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34 ADI | Engineering a More Sustainable Future Motor Encoder Types, Technologies, and Performance Metrics Absolute single-turn encoders return the absolute position, either within mechanical or electrical 360° once power is applied. The position of the motor shaft can be read immediately. Absolute multiturn encoders include both absolute functions and count the number of 360° turns. In contrast, incremental encoders provide position relative to the rotation starting point. Incremental encoders provide an index pulse to indicate 0° and a single pulse to count turns or a dual pulse to give direction information. The resolution of an encoder is the number of positions that can be distinguished per 360° rotation of the motor shaft. In general, the highest resolution encoders use optical technology, while medium resolution/high resolution encoders use magnetic or optical sensors. Resolvers (rotary transformers) or Hall sensors are used for low to medium resolution encoders. Optical or magnetic encoders use high resolution signal conditioning. The majority of optical encoders are incremental. Encoder repeatability is a key performance metric and is a measure of how consistently the encoder returns to the same commanded position. This is critical for repetitive tasks such as robotics or pick and place machines for semiconductor placement during PCB manufacture. Figure 3: Encoder types. 3 Table 1: Encoder Key Performance Metrics Metric Definition Comments Resolution The number of positions (n) an encoder can distinguish per revolution High resolution: 16 bits to 24 bits Medium resolution: 3 bits to 18 bits Low resolution: <12 bits Absolute Accuracy The difference between the actual position and the reported position through one rotation (like INL) Position control applications depend on absolute accuracy Differential Accuracy The difference between the reported distance between two neighbored positions and the ideal distance between positions (like DNL) Speed control applications depend on differential accuracy Repeatability How consistently the encoder returns to the same commanded position Repeatability is important for repetitive tasks such as those involving robotics