For some execution machines (such as heavy-duty CNC machine tool workbenches), although the semi-closed loop structure of the position servo system is easy to sketch, it is difficult to compensate the position error of the mechanical transmission part of the ground wheel, so that the position control accuracy cannot reach the required index; It is difficult to tune the system with a fully closed-loop structure, and it is difficult to eliminate the oscillation induced by various nonlinear factors in the loop after the system is closed.
Therefore, it is proposed that there are both semi-closed loop and fully closed loop in the system, that is, a hybrid closed-loop servo system. When the system is working, the semi-closed loop plays the main control role.Since the electrical automatic control part in the semi-closed loop is opposed to the execution machinery, a higher
Position gain makes the system easy to set, fast response and small tracking error.
The closed loop is only used for steady-state error compensation, and the position gain can be selected to be lower to ensure the stability of the system. The combination of the two can finally obtain higher position control accuracy and tracking speed. However, due to the simultaneous existence of two closed loops in the system, the control complexity of the system is greatly increased, and the coordination and gain adjustment between them must be carefully sketched, and the position servo system is therefore no longer versatile.
Among the above four basic structural forms of the position servo system, the semi-closed-loop servo system is the most widely used structure at present, and because its electrical automatic control part is relatively independent from the mechanical part, it can be divided into different types according to the mechanical inertia and load conditions.
The electrical parts are designed to be generalized independently. Therefore, in a narrow sense, people also habitually call the electrical automatic control part of the position servo in the semi-closed loop mechanism as the position servo system.
In principle, the servo system of CNC machine tools should include all the links from position command pulse given to actual position output, that is, including position control, speed control, drive motor, detection parts and other parts. But in many systems, for the convenience of manufacture, the position control part of the servo system is usually integrated with the CNC device.
Therefore, the servo feed system of CNC machine tools is commonly referred to as the speed control unit of the servo feed system. , Servo motor, detection components, but not the position control part.
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