Subject:
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Re: Cool robot platform and a question...
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Newsgroups:
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lugnet.robotics.handyboard
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Date:
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Tue, 3 Nov 1998 13:51:30 GMT
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Original-From:
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fthompson9@{Spamless}aol.com
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Viewed:
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2034 times
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Clem,
In a message dated 98-11-03 01:04:33 EST, you write:
<< I started probing around for continuity to try and figure out
what's what and I didn't get very far. >>
This is the right place to start. If you can see the circuit traces,
copy them to a piece of paper. The part your really trying to find is which
lines connect to the controlling chip. I figure that you have two choices
here, one is a simple H bridge type ciruit with one or two outputs from the
chip to the Amp per motor; the other choice is a linear amplifier driven from
the control chip with feedback from the power amplifier. A clue to the latter
would be finding a pair of resistors (called a voltage divider) connected near
the output. (This would have two resistors connected together in series, one
end connected to the output, the other end connected to ground and the middle
connected to the controller chip).
Once you have the outputs from the chip, it's time to find out what they
normally do. Attach minus of your volt meter to the minus terminal of the
battery. With the power on and the car off the ground, touch the plus lead of
your meter to these points on the controlling chip. See what happens when the
car is commanded to go forward, reverse, turning,... Do each test with DC,
AC, and Pulse detection on your meter. (I assume that you don't have an
O'scope. If you can get to one, use it to make these measurements.) Once you
know what the control chip does to drive this amplifier, you remove the chip
and you tap into its place and mimic what it did.
<< When I tried to check voltages
with the power applied, when I touched the circuit (or even got the probe
near the circuit) the car would start twitching wildly, which makes
debugging difficult.>>
Radio circuits are sensitive and low level RF noise can make them do
stuff. try twisting the meter leads together as much as you can, connect the
minus meter lead to battery minus and then test. You also might consider
touching the tip of the plus lead to the tip of the minus to discharge it,
although the meter resistance should have done this (sometimes the meter
impedance is high enough to make this worthwhile). If your hitting the gates
of FETs, you could be charging/discharging them enough to turn them on,
although I would be most surprised to find that you have a gate driven by a
high impedance circuit.
Be very careful not to touch two points with the probe tip at the same
time, Sensitive circuits toast easily.
<<Would it be easier to just start from scratch?>>>
I don't think so, though it might be easier to buy an H bridge circuit
and replace the existing one.
Hope this helps
Pherd
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