Precision CNC machine tools|Classification of CNC Machine Tools - Classification by Servo Control

⑴ Open-loop control of CNC machine tools
The feed servo drive of this type of machine tool is open-loop, that is, there is no detection and feedback device. Generally, its drive motor is a stepper motor. The main feature of the stepper motor is that the motor rotates one step every time the control circuit changes the command pulse signal. pitch angle, and the motor itself has self-locking ability.
The feed command signal output by the numerical control system controls the drive circuit through the pulse distributor, which controls the coordinate displacement amount by changing the number of pulses, controls the displacement speed by changing the frequency of the pulse, and controls the displacement by changing the distribution sequence of the pulse. direction.
Therefore, the biggest characteristic of this control method is convenient control, simple structure and low price. The command signal flow sent by the numerical control system is one-way, 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.
The early CNC machine tools all adopted this control method, but the failure rate was relatively high. At present, due to the improvement of the driving circuit, it is still widely used. Especially in our country, the general economical CNC system and the CNC transformation of old equipment mostly use this control method. In addition, this control method can configure a single-chip computer or a single-board computer as a numerical control device, which reduces the price of the entire system.
(2) Closed-loop control of machine tools
The feed servo drive of this type of CNC machine tool works according to the closed-loop feedback control mode. The drive motor can use two kinds of servo motors, DC or AC, and needs to configure the position feedback and speed feedback, and detect the actual displacement of the moving parts at any time during processing. It is compared with the command signal obtained by the interpolation operation, and the difference is used as the control signal of the servo drive, and then drives the displacement component to eliminate the displacement error.
According to the installation position of the position feedback detection element and the feedback device used, it is divided into two control modes: full-closed loop and semi-closed loop.
① Fully closed loop control
As shown in the figure, the position feedback device adopts a linear displacement detection element (at present, a grating ruler is generally used), which is installed on the saddle of the machine tool, that is, the linear displacement of the machine tool coordinates is directly detected, and the feedback from the motor to the saddle of the machine tool can be eliminated. The transmission error in the entire mechanical transmission chain is obtained, so as to obtain a high static positioning accuracy of the machine tool.
However, in the whole control loop, the friction characteristics, rigidity and clearance of many mechanical transmission links are nonlinear, and the dynamic response time of the entire mechanical transmission chain is very large compared with the electrical response time. This brings great difficulty to the stability correction of the entire closed-loop system, and the design and adjustment of the system are also quite complicated. Therefore, this full-closed-loop control method is mainly used for CNC coordinate machines and CNC precision grinders that require high precision.
② Semi-closed loop control
As shown in the figure, the position feedback adopts the rotation angle detection element (the encoder is mainly used at present), which is directly installed at the end of the servo motor or lead screw. Since most of the mechanical transmission links are not included in the system closed-loop loop, it is called to obtain relatively stable control characteristics. The mechanical transmission error such as the lead screw cannot be corrected at any time through feedback, but the software fixed value compensation method can be used to appropriately improve its accuracy. At present, most CNC machine tools adopt semi-closed loop control method.
(3) Hybrid control CNC machine tools
The characteristics of the above control methods can be selectively concentrated to form a mixed control scheme. As mentioned above, since the open-loop control method has good stability, low cost, and poor accuracy, while the full-closed-loop control method has poor stability, in order to complement each other and meet the control requirements of some machine tools, a hybrid control method should be used. There are more open-loop compensation type and semi-closed-loop compensation type for surface roughness.