Introduction to Industrial Automation

2.3.7   Encoders

Encoders are specific sensors that generate square pulses in response to a wide range of motion 

tasks. In many manufacturing and production processes, there is a need for precise shaft rotation 

positioning or linear motion position measuring. For this purpose, there are shaft or hollow shaft 

encoders that respond to rotation (rotary encoders) and linear encoders that respond to a linear 

movement along a straight-line motion. As electromechanical devices, encoders are able to follow 

the detected motion through mechanical coupling and provide a motion control system with 

feedback information, concerning position or distance, rotation speed or velocity, and direction. 

Linear and rotary encoders are both available with two main types of output interpretation: the 

absolute encoder and the incremental encoder.

The output of absolute encoders is a multi-bit digital code that indicates the actual position 

of the moving mechanical object directly. This means that an absolute encoder has a reference 

position. Even in the event of a power outage, it has the ability to maintain a record of its absolute 

position. After restarting, the motion system can resume motion immediately without the need 

for rehoming. On the other hand, the output of incremental encoders generates a series of pulses 

proportional to the rotation of the shaft (rotary encoder) or distance traveled (linear encoder). 

By counting the number of pulses per a desired period of motion and based on the resolution 

of the encoder—i.e., the number of pulses per revolution or millimeters per unit travel given by 

(a)

(b)

Figure 2.65  Typical view of an electromechanical counter from General Industrial Controls