Different classifications of servo systems： 

1. Open loop servo system

The open-loop servo system is a system without position feedback, and its driving components are mainly power stepping motors or hydraulic pulse motors. The essence of the working principle of these two driving elements is the conversion of digital pulses to angular displacements. It does not use position detection elements to achieve positioning, but relies on the driving device itself. The angle of rotation is proportional to the number of command pulses; the movement speed is determined by the advancement. The frequency of the pulse is determined.

The open-loop servo system has a simple structure and is easy to control, but it has poor accuracy, low speed and unevenness, and low torque. Generally used on light-loaded or economical CNC machine tools with little change in load.

2. Closed loop servo system

The closed-loop servo system is an error control follow-up system. The error of the CNC machine tool feed system is the difference between the position command output by the CNC and the actual position of the machine tool table (or tool post). The motion actuator of the closed-loop system cannot reflect the position of the motion, so a position detection device is required. The device measures the actual displacement or actual position, and feeds the measured value back to the CNC device, compares with the instruction, obtains the error, and in turn constitutes a closed-loop position control.

Since the closed-loop servo system is feedback control, the feedback measuring device has a high precision, so the error of the system transmission chain, the error of each element in the loop and the error caused by the movement can be compensated, thereby greatly improving the following accuracy and positioning accuracy.

3. Semi-closed loop system

The position detection element is not directly installed on the final moving part of the feed coordinate, but is converted through the position of the mechanical transmission part in the middle, which is called indirect measurement. That is to say, a part of the transmission chain of coordinate movement is outside the position closed loop, and the transmission error outside the loop is not compensated by the system, so the accuracy of this servo system is lower than that of the closed loop system.

The control structure of the semi-closed-loop and closed-loop systems is the same. The difference is that the closed-loop system includes more mechanical transmission components in the loop, and the transmission error can be compensated. Theoretically, the accuracy can be very high. However, due to mechanical deformation, temperature changes, vibration and other factors, the system stability is difficult to adjust. In addition, after the machine tool has been running for a period of time, due to changes in mechanical transmission components, wear, deformation and other factors, it is easy to change the stability and accuracy of the system. Therefore, more semi-closed loop systems are currently used. Only use the fully closed-loop servo system on high-precision CNC machine tools with high compactness of transmission components, stable performance, and little temperature difference during use.