Analysis of Full-closed-loop Control of Linear Coordinate Axis of NC Machine Tool

With the rapid development of modern manufacturing, more and more CNC machine tools are widely used. At the same time, the positioning accuracy and repeatability of CNC machine tools are also increasing. The semi-closed loop control system with precision ball screws and encoders is no longer available. Meet the needs of users. The semi-closed-loop control system can not control the transmission error caused by the transmission mechanism of the machine tool, the thermal deformation error caused by the transmission mechanism during high-speed operation, and the error caused by the wear of the drive system during the machining process. These errors have seriously affected the machining accuracy of CNC machine tools. And its stability. The linear scaler performs full-closed-loop control on the linear axes of the CNC craving bed, eliminates the above-mentioned errors, and improves the positioning accuracy, repeatability and accuracy of the machine tool. As a key component to improve the position accuracy of CNC machine tools, it is increasingly favored by users.

Below the linear scale selection, installation of special tool design, installation and CNC system parameter measurement and Other contents are discussed, there may be flaws, please readers have educated us.

A, linear encoder selection

(1) Selection of Accuracy Level The linear scale of the CNC machine tool is used to increase the accuracy and re-positioning accuracy of the linear coordinate axis. Therefore, the accuracy level of the grating scale is the first consideration. The scale accuracy level is ± 0.01mm, ± 0.005mm, ± 0.003mm, ± 0.02mm. While we design the CNC machine tool, we choose the accuracy grade according to the design accuracy requirements. It is worth noting that the thermal performance of the grating ruler should be taken into account when selecting a high-precision grating scale. It is the key link of the precision of the machine tool, that is, the grating scale is required. The coefficient of thermal expansion of the reticle carrier is consistent with the coefficient of thermal expansion of the mounting base of the tool scale to overcome thermal deformation due to temperature.

In addition, the maximum moving speed of the scale can reach 120m/min, which can fully meet the requirements of CNC machine tools. The maximum length of a single scale is 3040mm. If the linear coordinate axis is larger than 3040mm, the scale can be used to reach the required length.

(2) Selection of measuring method The measuring method of the scale is divided into two kinds: incremental grating scale and absolute grating scale. The so-called incremental grating scale is the position of the raster scanning head obtained by reading the relative movement distance to the initial point. In order to obtain the absolute position, this initial point will be engraved on the scale of the scale as a reference mark, so the machine must be returned to the reference point in order to carry out position control. Absolute grating ruler uses absolute flash position data with different widths and different spacings to directly write absolute position data to the grating in the form of code. After the grating ruler is energized, the position information can be obtained by the subsequent electronic device. It is not necessary to move the coordinate axis to find The reference position, the absolute position value is obtained directly from the grating line.

Absolute grating scale is 20% more expensive than incremental grating scale. Machine tool designer considers the cost-effectiveness of CNC machine tools and generally chooses incremental grating scale, which can not only ensure the precision of machine tool movement but also reduce the cost of machine tool. However, the advantages of the absolute scale without the reference point after being turned on are that the incremental scale cannot be compared. The machine can be directly executed from the point of interruption when the machine stops or the power is turned off, which not only shortens the non-processing time, but also improves the production efficiency. , And reduce the scrap rate of parts. Therefore, it is most desirable to use absolute scales in the production of beats or automatic production lines composed of multiple CNC machine tools.

(3) Selection of output signal The scale output signal is divided into four types: current sine wave signal, voltage sine wave signal, TTL square wave signal and TTL differential rectangular wave signal. Although the waveform of the output signal of the grating scale is different, the linear coordinate of the CNC machine tool is different. The positioning accuracy and repeatability of the axis have no effect, but it must be matched with the CNC machine system. If the waveform of the output signal does not match the CNC machine system, the machine system cannot process the output signal of the scale, feedback information, and compensation error to the machine tool. There is no way to talk about full-closed-loop control of linear axes. In practice, there is a case where the waveform of the output signal does not match with the CNC machine system. However, there are also ways to deal with this situation by adding a digital electronic device between the output signal and the machine tool system (such as HEIDENHAINDE's IBV600 series subdivision and Digital electronic devices) are easy to solve.

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SCR3923 grating ruler

Second, the linear scale structure design

The structure design and installation of the grating ruler is more important than the selection of the grating ruler. No matter which part is handled improperly, it will seriously affect the accuracy of the grating ruler, and the full closed-loop accuracy may not be as good as a semi-closed ring.

