890Views3Replies

Author Options:

Hello,

I was wondering if someone could tell me what the specific electronic components of a typical cycling power meter are and how they work together?  The picture is of one by Power Tap that is located in the rear wheel hub.  I'd like to try to build something similar.  All I know is that it uses strain gauges.  I have zero electronics experience but I'm hoping to learn by working the problem backwards.

Tags:

Discussions

The amount of instantaneous mechanical power P(t) flowing through a rotating shaft is equal to the amount of torque τ in the shaft multiplied by the angular speed ω at which it is rotating.
http://en.wikipedia.org/wiki/Power_(physics)#Mechanical_power

P(t) = τ*ω

To actually measure mechanical power, in an actual mechanical shaft, you need transducers to measure both those numbers.

Measuring angular speed is pretty easy. This is done by counting the number of times the shaft turns in a certain time interval.
http://en.wikipedia.org/wiki/Angular_velocity
http://en.wikipedia.org/wiki/Rotary_encoder

Measuring torque is much more difficult.   To measure torque there must be a section of the shaft capable of being twisted, and some way to actually measure the torque in that twisted, rotating,  element. It seems that the makers of the Power Tap(r), are using strain gauges to do this.
http://en.wikipedia.org/wiki/Torque
http://en.wikipedia.org/wiki/Torque_sensor
http://en.wikipedia.org/wiki/Strain_gauge

The next step is to multiply those two numbers together, and I would assume the easiest way to do that is in discrete time, using a microprocessor.  That is, at some stage in the game, your sensor outputs are converted to digital samples, with some sample frequency that is much faster than the dynamics of the system.
http://en.wikipedia.org/wiki/Discrete_time
http://en.wikipedia.org/wiki/Zero-order_hold

After doing all that, technically you have what you wanted:  P(t), or you know, the discrete time version of that signal, P[n*Ts] where Ts is the sampling period ( fs = 1/Ts is the sample frequency).

But of course that's not good enough because you want to do something with that data.  You need to feed it to an interface for the user, to show how hard he or she is pedaling, or store those data points on some other computer, or whatever it was you wanted the data for in the first place.

I think building one of these from scratch would be very challenging.

Sorry, this is way to advanced for someone with zero electronics knowledge.

In close to mass production like these things are, miniaturisation of the whole thing is do-able, if not straightforward.

If you WANT to do this, I'd instrument the chain wheel at the pedals.

Steve