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Classification Of Position Detection Devices Of CNC Machine Tools

Position detection devices are crucial components of CNC (Computer Numerical Control) machine tools as they provide accurate and reliable feedback on the machine’s position in the coordinate system.

These devices help ensure precise movement of the tool and workpiece during machining operations.

Position detection devices used in CNC machine tools can be broadly classified into several categories based on their principles of operation. The main types of position detection devices are as follows:

Linear Encoders

Linear encoders are used to measure linear movement along the X, Y, and Z axes of the CNC machine. They typically consist of a scale and a read head. The scale contains a series of equally spaced marks, while the read head contains sensors that detect the marks’ positions. As the machine moves, the read head sends signals to the CNC control system, which calculates the machine’s position accurately.

Rotary Encoders

Rotary encoders measure the angular displacement of rotating elements in the CNC machine, such as the spindle or the motor shaft. They convert rotary motion into electrical signals, providing feedback to the control system about the machine’s position and rotation speed.

Resolver Sensors

Resolvers are electromagnetic devices that measure angular displacement. They are commonly used in CNC machine tools to detect the position and speed of rotating elements. Resolvers are robust and can withstand harsh operating conditions.

Glass Scales

Glass scales are precision linear scales that use an optical principle for position detection. They consist of a glass scale with a pattern of fine lines and a read head that detects the lines’ positions as the machine moves. Glass scales provide high accuracy and are widely used in high-precision CNC machine tools.

Inductive Linear Sensors

Inductive linear sensors use the principle of electromagnetic induction to measure linear displacement. They are suitable for applications where dust, dirt, or oil may be present as they are relatively immune to environmental contaminants.

Magnetic Scales

Magnetic scales use magnetic fields to measure linear displacement. They are often used in CNC machine tools operating in harsh environments where other types of encoders may be less reliable.

Pulse Encoders

Pulse encoders use optical or magnetic sensors to detect rotary or linear movement. They provide position feedback in the form of electrical pulses, and the CNC control system converts these pulses into machine position and motion data.

The choice of position detection device depends on factors such as the required accuracy, environmental conditions, operating speed, and the type of CNC machine tool. Each type of position detection device has its advantages and limitations, and selecting the appropriate one is crucial to ensuring accurate and efficient CNC machining operations.

For different types of CNC machine tools, the following different detection methods can be used due to different working conditions and detection requirements.

Incremental And Absolute Detection Methods

The incremental detection method only measures the displacement increment, and uses the satisfactory number of numbers to represent the number of unit displacement (the minimum set unit), and sends a measurement signal every time a measurement unit is moved. The advantage is that the detection device is relatively simple, and any centering point can be used as the measurement starting point. But in this system, the shift distance is read out after accumulating the measurement signal. If the accumulation is wrong, the subsequent measurement results will be all wrong. In addition, when a fault occurs (such as power failure), the correct position before the accident cannot be found.

After the accident is eliminated, 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 gratings, laser interferometers, etc. are all incremental detection devices.   The absolute detection method measures the absolute coordinate position value of the component under test in an absolute coordinate system, and it is expressed as a binary or decimal digital signal. Generally, it must be converted into a pulse digital signal before it can be sent for comparison and comparison. show.

In this way, the higher the resolution requirement is, the more complicated the structure is. Absolute pulse encoder discs, three-speed absolute encoder discs (also known as multi-cycle absolute encoder discs), etc. all have absolute detection devices.  

Digital And Analog Detection Methods

The numerical control detection method is to quantify the measured unit and then represent 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 inspection has the following three characteristics.  

  • The measured number is converted into the number of pulses, which is convenient for display and self-care;
  • The measurement accuracy depends on the measurement unit, which is basically irrelevant to the range, but there is a cumulative error error;
  • The detection device is relatively simple, and the anti-interference ability of the pulse signal is strong.

The analog detection method is to represent the measured variable, such as the amplitude change of the voltage, the phase change and so on. When a large number of accurate analog inspections are performed, there are higher requirements for technology. Analog inspections in CNC machine tools are mainly used for small-scale measurement. Analog detection devices include hole generators, resolvers, induction synchronizers and magnetic rulers. The main features of analog detection are as follows.  

  • Directly detect the measurand without quantification;
  • High-precision measurement can be achieved in a small range;
  • Direct detection and indirect detection are possible.

Direct And Indirect Measurements

Direct Measurement

Direct measurement is to install the detection device directly on the execution part, such as grating, induction synchronizer, etc., to directly measure the linear displacement of the worktable, and the position detection device is installed on the execution part (ie the end piece) to directly measure the end of the execution part. The linear displacement or angular displacement of the parts can form a closed-loop feed servo system.

The measurement methods include linear gratings, linear induction synchronizers, magnetic gratings, laser interferometers, etc. to measure the linear displacement of the executive components; The detection device measures the linear displacement of the machine tool. Therefore, the advantage is that it directly reflects the linear displacement of the worktable. The disadvantage is that the detection device and the stroke are required to be the same length.  

Indirect Measurement

The indirect measurement device is to install the detection device on the ball screw or drive motor shaft, and indirectly measure the linear displacement of the executive member by detecting the angular displacement of the rotating part. The position detection device is installed on the transmission element or the drive motor shaft in front of the actuator to measure its angular displacement. After the transmission ratio is transformed, the linear displacement of the actuator can be obtained, which can constitute a blue closed-loop servo feed system.

If the pulse encoder is installed on the motor shaft. Indirect measurement is reliable and convenient to use, and has no length limit; its disadvantage is that the transmission chain error of linear transformation into rotary motion is added to the detection signal, thereby affecting the measurement accuracy.

Generally, it is necessary to compensate the transmission error of the CNC machine tool in order to improve the positioning accuracy. In addition to the above position detection devices, the servo system often also includes components for detecting speed to detect and adjust the rotational speed of the engine. A commonly used hole element is a tachometer.

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