An encoder is a device that encodes signals, such as bitstreams, or data and converts them into signals that can be used for communication, transmission, and storage. The encoder converts angular or linear displacements, known as code disks, into electrical signals. According to the readout mode, the encoder can be divided into contact type and non-contact type. According to working principle, encoders can be divided into incremental and absolute encoders. The incremental encoder converts the displacement into a periodic electrical signal, and then converts the electrical signal into a counting pulse, and uses the number of pulses to represent the displacement. Each position of the absolute encoder corresponds to a certain digital code, so its indication value is only related to the starting and ending positions of the measurement, and has nothing to do with the intermediate process of the measurement.

I. Classification of encoders

According to the principle of detection, encoders can be divided into optical type, magnetic type, inductive type and capacitive type, and can be divided into incremental type, absolute type and hybrid type according to their calibration method and signal output form.

1.1 Incremental Encoder Incremental encoder outputs three groups of square wave pulse phase A, B and Z directly by photoelectric conversion principle; The phase difference between A and B pulses is 90 degrees, so the rotation direction can be easily judged, while the Z phase is one pulse per rotation, which is used for positioning the reference point. Its advantages are simple principle structure, average mechanical life can be more than tens of thousands of hours, strong anti-interference ability, high reliability, suitable for long distance transmission. Its disadvantage is that it cannot output the absolute position information of shaft rotation.

Absolute encoder is 1.2, absolute encoder directly output digital sensors, along the radial in its circular encoder has several concentric encoder, each have pervious to light and opaque sector and white composition, adjacent code word sector trees are double relationship and the code number on the encoder is its binary digital digits, on one side of a dish is a light source, the other side corresponding to every yard there is a light sensor, when a dish is in different position, the photosensitive element according to the light or not proper level signal conversion, form a binary number. The characteristic of this kind of encoder is not to counter, in any position of the rotation axis can read a fixed number corresponding to the position code. Obviously, mando must have N mando paths. At present, there are 16 bit absolute encoder products in China.

1.3 Hybrid absolute encoder. It outputs two sets of information. One set of information is used to detect the position of magnetic pole and has the function of absolute information. The other set is exactly the same as the output information of the incremental encoder.

Second, the application of photoelectric encoder

Difference between incremental encoder and absolute encoder

1. Angle measurement

In the driving simulator, the photoelectric encoder is used as the sensor to measure the steering wheel rotation Angle. The gravimeter uses a photoelectric encoder to connect its rotating shaft to the compensation knob shaft in the gravimeter. The torsional angulator uses the encoder to measure the change of torsional Angle, such as torsional experiment machine and fishing rod torsional fishing ability test. Pendulum impact experiment machine, using encoder to calculate the impact is the pendulum Angle change.

2. Length measurement

Meters, using the wheel circumference to measure the length and distance of the object.

The draw line displacement sensor uses the reel circumference to measure the length distance of the object.

Directly measured with the spindle of the power plant that drives the linear displacement, measured by the output pulse number.

Medium detection, in the straight rack, rotating chain sprocket, synchronous belt wheel to transfer the linear displacement information.

3. Speed measurement

Linear velocity, by connecting to the meter, measure the linear velocity of the production line

Angular velocity, measured by an encoder to measure the velocity of a motor, shaft, etc

4. Position measurement

Machine tool, the memory of the coordinates of the machine points, such as the position of the drill press

In automatic control, the control performs the specified action at the pastoral location. Such as elevators, elevators, etc

5. Synchronous control

By means of angular velocity or linear velocity, the transmission link is synchronously controlled to achieve tension control

Iii. Incremental Encoder (rotary type)

1. Working Principle:

By A central shaft of light code disc, on which there are circular and dark scribed line, have read photoelectric emission and receiving devices, obtain four groups of sine wave signal is combined into A, B, C, D, each sine wave 90 degree phase difference (as opposed to A cycle of 360 degrees), will reverse, C, D signal superimposed on A, B two phase, can strengthen stability of signal; Another Z phase pulse is output at each turn to represent the zero reference point.

