Mechatronics - DC Servomotor Transfer function for electrical components

Thread Starter


Joined Nov 26, 2021
I'm looking to obtain the electrical transfer function of current/voltage for a DC servo motor.

The circuit feeding the servo motor has a resistor and inductor in series.
The ordinary differential equation for this is:
[di/dt]L + iR + Cmotorϴ= Vsupply

So, voltage of inductor + voltage across resistor + motor constant multiplied by angular velocity for the back emf from the motor = DC supply voltage.

When I transfer this to laplace to get the transfer function, I get:
Lsi(s) + Ri(s) + Cmotorsϴ(s) = Vsupply

The issue is, when I attempt to remove current and voltage to get i/v =...... They cannot be removed.

I know I must be missing something to do with the motor circuit itself, that would negate parts of this equation, to allow me to find i/V.

I need to look at just the electrical components of this as opposed to the mechanical parts. I expect my final equation will only require just Cmotor, Ls, and R.

Can someone help me out please? Is there something obvious I am missing? Any advice would be appreciated :)