sensing torque
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Posts: 46,084
Say we have a motor of some type (servo, stepper, dc, ac) doing a job such as
raising and lowering a weight.
Now suppose the weight changes. Is there a way to electrically detect the
change in the load on the motor?
I am thinking specifically about a small motor turning a balanced load, like a
flywheel, that gradually, over
time, goes out of balance. Could the change be detected and monitored with the
Stamp?
Thank you for your insights. Michael
raising and lowering a weight.
Now suppose the weight changes. Is there a way to electrically detect the
change in the load on the motor?
I am thinking specifically about a small motor turning a balanced load, like a
flywheel, that gradually, over
time, goes out of balance. Could the change be detected and monitored with the
Stamp?
Thank you for your insights. Michael
Comments
would change -- instead you will see dynamic reaction forces (i.e., vibration)
on the motor shaft and/or bearings, depending on your geometry.
Clark Hughes
Michael Gianturco wrote:
>
> Say we have a motor of some type (servo, stepper, dc, ac) doing a job such as
raising and lowering a weight.
>
> Now suppose the weight changes. Is there a way to electrically detect the
change in the load on the motor?
>
> I am thinking specifically about a small motor turning a balanced load, like
a flywheel, that gradually, over
> time, goes out of balance. Could the change be detected and monitored with
the Stamp?
>
> Thank you for your insights. Michael
>
unbalanced the current would follow a sine path at a frequency set by
the speed of rotation. You could by using ac coupling, remove the DC
component, so you're left with an AC wave proportional to out of balance
current and at a frequency of the RPM of the motor. Some noise filtering
would be necessary
To remove communtation noise etc.
Rob
Original Message
From: Michael Gianturco [noparse]/noparse]mailto:[url=http://forums.parallaxinc.com/group/basicstamps/post?postID=uzRUWA6-ioWR5rlEDijLiff1MU2c9RPKgnFUDyV_PEuA4aIj9F4pp7j126aq72dGyHa8NEFOFLBZaLaFvxmz5w]michcg@m...[/url
Sent: 26 March 2002 19:58
To: basicstamps@yahoogroups.com
Subject: [noparse][[/noparse]basicstamps] sensing torque
Say we have a motor of some type (servo, stepper, dc, ac) doing a job
such as raising and lowering a weight.
Now suppose the weight changes. Is there a way to electrically detect
the change in the load on the motor?
I am thinking specifically about a small motor turning a balanced load,
like a flywheel, that gradually, over
time, goes out of balance. Could the change be detected and monitored
with the Stamp?
Thank you for your insights. Michael
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>>From: Michael Gianturco [noparse]/noparse]mailto:[url=http://forums.parallaxinc.com/group/basicstamps/post?postID=OV98LnZ6TgqiBC55MI269rl58hyv9o4vY6lB6hjt4cb1KPbFESfnzCimzDOPTB7S_XRMX9LQg5eguZ1t]michcg@m...[/url
>>Sent: 26 March 2002 19:58
>>To: basicstamps@yahoogroups.com
>>Subject: [noparse][[/noparse]basicstamps] sensing torque
>>
>>Say we have a motor of some type (servo, stepper, dc, ac)
>>doing a job such as raising and lowering a weight.
>>
>>Now suppose the weight changes. Is there a way to electrically
>>detect the change in the load on the motor?
>>
>>I am thinking specifically about a small motor turning a balanced
>>load, like a flywheel, that gradually, over time, goes out of balance.
>>Could the change be detected and monitored with the Stamp?
>>
>>Thank you for your insights. Michael
>You could monitor the current drawn by a DC motor, as the load
>becomes unbalanced the current would follow a sine path at a
>frequency set by the speed of rotation.
Won't work. There is no change in torque as an unbalanced rotor turns,
there is just an added sideways force on the motor shaft.
>You could by using ac coupling, remove the DC
>component, so you're left with an AC wave proportional
>to out of balance current and at a frequency of the RPM
>of the motor. Some noise filtering would be necessary
>To remove communtation noise etc.
>
>Rob
If sensing the vibration caused by an out-of-balance rotor is acceptable,
maybe a piezo type sensor would work.
