Absolute rotary photoelectric encoders , due to their absolute uniqueness at each position, anti-interference capabilities, and lack of power-off memory, are increasingly widely used in angle and length measurement and positioning control in various industrial systems.
Absolute encoder principle
Absolute rotary photoelectric encoders, due to their absolute uniqueness at each position, interference resistance, and lack of power-off memory, are increasingly widely used in various industrial systems for angle and length measurement and positioning control. An absolute encoder's code disk has many etched lines, each arranged sequentially with 2, 4, 8, 16 lines, and so on. Thus, at each position of the encoder, by reading the on/off state of each etched line, a unique binary code (Gray code) from 2^0 to 2^(n-1) is obtained, which is called an n-bit absolute encoder. Such an encoder is determined by the mechanical position of the code disk and is unaffected by power outages or interference. The uniqueness of each position determined by the mechanical position of an absolute encoder means it requires no memory, no reference point, and no continuous counting; the position is read only when needed. This greatly improves the encoder's interference resistance and data reliability.
Because absolute encoders are significantly superior to incremental encoders in positioning, they are increasingly used in industrial control positioning. Due to their high precision and large number of output bits, absolute encoders require parallel outputs. Each output signal must be securely connected, and isolation is necessary for complex operating conditions. The large number of connecting cable cores leads to numerous inconveniences and reduced reliability. Therefore, absolute encoders with multi-bit outputs generally use serial or bus-type outputs. The most common serial output for absolute encoders manufactured in Germany is SSI (Synchronous Serial Output).
A single-turn absolute encoder measures each line on the optical code disk during rotation to obtain a unique code. When the rotation exceeds 360 degrees, the code returns to the origin, which does not conform to the principle of unique absolute coding. Such an encoder can only be used for measurement within a rotation range of 360 degrees and is called a single-turn absolute encoder.
If you need to measure rotations exceeding 360 degrees, you will need to use a multi-turn absolute encoder.
Encoder manufacturers utilize the mechanical principle of clock gears. When the central code disk rotates, it drives another set of code disks (or multiple sets of gears and multiple sets of code disks) through gear transmission. This adds more turns of encoding on top of the single-turn encoding, thereby expanding the encoder's measurement range. Such an absolute encoder is called a multi-turn absolute encoder. It also determines the encoding by mechanical position, and each position encoding is unique and non-repeating, so there is no need to memorize it.
Another advantage of multi-turn encoders is that due to their large measurement range, there is often a lot of margin in actual use. This means that there is no need to painstakingly find the zero point during installation. You can simply use a certain intermediate position as the starting point, which greatly simplifies the installation and debugging process.
Multi-turn absolute encoders have significant advantages in length positioning and are increasingly being used in industrial control positioning.
Mechanical installation and use
Absolute rotary encoders can be mechanically mounted in various ways, including high-speed end mounting, low-speed end mounting, and mounting with auxiliary mechanical devices.
High-speed end installation:
Mounted on the motor shaft (or via gear connection), this method offers the advantage of high resolution. Since the multi-turn encoder has 4096 revolutions, the motor's rotation count falls within this range, fully utilizing the range to improve resolution. The disadvantage is that backlash occurs during the reciprocating motion of the moving object through the reduction gear. It is generally used for unidirectional high-precision control and positioning, such as roll gap control in steel rolling. Additionally, when the encoder is directly mounted on the high-speed end, motor vibration must be minimal to avoid damaging the encoder.
Low-speed installation:
Installed after the reduction gear, such as at the shaft end of a hoisting wire rope drum or the shaft end of the last reduction gear, this method eliminates gear backlash, making the measurement more direct and accurate. This method is generally used for long-distance positioning, such as positioning of various lifting equipment and feeding trolleys.
Auxiliary mechanical installation:
Commonly used methods include gears and racks, chains and belts, friction wheels, and rope winding mechanisms.
Selection Considerations
I. Standard dimensions of absolute encoders: 38mm, 58mm, 66mm, 80mm, 100mm.
II. Absolute encoders are divided into: single-turn and multi-turn.
III. Absolute encoders are classified according to their operating principle into: magnetic absolute encoders and photoelectric absolute encoders.
IV. Absolute encoders have two types of cable exit methods: side exit and rear exit.
V. Absolute encoder shafts are available in the following sizes: 6mm, 8mm, 10mm, 12mm, 14mm, and 25mm.
VI. Absolute encoders are divided into: shaft, blind hole, and through hole.
VII. The protection rating of absolute encoders is divided into: IP54-68.
8. Absolute encoder mounting methods include: clamping flange, synchronous flange, clamping flange with synchronous band, blind hole (spring plate, clamping), and through hole (spring plate, key pin).
9. Absolute encoder accuracy is divided into single-turn accuracy and multi-turn accuracy, which together constitute the total accuracy, also known as the number of bits (typically 24-bit, 25-bit, 30-bit, 32-bit, etc.).
10. Absolute encoder communication protocol baud rate: 4800~115200 bit/s, default is 9600 bit/s. Refresh cycle is approximately 1.5ms.
11. Absolute encoder output options include: SSI, 4-20MA, PROFIBUS-DP, DEVicenet, parallel, binary, BiSS, ISI, CANopen, Endat, and Hiperface, etc.
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