Stepper and Encoder noobie question

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11 May 2018 19:04 #110586 by KcChris
First, I'm new to the forms and not sure this is in the correct thread.

My question is, I've read a lot on the setup of stepper with encoders. I have a working mill with steppers.
I understand that with the encoder, you can make up or see missed steps.

Is that it? or can you "tune" the stepper like you can a servo?

I know on servo motors ( large CNC machines ) you can tune for servo mismatch.

I have encoders but not sure it's worth the time to put it on the machine. Just trying to get a better understanding.

Thank you!
Chris

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11 May 2018 21:23 #110587 by Mike_Eitel
Hi Chris
When a stepper ist missing steps ( because it can not follow the commanded steps caused by overload) it normaly stalls.
So it's a black and white, go/nogo situation. As long as you stay in the physical given conditions a stepper is digitaly precise.

Servos are different. They are driven based on slippage principle. There you have always a definened error that is corrected by a regulating device. For this you need feedback = encoders. In principle you will never be as precise as with a stepper, but can spare a lot of energie.

Stepper always go full throttle, servos are more economic but have to hide their error.

Every stepper motor can be run in slippage (servo) mode when according driver is used. Closed loop steppers are such a chimeira.

m5c Mike

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11 May 2018 21:34 #110588 by rodw
A properly designed and configured stepper system should never loose steps. If you have enough load to cause the machine to miss steps, the motor's power is maxed out so it is unable to catch up the missing steps anyway. So the whole concept of closed loop stepper control is pretty useless. My vote is not to bother with adding your encoders.

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11 May 2018 21:47 #110589 by KcChris
Ah, ok. I thought maybe there was something big I was missing out on, but that doesn't sound like the case.

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13 May 2018 03:00 #110632 by jmelson
OK, the big difference is the motor is limited by heat. Steppers run at full current all the time, unless the drive has a current reduction feature when that axis is not moving. Servos apply just enough current to get the error down to the desired level. Therefore, they usually run MUCH cooler than steppers, generally the servos will appear completely cold. Because they are cool, they can be asked to "try harder" by increasing current when needed, such as when accelerating or a heavy load is applied.

The ratings on steppers are the holding torque, ie. the torque that is required to make them jump steps at standstill. When moving, the available torque is less, and at a certain "crossover" speed, the torque begins to drop rapidly.

The ratings on servos are typically the continuous stall torque, but the peak rating can be much higher, often 4X is common. So, when positioning error is detected, more current is delivered by the drive to correct the error. This error can be extremely small, just a few encoder counts. Because of the way servo motors are built, the speed at which available torque falls off is typically much higher. The typical stepper motor is a 50-pole motor, so the current in each winding reverses every 2 steps, or 100 times/revolution. At 600 RPM, or 10 revs/second, each motor phase needs to be reversed 1000 times/second. A two-pole brush motor would only reverse current in each armature winding twice/rev. At 600 RPM, that would be 20 times/second. Even an 8-pole brushless motor would only require the phases to reverse 160 times/second.

Since all windings have inductance, which resists change in current over time, the more time you have to reverse the current, the faster the motor can spin.

Jon

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