PURPOSE: To make a control function of a piezoelectric flexural transducer independent from mechanical output sent by motion of the flexural transducer.

CONSTITUTION: The piezoelectric flexural transducers 24 are each arranged for operating movement perpendicularly to a movement path of the associated arresting elements between a blocking position 24' of being disposed in the movement path of the associated arresting elements 12, 13 and a non-blocking position of being disposed outside the movement path of the associated arresting element.