Incremental Encoder

Incremental Encoder

Incremental encoders provide incremental information about the motor shaft position via 2 channels A and B. These channels output a defined number of pulses for a specific shaft movement. These pulses are generated in the incremental encoder via an optical disk mounted on the motor shaft. Incremental encoders generate a square wave output whose state changes each time the shaft rotates through a certain angle. The number of peaks per revolution defines the resolution of the sensor. Unlike the absolute encoder, the incremental encoder does not provide an absolute value of the position (only relative). This has the advantage of making the sensor less expensive because it is less technologically complicated.

The first application of incremental encoders is position tracking. This is done by counting the number of peaks related to the initial position. But they can just as easily be used to measure the speed of a component. This is done by calculating the ratio between the number of peaks and the total operating time. Common incremental encoders are incremental TTL encoders. The output is a digital pulse train with variable resolution. Resolutions of 50 … 5000 lines / rev. are standard with most providers. Special types with up to 100,000 lines / rev. are also available. Incremental encoders deliver the signals A, B and zero pulse (zero). The donors usually also provide a complementary signal. The complementary signal reduces the susceptibility to interference. The resolution of the encoders with complementary signals can be quadrupled by evaluating the edges.

An incremental encoder has a decisive weakness if it is to be used for commutating servomotors. It does not provide an absolute position at the moment it is switched on. This means that when the servo amplifier is switched on, for example in a system with absolute encoders, it knows where the axis “is”. Incremental encoders are incremental devices, which means that there is no way to determine the exact position, only to what extent the position has changed since the start of the evaluation. However, the servo amplifier requires a precise position signal for the correct energization (commutation) of the windings in relation to the position of the permanent magnets in the rotor.

One possibility of determining the absolute position of the motor shaft after switching on is the so-called ” wake and shake “. During the function, the motor shaft is moved back and forth minimally. A position is determined from the values ​​determined in this way. For this reason, incremental encoders are often equipped with additional commutation channels.