Wave Tank

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Outline

We have a version 1 Wave Tank on the Exploratorium floor, designed and made by my friend and Explo alum Jesse Marsh. The design was included in a suite of exhibits for the Science Center of Singapore. The gallery is focused on Earth Science, so the exhibit learning outcomes were modified slightly to include the dampening effects of foliage on the wave energy.

Additionally, there were certain shortcomings of the version 1 of this design that i wanted to address if i could.

Generally speaking, the paddle is actuated with a 3:1 timing belt reduction, and a couple linkages. The Beach lift makes use of a 5 start, acme threaded leadscrew, with a pivoting acme nut on the end of the beach spine.

V1 issues, Explo floor version

  1. Tank had multiple tapped holes, as well as clearance holes. These have held up ok on our tank (which is a couple years old), but adds risk of damage from improper maintenance or careless manufacturing. Our tank has not leaked, but the risk is significant with the many holes below the waterline.

  2. The axles for beach use an expansion/jack system which increases stress on the acrylic tank walls.

  3. The beach and paddle motors are independently controlled. The paddle motor logic is running in the integrated motor controller, rather than having a single central logic and control.

  4. The beach lift motor is a DC gearmotor controlled by a simple relay circuit which reverses polarity. There are no end stop switches, so to prevent the motor from burning out at the top and bottom of the travel, a torque limiter simply allows the motor to continue spinning, but the limiter slips and prevents further leadscrew rotation. This clutch would wear out, changing the effective torque limiting, as well as dropping clutch plate dust into the tank.

  5. Beach lift motor pivot is off the centerline of the screw, creating a constant side load on the motor and delrin bearings, which is wearing the delrin out prematurely.


The CAD model was redrawn from scratch, despite being quite similar to the prior design. This was probably not necessary, though the frame needed to be redrawn to allow the end door panels to be flush with the end of the tank. The frame, tank and top vitrines were made out of house, but i machined, welded, and finished, most of the other parts myself.

Singapore V2 Features

To address the issues from the floor version of this exhibit, i modified the design in the following ways.

  1. The tank has only through holes, and the minimum count below the waterline. There are two holes below the waterline, for filling and draining. The top flange has through holes only.

  2. A HDPE false bottom is installed which has a stainless subplate inset into it. This allows really strong, durable threads that don’t go through the tank. These locations are used to attach the clamps which constrain the beach and paddle axles. Easy to install and remove, and burly. No leak potential due to the lack of through holes.

  3. Both beach and paddle motors are controlled from a single arduino, which allows co-ordination. One could program certain wave frequencies at certain angles of beach. One could have a specific reset “attractor” setting that each moves to.

  4. Since the beach motor is not a Teknic Clearpath servo motor, we can use hardstop homing to find the mechanical limits once each boot. From there, the encoder and arduino control lets us move within the acceptable range, without hitting the stops again, and the torque limiting clutch is eliminated, and therefor can’t wear out.

  5. Bronze bearings added for wear resistance, and pivot mechanism redesigned to allow the pivot axis to pass through the leadscrew axis.



v3 Improvements

  1. The vitrines on top should really have either a lower acrylic tapped block to better constrain the bottom corners, or single forgo the side panel doors altogether. Removing the doors creates a cleaner look, but means access to the motors is not quite as good. I hindsight, i think the simpler single vitrines would be a smart, clean move.

  2. Integrated heating of the top vitrines would be a good idea to drive off condensation. One clever solution might be to use closed loop stepper motors and my TMC-5160 driver boards instead of ClearPath servos. This way, most of the benefits of the ClearPath motors could be realized (low noise, sensor-less homing), but the motors could be commanded to have a significant holding torque applied, which would create a fair bit of waste heat, actively heating the vitrines and driving off moisture/condensation. Additionally, the motors in the condensation area would be cheaper, so replacement due to condensation damage would be less costly.

  3. Single mounting plate for the paddle motor and pulley. This would increase stiffness, and reduce noise. If we went with a NEMA 34 stepper motor, we could get away with a direct drive solution as well, which reduces noise by eliminating the 3:1 belt reduction altogether.



For some additional context on the exhibit, there are a couple videos shot for SCS maintenance crew that talks a little more about the maintenance.