Servo - February 2009

Where:

RPM = Desired motor shaft speed Pole Pairs = Number of motor poles divided by 2

The code says we need to spin the BLY171S-24V-4000 motor shaft at 305 RPM:

tmr5val = 60 / 305 4 6 = 8.2 milliseconds

We can't readily load the raw 8. 3 ms into our PIC18F2431 TMR5 registers. So, we'll have to do a bit of conversion. We simply need to convert 8. 3 ms into TMR5 ticks. Here's what we need to load into the PR5 registers:

tmr5val = 8.2 ms (FOSC / 4) = 8.2 5000000 = 40984h = 0xA018

The stage is now set to run this code:

SCREENSHOT 1: This CleverScope capture was taken from the PHASE_C tap. This trapezoidal pattern is what we ultimately want to see when the BLDC motor is running at speed. We'll have to ramp up and spin a bit faster to get enough BEMF for the PIC18F2431's analog-to-digital converter module.

//**********************************************
//* START MODE
//**********************************************
case STARTMODE:
 if(voltage < 570)
 {

++voltage;
PDC0H = make8(voltage,1);
PDC0L = make8(voltage,0);
PDC1H = make8(voltage,1);
PDC1L = make8(voltage,0);
PDC2H = make8(voltage,1);
PDC2L = make8(voltage,0);

}
break;

Again, I used physical touch coupled with a minimal current draw to come up with the 570 PWM voltage value. Right now, all I want to do is smoothly spin the BLDC motor shaft. So, I dialed in the PWM voltage value for optimal symmetry of the waveform you see in Screenshot 1. The initial START mode voltage ramp is controlled by the tmr5val as the STARTMODE function is called with every TMR5 interrupt. The rotational speed is very slow and there isn't enough BEMF being generated to be recognized above the noise by the PIC18F2431' A-to-D module. However, my CleverScope picked up the beginnings of the trademark BLDC motor trapezoidal commutation waveform in Screenshot 1.

Spinning Out

The BLDC motor controller hardware is verified and the BLDC motor driver firmware is well along the way to being

viable. The next step is to transition from the START mode to the RUN mode. I think you have enough BLDC motor theory in your battery compartment to follow along in the complete version of the BLDC motor controller code that I'll put in the download package on the SERVO website (www. servomagazine.com) along with the ExpressPCB file. SV

Fred Eady can be reached via email at fred@edtp.com.

SOURCES

CleverScope
Saelig
www.saelig.com
A Microchip ICD3 was employed as the
debugging device.

ICD3 Programmer/Debugger PIC18F2431 MCP6002 MCP6542 TC4428 Microchip www.microchip.com

NDS9952A Fairchild Semiconductor www.fairchildsemi.com

BLY171S-24V-4000 BLDC Motor
Anaheim Automation
www.anaheimautomation.com
The code for the BLDC motor driver is compiled
using HI-TECH PRO for the PIC18 Family.