With the rapid development of modern manufacturing, CNC machine tools are increasingly widely used, and the requirements for positioning accuracy and repeatability of CNC machine tools are also constantly increasing. The original semi-closed-loop control system based on precision ball screws and encoders can no longer meet user needs. The semi-closed-loop control system cannot control the transmission errors generated by the machine tool's transmission mechanism, the thermal deformation errors generated by the transmission mechanism during high-speed operation, and the errors caused by wear of the transmission system during machining. These errors have seriously affected the machining accuracy and stability of CNC machine tools. Linear grating rulers provide full closed-loop control of each linear axis of the CNC machine tool, eliminating the above errors and improving the positioning accuracy, repeatability, and reliability of the machine tool. As a key component for improving the positional accuracy of CNC machine tools, it is increasingly favored by users.
I. Selection of Linear Grating Rulers
(1) Selection of Accuracy Class The purpose of configuring linear grating rulers on CNC machine tools is to improve the setpoint accuracy and repositioning accuracy of linear coordinate axes. Therefore, the accuracy class of the grating ruler is the first consideration. The accuracy classes of grating rulers are ±0.01mm, ±0.005mm, ±0.003mm, and ±0.02mm. When designing CNC machine tools, we select the accuracy class according to the design accuracy requirements. It is worth noting that when selecting high-precision grating rulers, the thermal performance of the grating ruler must be considered. This is a key factor in the working accuracy of the machine tool. That is, the coefficient of thermal expansion of the grating ruler's engraving carrier must be consistent with the coefficient of thermal expansion of the machine tool's grating ruler mounting base to overcome thermal deformation caused by temperature.
In addition, the maximum moving speed of the grating ruler can reach 120m/min, which can fully meet the design requirements of CNC machine tools. The maximum length of a single grating ruler is 3040mm. If the control of the linear coordinate axis is greater than 3040mm, the required length can be achieved by grating ruler docking.
(2) Selection of Measurement Method There are two types of measurement methods for grating rulers: incremental grating rulers and absolute grating rulers. Incremental grating rulers obtain position information by reading the relative movement distance from the initial point using a grating scanning head. To obtain the absolute position, this initial point must be engraved on the scale of the grating ruler as a reference mark. Therefore, the machine tool must return to the reference point when powered on to perform position control. Absolute grating rulers, on the other hand, use flashing grating lines of different widths and spacings to directly encode the absolute position data onto the grating. When the grating ruler is powered on, the subsequent electronic equipment can obtain the position information without moving the coordinate axes to find the reference point; the absolute position value is directly obtained from the grating lines.
Absolute linear encoders cost approximately 20% more than incremental linear encoders. Machine tool designers, considering the cost-effectiveness of CNC machine tools, generally choose incremental linear encoders, which ensure machine tool motion accuracy while reducing costs. However, the advantage of absolute linear encoders—that they do not require returning to a reference point after startup—is unmatched by incremental linear encoders. After a machine tool has stopped or experienced a power outage, it can directly resume the machining program from the point of interruption, not only shortening non-machining time and improving production efficiency but also reducing the scrap rate. Therefore, absolute linear encoders are the most ideal choice for production lines with strict cycle time requirements or automated production lines consisting of multiple CNC machine tools.
(3) Selection of Output Signal: The output signal of the grating ruler is divided into four types: current sine wave signal, voltage sine wave signal, TTL rectangular wave signal, and TTL differential rectangular wave signal. Although the different waveforms of the grating ruler output signal do not affect the positioning accuracy and repeatability of the linear coordinate axis of the CNC machine tool, they must be matched with the CNC machine tool system. If the waveform of the output signal does not match the CNC machine tool system, the machine tool system will be unable to process the output signal of the grating ruler, and feedback information and error compensation will be impossible for the full closed-loop control of the linear coordinate axis of the machine tool. In practice, there are indeed cases where the waveform of the output signal does not match the CNC machine tool system. However, there is a way to deal with this situation. It is easy to solve by adding a digital electronic device (such as the subdivision and digital electronic device of HEIDENHAINDE's IBV600 series) between the output signal and the machine tool system.
Taking the Z-axis linear coordinate axis of a horizontal machining center as an example, this paper introduces the adjustment and installation process of the grating ruler.
(1) Machining of the mounting reference surface of the grating ruler: Machining the mounting reference surface of the fixed and moving grating ruler to ensure that the parallelism with the guide rail is within 0.02mm. Machining the fixed ruler connecting screw hole according to the coordinate dimensions.
(2) Clean each installation reference surface, fix the special installation tool 1 on the fixed length installation surface, reliably connect the movable length bracket 2 with the special installation tool, grind the adjustment pad 3 according to the actual measured size, and match the movable length bracket and slide block with screws and tapered pins.
Laser interferometer measurement).
Parameter: P1852. Backlash compensation value for each axis during high-speed operation.
Setting value: Set according to the backlash value detected at high speed (e.g., 10000 mm/min) (measured with a laser interferometer).
Parameter: P1800#4RBK.
Setting value: When this parameter is set to 1, cutting and rapid backlash can be activated separately.
2. Compensation for pitch error
CNC systems typically have up to 128 pitch error compensation points per axis. When necessary, compensation can be applied to a specific axis, usually at 50mm or 100mm intervals. For better accuracy, 5mm or 10mm intervals are recommended.
3. Setting the compensation counter
In full closed-loop control, a compensation counter is usually set. Taking the FANUC Oi system as an example, the following is an explanation: Parameter: P2010#5HBBL backlash compensation value is added to the error counter.
Setting value: Setting it to 0 indicates semi-closed-loop mode (standard setting).
Parameter: The pitch error compensation value of P2010#4HBPE is added to the error counter.
Setting value: Setting it to 0 indicates full closed-loop mode (standard setting).
4. Increase gain setting
Under the premise of no vibration, try to increase the position loop gain P1825, velocity loop gain P2043, P2045 and load inertia ratio P2021 as much as possible.
V. Conclusion
In summary, the above factors have a unified aspect: the rational selection and correct use of the grating ruler can maximize its performance. However, they also have a contradictory aspect: the installation position should be as close as possible to the drive axis, yet as far away as possible from the machine tool's heat sources (such as the lead screw). This requires the machine tool designer to balance and compromise various factors, comprehensively considering the selection, design, installation, and testing of the grating ruler to achieve a more reasonable cost-performance ratio, which will inevitably lead to better control and detection results.
Through the above steps of debugging, a CNC machine tool can generally achieve good positional accuracy (positioning accuracy, repeatability) and meet the machine tool design requirements, easily satisfying user needs. This has extraordinary practical significance for CNC machine tool manufacturers and users.
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