REVIVING AN ANDROBOT BOB: Part 2
Original TOPO battery monitor board.
seemed to be right on the top of the chassis like the original TOPO controller board
mounted to its base in the RF model (see
the November ‘07 SERVO). I believe that
most of the original boards would have
mounted in this area. I didn’t want to drill
into the chassis to mount the boards.
Instead, I wanted to have a separate plate
which I could secure all the electronics to
and that plate could bolt to the existing
mounting studs on the chassis.
After taking a look at the base, I
headed out to the garage to sort
through some old aluminum plates I
had been saving. I found a rectangular
aluminum panel (salvaged from some
odd piece of computer gear) which
looked like it would do the job.
The custom power board (top).
44 SERVO 03.2008
Unbelievably, it was just the right size!
It already had a hole drilled in each corner and those almost lined up perfectly
with the mounting studs on the base. I
only had to drill out the holes a little
more to use them and bolt it in place.
That plate was pulled back out and
placed on the workbench to figure out
how to secure all the electronics to it.
First, I just set all the boards out on the
plate to determine what locations made
the most sense for the placement of
each board. The final result had the
dual LMD18200 based H-bridge board
near the middle with the Handy Board
next to it toward the back. The portion
in front of the H-bridge is reserved for
a co-processor and speech, another
H-bridge for the head, and sound amp
which will be discussed next month.
Behind the Handy Board is the
power supply and distribution section.
This includes a 24V to 5V DC/DC converter, barrier strip, and custom power board
to handle the 6V and 9V power needed
by some sections of the robot. There
really wasn’t a place for the Polaroid
sonar module or the custom board to
multiplex the five sonar transducers.
For that, I used a set of tall stand-offs and another salvaged aluminum
part. This was a cover that was recycled
from an old video to fiber adapter. It
seems to shield the sonar and keep it
from interfering with the Handy Board
and the other electronics.
Power is provided from a pair of
12V 7 Ah batteries just like the TOPO
robots. These are going to power both
the logic and the drive system. The robot
drive motors are meant to run at 24V,
but the logic and everything else runs at
lower voltages. A choice had to be made
The custom power board (bottom).
right here. Was everything going to run
off 24V and be stepped down to a
useful level, or would it be better to tap
off 12V from the first battery? Other
voltages can be derived from these.
The disadvantage of this method is
that the two batteries may discharge at
different rates since the first battery
will have more load on it. If the charging system can handle it and charge
each battery separately, then this can
be an easier way to go. Since the
power for the overall robot would be
switched from 24V, I was going to use
a large relay that would switch on the
12V supply from the center tap so that
would be off when the robot was off.
I built this section on a proto board
and it worked fine. However, before
implementing this solution I changed
my mind. I had some extra blank PCBs
for the TOPO II battery monitor board
and I built one to use on BOB. This has
several advantages. It acted correctly
for this type of robot (one switch to
push on and another to push off). It
also handled charging of the two
batteries, and would automatically shut
down the robot when the battery
power was drained. The input had both
the full 24V and the 12V from between
the two batteries and an onboard relay
would switch them both on at once.
Now, the extra relay to turn on the
12V supply on the custom power
board was no longer needed. Instead
of just yanking it off the proto board, I
just re-wired it so that it can be directly
controlled from the Handy Board or
extra co-processor board. It will be
available for future expansion to switch
on and off a large 12V or 24V load,
depending which is eventually needed.
Whether designing a full blown
commercial project or just wiring up a
homebrew one from leftover parts, it is
important to think about safety. A lot of
people ignore this and may get away
with it for a while. It is best to engineer
in at least some sort of protection for
your circuits. On the output of each one
of the batteries, a fuse is installed to
provide short circuit protection. It is much
better that a fuse should melt instead of
your wiring harness! The Handy Board
has a Polyfuse that acts as a safety in
case you overload that particular board