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Please verify the logic

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electronicsman

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Hi, for understanding purpose i have done the following to drive a motor. Please tell me which portions are absolutely wrong and which are to be modified. I have created a sine look up table and in a timer interrupt of 10ms i am loading one value each upto 256. So it is a kind of from 0 degrees to 360 degrees so one degree each every 10ms interrupt. To drive the legs for switching a vague method i have created is calculate the difference between previous voltage to present voltage and increase the duty cycle by some value of 20. I plan to change this switching sequence. I can see the motor rotating. Please suggest.
 

I have created a sine look up table and in a timer interrupt of 10ms i am loading one value each upto 256...
Click to expand...

The concept seems correct at all, I used the same look-up logic with an inverter. I'm assuming you mean by 256 to the maximum allowed value to the duty-cycle register of some PWM module.

To drive the legs for switching a vague method i have created is calculate the difference between previous voltage to present voltage and increase the duty cycle by some value of 20.
Click to expand...

There are issues on this method; the most critical is that you are using a constant correction value instead of using some kind of loopback control proportional to the difference.
 

Thank you very much for the reply.

andre_teprom said:
The concept seems correct at all, I used the same look-up logic with an inverter. I'm assuming you mean by 256 to the maximum allowed value to the duty-cycle register of some PWM module.
Click to expand...

No i am not loading duty cycle. can you please see the code i have attached. My doubt is should i load duty cycle?
Code: 
void Sinewave(void)
{
    static int index=0;
    
    if(index <= 256)
    {
         voltage_a= sin(2*PI*index/256);
         voltage_b= sin(2*PI*index+85/256);
         voltage_c= sin(2*PI*index+170)/256);
         index++;
    }
    else
    {
         index = 0;  
    }    
}

The above sine function i am calling in 10ms.
There are issues on this method; the most critical is that you are using a constant correction value instead of using some kind of loopback control proportional to the difference.
Click to expand...
Code: 
void pwm(void)
{
    double diff_phaseA=0.0;

   
   previous_phaseA=present_phaseA;
   present_phaseA=  voltage_a;
   
    if(present_phaseA >  previous_phaseA)
    {
     diff_phaseA= present_phaseA -  previous_phaseA;
     PDC1 += diff_phaseA * 200; /* diff_phaseA is in the range of 0.00 hence multiplying with 200*/
     if(PDC1 >= 500)
         PDC1 = 500;
    }
    else
    {
        
       diff_phaseA = previous_phaseA -  present_phaseA ;
        PDC1 -= diff_phaseA  * 200;
        if(PDC1 <= 2)
            PDC1=2;
    } 
}
Based on voltage difference i am loading the pdc value. And i am not getting constant value. But the motor is rotating very slowly. please help.
 

Many unclear points. Your are initially talking about a sine look-up table, but Sinewave() seems to calculate three phase voltages on the fly. The calculation is expectable very slow, probably taking a multiple of 10 ms, depending on the processor power.

Also what's the intended speed? 256 steps per 360° and 10 ms step rate gives only 23 rpm / number of pole pairs.

Don't understand yet what pwm() is doing...
 

I am sorry for not being very clear. Actually i am trying to generate three sine waves 120 apart for three phases. Now i want to switch the legs based on this voltage but don't know how to do it. I want to acheive this by pwm.
 

There are in total 6 pwm legs H1,L1, H2,L2,H3, L3 driving the 6 mosfets. The configuration is H bridge. The position i can read using an encoder.
 

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