Development Trend of Machine Tool Measurement System

With the popularization of CNC machine tools in the field of machine tool manufacturing, modern machine tools have also greatly improved in terms of processing speed, processing accuracy and reliability, which is largely due to the grating measuring elements used in machine tools.

Single-field scanning technology and absolute measurement technology are the two main directions in the development of current angle and length measurement technology, and single-field scanning technology and absolute measurement technology can also be used in combination. Single-field scanning absolute measurement equipment that uses both technologies is far superior to traditional measurement equipment in terms of signal quality, anti-pollution ability, measurement speed and reliability. The rapid promotion of such products in the market also confirms this.

Single-field scanning technology

In the four-field imaging scanning method used by traditional angle and length measurement equipment, the grating scale moves relative to the scanning mask with a similar or identical grating structure. The brightness of the light passing through the scale and mask changes regularly according to the relative position of the scale and mask: when the gap between the scale and the mask coincides, the light can pass through; when the grating line coincides with the gap, no light passes through. The photosensitive element converts the change in light intensity into an electronic signal. There are four scanning areas on the scanning mask, and the gratings of each scanning area are staggered by 1/4 of the grating pitch. The photosensitive elements corresponding to these four scanning areas generate four sinusoidal signals with a phase difference of 90°. These four scanning signals do not use the zero line as the center line, so it is necessary to subtract the four signals in pairs to obtain two output signals l1 and l2 with a 90° phase difference and a center line of zero line.

In the new single-field scanning technology, the scanning mask has a large-size grating, and its grating pitch is slightly different from the grating pitch of the grating scale (Figure 1). As a result, alternating light and dark phenomena will occur on the length of the scanning mask grating: in some places, the grating lines overlap with the grating lines, and the light can pass through; in some places, the grating lines overlap with the gaps, and the light cannot pass through; in between, the gaps are partially blocked, which plays the role of optical filtering, making it possible to generate uniform high-sinusoidal signals. Special grating photosensitive elements replace independent photosensitive elements to generate four scanning signals with a phase difference of 90°.

Figure 1 Principle of four-field imaging photoelectric scanning method

The single-field scanning optical scanning system plays a decisive role in improving the performance of angle and length measuring equipment. Its large scanning area and special optical filtering can produce a stable quality scanning signal throughout the entire stroke of the measuring device, which is the prerequisite for the following points: small position error within the signal period; high grating operation speed; high control quality when using direct drive. [page]

This advantage of single-field scanning can be clearly seen in the oscilloscope XY display mode (Figure 2): The output signal of the single-field scanning grating ruler has better roundness and less signal noise, which means higher positioning accuracy and better control quality. For linear motors, speed control can be smoother after being equipped with a single-field scanning grating ruler.

Figure 2 Principle of single-field imaging photoelectric scanning method

The large scanning surface covering the full width of the grating scale and the alternating and repetitive strip scanning areas make the measuring equipment using the single-field scanning principle particularly insensitive to pollution interference. This can be confirmed by anti-pollution tests: even in the presence of large-area pollution interference, the measuring equipment can still provide high-quality measurement signals (Figure 3), and the position error is much lower than the error value corresponding to the calibration accuracy level of the measuring equipment.

Figure 3 Comparison of anti-fouling capability between four-field scanning and single-field scanning

Compared with four-field scanning, single-field scanning can avoid the situation where the device has no signal output in certain pollution interference situations.

Absolute measurement technology

The so-called "absolute measurement (Figure 4)" is relative to incremental measurement. Incremental measurement equipment obtains the distance data of relative movement by counting the grating lines scanned by the grating probe. In order to obtain the absolute position, the incremental measurement device must perform a reference point action after turning on the machine, while the absolute measurement device uses grating lines of different widths and spacings to directly encode the absolute position data into the measurement device, and the measurement device can provide absolute position information after turning on the machine. Usually, absolute measurement equipment is equipped with incremental tracks in addition to absolute tracks to further improve the accuracy and resolution of the measurement equipment. [page]

Figure 4 Comparison of anti-fouling performance of single-field/four-field scanning technology (green: single-field; red: four-field)

In recent years, absolute optical encoders have been increasingly widely used due to their irreplaceable advantages. These advantages are mainly reflected in the following aspects:

1. Reducing the non-productive time of machine tools

. Machine tools are often forced to shut down and restart due to failures or other reasons during use. For machine tools equipped with only incremental optical encoders, each axis must pass through the reference point after startup. Unlike passing through the reference point after normal startup, the machine tool's tool is usually still in the processing position at this time, in direct contact with the workpiece, and sometimes even inside the workpiece (such as drilling, tapping, etc.). In order to safely pass through the reference point, the tool must first be manually moved out of the processing position. This is more difficult for multi-axis machine tools with tilted tools, and often takes a lot of time and manpower, because when the tool pointing is not parallel to the X, Y, and Z axes, it increases the difficulty of manually moving the tool out (after a failure).

For an automatic production line consisting of multiple processing equipment, when one of the equipment fails and needs to be restarted, it may be necessary to pass through the reference point for all equipment in the production line, and special treatment of the workpiece that has not been fully processed may be required. A machine tool or production line equipped with an absolute grating can immediately regain the absolute position of each axis and the spatial orientation of the tool after restarting, so it can immediately continue the original processing program from the interruption point, greatly improving the effective processing time of the machine tool.

Figure 5 Absolute measurement schematic

2. Improve the reliability of machine tools

The reliability of machine tools directly determines the efficiency and economy of parts production. Every minute of machine tool failure means increased costs for users. Therefore, a trend in machine tool design is to monitor the status of important components in real time. Heidenhain's absolute grating supports this kind of real-time monitoring function through its bidirectional communication EnDat data interface. In addition to the absolute position value, the EnDat data interface also transmits the working status of the grating to the CNC system, such as whether the adjustment margin of the light source of the reading head is close to the limit, whether the maximum allowable moving speed of the reading head exceeds the standard, and whether the operating temperature exceeds the allowable range. When the above situation occurs, EnDat can send a warning signal to the CNC system. At this time, the grating can still work normally, but the user can take measures in advance based on the warning signal. When

the grating has a serious fault, EnDat will also send an alarm signal to the CNC system, and the CNC system can decide whether to stop processing based on the signal to protect the machine tool and workpiece. When troubleshooting, maintenance personnel can find the root cause of the fault in the shortest time according to the information provided by EnDat, such as too low grating power supply, abnormal reading head operation, and too low output signal amplitude.

By sending two levels of fault signals, warning and alarm, the absolute grating can greatly improve the efficiency of user maintenance work, reduce the probability of downtime caused by grating failure, and thus improve the reliability of machine tools.

3. Improve the safety of machine tools

In order to protect the mechanical moving parts of the machine tool, a travel switch can usually be used to ensure that the movement of each part of the machine tool does not exceed the allowable range. The travel switch is in a harsh environment (coolant, metal chips, etc.) for a long time and is easily damaged. In addition, the travel switch is rarely used during normal operation of the machine tool, so it is impossible to determine whether its function is still normal at any time. In order to ensure work safety, users have to check the status of the travel switch regularly. Machine tools using absolute gratings can determine the position of the moving parts of the machine tool at any time. By making corresponding settings in the CNC system, the travel switch can be omitted, which not only saves the equipment maintenance time of the machine tool user, but also avoids the safety problems that may be caused by damage to the travel switch, and improves the safety of the machine tool when it is used.