(spring 2000)

Bob is a 3D passive dynamic biped with knees and actuated ankles. He is the result of the MSc-project of Martijn Wisse. At the end of each step, the ankle extends to input energy into the cyclic walking motion. The knees and hips are entirely passive joints. In order to prevent falling over sideways, a hipmass is driven left-to-right by a third motor. All three motors are commercially available servo's, controlled by a cheap microprocessor.

 

Compensation against lateral instability by a driven hipmass (the batteries).

Joints

The hips are just passive hinges. The knees are passive hinges with unilateral constraints to prevent hyperextension. Springs give the knee joint (almost linear) negative stiffness, slowing down the shank so that the swing knee is still bent when the legs pass each other, providing more ground-clearance.

 

Bob's knee in extended and bent orientation.

The ankle is a passive joint during the first part of the step. At the end of the step, the "achilles" tendon stretches and signals the servomotor to pull the tendon and so extend the ankle, lifting the entire robot a few millimeters. This energy input should be sufficient to compensate for the impact losses at knee strike and heel strike.

 

Ankle actuation scheme.

Actuation

The robot is actuated with three commercially available model airplane servo's ($150 each). These require a 5V DC power supply plus a pulse modulated signal defining the target orientation. With a frequency of 50 Hz, the servo must receive 1-2 ms pulses of 5V. These are generated with a cheap microprocessor. The inputs come from the two achilles tendons.

 

Servo and microprocessor.

Simulations

A fully 3D numerical analysis has been performed to determine the required mass distribution and other parameter settings for Bob. The parameter settings have to be such that the walking motion emerges naturally when started with the correct initial conditions. Therefore they have to fulfill a number of criteria:

• Existence of periodic gait,
• Stability of this periodic gait,
• Foot clearance,
• Positive stance knee torque; to press the knee in its constraint and prevent buckling,
• Lateral stability; obtained by keeping the center of pressure inside the foot contact area,
• Manufacturability; realistic parameter settings.

VRML representation of the simulation model.

The necessary knowledge of multibody dynamics all comes from our cooperation with the group of Engineering Mechanics (in Dutch).

The result of the simulation is given in the next simulation movie:

Preliminary simulations

Prior to the final simulation mentioned above (resulting in the design of Bob) simulations were performed with quite a differently designed biped, called Hoelahoep. This simulated biped was not constructed as it showed some difficulties in the simulations. Different aspects of these simulations are mentioned here.

Problem: rotation by counterswinging legs

Bipeds have a tendency to rotate around the vertical axis (yaw), because of the change in angular momentum from counterswinging legs. This is demonstrated in the VRML-simulation below (click the green square floor to start the simulated motion). To view VRML simulations, you need a VRML-plugin for your browser, like Cosmo Player.

Movies

These movies show how Bob walks, he still needs a little help.