I visited the web site which Michael Waters mentions.

Mr. Alistair Riddell built an electric striking and sustaining mechanism
which replaces the traditional keyboard and hammers in a grand piano.
Each note has a simple solenoid which strikes the string directly.
A separate, independently controlled solenoid lifts the damper from the
string.

Most of his paper, "A Meta-Action for the Grand Piano", presented at the
International Computer Music Conference Glasgow, 1990, is taken up with
the details of construction and the design of the controlling computers.
The essential approach is described thus:

  "Functionally, the action consists of approximately 160 solenoids, one
  per hammer and one per damper, mounted on 2 parallel but different
  shaped rails. ...  The action frame was machined from aluminium which
  gives it an assembled weight of 35 Kg."

  "The actual hammer is a single moving part consisting of the solenoid
  `plug' or `core' at one end and the hammer head at the other end of a
  threaded rod. It is technically capable of operating very rapidly. The
  hammer tips have a base of aluminium and although vary in size are
  smaller than conventional hammers. They can be covered in a variety of
  materials to produce different timbres. The hammer travels approximately
  18 mm and weighs slightly more than the corresponding conventional
  hammer."

In conclusion, Mr. Riddell says:

  "From the above discussion, it is obvious that the action was not
  designed to necessarily produce a traditional piano sound, nor in fact,
  to be necessarily played in a conventional manner.  For the most part,
  the piano provides a convenient, abundant and aesthetically entrenched
  musical vehicle upon which to expedite ideas about humans, machines and
  art."

Mr. Riddell's web site is at  http://farben.latrobe.edu.au/alistair/

My own comments:

1.  The iron solenoid core with a hammer-head material would be
significantly stiffer than the traditional hammer and shank.  This
might lead to noticeably different tone.

2.  The independently controlled damper is like a super sostenuto,
allowing selected notes to resound sympathetically when a lower note
is struck.

3.  Mr. Riddell says that his system enables exploration of new forms
of piano music; but, other than the independent damper, I don't see the
advantage of his system over a solenoid player and traditional action.

Robbie Rhodes