An way to sense the torque of a stepping motor?

I am starting on the development of what is basically a DC servo, which uses a FAULHABER microstepped stepper driving an inline gearbox.

There is a requirement to measure the torque on the output shaft of the gearbox. Not accurately; within 20% would do.

Existing implementations of what I am trying to do have the whole motor+gearbox swivelling on the gearbox output shaft, and deforming a steel leaf with strain gauges on it. This works, but is weak (the gearbox output shaft needs to be thick :)) and (this is a separate problem) the strain gauges easily come unglued from the leaf.

It would be great if one could somehow use the stepper current, but I don't think it will change much.

I am going to use an off the shelf stepper controller chip to do the microstepping, e.g.

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Has anybody done this?

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This is how I've done it in the past.

The "gauge" of the "leaf" can be whatever you want -- subject to the magnitudes of the strain you are measuring. Get creative: look at existing aspects of the mechanism that you can exploit to give you this information. But, you have to understand how the motor is likely to "twist" in a particular mounting configuration. I.e., you don't want to lose "signal" because the motor is squirming in a way that you hadn't considered in your instrumentation.

E.g., the mounting bolts for the motor can be instrumented.

It depends on how much sensitivity (gain) you need in the torque transducer. And, the nature of the load -- i.e., does it vary over time? What sorts of timescales?

In tablet presses, the strain in a ~1" diameter being "stretched" as the granulation (powder from which tablets are made) is compressed is measured to determine the force (many tons!) being exerted on the tablet. The deformations are tiny yet easily measurable.

(of course, you are looking for forces in a different orientation but the issues are the same)

At a given speed, current drives torque. But, you may find that there is too much "slop" in the signal for you to be able to extract the information you need. Also, you probably want to

*control* current (unless you are willing to let the motor "slip"?) so you have better control over the angular position of the shaft at all times (?)
Reply to
Don Y

Not quite. Speed, current and angle, these three. And very little on current

At rest, there will be a torque from the current times some function of the angle of the shaft with respect to the frame. This angle-dependent function will repeat around the circle as many times as there are steps in the motor. For most steppers there will also be a torque generated by the field magnets and the geometry of the armature; this torque will also depend on the angle, and will also repeat by the number of steps around the circle.

Things get wacky with speed, which is one of the reasons that I detest working with steppers.

But the main point is that without knowing the angle of the shaft with respect to the frame, you have no clue of what the actual torque is -- just the pull-out torque.

If you were designing this from the ground up I'd suggest you consider a DC motor with an encoder driven from a servo loop -- because with the right DC motor, torque is quite proportional to current. But even then I wouldn't just flat out tell you it would have to be better -- only that it'd be worthwhile to think about. I suspect that with what you're already making, your best bet is to stick with the stepper, and (perforce) use some sort of torque measurement hardware.

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Reply to
Tim Wescott

On an *instantaneous* basis, yes. However, if the OP is looking at a reasonably static load, then average current correlates with torque quite well. This is why I specifically asked what the nature of the load would be and the timescales over which it is likely to vary. If, for example, the load is livelier than the shaft (i.e., load changes more frequently than the shaft angle), then its an entirely different measurement problem than when the load remains relatively constant as the shaft rotates.

E.g., we drive stepper motors as if they were "brushless DC motors". Tell the control loop what constraints you want to place on its performance (within the range supported by the motor, load and driver) and the system drives it as if it was a very clever DC motor.

Want to control speed? Then the control loop supplies current for the instantaneous torque required to drive the load through the required number of radians per second. Want to control *torque*? Then the control loop adjusts the speed to "provide" the torque desired.

[of course, subject to acceleration profiles, angular positional constraints, etc.]

You need to ensure you have enough voltage compliance to overcome the back EMF as speed increases. I.e., you can't arbitrarily change speed without concern for how it will affect available torque. There is a locus of operating conditions within which you have to remain ("SOA"?) just like any system.

