Encoders for a Stepper System with a 7i85s

Closed loop stepper drives _are_ servo systems. They just use 2 phase 50 pole
motor rather than 3 phase 4 or 8 pole motors. Because of the high number of
poles they may have advantages on smaller systems that need higher torque
at a lower speed. (a side effect of having 50 poles)

When closed loop stepper motor stalls can driver recover the motion(to some degree like servomotors) or will throw an alarm and stop?
Higher number of poles in closed loop steppers except from increased torque have also increased positional accuracy (easier tunning)?

Utilitas wrote:

The more pertinent detail that still hasn't been covered is encoder compatibility with a 7i85s, more specifically, what is the
practical minimal signal current that the mesa input can reliably read. From what I saw on the CUI documentation in the first
post, it looks like that type of encoder may not have very strong signal output.
 

Well that type of info is always clearly specified in the Mesa card manual. I didn't look at it but PCW's the guy that makes the cards and he says it needs to be able to sink 2.5mA while your datasheet says the max output current of that encoder is 2mA it's probably not the greatest match. Probably a good thing you looked at that as I never really do, 2mA max output seems extremely low to me.

Utilitas post=300264 userid=38943

My comment regarding "fullstep output" was not meant as "encoder output" but rather as an alternative to a closed
feedback loop, ie. monitoring the "fullstep output" from the driver to throw a fault in the event of a stall. This was a thought 
that came to mind after reading discussions in several places that suggest closed loop stepper systems are not worth it.
Anyhow, please forget that I brought that up, the more useful topic is bebunking the usefulness of a capacitive encoder,
and outlining what the minimal signal current constraints for a 7i85s really are.

As PCW mentioned, that's the purpose of the CL stepper drive's alarm output. You don't have to monitor pulses and deal with a control loop, the drive does that, you just monitor a digital output...pretty simple.

Not sure where you are reading that closed loop stepper systems are "not worth it" and what the context is but the sentiment around here is that it's more worth it than the setup you want to use, that includes the guy who makes the Mesa cards. If you already have the 7i85s and want to use it stepper online sells optical motor encoders separately or you can just buy the closed loop motor, they'll all output at least 5mA

PCW wrote:

Closed loop stepper drives _are_ servo systems. They just use 2 phase 50 pole
motor rather than 3 phase 4 or 8 pole motors. Because of the high number of
poles they may have advantages on smaller systems that need higher torque
at a lower speed. (a side effect of having 50 poles)
 

I think everybody generally understands that a "servo motor" is technically any motor with a feedback device. Colloquially even the motion control industry typically does not refer to a closed loop stepper as a "servo motor"

.....but yeah, I've found AC servos tend to struggle with stall torque a much smaller stepper has no problem with.

vre wrote:

When closed loop stepper motor stalls can driver recover the motion(to some degree like servomotors) or will throw an alarm and stop?
 

Yes

vre post=300272 userid=18514Higher number of poles in closed loop steppers except from increased torque have also increased positional accuracy (easier tunning)?

No not really. Stepper can only position itself as an increment of a microstep. AC servos are practically infinitely able to posititon as long as the drive can interpolate the position vs the encoder resolution. Difference is a stepper can hold that position like a rock until the torque is overcome. Servos tend to just try to snap back into position like a rubber band. Motor brakes help out a bit.

As with everything, quality industrial motors cost more but from expereince, there is no comparison between Steppers on Line commodity drives and industrial quality drives from the likes of Moons, Motion King, Lam Technologies, Sanyo Denkai et al.

PCW touched on it  but one advantage of steppers is their high torque at low velocities. This translates to higher velocity and accelleration. I have pushed steppers to the limits (60 m/min rapids, 8 m/sec/sec accelleration. (You will not get that performance from Steppers online motors.) I found that the ony time open loop steppers loose steps is if they are asked to do too much (eg Accelleration and Velocities too high). So then you have to ask yourself will the motor have enough torque to correct for missed steps when it happens (eg when torque is maxed out anyway.) In my experience it won't!

A well designed open loop stepper system will never loose steps and will utilise that low down torque. However, the engineering design is more complex than a servo system. So the additional cost of closed loop steppers is not worth it in my testing if you get the design right! Particularly if you use smart steper drivers like the Lam Technologies DS1076 et al where the current can be reduced significantly (perhaps as much as 85%) so the motor is idling at constant velocity.

rodw wrote:

As with everything, quality industrial motors cost more but from expereince, there is no comparison between Steppers on Line commodity drives and industrial quality drives from the likes of Moons, Motion King, Lam Technologies, Sanyo Denkai et al.
 

I said motors, you said drives.

In no practical way does a high end stepper motor really matter over decent chinesium except if ya wanna be all "industrial' about it. I'd be inclined to thing the "industrial" motors just come out of the same Chinese factory with different labels anyway. Drives......yeah maybe but most StepperOnline drives are rebadged Leadshines which are A-OK in my book. Leadshine is like the Meanwell of drives, it's good Chinesium.

blazini36 wrote:

rodw wrote:

I said motors, you said drives.

 

My mistake. I meant motors.

I disagree on  motor quality. You just have to pick up a quality motor to appreciate the difference. Then appreciate how quiet they are. Plus if you take the motor parameters and do the engineering, the improved performance just leaps out at you from the page. Find a 2 amp Sanyo Denki NEMA 24 and it will almost  hold its own with a Small NEMA34. Its the rotaional inertia that is the key despite what people say about inductance.

After I read spumco's post mentioning the capacitive encoder on a lathe, I decided to go back and give the CUI
catalog a more thorough look, and sure enough, there was a differential model I had not noticed. And this one
has higher current ratings.

CUI AMT113Q
(datasheet info attached below)
I also noticed these have some level of programming support and can accept zero state input (connector pins 1,2,14)
Mouser sells some form of UART programming cable for the various versions of these encoders, software is on the
CUI webpage but only for windows systems I believe.

I also did some more digging and found the usdigital website again. (I had stumbled upon it much earlier while I was
looking at steppers a couple weeks ago, then forgot what their website was) They also have a somewhat modular
system of encoders but of an "optical" type. They are notably more expensive, but do represent another potential
option in the US.

rodw wrote:

blazini36 wrote:

rodw wrote:

I said motors, you said drives.

 

My mistake. I meant motors.

I disagree on  motor quality. You just have to pick up a quality motor to appreciate the difference. Then appreciate how quiet they are. Plus if you take the motor parameters and do the engineering, the improved performance just leaps out at you from the page. Find a 2 amp Sanyo Denki NEMA 24 and it will almost  hold its own with a Small NEMA34. Its the rotaional inertia that is the key despite what people say about inductance.
 

Lol dude you have a way of oddly oversimplifying things while overcomplicating others. I've been dealing with Industrial machinery my entire life, but that's not the point. Stepper motors are so heavily manufactured these days, if the shaft is straight and the paint is good, it's gold. IF you're running it off a modern drive that is, and sometimes that's not always a possibillity.....

Describing the rotor mass of a motor doesn't say anything about the quality of the motor, I have no idea why you're using that as a metric. That's part of specifying a motor. What you basically said is "Toyota's suck cuz they aren't green". Not sure why you think Chinese people can't make a high inertia motor. And BTW, quiet motors are a function of the stepper drive, not the motor itself.

Coil characteristics do matter, it's just these days you can just choose not to care. On on older amps you could wind up not even being able to spin the motor at all if coil resistance was much lower than what it was intended for. I wound up having to design this stepper drive to take care of some 90s era OEM stepper amp shenanigans...