Tony and Frank Decap demonstrated to me the electric actuators
in a dance hall organ built a few years ago by Decap Herentals.
As I recall, each of the larger percussion instrument beaters is
propelled by a big solenoid with a velocity feedback transducer,
so that the beater can follow MIDI 'velocity' commands and also be
repeatable under any condition of friction or power supply variation.
This method is much the same as is used in the Stahnke SE solenoid
piano system.

Robbie Rhodes