Jim H
>
> >>From: Michael Gianturco [noparse]/noparse]mailto:[url=http://forums.parallaxinc.com/group/basicstamps/post?postID=9RY9txe-LbyKxHMEsgPDO2JN90IUnM5DRR5_IibCPM2FtIlIISTjXckd2eL2KTZZ7zu_kwjSLPW6mIQe0Y0]michcg@m...[/url
> >>Sent: 26 March 2002 19:58
> >>To: basicstamps@yahoogroups.com
> >>Subject: [noparse][[/noparse]basicstamps] sensing torque
> >>
> >>Say we have a motor of some type (servo, stepper, dc, ac)
> >>doing a job such as raising and lowering a weight.
> >>
> >>Now suppose the weight changes. Is there a way to electrically
> >>detect the change in the load on the motor?
> >>
> >>I am thinking specifically about a small motor turning a balanced
> >>load, like a flywheel, that gradually, over time, goes out of balance.
> >>Could the change be detected and monitored with the Stamp?
> >>
> >>Thank you for your insights. Michael
>
> >You could monitor the current drawn by a DC motor, as the load
> >becomes unbalanced the current would follow a sine path at a
> >frequency set by the speed of rotation.
>
>Won't work. There is no change in torque as an unbalanced rotor turns,
>there is just an added sideways force on the motor shaft.
For the most part I agree with you Jim, but your answer is not
completely absolute. Rob's observation would depend on how
unbalanced the rotor is vs the rating of the motor and if the
"pendulum's" orientation must travel up/down hill as it rotates.
> >You could by using ac coupling, remove the DC
> >component, so you're left with an AC wave proportional
> >to out of balance current and at a frequency of the RPM
> >of the motor. Some noise filtering would be necessary
> >To remove communtation noise etc.
> >
> >Rob
>
>If sensing the vibration caused by an out-of-balance rotor is acceptable,
>maybe a piezo type sensor would work.
>
>
>Jim H
Beau Schwabe Mask Designer IV - ATL
National Semiconductor Enterprise Networking Business Unit
500 Pinnacle Court, Suite 525 Wired Communications Division
Mail Stop GA1 Norcross, GA 30071
pulsin for sensing a frequency change.
Original Message
From: "J. Clark Hughes" <jchughes@a...>
To: <basicstamps@yahoogroups.com>
Sent: March 26, 2002 12:35 PM
Subject: Re: [noparse][[/noparse]basicstamps] sensing torque
In the case of your flywheel, I don't believe the torque required
of your motor
would change -- instead you will see dynamic reaction forces
(i.e., vibration)
on the motor shaft and/or bearings, depending on your geometry.
Clark Hughes
Michael Gianturco wrote:
>
> Say we have a motor of some type (servo, stepper, dc, ac) doing
a job such as raising and lowering a weight.
>
> Now suppose the weight changes. Is there a way to electrically
detect the change in the load on the motor?
>
> I am thinking specifically about a small motor turning a
balanced load, like a flywheel, that gradually, over
> time, goes out of balance. Could the change be detected and
monitored with the Stamp?
>
> Thank you for your insights. Michael
>
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Original Message
From: Jim Higgins [noparse]/noparse]mailto:[url=http://forums.parallaxinc.com/group/basicstamps/post?postID=aSVzyfaX2SZbToR56TIVUw4Jz80oo2aBKnFdCSKZMOIkZVGrVKPzdQ5-CoN7DLD_dpN-eXpqLVpF]HigginsJ@s...[/url
Sent: 26 March 2002 21:00
To: basicstamps@yahoogroups.com
Subject: RE: [noparse][[/noparse]basicstamps] sensing torque
At 15:46 03/26/02, Rob wrote:
>>From: Michael Gianturco [noparse]/noparse]mailto:[url=http://forums.parallaxinc.com/group/basicstamps/post?postID=gxdbHTEvtUy1b2ghhZtBlWOq8rsQv45dSApZavHXcvH9WyIM1m9jbSuRYune3nxNR4020Eccif__gKauw3c7]michcg@m...[/url
>>Sent: 26 March 2002 19:58
>>To: basicstamps@yahoogroups.com
>>Subject: [noparse][[/noparse]basicstamps] sensing torque
>>
>>Say we have a motor of some type (servo, stepper, dc, ac)
>>doing a job such as raising and lowering a weight.
>>
>>Now suppose the weight changes. Is there a way to electrically
>>detect the change in the load on the motor?
