There are many competing technologies.  My sense is that pneumatic
systems are probably faster-acting for the level of mechanical output
needed for a piano.  We might make a comparison with early computer
printers, which used advanced solenoid technology but still made a
tremendous amount of noise, tended to become quite hot, and required
a lot of power.

The fundamental difficulty in all of this is that while we've made
tremendous progress in electronics, we haven't done nearly as well with
mechanical actuators.  We're still confined to hydraulics, pneumatics,
electromagnets, and motor-driven mechanical actuators.

This last category, which probably has a better name than I've assigned
to it here, is interesting to explore, and I was wondering if it had
ever been used for player pianos.  It's the principle used in early
electric typewriters: a fairly large electric motor would rotate a flat
bar that extended across the machine.  Pressing a key would swing a
metal tab out such that the rotating bar would strike it and thus whack
the type bar against the paper.  The action was powerful and very fast,
and little electric power was needed to actuate a lot of keys in rapid
succession.  The difficulty, of course, is that it can't be throttled
quite as easily as a pneumatic system.

Solenoids are problematic because they're tough to design with an
appropriate force vs. position characteristic.  The force starts out
too weak and ends too strong, instead of giving us the initial oomph
we need to actuate a mechanical piano action.  Thus a lot of current
is needed to get them going.  The effect of the inductive kick on the
driver electronics must be dealt with, but my guess is that the heat
build-up is more of a problem.

Mark Kinsler
Lancaster, Ohio
http://www.frognet.net/~kinsler

 [ In Bowers' Encyclopedia, page 382: "The Pianotist [ca. 1900] was
 [ a roll-operated piano which transmitted power to the piano hammers
 [ by means of a rotating power roller similar to that used in certain
 [ electric typewriters today."  The drawing shows a key frame with
 [ feeler fingers.  Hupfeld sold a similar instrument, too.
 [
 [ Karl Petersen evaluated the 'electric typewriter' method for
 [ a reproducing piano system and judged it feasible.  I think he
 [ proposed using two power rollers at separately controlled speeds,
 [ a la Duo-Art, for "Theme" and "Accompaniment" hammer velocities.
 [ The two motors must have low inertia.  -- Robbie