PWM duty cycle update for Sinewave UPS

[UroBoros]

I am using a Dspic for doing PWM for a Sinewave inverter.

The sine reference update part I have some problems.

I read the reference value from a sinetable at 104 usec interval. I have the output voltage and current ADC value at that instant in hand. Can anybody suggest a simple method to calculate the duty cycle update value?

Thanks

 

[FvM]

You forgot to mention, what's your intended algorithm to calculate the PWM duty cycle, e.g. how an ADC is involved.

 

[andre_luis]

The duty-cicle is exacly the sine value recorded at lookup table.
I did it in an UPS firmware and works fine.

+++

 

[btbass]

A method to vary the amplitude of a sine wave.

#define MAXPWM  1000    /* Whatever your max pwm is? */
#define MAXVALUES 359   /* Size of look up table */

#define MAXVOLTS  16500U        /* Maximum pwm amplitude, choose to suit application */
#define MINVOLTS  5000U         /* Minimum pwm amplitude */

extern sinetable[];   /* Your sine table look up values */

unsigned int duty_cycle;
unsigned int amplitude;
unsigned int sine_table_index = 0;

/*--- Set duty cycle using a timer interrupt, change period register for frequency ---*/

duty_cycle = sinetable[sine_table_index];
duty_cycle = ((unsigned long)duty_cycle * (unsigned long)amplitude) >> 15;
duty_cycle = ((unsigned long)duty_cycle * (unsigned long)MAXPWM) >> 15;

if(++sine_table_index > MAXVALUES){
  sine_table_index = 0;
  }


/*--- Sinewave lookup table, 360 1 degree steps ---*/

const unsigned int sinetable[] = {
32500,33067,33634,34200,34767,35332,35897,36460,37023,37584,
38143,38701,39257,39810,40362,40911,41458,42002,42543,43080,
43615,44146,44674,45198,45718,46235,46747,47254,47757,48256,
48750,49238,49722,50200,50673,51141,51603,52058,52508,52952,
53390,53821,54246,54664,55076,55480,55878,56268,56652,57028,
57396,57757,58110,58455,58793,59122,59443,59756,60061,60357,
60645,60925,61195,61457,61710,61955,62190,62416,62633,62841,
63040,63229,63409,63579,63741,63892,64034,64167,64289,64402,
64506,64599,64683,64757,64821,64876,64920,64955,64980,64995,
65000,64995,64980,64955,64920,64876,64821,64757,64683,64599,
64506,64402,64289,64167,64034,63892,63741,63579,63409,63229,
63040,62841,62633,62416,62190,61955,61710,61457,61195,60925,
60645,60357,60061,59756,59443,59122,58793,58455,58110,57757,
57396,57028,56652,56268,55878,55480,55076,54664,54246,53821,
53390,52952,52508,52058,51603,51141,50673,50200,49722,49238,
48750,48256,47757,47254,46747,46235,45718,45198,44674,44146,
43615,43080,42543,42002,41458,40911,40362,39810,39257,38701,
38143,37584,37023,36460,35897,35332,34767,34200,33634,33067,
32500,31932,31365,30799,30232,29667,29102,28539,27976,27415,
26856,26298,25742,25189,24637,24088,23541,22997,22456,21919,
21384,20853,20325,19801,19281,18764,18252,17745,17242,16743,
16250,15761,15277,14799,14326,13858,13396,12941,12491,12047,
11609,11178,10753,10335,9923,9519,9121,8731,8347,7971,
7603,7242,6889,6544,6206,5877,5556,5243,4938,4642,
4354,4074,3804,3542,3289,3044,2809,2583,2366,2158,
1959,1770,1590,1420,1258,1107,965,832,710,597,
493,400,316,242,178,123,79,44,19,4,
0,4,19,44,79,123,178,242,316,400,
493,597,710,832,965,1107,1258,1420,1590,1770,
1959,2158,2366,2583,2809,3044,3289,3542,3804,4074,
4354,4642,4938,5243,5556,5877,6206,6544,6889,7242,
7603,7971,8347,8731,9121,9519,9923,10335,10753,11178,
11609,12047,12491,12941,13396,13858,14326,14799,15277,15761,
16250,16743,17242,17745,18252,18764,19281,19801,20325,20853,
21384,21919,22456,22997,23541,24088,24637,25189,25742,26298,
26856,27415,27976,28539,29102,29667,30232,30799,31365,31932
};

 

[andre_luis]

Take a look here where I posted pice of code I used at above mentioned UPS :
https://www.edaboard.com/threads/104410/#post455712

This table was generated with that file :
https://www.edaboard.com/threads/211665/#post894464

+++

 

[UroBoros]

Thanks for all the replays and especially that sin table.
I can generate Sine wave now.Got it.But how to adjust the duty cycle for voltage correction based on inverter load.
I am reading inverter output volt and current using two channel ADC every 104 uS (ON every sinetable update - I have
sinetable with 192 points) to track the waveform. But how to apply PID or any simple method to compensate load changes?

Thanks

 

[btbass]

If you are using MPLAB C30 compiler, in the \src\include\ directory you will find 'dsp.h'. At the end of that file you will find the function prototypes for a PID controller that is included in the library.
You could use that PID controller to control the amplitude of your sine wave.

 

[UroBoros]

Thanks btbass. I will try that file.

Thanks for the help

 

[FvM]

I think, the multiple table solution provided by andre is good for tiny processor like pic16 or pic18, that has problems to carry out a multiply each PWM cycle. At PIC24 or dsPIC has a fast 16-Bit hardware multiplier and can use a single table, scaling the output. The voltage controller should use a measurement of averaged rectified value or even rms to adjust the duty cycle.

 

[UroBoros]

I think, the multiple table solution provided by andre is good for tiny processor like pic16 or pic18, that has problems to carry out a multiply each PWM cycle. At PIC24 or dsPIC has a fast 16-Bit hardware multiplier and can use a single table, scaling the output. The voltage controller should use a measurement of averaged rectified value or even rms to adjust the duty cycle.

I am trying to implement this in assembly in PIC16F73.
Can you elaborate this method little bit more?
Thanks

 

[FvM]

My assumption holds that PIC16 has difficulties to multiply table values on the fly during pwm generation. So the multiple table method suggested in post #5 would be a more promising solution.