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 Measurements
The incremental detection method only measures displacement increments, and uses the number of digital pulses to represent the number of unit displacements (ie, the minimum setting unit). A measuring signal is sent for every measurement unit moved. 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. Once accumulated, the measurement result will be completely wrong. In addition, in the event of a fault (such as a power failure), the correct position before the accident can no longer be found. After the accident is removed, the workbench must be re-counted at the starting point to find the correct position before the accident. Pulse encoders, resolvers, inductive synchronizers, gratings, magnetic grids, laser interferometers, etc., are incremental detection devices.
The absolute measurement method measures the absolute coordinate position value of the measured 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 show. In this way, the higher the resolution requirement, the more complex the structure. Such measurement devices include an absolute pulse code disk, a three-speed absolute code disk (or a multi-turn absolute code disk), and the like.
2, digital and analog measurement
Digital detection is to quantify the measured unit and then it is expressed in digital form. The measurement signal is generally an electrical pulse, which can be sent directly to the numerical control system for comparison and processing. Such detection devices include pulse encoders and gratings. Digital detection has the following characteristics:
(1) It is converted into pulses for measurement and processing;
(2) The measurement accuracy depends on the unit of measurement and has nothing to do with the measurement range; however, there is a cumulative error code error.
(3) The detection device is relatively simple, and the pulse signal has strong anti-interference ability.
Analog detection is to be measured with continuous variables, such as voltage amplitude changes, phase changes, etc. When performing accurate analog detection in a large number of processes, there is a high demand for technology, and the analog detection in CNC machine tools is mainly used for small-range measurement. Analog detection devices include tachogenerators, resolvers, inductive synchronizers and magnetic rulers. The main features of analog detection are:
(1) Directly measure the measured, without quantification.
(2) High-precision measurement can be realized in a small range.
3, direct detection and indirect detection.
The position detecting device is installed on the executing part (ie, the end piece) to directly measure the linear displacement or the angular displacement of the end piece of the executing component. It can be called a direct measurement and can constitute a closed-loop feeding servo system. The measuring mode includes a linear grating and a linear sensing synchronization. Instruments, magnetic grids, laser interferometers, etc., measure the linear displacement of the actuator; this type of detection uses a straight-line detector to measure the linear displacement of the machine tool. The advantage is that it directly reflects the linear displacement of the table. The disadvantage is that the inspection device and the stroke are required to be of equal length, which is a great limitation for large-scale machine tools.
The position detection device is installed on the transmission element or the drive motor shaft in front of the execution part to measure its angular displacement. After the transformation of the transmission ratio, the linear displacement of the execution part can be obtained. This is called indirect measurement and can constitute a semi-closed loop servo feed 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 a straight line to a rotary motion, is added to the detection signal, thereby affecting the measurement accuracy. Generally, it is necessary to compensate the transmission error of the machine tool in order to improve the positioning accuracy.
In addition to the above position detection devices, the servo system often includes elements for detecting the speed to detect and adjust the engine speed. Commonly used speed measuring elements are speed measuring engines.
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