Application of Encoder in CNC Machine Tool

Incremental encoders output pulses when they are rotated, and know their position by means of a counter. When the encoder is powered down, the value stored in the buffer or external counter is lost. This means that if the machine is forced to shut down due to get off work or maintenance, after the restart, the encoder will not know its exact location. The solution is to increase the reference point, the encoder every reference point, the buffer or counter is cleared, the CNC system to know the exact location. Before returning to the reference point, it is not possible to guarantee the accuracy of the position. To this end, in the NC machine control there is a boot every time the first reference point of the operation. The tool’s tool is usually in the machining position, usually in contact with the work piece, and sometimes even in the work piece (such as drilling, tapping, etc.), in order to safely carry out the reference point action, you must first manually Remove the machining position. If the tool is pointing at an angle to the X, Y, and Z axes (multi-axis machine), this operation becomes particularly difficult and often takes a lot of time and effort. As shown in Figure 1, for the FANUC encoder for incremental use, the use of iron back to zero. 

Figure 1 Schematic diagram of reference point return

The absolute encoder appears, it is a good solution to this problem. When the absolute encoder rotates, there is a code (binary, BCD, etc.) corresponding to the position, and the change in the code can be used to determine the position of the forward and reverse directions and the displacement, without the need for a direction circuit. It has an absolute zero code, when the power failure or shutdown and then boot, you can still accurately read out the power outage or shutdown location code, and accurately find the zero code, can significantly reduce the standby time and boot time, effectively shortened Home return time, but also omit the limit switch, cost savings. China’s Taiwan Province and foreign CNC machine tools, mostly using absolute encoder. Some examples of the use of encoders in CNC machine tools are as follows: 

Figure 2 the application of encoder in the positioning

The parameters of the incremental photoelectric encoder are known and the gear ratio of the pulley is known. If you want to process a workpiece immediately after processing another workpiece, you can calculate the number of pulses given by the encoder, the motor stops, and thus the workpiece. as shown in Figure 2

Figure 3 The application of encoder in the knife library selection

The angle encoder is connected to the rotary knife library, and the output of the encoder is the current knife tool number, as shown in Figure 3.

Figure 4 encoder in the application of servo motors

Using the encoder to measure the speed of the servo motor, and through the servo control system to control its various operating parameters, as shown in Figure 4.

Conclusion

At present, the market competition intensified, CNC machine tools, high precision and high efficiency is the direction of development. The use of absolute encoder can still read the absolute position of the machine while power is off, greatly reducing the boot time and standby time, improve efficiency, but also save the limit switch, cost savings. So the rotary encoder, especially the absolute encoder, in the CNC machine tool as a detection element is used in line with its functional requirements.