Transfer Function of an unknown device

[nebbione]

Hi everyone, in the last times, I'm really interested in controls engineering, and I wanted toknow how to find transfer functions of devices, or systems in the real world, let me explain better, for example, in my house i have various dc motors, and stepper motors, since i want to make an automatic control of the velocity, I'm blocked since i don't know the trasfer function of the motor... how can i proceed ? How can i find the transfer function of this motor ?
And in general how do i find a transfer function of a system, without knowing the matrixes A,B,C,D ?

 

[carlgrace]

What you're asking is non-trivial. The subfield associated with it is called "System Identification".

The easiest thing you can try if you have access to the input and output ports of the system is to input a fast pulse (approximating an impulse) and then record the impulse response. The transfer function would then be the Laplace transform of the impulse response.