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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Sun, 8 Aug 1999 23:39:15 GMT
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Viewed:
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1185 times
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On Wed, 17 Feb 1999, in lugnet.robotics, David Chen wrote:
> [...]
> I was reading about hooking up 2 motors A + B and arriving at a
> dynometer arrangement. ie. Turning A causes B to rotate.
>
> I tried this, but also hooked up both motors to an Input port that was
> set up for RAW readings (don't ask me why, I was just messing
> around...). Lo and behold, a RAW input reading was generated
> proportional to the speed at which the motors were turning.
>
> I re-configured for just 1 motor and I was able to replicate this, thus
> with this curcuit:
>
> +-----+
> |Motor|----->Sensor Input = proportional to Motor RPM
> | | Just measuring the voltage being
> +-----+ generated by turning the motor. I think.
>
> Three downsides.
> 1) You use up a perfectly good motor for a tach.
> 2) The Input reading maxes out at 1024 pretty fast so the motor has to
> be geared down (ie. slow down RPM's at the Motor relative to the input).
> 3) The motor imparts a significant frictional/mechanical load on the
> input being measured.
>
> Maybe I'll find a use for this, or maybe I'll just pony up the $$$ for a
> proper angle/rotation sensor. Let me know if anyone finds a use for
> this.
Dave, this use of a motor as an input is fascinating! (How safe is it?
Looking at some older threads, there seems to be a difference of opinions,
or at least a non-consensus that it's safe...)
I just hooked an 1x2 LEGO incandescent lamp up to a geared-motor to see what
would happen, and sure enough, the faster I turned the wheel, the brighter
the lamp shone.
One funny thing I noticed: I had to apply about twice as much torque to
turn the wheel with the lamp attached to the motor as I did when the motor
was hooked instead to a second motor. That is, it "felt" about twice as
hard to turn the wheel at the same speed. Is this because the lamp is
super-inefficient at converting electomagnetic energy to visible light,
while a LEGO motor is super-efficient at converting electromagnetic energy
back into kinetic energy?
Anyhow, I suppose it'd be possible to attach a motor (heavily geared down)
on the side of a small rodent exercise wheel and use the programmable brick
to graph how fast and how often the mouse/hamster/gerbil/etc. was exercising.
I wonder what other types of applications there are. . . .?
--Todd
p.s. A couple of related threads:
http://www.lugnet.com/robotics/?n=3309&t=i&v=b (Jan 99)
http://www.lugnet.com/robotics/?n=3371&t=i&v=b (Feb 99)
http://www.lugnet.com/robotics/?n=5240&t=i&v=b (Jun 99)
http://www.lugnet.com/robotics/?n=5254&t=i&v=b (Jun 99)
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Message has 3 Replies:
 | | Re: Using the Motor as a Tachometer
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| (...) I'm not a EE, but here's my take on the situation... Turning the motor increases the electrical potential (voltage) between its two contacts. If the motor isn't connected to anything, then its easy to turn - relatively little energy is (...) (25 years ago, 9-Aug-99, to lugnet.robotics)
| | | Re: Using the Motor as a Tachometer
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| (...) Try to short circuit the motor used as tarcometer / generator, and you'll see. Also try preventing the second motor from spinning, and you'll experience the same thing. About efficiency: this is not what you are experiencing, but the motor (...) (25 years ago, 9-Aug-99, to lugnet.robotics)
| | | Re: Using the Motor as a Tachometer
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| (...) SNIP (...) SNIP (...) Todd, The danger as I see it is not electrical but physical. If the sensor mode of the input port is "active" the port will provide power to the motor (sensor). Thus you could have 2 motors either working together or (...) (25 years ago, 9-Aug-99, to lugnet.robotics)
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Message is in Reply To:
| | Using the Motor as a Tachometer
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| I don't know if someone else has already written about this, if so, sorry about being repetetive. I was reading about hooking up 2 motors A + B and arriving at a dynometer arrangement. ie. Turning A causes B to rotate. I tried this, but also hooked (...) (25 years ago, 17-Feb-99, to lugnet.robotics)
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