Encoder basic knowledge

1. Classification of encoders

According to the detection principle, the encoder 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. Incremental encoder Incremental encoder directly uses 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 A and B is 90 degrees, so that the direction of rotation can be easily judged , And the Z phase is one pulse per revolution, 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.

2. Absolute encoder Absolute encoder is a sensor that directly outputs numbers. There are several concentric code discs along the radial direction on its circular code disc, and each track is composed of light-transmitting and opaque sectors. The sector trees of adjacent code channels have a double relationship. The number of code channels on the code disc is the number of binary digits. On one side of the disc is the light source, and the other side has a photosensitive element corresponding to each code channel. When the disc is in different positions, each photosensitive element converts the corresponding level signal according to whether it receives light or not to form a binary number. The characteristic of this kind of encoder is that it does not need a counter, and can read a fixed digital code corresponding to the position at any position of the shaft. Obviously, there must be N doctrines. At present, there are 16-bit absolute encoder products in China.

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

2. Application of photoelectric encoder

The difference between incremental encoder and absolute encoder

1. Angle measurement

In the car driving simulator, the photoelectric encoder is selected as the sensor for the measurement of the steering wheel rotation angle. The gravity measuring instrument uses a photoelectric encoder to connect its shaft to the compensation knob shaft of the gravity measuring instrument. The torsion angle meter uses the encoder to measure the torsion angle changes, such as torsion experiment machine, fishing rod torsion fishing performance test, etc. The pendulum impact test machine uses the encoder to calculate the impact is the swing angle change.

2. Length measurement

The meter counter uses the circumference of the roller to measure the length and distance of an object.

The cable displacement sensor uses the circumference of the winding wheel to measure the length of the object.

Direct coupling measurement, coupling with the main shaft of the power device driving linear displacement, and measuring by the number of output pulses.

In the medium detection, the linear displacement information is transmitted on the straight rack, the sprocket of the rotating chain, and the synchronous belt wheel.

3. Speed ​

​measurement

Line speed, measure the line speed of the production line by connecting with the meter

Angular speed, measuring the speed of motors, shafts, etc. through encoders

4. Position measurement

For machine tools, memorize the coordinate position of each coordinate point of the machine tool, such as a drilling machine

In terms of automatic control, control the designated action at the pastoral position. Such as elevators, hoists, etc.

5. Synchronous control

Synchronously control the transmission links through angular velocity or linear velocity to achieve tension control

3. Incremental encoder (rotary type)

1. Working principle:

A photoelectric code disc with an axis in the center, on which there are circular and dark engraved lines, is read by photoelectric transmitter and receiver, and obtains four groups of sine wave signals combined into A, B, C, D, each sine wave A phase difference of 90 degrees (360 degrees relative to a cycle), the C and D signals are reversed and superimposed on the A and B phases to enhance the stable signal; in addition, a Z-phase pulse is output every revolution to represent the zero reference Bit.

Since the phases A and B differ by 90 degrees, the encoder's forward and reverse rotation can be judged by comparing the phase A or the B phase. The zero reference position of the encoder can be obtained through the zero pulse.　

The materials of the encoder code disc are glass, metal, plastic. The glass code disc is deposited on the glass with very thin scribe lines, which has good thermal stability and high precision. The metal code disc is directly engraved with through and impassable lines and is not fragile. However, due to the certain thickness of metal, the accuracy is limited, and its thermal stability is one order of magnitude worse than that of glass. Plastic code discs are economical, and their cost is low, but accuracy, thermal stability, and life are worse. .

Resolution—The number of open or dark engraved lines provided by the encoder per 360 degree rotation is called resolution, which is also called resolution indexing, or directly called the number of lines, generally at 5 to 10,000 lines per revolution.

2. Signal output:

The signal output has sine wave (current or voltage), square wave (TTL, HTL), open collector (PNP, NPN), push-pull multiple forms, of which TTL is a long-line differential drive (symmetrical A, A-; B, B -;Z,Z-), HTL is also called push-pull and push-pull output. The signal receiving device interface of the encoder should correspond to the encoder.

Signal connection—The pulse signal of the encoder is generally connected to the counter, PLC, and computer. The modules connected between the PLC and the computer are divided into low-speed modules and high-speed modules, and the switching frequency is low or high.

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

A.B two-phase connection, used for forward and reverse counting, judgment of forward and reverse and speed measurement.

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

A, A-, B, B-, Z, Z- connections, due to the connection with symmetrical negative signals, the electromagnetic field contributed by the current to the cable is 0, the attenuation is the smallest, the anti-interference is the best, and it can be transmitted over a long distance.

For TTL encoders with symmetrical negative signal output, the signal transmission distance can reach 150 meters.

For HTL encoders with symmetrical negative signal output, the signal transmission distance can reach 300 meters.

　　 3. The problem of incremental encoder:

Incremental encoders have zero cumulative errors and poor anti-interference. The receiving equipment needs to be powered off and remembered when it is shut down, and the change or reference position should be found when starting up. These problems can be solved by using an absolute encoder.

General application of incremental encoder:

Speed ​​measurement, measurement of rotation direction, measurement of movement angle, distance (relative).

　　 4. Absolute encoder (rotary type)　　

There are many optical channel engraved lines on the optical code disc of absolute encoder, and each engraved line is in order of 2, 4, 8 and 16 lines. . . . . . Arrangement, in this way, at each position of the encoder, by reading the open and dark of each engraved line, a set of unique binary codes from the zero power of 2 to the n-1 power of 2 (Gray Code), which is called an n-bit absolute encoder. Such an encoder is determined by the mechanical position of the photoelectric code disc, and it is not affected by power failure or interference.

Each position determined by the mechanical position of the absolute encoder is unique. It does not need to be memorized, does not need to find a reference point, and does not need to be counted all the time. When it needs to know the position, when to read its position. In this way, the anti-interference characteristics of the encoder and the reliability of the data are greatly improved.

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

Rotate a single-turn absolute encoder to measure each engraved line of the photoelectric code disk during rotation to obtain a unique code. When the rotation exceeds 360 degrees, the code returns to the original point, which does not comply with the absolute code unique principle. The encoding can only be used for measurement within 360 degrees of rotation, which is called a single-turn absolute encoder.

If you want to measure rotation beyond 360 degrees, you must use a multi-turn absolute encoder.

The encoder manufacturer uses the principle of clock gear mechanism. When the center code wheel rotates, another set of code discs (or multiple sets of gears, multiple sets of code discs) are driven by gears, and the number of turns is added to the single-turn encoding. Encoding, in order to expand the measuring range of the encoder, such an absolute encoder is called a multi-turn absolute encoder, it is also determined by the mechanical position code, each position code is unique and does not repeat, without the need to remember.

Another advantage of the multi-turn encoder is that due to the large measuring range, the actual use tends to be richer. In this way, it is unnecessary to find the change point during installation. It is enough to use a certain intermediate position as the starting point, which greatly simplifies the difficulty of installation and debugging.