Eagle Tree data logger

[Tim Kearsley]

Finally managed to carry out some current measurements this morning to try and judge the accuracy of the data logger.  I put a digital multimeter, set to its 20A range, in series with the logger and ran the motor on a small Multiplex Minimag model.  To be fair, my method is a little crude, as what I did was to try and adjust the throttle position to give some "benchmark" readings as displayed on the multimeter - namely at a nominal 5A and a nominal 10A.  I then downloaded the data from this short session and graphed it.  The logger showed current measurement within a few % of the multimeter at those benchmark points.  Now the immediate question is - how accurate is the multimeter?  Answer - no idea!!  However, although I can't truly quantify the accuracy of the logger by this means, I get a "gut feeling" that's it's fairly close.  Next step is to install the data logger software on to a laptop and use a larger model so that I can run the logger in live mode and get a slightly wider range of readings.

I fully acknowledge that this is a small range of measurement with no guarantees of accuracy of my "reference".  I believe though that it indicates that the logger, at this lower end of its range at least, is not bad for accuracy.

Timbo - any more findings on this topic from your end?

Cheers,

Tim.

[Danny Fenton]

Tim thats much more accurate than mine was, which is good. when a known 10A was drawn it displayed just 6A.

A bit of innacuracy I can live with, but the difference between 1700watts and 2700watts nearly cost me my model as it didn't last 4 minutes in the air, which according to Eagle tree should easily have done six minutes. My A123's said empty after the first overshoot which meant putting my Hurricane down in the rough at the edge of the field, fortunately the damage was fairly straightforward to repair.

Its a good job the HK stuff is so cheap, I wouldn't have been using double spec gear otherwise (120A HV esc, and 3kw motor).

Danny

[Tim Kearsley]

Hi Danny,

Well the degree of inaccuracy you saw is just not on, from a unit that is declared to be factory-calibrated!  I suppose it's not so bad if you do have calibration equipment as the calibration can be set from software I believe.  Nonetheless, not everyone will have meters that will read current in the range that we encounter - my own meter has a maximum of 20A only.  The only thing I would like to check now is the accuracy of the current measurement higher up the range - say 30 to 40A.  I'm unlikely to be able to do that unless I can borrow a suitable meter from work!

Cheers,

Tim.