The accuracy of open-loop system is low, because the step error , start stop error and mechanical system error of servo driver will directly affect the positioning accuracy. This system has the advantages of both open-loop and closed-loop, that is, it has the stability of open-loop and the accuracy of closed-loop. The system will not vibrate because of the resonance frequency, crawling and malfunction of the machine tool. Feedback compensation open loop control does not need clearance compensation and pitch compensation.
Because the accuracy of open-loop control can not meet the requirements of machine tools, in order to improve the control accuracy of servo driver, the best way is to use closed-loop control. That is to say, there are not only the predecessor control channel, but also the feedback channel of the detection output. the deviation signal is obtained by comparing the command signal with the feedback signal, forming a closed-loop control system with deviation control.
For the closed-loop control system, reasonable design can achieve reliable stability and high accuracy, but the direct measurement of the position signal of the worktable needs to use the position detection device with high installation and maintenance requirements, such as grating, magnetic ruler or linear inductosyn. The equivalent feedback signal of position output can be obtained indirectly by measuring the angular displacement of drive shaft or lead screw. Because the error caused by this part of the transmission can not be compensated by the closed-loop system, which does not include the transmission chain from the rotating shaft to the worktable, the error caused by this part of the transmission can not be compensated automatically by the closed-loop system, the closed-loop control system composed of equivalent feedback signal is called semi closed-loop servo driver, and this control mode is called semi closed-loop control mode.
The principle of compensation of the servo driver is the same as that of the open-loop compensation system. Two independent measurement systems composed of resolver and inductosyn work in amplitude discrimination mode. The disadvantage of the system is high cost. It needs two sets of detection system. The advantage is that it is easier to adjust than the full closed-loop system and has good stability. It is suitable for the feed drive of high-precision large-scale CNC machine tools.