请高手逐行解释这个matlab程序，不胜感激！急用啊！

%PID Controller
clear all;
close all;

ts=0.001;
sys=tf(5.235e005,[1,87.35,1.047e004,0]);
dsys=c2d(sys,ts,'z');
[num,den]=tfdata(dsys,'v');

u_1=0.0;u_2=0.0;u_3=0.0;
r_1=rand;
y_1=0;y_2=0;y_3=0;

x=[0,0,0]';
error_1=0;

for k=1:1:3000
time(k)=k*ts;

kp=1.0;ki=2.0;kd=0.01;

S=1;
if S==1 %Triangle Signal
if mod(time(k),2)<1
rin(k)=mod(time(k),1);
else
rin(k)=1-mod(time(k),1);
end
rin(k)=rin(k)-0.5;
end
if S==2 %Sawtooth Signal
rin(k)=mod(time(k),1.0);
end
if S==3 %Random Signal
rin(k)=rand;
vr(k)=(rin(k)-r_1)/ts; %Max speed is 5.0
while abs(vr(k))>=5.0
rin(k)=rand;
vr(k)=abs((rin(k)-r_1)/ts);
end
end

u(k)=kp*x(1)+kd*x(2)+ki*x(3); %PID Controller

%Restricting the output of controller
if u(k)>=10
u(k)=10;
end
if u(k)<=-10
u(k)=-10;
end

%Linear model
yout(k)=-den(2)*y_1-den(3)*y_2-den(4)*y_3+num(2)*u_1+num(3)*u_2+num(4)*u_3;
error(k)=rin(k)-yout(k);

r_1=rin(k);

u_3=u_2;u_2=u_1;u_1=u(k);
y_3=y_2;y_2=y_1;y_1=yout(k);

x(1)=error(k); %Calculating P
x(2)=(error(k)-error_1)/ts; %Calculating D
x(3)=x(3)+error(k)*ts; %Calculating I
xi(k)=x(3);

error_1=error(k);
D=0;
if D==1 %Dynamic Simulation Display
plot(time,rin,'b',time,yout,'r');
pause(0.00000000000000000);
end
end
plot(time,rin,'r',time,yout,'b');
xlabel('time(s)');ylabel('rin,yout');
这是刘金琨《先进PID控制Matlab仿真》中位置式PID控制算法的程序，求高手逐行解释啊！满意答案加分！