EDM wire tensioning

I'm still making some progress on my micro wire-EDM, I now have 5000m
of 0.05mm tungsten EDM wire and a small RC based sinker set up based
on Ben Fleming's book but modified to use stepper motors and my
ballscrew actuators.

As a reality check I am going to try some single axis cutting putting
the wire through a simple guide head to work as a chop saw in effect.

The next stage is to work out the wire tensioning side of things.

On the machine I have used the wire spool is held on a moterized
spindle before passing around a roller with an electomagnetic
clutch/break before going through the rest of the machine and to a
motorized pinch roller that pulls the wire and determines the speed.

However with 0.05mm wire I think I can probably get better tensions
using gravity, I have done something a little like this before when
winding coils in 0.02mm copper from a spool that did not allow the
wire to be pulled from the end. I mounted the spool on a gear motor
and then threaded the wire through an eye at the end of a hinged and
weighted arm. The arm could trigger two reedswitches depending on
whether it was pulled up or alowed to drop. The upper switch latched
the wire spool motor on and the lower turned it off. It gave a bang
bang control that was enough to make breakages uncommon as long as I
wound at a speed slower than the wire spool motor.

I want to do something like this again but I don't want it to be
bang-bang as the tension will lower as the weight it lowered and
increase as it is raised, at least during the acceleration phases.

I guess a simple op-amp based feedback control to a bridge controlling
the motor would seem sensible much as you would find in an RC servo.
What should provide the feedback is what I need input on. I could use
a LVDT but they are a little costly, alternatively a single turn
"servo pot" perhaps, they are very easy to turn and a lot cheaper.

Suggestions appreciated. I even wondered about an optical approach
with an LED in a tube getting closer or further from a sensor.

Graham