>>
>>I am thinking specifically about a small motor turning a balanced
>>load, like a flywheel, that gradually, over time, goes out of
balance.
>>Could the change be detected and monitored with the Stamp?
>>
>>Thank you for your insights. Michael
>You could monitor the current drawn by a DC motor, as the load
>becomes unbalanced the current would follow a sine path at a
>frequency set by the speed of rotation.
Won't work. There is no change in torque as an unbalanced rotor turns,
there is just an added sideways force on the motor shaft.
In the horizontal plane yes, but in the vertical plane gravity will have
an effect on the load on the motor.
>You could by using ac coupling, remove the DC
>component, so you're left with an AC wave proportional
>to out of balance current and at a frequency of the RPM
>of the motor. Some noise filtering would be necessary
>To remove communtation noise etc.
>
>Rob
If sensing the vibration caused by an out-of-balance rotor is
acceptable,
maybe a piezo type sensor would work.
Jim H
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not very well defined. Let's
specify for the sake of conversation that the flywheel is mounted in the
vertical plane, with a big lead weight,
like an automotive wheel balance weight, bolted at a point on the rim.
In this system, it seems to me the weight would ascend against gravity and
descend with the help of gravity,
alternately accelerating and decelerating the motor.
Now the question becomes: Can these periodic accelerations and decelerations be
detected electronically?
Some supplier of stamp or robot accessories offers (I read this ad somewhere on
the web, but can't remember where)
a servo drive pc board that senses and reports out the torque developed by the
servo. Servos are instructed by
PWM, so I don't see how this torque detection would work, but evidently it can
be done.
Most do it with a Tachometer mounted on the motor. It is rated in so many
Volts per Revolution V/Rev.
As the motor spins, the Servo Amp monitors the speed command (from the Stamp)
and the Tach input, and modifies the motor speed to make the Tach match the
Command.
It is also possible to do this by mounting an encoder on the end of the motor
and counting the pulses Vs time. You can then alter the motor speed from that
input.
The Encoder method will also give direction and distance information if this
is needed.
The Tach and Encoder are items that have to be purchased with the motor. IM
not sure that they are easy to add on, although I have never tried.
Hope this helps,
Alan Bradford
Plasma Technologies
>It seems to me now that the problem as I originally stated it is ambiguous
>and not very well defined. Let's
>specify for the sake of conversation that the flywheel is mounted in the
>vertical plane, with a big lead weight,
>like an automotive wheel balance weight, bolted at a point on the rim.
>
>In this system, it seems to me the weight would ascend against gravity and
>descend with the help of gravity,
>alternately accelerating and decelerating the motor.
>
>Now the question becomes: Can these periodic accelerations and
>decelerations be detected electronically?
>
>Some supplier of stamp or robot accessories offers (I read this ad
>somewhere on the web, but can't remember where)
>a servo drive pc board that senses and reports out the torque developed by
>the servo. Servos are instructed by
>PWM, so I don't see how this torque detection would work, but evidently it
>can be done.
Given the refined definition, can the problem be reduced to determining
flywheel position while the motor is running? That strikes me as an easier
problem with less noise to filter out of the resulting signal.
Jim H
I acknowledge that you can detect this imbalance by sensing torque. But in
reality, you'll only find success if the imbalance is large and the speed of
revolution is very slow -- like 1 Hz (60 RPM).
Getting off-topic, but take a moment to think about the physics:
m = mass of your lump of lead or whatever
v = tangential velocity of the mass
r = distance the mass is from the point of rotation
g = gravitational acceleration
Reaction force:
m*v^2/r
Static force you're trying to measure:
m*g
Plug in some values and I think you'll see a huge advantage to using reaction
forces at high RPM rather than torque sensing of the static forces.
Yes, I'm familiar with motors that are powered by PWM, and some provide pickups
so you can in turn sense rotation speed. But this is all designed so you can
very accurately control speed -- for example, on a hard drive. Yes, I think you
can calculate torque based on this data, but I think you'll find that the
physics limit your ability to sense the imbalance via torque when the RPMs get
above 60 or so.
Find out how the equipment the tire shops use to balance tires works. I bet
they use a load cell to detect the varying reaction force at the bearing. Good
luck.
