Episode #51: Types of Feedback for Motion Control

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The Motion Control Show

By now you might be wondering…
  • What types of feedback are there?
  • What protocols do feedback devices communicate over?
  • What are the different form factors of feedback?
These and a couple other variations are what we're talking about today.  I'm Corey Foster at Valin Corporation.  Let's see what we can learn.  Let's start with a couple of variations because these will apply to all the different form factors that we're going to touch on here. 

Incremental versus absolute:  I may have mentioned this before, but it's definitely worth mentioning again.  If you have a guy running and he runs forward 10 steps and then he runs back 10 steps, that's incremental.  But if he runs to a position of minus 10, that's absolute, and he's going to end up a total of 20 steps back from where he ran out to.  So, coming back minus 10 is incremental.  Coming back to minus 10, the position of minus 10, which is 20 steps, that's absolute.

Single-turn versus multi-turn:  I mentioned this before for absolute encoders.  A single-turn absolute encoder is a lot like just hitting a stopwatch and only having 60 seconds, or one minute maybe, or one hour, showing on your stopwatch and you can't keep track of it beyond that.  Multi-turn however is more like a clock where you have the hour hand that keeps track of the hour.  Now it can only go up to 12 or 24 hours depending on the type of clock, but that's multi-turn.  Absolute encoders have that actual problem as well, where they can only go up to so many turns.  A lot of them go up to 4096 turns.

Resolution: like the resolution on a ruler, this is how fine of a distance you can measure.  How many counts per distance…which is often times how many counts per revolution of an encoder or other feedback device. 

Protocols: there are a bunch of different types of protocols.  There are the higher speed ones for motion control.  There are lower speed ones for process control.  Here are three that we've talked about:
  1. The quadrature signals for incremental.
  2. Analog for 0 to 10 volts like a tachometer.
  3. Then there are sine/cosine types that are analog.
And then there's a whole bunch of different communication protocols: Ethercat, Profibus, Profinet, SSI, serial ones like RS232 and 485; I mentioned SSI which is based on RS485; DeviceNet, EnDat, which is proprietary, CANopen.  A lot of these are closed protocols more than others.  Some are proprietary, but there are a whole bunch of other ones out there as well, so just kind of keep an eye out for that.

There are rotary encoders.  This one would go on the back of a motor.  It would bolt or screw onto the back of a motor here and it would go onto a rear shaft of the motor.  So that's a rotary encoder.  But then there are other types of rotary encoders where this would couple to the shaft of a motor, or maybe a conveyor belt or a rack and pinion or the wheel of something.  This is typically going to be mounted on to mechanics, and so it's going to have a more industrial body so it can be hit and hammered on.  Perhaps it's going to have a different type of communication protocol then this one.  This one is going to be more for the motion control, so it's going to have SSI or even EtherCAT that is high speed, whereas this one here is going to be more like Profibus or DeviceNet or CANopen…stuff that's going to be more for the process control.  They are for much different types of applications, but they're both rotary.

There are linear ones which have a lot of the same technology as rotary encoders, but this is a linear encoder here.  There are other options for linear.  There's LVDT, which has a lot of different meanings.  All have the same type of feedback.  There are wheels where this wheel rolls on a conveyor or a surface, and it translates from rotary to linear measurement.  There are cable-types where this cable comes out, again converting this linear into a rotary measurement.  There's basically a rotary feedback device in here and it just rotates with this cable.  Just pull it out.  Then there are even laser ones that can be used for high precision measurement over a linear distance.  That leaves me with one other type that is worth mentioning which is a ring.  Now I didn't put this with rotary because it's essentially the same type of technology as a linear encoder.  It's just that the tape for the linear encoder is put on the outside of the ring, so it's a bigger diameter then the Rotary ones that I was referring to earlier.

I'm Corey Foster at Valin Corporation.  I hope that helps.  Reach out to us at The Motion Control Show or this email address here.  We're 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.