Encoder is a device that compiles and converts signals (such as bitstreams) or data into signal forms that can be used for communication, transmission, and storage. The encoder converts angular displacement or linear displacement into electrical signals, the former being called a code disk and the latter being called a code ruler. According to the reading method, encoders can be divided into two types: contact type and non-contact type; According to the working principle, encoders can be divided into two types: incremental and absolute. Incremental encoders convert displacement into periodic electrical signals, which are then converted into counting pulses, and the number of pulses is used to represent the magnitude of displacement. Each position of the absolute encoder corresponds to a certain digital code, so its indication is only related to the starting and ending positions of the measurement, and is not related to the intermediate process of the measurement.
1、 Classification of encoders
According to the detection principle, encoders can be divided into optical, magnetic, inductive, and capacitive types. According to their calibration method and signal output form, they can be divided into three types: incremental, absolute, and hybrid.
1.1 Incremental Encoder Incremental encoder directly utilizes the principle of photoelectric conversion to output three sets of square wave pulses A, B, and Z phases; A. The phase difference between the two sets of pulses B is 90 degrees, making it easy to determine the direction of rotation, while the Z phase is one pulse per revolution, used for reference point positioning. Its advantages are simple principle and structure, an average mechanical life of tens of thousands of hours or more, strong anti-interference ability, high reliability, and suitable for long-distance transmission. Its disadvantage is that it cannot output absolute position information of shaft rotation.
1.2. Absolute encoder: Absolute encoder is a sensor that directly outputs digital signals. On its circular code disk, there are several concentric code disks along the radial direction. Each channel is composed of transparent and opaque sector areas, and the adjacent sector trees of the code channels are in a double relationship. The number of code channels on the code disk is the number of bits of its binary digits. On one side of the code disk, there is a light source, and on the other side, there is a photosensitive element corresponding to each code channel, When the disc is in different positions, each photosensitive element converts corresponding level signals based on whether it is illuminated or not, forming a binary number. The characteristic of this encoder is that it does not require a counter and can read a fixed numerical code corresponding to the position at any position on the shaft. Obviously, there must be N roads. At present, there are 16 bit absolute encoder products in China.
1.3. Hybrid absolute encoder: Hybrid absolute encoder outputs two sets of information, one set of which is used to detect the position of the magnetic pole and has an absolute information function; The other group is exactly the same as the output information of the incremental encoder.
2、 Application of photoelectric encoder
Difference between incremental encoder and absolute encoder
1. Angle measurement
A car driving simulator uses a photoelectric encoder as the sensor for measuring the rotation angle of the steering wheel. The gravity measuring instrument uses a photoelectric encoder to connect its rotating shaft with the compensation knob shaft in the gravity measuring instrument. The torsion angle meter uses an encoder to measure the changes in torsion angle, such as torsion testing machines and fishing rod torsion fishing performance tests. Pendulum impact testing machine, using an encoder to calculate the impact angle change.
2. Length measurement
A meter meter uses the circumference of a roller to measure the length and distance of an object.
A wire displacement sensor uses the circumference of the winding wheel to measure the length and distance of the object.
Coupling direct measurement, which is connected to the main shaft of the power device driving linear displacement, and measured through the output pulse number.
Media detection transmits linear displacement information through straight racks, rotating chain sprockets, synchronous pulleys, etc.
3. Speed measurement
Linear speed, measured by connecting to an instrument to measure the linear speed of the production line
Angular velocity, measured by encoder for motor, shaft, etc
4. Position measurement
In terms of machine tools, remember the coordinate positions of each coordinate point of the machine tool, such as drilling machines, etc
In terms of automation control, control the designated actions at the pastoral position. Such as elevators, elevators, etc
5. Synchronous control
Synchronous control of the transmission link through angular or linear velocity to achieve tension control
3、 Incremental encoder (rotary type)
1. Working principle:
A photoelectric code disk with a central axis, on which there are circular through and dark lines, and photoelectric emission and reception devices to read, obtains four sets of sine wave signals combined into A, B, C, and D. Each sine wave has a phase difference of 90 degrees (360 degrees relative to a cycle), and the C and D signals are reversed and stacked on the A and B phases to enhance stable signals; Another Z-phase pulse is output per revolution to represent the zero reference position.
