An way to sense the torque of a stepping motor?

Hi Peter,

On 4/24/2012 1:23 PM, Peter wrote:
> 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...

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.

> 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.

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)

> 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

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

> 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

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!

> 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.

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

> 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.

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

> 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 :)

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?

<frown>  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")

> 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).

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

> Oh another requirement is a -40C min temp, and I haven't yet found a

Ouch!

> stepper controller which does that... that Allegro chip I mentioned

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

> 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

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?

> 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.

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

> 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.

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.

> A pity I can't openly discuss the application.

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

On Apr 24, 1:23=A0pm, Peter <nos...@nospam9876.com> wrote:
> 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.
>
>...snip,,,

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.

On 24 Apr., 22:54, Don Y <t...@isnotme.com> wrote:
> Hi Peter,
>
> On 4/24/2012 1:23 PM, Peter wrote:
>
> > 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...
>
> Ah, OK. =A0You'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.
>
> > 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.
>
> 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)
>

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


> > 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
>
> ^^^^^^^^^^^^^^^^^^^^^ meaning "motor speed", indirectly?
>
> > 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
>
> Engineering is all about tradeoffs! =A0:> =A0It's a lot easier to just
> dump DC into a motor and watch/vary the current than it is to
> sequence a stepper/microstepper. =A0But, the mechanical commutation
> (vs. electronic) shows signs of wear!

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

>
> > 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.
>
> This is a (very) *small* motor, then? =A0(I don't imagine large
> *anything* in model aircraft)

they come in all sizes
this big enough?: http://www.hobbyking.com/hobbyking/store/__14427__Turnigy=
_CA120_70_Brushless_Outrunner_100cc_eq_.html

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

>
> > 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.
>
> Are you ultimately controlling something's "position"/orientation?
> Or, does the shaft "just spin" (like the wheels on a car)?
>
> > 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 :)
>
> Presumably, the rotation is "against a spring" for the torque
> measurement. =A0Just how much rotation are you seeing? =A0Could
> you, instead, let the motor frame rotate and sense the torsional
> strain exerted "off the back" against a fixed member?
>
> <frown> =A0No 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). =A0As the load *fights*
> the motor, the motor's frame will want to twist ABOUT IT'S
> SHAFT. =A0This 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*. =A0Google "poisson configuration" and
> "strain gauge")
>
> > 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).
>
> Is the motor "fastened" off the shaft end of the gearbox?
> Or, is there other support?
>
> > Oh another requirement is a -40C min temp, and I haven't yet found a
>
> Ouch!
>
> > stepper controller which does that... that Allegro chip I mentioned
>
> Does the lubricant in the gearbox exhibit a change in viscosity
> at -40F?
>
> > 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
>
> It seems to me that your approach suggests you are looking for
> lots of *motion* to indicate torque. =A0:< =A0What happens if the motor
> is ever "back driven" by an excessive load? =A0Does the support
> mechanism/sense scheme fail?
>
> > 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.
>
> Can you afford to calibrate each unit? =A0(to whatever degree of accuracy
> you ultimately need)
>
> > 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.
>
> 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). =A0But,
> 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.
>
> > A pity I can't openly discuss the application.
>
> <grin> =A0Welcome to my world! =A0:> =A0(Imagine having to live by
> that same constraint decades *after* the project's completion!)



-Lasse

Hi Lasse,

On 4/24/2012 3:39 PM, langwadt@fonz.dk wrote:
> On 24 Apr., 22:54, Don Y<t...@isnotme.com>  wrote:
>
>>> 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.
>>
>> 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)
>
> the stepper driver is constant current so even when you stall it'll be
> the same current

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

> 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

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

>>> 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
>>
>> 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!
>
> 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

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.  (?)

>>> 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.
>>
>> This is a (very) *small* motor, then?  (I don't imagine large
>> *anything* in model aircraft)
>
> they come in all sizes
> this big enough?: http://www.hobbyking.com/hobbyking/store/__14427__Turnigy_CA120_70_Brushless_Outrunner_100cc_eq_.html
>
> 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

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).

Peter wrote:

> 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.

You need to use a load cell.

Jamie

Just a followup...

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

   <http://en.nanotec.com/steppermotor_driver_SMCI47-S.html#top>

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

On 4/23/2012 8:50 PM, Don Y wrote:
> Hi Tim,
>
> On 4/23/2012 6:40 PM, Tim Wescott wrote:
>> On Mon, 23 Apr 2012 16:37:27 -0700, Don Y wrote:
>>
>>> On 4/23/2012 3:05 PM, Tim Wescott wrote:
>>>> On Mon, 23 Apr 2012 14:33:47 -0700, Don Y wrote:
>>>>
>>>>> On 4/23/2012 2:13 PM, Peter wrote:

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

>>>>> 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?
>>>
>>>>>> It would be great if one could somehow use the stepper current, but I
>>>>>> don't think it will change much.
>>>>>
>>>>> 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 (?)
>>>>
>>>> Not quite. Speed, current and angle, these three. And very little on
>>>> current
>>>
>>> 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.]
>>
>> 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:
>
> 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.
>
>> 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.
>
> 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.
>
>> 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.
>
> 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.

