The present invention relates to producing large, controllable, vibratory forces to compensate for sensed noise or vibrations, and more particularly to an active vibration control (AVC) system which is driven by an aircraft gearbox.
The dominant source of vibration in a helicopter is that generated by the main rotor system rotating at the blade passing frequency. Forces and moments are transmitted usually through the transmission via airframe attachments, to produce vibration in the airframe.
One conventional approach to reducing such vibration involves replacing a rigid gearbox mounting strut with a compliant strut and parallel hydraulic actuator. These actuator strut concepts “intercept” the main gearbox vibration before entry into the airframe or generate counteracting loads that partially suppress the vibration or loads. Interrupting a load path may disadvantageously permit relatively large motions between the main gearbox and the airframe. Interruption of the load path between the gearbox and the airframe may cause fatigue failures in high speed drive shafts that transmit shaft power from the vehicle engines to the main gearbox. Compliant mounts may permit the gearbox to vibrate at higher levels than desirable which may then be transmitted to the driving engines reducing their service life. Furthermore, unwanted motions may also induce unexpected control inputs by effectively deflecting the mechanical flight control system links.
Another conventional approach utilizes counter-rotating eccentric masses located within the airframe to rotate at the frequency of the primary aircraft vibration and generate a fixed magnitude vibration force. A second pair of eccentric masses phased relative to the first pair to yield a force magnitude from zero to maximum force. A control computer commands the masses such that the inertial forces are produced to minimize airframe vibrations. Although effective, this approach may be inadequate in a vehicle having a gearbox which is directly attached to the airframe.
Conventional actively controlled force generators are electrically driven by an electric motor. The electric motor and ancillary equipment are relatively heavy and require considerable electrical power. Due to their size and drive requirements, conventional actively controlled force generators are positioned within the relatively flexible airframe remote from the vibration source of the rotor assembly. This arrangement permits vibration to enter the flexible airframe and may limit effectiveness by requiring a large number of actuators to achieve significant vibration suppression. Reduction to vibration levels of 0.05 g are essentially unattainable with such conventional force generators.
Accordingly, it is desirable to provide an active vibration control system which generates relatively large controllable vibratory forces with a lower weight and smaller size than conventional systems.