step response by Transfer Function with variable time constant

I want to make a transfer function 1/(Ts+1) in simulink. The time constant T can vary with the time here. For example, T=2 at time 0~3s, T=3 at time 3~10s. I've tried 2 methods to realize it:

1. I define T in the block "Transfer Fcn". T is calculated from the block "MATLAB Function".

2. I calculate the transfer function into time domain. Then I use the basic blocks such as Integrator, Substract and Divide to realize the transfer function.

I've tested my model with step response. Now the values of T which I want can be obtained.

The problem is: The step response is not smooth and continuous. At time=3s there is a unexpected sudden spring. I'm very confused. Please help me to explain this. How can I make the step response smooth and continuous? Thanks a lot !!

Answer 1

[Nicholas]

Maybe try using a different solver. Are you using real time or simulation time?

[Huanchen]

Thank you. I've tried other solvers. It doesn't work... I use simulation time.