The promised excerpt (emphasis mine):

       "ClosedLoop current control (sinusoidal COMMUTATION VIA
       THE ENCODER) In contrast to conventional stepper motor
       positioning controls where only the motor is actuated or
       the position adjusted via the encoder, sinusoidal
       commutation CONTROLS THE STATOR MAGNETIC FIELD as in a
       servomotor via the rotary encoder. The stepper motor acts
       in this operating mode as nothing more than a high pole
       SERVOMOTOR, i.e. the classic stepper motor noises and
       resonances vanish. As the current is controlled, the motor
       can no longer lose any steps up to its maximum torque.
       IF THE CONTROLLER SEES THAT THE ROTOR IS FALLING BEHIND
       THE STATOR FIELD due to overload, adjustments are made
       with OPTIMAL FIELD ANGLE and increased current. In the
       opposite case, i.e. if the rotor is running forward due
       to the torque, the current is automatically reduced so
       that current consumption and heat development in the motor
       and controller are very much lower compared to normal
       controlled operation."

So, as I said:

> 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!  :> )

On Tuesday, April 24, 2012 4:50:01 AM UTC+1, Don Y wrote:
> Hi Tim,
> 
> On 4/23/2012 6:40 PM, Tim Wescott wrote:
> > On Mon, 23 Apr 2012 16:37:27 -0700, Don Y wrote:
> >
> >> On 4/23/2012 3:05 PM, Tim Wescott wrote:
> >>> On Mon, 23 Apr 2012 14:33:47 -0700, Don Y wrote:
> >>>
> >>>> On 4/23/2012 2:13 PM, Peter wrote:
> >>>>> 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 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?
> >>
> >>>>> It would be great if one could somehow use the stepper current, but I
> >>>>> don't think it will change much.
> >>>>
> >>>> 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 (?)
> >>>
> >>> Not quite.  Speed, current and angle, these three.  And very little on
> >>> current
> >>
> >> 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.]
> >
> > 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:
> 
> 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.
> 
> > 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.
> 
> 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.
> 
> > 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.
> 
> 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.
> 
> >>> 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.
> >>
> >> 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.
> >
> > Which is generally well within what you could do with a similar size and
> > weight DC brushless motor, or even a brushed DC motor.
> >
> >>> 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
> >>
> >> DC motors are a *lot* easier to control -- assuming you don't *also*
> >> need precise control over shaft angle.
> >
> > Definitely a different universe.
> >
> >> OTOH, if you ever want to "stop
> >> on a dime", change directions in a controlled manner, etc. the DC motor
> >> is a big headache.
> >
> > 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?
> 
> 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.
> 
> > 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
> 
> 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.
> 
> > 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.
> 
> 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.)
> 
> >> (this is why the "stepper as brushless DC motor"
> >> approach is so appealing -- in its operating range, it gives you
> >> characteristics of both, simultaneously)
> >
> > 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.
> 
> 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!
> :>

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.

How to choose a dc gear motor?
Nowadays dc gear motor has been widely applied in various fields, but also some people are still blind to dc gear motor. So they unable to choose a suitable motor. How to choose good dc planetary reducer? What are the specific dc motor deceleration parameters?
Here, let me explain how to choose dc gear motor for you. The first thing to determine the reduction ratio. If there is no suitable reduction ratio of conventional product, you can choose the customized products. The so-called customization is according to the requirements of customers to provide customized products, design and develop a new product. Determine after reduction ratio, please will you adopt servo motor rated torque on the reduction ratio, the value recorded in principle than the model on providing similar rated output torque of the speed reducer, should also consider the drive motor overload capacity and the largest in actual work torque. Maximum torque required to less than 2 times of the rated output torque. After meet the above conditions, please choose the smallest reducer, small volume of the reducer cost is relatively lower.
The next thing to consider dc gear motor return clearance. Return clearance, the smaller its precision is higher, the higher machining precision, the higher the cost. The user should choose to meet the accuracy requirement series reducer can be. Consider lateral/radial force and the average life expectancy. Lateral/radial stress of the reducer in the installation and use of high reliability, not a problem. Usually with servo motor by the average life expectancy is far more than life. After satisfied the above indicators, you can be produced according to the model provided by the company, select the installation size, diameter of axle and the input flange adapter with your motor reducer.
Finally you also consider the weight of the motor. A speed reducer is only allowed with less than a certain weight of motor, the motor is too heavy, long time operation will damage reducer input flange.