ode45 with multi functions

Actually i want a problem with ode45, but there is three function which express the problem here is my script: tspan = 0 : 0.001 : 3; y0 = [0;0]; [t,y] = ode45(@nonlinsys, tspan,y0); function f = force(wn_sq,damping,Umax,ydot,y,x_desire) f = wn_sq*(x_desire-y)+ damping*(-ydot);

        if f > Umax
          f = Umax;
         else 
            f < -Umax
            f = -Umax;

         end
 end

 function xd = x_desire(t)

 xd = heaviside(t-0.5);

 end

function dy = nonlinsys(t,y,wn_sq,damping,Umax,ydot,x_desire) dy(1)=y(2); dy(2) = force. dy = dy'; end

would you please help where the source errors are?