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.

Legs                                                                     

After much consternation, I settled on 34 inch legs with the knee half way. 

The lamp from A Christmas Story.

Robot legs.

Top of "thigh bone".

The “thigh bone” is made of three bolted together one inch square aluminum tubes snugly filled with pieces of hardwood. Four skate bearings are recessed into the top and bottom of the piece, such that 5/32” all-threaded turns effortlessly at the hip and knee joints. 

Knee joint.

The “shin bone” is also made from 1” square tubing and is reinforced laterally with light weight aluminum bar. At the moment, “the feet” consist of rubber crutch tips. There is a steel hanger at the top-back of the leg to hold the big piston and another bracket at the bottom of the leg to receive the end of the piston rod. Just below the knee there is a smaller bracket that attaches the heavy piston return springs to the lower leg. There is a short piece of 1” U-channel attached to the lower leg at the “knee-cap” which stops the leg from over extending. 

Altogether, there is very little play in any of the leg joints and the extended legs move less than a quarter inch in the sideways direction when wiggled. 

Robo Hips                                                                                                                                                                         

Well, before I build legs, I needed something to attach them to... hips if you will.

Snippet of Erin Mckeown's song – My Hips

Download complete Erin Mckeown songs at Amazon,com.

Temporary leg stand.

I built a three foot high temporary leg stand that holds the plank to which the legs will eventually be attached. Nine-inch diameter lazy-Susan swivels are mounted on each side of the plank and they turn in unison because they are firmly connected through a 4.5” diameter hole in the center of the plank. This swivel has a ratcheting/locking mechanism mounted on the top side and has hangers for the legs mounted on the bottom side.

Ratchet mechanism and drag adjustment.

I must admit, the ratcheting/locking mechanism is a bit odd. I needed a mechanism where the ratcheting direction on the right side was in the opposite direction of that used on the left side. What I did was to cut an unused 7¼” fine toothed circular saw blade in half, flip one half up-side-down and then weld the halves back together. Now half the saw teeth point one way and half the teeth point the other way. Steel ratchets are located on lateral aluminum “U” channel arms that can rotate in 30º bites around a central metal shaft made of 1” black pipe (shaft welded to the center of the saw blade). 

Ratchet arms.

A 1/8” nylon line threads through the end of each lateral arm and is attached to the ratchet release of the opposite arm. So when the right line is pulled, two things happen. First, the left ratchet is released allowing the hips to rotate to the right if so inclined. Second, the line tugging at the end of the right arm supplies the force to rotate the hips to the right with the help of  the right ratchet. When the right line is released, the right arm ratchets back to it’s lateral position with the help of a return spring. Now with both opposing ratchets engaged, the hips should be locked into its current position. The line though the end of the left arm works in the same way; ratcheting the hips to the left and when released, locking the hips in place. Pulling on both lines at the same time would release both ratchets allowing the hips to move in any way they wanted and that is probably not a good idea. I had to add an adjustable drag  mechanism  to the lazy Susan swivel to allow the swivel movements to be less erratic and more deliberate. 

Leg hangers.

A 5/16” all-thread axle runs though each vertical segment of two hefty “┌─┐” shaped steel bar hangers. Those bars are bolted to a thick plywood base and the bolts run through both lazy-Susans to the ratcheting saw blade above. The leg hangers and ratcheting mechanism move in unison with a range of + 90º from the central forward facing position.

So now I have hips and a place to hang the legs.