Due to the 90 degree difference between A and B phases, the encoder's forward and reverse rotation can be distinguished by comparing whether A phase is in front or B phase is in front. By using zero pulse, the encoder's zero reference position can be obtained.
The materials of the encoder encoder encoder disk include glass, metal, and plastic. The glass encoder disk is a thin scribed line deposited on the glass, which has good thermal stability and high accuracy. The metal encoder disk directly passes through and does not have scribed lines, and is not fragile. However, due to the certain thickness of the metal, the accuracy is limited, and its thermal stability is one order of magnitude worse than that of glass. The plastic encoder disk is economical and has low cost, but its accuracy, thermal stability, and lifespan are all slightly worse.
Resolution - The resolution refers to the number of through or dark lines provided by the encoder per 360 degrees of rotation, also known as the resolution resolution, or directly measuring the number of lines, typically ranging from 5 to 10000 lines per revolution.
2. Signal output:
The signal output includes sine wave (current or voltage), square wave (TTL, HTL), open collector (PNP, NPN), and push pull in various forms. TTL is a long line differential drive (symmetrical A, A -; B, B -; Z, Z -), and HTL is also known as push pull or 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 to the PLC and computer can be divided into low-speed modules and high-speed modules, and the switching frequency can vary from low to high.
For single-phase connection, used for one-way counting and one-way speed measurement.
A. B two-phase connection, used for counting forward and backward, judging forward and backward, and measuring speed.
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 contribution of current to the electromagnetic field of the cable is 0, with minimal attenuation and optimal anti-interference, and can be transmitted over long distances.
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. Problems with incremental encoders:
Incremental encoders have accumulated zero point errors, poor anti-interference, power off memory required for receiving equipment shutdown, and zero or reference position required for startup. These problems can be solved by using absolute encoders.
General applications of incremental encoders:
Measurement of speed, direction of rotation, angle of movement, distance (relative).
4、 Absolute encoder (rotary type)
There are many optical channel lines on the optical encoder disk of the absolute encoder, each line is arranged in order of 2 lines, 4 lines, 8 lines, 16 lines... In this way, at each position of the encoder, by reading the on and off of each line, a unique set of binary codes (Gray codes) from the zero power of 2 to the n-1 power of 2 are obtained, which is called an n-bit absolute encoder. This type of encoder is determined by the mechanical position of the photoelectric encoder, and it is not affected by power outages or interference.
The absolute encoder is unique for each position determined by the mechanical position. It does not require memory, reference points, and continuous counting. When the position needs to be known, its position can be read. In this way, the anti-interference characteristics of the encoder and the reliability of the data are greatly improved.
From single turn absolute value encoder to multi turn absolute value encoder
Rotate the single turn absolute value encoder to measure the various lines of the photoelectric code disk during rotation to obtain a unique encoding. When the rotation exceeds 360 degrees, the encoding returns to the origin, which does not comply with the principle of absolute encoding uniqueness. This encoding can only be used for measurements within the rotation range of 360 degrees, and is called a single turn absolute value encoder.
If you want to measure rotation beyond the 360 degree range, you need to use a multi turn absolute encoder.
Encoder manufacturers use the principle of clock gear machinery to drive another set of code discs (or multiple sets of gears, multiple sets of code discs) through gears when the central code disc rotates. On the basis of single turn coding, the number of turns is added to expand the measurement range of the encoder. This type of absolute encoder is called a multi turn absolute encoder, which is also determined by the mechanical position and the coding is unique and not repeated for each position, Without the need for memory.
Another advantage of multi turn encoders is that due to their large measurement range, there is often more margin in practical use. This eliminates the need to find zero points during installation, and a certain intermediate position can be used as the starting point, greatly simplifying the difficulty of installation and debugging.