Absolute vs. Incremental Rotary Encoders: Which is Best?

Rotary encoders sense changes in the position of a rotating shaft, then generate signals that send speed, direction, and position information to a receiving device such as a counter, drive, or controller such as a PLC. Rotary encoders come in two basic varieties: absolute encoders report the actual position of the shaft at a specific time, while incremental encoders indicate relative changes in the shaft’s position.

Depending on the system’s feedback requirements, either of these two types of encoder will work. But when the system powers down, absolute encoders retain their shaft position information and can pick right back up where they left off. Incremental encoders, however, must restart after power-off by establishing a starting point, or index, against which to report changes in position. This little difference becomes very important when operating conditions aren’t ideal.

What are the differences?

Both types of encoders are somewhat simple electromechanical devices, and they operate on similar principles. Each time the shaft rotates past a certain position, a sensor in the encoder registers this movement and sends either a digital signal or an electrical pulse through an output channel:

To identify the shaft’s position, absolute encoders can use magnetic or optical sensors. Either way, the encoder typically includes a magnet or patterned disk mounted to the shaft and a fixed-position sensor. As the shaft rotates, the unique disk pattern is read by the signal’s fixed sensor. The absolute position is generally represented as a binary word 2n, where n is the number of bits. The higher the number of bits, the greater the resolution, or more precise measurement of the shaft’s position and speed.

Incremental encoders typically have one or two signals, along with an index, or Z index signal. With one signal, the encoder can count the number of rotations the shaft makes over time to provide feedback on the speed of rotation — but cannot determine direction. With a second signal, offset in phase electrically of 90 degrees, the encoder can provide information on the direction of shaft rotation. Encoders with two signals that have phased electrical offset are called quadrature encoders.

If the system loses power, an absolute encoder can resume functionality immediately upon powerup. An incremental encoder, however, cannot begin relaying usable information until the shaft has made at least one turn. For some applications, this isn’t good enough. For those cases, incremental encoders can include an additional signal, or Z index pulse, which serves as a location index of the shaft's position as it occurs once during the 360-degree rotation of the shaft.

Which type is right for your purposes?

For some applications, like the precise orientation of radar or other sensing systems, there is no real debate: absolute rotary encoders are a must. For other purposes, including the simple measurement of shaft rotation speed, you may have an honest choice to make. The difference can often be measured in stability and cost.

Absolute encoders are uniquely able to begin operation immediately when a system is powered up. They also provide more information beyond speed and direction because they can let an operator know the exact location by supplying the position of the shaft in the 360-degree rotation and — in the case of multi-turn encoders — how many revolutions the shaft has performed.

For systems that use incremental encoders to determine location, there is typically additional software and programming needed to be able to calculate the same information that absolute encoders can instantly provide. That’s just one of the ways that incremental encoders may cost more in the long run than their absolute counterparts.

Upfront cost considerations typically favor incremental encoders by a wide margin. Absolute encoders simply contain more components to ensure higher resolution, and this means a higher up-front investment. But maintenance issues and downtime, along with the need to reinitialize incremental encoders each time the system regains power, may lead to performance losses which result in a higher overall cost option.

Wrapping it up

Absolute rotary encoders can do the job of incremental encoders and quite a bit more. On the other hand, they have a higher upfront cost, and their mechanical complexity means a higher level of care and maintenance are required.

Before you commit to one solution or the other, take a look at your system’s requirements and consider the conditions under which it will operate. On one hand, you may find that incremental encoders are just right for your job and that their simpler mechanical nature adds a helpful degree of reliability and confidence. On the other hand, if your system can take advantage of the benefits of absolute feedback, a wide range of options are available to you from which to choose.

For more information on all types of encoders and accessories, please visit Encoder Products Company.