Because phase A and Phase B differ by 90 degrees, the forward rotation and reverse of the encoder can be determined by comparing phase A and phase B in front. The zero reference bit of the encoder can be obtained through the zero pulse.

Encoder code disc material with glass, metal, plastic, glass encoder is in the deposition of thin scribed line on the glass, its thermal stability, high precision, metal encoder directly to scribe lines and impassability, non-friable, but because has certain thickness of metal, precision is limited, its thermal stability is an order of magnitude worse than glass, plastic encoder is economical, its cost is low, but the accuracy and thermal stability, life is less.

Resolution - The number of pass or dark lines provided by the encoder per rotation of 360 degrees is called resolution, also known as the analytic division, or directly called the number of lines, generally at 5~10000 lines per rotation division.

2. Signal Output:

Signal output is sinusoidal wave (current or voltage), square wave (TTL, HTL), open collector (PNP, NPN), push-pull type, where TTL is long line differential drive (symmetric A, A-; B, B -; Z, Z-), HTL is also called push-pull type and push-pull type output. The signal receiving device interface of the encoder shall correspond to the encoder.

Signal connection - the pulse signal of the encoder is generally connected to the counter, PLC, computer, PLC and computer connected modules have low speed module and high speed module, switching frequency has low and high.

Such as single - phase connection, used for one - direction counting, one - direction speed measurement.

A.B two phase connection, used for forward and backward counting, judge forward and backward direction and speed measurement.

A, B, Z three-phase connection for position measurement with reference bit correction.

A, A-, B, B-, Z, Z- connection. Due to the connection with symmetric negative signal, the electromagnetic field contributed by the current to the cable is 0, with the minimum attenuation and the best anti-interference, and can be transmitted over A long distance.

For TTL encoders with symmetric negative signal output, the signal transmission distance is up to 150 meters.

For HTL encoders with symmetric negative signal output, the signal transmission distance is up to 300 meters.

3. Problems of incremental encoder:

Incremental encoders have zero accumulative error, poor anti-interference, memory of power off when receiving equipment stops, zero change or reference point when starting up, etc. These problems can be solved if absolute encoders are chosen.

General applications of incremental encoders:

Measure speed, rotation direction, movement Angle and distance (relative).

Iv. Absolute encoder (rotary type)

There are many light channel lines on the optical code disk of absolute encoder. Each line is divided into 2 lines, 4 lines, 8 lines and 16 lines in turn. In this way, at each position of the encoder, a unique set of binary code (gray code) from 2 to the zero power to 2 to the n-1 power is obtained by reading the pass and shadow of each line, which is called an n-bit absolute encoder. Such an encoder is determined by the mechanical position of the photoelectric code disk, which is not affected by power failure and interference.

The absolute encoder is unique for every position determined by mechanical position, it doesn't need to remember, it doesn't need to look for reference points, it doesn't need to count all the time, it needs to know its position when, it needs to read its position when. In this way, the anti-interference characteristic and data reliability of the encoder are greatly improved.

From a single-turn absolute encoder to a multi-turn absolute encoder

The single-coil absolute value encoder is rotated to measure each line in the photoelectric code plate during rotation to obtain the unique code. When the rotation exceeds 360 degrees, the code goes back to the origin, which does not conform to the unique principle of absolute code. Such code can only be used for the measurement within the rotation range of 360 degrees, which is called the single-coil absolute value encoder.

If you want to measure a rotation in excess of 360 degrees, you use a multi-turn absolute value encoder.

Encoder manufacturers using the principle of gear mechanical clocks and watches, when the rotating center code through gear drive another set of encoder (or multiple sets of gear, multiple sets of encoder), on the basis of the lap coding number of laps to add code, to enlarge the measuring range of the encoder, the absolute encoder is called a coil more absolute encoder, it is also determined by mechanical position coding, each location coding not only repeat, without memory.

Another advantage of multi-coil encoders is that due to the large measuring range, the actual use is often more affluent, so it is unnecessary to bother to find zero in the installation, and a certain middle position can be used as the starting point, which greatly simplifies the installation and debugging difficulty.