BL motor tachometer using your digital multimeter

[cmpang]

As long as your digital multimeter can measure frequency, you can simple tap out any 2 of the 3 phase wire and with some simple R-C filtering, feeding to the dmm to read the frequency count.

After getting some physical constants like # of poles and the gear train ratio, you can easily convert the count to the headspeed of your heli or plane.

The good part other than cost over a cheap optical tach is that it can be used indoor under flourscent light without interference.

I permanently attached a JST connector (<1g) at the output of each of my esc and use it to connect the dmm whenever I like. Works great for me.

[llamatrails]

Interesting. Could you please give some specifics ? A complete example ?

TIA.

[cmpang]

Taking my 400DH as an example:

I use 2S LiPol measured = 8.3V
# of pole for 400DH = 9
Measured frequency by DMM = 1790Hz
Converted to RPM = 1790 * 60 / 9 * 3 = 35800 rpm
Hence no load KV = 107400 / 8.3 = 4313 rpm/V

This confirms all physical parameters choosen for this motor are correct and the Conversion factor for motor = 35800/1790 = 20.

My shogun's motor pinion = 10T and the main gear = 132T hence the Conversion factor for the motor with heli = 20 * 10 / 132 = 1.5151

In a real world application measuring the headspeed of my Shogun:

measured frquency = 1326Hz
equivalent HS = 1326 * 1.5151 = 2009 rpm.

All these sounds very complicated but once set in an Excel sheet, you only have to key in the measured frequency and get the heli's HS right away.

[ImaBiggles]

great idea, I was wondering if this could be done! u using 0.5uf caps across the lead for rf filtering or do you have a better system?

[cmpang]

one 1K resistor for each wire at the esc end and one 0.1u capacitor across at the dmm end.

[cmpang]

My idea really comes from one of the new esc design in China. The esc provides a serial link for programming and firmware upgrade similiar the the cc phoneix link.

However, there is one innoviative feature that it can be used to measure the motor rpm under your PC. With that, you can set the HS of your heli under governer mode very easily.