictive drive Giant magnetostrictive drive structure shown in Figure 1. Center of giant magnetostrictive material rod for the Beijing University of Science and Technology TbDYFe (φ12 × 110), the lower end of the fixed fixture, the upper end of the telescopic transmission shaft and spring magnetostrictive material prestressed. Around the magnetostrictive material is the excitation coil for driving field and the copper pipe for cooling coil. The outer periphery is a yoke (carbon steel), which forms a closed magnetic circuit with the magnetostrictive material to prevent magnetic flux leakage. Telescopic transmission shaft and other parts are made of non-magnetic stainless steel. The internal strain of Giant Magnetostrictive Materials is directly related to the strength of excitation magnetic field, material property constant and stress state. The external displacement and force output are the result of mutual coupling of magnetic field and elastic field. Therefore, the appropriate configuration of mechanical and electrical structural parameters (mainly pre-compression stress and bias magnetic field), the giant magnetostrictive material in the optimal electromechanical coupling state, improve energy conversion efficiency. When AC excitation is used, the Giant Magnetostrictive material is elongated under the action of positive and negative magnetic fields, and its mechanical motion is twice as frequent as the applied current. This is called "double frequency phenomenon." In order to make the mechanical movement of the actuator in the linear interval, a bias magnetic field is usually used to eliminate this phenomenon. However, when the driver is excited by PWM pulse voltage, no double frequency phenomenon will occur due to no negative magnetic field, so no bias magnetic field is needed. The purpose of applying a suitable prestress to the giant magnetostrictive material is to prevent the magnetostrictive material from working under tension and to increase the magnetostriction. According to the theory of ferromagnetism, this is because the applied stress increases the saturation magnetization of the material and increases the saturation magneto-coefficient. 2.2 switch valve structure. Usually as a hydraulic pilot valve required for the minimum displacement of not less than 0.5mm. Therefore, the displacement of the magnetostrictive actuator output is amplified by a lever mechanism. Magnification displacement at the same time, the driving force of the spool is reduced. However, due to the giant magnetostrictive actuator does not require spring reset, the spool drive only need to overcome the friction, so the driving force does not need large. On-off valve is a kind of existing two-way two-way solenoid valve made of transformation, the internal structure shown in Figure 3. The role of the spring 3 is to eliminate the displacement of the lever and other coordination gap, so do not need a large stiffness coefficient. 3 on-off valve characteristic test The on-off valve characteristic test mainly includes the magnetism drive static current - the output displacement, the output force and the on-off valve opens and closes the time the test, its principle block diagram like chart 4 shows. The test is carried out under constant temperature and vibration isolation conditions. 3.1 Magnetism drive quiescent current - output displacement, output force test Magnetostrictive rod pre-pressure 820N, that is, when the input current is zero, due to the force and reaction force equal relationship, then the magnetostrictive output Force is also 820N. When the input current is 1550mA, the output force reaches 980N, the displacement output is 40μm at this moment. Due to hysteresis in the material, the displacement and force output curves do not coincide when the magnetizing current is positive or negative. However, the on-off valve controlled by the PWM signal is not a linear element by itself, so the driver is not required to have linearity. It only requires that the output displacement of the driver also be equal to zero when the magnetizing current is reduced to 0 in the negative direction. For hysteresis, it is also possible to apply a negative voltage in reverse order to eliminate residual magnetization at the end of every forward PWM control signal. 3.2 The opening and closing time of the on-off valve The on-off time of the on-off valve has an important influence on the static and dynamic performance of the valve. The opening and closing time of the on-off valve can be determined by testing to determine its strain step response. The signal generator inputs a square wave signal to the on-off valve, and then measures its response curve through the strain gauge, as shown in FIG. 6. As can be seen from the figure, the rise time and fall time of the strain response are basically equal and only about 0.5 ms, and this time is the opening and closing time of the on-off valve. 4 Conclusion Through the above test results show that the use of giant magnetostrictive high-speed hydraulic valve on the switch, you can get a shorter switching time and higher switching frequency, and low power consumption. So when used in hydraulic system PWM control can get higher control accuracy and economic benefits. This article is only a preliminary study, for further application, there are still some problems to be solved, such as: magnetostrictive material hysteresis, the output displacement amplification and temperature effects. But it should be seen that this is a very promising hydraulic high-speed on-off valve.