[Editor's note] For different types of CNC machine tools, the following different detection methods can be used due to different working conditions and detection requirements.
1. Incremental and absolute detection methods
The incremental detection method only measures displacement increments, and uses the number of digits to indicate the number of unit displacements (minimum setting units). Each measurement unit sends a measurement signal. The advantage is that the detection device is relatively simple, and any one of the midpoints can be used as a measurement starting point. However, in this system, the shift distance is read by accumulating the measurement signal, and a cumulative error occurs. After that, the measurement result will be completely wrong. In addition, in the event of a failure (eg, power failure), the correct position before the accident cannot be found. After the accident is removed, the workbench must be moved to the starting point and counted again to find the correct position before the accident. Pulse encoders, resolvers, inductive synchronizers, gratings, magnetic grids, laser interferometers, etc. are all incremental detection devices.
The absolute detection method measures the absolute coordinate position value of the tested component in an absolute coordinate system, and it is represented by a binary or decimal digital signal. Generally, it must be converted into a pulsed digital signal before it can be sent for comparison and display. In this way, the higher the resolution requirement, the more complex the structure. Absolute pulse encoder discs, three-speed absolute encoder discs (also called multi-barrel absolute encoder discs) have absolute detection devices.
2. Digital and analog detection methods
The numerical detection method is to quantify the measured unit and then express it in digital form. The measurement signal is generally an electric pulse, which can be directly sent to the numerical control system for comparison and processing. Such detection devices include pulse encoders and gratings. Digital detection has the following 3 features.
(1) Measured and converted into pulses for easy display and self-care;
(2) The measurement accuracy depends on the unit of measurement, which is basically irrelevant to the range, but there is a cumulative error error;
(3) The detection device is relatively simple, and the pulse signal has strong anti-interference ability.
The analog detection method is to represent the measured country variables, such as voltage amplitude changes, phase changes, etc. When a large number of in-house precision analog tests are performed, there is a high requirement for the technology. The analog detection in CNC machine tools is mainly used for small-range measurement. The analog detection device includes a hole generator, a resolver, an inductive synchronizer, and a magnetic ruler. The main features of analog detection are the following.
(1) Directly measure the measured, without quantification;
(2) High-precision measurement can be achieved within a small range;
(3) Direct detection and indirect detection can be performed.
3. Direct measurement and indirect measurement
1) Direct measurement
Direct measurement is to directly install the detection device on the executing component, such as grating, inductive synchronizer, etc., to directly measure the linear displacement of the worktable. The position detection device is installed on the executing component (ie, not end piece) to directly measure the end of the executing component. The linear displacement or angular displacement of the part can constitute a closed-loop feed servo system. The measurement methods include straight-line grating, linear induction synchronizer, magnetic grid, and laser interferometer. The linear displacement of the measuring part is performed; this type of detection is linear. The detection device measures the linear displacement of the machine tool. Therefore, its advantage is that it directly reflects the linear displacement of the table. The disadvantage is that it requires the length of the inspection device and the length of the stroke. This is a great limitation for large-scale CNC machine tools.
2) Indirect measurement
The indirect measuring device is to install the detecting device on a ball screw or a driving motor shaft, and indirectly measures the linear displacement of the member of the board by detecting the angular displacement of the rotating member.
The position detecting device is installed on the transmission element or the driving motor shaft in front of the executing component to measure its angular displacement. After the transformation of the transmission ratio, the linear displacement of the implementing component can be obtained. This can constitute the blue closed loop servo feeding system. For example, the pulse encoder is mounted on the motor shaft. The indirect measurement is reliable and convenient, and there is no length limitation. The disadvantage is that the error of the transmission chain, which is changed from straight line to rotary motion, is added to the detection signal, which affects the measurement accuracy. Generally, it is necessary to compensate the transmission error of CNC machine tools in order to improve the positioning accuracy.