Selection criteria for absolute encoders and incremental encoders
Absolute encoders are more complex than incremental types and therefore are usually more expensive. Although the price difference is shrinking, incremental encoders are usually more suitable for simple monitoring of speed, direction or relative position. On the other hand, absolute encoders are a better choice in some cases.
The main advantage of absolute encoder is to maintain the position of the axis, so that the position data can be obtained immediately without waiting for the completion of the homing or calibration sequence. In this way, even if the shaft position changes when the encoder is turned off, the system can start up faster or recover from a power failure.
Another reason for choosing an absolute encoder is that the position information is needed immediately at startup before any mechanism is activated or moved. This may be a situation where rotating the shaft in the wrong direction from the starting position may damage the equipment or cause danger to the user.
In addition, since the absolute encoder provides the real position in real time, the digital system can poll the encoder through the central communication bus to capture the position with minimal delay. Using incremental encoders to continuously track position is more difficult, because external circuits are usually required to track all pulses using quadrature decoding, which increases the overhead of the host system, especially when multiple encoders must be monitored.
Figure 1: The absolute encoder generates a unique digital “word” for each position of the code disc, which is equivalent to the specified resolution.
Another advantage is that the use of absolute encoders helps reduce the system’s sensitivity to electrical noise. Unlike pulse counting incremental encoders, absolute encoders allow the system to read error checking codes from a binary output or digitally through a serial bus to calculate the position.
In addition, it is easier to combine multiple absolute encoders in the same system than using incremental encoders. Typical examples include factory automation or multi-axis robots. Monitoring the output of multiple incremental encoders can become complicated and require a lot of processing power, while the readings from individual absolute encoders are easier to interpret, especially when they can be connected to a central communication bus.
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