Error compensation technology to improve the machining accuracy of CNC machines

In the development and application of modern science and technology, there are two ways to ensure the accuracy of machining: one is to improve the quality of CNC machine tools, and the other is to use error compensation technology. to improve the accuracy through error compensation technology. Error compensation can generally be divided into error prevention technology and error compensation technology. Generally, error compensation technology uses the methods of modeling, measuring errors and realizing error compensation.

(1) Hardware static compensation method. Using the hardware static compensation method in controlling the machining accuracy involves adding an external hardware mechanism and allowing the machine tool to use the secondary position to generate a movement in the opposite direction to the error under the action of an external force to reduce machining errors. In screw processing, due to the error between the screws of the machine tool, using the pitch compensation ruler to adjust the pitch between the screws is a static compensation method. Due to the limitations of the static compensation method, compensation can only be achieved by adjusting the numerical values ​​or parameters of the equipment when stopping, and real-time compensation cannot be achieved during movement. This hardware static compensation method is relatively rarely used and is generally applied. is used with Other methods are used in combination.

(2) Integrated use of static compensation method and dynamic compensation method. As mentioned above, the static compensation method is to perform error compensation by adjusting the parameters when the CNC machine tool is at rest. This compensation method can systematically compensate and improve the accuracy, and cannot perform random error compensation during movement. The combination of methods can greatly improve the processing accuracy. Dynamic compensation is to perform the compensation amount or parameter compensation based on the working conditions of the machine tool, environmental conditions and spatial position changes during processing, and perform feedback compensation through the real-time movement state, such as when processing machine bearings, through timely modification of parameters and compensation to monitor heat, geometry and cutting degree, this is an error compensation method with practical significance. However, the technical level of CNC machine tools is extremely high and the investment cost is very high.

(3) Feed servo system compensation method. The servo system is a transmission device that drives each processing coordinate axis. This compensation system can work in both forward and reverse directions. It can realize forward or backward movement in real time according to the requirements of the processing path. The processing control accuracy can reach 0.1 micron. In addition, it has a wide speed. Range, fast response and no overshoot, low speed and high torque. In a typical CNC machine tool feed system, the open-loop control system is composed of a stepper motor. The angular or linear displacement of the stepper motor is proportional to the number of pulses, and its speed is proportional to the pulse frequency. turns the command pulse into a stepper motor. The rotary motion of the output shaft of the input motor is used to control the processing of the machine tool; the closed-loop feed position servo system is mainly driven by a DC servo motor or an AC servo motor. The offset of the machine table can be returned in time for comparison in the CNC device through the detection device. The difference between them is used as the control signal of the servo motor to control the worktable to eliminate the offset error; The semi-closed-loop feed position servo system is composed of a position control unit and a speed control unit. The pulses emitted by the pulse encoder are used as position feedback signals on the one hand and as speed measurement signals on the other hand. When the click load changes, the frequency of the feedback pulse signal will change accordingly. In actual machine processing, the accuracy error is reduced by controlling the speed of the servo motor.

(4) Change the compensation method of G code. G code is a language used to compile machine tool processing programs. G code has a tool compensation function, such as G44 and G43, which are tool length compensation. The principle of G code compensation is to compensate for the error range by changing the position information of the tool. This kind of compensation is also widely used to compensate for processing errors on CNC machine tools. For example, the five-axis machine tool error compensation model developed by Xu et al. is modified based on the model to change the original position of the tool generated by the model. CAM software and complete it by modifying the processing error compensation of the CNC machine tool. This compensation method requires G code programmers to define the workpiece geometry, determine the process and tool path. During actual work, if position deviation occurs, it is necessary to perform error compensation by modifying the G code. It is generally used for comparison. The shapes of simple machined parts are not complicated and are mainly contours consisting of straight lines and arcs. The amount of data processing is small. When dealing with complex parts with a large workload, they need to be modified by computer G code control. Programmers perform programming using a computer.

(5) Coordinate offset compensation method. The coordinate offset compensation method uses the offset of the coordinate origin of the CNC system to compensate for machine tool errors by taking into account feedback from signals such as position. During the programmer operation, the errors in the workpiece processing can be corrected by the visual display of the CNC system. If errors occur, the original coordinates can be reset by the CNC system to compensate for the errors. This compensation method is suitable for CNC machines with three-axis coordinates. This compensation method usually uses fixed side heads when using side heads, and also requires a certain software compensation to ensure the stability of the foundation.