If you are relatively new to the field of motion control and robotics, you are probably asking questions like, “What is an encoder?”, and more specifically, “What is an absolute encoder?” The goal of this article is to familiarize you with and give you an understanding of absolute encoders.
Let us start this discussion by stating that there are two broad categories of encoders, incremental and absolute. This article will focus in on the absolute type. The following will be discussed:
1. The meaning of the word “absolute” as it relates to encoders will be defined.
2. Where does this absolute characteristic of the absolute encoder come from?
3. Why the absolute encoder is chosen for use in an application over other types of encoders and sensors will be answered.
4. Various types of absolute encoders such as single turn, multi-turn, and linear will be described.
When It Comes To Encoders, What Is Meant By Absolute?
Let us start off by defining what an encoder is. Encoders are often defined as devices that are used to convert rotary or linear motion into electrical signals that are then converted into position and/or speed information. As was mentioned before, there are two categories of encoders, incremental and absolute.
If you are like me, when I hear the word absolute, I often think of the word relative as well. I think of absolute as being the opposite of relative. In the same way, think of absolute encoders as being the opposite of relative or incremental encoders.
They are opposite in the sense that absolute encoders do not lose their position when electrical power is lost. Their ability to track and report the current position is not dependent upon or determined relative to an outside reference point. The position that an absolute encoder reports is derived internally within itself.
Relative or incremental encoders do not provide this absolute position retention characteristic because they can track relative changes in position from a known starting point only. If electrical power is lost, the system that incremental encoders are a part of must perform an indexing home cycle or command a move to a hard stop to determine absolute position.
So when electrical power is restored, the absolute encoder will be able to report back the correct current position immediately to the control system it is a part of. Even if things moved while power was off, the absolute encoder will still know the correct position to report when power comes back on.
So What Makes An Absolute Encoder So Special On The Inside?
What gives the absolute encoder the ability to retain its position when electrical power is lost? One method for accomplishing that is to put a special Gray code pattern on the disk or linear strip inside of the encoder.
Here is an example of a disk with a special coded pattern of opaque black areas on a clear or reflective disk. This is an example of a disk that might be found in an absolute encoder.
A closer examination of the disk reveals that there are nine tracks of lines on nine concentric circles within the disk. The outermost circle contains 128 lines, the next circle in contains 64, then 32, then 16, then 8, then 4, then 2, and then the two innermost circles contain one line each.
This absolute encoder would have nine optical sensors all in a row extending out on a radius from the center of the disk. Each of the nine sensors would sense the presence or absence of a black line in the circle of lines it is positioned over. If a line is present, the sensor output would be high or in a logic “1” state. If the line is absent, the sensor output would be in a low or logic “0” state.
Using the outer nine circles of lines, this absolute encoder would be able to sense 512 unique positions in one 360-degree revolution of the disk. The 512 positions would have Gray code binary outputs of 000000000, 000000001, 000000011, 000000010, 000000110, 000000111, and so on. Each of the 512 positions around the disk could be identified by a unique 9-bit binary code that the encoder would present at its outputs.
Here is a simpler 3-track version of the 9-track disk shown above:
Having a 3-track pattern on the disk will make it possible to sense eight unique positions as the disk rotates in one complete 360 degrees turn. The eight binary Gray code outputs generated by an absolute encoder using this disk would be 000, 001, 011, 010, 110, 111, 101, 100, and then back to 000, and so on for each revolution of the disk.
After analyzing the two absolute encoder disks that we have looked at, it should now be apparent to you as to why an absolute encoder can retain its position even when electrical power is lost and even if movement happens during the loss of power. The combination of nine or three optical sensors looking at 512 or eight unique position codes is the key to it all.
When To Choose An Absolute Encoder For Your Application
Now that we know all of this about an absolute encoder, how do we determine if our particular application requires utilizing such an encoder?
Applications where it is critical that the current position of something be known immediately upon power up or after the loss of power, require the use of an absolute encoder. Or in applications where an indexing home cycle or running up against a hard stop cannot be performed to determine position, would also require the use of an absolute encoder.
A simple example of this would be where an encoder is being used to report the wind direction that a wind vane is indicating as part of a weather station. If power is lost and then regained to the weather station, the wind direction needs to be known immediately. It would be a bother and impractical to have someone manually re-calibrate the wind vane each time after a power loss.
Types Of Absolute Encoders
So far just single-turn absolute encoders have been discussed. Single-turn absolute encoders are good for use in applications where the position of something within a zero to 360 degrees range is all that is needed.
For applications where the number of revolutions must also be kept track of and reported, there are multi-turn absolute encoders.
One method of achieving a multi-turn absolute encoder is to include some gearing inside the encoder. If for example the gearing has a 10:1 ratio, then it would take ten turns of the outer shaft of the encoder to make the disk inside the encoder turn once. So now the range of the encoder would be ten turns or 3,600 degrees.
There are also linear absolute encoders for tracking and reporting straight-line back and forth movements. A typical application for an absolute linear encoder would be a set of digital calipers as shown in the photo below.
Get In Touch And Let’s Kick Some Ideas Around!
It is hoped that after reading this article, you now have a better understanding of what an absolute encoder is. Maybe you even have some ideas on how you could put one to use in an application of your own. I would love to hear about it.
If you are needing a single-turn absolute rotary encoder for an application that you have in mind, click here to check out the specifications and pricing on one made by Calt. If you need a multi-turn absolute encoder for your application with or without a digital display, click here.
Please feel free to leave comments below with your questions, thoughts, and application ideas. I would be more than willing to discuss the topic of absolute encoders and answer any questions you may have.
Thank you for taking the time to read my post.