Help for those with Circuititis

[Mike Mane]

For anybody who can help, I have yet another question about
electronics. I have been working on a small tank, as I may have
mentioned, which uses a simple-as-dirt 27MHz skid control system.
Anyway, the tank is powered by two 9VDC gear-motors, and I would like
to be able to control the trim of these engines to ensure equilibrium.
Can this be done with a potentiometer on each circuit? If so, what
kind of pot and how are they installed?

Thanks,

—Mike Måne

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[Phil]

I'm no techie, Mike, but I would question whether you need to bother?
In practice when driving a tank you are constantly correcting, and if
it has a slight (say) left bias then you're just correcting right
more.

Some tanks (eg Tiger II) have a pronounced bias one way (because of
the track configuration).

Phil "OK that wasn't actually any help" Palmer

[Gregory Pwneror]

It might work, it'd be better to use a transistor instead and have the pot go to the transistor. You might also be able to get away with making a voltage divider using a pot. I'm not really a motor sort of person.

-Gregory

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[Frank Pittelli]

Mike Mane wrote:
> Anyway, the tank is powered by two 9VDC gear-motors, and I would like
> to be able to control the trim of these engines to ensure equilibrium.
> Can this be done with a potentiometer on each circuit? If so, what
> kind of pot and how are they installed?

Yes, it can be done, but you'll need to describe the motors a little
more in order to determine the proper size pot. Specifically how much
current does each motor draw when running under load? We need that
information in order to determine the max wattage rating needed for the
resistor.

Of course, using resistance to decrease the voltage generates heat
(that's where the unused energy goes) and is therefore not a very
efficient way to go. You'd be better off going with an inexpensive
manual speed control. Here again, if we know the current draw for each
motor we can recommend an appropriate model. I've used manual speed
controls for the rotate/elevate motors in the Brumbar and now the
Navarrone Gun with great results.

Frank P.

[?]

You can modify most servos for continuous rotation..instant variable
speed geared motor... for small tanks.

[Mike Mane]

Pardon my ignorance, when you say "under load", do you kean the motor
connected with all its final resistance such as gears and tracks?

Also, I was not planning to go as far as digital control. This is a
cheap, but important project and I hope I can get it running.

—Mike M

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On Nov 22, 2009, at 6:03 PM, Frank Pittelli <frank.p...@gmail.com>
wrote:

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[Frank Pittelli]

Mike Mane wrote:
> Pardon my ignorance, when you say "under load", do you mean the motor

> connected with all its final resistance such as gears and tracks?

Yes. A direct measurement while underway is the best value to consider.

> Also, I was not planning to go as far as digital control. This is a
> cheap, but important project and I hope I can get it running.

Then, as previously suggested, you should manually compensate for any
drift while driving it. That's the cheapest and most effective way to
get a tank to go straight, because most brains can easily compensate for
many, many different situations, such as uneven terrain, bumps, poor
traction, etc. Electronic solutions can never come close, without
spending lots of money.

Frank P.

[Mike Mane]

I understand now, thank you. I should probably construct the tank
first as long as I leave room for custom electronics if necessary. As
for the physical correction of driving errors, I was planning to use
manual control for the most part, but while test-running the motors I
could easily hear a difference in pitch, therefore most likey speed. I
was hoping to take care of most of that with adjustable trim
controllers.

—Mike M

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On Nov 23, 2009, at 9:53 AM, Frank Pittelli <frank.p...@gmail.com>
wrote:

[Frank Pittelli]

Mike Mane wrote:
> I understand now, thank you. I should probably construct the tank
> first as long as I leave room for custom electronics if necessary. As
> for the physical correction of driving errors, I was planning to use
> manual control for the most part, but while test-running the motors I
> could easily hear a difference in pitch, therefore most likey speed. I
> was hoping to take care of most of that with adjustable trim
> controllers.

Not necessarily true. Motors will often run a different speeds under
"no-load" conditions, but as soon as you make them start working, they
may run at exactly the same speed. Moreover, by their nature, tank
tracks want to go in a straight-line. So, even if the motors are
slightly different speed, the mechanical system may still keep the tank
going straight.

In short, there are so many variables to consider, that until you get it
built, there is no reason to worry about it. R/C steering with mixed
track controls is easy enough that you won't even think about it in the
field.

Frank P.

[Mike Måne]

I see. I suppose I should at least begin building the tank and stop
fretting. I was just a bit worried that it would be difficult to
control with only one reversible speed.

Thanks again,

-Mike M

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