Reactionless rotary drive mechanism

1. Field of the Invention An articulating probe head includes motors and for driving respective output shafts and about respective orthogonal axes z and x to move a stylus over the surface of a workpiece under the control of a controller . At least one of the motors is inertia balanced by mounting the stator of the motor on bearings to allow it to rotate in opposition to the rotation of the rotor . Control of the speed of the spinning stator is achieved by connecting it to the winding assembly of an additional "back-to-earth" motor the magnet assembly of which is connected to the housing. The motor acts as a brake to prevent overspeeding of the rotatable stator, and can have power supplied to it to ensure that the stator does not slow down excessively when the main motor is running at constant angular velocity. Control of the power supply to the motors is achieved by the controller