In this article, an incremental vs absolute encoder comparison will be made. A brief description of both incremental and absolute encoders will be given along with pointing out some characteristics, advantages, and limitations of each.
An explanation of how adding a third channel INDEX to an incremental encoder turns it into a semi-absolute encoder, will also be given.
An encoder hybrid where an incremental encoder and absolute encoder are ganged together on the same shaft will be described, and an explanation as to why such a hybrid combination is sometimes necessary will be given.
The article will end with a discussion on how to choose the right type of encoder for your application.
It is hoped that by reading this article, you will be familiar enough with both incremental and absolute encoders, and their variations, to make an informed decision as to which one is best suited for your application.
The Incremental Encoder – Think Relative Change In Position
In a motion control system where an incremental encoder is being used as the position feedback device, the starting position of the encoder is not known when electrical power is first turned on to the system. So this is the first big difference between an incremental encoder and an absolute encoder that will be looked at.
When it comes to incremental encoders, think of the word “relative”. Incremental encoders are great for use in applications where measuring relative changes in position is what is important, and knowing the absolute position of the encoder when the system is first powered on is not required.
An example of such an application would be where an incremental encoder is being used to measure how much electrical wire a customer has pulled off of a spool down at the store before cutting it and making a purchase. In this application, the starting point is assumed to be zero before the customer starts pulling wire off the spool, and the relative change from that zero starting point is all that is needed to be measured.
In general, here are some advantages that an incremental encoder may have over an absolute encoder. Usually lower in cost, smaller, lighter, and capable of working at higher mechanical speeds or RPM’s than an absolute encoder.
The Absolute Encoder – Think Never Being Lost
Absolute encoders are used in applications where knowing the exact position of something at all times is required. Absolute encoders can retain their current position reading even when there is a loss of electrical power to the system that the absolute encoder is a part of. And if there is a change in position during the loss of electrical power, the absolute encoder will still be able to report the correct position reading immediately when electrical power is restored to the system.
Absolute encoders are usually rated in terms of their number of bits of resolution. For example, a rotary absolute encoder that has 12 bits of resolution, can report 2 to the 12th power, or 4096 different locations around a 360 degrees circle. This means that such an encoder can know and report its current position down to 0.088 of a degree resolution.
Absolute encoders are available with parallel outputs and serial outputs. A 12-bit resolution absolute encoder with parallel outputs would require a minimum of 14 wires going to it. 12 wires for the 12-bits of output, and then 2 power supply wires for power and ground. Additional wires are often included to include optional features, special functions, etc….
Absolute encoders with a serial output require fewer wires since, as in our example, the 12-bits of position data are sent out in a serial stream of bits. Also, since the position information is coming out as a serial stream of bits, the data can be reliably sent over a longer distance than what can normally be achieved with parallel outputs.
The Calt line of absolute encoders are good examples of reasonably priced absolute encoders that will work well in a variety of different motion control and robotic applications. Both single-turn and multi-turn versions of them are available. For specifications and pricing for the single-turn version click here. For specifications and pricing for the multi-turn version with or without a digital display, click here.
This ability to always know and report the current position reading is the main advantage that the absolute encoder offers, but there is a price to be paid for this advantage. Absolute encoders when compared to incremental encoders are in general more expensive, larger, heavier, and not as capable of operating at high mechanical speeds or RPM’s.
The Third Channel INDEX – Making The Incremental Semi-absolute
A clever option that some incremental encoders possess is a third channel output called the INDEX channel. A standard incremental encoder has two channels of output commonly referred to as the “A” and “B” channels. These two channels output a pulse or electrical square wave cycle for each line on the disk. There may be hundreds or even thousands of lines on the disk resulting in hundreds or thousands of pulses or electrical square wave cycles being generated for each complete revolution of the disk.
The “Z” or INDEX channel outputs a single pulse for each complete revolution of the disk, and always at the precise same position or point in the revolution. This is useful, because now each time an INDEX pulse occurs, the absolute position of the disk is known.
So the INDEX option makes the incremental encoder semi-absolute in nature. By semi-absolute, it is meant that when electrical power is first turned on to the motion control system, the absolute position of the encoder is still not known, but as soon as the encoder disk turns enough to cause the index mark on the disk to pass by the INDEX channel sensor, an output pulse on the “Z” channel will occur, and now the absolute position of the encoder will be known.
A typical application for an incremental encoder with the third channel INDEX option would be on the shaft of a stepper motor where it is desired to know the absolute position of the stepper motor’s shaft in degrees within a revolution. It would have to be an application where doing a home cycle on power up would be allowable.
In other words, when the motion control system first powers up, a command would be sent to the stepper motor controller to rotate the shaft of the stepper motor until the INDEX pulse is detected. Then the system would be positionally calibrated and could perform all the needed motion control tasks from that point.
The Hybrid Encoder – Ganging Incremental to Absolute
In some applications, the high operating RPM speeds that an incremental encoder is capable of operating at, and the absolute position reporting capability of an absolute encoder are both needed. In such a scenario, an incremental encoder and an absolute encoder can be ganged together on the same shaft.
When the motion control system first powers up, it gets the absolute position reading from the absolute encoder. The motion control system then switches away from the absolute encoder, and uses the incremental encoder to track changes in position from there.
This would be an example of using and taking advantage of the best features of both types of encoders in a single application.
Tips On Choosing The Right Encoder For Your Application
So in conclusion, here are some tips on choosing the right encoder for your application:
1. Ask yourself the following questions. In this application, is it required to know the position of the encoder immediately upon powering the system up? And performing a home cycle or moving to a hard mechanical stop is not acceptable or possible? If you answered yes to both questions, then you will need to choose an absolute encoder.
2. If you answered “no” to either of the above questions, then an incremental encoder should be chosen for your application.
3. If keeping track of position within a limited range of motion, such as within one revolution or less is all that is needed, then consider choosing an incremental encoder that has the third channel INDEX option.
4. If the application is multi-turn in nature, or involves measuring relative changes in position from any starting point, or involves doing a home cycle or moving to a hard mechanical stop at power on, then just a standard 2-channel incremental encoder without the INDEX option should work fine in your application.
Leave your questions and comments regarding this article below. I would enjoy getting your feedback and suggestions on how to make this article even more helpful and informative.
By the way, if you need a digital display to use with standard 2-channel incremental encoders, a company called CNL Devices Inc makes an excellent dual encoder version that is small and relatively inexpensive. For more information email email@example.com.