Patent GB-1032418-A (GB)
Title: Improvements in and relating to apparatus for controlling the drying of particulate material
Inventor: MATTHEWS JOHN
IPC: G05D22/02
Filed: 1961-12-15
Abstract: 1,032,418. Automatic control of humidity. NATIONAL RESEARCH DEVELOPMENT CORPORATION. Feb. 15, 1963 [Dee .15, 1961], No. 45089/61. Heading G3R. [Also in Division G1] The moisture content of a particulate material (which term includes agricultural material such as peas and beans) is determined by using a sample as the dielectric of a capacitor and the value thus obtained is used to control a drier. Electrode construction Fig.1 shows a plan view of a vertical drying column having two perforated metal sides 1,2 which act as electrodes for a capacitor and also allow the passage of a drying gas. The two sides 1,2 are electrically connected, the other electrode of the capacitor being an aluminium rod 10 which is free to move at one end to allow for expansion. The rod incorporporates a capacitor 8 connected in series with the measuring capacitor 1,2, 10, the capacitor 8 having a negative temperature coefficient to compensate for temperature changes. Alternatively a capacitor of the fringe field type may be used. A capacitor having a central plate electrode surrounded by an outer electrode covering both faces of the plate is described (Fig.6 not shown). The main di-electric of this fringe field capacitor may be epoxy resin the electrodes being brass. Measuring circuit. The measuring capacitor is connected in parallel with a fixed capacitor and a variable capacitor this parallel arrangement being in series with the compensating capacitor 8 in one arm of an A.C. bridge connected to a 2 Mc/s supply. The reference arm also includes a parallel arrangement of three capacitors one fixed and two variable so that a desired value of the moisture content can be set (Fig.2, not shown). The output of the bridge circuit is supplied to a phase sensitive detector which supplies a D.C. voltage which is then amplified to produce a signal whose amplitude and polarity indicate the magnitude and direction of the difference between the actual and desired moisture content. Control circuit The output of the D.C. amplifier is used to operate a relay 32 which couples a D.C. supply derived from an A.C. supply through a rectifier 36 to one of relays 23, 24 the particular relay operated corresponding to the polarity of the amplifier output. This relay 23 or 24 connects servo motor 40 to a further D.C.supply derived through a second rectifier this servo-motor 40 driving a potentiometer 29 through a gear train. The potentiometer 29 operates limit switches 27 at the ends of its travel thus disconnecting the motor 40. The output of the potentiometer is subtracted from the proportion of the amplifier output across a potential divider. The output controls the drier by altering the heating in the drier or the speed of passage of the material therethrough. The potential divider includes a diode 45 so that the polarity of the amplifier signal effectively changes the division ratios. Thus the response of the dryer is faster when the moisture content is below that desired. It is stated that this produces a more linear response as it allows for the non- linearity of the moisture content/drying rate relationship. Diodes 41 and capacitor resistor combinations 39 reduce the transient effects of the relay circuits. Several types of drier are mentioned and the material may be moved by an oscillating plate conveyer belt, paddle system or an archimedean screw (Fig. 5, not shown). Alternatively a batch drier may be used. The sensing head may be positioned either before the drier, in the drier, alternatively after the drier to produce a retroactive control.