Valve controlled hydraulic motor speed servo system
Valve controlled hydraulic motor speed servo system
This kind of system is essentially a throttling servo system, which adjusts the flow into the hydraulic motor by adjusting the opening size of the electro-hydraulic servo valve, and then adjusts the speed of the hydraulic motor to keep it consistent with the set value. Due to the high frequency response of the servo valve, this kind of system has fast response, high precision, simple structure, but low efficiency. It is generally used in medium and small power and high-precision occasions. This kind of system can be divided into the following three types according to its structural form: serial valve controlled hydraulic motor speed servo system (see Figure 54),
Throttle parallel valve controlled hydraulic motor speed servo system (see Figure 55) and oil replenishing parallel valve controlled crossover motor speed servo system (see Figure 56).
(1) Serial valve controlled hydraulic motor speed servo system. The composition of the system is that the servo valve is connected in series between the pump and the motor, and the speed of the hydraulic motor is detected by the speed measuring device, which forms a speed closed loop through feedback. The working principle of the system is: when the speed of the hydraulic motor changes, the speed measuring device will feed back the actual speed signal, compare it with the reference signal R and generate a deviation E. according to the size of E, the controller controls the current input to the servo valve through a certain control law to change the opening size of the servo valve, so as to change the output flow of the servo valve, that is, change the flow into the hydraulic motor, so that the speed of the motor reaches the expected value. The characteristics of this system are: because the servo valve directly controls the flow into the hydraulic motor, the frequency response of the system is faster; However, due to the existence of throttling loss in the system, the efficiency of the system is very low, and the maximum efficiency in theory is only 30%; Moreover, because the throttling losses are converted into heat, the system temperature rises rapidly. This system is only suitable for medium and small power occasions.
(2) Throttle parallel valve controlled hydraulic motor speed servo system. In this system, the servo valve is connected in parallel in the system, and its structure is shown in Figure 55. The working principle of the system is: first, give a pre opening to the servo valve. The size of the pre opening depends on the speed range of the hydraulic motor and the leakage of the system. The specific data can be determined according to the experiment. After the speed of the hydraulic motor is determined, make the leakage flow of the bypass part reach the maximum value that needs to be adjusted.
If the speed decreases by 15% when the load changes from zero to full load, the flow of bypass leakage part will be 15% of the total system flow. When the external load increases or the temperature increases, the speed of the hydraulic motor decreases. At this time, the opening of the servo valve is reduced to compensate for the flow reduction caused by the slip and leakage of the variable displacement pump driven motor, so that the motor speed returns to the set value. Conversely, when the external load decreases, the speed of the hydraulic motor increases. At this time, the opening of the servo valve will be increased to increase the external leakage of the system to maintain the constant speed of the hydraulic motor.
这种系统的特点是：①由于伺服阀本身不带负载，所以频响很高，可使系统的调节时间大大缩短；②系统从旁路流回油箱的流量不大，旁路功耗较小，效率比串联阀控系统高，一般可达80%左右；③旁路的泄漏增加了系统阻尼，从而提高了系统的稳定性；④该系统的刚度较差。如果采用合适的调节手段，来弥补节流式并联阀控液压马达调速系统刚度差的弱点，那么该系统就可获得较快的调节时间和较高的效率。可用于高精度、大功率场合。
(3)补油式并联阀控液压马达速度伺服系统。与节流式并联阀控系统相比，补油式并联阀控支路(见图56)有自己的单独能源，伺服阀工作于向系统补油状态。系统的工作原理是；当系统受到阶跃负载或负载扰动时，液压马达的转速发生变化，系统通过闭环控制方式调节旁路伺服阀的开口，从而调节进入马达的流量，实现对系统调速或稳速的目的。从系统原理来看，泵提供马达运转的主要流量，保证大功率系统稳态高效，伺服阀由于在旁路上未直接带动负载，能充分发挥其快速响应的特性，保证系统快速调节。当采用适当的控制率时，可使系统的调节时间变得很短。
Compared with the throttle parallel valve control system, this system has two outstanding advantages: ① the bypass servo valve has its own independent oil supply system, which always works in the state of filling oil to the system, so that the system can obtain better stiffness; ② The differential pressure at the port of the servo valve is not only determined by the system pressure, but also affected by the oil replenishment pressure. Increasing the oil replenishment pressure can improve the response speed of the system.
This system is suitable for solving the problem of efficient and fast regulation of high-power systems, especially when some systems focus on the dynamic regulation performance under step load.
