I asked previously about the speedometer and then I forgot about that thread, but I didn't get the answer I was searching for. [sorry..]
I am trying to develop an bicycle power meter using strain gauges for measuring torque and I need something to measure the rear wheel velocity [that's where the strain gauges will be] with a fast response and reasonable precision to obviously get the power calculation done.
My idea was to use a hall effect sensor, but then I realized that wouldn't give me the response time needed, so I changed my mind to using a light gate consisting of two light insulated LEDs that create a gate in between the single speed sprocket teeth, that is installed on the bicycle.
Since I can not use just one chip for both strain gauges and light gate due to interrupt, I need some advice using another chip [probably attiny85] to control the light gate and send signals about the rear wheel speed value to the arduino duemilianove I have using some sort of serial connection.
Which power do you mean? Power the rider is putting out? If so then the rear stays will have mechanical strain every time the cranks are turned. A strain gauge on any rear stay will give you an indication of that pull.
That sounds tricky... You need to measure the torque-only without measuring the stress caused by weight/gravity as the wheel go's around. If you could measure the stress continuously, the gravity-stress would probably cancel-out, but that could get difficult.
Maybe you can measure the force/stress on the pedals??? That would solve the gravity problem, but you still need continuous measurement on a rotating part...
GoForSmoke:
Which power do you mean? Power the rider is putting out? If so then the rear stays will have mechanical strain every time the cranks are turned. A strain gauge on any rear stay will give you an indication of that pull.
No, the strain gauges will be mounted on a torque tube between freewheel and the hub that I got machined.
I read the thing, I have no idea what that meant. I'm a newbie.
Is it an automated interrupt that runs in the background and senses change? I think it will be easier for me to simply spend additional 4$ on a ATtiny to make it "dual core". How complicated is this?
wait! I think I got it. So it is a counter that counts in the background to say how many times the digital pin has changed while other calculations were done or something? Or am I absolutely wrong.
It is code that runs whenever the pin state changes. The regular code stops, interrupts are disabled and the Interrupt Service Routine (a function you wrote, should be -very- short) runs. When it is done, interrupts are enabled again and your regular code picks up where it left off. Done well they are a great tool, done poorly they will cause very hard to trace bugs.
I bought 12 ATtiny85's from Mouser for $1.30-some each. If I bought 1 it would have cost $1.50-some. But that didn't include shipping which I spread over a $50 total order anyway.
Good luck with your project, I like the idea of using sprocket teeth as interrupter gears.