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frequencies tends to cause problems
with opto-isolators. They have an
inherent turn on and turn off delay,
which results in some edge shaping.
Too slow a response would result in
poor control, additional heating, and
possible damage. With the part chosen, we should be able to experiment
up to at least a 50 kHz PWM rate.
Of course, there is no guarantee
that opto-isolating the inputs will
prevent your controller from being
damaged if you smoke the driver, but
I’m hoping you will be hard pressed to
do it. I’ll leave that to Tim, our resident
smoke artist to test ...
The power section uses a DC/DC
converter to drop the input voltage
down to a steady 12V for the driver,
fan, and auxiliary relay without
wasting power. An additional three
terminal linear 5V regulator provides
the logic supply.
Two automotive fuse holders were
added to the circuit. If sized properly,
these fuses will offer some level of
protection from total catastrophe, but
aren’t a perfect solution. Some might
argue that this is a waste of time. We
have used this method in many high
power switching circuits before and it
does offer some protection; at least
it will stop a dead short across your
battery stack. Nothing generates
smoke faster than a locked rotor.
There is NO reverse polarity protection on the battery input. This
could have been done with a large
diode across the fuses and input, but I
didn’t want to give up the PC board
area for it. I may revisit this idea on the
next revision.
At first, I was tempted to try and
wire this up on a perf board, but a
friend that designs high power circuits
advised me not to waste my time.
Motor drivers are funny circuits with
many circulating ground currents. To
do it right, it should be on a PC board,
with 4 oz copper layers (see Figure 2
— available on the Nuts & Volts
website at www.nutsvolts.com).
Three days of layout produced a
compact package with heatsink and
fan, all in a 4. 25” x 4. 5” x 3” high package. It is a combination surface mount
and through hole design. This is due
to a combination of factors, including
cost and the high currents involved.
The boards are out for fabrication
and will be back soon.
(Phil again ...) It’s really great to
have an electronics wiz like Ken as part
of your robotics club; it certainly makes
some things a lot easier. Next month,
we’ll assemble one of the motor
controllers and see how it performs by
writing some control code and driving
one of the motors. Stay tuned. NV
RESOURCES
■ The HIP4081A Driver
www.intersil.com/cda/deviceinfo/0,0,
HIP4081A.html
■ The OSMC official site on Yahoo!
http://groups.yahoo.com/group/osmc/
■ International Rectifier, the MOSFETs
www.irf.com/indexsw.html
■ Bascom, for those interested
www.mcselec.com/
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