DC motors are a *lot* easier to control -- assuming you don't *also* need precise control over shaft angle. OTOH, if you ever want to "stop on a dime", change directions in a controlled manner, etc. the DC motor is a big headache. (this is why the "stepper as brushless DC motor" approach is so appealing -- in its operating range, it gives you characteristics of both, simultaneously)

Reply to
Don Y

I'm terribly sorry, you seem to be living in a different universe from the one I inhabit.

In _this_ universe, stepper motors have absolutely no way of sensing torque -- the current one puts through them affects the maximum torque at which they will work, and beyond that they cannot hold the load: generally what happens is that because with the shaft spinning with respect to the magnetic field, the average current is zero and the load just goes wherever it wants to unless or until it happens to get synchronized with the motor again.

So here in this universe, we have to design the motor drive for the maximum torque that the motor will ever have to generate, and accept the fact that any excess current will get burnt up as heat. In other words, the control system has to treat the stepper as a very _stupid_ brushless DC motor, that cannot feed its armature position back to the controller to be used in a sensible manner.

Which is generally well within what you could do with a similar size and weight DC brushless motor, or even a brushed DC motor.

Definitely a different universe.

So, having your overly large, overly heavy, and overly power hungry motor occasionally fall out of sync and damage equipment _isn't_ a big headache?

Or are you just going to say that overly larger, heavier and power hungrier is a good thing?

If you design your control law correctly, a design with a DC motor will _always_ be able to deliver _all_ of it's available torque, without requiring you to overdesign the motor or driver for fear that your motor will fall out of sync.

And you _cannot_ stop a rapidly moving stepper motor that has any kind of an inertial load "on a dime" -- at least not if "on a dime" means in one step. You must decelerate the mechanism, open loop, and hope that the motor doesn't fall out of synchronization.

What, the worst ones from each?

I admit, my experience with steppers has mostly been with attempting to rescue projects from mechanical engineers that designed them in without paying attention to the notion of "safety margin". And I've even used them in some low-demand applications with success.

But if you need small, light, low-power drives, steppers should be on the bottom of your list.

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Reply to
Tim Wescott

Exactly! And, when you notice that it isn't executing the step as expected, you are now operating at the torque associated with that current and can no longer increase speed (step rate). Set the drive current for the torque you desire/need and then let the controller increase the step rate until the armature starts to lag the rotating field.

This happens if you run the motor open loop without watching where the armature is "physically" in relation to the rotating field. If you operate a stepper like that, you risk the motor effectively *stalling* if it misses a step and is unable to accelerate to the current step rate within a cycle (likely!). So, you either artificially limit the top rate to something that you *know* the motor can keep up with regardless of load, or, you provide a means of detecting when this HAS happened (or, if you are clever, when it is *about* to happen) so that your drive can be adjusted to resynchronize with the motor.

No. You can sense the position of the armature with encoders or, if not completely driving all of the coils in the stepper, by sensing the back EMF on the "idle" coils to tell when the motor has effectively "taken" the step (this is how "sensorless" brushless DC motors operate). Your controller then watches how the armature is moving so that it doesn't advance to the next "step" (using that term loosely) until it knows that the motor can follow it there. Just like a brushless DC motor.

That;s why you track *when* the motor has actually taken the step. You don't drive it "open loop and hope". Otherwise, you end up with a motor poorly matched to the needs of the application.

I didn't say that the motor was "rapidly moving". I stated that you always have to operate within the design constraints of the system. That includes the acceleration/deceleration profiles of your loaded drive.

No! The beauty of operating it as a "brushless DC motor" is that you *remain* in closed loop control of the armature. It doesn't (can't) fall out of synchronization any more than your DC motor can be "overridden" by its load.

