Wednesday, August 21, 2013

Accuity Powermeter Testing Video and Data File

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Testing Accuity V4.1 Bicycle Powermeter

I’ve also made my Garmin Connect file public (Garmin here, Fit File here) so everyone can download it. There are anomalies and this is a first test!
  • Bad battery connection means that it loses it’s zero and causes massive offset showing as huge power numbers (up to 18000 watts!)
  • A few times during the video it’s noticeable that if the crank hasn’t been rotated in a while and during the first rotation there is an abnormally high power number. There is an overflow issue or double tripping of the Magnetic reed switch causing an erroneously high power for the first 1 – 2 rotations then it stops. I didn’t see this as much in the later part of the ride. I think it’s an ultra low cadence problem.
  • The bump issue is related to the magnet position I found out later during the ride. If I hold my legs level with my right leg on the back side it can trip the reed switch multiple times in a very short period with half my weight on it. Obviously a false 200+RPM reading combined with 100NM torque will give over 1000 watt reading. This is solved on SRM and Quarq by putting the magnetic pickup not in line with either crank arm. Smart for them, I missed this in designing.
  • Reads back a random value when told to calibrate. Minor but would be useful to implement an actual value. It’s standard with Quarq and SRM.
  • Have to calibrate twice – once for each side. Not really needed but it’s left as a debug measure. It could be useful to know the offset of each side as the ANT+ protocol only supports one value.
  • Sorry it’s hard to see the actual values in the video.
Some more pictures
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12 comments:

  1. Is the .fit file available to download?

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    1. Yes, I just updated it. See above or copy and paste this link. www.accuity.ca/2013-08-20-18-33-58.fit

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    2. Didn't realize people couldn't export from the Garmin Connect page. Only the actual owner can. Opps.

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  2. Cool stuff happening here.

    What are your thoughts on packaging the system on the crank?

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  3. Two part rapid prototyped enclosure. Battery housing would be below the right arm. O-rings on either side and between pieces to make watertight. It's still being designed sadly. Which is off schedule.

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  4. Way, Way, Way too cool!!! Fantastic job.

    Just don't get killed on the bike concentrating on electronic details, troubleshooting on the fly, keeping camera pointing appropriately, etc., and not where the edge of the pavement is, or potholes, or cars...I'm serious, I've had some close shaves when distracted, and you'd be major distracted.

    Is the repositioning of the magnetic pickups to fix the bump problem a trivial redesign?

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    1. Biking while trying to take video is literally one of the stupidest things I've done. I'm not going to be doing it again. It's worse than driving and texting and I consider that a terrible atrocity. I might have to put a disclaimer on my video.

      Thanks for the support.

      The pickup is designed to be either on the circuit board or off it, so it's a matter of integrating it properly into a housing which is changing my envisioned design. The problem goes away if you stop pedalling and don't have the pedals with the right one aft. It's also solved by moving the magnet itself. Quarq makes a cable mount for the bottom of the BB shell. That would solve it as well.

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  5. This is a great source of interesting tech!
    I've just checked out the crowd funding site for The Swedish Adrenaline Power Meter.
    Can you guys explain how Resonant String Gauge technology works? Is it likely to be as accurate as they say it will be?

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    1. Thanks.

      I think I can explain their RSG tech if in a basic way. Obviously these people would be a competitor if I was trying to enter the market so I'll likely come off critical. It'll have to happen in a post rather than a reply. It's good tech but they are too early for crowd funding. They lack engineering staff as far as I can tell and don't have a prototype. 3 graphic and industrial designers does not equal an engineer or prototype. Their axel design won't work. It's susceptible to ankle twisting. Same problem using a shear gauge. They need to do what Garmin vector did to eliminate moment measures. Look for a detailed post later this week.

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    2. File didn't get uploaded until late last night. http://www.youtube.com/watch?v=Abdz5Ycuj5E

      How Garmin Vector, Look Power Pedals, and Swedish Adrenalines new Resonant String Gauges work. 10 minutes of my ramblings in front of a whiteboard.

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  6. On the diagrams there is a mass attached to the string. I understand this in a conceptual way, i.e you have to calculate using the mass that is oscillating. But why does it need to be constructed in a literal way? The string (wire) has mass already -- why complicate the picture by appending another mass to it.? Also, if the wire is ferromagnetic, just put the detector coil next to it -- electric guitar pickups work simply and cheaply -- why not just steal that technology wholesale?

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    1. In reality, and the testing I've done, it's not needed. I'm just putting it in because they have it in their concept picture. I think it's a makes the vibrations easier to measure with an inductor coil, so could just be a ferromagnetic material (like an electric guitar string) or a magnet to induce voltage.

      I'm not proposing this is a good method. I think it's too bulky inside a pedal. It's a re-application but it needs work and frankly they need to explain it. I shouldn't be doing it for them. :P

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