Leg Harmonic Motion Problem                                               

I attached the legs to the cart thinking that when the legs were compressed by the weight of the cart, that it would dampen the harmonic motion that occurs at times. Wrong; it got worse, as can be seen in this video of a right leg test. The leg even becomes airborne at times, which would be handy if I was building a running bot, but this one is supposed to stay on the ground. In the test I make the right leg extend and contract several times and then I extend the leg incrementally. 


Visit   http://asylumstreetspankers.bandcamp.com/track/monkey-rag  to hear more or to purchase Asylum Street Spankers music.

Reasons for the harmonic motion may include:

1.    When a leg is compressed, more air is added to it’s piston causing upward movement (as intended). By the time the leg reaches it’s correct position, there is upward momentum that carries it past nominal, now causing air to be released from the piston. The leg contracts and the downward momentum causes it to overshoot the nominal position again. And so it goes.

2.    When the leg is nearly straight, small changes in piston length cause large changes in the knee angle. The resulting rapid lower leg movement, carries a larger amount of kinetic energy, making the harmonic movements more extreme.

3.    When the leg is nearly straight, the piston cylinder has the greatest volume. Large air volumes take less force to compress per unit of piston travel, so the leg is softer(more easily compressed) when nearly straight.

4.    When the leg is nearly straight, the lower leg’s mechanical advantage to compress the piston cylinder air is at the maximum. This also makes the leg easily compressed when it is extended.

Ways to dampen this harmonic movement:

1.    I thought adding mass to the leg would slow or stop this movement. The leg will bounce even with me sitting on top of it. (50lbs legs + 150lbs me = 200lbs of load!)

2.    Making the valve holes small would slow the movement speed. The leg would probably not overshoot it’s target position. But I need the legs to move fast if they are to walk.

3.    Changing the shape of the valve holes to taper-off the air flow as the target position is neared. This would probably help.

4.    I could monitor the knee position in real time and add or release air with a servo driven proportional valve. The speed could be calculated so as to not overshoot the target. This could take up a lot of microprocessor time and a timing lag could also cause harmonic motion.

So, I am going to rework the current servo-follower valves to make the air holes of a tapering tear-drop shape.  If this does not work, I will abandon my servo follower type of valve and instead, make proportional valves that regulate the airflow under direct microprocessor control.