一 ：tf([ ],[ ]) 函数
是传递函数的意思，一般学自动控制原理的时候经常用，在s域中，比如你要输入G（s）=1/（s^2+2s+1），就可以在matlab中输入G=tf（[1],[1 2 1]）;就OK了。不懂的话你可以在command窗口输入help tf 就行了
matlab中 help tf :
tf
Create or convert to transfer function model
Syntax
tf
sys = tf(num,den)
sys = tf(num,den,Ts)
sys = tf(M)
sys = tf(num,den,ltisys)
tfsys =tf(sys)
tfsys = tf(sys,'inv')
Description
tf is used to create real-or complex-valued transfer function models (TF objects) or to convertstate-space or zero-pole-gain models to transfer function form.
二：
sysd=c2d(sys,Ts,'z'); % c2d ---连续的时间系统转化为离散的时间系统,即离散化.'t'表示双线性变换法
% Find critical valuecrs Kcr,Wcr % 'z'表示带零阶保持器的Z变换法,可缺省
三：
sys=tf(5.235e005,[1,87.35,1.047e004,0]);%获得传递函数。
dsys=c2d(sys,ts,'z');%将连续的时间模型转换成离散的时间模型，采样时间是ts=0.001。
[num,den]=tfdata(dsys,'v');%获得离散还建模型的分子分母矩阵
四： PID MATLAB 仿真程序
>> ts = 0.001;
sys = tf (400,[1 ,50,0]);
dsys = c2d (sys,ts,'z');
[num,den] = tfdata(dsys,'v');
u_1 = 0.0 ;u_2 = 0.0; u_3 = 0.0 ;
y_1 = 0 ; y_2 = 0; y_3 = 0;
x = [0, 0 ,0];
error_1 = 0;
error_2 = 0;
for k=1:1:1000
time(k) = k*ts;
rin(k) = 1.0;
kp = 8;
ki = 0.10;
kd = 10;
du(k) = kp*x(1) + kd*x(2) + ki*x(3);
u(k) = u_1 + du(k);
if u(k) >= 10;
uk = 10;
end
if u(k) <= -10 ;
u(k) = -10;
end
yout(k) = -den(2)*y_1 - den(3)*y_2 + num(2)*u_1 +num(3)*u_2;
error = rin(k) - yout(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 - error_1;
x(2) = error - 2*error_1 + error_2 ;
x(3) = error;
error_2 = error_1;
error_1 = error;
end
plot(time,rin,'b',time,yout,'r');
xlabel ('time(s)');
ylabel ('rin,yout');