The principle and use of incremental encoder

An encoder is a device that converts an angular displacement or a linear displacement into an electrical signal. According to the working principle, the encoder can be divided into two types: incremental (SPC) and absolute (APC). The incremental encoder converts the displacement into a periodic electrical signal, which is then converted into a counting pulse, and the number of pulses is used to represent the magnitude of the displacement. Here is some introduction to incremental encoders.

1. working principle

The working principle of the photoelectric incremental encoder is as follows: the pulse code disc rotating together with the rotating shaft has a uniformly engraved grating, and a plurality of transparent sections and shading sections are evenly distributed on the code disc. Incremental encoders do not have a fixed starting zero. The output is a pulse proportional to the increment of the corner. A counter is required to count the number of pulses. Each time a light transmissive area is rotated, a pulse signal is sent, the current value of the counter is incremented by 1, and the count result corresponds to the increment of the corner. Incremental encoders are simple to manufacture, inexpensive, and sometimes used to measure absolute corners.

2. Incremental encoder classification

(1) A single-channel incremental encoder has only one pair of optocouplers inside, and only one pulse sequence can be generated.
(2) The AB phase encoder has two pairs of photocouplers inside, and outputs two sets of pulse sequences with a phase difference of 90°. The lead and lag relationship of the two pulses in the forward and reverse directions is just the opposite. As can be seen from the figure below, at the rising edge of the B-phase pulse, the level of the A-phase pulse is reversed when the forward and reverse are reversed. Therefore, using the AB-phase encoder, the PLC can easily recognize the direction of rotation of the shaft. When it is necessary to increase the accuracy of the measurement, the 4th frequency mode can be used, that is, the rising edge and the falling edge of the A and B phase waveforms respectively, and the resolution can be increased by 4 times, but the high frequency of the zui of the measured signal is correspondingly reduced.
(3) In addition to the two pairs of optocouplers with dual-channel incremental encoders, the three-channel incremental encoder has one light-transmitting section in the other channel of the pulse code disc, one revolution per revolution, and output 1 One pulse, called the Z-phase zero pulse, is used as the system clear signal, or the origin of the coordinates to reduce the accumulated error of the measurement.
The advantages of the incremental encoder are simple structure, long average life, strong anti-interference ability and high reliability, which is suitable for high-precision positioning control for continuous operation. The disadvantage is that the absolute position information of the shaft rotation cannot be output.

3. Encoder selection

First, the type of encoder is selected according to the measurement requirements. The number of pulses per revolution of the incremental encoder is equal to the number of lines of its grating. The number of encoder lines should be determined according to the speed measurement or positioning requirements and the encoder speed. The encoder is mounted on the motor shaft or mounted on a shaft after deceleration. The encoder speed is very different. It should also be considered whether the high frequency of the zui of the pulse it emits is within the range allowed by the PLC's high-speed counter.

3. Coordination problem between encoder and PLC high-speed counter

Taking the S7-200 as an example, when using a single-channel incremental encoder, the single-phase up/down counter mode (mode 0~5) of the high-speed counter can be selected, which can be subdivided into yes/no external direction input signals, with / No reset input and with/without start input signal. When using the AB phase encoder, the high-speed counter should select the A/B phase quadrature counter mode (modes 9 to 11), which can be used to count up during forward rotation and count down during reverse rotation.

4. How to judge whether the AB phase encoder is forward or reverse?

The high-speed counter of the S7-200 uses the current count direction status bit in the SM area to indicate the direction of rotation of the encoder. If the period of the encoder output pulse is greater than twice the scan cycle time of the PLC, the direction of the encoder rotation can be judged by judging the 0, 1 state of the A phase pulse signal on the rising edge of the B phase pulse.

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