robotmatrix.org

 Introduction

• Objective: Using wide rubber bands wrapped around the last links, this robot grips and twiddles a ball between its fingers. Each finger has three joints, which can bend forward and backward.
• Parts:
  • Robix Rascal Robot Kit/Construction Set
• To learn: Using balanced motions, the fingers can even roll the ball back and forth while holding it steady in space, similar to the way a human can roll a marble between thumb and forefinger.

 Step 1: Assemble and build the Fingers Robot Body and Muscle

 Step 2: Fingers Robot Teaching and Macro Programming

If you put your two palms together, then put the table-tennis ball between your two index ("pointer") fingers, you'll be able to roll it back forth with several different motions. Your finger joints can usually only bend forward by about 90 degrees for each joint, and not backward very much at all just using your finger muscles. And you'll notice that usually the last two joints on each finger bend at the same time.

By contrast, the robot joints can also bend backwards by 90 degrees, and can move fully independently of each other.

The main trick to making this project work well is to program the motions so that a light pressure is maintained on the ball all the time. Your first experiments will probably drop the ball at certain points in your motion. When this happens it's tempting to just make the grip tighter all through the motion, but this can make it too tight at some points and needlessly strain the servos. Instead, try adding an intermediate position or two around where the ball is dropped. Also try to see positions where servos seem to squeeze excessively and ease pressure there.