pid controller with daq

i want to use PID to make control on buck converter , output the control signal using daq card usb6008 .

any suggestion on building the controller in real time. or how to do this equation

vn= vn-1+ kp(en - en-1) + ki((en + en-1)/2)t + kd/t(en - (2en-1) + en-2),

[mado]

   i tried this equation but it isn't working 
   for i=1:3
      read=getsaample(ai)
      Vmeasured=read(:,1)
  e(i+1)=  Vmeasured*2-Vref

IE=sum(e); %Integral of  Error
%e= 18 - Vref
dt=.01
KP=2;
KI =1; 
Int(i+1)  = (e(i+1) + e(i))*dt/2; % integration of the error
 I(i+1)  = sum(Int); % the sum of the integration of the error

u=(KP*(e(i+1))+KI*I(i+1)) % control signal
 end