What is a photoelectric encoder Principle of photoelectric encoder

What is a photoelectric encoder?

Working principle: When the shaft of the photoelectric encoder rotates, both lines A and B produce pulse output, and the phase angle of the phase A and B pulses are 90 degrees. From this, the rotation direction of the photoelectric encoder and the motor speed can be measured. If the A-phase pulse is ahead of the B-phase pulse, the photoelectric encoder is forward, otherwise it is reverse. The Z line is a zero pulse line, and the photoelectric encoder generates a pulse per revolution. It is mainly used for counting. The A line is used to measure the number of pulses, and the B line and A line can be used to measure the direction of rotation.

N is the motor speed

Δn=ND test-ND theory

For example: the speed of our car is 1.5m/s, the diameter of the wheels is 220mm, C=D*Pi, and the motor is controlled at 21.7 revolutions per second. According to the index of the servo system, the motor speed is set to 1500 revolutions per minute, so it can be calculated properly When ND=21.7*60=130 rpm, the number of pulses output by the optical code disc per second is:

PD=130&TImes;600/60=1300 pulses

When the measured number of pulses deviates from the calculated standard value, the incremental voltage △U output to the servo system can be calculated according to the corresponding relationship between the voltage and the number of pulses, and after D/A conversion, the increase is calculated. Measure the number of pulses and subtract it later.

When the running time is longer, the route is longer, and it deviates more from our pre-made route. At this time, the system starts the position loop, by continuously measuring the number of pulses output by the photoelectric encoder per second, and comparing it with the standard value PD (ideal value), calculating the increment △P and converting it into the corresponding D/A Output digital quantity, reduce the number of pulses output to a motor through the controller, subtract the increment from the original output voltage, force the motor speed to drop, and stop the adjustment when the measured △P is approximately zero. The motor speed is always controlled within the allowable range.

According to the detection principle, encoders can be divided into optical, magnetic, inductive and capacitive. According to its scale method and signal output form, it can be divided into three types: incremental, absolute and hybrid.

1.1 Incremental encoder

Incremental encoders directly use the principle of photoelectric conversion to output three sets of square wave pulses A, B, and Z; the phase difference of the two sets of pulses is 90 per revolution. The phase difference of the two sets of pulses is 90. Pulse, used for reference point positioning. Its advantages are simple structure, average mechanical life of more than tens of thousands of hours, strong anti-interference ability, high reliability, and suitable for long-distance transmission. The disadvantage is that the absolute position information of the shaft rotation cannot be output.

1.2 Absolute encoder

The absolute encoder is a sensor that directly outputs digital quantities. There are several concentric code tracks along the radial direction on its circular code disc. Each track is composed of light-transmissive and opaque sectors. The fans of adjacent code channels The number of zones is a double relationship. The number of code channels on the code wheel is the number of binary digits. On one side of the code wheel is the light source, and the other side has a photosensitive element corresponding to each code channel; when the code wheel is in different positions When, each photosensitive element converts a corresponding level signal according to whether it receives light or not, and forms a binary number. The characteristic of this kind of encoder is that no counter is needed, and a fixed digital code corresponding to the position can be read out at any position of the shaft. Obviously, the more code channels, the higher the resolution. For an encoder with N-bit binary resolution, the code disk must have N code channels. At present, there are 16-bit absolute encoder products in China.

The absolute encoder uses natural binary or cyclic binary (Grey code) method for photoelectric conversion. The difference between an absolute encoder and an incremental encoder is the transparent and opaque line pattern on the disc. The absolute encoder can have several codes, and the absolute position can be detected according to the code on the read code disc. The coding design can adopt binary code, cyclic code, two's complement code, etc. Its unique features are:

1.2.1 The absolute value of angle coordinates can be read directly;

1.2.2 There is no cumulative error;

1.2.3 The position information will not be lost after the power is cut off. But the resolution is determined by the number of bits in the binary system, which means that the accuracy depends on the number of bits. There are currently 10 bits, 14 bits, and so on.

1.3 Hybrid absolute encoder

Hybrid absolute encoder, it outputs two sets of information: one set of information is used to detect the magnetic pole position, with absolute information function; the other set is exactly the same as the output information of the incremental encoder.

The photoelectric encoder is an angle (angular velocity) detection device. It converts the angle input to the shaft into the corresponding electric pulse or digital quantity by using the photoelectric conversion principle. It has the advantages of small size, high precision, reliable work, and digital interface. . It is widely used in devices and equipment that need to detect angles such as CNC machine tools, rotary tables, servo drives, robots, radars, and military target determination.