Subject:
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Balancing robots
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Newsgroups:
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lugnet.robotics
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Date:
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Mon, 26 Jan 2004 22:58:12 GMT
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Original-From:
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PeterBalch <peterbalch@AVOIDSPAMcompuserve.com>
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Viewed:
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975 times
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Hi Gordon and Jim
Motors really are tricky things.
Oh dear, I seem to have stirred up a hornet's nest.
I too have built magnetometers and they are far easier to understand and
model. The problem with motors is that they go round - and they have a
commutator.
As they go round, the torque produced varies depending on which coils are
currently connected via the commutator and what angle they are at relative
to the stator magnets. The magnetic field is nothing like the nice
straight lines shown in a physics textbook (so "sin(theta)" is worth next
to nothing). The back EMF depends on the which coils are currently
connected via the commutator and what angle they are at.
I got down at a range of textbooks from my bookshelf and "back emf" for
motors is defined as "the induced EMF due to the generator principle" (from
a book of 1907; a 1992 book gives much the same definition).
The same books talk about the EMF due to the collapse (or build-up) of the
magnetic field in a coil. They refer to it as the "induced EMF".
Measuring the "back EMF" to estimate the speed of rotation of a PWM
controlled motor ought to work both when the motor is "on" and when it's
"off". My questions were: how reliable is it as a way of estimating speed
and what's the best way to measure it.
Let's say I measure the EMF at the motor terminals with an op-amp (the
motor is in an H-bridge so I need to do a subtraction).
Measuring it when the PWM is in its "off" phase may be easier: You don't
have to worry about the drive current and the internal resistance of the
motor. But the transients due to "induced EMF" can be huge. Certainly big
enough to saturate the op-amp inputs.
Measuring "back EMF" when the PWM is in its "on" phase means that the
transients from the interaction of the PWM and the inductors are smaller
(because the transistors are on). But there will still be large transients
when the commutator connects and disconnects coils.
I would have to subtract a voltage which depends on the current and the
internal resistance. (Which a few extra resistors and the op-amp can do.)
Of course, the internal resistance of the motor is also affected by the
commutation.
My guess is that any voltage I try to measure will be bouncing around all
over the place. The ADC on my chip will have to measure it at some fixed
time (or times) in the PWM cycle. When is best? If the PWM period is long
then I can wait for the "induced EMF" to die down before I measuring the
"back EMF". But in practice, the PWM period will probably be so short that
I've got to put up with "induced EMF".
If I just put a capacitor in the feedback of the op-amp will that average
out all these effects? In theory, yes. In practice, the "induced EMF" may
well saturate the op-amp so the "average" is no longer valid.
My belief is that - like many problems in electrical engineering (rather
than physics or electronics) - you just can't tell until you've tried it. I
was hoping tha someone had tried it and would say: "here's a circuit that
worked for me and the results were accurate to 10%" or "I messed about for
a month and eventually gave up".
Peter
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Message has 2 Replies:
 | | Re: Balancing robots
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| Hi Peter, (...) To each their own. That they go round is called 'polar coordinates' if you have problems translating it to cartesian. The result is the same either way. The commutor is a (low) resistance. Now we could fall back on a lumped-parameter (...) (20 years ago, 27-Jan-04, to lugnet.robotics)
| | | Re: Balancing robots
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| Peter, take a look at these two papers. The first is the web page on the H bridge concept, and the second is an (oversimplified) description of an H bridge motor control. (URL) (NOTE PDF file) (These are very simple, not mathematical, but the (...) (20 years ago, 27-Jan-04, to lugnet.robotics)
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