Introduction
The system may record systematic machine-related deviations, but they may still occur or increase during subsequent use due to environmental factors such as temperature or mechanical load. In these cases, SINUMERIK offers several compensation features.
Compensating for deviations with measurements obtained using real position encoders (such as grids) or additional sensors (such as laser interferometers) can lead to better machining results.
This article provides an overview of common SINUMERIK compensation features. The practical SINUMERIK measurement cycle such as “CYCLE996 motion measurement” can provide comprehensive support to end users in continuous monitoring and maintenance of machine tools.
Reverse Gap Compensation
The transfer of force between moving parts and their driving components, such as ball screws, can cause discontinuity or delay. Backlash-free mechanical structures significantly increase machine tool wear and are technically difficult to achieve.
Mechanical backlash creates a deviation between the axis/spindle path and the measured value of the indirect measurement system. This means that when the direction changes, the shaft will move either too far or too close, depending on the size of the gap.
Additionally, the work platform and associated coders will be affected. If the encoder is positioned in front of the workstation, it will reach the instruction position ahead of time, shortening the actual moving distance of the machine tool.
In machine tool operations, the reverse backlash compensation function can be used on the corresponding axis to automatically activate the deviation from previous records during reversal. The deviation from previous records will then be superimposed on the actual position value.
Screw Pitch Error Compensation
The principle of indirect measurement in CNC control systems is based on the assumption that the ball screw pitch will remain constant within the effective travel range.
Therefore, in theory, the actual position of the straight shaft can be deduced based on the position of the drive motor's motion information.
However, ball screw manufacturing errors can cause deviations in the measuring system, also known as screw pitch error.
Measurement bias (depending on the measurement system used) and measurement system installation error (also known as measurement system error) on the machine tool can further exacerbate this problem.
To compensate for these two errors, an independent measurement system such as a laser measurement system can be used to measure the natural error curve of CNC machines. The required compensation value can then be stored in the CNC system for later compensation.
Friction compensation (quadrantal error compensation) and dynamic friction compensation
Quadrantal error compensation, also known as friction compensation, is suitable for all the situations mentioned above, as it can significantly improve contour accuracy during circular contour processing.
The reason for this is that in quadrant conversion, one axis moves at the highest feed rate, while the other axis remains stationary. As a result, different frictional behaviors of the two axes can lead to contour errors.
Quadrant error compensation effectively reduces this error and ensures excellent machining results. The compensation pulse density can be defined according to the acceleration-related characteristic curve, which can be determined and parameterized through circularity tests.
During the roundness test, the actual position of the circular contour and the deviation from the programming radius (especially when reversing) are quantified and graphically displayed on the human-machine interface.
In the new version of the system software, an integrated dynamic friction compensation function can dynamically compensate for the friction behavior of the machine tool under different rotational speeds. This helps to reduce actual machining contour errors and achieve greater control accuracy.
Curvature and angle error compensation
If the weight of a single part of a machine tool causes shifting and tilting of the moving part, sag compensation will be necessary as it may cause sagging of the relevant machine parts, including the steering system.
Angle error compensation is necessary when the moving axes are not correctly aligned with each other at the correct angle, such as when they are perpendicular.
As the zero point offset increases, the position error also increases. Both errors are caused by the weight of the machine tool or the weight of the tool and workpiece.
During the debugging process, compensation values are measured, quantified and stored in SINUMERIK in the form of a compensation table according to the corresponding position.
When the machine is in operation, the relevant axis position is interpolated based on the stored point compensation value. For each continuous trajectory movement there are basic and compensating axes.
Temperature compensation
Heat can cause parts of a machine to expand, with the range of expansion depending on the temperature and thermal conductivity of each part.
Different temperatures can lead to changes in the actual position of each axis, which can negatively impact part accuracy during processing.
To compensate for these changes in actual values, temperature compensation can be used, where error curves for all axes at different temperatures are defined.
For correct thermal expansion compensation, the temperature compensation value, reference position and linear gradient angle parameters must be transferred from the PLC to the CNC control system using function blocks.
The control system automatically eliminates unexpected parameter changes, preventing machine tool overload and activating the monitoring function.
Space Error Compensation System ( VC)
Systematic geometric errors of rotary heads and turrets can occur due to the position of the rotary axis, mutual compensation and tool orientation errors. In addition, small errors may also occur in the feed spindle guidance system of each machine tool.
Linear position errors occur for linear axes, while rotary axes can have horizontal and vertical straightness errors, as well as pitch, yaw, and roll angle errors. Other errors can also occur when aligning machine tool components, such as vertical error.
In a three-axis machine tool, there may be 21 geometric errors at the tip, which include six types of errors per linear axis multiplied by three axes, plus three angular errors. These deviations collectively form a total error, also known as spatial error.
Spatial error is the deviation between the tool midpoint position (TCP) of the actual machine tool and that of an ideal, error-free machine tool. Solution partner SINUMERIK can determine spatial errors using laser measuring equipment. However, it is necessary to measure the error of all machine tools in the entire machining space, not just in a single position.
It is essential to record the measured values of all positions and plot the curve, as the magnitude of each error depends on the position of the relevant feed shaft and the measured position. Even when the Y-axis and Z-axis are in almost the same position on the X-axis, the trend that results on the X-axis can be different when they are in different positions.
With the help of “CYCLE996 – motion measurement”, determining the rotation axis error only takes a few minutes. This means that the machine tool's accuracy can be continuously checked and corrected if necessary, even during production.
Drift compensation (dynamic feedforward control)
Deviation refers to the difference between the position controller and the standard when the machine axis is moving.
Axis deviation is the difference between the target and actual positions of the machine tool axis.
Deviation can result in unnecessary contour errors, especially when the curvature of the contour changes, such as in circular or square contours.
To reduce velocity-related bias to zero along the path, use the NC FFWON advanced language command in the part program.
Through feedforward control, path accuracy can be improved, resulting in better machining effects.
FFWON activates the feedforward control command.
FFWOF disables the feedforward control command.
Electronic counterweight compensation
In extreme cases, the electronic counterweight function can be activated to prevent shaft sagging from damaging machine tools, tools or workpieces.
On load axles without mechanical or hydraulic counterweights, the vertical axle may unexpectedly sag when the brake is released.
By activating the electronic counterweight, it can compensate for unexpected shaft tilt. The constant balancing torque maintains the tilted shaft position after the brake is released.
