Episode #50: Feedback Technologies for Motion Control Explained?

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The Motion Control Show

The Internet is already awash with just a ton of information about feedback, feedback technology, and new products.  So, I'm just going to provide you here with some basic terminology and technologies that most everyone in the industry refers to.  That's going to be your foundation for learning about other products and technologies out there on the Internet.  I’m Corey Foster at Valin Corporation.  I hope this helps.

A few different sensor technologies are:
  • Optical: a light source going through typically a slit to some photo detectors.
  • Magnetic: typically a magnet being turned near a sensor like a little chip right here detecting the transition from North to South.
  • Inductive: like an inductive proximity sensor, detects whether there is metallic material there or not.

Tachometers are typically used on the back of AC motors to measure how fast they're going.  Like the tachometer on a car, it gives a voltage per RPM, and that voltage is an analog output.  It's linear from the lowest to highest speed until maybe it gets to the highest speeds. 

Incremental encoders have slits in them typically, so this disk may have 1000 lines, or 1000 slits, in it where the light source is shining through those slits.  Then there are photo detectors on the other end.  Now you see here that it puts out a sine and a cosine wave.  The B sensor is actually offset by 90 degrees electrically, and as the light goes past that split, it doesn't just immediately go off, it actually diffracts a little bit, so you get a fade in and fade out of that light.  So that's why it's basically a sine wave.  It has to go through some squaring circuitry just to measure the voltage and compare it against itself and that produces square waves.  So, if you have 1000 lines, or 1000 slits, around this disk, you're going to get 1000 pulses on the phase A and 1000 pulses on phase B.  Again, notice that they are offset by 90 degrees.  This is called pre-quadrature.  A 1000-line encoder is 1000-lines pre-quad.  However, after you square it, the electronics that are reading this actually count it for every transition: 1, 2, 3, 4.  That's four counts for every line.  So, the post-quadrature count is actually 4000 counts per revolution. 

There is absolute versus incremental.  The incremental, like I already said, has each position that outputs an identical 5-volt square wave.  Absolute, on a similar disk, is going to have a discrete position at each different slot, so this is a binary code.  This is pretty much an old style.  There are newer encoders that do things different ways, but the concept is the same, where each location all the way around this disk is unique to that location on the disk.  Now there’s single-turn and there's multi-turn and there are a lot of different types of absolute encoders out there.  The single-turn can only read the absolute position within one turn of that motor.  The multi-turn has maybe a memory in there to remember how many times it's gone around, so it has to have a battery that backs it up.  Or maybe it has gears that keep track of how many times it's gone around so it knows the difference between one turn and 10 turns.  That may or may not be important in your application.

Resolvers are analog devices.  Here's the rotor.  As it turns, these stators read the rotation of the rotor.  There's an excitation voltage that goes into it and then the stators read that inductively much like a transformer.  Notice that these are offset here, so one creates a sine wave and the other one creates a cosine wave which is intentional.  This is through one electric cycle which may be 1 revolution of the motor, one revolution of the resolver.  It's actually absolute all the way around.  Now, this isn't necessarily used for absolute positioning, but it is used for commutation.  See a previous episode on that one, but this is an analog signal.  Resolves are typically much more robust then the incremental glass-based or disc-based encoders.

Now there's sine/cosine and there is SinCos.  Don't be confused so much by the terminology.  Some people will say SinCos referring to sine/cosine.  But SinCos is actually a trademarked name for one particular brand that uses a sine/cosine signal.  Sine/cosine looks just like the resolver output.  I use the same graph.  It has one signal that’s sine, another signal that's cosine.  It's typically only one-volt peak-to-peak though, so it's a pretty low level signal.  A drive can use the signal, or often times it's digitized.  It takes a certain slice of it and only gives so many pulses.  If it's incremental or absolute information, it gives so many pulses and then it can change the resolution with a little interpolator box and you get more counts here.  Now, SinCos essentially has that same technology, it's just a particular brand.

Those are a bunch of different protocols, but then there's other communication protocols that are used out there in the motion control world.  There's EnDat, there's SinCos, there's SSI, which is serial-based communication.  There's BiSS-C, which is one of the newer and more popular ones.  And then there's some slower ones that are really only used for conveyors and process control applications: Profibus, DeviceNet, Ethernet IP.  You're not going to see those on the back of a servo motor going to a drive for that communication.  They’re too slow.  And, in the case of EthernetIP, indeterminate, which means you don't know how often you're going to get that update.  Then there's a few other protocols.  You'll see encoders with CANopen and EtherCAT.  Now those are fast enough to put on the back of a servo motor, but I have yet to see them be used between a servo motor and drive.  However, they certainly are fast enough.  But EnDat, SinCos, SSI, BiSS-C, those are definitely ones that you'll see on the back of a servo motor.

That's it for some pretty basic encoder, feedback and resolver technologies and terminologies.  I’m Corey Foster at Valin Corporation.  Reach out to us here at TheMotionControlShow.com or this email address and follow our hashtag: #motioncontrolshow.  We are happy to help.

If you have any questions or are just looking for some help, we're happy to discuss your application with you.  Reach out to us at (855) 737-4716 or fill out our online form.