Constant Velocity unwinder

Hello,
I think this would be off topic, dont know so if you think it is,
please add the OT on the subject on your replies.

I have a client that has an unwinder, it unwinds flatstock about .25"
thick, it has a 5HP motor with a reducer, and a VFD. The client has it
right now as to supply the max amount of material it can, but
obviously, as the diameter of the roll gets smaller the less material
is being delivered. What would be a good way to supply constant
material flow? I was kind of thinking of replacing the motor and VFD,
with a servo operating in velocity mode, and attaching the encoder to
a following wheel that is in contact with the material being
delivered. Does anyone know if there are VFD that can accept a signal
that regulates the speed, that signal being an encoder or tach
attached to the advancing material?
Any suggestions would be greatly appreciated.
Regards
Fernando