Wenxiong Hao

Graduate research at the Xi Chen Lab, Columbia University, on fast-swimming soft robotic fish driven by a pre-stressed bi-stable "hair-clip" mechanism. Published as a co-authored preprint (arXiv:2206.14867).

I designed the CAD models, 3D-printed, and assembled two fish prototypes — one pneumatically actuated, one untethered and motor-driven. For the pneumatic fish, I iterated on the air actuator's geometry and wall thickness and drove it through a custom pneumatic test bench (solenoid valve manifold, regulators, and DC supply). During testing, the silicone actuators tended to balloon and leak at higher pressures and faster actuation; I used FEA to locate the high-stress regions driving that failure and fed the results back into the mold dimensions to reinforce those areas. I also evaluated a shape-memory-alloy (SMA) hybrid actuation scheme before determining that a pneumatic-only drive delivered the best performance.

Separately, I laser-cut and tested the bi-stable mechanism across multiple materials and dimensions, helping validate that its swing amplitude is scale-independent — a key result of the published work.

My final contribution was redesigning the 3D-printed fish body with internal cavities to tune its buoyancy and center of gravity, so the fish could swim in a straight line without the external foam float that earlier prototypes relied on. Removing that float was what made the target speeds achievable: the pneumatic fish reached 1.40 body-lengths/second (a 2× improvement), and the untethered motor-driven fish reached 2.03 BL/s — roughly 3× faster than prior untethered soft robotic fish.

Led a 5-person team to design and build an untethered, servo-driven robotic fish from scratch — driven by an Arduino Nano, a single servo, and a battery, all housed inside a 3D-printed body. I designed the body in SolidWorks and optimized the skeleton for structural integrity and FDM printability.

Much of the work was hands-on prototyping and iteration, especially on the tail. The first design used a pure-silicone tail swung by an internal servo-driven lever, but it came out too soft to push effectively. Under a tight budget I cut off the rear half of the silicone tail and coupled a stiff waterproof card directly to the internal linkage, so the linkage flaps the card as the working surface — then iterated the card into a C-shaped profile for the contact area a flat card lacked. The front section of silicone I kept, repurposed as a waterproof seal where the linkage exits the body. Inside the body, the servo linkage didn't fit the way the CAD assumed, because the actual rod stock I could source didn't match the designed dimensions; I reworked the internal cavity by hand (heat-forming and hollowing it out) to make the available hardware fit. Final waterproofing combined molded silicone with a waterproof coating over the printed body.

This was the first robot I built from scratch, and it became the base platform everything else grew out of — both the ground vehicle and the robotic fish started from what I learned here.

The cart is fully self-contained and runs untethered. Compressed air is stored onboard in a large tire mounted on top, which serves as the air reservoir, while a 12V battery powers the electronics. An Arduino times the cycle and switches an AirTAC solenoid valve to release air on each stroke, driving a pneumatic cylinder back and forth. Steering is handled separately by a servo: after the cart covers a set distance, it reads its heading from an onboard compass and the servo corrects its course, so it holds a straight line with no driver.

Turning that back-and-forth stroke into forward travel went through two iterations. The first drive used a one-way ratchet wrench: on the push stroke the cylinder advanced the cart, and on the return stroke the ratchet freewheeled, so the wheels didn't roll backward. It worked, but too much energy was lost in the mechanism — so I redesigned it to push directly against the ground on each stroke, which proved more efficient. I designed and built the wooden chassis, mounted and routed all of the pneumatics and electronics, and tuned the drive geometry so each stroke reliably produced motion.

My KTM 690 SMC R doesn't come with a tachometer from the factory. I found a small aftermarket trip computer that can serve as one, but it ships with no bracket — it's meant to be mounted however you like — so I designed and 3D-printed my own mount for it.

To get the geometry right, I 3D-scanned the handlebar area using my phone's scanner, brought that scan into SolidWorks, and modeled the bracket directly against the real bar profile rather than guessing at dimensions. The mount went through two iterations:

V1 (black): a curved section that wrapped around the handlebar and was bonded on with adhesive, with a square pocket on top to hold the trip computer's display. It worked, but relying on glue alone made it less stable than I wanted.

V2 (white): I addressed that directly. I added two small tabs so the mount could be secured a second way — zip-tied physically to the bar in addition to the double-sided tape — which made it far more solid. I also rotated the display-mounting face 90° relative to v1 — instead of lying flat along the curved bar clamp, it now meets that curve at a right angle — which gave a more sensible load path and held steady even in wind. And I angled the display face upward a little, so the reading angle is more comfortable while riding.

V2 was printed in white only because I'd run out of black filament — the color didn't matter functionally. Since I prefer black, I wrapped the finished part in black electrical tape, which also waterproofs it and keeps it visually consistent with the bike.

This is a straightforward demand-driven design: scan, model in SolidWorks, 3D-print, iterate twice, and it's been on the bike in daily use ever since. In the photos, the lit-up blue display is the trip computer mounted in the finished bracket.