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Interactive Leg Linkage Design powered
by JavaSketchpad
(Drag points A-F, X-Z to modify geometry in real-time) (Please be patient while applet downloads!) Hide the construction elements JavaSketchpad Home JavaSketchpad Construction Grammar Design Goals: Vertical locus at H; ABC inline when leg down; Point C vertically above same point at leg up & down.The leg linkage used on Ziggy is key to the operation of the robot. Using linkages has allowed me to convert the torque from the servos into vertical lift without the need for gears or belts. This is an important design feature for me as I intend to take Ziggy out to the Black Rock Desert for Burning Man, and it's verrrrry dusty out there... The servos are sealed with o-rings, and the leg joints are pretty well sealed so I am not anticipating too much dust angst. I'll let you know if this is so... I initially prototyped the linkage using Lego Technics, which is extremely fast and easy to use. When I had an approximate design, I transfered the geometry to the JavaSketchpad applet. This applet then allowed me to tweak the lengths of the various linkage bars until I had the best compromise between design goals. Go ahead and try it - you can drag any of the green or yellow points above and see what happens... Designing the linkage so that points ABC are inline when the leg is down gives me two advantages: firstly, it maximises the force downwards at H as the leg reaches its lowest point (think of straightening your arm against a weight). Secondly, it allows me to drive the linkage AB slightly over centre against a mechanical stop; then any force on H (the robots weight) will push the leg against the stop, holding the robot up. Thus, when Ziggy stands and locks his legs, he will support his weight even if the motors are turned off. The other two main design goals were to keep point C vertically above the same axis when the leg is up/down, so that it can be coupled to the servo using a universal linkage. Also, the leg locus at H should be as close to vertical as possible to prevent loss of force due to sideways dragging.
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