Unknown secundary peaks in motor control PWM signal.

[E]

Hey guys,

I'm currently working on a diy motor controller for brushless motors for fun. I'm trying to use Jianwen Shao's method (http://scholar.lib.vt.edu/theses/available/etd-09152003-171904/unrestricted/T.pdf) of measuring the rotor position on the third coil while the driving PWM signal is low.

I'm sending a PWM signal to the FAN7888 motor driver (http://www.fairchildsemi.com/ds/FA/FAN7888.pdf) and measure the voltage of the floating coil behind a 10kOhm resistor with an Atmel microcontroller.

It's working rather neat so far, but I ran into something weird that I can't explain: shortly before the PWM on the high MOSFET of the active coil rises, I get an unexpected edge going up about 2V before the PWM going up to 20V. It's a rather sharp edge modulated onto the voltage induced by the stators permanent magnets. (see pictures) These secondary peaks mess up my measurements of the rotor position.

The measured point is the floating coil directly, connected to the lower side of the motor drivers charge pump, the drain of the lower MOSFET, the source of the higher MOSFET and to the microcontroller via a 10k resistor. (And obviously the other two coils in the motor.)

Is this caused by the charge pump or am I not getting something? As you can see from the images, the width of those secondary peaks varies. Also with (much) higher RPM they luckily disappear.

Thanks for any help,

E

[Paul Price]

No, the waveform is correct, you are simply seeing the voltage across the diode float up to the output voltage level after the catch diode has completely discharged the inductor.

[Paul Price]

Also be aware of the very poor recovery time of the vertical digitizing circuitry to overrange signals, and thus any waveform you see that in peak amplitudes exceeds the top or bottom graticule line, will be severely distorted, unlike the correct pulse shapes and amplitudes shown a cheap old analog oscilloscope.