Servo - February 2009

security, I didn't apply power to the motor controller's

+ 12 VDC power input. Things seemed to go well without the BLDC motor and its power source. I tested with this configuration without incident.

After coding what I thought was enough to try to spin the BLDC motor shaft, the time had come to apply power to the three phase driver. My goal was to simply nudge the motor shaft by forcing it to assume the final (sixth) position in a BLDC motor electrical cycle. Recall from our previous discussion that the PWM outputs are selected using the PIC18F2431's OVDCOND and OVDCONS registers as follows:

PDC0H = 0x01;
PDC0L = 0xFF;
PDC1H = 0x01;
PDC1L = 0xFF;
PDC2H = 0x01;
PDC2L = 0xFF;
OVDCOND = 0b00100000;
OVDCONS = 0b00000000;

//PWM0 and PWM1

//PWM2 and PWM3

//PWM4 and PWM5

//PMW5 active only

Upon applying + 12 VDC BLDC motor power along with the + 5 VDC logic power, I noticed that the + 12 VDC power indicator LED dimmed and started to pulsate. After quickly powering everything down, I noticed that the TC4428s were very warm. I simply noted this as "odd" and moved on. I again applied full power and observed the pulsating LEDs which seemed to be in rhythm with a jerking of the BLDC motor shaft. After whiffing the aroma of "magic smoke,” I decided to "finger test" the warm parts. The TC4428s were hot enough to put a blister on my now-temperature-calibrated thumb. Something was wrong in a bad way. I figured I was somehow pushing too much current through the motor coils. Following some deep meditation, I came to the conclusion that the pulsating I was seeing was the wall wart switching power supply attempting to save itself from an overcurrent condition by discontinuing power to the load. As I'm trying to keep the nasty thought of "bad design" out of my mind, I'm burning up TC4428s at a fanatical rate. It came down to the fact that simply applying full power to the motor controller smoked the TC4428s. My feelings, my pride, and my thumb were hurt. For the next few days, I was clueless as to what was going on in a bad way with this project. Everything that had to do with polarity of power supply voltages and PWM signals looked fine on my CleverScope display and the NDS9952A MOSFETs were never getting warm. How could I be frying the TC4428s and not the MOSFET pairs?

More meditation led me to the conclusion that since I was only releasing the "magic smoke" when + 12 VDC was present, I was somehow overloading the TC4428's push-pull outputs. To kill the outputs meant that I had to be punching clean through the gates of the MOSFET pairs causing them to present a short to the TC4428 outputs. I whispered to myself, "No way."

Once again, I got down off of my donkey. The problem had to lie in the way I was driving the TC4428 inputs with the firmware. After a couple of days of intense datasheet scrutiny, I found that the problem was embedded in this set of firmware commutation patterns:

//* COMMUTATION PATTERNS
//* PHASES: USED OPEN
//*#define pos0 0b00111001 ;PWM1-PWM2 A-B  (C)
//*#define pos1 0b00101101 ;PWM1-PWM4 A-C  (B)
//*#define pos2 0b00100111 ;PMW3-PWM4 B-C  (A)
//*#define pos3 0b00110110 ;PMW3-PWM0 B-A  (C)
//*#define pos4 0b00011110 ;PWM5-PWM0 C-A  (B)
//*#define pos5 0b00011011 ;PWM5-PWM2 C-B  (A)

Do you see the problem? If you don't see the problem, don't feel bad. It took me two days to discover the source of my hardware pain. If you're still under the spell that was cast upon me, look back at the OVDCOND/OVDCONS code snippet I pulled from last month's discussion. Each and every commutation pattern has not one, not two, but four MOSFETs activated. I was lucky not to set fire to the bench! Here's the commutation pattern set that removed the spell and led me out of the magic-smoke-filled forest:

//* NEW COMMUTATION PATTERNS
//* PHASES: USED OPEN
//*#define pos0 0b00000110 ;PWM1-PWM2 A-B (C)
//*#define pos1 0b00010010 ;PWM1-PWM4 A-C (B)
//*#define pos2 0b00011000 ;PMW3-PWM4 B-C (A)

etc.

That's more like it! Now, only two PWMs are activated per commutation position. One of the PWM signals is always a + 12 VDC enabler and the other PWM is always pulsing the ground return MOSFET. I am happy to report that the + 12 VDC indicator LED no longer dances to the beat of an overloaded wall wart and the TC4428s are no longer burning holes in my thumb.

The Big Picture

The updated motor controller hardware is saying "cheese" in Photo 1. During the firmware test period, I discovered that the VM voltage output needed its own dedicated analog-to-digital (A-to-D) input. So, I moved the MOTOR_I voltage output point to the jumper block point that was occupied by the VM voltage output. For the purposes of a fault condition, there's no reason to keep up with the motor current as the current op-amp/comparator monitor circuit can be used to detect and report an overcurrent condition to the PIC18F2431. The output of the MCP6542 comparator feeds the PIC’s active-low FLTA pin. When an overcurrent condition is sensed by the MCP6542, its output falls to a logical low level, which kicks in the PIC's automatic fault handler. Upon seeing a logical low at the FLTA pin, the hardware PWM outputs are driven to an inactive state until the fault condition is removed. With some simple register programming, we can choose to remain in a fault mode until the firmware clears the fault indicators or exits fault mode automatically when the overcurrent condition is removed. If you wish to monitor the current with the A-to-D module, you'll be pleased to find that the voltage in millivolts presented at the MOTOR_I node is identical in value to the current being drawn. For instance, if you read 37 mV at the MOTOR_I node, you're pulling 37 mA of current through the three phase driver