CNC Servo System And Machining Accuracy And Speed Compensation Function

The transmission mechanism of CNC machine tools is generally a ball screw pair. After ensuring the geometric accuracy of the machine tool, the reverse deviation of the machine tool axis and the pitch error of the ball screw are the main factors affecting the positioning accuracy and repeatability accuracy of the machine tool. Compensating for the reverse deviation of the machine tool axis and the pitch error of the ball screw can greatly improve the machine tool accuracy. The numerical control system also sets specialized parameters for these two compensation parameters and uses their compensation function as the basic control function of the control system.
1. Pitch error compensation
Pitch error compensation mainly compensates for the errors generated by the ball screws of each feed axis in the transmission process of CNC machine tools. Pitch error compensation is performed after adjusting the geometric accuracy of the machine tool (such as bed level, parallelism, and perpendicularity).
The pitch error compensation function of machine tools includes two methods: linear axis and rotary axis, which can compensate for the positioning accuracy of linear axis and rotary worktable respectively. Compensate the origin by taking the machine tool origin of each coordinate axis as the reference point, setting compensation points centered on the origin, with equal compensation intervals, and compensating at the compensation points.
The pitch error compensation of the CNC system needs to set the following parameters: the number of pitch error compensation points, compensation interval, the farthest compensation position at the negative end of the axis (machine coordinate system), the farthest compensation point position at the positive end of the axis (machine coordinate system), the compensation value of the compensation point, and the distance between the compensation points of the axis.
2. Reverse gap compensation
In the feed transmission chain of CNC machine tools, there are reverse clearances in gear transmission, ball screws, and nut pairs. The servo motor idles during reverse motion while the worktable does not actually move.
For CNC machine tools using a semi closed loop servo system, the presence of reverse clearance can affect the positioning accuracy and repeatability accuracy of the machine tool, thereby affecting the machining accuracy of the product. The reverse clearance compensation of CNC systems can automatically compensate for some regular errors during the machining process, improving the machining accuracy of parts. And as the use time of CNC machine tools increases, the reverse clearance will gradually increase due to the increase of motion pair clearance caused by wear, so it is necessary to regularly measure and compensate for the reverse clearance of each coordinate axis of CNC machine tools.
3. Thermal compensation
Thermal compensation refers to compensating for the amount of thermal deformation error caused by temperature changes caused by machine tool motion or environmental temperature changes. Temperature sensors installed in different parts of the machine tool send temperature information to the temperature measurement device. Based on the received information and the operating status of the machine tool, the CNC system calculates the current thermal deformation. Compensate for the position output of each axis based on the calculated results. The temperature measuring device is generally installed near the transmission mechanism of each feed axis, near the spindle box, and on non moving parts of the machine tool (such as the bed).
The basis for calculating thermal deformation in numerical control systems is the thermal deformation error compensation model. Due to the different structures of machine tools, the model between their deformation and temperature changes is also different. Measure the positioning errors of each feed axis of the machine tool at different temperatures, and establish a model between the temperature changes of the machine tool after heating, the environmental temperature changes, and the positioning errors of each axis. The numerical control system can calculate the instantaneous thermal deformation error based on the real-time collected temperature and the established model. Then, use the compensation value (the inverse of the thermal deformation error) to compensate for the displacement of each axis of the machine tool.
4. Nanotechnology
Nano interpolation technology is adopted in some high-end CNC systems to improve the machining accuracy of machine tools. Nano interpolation refers to the CNC system generating instructions in nanometer units to the servo controller, making the position instructions of the servo controller smooth, thereby improving the smoothness of the machined surface.