When you are planning
your mouse, where are you going to put all the bits? There are a few key
points to consider but, in the end, you are going to be constrained by the
availability of parts and the available mechanical skills required to build
the beast.

Centre of mass
You will need to keep the mass low for better acceleration with small
motors. Keep the centre of mass low to improve cornering performance and
to prevent excessive weight transfer off the wheels under acceleration
and braking. Keep the components close to the centre of mass to avoid
excessive moments of inertia for turns. In particular, look-down sensors
on wings can contribute excessive moments, limiting your ability to start
and end a turn.


The greatest mass is likely to come from your batteries and, if you are
using stepper motors, from those. Stepper motors benefit from high voltages
and that means more batteries. Remember that you mouse only needs to run
for fifteen minutes or so. There is not much point in putting in, say,
1500mAh batteries if they will keep it running for an hour or two. Dry
cells have a better energy storage density than NiCds or NiMH cells and,
for the competition, may well be worth the expense although they have
very poor voltage regulation. NiCds are, of course, much more cost effective
for development. NiCd cells can be soldered together in any combination
you want so you should be able to make conformal batteries that can be
placed close to the motors. Make the batteries easy to replace and keep
a set (or two) of fully charged spares to hand. Everyone forgets to turn
things off sooner or later.


The main processor board may well turn out too large to make turning
in the maze convenient. Make sure you mount it high enough (6cm or more)
so that it can clear the tops of the walls during turns or near misses.
A handy option is to make the circuit board part of the chassis construction.
Use a multi-layer approach if you have to. Several small boards can be stacked together. An advantage of this approach is that you can modularise your mouse. Use a board for the processor, another for the sensors and a third for the motor drivers.


Some time or another, your mouse is likely to crash. It could do that
at top speed into an immovable object. Will it survive? If not, making
it stronger may not be the best answer. Weight (or mass anyway) is the
enemy of acceleration. Build light and resilient. Allow things to bend
or break off easily to absorb crash forces. Those forces are larger for
heavy items. Have a look at Dave Woodfield’s Voyager and you will see
a mouse where the main chassis is strong but all the bits that stick out
are held on with elastic bands. Make delicate parts easy to replace and
keep spares handy. Put a fuse in your battery supply to prevent a crash
shorting the battery. Nicads release huge amounts of energy when shorted.
Once the smoke has been let out, it is very difficult to put it back.

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