1. The influence of walking posture on the detection accuracy of grating ruler

(1) The coincidence degree between the drive axis and the midpoint of the load is well-known. When the object is moved, if it is not pushed to the midpoint position, the object may be easily rotated and unstable phenomena such as swing may occur. The dynamic axis is required to coincide with the midpoint of the load as much as possible. In practice, due to structural and machining errors, the drive axis has a certain distance from the midpoint of the load, which causes the phenomenon of oscillation when the screw is dragging the load.

(2) Consistency of the damping characteristics of the guide rails The consistency of the damping characteristics of the two guide rails is also a very important factor. The inconsistency of the damping characteristics of the two guide rails can easily cause the phenomenon of object oscillation, as shown in Figure 1.

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(3) The installation position of the grating ruler should be as close to the drive axis as possible. Most linear coordinate axes of the machine tool drive system generally use precision ball screws. Theoretically, the grating ruler should be installed as close to the axis of the screw as possible. The installation of the scale complies with the principle of Abbe's error minimization, which means that the mounting position of the scale is close to the working reference plane of the control axis. The closer to the formed Abbe error, the higher the accuracy of position control by the grating ruler. The better the positioning accuracy. However, in practice, due to the limitations of structure and space, there are only two installation methods for the scale, one is installed on the near side of the screw, and the other is installed on the outside of the guide. In order to obtain the minimum Abbe error, it is recommended to select the first installation method as much as possible. On the other hand, high-precision grating scales have been chosen, but they have not actually reached the precision required by CNC machines.

Although the installation position of the scale is relatively close to the drive axis, the installation position has a certain distance from the drive axis after all. This combination of the distance and the swing of the object during driving brings a great deal of trouble to the detection and control of the scale. When the driving object swings toward the installation side of the grating scale, the grating scale mistakenly considers that the moving speed is insufficient when the detection is performed, the system gives an acceleration signal, and the driving object immediately swings to the other side, and the grating scale mistakenly considers that the moving speed is too high during detection. Fast, the system will give a deceleration signal, so that repeated operation, actually did not improve the control of the CNC machine tool linear axes, but intensified the vibration of the drive object, resulting in a full closed-loop than semi-closed loop strange phenomenon. From this point of view, the position design of the drive axis of the drive object, the mounting position of the scale, and the damping characteristics of the two guide rails are of utmost importance and must be given great attention by the machine tool designer. Care must be taken when designing the machine tool. Get a good, satisfying effect of setting the juice.

2. The size of the grating scale, the mounting surface of the slider and the slider bracket have enough rigidity and strength

The rigidity and strength of the installation position of the grating ruler are also the key to ensure the normal operation of the scale. The grating scale works through the principle of photoelectric scanning. Therefore, the grating scale cannot be in a strong vibration state. The instability of the light source caused by the vibration affects the control accuracy of the grating scale. Therefore, the installation position is preferably integrated with the solid casting of the machine tool. Even if the connection is required due to structural reasons, it is required that the entire joint between the connection member and the body be well contacted and the rigid connection be sufficient to prevent the weak link between the joint and the joint. Causes strong vibrations to affect the normal operation of the scale and ultimately leads to a reduction in the positioning accuracy of the machining center.

3. The installation position of the grating rule should be far away from the heat source of the machine tool

The installation position of the grating ruler should be far away from the heat source of the machine tool to avoid the influence of temperature. The grating ruler is not afraid of heat. The influence of the overall environment temperature on the grating ruler is small, but the heat source of the machine tool (such as the ball screw) is locally generated. Uncertain temperature rise produces errors, and this error is difficult to control and it is difficult to correct and compensate in real time. If the scale is close to these places, it will inevitably affect the accuracy of the scale.

4. The protection of the installation position of the scale is very important

(1) In modern machine tools, users generally require a large flow of cooling, and in the case of large flow flushing, cutting fluid splashes on the scale. The work environment of the scale is also filled with moist, air with cooling spray, In this environment, the grating is prone to condensation, and a thin film is likely to form on the scan head. In this way, it will lead to poor light projection of the grating scale, coupled with the grating will easily leave traces of water, seriously affecting the measurement of the grating. If the processed chips accumulate in the vicinity of the grating and cause chip evacuation and poor drainage, the grating scale will be immersed in the cutting fluid and impurities, which will affect the use of the grating, which will seriously damage the grating ruler and make the whole machine瘫痪 state.

(2) If the scale is in a strong cooling spray or dust, it can be treated by compressed air, but the compressed air must be purified through the filter and meet the following impurity quality level requirements according to ISO 8573-1, as shown in the attached table.

