BLDC transfer function from step response

hello,

I have managed to obtain a measured step response (speed in revs/sec vs time) of a BLDC motor system (motor, driver circuit) and now need to obtain a transfer function in order to design a controller in matlab. Does anybody know how to work out a systems transfer function from a step response?

I have worked out the mathematical model of the motor using the motor parameters and got a transfer function, but i would like to get one for the whole real system.

Any help much appreciated!