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Message |
Paul E. Bennett
science forum beginner
Joined: 13 Jun 2005
Posts: 45
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 Posted: Fri Mar 31, 2006 10:40 pm Post subject:
Stepper Motor Behaviour
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Just come across a stepper motor behaving in a strange way and feel at a
loss for an explanation of how it might be able to do this.
The stepper motor is rated at about 1kW is a bipolar device with two pairs
of windings (four isolated coils) connected in series. All eight flying
leads are connected as per the motor manufacturers guidance and is driven
from a stepper driver package recommended by the motion controls company
from which the stepper, controller and linear thrust unit were supplied.
The installation seems to be connected in accordance with the installation
instructions. The whole assembly is expected to provide a significant
linear compressive force to the material that the end of the linear thrust
unit contacts.
Our investigations so far have stripped the equipment down to just the
stepper motor itself (removed from the linear thrust unit) driven from the
stepper drive unit. The stepper drive unit takes a direction and step
signal from a PLC and translates that into the appropriate motor coil power
sequence. We still have a few things to measure on Monday when we return
but here is what we have so far.
I will concentrate on the stepper motor part only as this exhibits the
behaviour by itself without the lnear thrust unit attached.
When commanded to continuously step, in one direction only, the motor
operates smoothly (for the 400 of steps per revolution it is expected to
do) and continuously.
We have provided a mechanical stop to the motors ability to rotate at one
position in the rotational arc. When this position is encountered the
stepper motor slips back in the opposite direction before resuming to
rotate up to the stop again. The expectation was that the motor would
torque up and hold hard against the stop. The slip back is almost like a
cog slipping (only there aren't any). The effort applied before slippage
occurs is near maximum torque (as far as can be determined).
Any ideas or explanations for this behaviour?
TIA
--
********************************************************************
Paul E. Bennett ....................<email://peb@amleth.demon.co.uk>
Forth based HIDECS Consultancy .....<http://www.amleth.demon.co.uk/>
Mob: +44 (0)7811-639972
Tel: +44 (0)1235-811095
Going Forth Safely ..... EBA. www.electric-boat-association.org.uk..
******************************************************************** |
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Stan Katz
science forum beginner
Joined: 31 Mar 2006
Posts: 1
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| Posted: Fri Mar 31, 2006 11:15 pm Post subject:
Re: Stepper Motor Behaviour
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Paul Bennett Wrote
| Quote: | Just come across a stepper motor behaving in a strange way and feel at a
loss for an explanation of how it might be able to do this.
|
snipped some details on the system
| Quote: | I will concentrate on the stepper motor part only as this exhibits the
behaviour by itself without the lnear thrust unit attached.
When commanded to continuously step, in one direction only, the motor
operates smoothly (for the 400 of steps per revolution it is expected to
do) and continuously.
We have provided a mechanical stop to the motors ability to rotate at one
position in the rotational arc. When this position is encountered the
stepper motor slips back in the opposite direction before resuming to
rotate up to the stop again. The expectation was that the motor would
torque up and hold hard against the stop. The slip back is almost like a
cog slipping (only there aren't any). The effort applied before slippage
occurs is near maximum torque (as far as can be determined).
Any ideas or explanations for this behaviour?
|
If the amount of slip is small (a few degrees) then it is because there is
effectively
a magnetic cog slipping. The stepper works by moving the electromagnetic
field
generated by the coils and this pulls the fixed magnetic field of the rotor
(generated
by the permanent magnets) along with it.
The number of "teeth" or poles is determined by the mechanical construction.
A 2 phase
motor usually has 50 poles. The 2 phases of the windings can generate the
same number
of magnetic poles and moves the field by changing the phase of the winding
currents. For
a simple driver where each winding can have a fixed direction there are 4
"steps" per cycle
which gives 200 steps per rev.
If the rotor is up agains the stop then it can't move forward with the
field, but the driver
continues to move the field. After about half a cycle, the electrical field
is now effectively
behind the rotor so it jumps back into alignment (slips a magnetic cog).
Stan |
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Tim Wescott
science forum Guru Wannabe
Joined: 03 May 2005
Posts: 292
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| Posted: Fri Mar 31, 2006 11:36 pm Post subject:
Re: Stepper Motor Behaviour
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Paul E. Bennett wrote:
| Quote: | Just come across a stepper motor behaving in a strange way and feel at a
loss for an explanation of how it might be able to do this.
The stepper motor is rated at about 1kW is a bipolar device with two pairs
of windings (four isolated coils) connected in series. All eight flying
leads are connected as per the motor manufacturers guidance and is driven
from a stepper driver package recommended by the motion controls company
from which the stepper, controller and linear thrust unit were supplied.
