Pneumatics                                                                                       

Pistons.

The pistons are made out of 3” ABS pipe. The pistons pivot smoothly on solid 3/8” steel axels that runs through 13/32” brass tubing horizontally epoxied into the ABS caps. The caps also have ¼” brass air line ports and 0-60 lb pressure sensors (Mouser 785-NBPDLNN060PAUNV $12.02) epoxied into them. I bought 3” rubber piston cups from herculesus.com  ($7.50 each) that are mounted on ½” aluminum round tubing shafts. I fashioned aluminum holder blocks for the 7/8” bearings on the ends of the piston rods. The pistons have a working range of 15.5” to 27” on-center.

Servo-follower valves

The piston servo-follower valves are located on the front side of the “thigh bones”. I made the valves very cheaply out of telescoping 3/8” and 13/32” brass tubing epoxied into 1/2” PVC T-fittings. The inner tube can be driven back and forth 1.5 inches by a high-torque servo (Power HD 1501MG $15.66) and the outer tube, T-fitting and holder are moved back and forth the same distance by the movement of the lower leg.

When the leg is under-extended, pressure in the inner tube flows though holes lined up with holes in the outer tube allowing pressurized air to enter the piston, extending the leg. When the leg is over-extended, different holes are aligned that allows pressure to escape from the piston, retracting the leg. This process causes the leg to mimic whatever position the servo happens to be in, regardless of the dynamic changes in load that the piston may be going though. Push in on the piston and the air pressure will be increased to compensate exactly. Release force on the piston and it remains steadily in place. Way cool! 

                                          Click images for bigger views of valve action.