Caddock  (Available through allied avnet)
makes an SR10 current sensing resistor for $2.91

This is a 4 terminal resistor. The idea is that there
are two heavy terminals to pass the current through, and
off to the side are two sensing terminals. The resistance between
the sensing terminals is accurately controlled.

Couple this to a hi-Z in op-amp through a couple hi-ohm resistors
and use to PWM a optocoupler.

or, if money's no object, you can buy clamp on inductive current
sensors for $95 ea frm FW Bell.

Alltronics is nice folks, but it's a big warehouse full of
junk they got from somebody. They usually have no idea what
it is or what it does.

--
Anniepoo
Need loco motors?
http://www.idiom.com/~anniepoo/depot/motors.html