Adaptive cancellation of tonal vibrations

Tonal vibrations are present in many industrial installations such as power transformers, rotating machines and computer disc drives [1]. Noisy machines can cause many problems not only by radiating the sound into the air but by also causing parts of the structure to vibrate and possibly resonate. In particular, there are many noise effects within disc drives primarily caused by the ball bearings in the disc drive housing [3]. These ball bearings cause tonal vibrations and as a result, these drives would need a good linear controller for head position control. One of the major problems with a linear controller is that Bode's theorem states that a linear controller would amplify these tones and disturb head position. Adaptive cancellation of tonal vibrations is the topic of this thesis. This thesis will investigate new non-linear control architecture for adaptive control of tonal vibrations in structures where the tone is high in frequency relative to the structural modes. The tone can drift in frequency. A three plate compound mount adaptive tonal control system consisting of a disturbance (motor) and an intermediate mass (actuator) was built, modeled and tested. The disturbance produces a tonal vibration which propagates through the structure and is to be eventually suppressed at the bottom plate. A tone sensor senses the tone and provides an input signal to a phase shifter. The phase shifter has the capability of phase shifting the "tracked" tone signal by a command from the controller. A power amplifier, which has a commandable gain, amplifies this signal which in turn drives an actuator located on the intermediate, "middle", plate. The actuator is now vibrating at the tonal frequency and is now phase-locked to the rotating machine with a commandable amplitude and phase.; There will be a tone null sensor accelerometer attached to the bottom plate sensing the tone of the system. This signal will be measured with a true RMS voltmeter, and this value is fed into a computer where a search in gain and phase is performed to null the RMS value of the sensor attached to the bottom plate. The properties of the controller were determined experimentally and through matlab/simulink analysis. By tuning gain and phase values through extensive trials, large decreases in RMS values were calculated for the bottom plate. One trial produced a decrease as high as 92.85% in RMS value. The greatest measured tonal attenuation in this experiment is 22.95dB.