How the pulse encoder works

The pulse encoder is an optical position detection element. The encoder disk is directly mounted on the rotating shaft of the motor to measure the rotation angle position and speed change of the shaft. The output signal is an electric pulse. It is a commonly used angular displacement sensor. It can also be used as a speed detection device.

The advantages of pulse encoders are no friction and wear, small driving torque and fast response speed. The disadvantage of the pulse encoder is its poor anti-pollution ability and easy damage.

Classification and structure of pulse encoder

Pulse encoders are divided into three types: photoelectric, contact and electromagnetic induction. The accuracy and reliability of the photoelectric type are better than the other two, so only the photoelectric pulse encoder is used on the CNC machine tool.

The structure of the pulse encoder: equidistant lines are engraved on the circumference of a disc, which is divided into transparent and opaque parts, called circular gratings. The circular grating rotates with the working axis. Opposite to the circular grating, a fixed fan-shaped sheet is placed in parallel, called an indicator grating, on which two slits (differentiating slits) with a 1/4 pitch difference are made. In addition, there is a zero slit (one pulse is transmitted every time). The pulse generator is connected to the servo motor through a cross connector or key.

The working principle of pulse encoder

When the circular grating rotates with the working shaft, the light passes through the line pattern of the two gratings to form light and dark stripes. The photoelectric element receives these light and dark light signals and converts them into alternating electrical signals. The electrical signal is two sets of current signals A and B that are similar to sine waves, as shown in Figure 4-17. The A and B signals have a phase difference of 90°, and are amplified and shaped into a square wave. The signal passing through the two gratings also has a "pulse per revolution" called the Z-phase pulse, which is also obtained through the above processing. Z pulse is used to generate the reference point of the machine tool. The subsequent pulses are sent to the counter, and the rotation angle and speed of the working shaft can be measured according to the number and frequency of the pulses. Its resolution depends on the number of turns of the circular grating and the subdivision multiple of the measuring circuit.

Application of pulse encoder

The pulse encoder is used as a position detection device on the CNC machine tool, and the detection signal is fed back to the CNC system. There are two ways to feed back to the numerical control system: one is to adapt to a reversible counter with up and down counting requirements to form up and down counting pulses; the other is to adapt to a counter with counting control and counting requirements to form direction control signals and count pulses .