You can understand how a brushless DC motor operates, right? How it recovers from a momentary stall? How it can adjust its speed to match the torque available from the current flowing through its windings?? The same technique applies when you operate a stepper motor like this -- except *you* are controlling the commutation instead of a hall effect sensor in the motor (or a rotary encoder, etc.)

Stepper motors are a win when you want precise control over the angular position of the shaft. And, to be able to operate, reliably, at very low angular velocities. Getting a DC servo motor to give you precise angular control requires high resolution encoders. A stepper operated with feedback from the armature itself allows you to get finer resolution than available with a given encoder and very low angular velocities.

When you can also operate the stepping motor as a brushless DC motor, then you can drive it CLOSED LOOP at whatever speeds and accelerations the load will tolerate -- without resorting to open loop (de)acceleration profiles, etc.

You push a fixed current into your DC servo motor and, with a stalled armature, the motor will exert a certain amount of torque. With a particular load and that same current, the motor will achieve a particular top speed (assuming you have enough voltage compliance). The stepper as brushless DC motor gives you the same sorts of characteristics -- with the addition of fine control over armature position and great holding torque.

We've used this technique on tiny steppers as well as "fractional horsepower" units (e.g., NEMA 34 frame sizes, *hundreds* of oz-in of torque, etc.). Cool to watch the same motor scream along at a few thousand RPM -- then microstep to 15 minutes of arc a few seconds later!

But, the controls get significantly nasty!

Same universe, you just haven't looked in all the right corners! :>

Reply to
Don Y

Two possible ways i can think of: monitor current to derive power to motor; know resistance for static IR loss and steady state power drive. You will need to model the motor - back EMF etc and do dynamic calculations. or.. Use a laser to create holographic stress pattern on the shaft - the greater the torque the greater the stress.

Reply to
Robert Baer

I would search to see what strain-gauge torque sensors are out there and look at the designs.

Another possibility might be to use a bearing or pivot combined with something like this:

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so that the torque is converted to a linear force on a load-cell with a stud.

Strain gauges on leaf springs seems like a bad design.

The material used for a load cell has to deform somewhat, but usually the thing is designed to still be quite rigid (i.e. the deformation won't be visually perceptible). I don't recall ever using a leaf spring with strain gauges.

Also, some of the designs you find out there may give you some ideas. There is some model of helicopter I saw once that had a rotor torque gauge in the cockpit (one of many gauges, as you might guess with an aircraft). I asked the pilot how in the hell they can sense torque on the main shaft (because it is structural and rotates). He indicated that there is some sort of a gauge in the shaft that is AC-coupled, so you can sense torque without having to have a direct connection. I thought it was pretty clever.

You may find something interesting and useful by examining existing designs.


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David T. Ashley

Sounds like a in-house, poorly implemented, reinvention of a reaction torque sensor. See:

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They will increase the cost of the stepper "micro-universe" by only one or two orders of magnitude.

-- Roberto Waltman

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Roberto Waltman

Thank you all for the responses.

My view is that measuring the stepper current is not going to work not least because it will be very lightly loaded. The axial gearbox on the end of it will be something of the order of 100:1...

Also it will be driven from one of those chips so presumably the only way to get a grip on the current will be to measure the overall current drawn by the chip. Actually I can try that. It will be easy enough to see if there is any useful change.

Regarding the motor type....

The "thing" I am designing is a redesign of an existing product (not mine) which is a crap design and with very poor reliability. There is an external signal involved (which I can't change) which controls the gearbox output speed and which contains a lot of noise. Some of the noise is high frequency and even RF (which is trivial to filter out) and some of it is within the passband of the whole control system so one needs to be a little careful about filtering that out too keenly. The latter noise creates problems with the existing design which uses a crappy (Globe) DC brush motor whose brush life is similar to its commutator life (!) and which ends up waggling back and forth trying to follow the spurious signal. It's like taking a dog for a walk; the dog covers about 10x the distance *you* walk.

