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Hi,

I’m involved in a project to build a pedal powered aircraft (no, seriously!) and we want to add a power meter.

Commercial cycle power meters are available, but are all very, very expensive and most of them still seem to be in the development stage. Power meters also seem to be popular in the hacking community, but it seems they’re a lot more difficult than they first appear (there are plenty of blogs/website that describe work-in-progress, but few that describe a finished product).

There seem to be several approaches to this problem including.....
Measure the pressure on the pedals
Measure the strain in the crank.
Measure the tension in the chain.

I want to try something different. I want to place a pulse-counter of some sort on the crank by the pedals, and a second similar pulse counter on the drive shaft by the propeller. At low power I’d measure the time between the pulses produced by each device, as the power increases, the tension in the drive mechanism will increase, the drive shaft will twist, the chain will stretch and the time difference between the pulses should increase.

The time increase should be proportional to the torque, which should in turn be proportional to the power.

The problem with this method is that it would rely on very accurate timing information, I had planned to use a magnet and hall probe to count each pulse, but I understand from looking at some technical data sheets that hall probes aren’t suitable for this because the magnetic flux that triggers them can vary with environmental fluctuations.

In summary, what can I use to accurately and repeatably measure shaft rotation?

Comments on what I’m planning and/or alternative methods of doing it are also welcome.
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Have you done any calculations to determine what the change in pulse rate would be between no-load and full load?
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The problem with this method is that it would rely on very accurate timing information
I think you are correct about the high accuracy required and my guess is that the difference will be so small that the signal will be lost in the "noise" of varying human power supply, changing load conditions, vibration, and flexing of the presumably lightweight structure.
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My mental image of this is essentially a bicycle setup tied to a propeller of some sort.

You may be able to do this fairly cheaply.

This sensor detects the presence or absence of an object:
http://www.pololu.com/catalog/product/1134

It's basically a range finder, but it's either "ON" or "OFF" - meaning it can be used as a counter.

If your gear has slots in it, this could be used to count slots - once all the slots are counted, you have completed one rotation.  This can be easily converted to RPMs.

Psuedo code:

startTimer

while( variable < numberOfSlots)
{
     if(beamBroken = true)
     {
          variable++
     }
}

stopTimer

rpm = 1min x timerMinutes     //Need to work the units, probably be in microseconds
                                             //Also, it's been a while since I've done algebra, but this should be right

Once you have the RPMs, you should be able to calculate other values.

There are also compass, acceleramator and GPS units for Arduino that could be integrated depending on what values you need.
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Hi,

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My mental image of this is essentially a bicycle setup tied to a propeller of some sort.
The drive chain consists of a set of pedals on a gearbox, this drives a drive shaft, which then drives a chain which is connected to the propeller.

Quote
Have you done any calculations to determine what the change in pulse rate would be between no-load and full load?
If I assume the pedal speed is about 80rpm, this equates to 1.33 revs/sec, or 480 degrees/sec, or 0.48 degrees per millisecond.

Now lets assume that under maximum torque the propeller end of the shaft is now lagging behind the pedal end by about 1 degree, (due to the increased twist in the shaft and the stretch of the chain) this will equate to approximately 2 milliseconds delay. The arduino has a micros() function for recording time down to millionth’s of a second, but I’m not sure how accurate this is

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the difference will be so small that the signal will be lost in the "noise" of varying human power supply, changing load conditions, vibration, and flexing

Yes, this is the big danger, I think this is where most other systems fall down to.

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It's basically a range finder

The problem with using a ranger finder, is that it needs to trigger on exactly the same distance every time. I suspect that in practice they trigger on the same distance + or - a few percent each time.

I’ve also considered strain gauges on the pedals or cranks, but there are a number of issues here, firstly the sensors need to be wireless (or some sort of commutator is needed) and secondly the pressure/strain measured here would be very variable because of the way we cycle i.e. applying force with alternate legs.
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The sensor I linked is a digital sensor - it's just detecting the presence of object/gap.  That's what would work with a slotted gear - it just counts the gaps.