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Third, the installation of linear scale

The structure of the grating scale is shown in Fig. 2. It consists of a constant-length body 1 and a reading head 2 of a moving scale. The scale of the scale is an aluminum housing that protects the scale, scanning unit and its guide rails from chips, dust and splash water. The moving scale reading head consists of a scanning unit, a precision connector and a mounting block. The precision connector connects the scanning unit with the mounting block to compensate for a small amount of mechanical error of the guide rail.

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1. The design of the installation

From Figure 2, it is easy to see that the measurement of 1.5mm±0.2mm and 2mm dimensions on the scale is very important and must be ensured during installation. On the other hand, the scale body and the moving scale reading head are not rigidly connected, and the scanning unit and the mounting block are elastically connected with the precision connector. From this point of view, correct installation of the scale is not an easy task. To this end, after many studies designed a special economical and practical installation tool, as shown in Figure 3.

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2. Adjust the installation

The installation and adjustment of the grating ruler is an important step in the use of the grating ruler. Adjusting the installation quality directly affects the quality of the grating ruler detection and control work, so it must be given enough attention. The Z-axis linear coordinate axis of the XH768 horizontal machining center is taken as an example to introduce the adjustment and installation process of the scale.

(1) The installation of the reference surface of the grating ruler The mounting reference surface of the ruler of the grating scale and the moving ruler is ensured to be within 0.02 mm in parallel with the guide rail, and a fixed-length combined screw hole is machined according to the coordinate size.

(2) Clean up the installation datum planes, fix the special installation tool 1 on the fixed installation surface, and connect the movable scale support 2 with the special installation tool. Match the grinding adjustment pad 3 according to the measured dimensions to match the moving scale support and the sliding seat. Combine screws and taper pins, as shown in Figure 4.

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(3) Remove the special fixture and install the scale 4 to the position.

IV. Adjustment of CNC System Parameters

1. Compensation for backlash

First of all, it is required that the backlash after the mechanical installation is completed must be within a certain range. The backlash has different values ​​when the direction is switched at different speeds, so backlash compensation compensates for feed and rapid movement separately. Traditionally, the former is only set to the former, which is unscientific. Take the FANUC Oi system as an example. The explanation is as follows:

Parameters: P1851: Backlash compensation value for each axis feed.

No setting: Set the backlash value (measured with a laser interferometer) at the cutting feed (usually 500 to 1000 mm/min).

Parameter: P1852. Backlash compensation value for each axis at rapid speed.

Setpoint: Set the backlash value detected at rapid (eg, 10000mm/min) (measured with a laser interferometer).

Parameter: P1800#4 RBK.

Set value: If this bit parameter is set to 1, cutting and rapid backlash can take effect separately.

2. Pitch error compensation

The numerical control system generally sets up to 128 pitch error compensation points per axis. If necessary, an axis can be compensated. The general practice is to compensate at an interval of 50mm or 100mm. To improve the accuracy, it is recommended to use 5mm or 10mm intervals for compensation. The effect is better.

3. Compensation counter setting

In full-closed-loop control, the compensation counter is usually set. Take the FANUCOi system as an example. The parameters are as follows: Parameter: P2010#5 HBBL The reverse gap compensation value is added to the error counter.

Set value: Set to 0 to indicate a semi-closed loop mode (standard setting).

Parameter: P2010#4 HBPE pitch error compensation value is added to the error counter.

Set value: Set to 0 to indicate full closed loop mode (standard setting).

4. Improve gain settings

On the premise of no vibration, try to increase the position loop gain P1825, speed loop gain P2043, P2045 and load inertia ratio P2021 and other parameters.

V. Conclusion

Summarizing and analyzing the above factors, they have a unified facet of each other, a reasonable choice of scales, the correct use of the material to make its best; there are also contradictory aspects, the installation position should be as close as possible to the drive axis, but also try to stay away from the heat source of the machine tool ( Such as screw vice), this depends on the machine tool designer, how to take into account the trade-off consideration of various factors, comprehensive consideration of the grating size selection, design, installation, test and other factors, get a more reasonable cost performance, is bound to get a better control Test effect.

Through the above several steps of debugging, a CNC machine can generally get a good position accuracy (positioning accuracy, repeatability positioning accuracy), to meet the design requirements of the machine tool, can easily meet the needs of users, manufacturers of CNC machine tools and The use of users has extraordinary practical significance.

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