Subject:
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Re: Balancing robots
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Newsgroups:
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lugnet.robotics
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Date:
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Thu, 22 Jan 2004 15:41:13 GMT
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Original-From:
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Jim Choate <RAVAGE@EINSTEIN.SSZ.spamcakeCOM>
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Viewed:
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1844 times
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On Thu, 22 Jan 2004, Steve Hassenplug wrote:
> In lugnet.robotics, PeterBalch <PeterBalch@compuserve.com> wrote:
> >
> > How does a balancing robot like Legway balance on a patterned floor? Or can
> > it?
> >
> > But the Legway (for instance) can't measure the position of wheel or the
> > wheel velocity. You can pretend that the motor commands correspond to the
> > wheel velocity and smooth or integrate those numbers. That helps with
> > balance but the results (in my simulation) are never as good as a robot
> > that has wheel encoders. Is it possible to measure wheel velocity by
> > measuring the motor back-emf?
>
> I don't think there's a good source for this data (motor back-emf) using an
> RCX.
Um, the back-EMF of the motore isn't related to the motor load per se but
rather the relaxation of the current in the coil when it moves off the
stators. It's also very fast (ie E=Bc) so the RCX isn't likely to get good
readings for this. It's very hard to measure back-EMF accurately without
some sort of delay line on the measurement channel (ie what is being
measured), with regard to the trigger channel (ie when to measure). On a
scope one uses a delay line between the trigger input and the actual
vertical amp input.
Actually it's pretty easy, measure the current on the motor. As the
velocity goes up the current goes up, the catch is if the motor stalls.
You're solution must include a 'step function' test to look for this.
It's reasonably easy to equate the F=ma for the robot to the F put out by
the motor, derived from F=BILsin(theta).
The difference between resistance and the motor force gets converted into
motion.
The simplest way to measure the current is to put a small resistor in
series with the motor and measure it's voltage drop (ie E=IR).
Another way to measure wheel velocity is to have a switch on the axle and
measure how often it gets closed. Working issues of geometry from circular
to linear acceleration is reasonably simple so I won't bore you with
details. I like the wheels that have good tread, they give you a lot
better place to put such a switch. More data points (ie ridges in the
tread) mean more precise measurements, integration functions related
to dx. The distance between sample points on the axle is the minimum dx
that you can use and get non-GIGO numbers out the other side. More points
per circumfrence means more accurate dx measures.
Good luck with it.
-- --
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James Choate 512-451-7087 ravage@ssz.com jchoate@open-forge.com
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Message is in Reply To:
 | | Re: Balancing robots
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| (...) I don't think there's a good source for this data (motor back-emf) using an RCX. Yes, Legway just used the motor commands to simulate the wheel velocity. It would have greatly increased the complexity of the wheel assembly to add rotation (...) (20 years ago, 22-Jan-04, to lugnet.robotics)
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