ACTIVE CONTROL OF FRICTION-DRIVEN OSCILLATIONS

This paper introduces an active control technique that combats oscillations driven by dry friction forces. Dry friction can act as the excitation mechanism for some high amplitude oscillations; curve squeal from trains is a well known and notorious example. Another, more elementary, example is the oscillation of a mass-spring system sliding on a moving belt. A model which predicts the stability behaviour of this system is presented. The model is then extended to include an active control system. The active control system is a feedback loop which consists of the following components. A transducer senses the velocity of the mass. The transducer output is passed to a filter, then to a phase shifter which applies an adjustable phase-shift, to a variable-gain amplifier, and finally to a shaker which is attached to the mass. The shaker exerts a control force in the tangential direction, and this force superimposes on the friction force. The stability behaviour of the controlled system is analyzed in two ways: by calculating of the complex eigenfrequencies and by considerations based on a balance equation for the oscillatory energy. The differences between this form of active control and conventional anti-vibration methods are discussed. Further studies, both on the theoretical and experimental front, are under way to extend the application of this kind of active control to curve squeal from trains.