Thursday, April 1, 2010

Powertap rewired!

My off-season (well, not so off-season at this point) focus on trail running this year has really killed my legs. As a result, I've been generally suffering on Old La Honda during the Wednesday Noon Ride. Guys I am accustomed to staying with have been simply riding away from me, chatting in a relaxed voice, while I suffer, gasping, in their wake.

But now that's going to change.

It occurred to me during my recent Pirates Cove Trail Run that my Powertap, which as I've documented hasn't been reporting accurate power anyway, can be put to a far better use. The key is the piezoelectric effect. The Piezoelectric effect, as applied to measuring power, results in the following relationship:

voltage = (torque ‒ τ0) × K,

for some τ0 and K. However, trivial algebraic manipulation yields:

torque ‒ τ0 = voltage / K,

where torque ‒ τ0 is "useful torque" sufficient to overcome the built-in tension in the powertap hub.

What this says is if instead of forcing a certain torque and measuring the resulting voltage, if I force the voltage I will instead generate a torque. This idea is hardly new, as it's the basis for piezoelectric motors.

Despite my overeducation in electrical engineering, I'm not very good with electronic hackery. But this mod was just too simple. A simple swap of connectors, and I was, you might say, ready to roll:

powertap mod
Simple Powertap mod

I weighed the hub before and after on a vibration-controlled microbalance in the class-100 clean room at the Stanford integrated circuit processing facility (don't tell!), and with my somewhat sloppy soldering technique, this added 47.6 mgrams, which adds around 0.4 msec to my Old La Honda time. But the payback is worth it.

Next there's the battery issue. Sure, the Powertap comes with a battery, but an actuator obviously demands more capacity than a sensor. The "obvious" approach would be to wire a connection to a frame-mounted battery. But a wire from the rear hub to the frame isn't the best approach, and in any case USA Cycling rule 1M(c) forbid the use of any "stored energy" for propulsion.

Fortunately it pays to have friends. In this case a grad student I know at Stanford is doing his PhD in "energy harvesting". This basically involves using sensors to harvest vibrational energy and converting it to useful electronic power. I lent him my wheel for a week, and when it came back, he'd cleverly installed silicon-based microsensors inside my moderate deep Reynolds rims (in return, I act as a test subject for his thesis, and help him with data analysis). The spokes act as perfectly useful wires with a bit of epoxy in the spoke holes as insulation on the "power" spokes (radial non-drive side). The non-drive-side, still touching the rim and hub, act as ground. This worked so well only because the Reynolds rim has the nipples internal to the rim, rather than extending from the rim, making the installation relatively straightforward. The weight penalty was a bit more here: 17.5 grams on my Scout, sensors + epoxy. But oh-so-worth it.

Okay, so in theory if you ride on perfectly smooth roads you don't get anything out of this. But the roads are never perfectly smooth, and on my favorite climb, Old La Honda, even with the relatively recent road work, I'll be than adequately powered for the task. The real beauty of this is if I stop, the vibrations cease, and the power shuts down. To get it going again I simply need to start the bike moving. No energy "stored and released": direct generation from road vibrations to voltage to torque to climb-slaying power.

So there it was. Did it work? Of course: the physics is just too simple. And the result? Let's just say based on preliminary tests in the Marin Headlands yesterday I'll be posting a new PR report on Old La Honda soon. Watch out, Diablo Hill Climb Time Trial in June!


Hefner said...

You crack me up. How much are you gonna charge???? Has Lance been using that contraption all these years??

Ron said...

April Fools? :)

茂一 said...
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