Sisyphish was inspired by the Sisyphus Kinetic Art table by Bruce Shapiro. There’s not much conceptual depth here. Sisyphish is like the Sisyphus Table… if it were a fish tank. A magnetic ball is actuated by a magnet beneath the tank to draw patterns in the sand. Sisyphish, as you might have guessed, is a portmanteau of Sisyphus and fish.
I haven’t seen the motion system used for Sisyphus elsewhere. It’s a modified polar gantry that uses a rotating arm for rho (linear) positioning and a spur gear for theta positioning. The center spur gears are made out of Waterjet Cut Delrin, and the pinion gears are off-the-shelf from Mcmaster Carr.
The axes are driven by stepper motors running on a Einsy Rambo board. I had the board leftover from a previous build, and it has TMC stepper drivers on board which support StallGuard. One challenge with designing this gantry was eliminating wires routing to any of the rotational stages as that would necessitate adding slip rings to the design. TMC drivers with StallGuard allow me to home the rho axis without the need for additional hardware limit switches.
The Einsy Rambo uses a Raspberry Pi running Octoprint as a host. I can upload my modified polar gcode files to the octopi server and run the machine headless. Paths for this machine aremade using Sandify. Sandify outputs a file with a theta rho coordinate pair, and a script that I wrote manipulates it into gcode that’s usable on Sisyphish.
The bottom of the system is CNC routed acrylic, and the base for the fish tank is a 12” diameter piece of schedule 80 PVC pipe. I cut the PVC pipe on a horizontal bandsaw, and to my knowledge it holds the record for the largest part cut on my company’s bandsaw. I had to extend the vice jaws upwards with plates of aluminum so that the jaws were proud of the midpoint of the PVC.
The final part attached to the base is a Tetra air pump. Stationary water isn’t great in a fish tank, so the air pump routes to a bubbler inside the tank which circulates the water. The tank itself is an off-the-shelf 12” x 12” Acrylic Cylinder. All of the hardware components were designed around this tank geometry.
On a basic level, there are three levels of filtration in most fish tank filters: mechanical, chemical, and biological. Mechanical filters usually take the form of open-cell foam which separate large particulate matter from the water. The next stage is usually chemical which takes the form of things like activated charcoal that removes chemicals and impurities from the water. The final stage is biological which represents all of the nitrifying bacteria that convert waste byproducts into less harmful compounds like nitrites. In a well-designed tank, nitrites will be consumed by plants, and the tank can run for months without the need for water changes.
In my case, I didn’t like the aesthetic of a large filter hanging off of the back of the tank, or large filter lines routing up the sides. I routed the air up the side of the tank with a clear PVC tube, but beyond that nothing routes into the tank.
I designed a custom filter to keep the tank inhabitants happy. It features an open cell polyurethane foam for mechanical filtration and a significant volume of lava rock and Seachem Matrix for biological filtration. The air pump doesn’t provide the same amount of water flow that even a low-strength filter might, so I compensated with a significant amount of surface area for biological and mechanical filtration. The tank cycled quickly, and I can get away with very infrequent water changes as the nitrifying bacteria make quick work of ammonia and nitrates.
The veneer of the tank was made using the same technique I used for the Veneer Vases project. I grabbed some Teak wood and cut small 2-dimensional lines into it to mechanically compromise the wood in one axis. This allowed me to bend the wood over the curvature of the PVC and the tank. After a few hours fiddling with hose clamps and low-surface energy optimized epoxy, I had the veneers attached.
The most important feature of the project, the livestock, are Taiwan Bee Shrimp. These are among the most colorful of the Caridina and Neocaridina shrimp families. The main benefit is that they can breed with differently colored shrimp without losing their coloration over generations. More common Neocaridina shrimp like Cherry Reds can’t be mixed with other colors of Neocaridina shrimp because the interbreeding will result in brown “wild type” offspring after a few generations. This is not the case with Taiwan Bee shrimp, and I can expect vivid coloration for as long as I keep the tank going.
As well as this project ended up, I’m not sure that I’m going to keep this tank going, and I’ll probably make a new tank for the Bee Shrimp. The main challenge with this tank is fairly simple; shrimp poop. I was hoping the volume wouldn’t be quite as significant, and I was also hoping that biological filtration would quickly result in the poop fully decomposing. I was incorrect on both fronts, and the sand at the bottom quickly loses its luster due to a surplus of shrimp poop.
Vacuuming the bottom of the tank frequently cuts down on this, but it also runs the risk of sucking up baby shrimp which like to hang out imperceptibly in the sand, so I prefer to do water changes using water at the top of the tank where shrimp are less likely to hang out.
I’m really happy with how this came out, but the maintenance required to keep it clean will likely motivate me to move towards a more conventional setup. Maybe I’ll come up with a more sustainable way to mix mechanics and fish in the future.