The difference between incremental encoder and absolute encoder

Encoders can be divided into incremental encoders and absolute encoders based on signal principle.

Incremental encoder (rotary type)

 

working principle:

A photoelectric code disc with an axis in the center, on which there are circular and dark engraved lines, is read by photoelectric transmitter and receiver, and obtains four groups of sine wave signals combined into A, B, C, D, each sine wave Phase difference of 90 degrees (relative to a cycle is 360 degrees), 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 output per revolution to represent the zero reference Bit.

 

Since the difference between A and B phases is 90 degrees, the encoder's forward and reverse rotation can be judged by comparing whether the A phase is the first or the B phase. The zero reference position of the encoder can be obtained through the zero pulse.

 

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 resolution, also known as resolution division, or directly called the number of lines, usually 5--10,000 lines per revolution.

 

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, and the switching frequency is low or high.

 

Such as single-phase connection, used for single direction counting and single direction speed measurement.

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

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 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 long 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.

 

Problems with incremental encoders:

 

Incremental encoders have zero cumulative errors and poor anti-interference. The receiving equipment needs to be powered off and memorized when shutting down, and the change or reference position should be found when starting up. These problems can be solved by using an absolute encoder.

General application of incremental encoder:

Speed ​​measurement, measurement of rotation direction, measurement of movement angle, distance (relative).

 

Absolute encoder (rotary type)

There are many optical channel engraved lines on the optical code disc of absolute encoder, and each engraved line is divided into 2 lines, 4 lines, 8 lines and 16 lines in sequence. . . . . . Arrangement, in this way, at each position of the encoder, by reading the open and dark of each engraved line, a set of unique binary codes from the zero power of 2 to the n-1 power of 2 (Gray Code), 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. When it needs to know the position, when to 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

 

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 360 degrees of rotation, which is called a single-turn absolute encoder.

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

 

The encoder manufacturer uses the principle of 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. 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 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 measurement range, the actual use is often richer. In this way, it is unnecessary to find the change point during installation. It is enough to use a certain intermediate position as the starting point, which greatly simplifies the difficulty of installation and debugging.