What is the difference between incremental and absolute encoders?

What exactly is the difference between incremental and absolute encoders? Both can be used to measure distance, speed, acceleration and position of a mechanical system. The key difference is what happens if that system has a loss of power.

After power cycling, incremental encoders are not going to be able to report exact position of the system until they find the index/home position – if they have one. Absolute encoders will be able to report the exact position without returning to a home position after power loss.

Let’s get into how that works.

How Incremental Encoders Work

Our incremental optical encoders use LEDs, photo sensors and a patterned disk to report shaft position.
Our incremental optical encoders use LEDs, photo sensors and a patterned disk to report shaft position.

There are quite a few different ways to design an incremental encoder. For the sake of this blog post we are going to be using our rotary incremental encoders as an example, which use transmissive optical disks.

Our incremental encoders have a series of LEDs on one side of a transparent disk and photo sensors on the other. The disk has a series of lines and windows on it. The windows allow light to be transmitted between the LED and the sensor. The lines do not. This alternating pattern of light is how the encoder reports position information back to the mechanical system.

If the system power cycles, the encoder doesn’t know where in the rotation it is because each set of lines and windows is identical to the next.

In some applications – for instance, when using an encoder to determine speed, distance or direction of movement – this is perfectly fine.

In other applications an exact position is needed. Adding an index to an encoder provides a “home” to which the encoder may rotate after power cycling to reorient itself and then report its exact location. (Not all incremental encoders have an index position and some can have more than one, but that is a different blog post).

If you’re designing a system where it’s critical that you know exactly where the shaft position is, including after a loss of power, you should consider an absolute encoder.

How Absolute Encoders Work

Absolute optical encoders can use a series of lines and windows and multiple bands to create a binary pattern that gives a unique position.
Absolute optical encoders can use a series of lines and windows and multiple bands to create a binary pattern that gives a unique position.

Absolute optical encoders can use similar technology to incremental encoders, but use multiple bands to create a unique pattern on the disk so the exact position is known and there is no need to find the home position after a loss of power.

For instance, if you are building a robotic arm you may not have the clearance or flexibility for it to move to a home position after power loss. In that case, having an absolute encoder would ensure that you know exact position of the arm at all times.

Of course there is much more to learn about absolute encoders than we can fit into one blog post. Lucky for you we have an expanded intro to absolute encoders blog post.

And hey – if you are looking for an encoder and still have more questions, be sure to check out our latest white paper that helps you find the right encoder for your project. In it we go through resolution, type of motion, CPR vs. PPR and just about everything else you could imagine related to encoder selection.


Published in Blog Posts on Monday, February 10, 2020

 

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