The installation seems to be connected in accordance with the installation
instructions. The whole assembly is expected to provide a significant
linear compressive force to the material that the end of the linear thrust
unit contacts.
Our investigations so far have stripped the equipment down to just the
stepper motor itself (removed from the linear thrust unit) driven from the
stepper drive unit. The stepper drive unit takes a direction and step
signal from a PLC and translates that into the appropriate motor coil power
sequence. We still have a few things to measure on Monday when we return
but here is what we have so far.
I will concentrate on the stepper motor part only as this exhibits the
behaviour by itself without the lnear thrust unit attached.
When commanded to continuously step, in one direction only, the motor
operates smoothly (for the 400 of steps per revolution it is expected to
do) and continuously.
We have provided a mechanical stop to the motors ability to rotate at one
position in the rotational arc. When this position is encountered the
stepper motor slips back in the opposite direction before resuming to
rotate up to the stop again. The expectation was that the motor would
torque up and hold hard against the stop. The slip back is almost like a
cog slipping (only there aren't any). The effort applied before slippage
occurs is near maximum torque (as far as can be determined).
Any ideas or explanations for this behaviour?
TIA
This is perfectly normal behavior, per Stan's post. |
The way a stepper works is to induce a preferred position for the rotor,
then step that preferred position around the circle. The rotor will
always fall toward the closest preferred position, as fast as it can.
To get fine stepping, the rotor and windings are arranged so there are a
number of identical cycles of this preferred position as you go around
the circle.
When you block the rotor from moving in one direction and keep stepping,
the rotor will press up against your stop as long as the closest
preferred position is in front of it -- once you step through more than
1/2 cycle then the closest preferred position will be behind, and it'll
go there.
This behavior of a stepper is both a blessing and a curse. Because it
is so predictable you can use a stepper in open loop, trusting that it
will go only the amount you commanded it, no more and no less. But
because it does have a maximum torque you can run into problems, either
because you need too much static torque or because you're stepping too
fast. In either case your motor loses synchronization and life gets bad.
Worse, should your motor be stepping faster than it can keep up not only
do you lose the dynamic torque from the coils, but your holding torque
from the variable reluctance between rotor and armature can effectively
go to zero. I had this happen to me with a small stepper driving a
linear actuator -- the carrier that it was driving would actually fall
back instead of driving forward. The root cause of this was a spec that
called for moving the mechanism faster than the stepper could support.
Since every stepper motor that I've ever had to drive has been pushed
past its reasonable limits I have developed a healthy dislike for them.
If you need to drive a mechanism to the limits of its actuator then a
stepper is simply the wrong thing to use -- a DC motor, brushed or
brushless, with feedback, is a much more sensible choice.
DC motors will always give you what torque they can, and real position
feedback won't leave you guessing about where your mechanism is. You
don't have to pre-compute an acceptable acceleration profile and worry
that after your system has had a few years for the grease to dry that
it'll still work as designed. You don't have to worry that a bit of
grit will fall into the mechanism and make it happily fail (it may still
_unhappily_ fail, but at least your controller will know). You know
that whatever the situation, a full torque command will get you full
torque -- not a jittery mechanism that sags to whatever local minimum is
closest.
--
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
Posting from Google? See http://cfaj.freeshell.org/google/ |
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Paul E. Bennett
science forum beginner
Joined: 13 Jun 2005
Posts: 45
|
| Posted: Fri Mar 31, 2006 11:37 pm Post subject:
Re: Stepper Motor Behaviour
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|
Stan Katz wrote:
| Quote: | Paul Bennett Wrote
|
Thanks sent by email.
--
********************************************************************
Paul E. Bennett ....................<email://peb@amleth.demon.co.uk>
Forth based HIDECS Consultancy .....<http://www.amleth.demon.co.uk/>
Mob: +44 (0)7811-639972
Tel: +44 (0)1235-811095
Going Forth Safely ..... EBA. www.electric-boat-association.org.uk..
******************************************************************** |
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Jerry Avins
science forum Guru
Joined: 03 May 2005
Posts: 534
|
| Posted: Sat Apr 01, 2006 3:10 am Post subject:
Re: Stepper Motor Behaviour
|
|
|
Tim Wescott wrote:
| Quote: | Paul E. Bennett wrote:
Just come across a stepper motor behaving in a strange way and feel at a
loss for an explanation of how it might be able to do this.
The stepper motor is rated at about 1kW is a bipolar device with two
pairs
of windings (four isolated coils) connected in series. All eight flying
leads are connected as per the motor manufacturers guidance and is driven
from a stepper driver package recommended by the motion controls company
from which the stepper, controller and linear thrust unit were supplied.
The installation seems to be connected in accordance with the
installation
instructions. The whole assembly is expected to provide a significant
linear compressive force to the material that the end of the linear
thrust
unit contacts.
