Speed Control of Separately Excited DC Motor by Controlled Rectifier

there is s function to generate the firing angle to the three phase controlled rectifier

the inpu to this s function block is the controlled voltage

can anyone explain this code of s function

function [sys,x0,str,ts]=abcrtoqdor(t,x,u,flag)

%dfsfgsfdgdghdhdfg

switch flag

case 0

[sys,x0,str,ts]=mdlInitializeSizes;

case 3

sys=mdlOutput(t,x,u);

case {1,2,4,9}

sys=[];

otherwise

error(['Unhandel flag =',num2str(flag)]);

end;

%========================================================================

%========================================================================

function [sys,x0,str,ts]=mdlInitializeSizes

sizes=simsizes;

sizes.NumContStates= 0;

sizes.NumDiscStates= 0;

sizes.NumOutputs= 1;

sizes.NumInputs= 2;

sizes.DirFeedthrough=1;

sizes.NumSampleTimes=1;

sys=simsizes(sizes);

x0=[];

str=[];

ts=[-1 0];

%=========================================================================

%=========================================================================

function sys=mdlOutput(t,x,u);

%=========================================================================

vc=u(1);

vm=u(2);

q=(vc*pi)/(3*vm);

if q>1

q=1;

end

if q<-1

q=-1;

end

a=acos(q);

sys=a*180/pi;