One minor and one major update…
The minor update: a recent pool test showed Poolbot making only starboard turns; this turned out to be due to a broken starboard propeller.
The more significant issue involves ballast.
Both the ComPod and the main body of Poolbot were recently analyzed for displacement and overall ballast neededto dive. Recall that ComPod has to protrude from the water at least partially in order to allow the XBee inside to communicate with the shore.
Previous displacement checks were done on Poolbot alone, less to get an actual displacement number and more simply to visually check buoyancy, and adjust trim and freeboard in the water. The ComPod was tested for actual displacement in the kitchen at least once, but using some dificult and questionable methods. To get a better number, the following extremely high-tech test bed was assembled:
The hard plastic bowl is filled to just before overflowing. The softer plastic bin will catch the water displaced by the ComPod once it’s inserted into the bowl. Measure the water in the bin to get displacement.
This measurement went exceptionally well, and resulted in a displacement within about 12 mL of the previous ComPod checks. The final numbers:
- ComPod’s volume (displacement): approximately 1470 mL
- ComPod’s weight: 549 g (including internal electronics)
For neutral buoyancy, the item weight must equal the weight of the volume of water displaced. As 1 kg of fresh water = 1 L of fresh water (pretty clever that metric system is), ComPod’s required ballast is… 921 g, or 2 lbs, 0.5 oz.
To measure Poolbot’s displacement, aside from the ComPod, is harder. Poolbot is rather large, requiring a tank at least 25 inches (sorry metric system) long and 11 inches wide, with about the same depth. Any catch basin must be larger still, to accommodate the overflow without losing any.
Thanks to several smaller bins, four plastic bags, and some tape, a reasonable up-scaled version of the previous dunk tank was created; there’s just enough clearance to allow water to overflow straight down into the smaller bins. The flaw in this design will be obvious from this post-dunking picture.
- Use a rigid container, if possible, for the actual dunking. The soft plastic deformed enough to raise concerns about accuracy. The strap shown above is wrapped around the bin and several pieces of acrylic plastic and plywood, to make the sides more stable. The risk is that the plastic will deform, rather than water overflow, when the object is dunked.
- Poolbot floats, so is obviously guilty of witchcraft.
Here are the numbers, +/- 100 mL or so for water loss and bin expansion:
- Poolbot’s volume (displacement, without ComPod): 8050 mL
- Poolbot’s weight (without ComPod or batteries): 3.6 kg
This means that Poolbot’s ballast must be 4.45 kg, or 9.82 lbs, to compensate for buoyancy. This number includes all of its current electronics, but does not include batteries.
The challenges that remain:
- Can the ComPod be kept sufficiently above the waterline to transmit and receive messages? Or is it time for the extended antenna?
- If ComPod protrudes enough for XBee communications, are the motors powerful enough to allow for diving without dive planes or some sort of ballast compensation system?
Unfortunately, there’s a third problem lurking, which will be addressed in the next post…