SMA Actuated Pantograph Robot

Pantograph Robot
   

This picture shows the complete pantograph robot, and there are two close-up pictures below showing some of the details.  This device is a test rig for our experiments into fast, accurate motion control of SMA actuators.  It features:

  • two antagonistic pairs of 0.1 mm SMA (Flexinol) wires, each pair operating a drive shaft via a pulley;
  • a lightweight pantograph mechanism made from carbon tubes, which serves as a coupled, nonlinear dynamic load for the two actuator pairs to try and control; and
  • high-resolution optical shaft encoders for accurate measurement of drive-shaft motions.

We have used this system to test a new rapid-heating algorithm that doubles the speed of the wires, and we have implemented several different control laws in an effort to find one that produces highly accurate motion.

Here is a movie (9MB) of one of our earlier attempts, which used a relay controller and a phase-lead compensator.  It makes the pantograph tip trace a small square at 1 Hz.  The speed is very good, but the accuracy needs improvement.

We have since implemented a modified proportional controller, which is much more accurate.

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Front view, showing pulleys, chords, eyelets and (barely visible) SMA wires

Rear view, showing optical encoder discs
Front View Showing Pulleys Rear View Showing Encoders