Differences between an Absolute and Incremental Encoder

Introduction

An absolute encoder is an electronic sensor that tracks position data and retains the data even if the power is removed from it. In contrast, incremental encoders cannot retain the position data once the power supply is removed. This difference is a result of their difference in design. A closer look into their design will show why each encoder behaves as they do. Also, an absolute encoder records the absolute location of the device, whereas an incremental encoder can measure only the relative position.

Differences | Absolute Encoder vs Incremental Encoder

Measuring the Absolute Position

Absolute encoders can track and measure the location with just an initial calibration. An absolute encoder set up doesn’t have to calibrate again after a power restart.

An incremental encoder, even though it tracks only relative movement, can be used to track and measure absolute location with the help of external circuitry.

The Design Fundamentals

Rotary encoders can be manufactured using a wide range of techniques. Let’s stick to the optical method for the sake of explanation.

Incremental Encoder

An optical incremental encoder has 4 main sections, an LED, a lens(to produce collimated light), a transparent disk & a detector chip with a photodetector area. The transparent disk has a series of lines and windows, of the same dimensions. The encoder is set up to the index position before powering it on. Once the tracking starts, the LED illuminates the transparent disk. The lines and windows in the transparent disk block or allow the light, respectively. The photodetector at the other side of the disk detects the incoming light.

Since the lines are separated uniformly, each position will have almost similar detection at the photodetector end. This is why incremental encoders cannot track the absolute position. Incremental encoders are best suited for applications involving speed, distance, or direction-measurement.

Absolute Encoder

An absolute encoder has a design partially similar to the incremental encoder. It has an LED, a lens, a transparent disk, and a photodetector. The lens in the absolute encoder is used to make columnated light, in contrast to the collimated light in the incremental encoders. Also, the transparent disk uses multiple bands unlike the uniform lines in the incremental encoders.

Each alignment of the disc can be mapped to a distinct position data. Otherwise, for each position in the disk, the photodetector reads a unique signal. This way, when the power is removed, the position is retained. And when the device restarts, the encoder doesn’t need any calibration to resume its work.