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[DPRG] More ProtoBot developments to share...

Subject: [DPRG] More ProtoBot developments to share...
From: Kenneth Maxon kmaxon at qwest.net
Date: Sun Jul 29 15:53:31 CDT 2007

Hello DPRG...

I thought to take a minute and share some progress on my outdoor robot,
Protobot, in the hopes that in some small way it may inspire at least one
other amateur roboticist to get out to their garage / attic / basement and
work on their robot as well.

The first piece is pretty basic but fun as it is up and running.  (sorry for
the blurry picture I'll re-take later...)  The board in the photo is a radio
interface board.  It uses a pre-made, pre-certified module from Linx
technologies that has a digital decoder on the back end.  The decoder
listens for modulated pulse trains of coming from the radio section and when
the appropriate number of bits have been received, look at the first 10 for
an encoding pattern (much like a garage door opener.)  On the sending side I
am using a nice little keychain side pre-made keyfob.  On the board if one
looks closely they can see a set of red hexadecimal displays showing the
currently captured codes.  The large white monstrosity in the middle is a
break out adapter and all of the wires off the top of it are routed back to
a digital logic analyzer.

http://nikita.argia.net/kmaxon/proto_bot_129.jpg

The next one to share is the new motor driver I have just finished working
on.  In this photo (sorry this one is blurry also) the heat sinks have been
removed to give a better image.  The analog section includes a pair of back
to back inverting op-amps on the sense resistor.  The first op-amp is
configured in differential mode.  The second op-amp references the signal to
board ground and I have split the gain between the two of them so as not to
surpass the GBWP of the op-amps used.  (something missed in the original
design consideration that I'll update the schematics on my web site with
this afternoon.)   The amplified analog signal is driven off board to an
AtoD converter for monitoring by the robots embedded processor system.  Both
stages of the amplifier now contain low pass filtering such as not to exceed
the slew rate of the AtoD converters used.

In the near right section of the board are a pair of op-amps.  These op-amps
locally monitor for maximum current levels and control local shutdown
functions on board for short circuit protection.  The two stage low pass
filtering (mentioned above) not only helps with the slew rate limiting of
the signals into the off-board AtoD, but they additionally keep transients
from reaching the sensitive inputs of the comparators thus helping with
false trigger problems.

The last piece that just doesn't show well in this photo are all of the
components involved with gate driving and signal shaping for moving the
significant charge needed to punch these MOSFETs on an off in a proper
manner.  Again a change or two from the original design, I've lowered the
Zener clamps to 15v which when combined with the forward diode drop of the
back to back diode sets the trip point only slightly above the operating
range clamping below the damage level of the MOSFETs.  If I had it to do
again I'd go with 16V Zeners for slightly more operational range.

There are more components on the bottom of the board including (barely
visible black edges, front, left) isolated switching supplies and
pi-filtering to provide clean split rails for the op-amps and large 5-Watt
surface mount current sense resistors as well as some other miscellaneous
parts...

http://nikita.argia.net/kmaxon/proto_bot_146.jpg

Anywho, Hopefully this work will inspire someone to pick up a soldering
iron, write a line of code or grab a hack saw and start fabrication on their
own robot project...

-Kenneth
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