Tuesday, November 13, 2012

V3–Update and Naming

Sorry it has been a little while since I’ve updated this. So everyone knows I’m very much committed to this project however I’m constantly running up against minor issues.
Recently I found out that I missed a 0.1 uF cap on the voltage reference causing it to drift and as a result there was a massive loss of resolution but it was hard to detect when you average 10000 samples. Also I’ve come to a realization that commercial units sample WAY WAY WAY less which is why they only claim 1 – 2 % accuracy. Strain gauges are good for way higher accuracy but it’s this combined with inaccuracy of rotational measurement and trying to keep consumption down that drives inaccuracy. So it’s a trade off, a real balancing act.
The latest issue I’m still debugging is that on V3 when I read the ADC’s the AP2 module is not consistent with transmitting. Essentially my serial output is turning to garbage. As a result it’s not transmitting consistently. I’m having a hard time debugging this and that is what I’m working on now.
I’ve gotten several emails over the last couple of weeks. I know right now that some of them have gotten lost in the inbox. I’m going to get to finding and replying. I apologize for this. Unfortunately my day job, exercise, and this project are all competing for attention.
Once this problem is sorted an updated schematic, gerber files and part list will be posted along with some basic code. It will by no means be the end of the project.
Back to debugging!

Also V4+ will have a new name, a more official name.


  1. Hi Keith

    Yes, your sample rate seems WAY too high! Here's a list of the sample rates of some of the commercial power meters along with their battery life and battery types:

    Quarq: 60Hz (Battery life: 300 hours with one CR2450)
    SRM: 200Hz (Battery life: 1400 hours)
    Rotor: 500Hz (Battery life: 300 hours using two CR2477)
    StageOne: ? (Battery life: 200 hours using one CR2030)
    Power2Max: ? (Battery life: 300-400 hours using one CR2450N)

  2. After searching a bit on the web I could confirm the SRM and Rotor numbers but not the Quarq. Summarizing this data for me is quite appreciated. Thank you.

  3. Hi Keith,

    I've seen the 60Hz sampling rate for Quarq mentioned in several places. Here is one of the references: http://www.naisan.net/blog/2010/04/09/detailed-discussion-with-jim-from-quarq-on-cinqo-and-edge-500/

    1. Interesting. These seem all very very low in my opinion. Though in regard to the Quarq it is sampling at least 5 bridges 60 times, or possibly 10 bridges so it's really doing 300 hz or 600 hz total and applying an algorithm to decouple. Had no idea the Rotor was using such a large battery in their prototypes either!