The systematic mechanical deviation of the machine tool can be recorded by the system, but due to environmental factors such as temperature or mechanical load, the deviation may still appear or increase in the subsequent use process. In order to obtain a better CNC manufacturing effect, the measured values obtained by actual position encoder (such as grating) or additional sensors (such as laser interferometer) are used to compensate the deviation.
Reverse clearance compensation
The transmission of force between the moving parts and the driving parts of the machine tool, such as the ball screw, will be interrupted or delayed, because the mechanical structure without clearance will significantly increase the wear of the machine tool, and it is difficult to achieve from the process. The mechanical clearance results in the deviation between the motion path of the shaft / spindle and the measurement value of the indirect measurement system. This means that once the direction changes, the axis will move too far or too close, depending on the size of the gap. The worktable and its related encoder will also be affected: if the encoder is ahead of the worktable, it will arrive at the command position ahead of time, which means that the actual moving distance of the machine tool is shortened. When the machine tool is running, by using the reverse clearance compensation function on the corresponding axis, the previously recorded deviation will be automatically activated when reversing, and the previously recorded deviation will be superimposed on the actual position value.
Lead screw pitch error compensation
The measurement principle of indirect measurement in the CNC control system is based on the assumption that the pitch of ball screw remains unchanged in the effective stroke, so in theory, the actual position of linear axis can be deduced according to the motion information position of driving motor. However, the manufacturing error of the ball screw will lead to the deviation of the measurement system (also known as the screw pitch error). The measurement deviation (depending on the measurement system used) and the installation error of the measurement system on the machine tool (also known as measurement system error) may further aggravate this problem. In order to compensate these two kinds of errors, a set of independent measurement system (laser measurement) can be used to measure the natural error curve of CNC machine tool, and then the required compensation value is saved in the CNC system for compensation.
Friction compensation (quadrant error compensation) and dynamic friction compensation
Quadrant error compensation (also known as friction compensation) is suitable for all the above situations, so as to greatly improve the contour accuracy when machining circular contour. The reason is as follows: in quadrant conversion, one axis moves at the maximum feed speed, and the other axis is stationary. Therefore, different friction behaviors of the two axes may lead to profile errors. Quadrant error compensation can effectively reduce this error and ensure excellent machining effect. The density of compensation pulse can be set according to the characteristic curve related to acceleration, which can be determined and parameterized by roundness test. In the roundness test, the deviation between the actual position of the circular contour and the programmed radius (especially when reversing) is recorded quantitatively and displayed on the human-machine interface through graphics.
Sag and angle error compensation
If the weight of a single part of each machine tool will cause the displacement and inclination of the moving parts, sag compensation is needed, because it will cause the relevant parts of the machine tool (including the guiding system) to sag. Angle error compensation is used when the moving axes are not aligned with each other at the correct angle (for example, vertical). As the offset of the zero position increases, the position error also increases. These two kinds of errors are caused by the weight of the machine tool, or the weight of the tool and workpiece. After the measured compensation value is quantified during debugging, it is stored in SINUMERIK according to the corresponding position in some form, such as compensation table. When the machine tool is running, the position of the relevant axis is interpolated according to the compensation value of the storage point. For each continuous path movement, there are basic axis and compensation axis.
Heat may cause the parts of the machine to expand. The expansion range depends on the temperature and thermal conductivity of each part of the machine tool. Different temperature may cause the actual position of each axis to change, which will have a negative impact on the accuracy of the workpiece. These changes in actual values can be offset by temperature compensation. The error curves of each axis at different temperatures can be defined. In order to compensate the thermal expansion correctly, the temperature compensation value, reference position and linear gradient angle parameters must be continuously transferred from PLC to CNC control system through function block. Unexpected parameter changes will be automatically eliminated by the control system to avoid machine overload and activate the monitoring function.
Space error compensation system (VCS)
The position of the rotary axis, their mutual compensation and tool orientation error may cause systematic geometric errors of the rotary head and the rotary head. In addition, there will be small errors in the guiding system of the feed axis in each machine tool. For the linear axis, these errors are linear position errors; horizontal and vertical straightness errors; for the rotating axis, there are pitch angle, yaw angle and roll angle errors. Other errors may occur when aligning machine components with each other. For example, vertical error. In a three-axis machine tool, this means that 21 geometric errors may occur on the tool tip: Six error types per linear axis multiplied by three axes, plus three angular errors. These deviations work together to form a total error, also known as spatial error. Spatial error describes the deviation between the actual tool midpoint (TCP) position and the ideal error free tool midpoint position.
Deviation compensation (dynamic feedforward control)
Deviation refers to the deviation between the position controller and the standard when the machine axis moves. The axis deviation is the difference between the target position and the actual position of the machine axis. Deviation leads to unnecessary contour errors related to velocity, especially when the curvature of contour changes, such as circular and square contour. With the NC high-level language command ffwon in the part program, the speed related deviation can be reduced to zero when moving along the path. The feed-forward control is used to improve the path accuracy and obtain better machining effect.
Electronic counterweight compensation
In extreme cases, the electronic counterweight function can be activated in order to prevent the machine tool, tool or workpiece from being damaged due to shaft sagging. In the load shaft without mechanical or hydraulic counterweight, once the brake is released, the vertical shaft will sag unexpectedly. After the electronic counterweight is activated, it can compensate for the unexpected shaft droop. After releasing the brake, the position of the droop shaft is maintained by a constant balance torque.