Our investigations so far have stripped the equipment down to just the
stepper motor itself (removed from the linear thrust unit) driven from
the
stepper drive unit. The stepper drive unit takes a direction and step
signal from a PLC and translates that into the appropriate motor coil
power
sequence. We still have a few things to measure on Monday when we return
but here is what we have so far.
I will concentrate on the stepper motor part only as this exhibits the
behaviour by itself without the lnear thrust unit attached.
When commanded to continuously step, in one direction only, the motor
operates smoothly (for the 400 of steps per revolution it is expected to
do) and continuously.
We have provided a mechanical stop to the motors ability to rotate at one
position in the rotational arc. When this position is encountered the
stepper motor slips back in the opposite direction before resuming to
rotate up to the stop again. The expectation was that the motor would
torque up and hold hard against the stop. The slip back is almost like a
cog slipping (only there aren't any). The effort applied before slippage
occurs is near maximum torque (as far as can be determined).
Any ideas or explanations for this behaviour?
TIA
This is perfectly normal behavior, per Stan's post.
The way a stepper works is to induce a preferred position for the rotor,
then step that preferred position around the circle. The rotor will
always fall toward the closest preferred position, as fast as it can. To
get fine stepping, the rotor and windings are arranged so there are a
number of identical cycles of this preferred position as you go around
the circle.
When you block the rotor from moving in one direction and keep stepping,
the rotor will press up against your stop as long as the closest
preferred position is in front of it -- once you step through more than
1/2 cycle then the closest preferred position will be behind, and it'll
go there.
This behavior of a stepper is both a blessing and a curse. Because it
is so predictable you can use a stepper in open loop, trusting that it
will go only the amount you commanded it, no more and no less. But
because it does have a maximum torque you can run into problems, either
because you need too much static torque or because you're stepping too
fast. In either case your motor loses synchronization and life gets bad.
Worse, should your motor be stepping faster than it can keep up not only
do you lose the dynamic torque from the coils, but your holding torque
from the variable reluctance between rotor and armature can effectively
go to zero. I had this happen to me with a small stepper driving a
linear actuator -- the carrier that it was driving would actually fall
back instead of driving forward. The root cause of this was a spec that
called for moving the mechanism faster than the stepper could support.
Since every stepper motor that I've ever had to drive has been pushed
past its reasonable limits I have developed a healthy dislike for them.
If you need to drive a mechanism to the limits of its actuator then a
stepper is simply the wrong thing to use -- a DC motor, brushed or
brushless, with feedback, is a much more sensible choice.
DC motors will always give you what torque they can, and real position
feedback won't leave you guessing about where your mechanism is. You
don't have to pre-compute an acceptable acceleration profile and worry
that after your system has had a few years for the grease to dry that
it'll still work as designed. You don't have to worry that a bit of
grit will fall into the mechanism and make it happily fail (it may still
_unhappily_ fail, but at least your controller will know). You know
that whatever the situation, a full torque command will get you full
torque -- not a jittery mechanism that sags to whatever local minimum is
closest.
|
Paul,
Tim is leerier of steppers than I am, although he gives good advice. One
approach is adding a digital absolute encoder (or even an incremental
one if you know how to make them foolproof -- another discussion) to at
least warn of trouble or perhaps allow recovery.
One rather standard approach is to ramp the speed. A stepper can reach
and run at a higher speed than it can start at. Given its elastic
operation, that's no surprise.
If a stepper has been disassembled, replace it. Its magnets were
polarized with the entire magnetic structure in place. Separating the
parts diminishes the flux, and it won't completely recover after reassembly.
I assume that your device worked once. That makes it likely that there
is a new defect somewhere. Check the electronics. Check the currents in
each winding that result from eight consecutive steps. If you discern no
pattern, post it. If there is no pattern, you probably have a defective
driver. Good luck.
Jerry
--
Engineering is the art of making what you want from things you can get.
ŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻ |
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nospam
science forum beginner
Joined: 25 Apr 2005
Posts: 5
|
| Posted: Sat Apr 01, 2006 1:29 pm Post subject:
Re: Stepper Motor Behaviour
|
|
|
"Paul E. Bennett" <peb@amleth.demon.co.uk> wrote:
| Quote: | We have provided a mechanical stop to the motors ability to rotate at one
position in the rotational arc. When this position is encountered the
stepper motor slips back in the opposite direction before resuming to
rotate up to the stop again. The expectation was that the motor would
torque up and hold hard against the stop. The slip back is almost like a
cog slipping (only there aren't any).
|
Expected behaviour. The only way you are going to get a stepper to apply an
adjustable torque or force is by putting a spring somewhere in the
mechanics.
-- |
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