Mark de Jonge wants to know what the pressure distribution on the blade of his kayak paddle looks like. Why? To see if anything interesting can be learned. For one, knowing the pressure distribution allows you to find the centre of force. It’s important to know where this is to quantify the effects of hand positions on the paddle.
We decided to try to build something to measure pressure distribution. Will the pressure be fairly uniform? Probably. Will the centre of force end up being right at the centroid of the blade? Almost certainly. Will we get to see some cool graphs? We wouldn’t have even started if that wasn’t a possibility. Let’s dig in.
Two circuit boards were constructed, one with the microcontroller and SD card logger, and the other with the amplifiers. Mark did the bulk of the soldering on the boards, including the painstaking blue wiring. We also hired soldering expert Alexa Irvin to do some board assembly while Mark and I laid out the sensor wiring.
This mess cost us more than a few burnt fingers. Luckily, when it was time to wire up the other board, Nicole Brown showed up with her electronics toolkit (makeup bag) and lent us some tweezers.
At this point in time, applying a force to the sensor gives us a “Force Number” between 0 and 1023. Calibrating the sensor will tell us what that number means in Newtons. To do this, our data acquisition system was hooked up to the computer to show us the outputs from each sensor. We placed small weights on the sensors and wrote down the values that were output. As it turns out, these things are REALLY sensitive. Several factors affected the values significantly:
- Tilting the paddle a tiny bit
- Placing the weight slightly off centre
- Touching anything
- Talking, breathing, sneezing, opening the door, and looking at it funny
We quickly came to realize that despite our best efforts, our (barstool/duct tape/hot glue/loose change) calibration rig just wasn’t going to cut it.
Olympic kayaker Rhys Hill enthusiastically volunteered to be our test subject. Mark and I prepared the equipment while Rhys performed his thorough pre-race warm up. When ready, we presented him with the instrumented paddle, placed the data acquisition system in his boat, and turned it on. It was pretty anti-climactic. The next version should beep or something.
We knew he had about 9 minutes before the SD card would run out of space, causing either corrupted data or a violent explosion. Not wanting to take any chances, Rhys kept an eye on the time and paddled around. Mark and I watched, and hoped for him not to tip.
He looks pretty stable here.