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Feb 19, 2010, 12:43 PM
aka Space Cowboy
mpboost's Avatar

Sensored Brushless motor feedback into eLogger V3

I recently purchased an eLogger V3 so I could do some datalogging on my 1/10th scale touring cars. Both the cars I plan to use it on have sensored brushless motors and I was wondering if I could tap into the existing sensor harness in order to get an RPM signal.

The pinout for the motor sensor wire is as follows (as per ROAR specs):
Pin #1- ground potential
Pin #2- phase C
Pin #3- phase B
Pin #4- phase A
Pin #5- temp control, 10 k Thermistor referenced to ground potential
Pin #6- + 5.0 volts =/- 10%

Could I possible connect Pin #2/3/4 to the white wire RPM input on the eLogger?

Also could the temp control Pin #5 be used similarly to feed the eLogger temp info instead of using the external sensor?
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Feb 21, 2010, 01:08 PM
billpa's Avatar
Hi mpboost, I am not positive, but I would think that connecting the two wires of our brushless RPM sensor to any two of the three "phase" wires would work.

Or, if the phase pulses at those pins are 3.3 to 5V level, you might be able to connect just one of those phase outputs to the white pin of our RPM sensor input. Also, you'd need to connect pin 1 (ground) to the RED pin of the sensor input (ground = center pin), which will create a common ground between your motor sensor and eLogger.

Once you have it hooked up, you would need to experiment with calibrating the RPM using an optical tach or similar, and entering the gear ratio or # of poles so that it matches your motor.

Note that all these would be done "at your own risk" and could damage the equipment.

Re the temperature output, that won't work due to the way they have it wired.
Feb 21, 2010, 03:16 PM
aka Space Cowboy
mpboost's Avatar

thanks for the info! This will help me greatly!

I will report back on how it goes. I am pretty sure the sensor wire outputs a square wave 3.3v or 5v signal, but I will check with an o-scope before I get carried away.
Feb 24, 2010, 10:18 PM
aka Space Cowboy
mpboost's Avatar

The tech support you provided has helped me greatly

Today I received my eLogger V3 so I decided to get started on testing out the idea of using the existing sensor cable as an RPM input. First thing I did was some investigation into how the sensors work in a sensored brushless motor. I just happened to have a spare Tekin 7.5t sensored brushless motor laying around, so I decided to grab my allen wrenches and disassemble the can. Below is a picture of the circuit board inside the motor can which is used to determine rotor position:

As you can probably see, there are three hall effect sensors conveniently located on this board. These are used to tell the speed control rotor position so it knows which coils to energize. For those that aren't familiar with sensored brushless motors, the sensors allow the ESC to reduce "cogging" at low speeds and also allow some of the higher end ESC's to electronically advance motor timing based on rotor position in order to increase acceleration and max motor rpm.

Next I took one of my spare sensor cables and tapped into it using a spare servo lead. Specs for sensored motors are defined by ROAR rules and I found the diagram of the sensor cable wiring in the 2010_ROAR_Rule_Book.pdf document. Based on the advice you gave me I picked up the common ground from PIN 1 and decided to use PIN 2 from the phase C sensor as my rpm input to the eLogger. Below is a picture of the sensor harness with the two wires soldered on:

Once I had this all done, I hooked the motor and sensor cable back up to the ESC, pulled out my trusty multimeter, powered up the speed control so the sensor board would get power and slowly rotated the motor by hand. While watching the multimeter, it was clear the sensor cable outputs a discrete 3.3v when the rotor is in the proper position. This happens once per a revolution.

So I decided to go ahead and hook the sensor cable up to the eLogger and move forward with the calibration of the sensor. Since the sensor board in the motor uses hall effect sensors I decided to setup the eLogger as if I was using the Eagle Tree hall effect RPM sensor. The setting I chose in the software were for 1 magnet, and I also set the sensor location as "on the pinion" for speed and rpm calculations. After running the car through a bench test, this proved to work fine and after reviewing the datalogs, everything appears to be working properly. I don't recall the peak rpm I saw in the log, but it was on par with what I expected to see for the motor I was testing with. Below is a picture of my eLogger with the Power Panel, temp sensor loop, sensored brushless motor, and sensor cable with rpm taps for the eLogger.

So again, thanks for the help.
Feb 25, 2010, 10:54 PM
billpa's Avatar
Wow, great news! Glad you got it working.
Sep 16, 2010, 06:49 PM
Registered User
I've tried this method with mine (couldn't get the Eagle Tree brushless RPM sensor to work). I'm getting data, but it's not always believable. It sometimes shows the car goes from 5000 to 55000RPM in 1/10 of a second. Also the shape of the RPM graph doesn't seem to be a good match to the amp draw graph or the layout of the track. Is there an upper limit to RPM that could be causing issues here? My motor probably has a Kv of around 9000 (no official manufacturer specs) and is running on 2s LiPo. Maximum RPM reported is 65482, which seems a bit much. Also, it's EXACTLY the same on every run.
Aug 29, 2014, 03:20 AM
Registered User
Thank you so much for this informative thread. I soldered onto my brushless motor sensor wire. I did pin 2 and 3. Said 1 pole motor and verified against my tekin spec ESC built in data logger.

I only seem to get the rpm sensor setup once, can't find how to edit this rpm setting after the fact.. When I said 2 pole the RPM was half.

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