Shoot, depending on the distances involved, you could use it (or something like it) to count the RPMs of the propeller itself.  Using any distance sensor, just count the number of times the distance dropped from "infinity" to anything less - that would be each time the propeller passed the sensor.

That would give you the for-sure RPMs of the propeller shaft.
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Comments on what I’m planning and/or alternative methods of doing it are also welcome.

I can only see that working if you use a quill shaft that flexes through tens of degrees under normal loads.

I suspect you would get more useful results if you captured the air speed and prop rotational speed. I think you could calculate the applied power from those two, although you'd need to do some calibration to find the exact relationship between those two and the applied power. I suspect that simply knowing those two metrics separately would tell you more or less what you're looking for, anyway.
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captured the air speed and prop rotational speed.

I've already got that data (plus heart rate, altitude and rate-of-climb)

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find the exact relationship between those two and the applied power

There is a close relationship between these values, but to work out how efficient the aircraft is we need to know the applied power (i.e. the input power). Don't forget that a heavier pilot will have to put in a lot more work to achieve the same flight characteristics as a lightweight pilot.
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In that case you need some way to measure torque in your drive line, as you suggested. How much rotational flex do you have?
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Don't forget that a heavier pilot will have to put in a lot more work to achieve the same flight characteristics as a lightweight pilot.

Probably not a helpful suggestion but; use pilots of different masses then, combined with the air speed and prop rotational speed, calculate the efficiency of the aircraft.
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What about a spring loaded joint in the driveshaft that would deflect 10°-20° under load?  That would be easier to measure with simple hall effect sensors.  I would add a viscous damper to the mechanism to damp out the oscillating force of the person pedaling.
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Comments on what I’m planning and/or alternative methods of doing it are also welcome.

What do you care about power to the pedals? What you want is thrust! Since thrust depends entirely on propeller rotation, all you have to do is measure the propeller rpm.
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thrust depends entirely on propeller rotation
No! Thrust depends on lots of factors, i.e. the aerodynamic efficiency of the prop

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What about a spring loaded joint in the driveshaft that would deflect 10°-20° under load?
This idea could be worth exploring - insert a joint or coupling specifically designed to twist a known amount. The only catch with this one is that keeping the weight down is of paramount importance, every few extra grams count. So far the entire instrument package (altimeter, cadence, airspeed, GPS & heart rate) plus batteries and display weight less than 150g

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What do you care about power to the pedals?
Without knowing the input power, it's impossible to work out if improvements to the aircraft (different propeller designs, changes to wingspan etc) have actually made a difference. If you put a new propeller on aircraft and then fly the length of an airfield 10 seconds quicker than you did last time, you might think the new propeller is better, but if it's actually taken 20% more input power on the 2nd flight, then the propeller probably isn't the cause of the speed increase!

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How much rotational flex do you have?
I'm not sure, in static tests on the ground, maybe a degree or two, but in flight under load I've no idea.
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What about a spring loaded joint in the driveshaft that would deflect 10°-20° under load?

As you mentioned solutions like this all add weight. Also, at the end of the day, is the most desirable measurement not the torque at the prop shaft. Are other measurments, not just more complex solutions?
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How much rotational flex do you have?
I'm not sure, in static tests on the ground, maybe a degree or two, but in flight under load I've no idea.

That's nowhere near enough IMO to be a useful indication of torque. Somewhere in your transmission is a drive shaft that is heavier and stiffer than it needs to be. Replace whatever-it-is with a thin fibreglass rod that is soft enough to deflect a few tens of degrees under the loads you're expecting but strong enough to be in no risk of breaking, and you could have a viable solution.

If all you want this for is comparison of power output between similar runs, I think that air speed, height and prop speed would give you everything you need to see where the pilot was putting out more or less effort.
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