Only by sensing current, not via a chopper wheel or mag input.

Most SCR or Triac or Quadrac type controllers are like this to most
extents.
Remember the line to the motor is inductive as well. Be sure the unit
is
rated for inductive loading - so the inverse kick of the motor doesn't
zap it.

Might not be on the label - so be prepared. Always best not to force
stop
as the electric field in the windings has to go somewhere. Normally
into the
Semiconductors - but if it coasts down or stops by itself it is best -
lest there
is protective circuits - e.g. buy a power plug strip that has lightening
strike
protection - and use it between the speed controller and the motor.
That will
protect both motor and speed controller to a great extent.

Martin
--
Martin Eastburn, Barbara Eastburn
@ home at Lion's Lair with our computer
NRA LOH, NRA Life
NRA Second Amendment Task Force Charter Founder