Pneumatic Segmented Caterpillar
Caterpillar moves by expanding and contracting pistons in a coordinated fashion. This moves the feet forward one at a time. It moves forward very slowly.
The crawing mechanism is based on a 7 cylinder pneumatic motor. In the motor, each of the cylinders expands until all the cylinders expand, then each of the pistons contracts until all the cylinders contract. Once all the pistons contract, the cycle repeats itself. I thank Dr. C.S. Soh, for his web page showing me how to make a pneumatic motor.
Each cylinder of the motor drives two switches, one for the motor, and one to drive the pistons that make the caterpillar move.
The body of my caterpillar has 7 segments connected together by 6 movement pistons. These pistons are the ones that make the caterpillar move forward. Each segment contains a foot (made up of a wheel and tyre), two pistons and two valves.
The pressure that makes the first cylinder in the motor expand also makes the first movement cylinder expand. Since each motor cylinder has to press on two pneumatic switches to expand, the movement cylinder expands before the motor cylinder. This pushes the first foot forward. When the motor cylinder expands, it flips the motor and movement switches.
When the movement switch flips, it makes the movement cylinder contract. At the same time motor switch flips and it starts to make the next motor cylinder and movement cylinder expand. Since the first movement cylinder is contracting while the second movement cylinder is epxanding, this has the effect of pulling and pushing the second foot forward.
When the second motor cylinder expands it switches the attached switches and movement happens for the third foot. This simultaneous expansion/contraction movement propogates from foot to foot until it reaches the tail end of the caterpillar.
When the last motor cylinder expands, the last motor switch is flipped, which makes each of the motor pistons contract in turn. No movement happes to the caterpillar during this part of the motor cycle. Once the motor cylinders are all contracted, the movement half of the motor cycle starts.
Caterpillar moves forwards very slowly, at about a half a cylinder length per pneumatic motor cycle.
The movement works because each of the feet has about the same amount of friction with the ground as any other foot, plus the amount of friction any given foot has with the ground is much smaller than the sum of the frictions of the other feet. To help keep the two previous sentences hold true, I made the RCX tethered.
All the movement is controlled by the pneumatics. All the RCX program does is run the compressor all the time.