Mind / Iron
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ERVO FOR THE ROBOT INNOVATOR
6 SERVO 10.2014
Sensing Sensibly
I recently joined an Iaido club to learn how to wield a Japanese long sword, or
Katana. My first challenge was to learn to accelerate the sword quickly, and then stop
it before the blade hit the floor or a limb. For some reason, this didn't come naturally,
and I quickly formulated a plan to mount an accelerometer on the sensei's sword and
one on mine to compare the acceleration patterns. Before I could put the plan into
action, I discovered that I was going about it all wrong.
The sensei could tell that I was gripping the sword too tightly at the start, and
therefore slowing acceleration of the sword — even when he was observing someone
else. So, clearly, he was cueing on something other than sight. On questioning the
sensei, it turns out that he could tell the acceleration pattern by sound alone, and
from experience knew that improper grip was the most common culprit.
As shown in the photo, a bokken or wooden sword for Iaido practice has
grooves on either side of the blade to replicate the groove on a metal blade. The
blade itself makes a noise, and each of the two grooves make a noise as well — that
is, if the sword blade is oriented properly during a cut.
So, instead of taking the time to wire an accelerometer to an Arduino and
compare patterns, I simply recorded his sword making a downward cut and
compared the sound to mine. I used a common audio editing program that ships
with the Mac to view the spectral distribution and then — using the sensei's pattern
as a template — worked with my sword until the sound patterns generally matched.
It'll probably take many months before the frequency distribution patterns are
indistinguishable, but at least I have a quantitative goal to reach.
The point of all this is that you shouldn't box yourself into a particular sensor
solution. My problem was measuring acceleration and so, naturally, I first thought
accelerometer. However, the best — that is easiest, quickest, least intrusive, and
cheapest — solution turned out to be a simple digital recorder and common sound
analysis software. In retrospect, an accelerometer would have added mass to the
sword, the wires would have possibly become tangled, and the balance of the sword
would have been affected.
As a practical example, have you ever noticed how the sound changes as you fill
a container with water from the tap? As the volume of air diminishes, the frequency
of the noise increases. I'm sure that instead of a direct measurement device, you
could create a sound-activated switch that would sense a nearly filled container.
Another approach would be to use a pair of wires and sense the resistance change.
Or, perhaps use an LED and phototransistor to detect the fluid. Or, perhaps a scale to
determine when the glass is nearly full of fluid. I suppose you could also bounce an
IR or US signal off the surface of the fluid if the glass or other container was large
enough.
So, next time you set out to use one of those new inexpensive miniature sensors
that are all the rage today, think about your options. They're probably a lot greater
than you think. SV
