Transfer function of motorized valve

How to find transfer function of motorized control valve. I need to derrive its transfer function for the control loop calculations, simulations.
Thanks you.
Hi Amornrat...
I can point you in a direction "Step change and transfer function" The step would be a go to position x and zero and then you would plot the position of the valve vs time......I think that would be a first-order system( if you ignore energy source from flow )
...that's for the valve, not the entire loop....that's the measured transfer function. ..there are some vids on youtube that talk about how to get the transfer function using this method hope that helps.. "bump test"...impuse test ....step test