What is the difference between an incremental and absolute encoder?

Short Answer:

An incremental encoder provides information about the change in position, giving output as a series of pulses as the shaft rotates. It tells how much movement has occurred but does not retain the actual position if power is lost. You need to set a reference point every time it restarts.

An absolute encoder, on the other hand, gives a unique code for every position, meaning it always knows the exact position even after power loss. It is more suitable for applications where precise position tracking is required without needing to reset the system.

Detailed Explanation:

Difference between incremental and absolute encoder

Encoders are devices used to measure the position, speed, or direction of a rotating shaft or moving object. The two most commonly used types are incremental encoders and absolute encoders. Both convert mechanical motion into electrical signals but differ in how they represent position and how they behave during power loss or restarts.

Incremental Encoder:

An incremental encoder works by generating a fixed number of pulses per revolution. It typically consists of a code disk with evenly spaced transparent and opaque lines, an LED light source, and a photodetector.

Working:

  • When the shaft rotates, the light from the LED passes through the code disk and reaches the sensor.
  • Every time the light is interrupted or passed, it generates a pulse.
  • These pulses are counted to determine how far the object has moved.
  • Two output channels (A and B) are often used in quadrature to also detect direction.

Key Features:

  • Measures relative position
  • Requires a reference point (zeroing) at startup
  • Simple and cost-effective
  • Good for speed and direction sensing

Limitation:

  • Loses position data when power is lost
  • Needs re-homing after power-up

Absolute Encoder:

An absolute encoder provides a unique binary or digital code for each shaft position. The disk in an absolute encoder has multiple tracks, each with its own sensor to detect specific position data.

Working:

  • As the shaft rotates, different patterns of light and dark areas are detected.
  • Each position on the shaft corresponds to a unique combination of bits.
  • This code is sent as output and directly represents the current position.

Key Features:

  • Measures absolute position
  • Retains position after power loss (no need to reset)
  • Can be single-turn or multi-turn
  • More complex and costly, but highly reliable

Limitation:

  • More expensive than incremental encoders
  • Slightly slower response due to code conversion

Main Differences at a Glance:

  • Data Output:
    Incremental gives pulses, absolute gives unique position codes.
  • Position Memory:
    Incremental forgets position after power-off; absolute remembers it.
  • Startup Requirement:
    Incremental needs re-zeroing; absolute does not.
  • Complexity:
    Incremental is simpler; absolute is more complex but informative.
  • Application Suitability:
    Incremental is good for speed and direction tracking; absolute is better for precise position control.
Conclusion:

The main difference between incremental and absolute encoders is how they represent position. Incremental encoders provide relative position using pulses and require a reference after power-up. Absolute encoders give the actual position value at all times, even after power loss. Choosing between them depends on the specific needs of the system—whether continuous position tracking or cost-saving simplicity is more important.