What is an Encoder?

Catalog

Ⅰ Classification of encoders

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

encoder 1

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 determined. 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

The absolute encoder is a sensor that directly outputs numbers. There are several concentric code discs along the radial direction on its circular code disc. Each track consists of light-transmissive and opaque sectors. The sector trees of adjacent code channels are in 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.

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 absolute information function; the other set is exactly the same as the output information of the incremental encoder.

Ⅱ Application of photoelectric encoder

encoder 2

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 the 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: measuring 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, it can be used to memorize the coordinate position of each coordinate point of the machine tool, such as a drilling machine, etc.

In terms of automatic control, it can 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

Ⅲ Incremental encoder (rotary type)

encoder 3

1. Working principle

There is a photoelectric code disk with an axis in the center of the incremental encoder, on which there are circular and dark engraved lines. They are read by the photoelectric transmitter and receiver devices, which obtains four groups of sine wave signals combined into A, B, C, D. Each sine wave 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 an output every revolution to represent the zero reference bit.

Since 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 pulses.

The materials of the encoder code disc are glass, metal, plastic. The glass code disc is deposited on the glass with very thin score 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 the resolution, also called resolution indexing, or directly called the number of lines, usually 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, among which TTL is 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.

Single-phase connection is used for single direction counting and single direction speed measurement.

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

A, B, Z three-phase connection is 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 longer 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.

Ⅳ  Absolute encoder (rotary type)

There are many optical channel markings on the optical code disc of the absolute encoder, and each marking line is arranged in 2 lines, 4 lines, 8 lines, 16 lines... At each position of the encoder, by reading each track, the opening and darkening of the engraved lines obtain a set of unique binary codes (Gray codes) from the zero power of 2 to the n-1 power of 2, 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. It can read its position when it needs to know the 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 disc during rotation to obtain a unique code. When the rotation exceeds 360 degrees, the code returns to the origin, which does not comply with the absolute code unique principle. The encoding can only be used for measurement within the rotation range of 360 degrees, which is called a single-turn absolute encoder.

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

The encoder manufacturer uses the principle of the 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 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 of the 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 an intermediate position as the starting point, which greatly simplifies the difficulty of installation and debugging.

Ⅴ Common faults

1.Encoder fault: it refers to the fault of the encoder's own components, which causes it to fail to generate and output the correct waveform. In this case, you need to replace the encoder or repair its internal components.

2.Encoder connection cable failure: This type of failure has the highest probability of occurrence, and it is often encountered during maintenance. Usually, the encoder cable is open-circuited, short-circuited, or poorly contacted. At this time, the cable or connector needs to be replaced. Special attention should also be paid to whether the cable is not tightly fixed, causing loosening or disconnection. At this time, the cable needs to be clamped.

3.Encoder +5V power supply drop: it refers to the +5V power supply is too low. The cause of the too low is that the power supply failure or the power transmission cable resistance is too large and the loss is caused, then the power supply needs to be repaired.

4.Absolute encoder battery voltage drop: This type of failure usually has a clear alarm. At this time, the battery needs to be replaced. If the memory of the reference point position is lost, the operation of returning to the reference point must be performed.

5.The shielded wire of the encoder cable is not connected or falls off: It will introduce interference signals, making the waveform unstable, and affect the accuracy of communication. It is necessary to ensure that the shielded wire is reliably welded and grounded.