Clark Hughes
Michael Gianturco wrote:
>
> It seems to me now that the problem as I originally stated it is ambiguous and
not very well defined. Let's
> specify for the sake of conversation that the flywheel is mounted in the
vertical plane, with a big lead weight,
> like an automotive wheel balance weight, bolted at a point on the rim.
>
> In this system, it seems to me the weight would ascend against gravity and
descend with the help of gravity,
> alternately accelerating and decelerating the motor.
>
> Now the question becomes: Can these periodic accelerations and decelerations
be detected electronically?
>
> Some supplier of stamp or robot accessories offers (I read this ad somewhere
on the web, but can't remember where)
> a servo drive pc board that senses and reports out the torque developed by the
servo. Servos are instructed by
> PWM, so I don't see how this torque detection would work, but evidently it can
be done.
>
complex. A wheel with a weight attached to the perimeter can be modeled
in its most basic form as a rigid pendulum oscillating in a vertical
plane. The pendulum equation is: ml^2v^2 - mgl[noparse][[/noparse]cos(theta)] = E, where v
is the derivative of theta, g is gravity, m is the weight mass, l is the
distance of the weight from center, and E is the driving force. The
additional energy of the wheel without the weight is a constant additive
term, and can be ignored as a first approximation. When E > mgl, the
pendulum has enough energy to swing around in a complete circle, with v
oscillating between a maximum and minimum value. This can be solved by
an integral equation in terms of elliptic functions. This equation is
the basis for planetary orbit eccentricity, so solutions have been known
since the time of Kepler. (At certain velocities, a resonance condition
is reached, which can be felt when driving a car with an unbalanced
front wheel.) Note that l is important in addition to v and m, for
predicting the wobble factor. Intuitively, a small-diameter wheel with
a large mass will wobble more than a large-diameter wheel with a small
mass. This can be shown analytically by attacking the above equation
with an equation solver.
Dennis
Original Message
From: J. Clark Hughes [noparse]/noparse]mailto:[url=http://forums.parallaxinc.com/group/basicstamps/post?postID=NlvyacQAuhDx7DqXqGXfld9gUUnYVsQE9wtGvbhv9Ym-5AGTAJmo9vFIjuyXpuJNtDxcYjnVJYD5DQrsCS6ljQ]jchughes@a...[/url
Sent: Wednesday, March 27, 2002 6:24 AM
To: basicstamps@yahoogroups.com
Subject: Re: [noparse][[/noparse]basicstamps] sensing torque
Michael:
I acknowledge that you can detect this imbalance by sensing torque. But
in reality, you'll only find success if the imbalance is large and the
speed of revolution is very slow -- like 1 Hz (60 RPM).
Getting off-topic, but take a moment to think about the physics: m =
mass of your lump of lead or whatever v = tangential velocity of the
mass r = distance the mass is from the point of rotation g =
gravitational acceleration
Reaction force:
m*v^2/r
Static force you're trying to measure:
m*g
Plug in some values and I think you'll see a huge advantage to using
reaction forces at high RPM rather than torque sensing of the static
forces.
Yes, I'm familiar with motors that are powered by PWM, and some provide
pickups so you can in turn sense rotation speed. But this is all
designed so you can very accurately control speed -- for example, on a
hard drive. Yes, I think you can calculate torque based on this data,
but I think you'll find that the physics limit your ability to sense the
imbalance via torque when the RPMs get above 60 or so.
Find out how the equipment the tire shops use to balance tires works. I
bet they use a load cell to detect the varying reaction force at the
bearing. Good luck.
Clark Hughes
Michael Gianturco wrote:
>
> It seems to me now that the problem as I originally stated it is
ambiguous and not very well defined. Let's
> specify for the sake of conversation that the flywheel is mounted in
> the vertical plane, with a big lead weight, like an automotive wheel
> balance weight, bolted at a point on the rim.
>
> In this system, it seems to me the weight would ascend against gravity
> and descend with the help of gravity, alternately accelerating and
> decelerating the motor.
>
> Now the question becomes: Can these periodic accelerations and
> decelerations be detected electronically?
>
> Some supplier of stamp or robot accessories offers (I read this ad
> somewhere on the web, but can't remember where) a servo drive pc board
> that senses and reports out the torque developed by the servo. Servos
> are instructed by PWM, so I don't see how this torque detection would
> work, but evidently it can be done.
>
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