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Ocean-Tolerant Precision Latching Mechanism

Andrew Cai, Chelwei Jang, Tamara Kawa, Mithulesh Ramkumar

Sponsored by Prof. Andrew Lucas, Scripps Institute of Oceanography

Executive summary : Link

Background

The Wirewalker is a wave-powered, ocean-profiling vehicle created by the Multiscale Ocean Dynamics (MOD) group at the Scripps Institute of Oceanography. Because movement is powered entirely by ocean waves, the Wirewalker is an economical, environmentally friendly means of monitoring ocean waves, and is used by researchers around the world. The Mini Wirewalker, true to its name, is a miniaturized version of the Wirewalker; as a smaller vehicle, it is easier to place in the water without any special equipment, is lighter and cheaper, and can be used by research teams with less or smaller equipment. However, unlike its predecessor, the Mini Wirewalker does not profile reliably due to flaws in its design. The team was tasked with redesigning the current mini Wirewalker to fix these flaws.

How the Mini Wirewalker works?

The Mini Wirewalker is placed in the water, suspended on a wire between a buoy at the top and a weight at the bottom and travels between the two. In the beginning of the cycle, the vehicle starts at the top of the wire and is pushed down by a wave. As the Mini Wirewalker starts to move up due to its internal buoyancy, an internal cam latch system pushes a set of races downward through the jaws, pushing the races together to clamp onto a wire. The next wave pulls the wire up, and the races prevent the Wirewalker from moving upward relative to the wire. Upon reaching the bottom of the wire, the plunger is pushed by the weight, changing the position of the push rod and causing the race to disengage from the wire. The Mini Wirewalker then returns to the top due to its buoyancy.

However, the Mini Wirewalker does not properly follow this behavior at all times. Common failure modes of the Mini Wirewalker include failure to reach bottom depth - the most common failure mode; improper descent - in which descent takes fare too long;and failure to reach top depth - the least common issue. We also identified what we called "compounding failure," in which failure happens in rapid succession and occasionally gets worse over time. We believe that the failure to reach bottom depth is caused by false triggering of the latch. We believe that the slow descents, particularly when compounded with a brief directional change, are caused by a "zombie mode" in which the push rod does not hold in one position or the other, which could potentially cause improper wire engagement. On the bottom right we have a comparision of the mini Wirewalker profiling is supposed to look like (Wirewalker motion) versus how it currently is (Mini Wirewalker motion).

Currently, the main problems with the Mini Wirewalker lie with the spring latch, the actuator, and the races. The directional control  relies on magnets, one in a stationary part of the Mini Wirewalker and another in the push rod. The collars on the push rod prevent the races from going up through the jaws during descent, or from going down during ascent, and the push rod is held in proper position with magnets. However, evidence shows that the push rod may be getting stuck in a position where it does not engage or remain engaged with the magnets at the top or bottom, triggering a "zombie" mode that causes slow or uneven descent. Furthermore, evidence from the data shows that that the latch is being triggered before reaching either the top or bottom, causing sudden, improper changes in direction. Even with proper function of the actuator and latch, issues may arise from the deformation of the race. During assembly, steel pins are press-fitted into the plastic race, causing the races to become bowed out due to the stress; this would potentially cause the races to lost contact with the wire.

Criteria

A successful redesign of the Mini Wirewalker ideally:

1. Cannot false trigger

2. Switches positions at a specific force threshold, with the ability to withstand higher forces

3. Can withstand long submersion periods without significant material corrosion or fatigue

4. Operates purely mechanically

5. Has specific/unchanging thresholds for direction reversal

6. Can last 10,000-1,000,000 cycles

Design Solutions

After coming up with 3-5 redesign ideas for the spring latch system, the races, and the operator, we settled on designs that would work most effectively with limited space, long periods of ocean submersion, and variable forces. Our redesign of the Mini Wirewalker will feature roller races with centered holes and pins press-fit into the rollers; an internal deformation follower that moves through side rails; and a larger buoy and weight to reduce or prevent angular displacement. Below we have images of each design solution in its present state - for more information, check the Final Design page.

We presented these designs to Professor Lucas and the rest of the Multiscale Ocean Dynamics (MOD) lab at Scripps on Friday, June 5. While some more refining and testing will be needed for the operator and deforming cam mechanism, [finish after meeting]

Expected Performance

Poster Presentation