So I originally went to a brushless motor (3-phase) which I know pretty well; my son is heavily into model aircraft and that business has gone that way, and the motors are amazing. The controllers are of poor quality (like all model aircraft electronics) but it appears feasible to build your own.

There are two issues (in fact I kicked off a long thread here some months ago on controlling 3-phase brushless motors :)) with these. One is poor low speed control (unless one uses a tacho of some sort) and the other is that they do need a relatively clever controller (relative to a stepper which gives you implicit precise speed control). I do need a fairly accurate (few %) transfer function in this case.

The existing product uses a stupid brush tachometer to achieve that, which is gear coupled (and the gears wear out) and whose brushes wear out, and because the whole assembly rotates (to sense the torque) the tacho wires tend to come off :)

So I want to avoid the tacho. Also there is a space limit and I can't add another inch or two onto the end of the motor+gearbox assembly (which is probably why the existing tacho is geared).

Oh another requirement is a -40C min temp, and I haven't yet found a stepper controller which does that... that Allegro chip I mentioned earlier is -20C. I better sort this bit before doing anything else. On another project, I had a bit of "fun" finding a plain old 7805 TO220 regulator which was specced down to -40C :)

A "brushless" motor (brushless or stepper) will neatly sidestep the problem with the in-band spurious signals coming in, because apart from the bearings, and the gearbox, there isn't much to wear out.

A stepper seems to be an easy solution to the transfer function too.

Regarding sensing the torque, I could use a version of the existing steel leaf system (with the motor+gearbox swivelling around the gearbox output shaft) but with Hall sensors around it. Or some magnetic-proximity sensing (a form of LVDT) which would work better over the temp range than Hall sensors. There are probably several ways to do it. I know about the shaft torque sensors; they are a damn clever solution but pricey and too bulky for this. I don't have the room to put one in.

Actually I think sensing the stepper current is a useless idea because the torque sensing has to work even when there is no control voltage i.e. the motor is stopped, and in that situation the stepper controller won't be doing any stepping; it will just be holding the armature in position.

A pity I can't openly discuss the application.

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Ah, OK. You'd not mentioned this nor the frame size of the motor/load magnitude, etc.

Keep in mind that a gearbox can influence the backlash that you see -- if this is important in your control.

I don't think you will see much (since you're hiding behind the gearbox *and* the driver chip) -- uless you drastically vary the mechanical load on the motor shaft (e.g., grab it with some vice grips and "try" to stall it -- that's the *biggest* signal you'll encounter)

^^^^^^^^^^^^^^^^^^^^^ meaning "motor speed", indirectly?

Engineering is all about tradeoffs! :> It's a lot easier to just dump DC into a motor and watch/vary the current than it is to sequence a stepper/microstepper. But, the mechanical commutation (vs. electronic) shows signs of wear!

This is a (very) *small* motor, then? (I don't imagine large

*anything* in model aircraft)

Are you ultimately controlling something's "position"/orientation? Or, does the shaft "just spin" (like the wheels on a car)?

Presumably, the rotation is "against a spring" for the torque measurement. Just how much rotation are you seeing? Could you, instead, let the motor frame rotate and sense the torsional strain exerted "off the back" against a fixed member?

No graphics, here...

Imagine the front of the motor sitting in a pillow block. Imagine a plate fastened to the back of the motor with a "shaft" protruding coaxially wrt the actual output shaft. Fasten the far end of that shaft to a stationary member. I.e., the front of the motor is captive and supported by the pillow block (or equivalent). As the load *fights* the motor, the motor's frame will want to twist ABOUT IT'S SHAFT. This torsion is transfered to the fixed mount at the rear of the motor where a *stationary* sensor can be mounted (i.e., you are measuring the deformation in the "rear shaft").

Alternatively, the rear shaft can be "point-attached" to a cantilevered member who's deformation is measured.

(You're not looking for *motion* as much as you are

*deformation*. Google "poisson configuration" and "strain gauge")

Is the motor "fastened" off the shaft end of the gearbox? Or, is there other support?


