Encoders can be classified according to their signal principle into incremental encoders and absolute encoders. Incremental encoders (rotary type)
Working principle:
A photoelectric code disk with a central axis and circular light and dark markings is used to read four sets of sinusoidal signals, A, B, C, and D. Each sinusoidal wave is 90 degrees out of phase (360 degrees in one cycle). The C and D signals are reversed.
The pulse is superimposed on phases A and B to enhance signal stability; additionally, a Z-phase pulse is output per revolution to represent the zero-position reference position.
Since phases A and B are 90 degrees out of phase, the forward and reverse rotation of the encoder can be determined by comparing whether phase A or phase B comes first. The zero-position reference position of the encoder can be obtained through the zero-position pulse.
Encoder disks are made of glass, metal, or plastic. Glass disks have very thin lines deposited on glass, resulting in good thermal stability and high precision. Metal disks have lines directly engraved with both through and non-through surfaces, making them less prone to breakage. However, due to the thickness of the metal, their precision is limited, and their thermal stability is an order of magnitude worse than that of glass. Plastic disks are economical, with low cost, but their precision, thermal stability, and lifespan are all inferior.
Resolution—The number of through or dark lines provided by an encoder per 360 degrees of rotation is called resolution, also known as resolution scale or simply the number of lines. It is generally 5 to 10,000 lines per revolution.
Signal output:
Signal outputs include sine waves (current or voltage) , square waves (TTL, HTL) , open collector (PNP, NPN) , and push-pull types. TTL is a long-line differential drive (symmetrical A , A-; B , B-; Z , Z-) . HTL is also called push-pull or push-pull output. The signal receiving device interface of the encoder should correspond to the encoder.
Signal connection - The encoder's pulse signal is generally connected to the counter, PLC, and computer. The modules that connect 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.
A and B are connected together for counting forward and reverse directions, determining forward and reverse direction, and measuring speed.
The A, B, and Z phases are connected for position measurement with reference correction.
A, A-, B, B-, Z, Z- connections, due to the symmetrical negative signal connections, result in zero electromagnetic field contribution from the current to the cable, minimal attenuation, optimal anti-interference, and the ability to transmit 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.
Problems with incremental encoders:
Incremental encoders suffer from zero-point cumulative error, poor anti-interference, and require power-off memory when the receiving device is stopped, as well as zeroing or finding a reference position when powered on. These problems can be solved by using an absolute encoder.
Common applications of incremental encoders :
Speed measurement, rotation direction measurement, and movement angle and distance (relative).
Absolute encoder (rotary type)
An absolute encoder has many optical channels etched on its code disk. Each channel is arranged sequentially with 2 lines, 4 lines, 8 lines, 16 lines, and so on. Thus, at each position of the encoder, by reading whether each channel is on or off, a unique binary code (Gray code) from 2^0 to 2^(n-1) is obtained. This is called an n-bit absolute encoder. Such an encoder is determined by the mechanical position of the photoelectric code disk and is unaffected by power outages or interference.
An absolute encoder's position is unique, determined by its mechanical position. It requires no memorization, no reference point, and no continuous counting; it reads the position only when needed. This significantly improves the encoder's anti-interference capabilities and data reliability.
From single-turn absolute encoders to multi-turn absolute encoders
A single-turn absolute encoder measures each line on a 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 conform to the principle of unique absolute coding. Such coding can only be used for measurements within a rotation range of 360 degrees, hence the name 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 significant margin of error in practical applications. This eliminates the need for painstaking zero-point finding during installation; a midpoint can be used as the starting point, greatly simplifying the installation and debugging process.
