It took me about 6 hours to create the horizontal and vertical piston geometries needed to be able to make it walk. It then took me a week-and-a-half to design the pneumatic circuit to make it walk the way I wanted. :^)
In typical bipeds, quadrapeds and hexapods, the total leg count is split into two groups. When one group is up, the other group is down. The groups take turns being up and down, and the up group moves down at the same time the down group moves up. This makes body of the walker raise and drop each time the leg groups switch weight bearing responsibilities.
Years ago JP Brown threw down the challenge to me to try to make a pneumatic walker that didn't suffer from this up down lurching that he called a gravity well. I decided that with Quad242 I would have no gravity well.
To acheive this, I'd need my quad go from having two feet down (and two feet up) to four feet down (and weight bearing), and then the other two feet down (and two up). This is the part that took me a week and a half to design.
Each leg of Quad242 has two pistons: one to raise and lower the foot, and the other to sweep the foot forward and backwards. The front left leg and the back right leg are paired together. They both raise and lower the feet and sweep them forward and backward at the same time. The front right leg and back left leg are also paired together.
One of the things that makes the pneumatic circuit tricky to design is that no two pistons expand or contract at the same rate, due to differences in manufacturing and wear. Also assymetries in the friction of the feet assemblies make the pistons expand and contract at different rates.
While we cannot make two pistons expand and contract at the same rate, we can make sure that both pistons stay coordinated throughout the walking process. Without coordinating the pistons' expansion and contraction, our walker will walk with a limp, or partially walk the wrong way or worse.
I've developed a method for keeping groups of pistons coordinated. Pneumatic Sequencers describes a number of simple sequencers, and how to design complex pneumatic circuits that go through repeated patterns over time, just by being pressurized.
Quad242 uses a repeating six step sequence to achieve walking. The walking forward mechanism is captured in this six part collage. This table describes the sequence of leg movements to achieve walking forward. Notice that the last entry in the table is the same as the first entry in the table showing how the sequence repeats.
| Front-Left/Back-Right | Front-Right/Back-Left |
| Down/Forward | Down/Back |
| Down/Forward | Up/Back |
| Down/Back | Up/Forward |
| Down/Back | Down/Forward |
| Up/Back | Down/Forward |
| Up/Forward | Down/Back |
| Down/Forward | Down/Back |
The design for Quad242 looks like this in my LEGO log book:
To make sense of this you might want to review my pneumatic sequencers page.
I originally designed Quad242 to walk forward. I then added pneumatic polarity reversers to swap the expand/contract inputs and switch outputs for the hips. This allows Quad242 to walk backwards.
I added two more polarity reversers to reverse the hip activity and switch outputs for one side of the body. This allows Quad242 to turn right, and left.
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