Page 123 - The Unofficial Guide to Lego Mindstorms Robots

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The arm also needs to be strong, as its parts are under a fair amount of stress. You may notice, as you use Minerva, that the
p ieces of the arm loosen up after time. You could make the arm stronger, but it would also be bulkier. See Ben Williamson's
FetchBot (in the "Online Resources" section) for the original arm design, which is stronger and larger than Minerva's.

Balance

One of the fundamental issues Minerva face s is balance. The grabber arm sticks out in front of Minerva. Without some kind of
co unterbalance, Minerva falls right on her face. One way to fix this is to move the drive wheels closer to the front of Minerva.
In this case, howe ver, the directional transmission and the grabber arm gearing would not have enough space to coexist.
M inerva solves the problem of balance by mounting the RCX near the back of the robot. The weight of the RCX (the batteries,
mostly) more than compensates for the weight of the grabber arm.

Drivetrain

Although the directional transmission is relatively s imple, Minerva's drivetrain is a little more complex. When the motor runs
o ne way, the two drive wheels both turn forward. When the motor runs the other way, the drive wheels turn in opposite
directions, causing Minerva to spin in place. Minerva uses a fairly complex system of gears to translate the outputs of the
directional transmission into the correct wheel movements. Only one of the outputs drives a wheel directly.

Two Sensors, One Input

You might think that the RCX's three inputs limit you to designing robots with only three sensors. In this section, I'll describe
o ne way to move beyond the three-input barrier: attaching more than one sensor to a single input. Minerva, for example, has a
touch sensor and a light sensor attached to a single input. If you have ambitious plans (and parts) for expanding Minerva,
you've got two inputs, not just one, with which to play.

T here are two common variations on the theme of attaching multiple sensors to one input, both of which are discussed here.

Multiple Touch Sensors

T he first and easiest possibility is to attach multiple touch sensors to a single input. With the input configured for a touch
sensor, a press on any one of the attached sensors will produce a value of 1 on the input.