MarkT:
I've been thinking about this some more and looking at what the Arduino
timers can do.I've realised the Mega has some timers with 3 output pins, and this allows a
clever way to modulate 3 output signals for doing this.It relies on having an inverter module that takes one input per phase (in
other words it generates its own high/low gate signals with appropriate
dead-time automatically generated - without this you'd need some
circuitry to do this).Using phase-correct mode you can effectively get two control pulses per
cycle (where the pulses are differential between separate phase outputs).So here's some test code - it drives pins 6/7/8 which are the pins for timer4.
// For the MEGA!
#define HALF 400
#define PERIOD 800
void setup ()
{
setup_cosines () ;
TCCR4A = 0xFE ; // phase correct (mode 1010)
TCCR4B = 0x11 ; // prescale by 1
TIMSK4 = 0x01 ; // overflow interrupt
ICR4 = PERIOD+1 ; // 100us cycle time, so effectively 50us.
OCR4A = HALF ;
OCR4B = HALF ;
OCR4C = HALF ;
pinMode (6, OUTPUT) ; // the OCR4A pin, our U phase
pinMode (7, OUTPUT) ; // the OCR4B pin, our V phase
pinMode (8, OUTPUT) ; // the OCR4C pin, our W phase
}
// control time values, in units of 62.5ns
volatile int u = HALF ;
volatile int v = HALF ;
volatile int w = HALF ;
// every complete cycle we update the registers.
ISR (TIMER4_OVF_vect)
{
OCR4A = u ;
OCR4B = v ;
OCR4C = w ;
}
// a cosine table, 1024 entries in range +/-127
char cosine [0x400] ;
void setup_cosines ()
{
for (int i = 0 ; i < 0x400 ; i++)
{
float a = PI * i / 0x200 ;
cosine [i] = round (127.0 * cos (a)) ;
}
}
int phase = 0 ; // taken modulo 1024
int amplitude = 200 ; // 201 is maximum value without overflow.
void loop ()
{
phase ++ ; // phase increment, normally this would be done by DDS loop
int newu = (cosine [phase & 0x3FF] * amplitude + 0x1F) >> 6 ;
int newv = (cosine [(phase + 0x155) & 0x3FF] * amplitude + 0x1F) >> 6 ;
int neww = - newu - newv ;
newu += HALF ;
newv += HALF ;
neww += HALF ;
noInterrupts () ; // interrupt-safe updating of u,v,w
u = newu ;
v = newv ;
w = neww ;
interrupts () ;
delay (2) ;
}
[ edited to fix mistake in the ISR ]
I have tested this Code and it really works very good. That was a great favor for my project.
Please help to define the base frequency of this code. I mean I have connected to 3 phase motor and it run with approx 0.5Hz. How it happens? any justification???