Ok, I've got dents in my wall and lumps on my forehead. It's time to tap
the collective wisdom of the list.

I'm building a system to control a sensored NEMA23 size brushless motor
based on a dsPIC33F128MC804.
I've got the motor spinning and I'm working out my control loops (speed and
position).
Earlier, when I was doing simple bench testing, the motor
was noticeably smoother in one direction
than the other. Now that I have the gearbox and a load connected to it,
it's _much_ worse.
Clockwise, it's smooth as silk and moves my load at about 1.5 Amps.
Counter-clockwise it's noticeably
noisier and draws upwards of 7 Amps. The load is very close to the same in
both directions.

I'm completely at a loss for why it would behave differently depending on
direction of rotation.

I'm using brand new, Electrocraft motors, (two different motors,
from different production batches have been tried with the same results)

The board is our design. I've run several tests on it and I'm confident
that the FET drivers and FETs are operating correctly.
I've got a small logic analyzer and I've looked at the halls (clean, well
sequenced, uniform widths - both directions) and the control lines to the
FET drivers (exactly the expected sequence).
I've scoped the gates of the FETs and I don't see anything odd.

The Hall sensors and commutation is managed with the CN interrupt.
Assembled into a three bit value and used as lookup in a sector table.
The sector value is then incremented or decremented (CW vs CCW) and used to
lookup the OVDCON value. This is all very much strait out of the appnotes.

I'm fresh out of ideas. Any good suggestions?

-Denny




Here's some snips of the relevant code sections:

//globals
char SectorTable[] =3D {0,6,2,1,4,5,3,0};
unsigned int StateLoTable[] =3D { 0x0000,  /* All off, guards from bad hall
data*/
                                0x0204,  /* PWM2L -> 1, PWM1H -> PWM */
                                0x0210,  /* PWM3L -> 1, PWM1H -> PWM */
                                0x0810,  /* PWM3L -> 1, PWM2H -> PWM */
                                0x0801,  /* PWM1L -> 1, PWM2H -> PWM */
                                0x2001,  /* PWM1L -> 1, PWM3H -> PWM */
                                0x2004,  /* PWM2L -> 1, PWM3H -> PWM */
                                0x0000}; /* All off, guards from bad hall
data*/





//and in the CN interrupt

tempC =3D PORTC;                                  //grab the pins as quick =
as
possible
tempB =3D PORTB;                                  //according to stopwatch,
assembly takes 15 cycles
HallValue =3D (0x0007 & (((tempC & 0x0040) >> 4) | ((tempB & 0x0200) >> 8) =
|
((tempB & 0x0100) >> 8)));

CurrentHallSector =3D SectorTable[HallValue];     //linearize hall reading =
to
sector number

if (MotorDirection =3D=3D CW)                       //Calculate the next se=
ctor
based on current sector and direction
    {
    NextHallSector =3D CurrentHallSector + 1;
    if (NextHallSector =3D=3D 7)                    //the numbers wrap arou=
nd...
        NextHallSector =3D 1;
    }
else
    {
    NextHallSector =3D CurrentHallSector - 1;
    if (NextHallSector =3D=3D 0)
        NextHallSector =3D 6;
    }

P1OVDCON =3D StateLoTable[NextHallSector];        //Load a value into OVDCO=
n
based on next position
--=20
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