Applying limits in matlab

I have created a transfer function describing the movement of a piston in a cylinder. The piston can only move a max distance of 400mm. My mathematical model for the cylinder piston Q(s)=AsX(s) were Q is flow rate A is piston area and X is displacement, hence transfer function is X(s)/Q(s)=1/As (in matlab num=1, den=10 (when A is 10) function=tf(num, den). For step input step (function) gives a straight line inclined to the right. How do I tell matlab that the max displacement is 400mm so when the line gets to this limit it becomes flat?

I also need to apply PID tuning to this function, can anyone help with this?

Answer 1

[Matt Fig]

Displacement = min(400,X);

For example:

X = [1:20:800];
plot(min(X,400));
ylim([0 800])

[Tim]

Do I add this line after function=tf(num, den)before doing the step input? Sorry for asking these type of questions, but first time using matlab.