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October 28, 2019

Servo control mode to classify CNC machine tool.

(1) open loop control CNC machine tool

The feed servo drive of this kind of machine tool is open-loop, that is, there is no detection feedback device. Generally, its driving motor is a stepping motor. The main feature of stepping motor is that every time the control circuit changes the command pulse signal, the motor will rotate a step angle, and the motor itself has self-locking ability.

The drive circuit is controlled by the pulse distributor, which controls the coordinate displacement by changing the number of pulses, the displacement speed by changing the frequency of pulses, and the direction of displacement by changing the distribution sequence of pulses.

Therefore, this control mode is characterized by convenient control, simple structure and low price. The command signal flow from the CNC system is unidirectional, so there is no stability problem of the control system, but because the error of the mechanical transmission is not corrected by feedback, the displacement accuracy is not high.

Aluminum Turning & Milling Parts
Aluminum Turning & Milling Parts

(2) closed loop control CNC machine tool

The feed servo drive of this kind of NC machine tool works in the way of closed-loop feedback control. Its drive motor can be DC or AC servo motor, and need to configure position feedback and speed feedback. In the process of machining, the actual displacement of the moving parts is detected at any time and fed back to the comparator in the numerical control system in time. It is compared with the instruction signal obtained by interpolation operation, and its difference is used as the control signal of servo drive, which drives the displacement parts to eliminate the displacement error.

According to the difference between the installation position of the position feedback detection element and the feedback device used, it can be divided into two control modes: full closed loop and half closed loop.

CNC milled workpiece
CNC milled workpiece


As shown in the figure, its position feedback device adopts linear displacement detection element (at present, grating ruler is generally used), which is installed on the bed saddle of the machine tool, i.e. directly detecting the linear displacement of the machine tool coordinate. Through the feedback, the transmission error in the whole mechanical transmission chain from the motor to the bed saddle of the machine tool can be eliminated, so as to obtain high static positioning accuracy of the machine tool.

Aluminum Precision Machining Part
Aluminum Precision Machining Part

However, in the whole control ring, the friction characteristics, rigidity and clearance of many mechanical transmission links are nonlinear, and the dynamic response time of the whole mechanical transmission chain is very large compared with the electrical response time. This brings great difficulty to the stability correction of the whole closed-loop system, and the design and adjustment of the system are also quite complex. Therefore, this kind of full closed-loop control mode is mainly used for CNC coordinate building machine and CNC precision grinder with high precision requirements.


As shown in the figure, its position feedback adopts angle detection element (currently mainly encoder, etc.), which is directly installed at the end of servo motor or lead screw. Because most of the mechanical transmission links are not included in the closed-loop system, it is called to obtain more stable control characteristics. The mechanical transmission errors such as lead screw can not be corrected at any time by feedback, but the software fixed value compensation method can be used to properly improve its accuracy. At present, most CNC machine tools adopt semi closed loop control.

(3) hybrid control CNC machine

The characteristics of the above control methods can be selectively centralized to form a hybrid control scheme. As mentioned before, because the open-loop control mode has good stability, low cost and poor accuracy, while the full closed-loop control mode has poor stability, in order to make up for each other and meet the control requirements of some machine tools, the hybrid control mode should be adopted. There are two kinds of open-loop compensation and half closed-loop compensation.

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