Patent GB-1301301-A (GB)
IPC: B04B9/14
Filed: 1969-01-07
Abstract: 1301301 Controlling a vibratory body; electrooptical path THOMAS BROADBENT & SONS LTD 6 Jan 1970 [7 Jan 1969] 850/69 Headings G1A, G1N [Also in Divisions B1, G3, H2] An arrangement for maintaining constant the magnitude of vibrations of a vibratory body of non-constant inertia operating at or near resonance comprises a sweep frequency oscillator whose output determines the frequency of vibration of the body and which is controlled so that as the inertia of the body and hence the frequency for resonance varies the sweep frequency range varies so as to be centred on that frequency. The term "vibratory body" includes within its scope a body rotating at constant angular velocity and having an oscillatory motion about the same axis imposed thereon. As shown a centrifuge basket 4 is carried on a shaft 1 and rotated by a motor 5 at a rotational speed of about 200 r.p.m., and a rotor/stator, arrangement is provided such that the stator provides a rotating field which, if it is not synchronous with the shaft, causes an alternating torque which produces a sinusoidal vibration superimposed on the rotation of the shaft. The assembly of basket rotor and shaft, all of known inertias, constitute an arrangement having a resonant frequency, and if the imposed frequency matches this resonant frequency maximum amplitude of oscillation of the basket will occur. During movement of the basket solid matter from a slurry fed to the basket is discharged at the open end, and hence the discharge rate is dependent on the amplitude of vibration. Since material is being continually fed to and discharged from the basket the mass and hence the inertia of this varies, and to maintain maximum amplitude of vibration the imposed vibrational frequency must be varied to maintain resonance. To effect the necessary control, a sensor 12, whose output depends upon the amplitude of oscillation, feeds, through an amplifier-rectifieramplifier arrangement, one side of a differential amplifier 24 whose other side is fed from a preset potentiometer 25 so that at maximum output from sensor 12 the two inputs are equal and little or no current flows through a lamp 26. The lamp 12 shines on a photo-cell 27 the left hand side of which is periodically switched every ¢ second between +ve and -ve supply lines by means of circuits 28, 29, 30, 31 and the right hand side of which is connected to capacitor 32, which is charged first in one sense and then in the other every second to a voltage dependent on the photo-cell resistance, and this voltage controls the output frequency of a uni-junction transistor oscillator 38,39,40. This frequency in turn controls the exciting frequency of the stator 11.  Hence once per second the oscillator sweeps a frequency band dependent on the voltage excursion at the capacitor 32. During this sweep the resonant condition will be passed through, resulting in a dim light, and a period during which cell 27 has a high resistance, so that the next capacitor voltage excursion and hence frequency sweep is reduced. Eventually only a small frequency sweep around the resonant frequency obtains. The arrangement may be modified by maintaining a constant stator excitation frequency and controlling an electromagnetic brake on the shaft. The capacitor may be replaced by an inductive arrangement, and the photo-cell may be replaced by a thermistor, voltage dependent element, field dependent element or the like.