An automatic player piano includes keys driven for the angular motion
through a hybrid feedback control loop; a controller, a key position
sensor, a plunger velocity sensor and a solenoid-operated key actuator
form parts of the hybrid feedback control loop for each key, and a
current key position and a plunger velocity are reported to the
controller; the controller determines a series of target position or a
reference trajectory and a target velocity, and periodically compares a
composite current position and a composite current velocity, which are
determined on the basis of the current key position and current plunger
velocity, with the target position and target velocity to see whether or
not the key travels on the reference trajectory; if the answer is
negative, the controller adjusts the driving signal to a proper duty
ratio so as to force the key to travel on the reference trajectory.