The Stewart Platform

This was a summer project that explored the use of a Stewart platform to simulate squash-and-stretch motion. We wanted to use this type of platform to create an expressive and animate robot for social interaction.

I first learned about the composition of a Stewart platform in order to design this custom robot. There are usually six linear actuators oriented at opposing angles that allow the top platform to move in six degrees of freedom (translation and rotation). Most of the linear actuators that I found did not satisfy our functional requirements, so I created my own.

Each actuator is a combination of an IG16 DC gearmotor and winching mechanism. The gearmotor is fitted with a custom-cut acetal pulley that pulls a nylon cord through an acrylic piston. The piston is fitted with two hobby springs and an acetal piston rod that runs through an acetal cap. As the motor pulls the cord, the rod is pulled downward and compresses the springs; therefore, the actuators are one-directional active. The piston rod is also fitted with a slider potentiometer to track compressions, which was a necessary function for this platform. If someone wanted to puppeteer the robot manually, they could compress the robots in different motions, and the robot could repeat the motions with playback. The motors' rotations are tracked using AS5030 magnetic encoders and magnets embedded in the acetal pulleys

The actuators were then assembled together using hobby ball sockets, placed at the correct angles for coordination. Once handed off, the coding for controlling the platform was handled by the group. The video to the left shows an example of its movements.