The systematic mechanical related deviations of machine tools can be recorded by the system, but due to environmental factors such as temperature or mechanical loads, deviations may still occur or increase during subsequent use. In these cases, SINUMERIK can provide different compensation functions. Compensate for deviations by using actual position encoders (such as gratings) or additional sensors (such as laser interferometers) to obtain better machining results. In this issue, we will introduce the common compensation functions of SINUMERIK. Practical SINUMERIK measurement cycles such as "CYCLE996 motion measurement" can provide comprehensive support for end users in the continuous monitoring and maintenance process of machine tools.
Reverse gap compensation
There may be interruptions or delays in the transmission of force between the moving parts of the machine tool and its driving components, such as ball screws, because a mechanical structure with no gaps at all will significantly increase the wear of the machine tool, and it is also difficult to achieve from a process perspective. The mechanical clearance causes a 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 encoders will also be affected: if the encoder is positioned on the worktable, it will arrive at the commanded position ahead of schedule, which means that the actual distance traveled by the machine tool will be shortened. During the operation of the machine tool, 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.
Screw pitch error compensation
The measurement principle of indirect measurement in CNC control systems is based on the assumption that the pitch of the ball screw remains constant within the effective stroke, so theoretically, the actual position of the linear axis can be derived based on the motion information of the driving motor. However, manufacturing errors in ball screws can lead to measurement system deviations (also known as screw pitch errors). 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 exacerbate this problem. To compensate for these two types of errors, an independent measurement system (laser measurement) can be used to measure the natural error curve of the 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, in order to significantly improve the contour accuracy when machining circular contours. The reason is as follows: In quadrant conversion, one axis moves at a high feed rate while the other axis remains stationary. Therefore, the different friction behaviors of the two axes may lead to contour errors. Quadrant error compensation can effectively reduce this error and ensure excellent machining results. The density of compensation pulses can be set based on a characteristic curve related to acceleration, which can be determined and parameterized through roundness testing. In roundness testing, the deviation between the actual position of the circular contour and the programmed radius (especially during commutation) is quantified and recorded, and displayed graphically on the human-machine interface.
In the new version of the system software, the integrated dynamic friction compensation function can dynamically compensate based on the friction behavior of the machine tool at different speeds, reducing actual machining contour errors and achieving higher control accuracy.
Compensation for sag and angle errors
If the weight of individual components of each machine tool causes displacement and tilting of moving parts, sag compensation is required because it can cause the relevant machine parts (including the guidance system) to sag. Angle error compensation is used when the moving axes are not aligned with each other at the correct angle (e.g. perpendicular). As the offset of the zero point position continues to increase, the position error also increases. Both of these errors are caused by the self weight of the machine tool or the weight of the tool and workpiece. The compensation values measured during debugging are quantified and stored in SINUMERIK according to their corresponding positions in some form, such as a compensation table. When the machine tool is running, the position of the relevant axis is interpolated based on the compensation value of the storage point. For each continuous path movement, there are fundamental axes and compensation axes.
temperature compensation
Heat may cause expansion of various parts of the machine tool. The expansion range depends on the temperature, thermal conductivity, etc. of each machine part. Different temperatures may cause changes in the actual position of each axis, which can have a negative impact on the accuracy of the workpiece during machining. These actual value changes can be offset by temperature compensation. The error curves of each axis at different temperatures can be defined. In order to always compensate for thermal expansion correctly, it is necessary to continuously retransmit temperature compensation values, reference positions, and linear gradient angle parameters from the PLC to the CNC control system through functional blocks. Unexpected parameter changes will be automatically eliminated by the control system, thereby avoiding machine overload and activating monitoring functions.
Space Error Compensation System (VCS)
The position of the rotary axis, their mutual compensation, and tool orientation errors may lead to systematic geometric errors in the components of the rotary head and rotary head. In addition, there will be small errors in the guidance system of the feed axis in each machine tool. For linear axes, these errors are linear position errors; Horizontal and vertical straightness errors; For the rotation axis, pitch angle, yaw angle, and roll angle errors will occur. Other errors may occur when aligning machine tool components with each other. For example, vertical error. In a three-axis machine tool, this means that there may be 21 geometric errors on the tool tip: six error types per linear axis multiplied by three axes, plus three angular errors. These deviations work together to form the total error, also known as spatial error.
Space error describes the deviation between the tool center point (TCP) position of an actual machine tool and the tool center point position of an ideal error free machine tool. SINUMERIK solution partners can use laser measurement equipment to determine spatial errors. Measuring only the error of a single position is far from enough, all machine tool errors within the entire machining space must be measured. Usually, it is necessary to record the measurement values of all positions and plot them into curves, as the magnitude of each error depends on the position of the relevant feed axis and the measurement position. For example, when the y-axis and z-axis are in different positions, the deviation caused by the x-axis will be different - even at almost the same position on the x-axis, errors will occur. With the help of "CYCLE996- Motion Measurement", it only takes a few minutes to determine the rotation axis error. This means that the accuracy of the machine tool can be continuously checked, and if necessary, the accuracy can be corrected even during production.
Deviation compensation (dynamic feedforward control)
Deviation refers to the deviation between the position controller and the standard during the movement of the machine tool axis. Axis deviation refers to the difference between the target position of the machine tool axis and its actual position. Deviation leads to unnecessary contour errors related to speed, especially when the contour curvature changes, such as circular, square contours, etc. With the NC language command FFWON in the part program, speed related deviations can be reduced to zero when moving along a path. By using feedforward control to improve path accuracy, better machining results can be achieved.
FFWON: Command to start feedforward control
FFWOF: Command to disable feedforward control
Electronic weight compensation
In extreme cases, the electronic counterweight function can be activated to prevent damage to the machine tool, tool, or workpiece caused by axis sagging. In a load shaft without mechanical or hydraulic counterweights, once the brake is released, the vertical axis will unexpectedly sag. After activating the electronic counterweight, it can compensate for unexpected shaft sagging. After releasing the brake, maintain the position of the drooping shaft with a constant balance torque.
