So I have learned much over the past few days about BLDC motor control, what is available in existing hardware, and some of the concepts that are common across all the available controllers. With that new knowledge, I went back to some of the earliest research I did on this subject; and I now have another potential solution.
Early on, I made contact with a user here named "oldraven", who had fixed a dead SX3 controller board; and in the process made schematics of the power board and the controller board. He was kind enough to share them with me, but I had little knowledge of the subject at the time and they meant nothing to me then. However, this week I dug them back out, and something jumped out at me - the SX3 controller uses a standard MC33035 BLDC motor controller chip. This chip is likely used in many of the off-the-shelf servo and BLDC controllers because it handles commutation and other motor control issues; but what struck me was that it also includes speed control by DC voltage.
Pin #11 of the MC33035 is typically attached to the wiper of a pot, which when varied between 0 volts and the reference voltage supplied on pin #8, controls the speed of the motor. Since the SX3 includes this chip, voltage control should be just a simple matter of injecting the control voltage into pin #11 of the chip.
I still know that I don't know much about this subject, so before I go tempting fate by hacking into this PC board, I thought I would run this by any of you motor control experts out there to see if this seems to be a reasonable idea.
I have attached a jpg of the relevant portion of the schematic that oldraven had produced. It shows pin 11 basically going to 6V. It would seem to me that just disconnecting this trace, and instead feeding a 0-6 volt signal either through a pot or an external source should provide speed control. It should be relatively easy to test this, but before I tried it I thought I would see if anybody has any concerns or thoughts about this approach.
I have attached a couple of PDF's of the schematics, and the datasheet for the MC33035 IC is here:
http://www.onsemi.com/pub_link/Collateral/MC33035-D.PDF
Any advice as to how an external signal should best be brought in to the circuit board? Any buffering required? Scaling to 10V and buffering could probably easily be done using a simple op-amp if necessary.
Dave