Encoders are a necessary element in precision motion control. They are required to be used on servo motors to commutate the motor and give precise feedback of position. Additionally, they can be useful on stepper motors even though they aren’t needed for commutation. However, there are several different varieties of encoders available, and they all have advantages and disadvantages depending on the motion application in question. In fact, choosing the proper encoder will have an impact on how effective the application can turn out.
So, the question then becomes, “What type of encoder should someone choose for their motion control application?”
The answer lies both in the types of encoders that you can use and the other components that you select to run your motor. Selecting an encoder, however, has more nuance to it than one may think.
One of the most utilized types of encoders for a rotary or linear servo motor is a digital incremental encoder. Not only are they the most common type of encoder used on servo motors, but the technology is so mature and robust that they are compatible with almost every servo drive on the market.
Digital incremental encoders
Digital incremental encoders offer a wide range of resolutions, physical and electrical form factors, and most importantly, price points. They are available from a very wide range of manufacturers and are relatively easy to use. This, of course, adds to their appeal.
There are, however, some drawbacks to using incremental encoders. First and foremost, they require the operator to home the motor on system power up. This can be a cumbersome task, especially if there is a lack of experience doing so.
Next, with a digital incremental encoder, there is the potential for electrical noise interference. This is typically only a problem if the cabling is not shielded, though. This type of encoder requires a large number of individual wires inside the encoder cable itself to carry the electrical signals back to the servo drive. Finally, the length of the encoder cable is limited. This is mainly due to increased electrical resistance present over long cable runs.
Analog encoders
The second most common encoder technology on the market is the analog encoder. Instead of using two digital square wave pulses to keep track of position, like a digital incremental encoder uses, the analog encoder uses a pair of offset sinusoidal waveforms. These encoders provide a voltage or current output that will vary linearly depending on its position. Similar to digital incremental encoders, an analog encoder is a mature encoder format that can be used in a wide range of servo drives.
One of the biggest advantages inherent to an analog encoder is the higher resolution that can be achieved when compared to digital incremental encoders. Furthermore, this achievable higher resolution is usually not fixed. Alternatively, it can be specified by the user from a fixed number of options depending on the type of motion controller or servo drive that is used.
There are some noticeable drawbacks to using analog encoders, though. Specifically, the servo drive cost may increase as having an analog encoder input is usually a more expensive option on some servo drives.
The maximum speed of the motor also may be limited by the maximum frequency that the servo drive is capable of taking in. The cost is another area of concern. The price associated with the encoder read head and analog tape scale is almost certainly going to be more than using a comparable digital incremental encoder.
Similar to the digital incremental encoder, an analog encoder requires the motor to be homed on system power up and again, the encoder cable overall length is limited due to the increased electrical resistance over long cable runs.
Absolute encoders
Recent improvements in encoder technology now allow users to avoid re-homing the motor every time it’s powered on, while also enabling more accurate position monitoring in their motion through increased resolution of the encoders themselves. These types of encoders are called absolute encoders, and they can keep track of position through power cycles of the control electronics that run the encoders.
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There are various types of absolute feedback, including EnDAT, SSI, BiSS, Hiperface, Panasonic, Sanyo Denki and Tamagawa. Additionally, these different types of absolute feedback can come in either single-turn or multi-turn formats. Single-turn absolute encoders keep track of position across one revolution of the motor shaft only and will effectively reset after crossing a 360° position back to zero. Multi-turn encoders, however, do the same thing as a single-turn, but they will also keep track of how many total revolutions have gone by.
Absolute encoders have their own set of clear advantages, with one of the most important features being the fact that there is no need to home the motor on system power up. With absolute encoder technology, the position is saved by the encoder during power down situations.
There is also a higher possible encoder resolution with this technology when compared to a digital incremental encoder. The resolution is very much on par, and sometimes even greater than analog encoder capability.
As opposed to the other two encoders discussed, absolute encoders also have a lower total number of electrical wires overall. These wires also have relatively good electrical noise resistance if the cable is shielded and grounded properly.
There are, of course, a few drawbacks when selecting an absolute encoder. There is typically a higher overall cost of both the encoder itself, and the servo drive as it must now be configured to take in absolute encoder feedback.
There is also a convenience/compatibility factor at play. Not all servo drive manufacturers offer absolute encoder feedback options, and very few manufacturers offer the entire range of absolute encoder feedback options on a single drive. Also, some absolute formats are proprietary to the manufacturer.
The key takeaway is that decision-makers don't always have a clear understanding of what their application truly requires. It is always best to work with a professional in the industry that can thoroughly and clearly walk someone through the decision-making process to ensure that the best encoder technology is selected for a given motion control application. As an example, I once had a customer that was running a limited travel, linear ball-screw stage and they wanted to be able to immediately run the stage after a power down event. This customer did not want to re-home the stage after power-up. This stage had limited travel that was equal to about 50 mm overall, so we were able to select a rotary servo motor with a less expensive multi-turn absolute encoder simply because we did not have to keep track of a large number of turns that a higher resolution multi-turn absolute encoder would be able to keep track of. The stage had a 2-mm pitch screw on it, so over the entire range of travel the rotary servo motor would only have to make 25 total revolutions to span the entire range of linear travel on the stage. The customer was able to save money by selecting a lower multi-turn resolution absolute encoder.
