My previous blog Encoders Know All the Angles introduced encoders that provide information about rotary positions. A diagram illustrated the phase relationship of signals from an incremental encoder. They indicate rotation direction and counts per revolution.
A careful look at the signals reveals a way to increase resolution fourfold. The figure shows the 2-bit binary codes produced by an incremental encoder's A and B outputs. Measurement systems that count pulse edges or use the binary codes can improve resolution. Thus if you have a 500-count-per-revolution (CPR) encoder, by monitoring the codes or counting pulse edges you obtain 2,000 CPR.
Out-of-phase detectors produce two pulse signals that indicate relative rotation as well as direction for an incremental encoder. Counting edges or monitoring binary values can increase resolution fourfold.
Because incremental encoders produce only pulses, they can continue to rotate through many 360-degree turns. A basic absolute encoder, though, will produce the same set of codes through each complete revolution. When you must track absolute position through several complete shaft rotations, a multi-turn absolute encoder will do the job. This type of encoder counts revolutions mechanically or electrically. Think of a 24-hour mechanical clock movement that indicates the day of the month and you understand the principle. The multi-turn absolute encoders also offer high resolutions.
A mechanical revolution counter uses a geared mechanism that rotates, say, 1/8 turn per complete shaft revolution. This mechanism would have its own 3-bit (23) code wheel that would count 0 to 7 revolutions. By cascading precision gears and adding encoding wheels, a multi-turn absolute encoder with four geared 3-bit encoders could count 4096 (212) revolutions, for an added 12 bits of resolution. This type of true multi-turn counter costs more than a basic encoder and the gear mechanism requires additional space. The mechanism inherently handles changes in rotation direction and it continues to keep track of any position changes during power loss. The electronic outputs include position and direction information in various formats.
An electrical approach removes gears and provides circuits that count the number of shaft rotations and determine the direction of rotation. This type of pseudo-multi-turn encoder requires either an internal battery or an external power source to maintain the rotation count. Batteries require replacement and as temperatures rise, battery life decreases.
A new technique, though, uses an encoder's shaft rotations and the Wiegand effect to generate electricity to power rotation-counter circuits. A coil wound around a small magnetically "soft" core of Vicalloy creates a miniature magneto. When exposed to a magnetic field, the core changes polarization in a few microseconds and creates a current sufficient to power the non-volatile counter. The counter wheel has alternating magnetic fields, N-S-N-S... around its perimeter. A Hall-effect sensor detects the direction of rotation. This type of encoder usually has a resolution of 12 or 13 bits and maintains position information even when not powered.
I hope to see more of the Wiegand effect encoders in the near future. It might be a good alternative to optical encoders in a closed loop stepper motor system. Which, by the way, are getting cheap enough for anyone to buy.
Great article - I particularly liked the mention of using hall effect sensors. Makes sense since historically they have been used as rotary position gear tooth sensors such as crank and camshaft sensing in automotive applications. They are usually a lot cheaper than optical encoders too.
Hello, TJ. As far as I can tell, the magnetic field for a Wiegand detector must be very localized, so magnetic interference shouldn't become a problem. Resolvers get used with equipment that uses electric motors, so I bet the manufacturers have added shielding as needed. Worth asking about before you buy a resolver, and worth the time to run some tests in proximity to specific equipment.
In my applications, I often need an axis with a hollow bore, so the position encoder either needs to be hollow. From what I can tell, the Wiegard effect only works when the rotating magnet axis of rotation is centered about the Wiegard wire. I'm sure I've misunderstood the tech in some way and hope that my understanding is wrong.
If my understanding is wrong and the axial rotations do not have to align, then I'd be inclined to merge an optical absolute encoder with a Wiegard sensor for counting rotations. You'd get the resolution of optical with the persistent turn counting of the Wiegard sensor.
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