Subject:
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Re: Using the Motor as a Tachometer
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Newsgroups:
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lugnet.robotics
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Date:
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Mon, 9 Aug 1999 18:41:55 GMT
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Original-From:
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Ben Williamson <BENW@nomorespamPOBOX.COM>
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Viewed:
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1061 times
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On Mon, 9 Aug 1999, Nate Ridderman wrote:
> I'm not sure if I understand the brake concept? Could you explain?
A motor is just some coils attached to the shaft, which move through the
field of a permanent magnet. When current flows through the coils, they
become magnets and react with the permanent field. But I'm sure you
already knew that. There are some good diagrams at:
<http://www.howstuffworks.com/motor.htm>
Start with just one motor, not connected to anything. Because no current
can flow through the coils, they don't react with the field and the shaft
spins freely.
Now short out the contacts of the motor. You can do this by attaching
the two connectors at the ends of a lead together at a right-angle, and
then attaching that to the connector on the motor. Or you can connect the
motor to an output of the RCX in the "Stop" state. Now the shaft is much
harder to turn. This is because current can flow through the coils, so
the coils become electromagnets with fields that react to the permanent
field, opposing the movement.
But what is creating the current in the coils if they are shorted out?
The answer is the coils. Moving a coil through a field creates a
potential difference at its terminals. Since the terminals are shorted
out, current flows. That same current through the coils creates a field
that opposes the motion through the permanent field.
If you like, the motor is being a generator and a motor at the same time.
Turning the shaft generates a current which wants to drive the motor in
the opposite direction, producing mechanical resistance.
This is Lenz's Law in action. Here's a good slide that explains it:
<http://webug.physics.uiuc.edu/courses/phys112/fall97/lectures/lect16/sld006.htm>
As an aside, there is a way of using the one motor as both a motor and a
tachometer. I'm guessing this technique is used in the CyberMaster. The
same motor is connected to both an output and an input. The output is
pulsed with wide pulses, so that most of the time the motor is being
powered. But in the narrow gaps between the pulses, the motor is being a
generator. (The output must be left floating here, not in "stop" mode.)
The voltage that it generates is measured at the input, which is
proportional to the speed of the shaft. Some time I'll get a chance to
try this out with an RCX, but perhaps one of you will beat me to it and
tell us the results. :)
Cheers,
- Ben.
-------------------------------------------------------------------
Ben Williamson benw@pobox.com http://www.pobox.com/~benw/
--
Did you check the web site first?: http://www.crynwr.com/lego-robotics
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Message is in Reply To:
 | | Re: Using the Motor as a Tachometer
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| In fact, if you hook up two motors using the black cords and turn one of them, the increased voltage will cause the other one to turn without any physical effort applied to it. That's pretty rad. I'm not sure if I understand the brake concept? Could (...) (25 years ago, 9-Aug-99, to lugnet.robotics)
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