Improved Mechanics

The mechanical devices that drove the three modules seen in the Cause and Effect display had been in place for years and were working perfectly.  And yet, the middle module – the Dancing Seahorses – had always bugged me.  Click here to see a video of the original Dancing Seahorses and the mechanism that drove them.  If you’ll notice, the up and down movement of the seahorses is not fluid: one seahorse rises quickly but falls slowly, the other seahorse falls quickly but rises slowly.  A close look at the mechanism will reveal the problem.  The bearing that drives the rocker arm is attached to a rotating wheel.  Since the angular velocity of the wheel is (nearly) constant, it is necessarily true that the rocker arm will rock more quickly when the drive bearing is closest to the rocker’s fulcrum.  Hence the uneven movement.

I solved the problem simply by attaching the rocker arm at a fixed distance from the fulcrum to the rotating wheel.  You can see the improved mechanism here.  I would have fixed this ages ago, but once an exhibit is in place and functioning, people are loathe to take it down for any reason.  Any reason, that is, except extra money for improvements.

As an added bonus to the viewer, you can click here to see a video showing the mechanism that drives the Waving Seaweed.  (Note: the mechanism is moving more quickly here than in the actuall display since I am using 12V for testing instead of the 6V that normally drives it.)  It was very important to me that the seaweed not merely rock to and fro, but that it undulate.  I accomplished this simply enough by having the seaweed attached to an eccentric that lifted the seaweed up and down with a brass shaft attached to the bottom of the seaweed that was constrained to a nearly vertical motion.  One problem I did not anticipate was that the seaweed was so heavy that it lugged the little electric motor quite badly, causing it to growl noticeably and slow visibly on the seaweed’s upstroke.  The attachment of the little counterweights you see solved the problem.

I also wanted the two branches of seaweed to rotate out of sync with one another, thereby creating a more natural feel.  I thought I might have to put a resistor in line with one of the motors, but it turns out that the ordinary variations in such mass-produced motors was enough to make them rotate at noticeably different speeds.