Magnetic Levitation/Suspension. PID control Help.

[PICMICRO]

I am trying to make a magnetic Suspension Device, very similar looking to this. Image
The objects position is sensed via IR-diode and buffered before feeding to ADC.
The PWM is setup at around 4Khz.
I am using PIC16f877A and also using LCD to display various diagnostic datas if required.
This is my first PID project, so, I may be missing the very basics.
Please, Have a look at this code, and teach me something.
Feel free to ask anything.

void main()
{
        	setup_ad();
	init_LCD();
	init_PWM(); //intialize PWM
	set_duty(514U); // set PWM dutycyle = 514. ( 512 means 50%)
	init_Compare(); // Use CCP2 compare module to set CCP2IF flag at 256Hz rate
                                          // used to trigger PID loop

GIE = 0;
PEIE = 1;
TMR1IE=0;

unsigned int voltage = 0; // variable for the ADC value (gives the object position)
unsigned int ref = 512;  // refrence postion (middle)
unsigned int K = 2;       // some constant
signed int error = 0;   
signed int Iterm,Dterm,Pterm;
signed int net_error=0;
signed int old_error=0;
signed int correction;
signed int new_duty=0;

		while(1)
        {
			
			if(CCP2IF){  // wait for PID Sampling trigger(done at 256 Hz)			              
                         CCP2IF = 0;
			// GoDone set automatically
			while(GODONE);
			voltage = ADRESH*256+ADRESL; // Objects Position Value
			error = ref - voltage;
			net_error += error;
			if(net_error > 1023) net_error = 1023;
			if(net_error < -1023) net_error = -1023;
			Iterm = net_error/4;
			Dterm = (error-old_error)*4;
			Pterm = error/4;
			correction = K*(Pterm + Iterm + Dterm);
			new_duty = 512 - correction;
			if(new_duty>1023) new_duty = 1023;
			if(new_duty<0) new_duty = 0;
			set_duty(new_duty);
			LCD_Goto(2,9);
			LCD_Write_Num(new_duty);
			old_error = error;
			}
		}
}

 

[Mosaic]

Search up PID here, there are 4 hits for u!
https://www.microchip.com/wwwcategory/TaxonomySearch.aspx?show=Application Notes&ShowField=no

 

[PICMICRO]

Should I be clipping the net_error within +-1023 or should I allow it more room?
To tune the circuit, I am varying the constants, K, Kd, and Ki at powers of 2 (to make multiplication fast). Should I need more smooth control?
(the Ki and Kd are 4 in above code)
Also, how do I determine, what Sampling frequency I require?
Thanks.

 

[Mosaic]

have u read up any PID theory?

D is never a big factor, too much d makes it unstable.

A lot of control just needs PI tuned alone. D comes after PI is tuned.

A custom PID control means u experiment to determine the parameters. It has a lot do do with the speed of iteration based on the rate of I accumulation. A faster loop makes I accumulate faster. Therefore i suggest u standardise your loop speed with an interrupt and then tune with different values for P I.

 

[large_ghostman]

i was given one of theese machines for christmas! its realy cool, when i get better at electronics i would like to have a go at building one. good luck for your project