Calibration taboos of electro-hydraulic servo systems
1) Taboos in Choosing Between Lead and Lag Compensation: In electro-hydraulic servo systems, series lag compensation is often used to improve low-frequency gain and enhance system accuracy. Alternatively, while maintaining system accuracy, the mid-frequency gain can be reduced to decrease the crossover frequency and ensure system stability. However, for hydraulic servo systems, which are primarily characterized by integral and oscillatory components, phase stability margin is relatively easy to guarantee, while assigned stability margin is not. Therefore, series lead compensation, which focuses on improving phase margin, is not recommended; instead, series lag compensation should be used.
(2) Series lag compensation should not be used alone. After adopting series lag compensation, the system response characteristics are not improved. The bandwidth of the actual system will be reduced, and due to the reduction of phase margin and the change of cutoff frequency during operation, the system may become a conditionally stable system. Therefore, series lag compensation should be used in conjunction with other compensation measures.
(3) Do not overlook the difference between velocity feedback correction and acceleration feedback correction. Velocity feedback correction mainly improves the stiffness and bandwidth of the system and reduces the interference and nonlinear effects of the components enclosed in the velocity feedback loop. Acceleration feedback, on the other hand, mainly improves the damping of the system.
4) Velocity feedback correction and acceleration feedback correction are effective. Depending on the needs, velocity feedback correction and acceleration feedback correction can be used individually or simultaneously. Acceleration feedback can increase the system's damping, while velocity feedback can increase the system's natural frequency but decreases gain and damping. If both velocity and acceleration feedback are introduced simultaneously, and the values of K1 and K2 are appropriately selected, the damping ratio can be increased, thus lowering the resonant peak value and comprehensively improving both dynamic and static performance indicators.
(5) Dynamic pressure feedback is better than pressure feedback. Using a dynamic pressure feedback electro-hydraulic servo valve can improve the damping ratio. If the structure of the electro-hydraulic servo valve is not changed, but a pressure sensor is used to measure the load pressure and convert it into a voltage signal, and then a voltage signal proportional to the voltage is extracted by a differentiating circuit, and then the feedback loop surrounds more components, thereby improving the damping ratio. Dynamic pressure feedback only plays a feedback role in the dynamic process. The function of pressure feedback is the same as that of a pressure feedback servo valve, which is to generate an accessory pressure flow coefficient to increase K, thereby increasing the damping ratio. However, external disturbance forces and pressure signals can also be fed back to the input end, causing the system to have a larger static error. Obviously, dynamic pressure feedback is better than pressure feedback.