Does the lubricant in the gearbox exhibit a change in viscosity at -40F?

It seems to me that your approach suggests you are looking for lots of *motion* to indicate torque. :< What happens if the motor is ever "back driven" by an excessive load? Does the support mechanism/sense scheme fail?

Can you afford to calibrate each unit? (to whatever degree of accuracy you ultimately need)

That doesn't preclude using torque -- see my description, elsewhere, of a scheme by which you can effectively measure/control the present "holding torque" (with shaft position feedback). But, I agree, in the situation you've outlined, it seems like your best bet is a sensor+mechanism combination that gives you a signal more directly proportional to instantaneous torque.

Welcome to my world! :> (Imagine having to live by that same constraint decades *after* the project's completion!)

Reply to
Don Y

I didn't catch until now that you are using a chip. Presumably something like Rifa? Nice chip with H drive output that let's you forget about excess drive current, because it automatically goes into switch mode output as the stepper seats and you don't know it.?

Those chips are handy. You can use high voltage that gives high initial torque, and then count on that switching function to limit the drive current. DON'T DO THAT!!!!

I naively did that on a first design and got caught with the reality that all the damping goes away! Once you no longer are driving the stepper with a voltage source but what has effectively become a current source, you now have ringing on that last step you cannot believe! The ringing was so bad that sometimes combined with load the stepper actually jumped a cog. Took something like more than ten cycles of that ringing oscillation to get anywhere near final position, too.

Instead you could match the voltage drive to the chip to the expected time constant of your system. That way the voltage to the drive, and therefore the motor as it seats, drops for each step.THEN you getcurrent limit *and* damping.

Probably preaching to the choir, but still, a heads up.

Reply to
Robert Macy

the stepper driver is constant current so even when you stall it'll be the same current

unless theres an encoder of some sort to tell you where the motor is compared to where you trying to make it go, I can't see how you could measure torque

with numerous stepper ic's with sequencer and all the bells and whistles you can image driving a stepper the difference isn't so big I think

they come in all sizes this big enough?:

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there are several open source projects doing BLDC with small MCUs

I think with an encoder so you could calculate torque from measuring/ controlling current and motor position vs. driving waveform


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Some drivers are voltage mode -- though that doesn't change the (faulty) answer, here.

Yes, sorry, I misspoke (thinking about how *I* drive motors and not the application at hand)

It depends on the size of the motor. E.g., I've dealt with 34 and

43 frame sizes -- *lots* of power delivered to the motor (i.e., many amps per phase). I don't believe there are any integrated solutions that can address this range of devices. (?)

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It's still not trivial. You have to decide where you are willing to put your development and recurring dollars. Complexity in the controller vs. cost in a transducer. :<

I don't like "mechanism". It tends to add recurring labor, cost, failure points, etc.

OTOH, it is usually a lot easier to understand and "relate to".

Ideally, come up with a hybrid that lets you save cost/complexity by integrating the two approaches. E.g., I've designed systems where a mechanism drove a known "calibration spring" so the drive could observe its feedback and calibrate itself (instead of using, e.g., NBS traceable transducers).

Reply to
Don Y

You need to use a load cell.


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Just a followup...

A chap watching this discussion dropped this in my mailbox this morning:

An excerpt from which appears at the end of the post...

The promised excerpt (emphasis mine):

"ClosedLo> Same universe, you just haven't looked in all the right corners!

Unless, of course, you truly *are* operating in a different universe and the Nanotec firm hasn't managed to set up a sales office in yours, yet! ;-) (This could be an excellent business opportunity -- the first, in *your* universe, with CLOSED LOOP control! :> )

Reply to
Don Y

Sorry Don but Tim is right!! you are in a different universe. It is you that does not understand how a brushless dc motor works and why they are different to stepper motors and why they are the same as brushed DC motors. A little study is required.

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