Wednesday, April 24, 2013



I’ve created a relatively aggressive schedule for myself which should have prototypes for beta testers to use around the end of June or early July. If you look closely to the picture above you will see that I’m on track with the coding, the strain gauge design evaluation, and ahead of things on the thermal side. However, the gyro evaluation isn’t started – and it’s due to happen yesterday.

At this point I don’t see it as likely to start. The problem is that the gyro has to be running at all times. Stable startup times are long, on the order of 100ms, so you can never really shut it off. When it’s on, it's 4ma. The gyro alone would kill a CR2032 in under 50 hours without readout enabled. In short, current draw is high too. But these were all issues I mentioned before and said I would work with and maybe find a way around. That is looking less likely to happen to meet the build schedule.

In news, SRM is evaluating a rechargeable power meter for mountain biking. SRM generally uses the idea of primary cell lithium that are not user replaceable. IE: Has to be sent to Germany or Colorado. However seems they are warming to the idea of rechargeable.

This brought a single thought to me. It comes down to if I went with a rechargeable I could have my gyro, but sacrifice operating time (50 – 80 hours depending on battery), or pursue coin cell operation but still potentially be forced into rechargeable due to not being able to source 1000 ohm gauges (especially the double shear gauge for the left torque measurement).

I have ran all the tests on the strain gauge setups for rejection of unwanted forces and I have to make a choice on which arrangement will be used. In order to do this I need to measure total current draw ASAP on the current setup. The choice of battery and gyro are coming by the end of the week or early next. This will lock me into a path. I’m close to a decision, but I’m not there.

Sadly there is little advantage to gyro, thought it might increase the accuracy over a magnet setup. The battery choice is the answer to all of this.

Lots of parts on the way though in the meantime.


  1. As a potential user/tester, I'll chime in to say that recharging the battery isn't bothersome to me. If I had to choose between periodic recharging and sending the unit back for service every other year, recharging wins hands down. A USB charging interface has an added advantage in that it could make it easier to push out firmware updates to end users.

    1. Thanks Duane.

      I'm really having an internal battle over this battery thing. There seems to be no clear winner, but most of the market is doing coin cells. However, even new Quarq units still require magnets so they haven't solved the gyro problem - though I don't know if they attempted.

      Brim Brothers seem forced into rechargeable as they have to have the orientation of each foot, sort out the force vector using trig for each sample. Rotor is using two coin cells. One per leg. Hard decision.

  2. I also wouldn't mind periodic recharging, depending on how frequent. As a serious cyclist who has held out on buying a power meter because of cost, I won't buy one that requires me to send the unit back when the battery dies. I also wouldn't buy one if I had to run an extension cord to my bike. Would you plan to make it removable somehow for charging?

    Also, just my perspective but if the gyro offer's little advantage, why bother? If it doesn't solve a real problem that users have, why allow this to drive your decision about components?

    1. Hi Mike,

      Thanks for the input. I did some calculations and if I went with a rechargeable battery I have estimated 100 - 140 hours with a gyro and minimal impact on weight. However, I want to state explicitly that targeting a lower end market compared to SRM I am not as obsessed with weight.

      I had not even thought of a removable rechargeable battery. This seems so blatantly obvious now that you mention it but honestly never crossed my mind! External charger, include 2 - 3 batteries with unit and all problems seem to be solve.

  3. I also think that the gyro is the one that is driving the power decision, and it may be better to use an accelerometer with lower startup time. This has the advantage of lower current draw all the time, and also lower startup time.

    Alternatively you could use a magnet and coil, to charge a capacitor on your PCB. This will then induce charge each revolution of the pedals. You can then use this charge to run your board without using the battery. The battery then is only used when pedaling too slowly (in which case you can start to go into power save mode and only periodically poll the gyro), or use the battery when detecting start of ride conditions.

    The charging of a capacitor from a magnet and coil has been used by Polar on its wireless transmitters to avoid needing to replace the coin cell battery for years.

    1. Hi Paul,

      I've actually thought on this type of charging setup. I could use the same magnet to detect rotation to power the coil. I didn't know Polar had already been doing this. It's patent is likely expired then, which would be good news for me.

      I believe stages is using an accelerometer. Radial acceleration should correlate to rotational velocity though it will have a periodic change due to gravity depending on position. I haven't figured out how they deal with this without a Kalman filter to estimate position.

    2. The downside of using the magnet to detect rotation is that it will not give accurate power output for different parts of the pedal stroke. Taking only a single point on the rotation, assumes velocity is constant around the stroke.

      As power = torque x radial velocity and you are constantly reading torque, I think you also want to be constantly reading velocity. I know for myself, if I am in a very light gear I find it tough to have smooth pedal speed.

      I was thinking of only using the magnet to avoid having to charge the battery.

      You could you use an IC that has both accelerometer and gyro, and use the gyro only periodically to re-calibrate the accelerometer. This way you get the best of both worlds. Both low power, but still accurate.

      The gyro readings could be done say every 3-5 even 10 seconds, which would save a tremendous amount of power over using the gyro all the time. (assuming a 300ms startup time, 3 seconds = 10% power, 5 seconds gives 6% power, and 10 seconds 3% power, which is pretty massive savings). Using 4mA ON consumption, this is 400uA, 240uA and 120uA. Or in time (220mAh) 550 Hours, 916 Hours, 1833 Hours (at which point its not the gyro killing your battery).

    3. This is true about accuracy being compromised because the magnetic sensor is basing it on average rotational velocity. The smoothness of the pedal stroke is the main reason to use gyro like you are indicating.

      I think though that the gyro really needs to be reading multiple times a second. Generally a rule of thumb for aliasing is that you need 5 times the rate of what you expect to see. So if we wanted to match up to ant+ of 4Hz, it needs to be sampled at at least 20Hz which means 50ms. So it can't ever shut down. Realistically I'd prefer to have at least 16 samples per revolution with Gyro but rather sample once per strain gauge sample. IE: Left, Right, Gyro, Repeat. This will give me the most accurate.

      Strain gauges are being the big hog now. 350 ohm isn't cutting it and I can't find a 1000 ohm double shear gauge for the left sensor. Only option is to drop the voltage down really low and use a separate voltage regulator, but if it's a linear regulator it won't help.

  4. Ahh, you mis-understood me.

    I was planning on using the accelerometer all the time. But then use the gyro to re-calibrate the accelerometer as it drifts over time. This way the accelerometer is giving you the once per sample for angular velocity, and the gyro is hopefully allowing you to re-calibrate for offsets due to gravity/climbing.

    With regards to power and 350 Ohm strain gauges. Any design that comes up with a DC/DC converter will never win in low power devices. (is the 350 strain gauge, both the top and bottom resistor? ie 175 Ohm each leg? or is it 350 Ohm a leg?). Either way, we are talking about 1mA. A DC/DC converter won't be able to drop this lower due to switching losses, along with needing to power the DC/DC chip.

    Is it possible just to place a 1% (or 0.1%) resistor in series? This will decrease your overall range of your ADC (due to the voltage divide), but this would happen anyway if you were to go with the DC/DC option and a low voltage (say 1V).