The word “encoder” comes from the word “encode”. Reading the dictionary definition of the word “encode” leads one to conclude that to encode something means to change it from one form into another. In the realm of motion control and robotics, if someone were to ask, “What is an encoder?”, a good starting point would be to say it is a device that changes or converts mechanical motion into coded electrical signals.
This article will take a look at a few different models of encoders that are on the market today. By examining their features and specifications, the type of electrical output they generate, and which applications they are best suited for, the reader should come away with a better grasp and understanding of encoders in general.
This better understanding and increased knowledge of encoders should help when it comes time to actually purchase one for the application you may be working on.
Quick Assembly Kit Encoders
Probably the most popular and widely used encoders on the market today are the quick assembly kit encoders. They are the least expensive, highly reliable because of their simplicity and lack of bearings to wear out, and can be easily installed onto a shaft.
They are most often found installed onto the shafts of stepper motors or other types of electric motors. The encoders, more often than not, are of the incremental type that output 2-channel A/B quadrature signals.
The quadrature signals are used by the motor control electronics to determine position, speed, and acceleration of the motor shaft. In this case, the incremental encoder is converting rotational motion into a stream of 2-bit binary Gray code.
If you are not familiar with terms like 2-channel A/B quadrature and 2-bit binary Gray code, please refer to the article covering those topics found at:
US Digital manufactures quick assembly kit encoders. Their E2 encoders are widely used in motor control applications. As can be seen in the drawings below, the E2 is available with many housing options that the customer can choose from to best fit the application at hand.
Here is a photo of the E2 encoder showing it with the cover removed.
The Hub/Disk assembly is shown along with the brown optical encoder module. Notice that there is a setscrew in the hub of the Hub/Disk assembly. The purpose of the setscrew is to secure the hub to the motor shaft.
The hubs are available in bore sizes to fit shaft diameters ranging from 2 mm to 10 mm in diameter. The disks are available with as few as 32, to as many as 5000 lines on the disk.
The electronics inside the encoder module generate the quadrature signals that are sent to the motor control electronics. The output signals from the module are 5V TTL compatible.
The electronics of the module can electrically track at rates of up to 360,000 lines per second going by. That means that a disk with 5000 lines on it would work with the module at speeds up to 72 revolutions per second, or 4,320 RPM.
Shafted And Panel Mountable Encoders
The next type of encoder that will be looked at more closely is what is known as shafted or sometimes panel mount encoders. In terms of the electronics and type of output, they are identical to the E2 encoder described above. The big difference is that encoders of this type have a shaft built into them. This means that they have either a shaft that turns on ball bearings with very little friction (spins free), or they have a sleeve bushing movement with drag on the shaft to make it feel like a potentiometer when turned.
US Digital’s S1 encoder is a good example of this type of shafted or panel mount encoder. The S1 is pictured in the photo to the right. You can see that it looks like an E2 encoder, but with a shaft incorporated into it.
Hollow Shaft Encoders
Hollow shaft encoders are a kind of hybrid between the quick assembly kit encoders like the E2, and the shafted or panel mount encoders like the S1. Hollow shaft encoders have a shaft built into them, but the shaft is hollow so that the encoder can be installed onto a shaft such as the shaft of an electric motor.
Shown in the photo is the HB5M model of hollow shaft encoder made by US Digital. The hollow shaft has a ball bearing movement for low drag. The slotted mounting bracket shown serves the purpose of anchoring the encoder in place keeping the encoder from trying to turn with the shaft. A setscrew in the hollow shaft locks it to the shaft that the encoder is mounted on.
Electrically, this HB5M encoder is identical to the E2 and S1 models that have been previously discussed above.
Inclinometers – Turning Tilt Angles Into Electrical Signals
The last type of encoder we will look at in this article is an absolute inclinometer. As you might have guessed by the name, inclinometers convert tilt angles into electrical signals.
The example of an inclinometer that will be used for this discussion will be the A2T absolute optical inclinometer manufactured by US Digital. This inclinometer has a weighted disk that is rotated by gravity as the tilt angle of the inclinometer changes. Very low friction ball bearings are used to make the A2T very sensitive to small tilt angle changes, and to help minimize bearing stiction.
There is a special barcode-like pattern printed on the disk that the optical array of sensors in the A2T can translate into up to 4,096 unique positions in one 360 degrees rotation of the disk. This resolution of 4,096 positions or codes per revolution means that the A2T inclinometer can report changes in tilt angle as small as 0.088 of a degree.
Instead of outputting 2-channel A/B quadrature, the A2T outputs two binary bytes of position code each time the system host sends a position request command to it. It does this serially over a 6-wire RS485-like interface consisting of signals DATAH, DATAL, BUSY+, BUSY-, +V POWER, and GROUND. A serial bus to USB adapter is available to make it easy to communicate with the A2T by way of a standard USB port.
An optional analog voltage output feature is also available with the A2T. The analog output option comes factory set to output 0 to +3.599 volts in a tilt angle range of 0 to 359.9 degrees. So each milli-volt of change at the analog output represents 0.1 degree of tilt angle change.
Let’s Sum It All Up
I hope that you have enjoyed taking this tour with me of this sampling of various kinds of encoders available on the market today. There are many more types and models of encoders that were not mentioned, but the examples we did cover should give you a good foundation for doing further research.
I would more than welcome your comments and questions. You can leave your comments in the comments section below. Thank you for taking the time to read this article.