A rotary encoder is a rotary encoder that usually provides digital output signals that need to be decoded at the other end of the sensor line in the evaluation device. Therefore, it is also called a rotary encoder. “Encoder” is sometimes used as a generic term for all angular position encoders.
Differences in the Way Signals are Evaluated
Incremental Encoders
Incremental encoders deliver a certain number of step pulses as well as a so-called zero pulse per revolution. Here, the recording takes place via a disc, a magnetic wheel (pole wheel) or via gear flanks. Two digital output signals (A and B track) or analogue sine or cosine signals are output, offset by a quarter pulse (90° electrical). The value is generated by counting the pulses in the evaluation device. The absolute angle can only be determined after processing the reference mark or detecting the mark of the zero point (zero pulse). By detecting the signal edge, one signal can be calculated as a “clock” or “step” and another for the “direction”. Incremental encoders are often used as tachometers by measuring the time of one revolution (time between two zero pulses) or between two increments.
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Incremental encoders with UVW
In the case of an incremental encoder with UVW signals, in addition to the AB traces of the incremental encoder, there is also a so-called U, V and W track.
These are usually used for commutation in electric three-phase machines. The UVW signals change as they rotate according to the 3-bit gray code, with the states and not occurring. In a three-phase machine with only one pair of poles, six states are distinguished per mechanical revolution. This means that the rotor position is known with an accuracy of at least 60°, which is sufficient to move the three-phase machine to the next UVW flank with field-oriented control. There, the exact rotor position is known.
Absolute Encoders
In the case of the absolute encoder, the acquisition is carried out via a coding disc. Multiple revolutions are detected by a built-in gearbox or other methods; this is followed by the serial transmission of the measured values to the evaluation device. The measured value is available as soon as it is switched on.
Absolute encoders are length or angle measuring devices that are used as displacement measuring devices on machine tools, in handling and automation technology and on measuring and testing equipment. The absolute measured value is available immediately after switching on without referencing.
In the case of rotary encoders, a distinction is made between those that can only resolve one revolution and then start again at 0 (single-turn encoders) and those that can resolve several revolutions (multi-turn encoders).
A single-turn encoder assigns a coded position value to each angle position. This means that the angle of rotation is only known within one revolution. To ensure that the absolute position remains known after several revolutions, the revolutions must be counted by the controller and saved when switched off. In addition, it must be ensured that the measuring system is not accidentally adjusted after switching off.
A multiturn encoder assigns a position value to each angular position and each full revolution. There is no need for a zero setting or reference run. To ensure that the absolute position remains known after several revolutions, it must be possible to continue to determine the revolutions in the measuring system even after switching off. There are two fundamentally different methods for this.
Differences in mechanical design
- Encoder with self-bearing
– single-sided or continuous hollow shaft with attached stator coupling. They are placed directly on the shaft to be measured.
– with short shaft and synchro flange or clamping flange. They are connected with a separate shaft coupling. - Encoders without self-bearing, e.g. built-in encoders or built-in measuring devices. In these cases, the rotor is mounted directly on the shaft to be measured. To do this, the stator must be aligned with the machine.
