The systematic mechanical-related 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 during subsequent use.
In these cases, SINUMERIK can provide different compensation functions. Use actual position encoders (such as gratings) or additional sensors (such as laser interferometers, etc.) to obtain measured values to compensate for deviations, so as to obtain better processing results. In this issue, I will introduce the common compensation functions of SINUMERIK. Practical SINUMERIK measurement cycles such as “CYCLE996 motion measurement” can provide end users with comprehensive support during the continuous monitoring and maintenance of the machine tool.
Backlash compensation
There will be discontinuities or delays in the transmission of force between the moving parts of the machine tool and its driving parts, such as ball screws, because a mechanical structure with no gap at all will significantly increase the wear of the machine tool, and it is also difficult to achieve in terms of technology. . The mechanical backlash causes a deviation between the movement path of the axis/spindle and the measured value of the indirect measuring 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 workbench and its related encoders will also be affected: if the encoder position is ahead of the workbench, it reaches the commanded position ahead of time, which means that the actual distance of the machine tool is shortened. When the machine tool is running, by using the backlash compensation function on the corresponding axis, the previously recorded deviation will be automatically activated when commutation, and the previously recorded deviation will be superimposed on the actual position value.
Screw pitch error compensation
The measurement principle of indirect measurement in the CNC control system is based on the assumption that the pitch of the ball screw remains unchanged within the effective stroke. Therefore, in theory, the actual position of the linear axis can be derived from the position of the drive motor’s motion information.
However, the manufacturing error of the ball screw will cause a deviation in 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 called measurement system error) may further exacerbate this problem. In order to compensate for these two kinds of errors, an independent measurement system (laser measurement) can be used to measure the natural error curve of CNC machine tools, and then the required compensation value can be saved in the CNC system for compensation.
Friction compensation (quadrant error compensation) and dynamic friction compensation
Quadrant error compensation (also called friction compensation) is suitable for all the above situations, so as to greatly improve the contour accuracy when machining circular contours. The reason is as follows: In the quadrant transition, one axis moves at the highest feed rate, while the other axis is stationary. Therefore, the different friction behavior of the two shafts may cause contour errors. Quadrant error compensation can effectively reduce this error and ensure excellent processing results. The density of the compensation pulse can be set according to a characteristic curve related to acceleration, and the characteristic curve can be determined and parameterized through a roundness test. In the roundness test, the deviation between the actual position of the circular contour and the programmed radius (especially when commutation) is quantified and recorded, and graphically displayed on the man-machine interface.
In the new version of the system software, the integrated dynamic friction compensation function can dynamically compensate according to the friction behavior of the machine tool at different speeds, reduce the actual machining contour error, and achieve higher control accuracy.
Sag and angle error compensation
If the weight of a single part of each machine tool will cause the displacement and tilt of the movable part, sag compensation is required, because it will cause the relevant machine tool parts (including the guide 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 continues to increase, the position error also increases. Both of these errors are caused by the weight of the machine tool, or the weight of the tool and workpiece. The compensation value measured during debugging is quantified and stored in SINUMERIK in some form, such as a compensation table, according to the corresponding position. 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.
Temperature compensation
Heat may cause the expansion of various parts of the machine tool. The expansion range depends on the temperature and thermal conductivity of each part of the machine tool. Different temperatures may cause the actual position of each axis to change, which will have a negative impact on the accuracy of the workpiece in processing. These actual value changes can be offset by temperature compensation. The error curve of each axis at different temperatures can be defined. In order to always correctly compensate for thermal expansion, the temperature compensation value, reference position and linear gradient angle parameters must be continuously retransmitted from the PLC to the CNC control system through the function block. Unexpected parameter changes will be automatically eliminated by the control system to avoid overloading the machine tool and activate the monitoring function.
Spatial Error Compensation System (VCS)
The position of the rotary axis, their mutual compensation, and tool orientation errors may cause systematic geometric errors in parts such as the rotor and the rotary head. In addition, there will be small errors in the guide system of the feed axis in each machine tool. For linear axes, these errors are linear position errors; horizontal and vertical straightness errors; for rotating axes, pitch, yaw, and roll angle errors will occur. When the machine components are aligned with each other, other errors may occur. 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 for each 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 of the tool midpoint (TCP) position of the actual machine tool and the tool midpoint position of the ideal error-free machine tool. SINUMERIK solution partners can use laser measuring equipment to determine spatial errors. It is not enough to measure the error of a single position. It is necessary to measure all machine tool errors in the entire processing space. It is usually necessary to record the measured values of all positions and draw them into a curve, because the magnitude of each error depends on the position of the relevant feed axis and the measuring position. For example, when the y-axis and the z-axis are at different positions, the deviation caused by the x-axis will be different-even if the x-axis is almost the same position, there will be an error. With the help of “CYCLE996-Motion Measurement”, the error of the rotary axis can be determined in just a few minutes. This means that the accuracy of the machine tool can be constantly checked, and if necessary, the accuracy can be corrected even in production.
Deviation compensation (dynamic feedforward control)
Deviation refers to the deviation of the position controller from the standard when the machine axis is moving. The axis deviation is the difference between the target position of the machine axis and its actual position. Deviations lead to unnecessary contour errors related to speed, especially when the contour curvature changes, such as round, square contours, etc. 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. Improve the path accuracy through feedforward control, so as to obtain better processing results.
FFWON: Command to start feedforward control
FFWOF: Command to turn off feedforward control
Electronic counterweight compensation
In extreme cases, in order to prevent damage to the machine tool, tool or workpiece due to shaft sagging, the electronic counterweight function can be activated. In load shafts without mechanical or hydraulic counterweights, once the brakes are released, the vertical shaft will sag unexpectedly. After activating the electronic counterweight, it can compensate for accidental shaft sagging. After releasing the brake, a constant balance torque is used to maintain the position of the sagging shaft.
Post time: Mar-16-2021