Requirements of CNC machine tools for AC servo system
The servo system is an important part of the CNC machine tool. The accuracy and speed indicators of the CNC machine tool are often determined by the servo system. The servo system has gone through the step servo to DC servo and then to the ac servo system development process. With the development of technology, the linear drive of high-speed and high-precision machining has become a new trend in the development of servo systems. CNC machine tools require the following points of servo system:
(1) Good stability: Stability means that the system can reach new or return to the original equilibrium state after a short adjustment process under the given input or external disturbance.
(2) The output position accuracy is high: the accuracy of the servo system refers to the accuracy with which the output can follow the input amount. As a precision machined CNC machine tool, the required positioning accuracy or contour machining accuracy is passed.Often higher, the allowable deviation is generally between 0.01 and 0.00lmm. Statically requires high positioning accuracy and repeat positioning accuracy, that is, positioning error and repeated positioning error are small (to ensure size accuracy). Dynamically, the following accuracy is required, that is, the following error is small, which is a dynamic performance indicator (to ensure contour accuracy). In addition, the sensitivity is required to be high enough to have a high resolution.
(3) Fast response, fast response and no overshoot: Fast response is one of the indicators of the dynamic quality of the servo system, that is, the response of the tracking command signal is required to be fast, on the one hand, the transition is required.The process time is short, generally less than 200ms, or even less than tens of milliseconds; on the other hand, in order to meet the overshoot requirement, the front edge of the transition process is required to be steep, that is, the rate of increase is large. This is the requirement of ac servo system dynamic performance, that is, without the overshoot, the settling time tp of the moving speed of the executing component should be as short as possible. Usually it is required to be from 0→Fmax(Fmax→0), its time should be 200ms,and there can be no overshoot, otherwise it is not good for mechanical parts and is harmful to the processing quality.
(4)The speed range should be wide and have good stability (within the speed regulation range). Speed range: General requirements: Stability means less fluctuation of output speed, especially at low speed,it is especially important.
(5)The load characteristics should be hard. In the system load range, when the load changes, the output speed should be basically the same. That is, ΔF is as small as possible; when the load is abrupt, the recovery time of the required speed is short and no oscillation. That is, Δt is as short as possible; in addition, there should be sufficient overload capability. This is required for CNC machine tool servo systems to have good static and dynamic stiffness.
(6). It can be reversible and frequently started and stopped.
(7). The system has high reliability, convenient maintenance and low cost.
To sum up: the requirements for the servo system include both static and dynamic characteristics; for high-precision CNC machine tools, the dynamic performance requirements are more stringent.