The difference between incremental encoder and absolute encoder

The difference between incremental encoder and absolute encoder

Encoders can be divided into incremental encoders and absolute encoders based on signal principles. The working principle of
the incremental encoder (rotary type) : a photoelectric code disk with a central axis, on which there are circular open and dark engraved lines, which are read by photoelectric transmitting and receiving devices to obtain four groups of sinusoidal wave signals combined into A, B, C, and D. Each sinusoidal wave has a 90-degree phase difference (relative to a cycle of 360 degrees). The C and D signals are reversed and superimposed on the A and B phases to enhance the stability of the signal; in addition, a Z phase pulse is output for each rotation to represent the zero reference position. Since the A and B phases differ by 90 degrees, the forward and reverse rotation of the encoder can be determined by comparing whether the A phase is in front or the B phase is in front. The zero reference position of the encoder can be obtained through the zero pulse. The materials of encoder discs include glass, metal, and plastic. Glass discs are very thin lines deposited on glass, which have good thermal stability and high precision. Metal discs are directly engraved with open and closed lines, which are not easy to break. However, due to the certain thickness of metal, the precision is limited, and its thermal stability is one order of magnitude worse than that of glass. Plastic discs are economical, with low cost, but the accuracy, thermal stability, and life are all worse. Resolution - The number of open or dark lines provided by the encoder for each 360-degree rotation is called resolution, also known as analytical division, or directly called the number of lines, generally 5~10000 lines per rotation. Signal output: Signal output has multiple forms such as sine wave (current or voltage), square wave (TTL, HTL), open collector (PNP, NPN), and push-pull. Among them, TTL is a long-line differential drive (symmetrical A, A-; B, B-; Z, Z-), and 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 to the PLC and computer are divided into low-speed modules and high-speed modules, and the switching frequency is low or high. For example, single-phase connection is used for single-direction counting and single-direction speed measurement. AB two-phase connection is used for forward and reverse counting, forward and reverse judgment and speed measurement. A, B, Z three-phase connection is used for position measurement with reference position correction. A, A-, B, B-, Z, Z-connection, due to the connection with symmetrical negative signals, the electromagnetic field contributed by the current to the cable is 0, the attenuation is minimal, the anti-interference is optimal, and it can be transmitted over a long distance. For TTL encoders with symmetrical negative signal outputs, the signal transmission distance can reach 150 meters. For HTL encoders with symmetrical negative signal outputs, the signal transmission distance can reach 300 meters. Problems with incremental encoders: Incremental encoders have zero-point cumulative errors, poor anti-interference, the receiving device needs to be powered off and memorized when it is shut down, and the zero or reference position should be found when it is turned on. These problems can be solved by using absolute encoders. General applications of incremental encoders: measuring speed, measuring rotation direction, measuring moving angle and distance (relative). Absolute encoder (rotary type) There are many light channel lines on the absolute encoder optical code disk, and each line is arranged in 2 lines, 4 lines, 8 lines, 16 lines, etc. In this way, at each position of the encoder, by reading the pass and dark of each line, a set of unique binary codes (Gray code) from 2 to the power of 0 to 2 to the power of n-1 is obtained, which is called an n-bit absolute encoder. Such an encoder is determined by the mechanical position of the photoelectric code disk, and it is not affected by power outages and interference. Each position of the absolute encoder determined by the mechanical position 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 you need to know the position, you can 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 Rotating single-turn absolute encoder measures each line of the photoelectric code disk during rotation to obtain a unique code. When the rotation exceeds 360 degrees, the code returns to the origin, which does not meet the principle of absolute code uniqueness. Such code can only be used for measurement within a rotation range of 360 degrees, which is called a single-turn absolute encoder. If you want to measure a rotation range exceeding 360 degrees, you must use a multi-turn absolute encoder. Encoder manufacturers use the principle of clock gear machinery. When the center code disk rotates, another set of code disks (or multiple sets of gears, multiple sets of code disks) are driven by gears. On the basis of the single-turn code, the number of turns is increased to expand the measurement range of the encoder. Such an absolute encoder is called a multi-turn absolute encoder. It is also determined by the mechanical position. The code is unique and non-repetitive for each position, and no memory is required. Another advantage of multi-turn encoders is that due to the large measurement range, they are often more abundant in actual use. In this way, it is not necessary to find the zero point during installation. A certain middle position can be used as the starting point, which greatly simplifies the difficulty of installation and debugging.