GB591172A - Improvements relating to the electrical measurement or variation of physical states of materials such as moisture contents thereof - Google Patents

Abstract

591,172. Automatic control systems. FERRANTI, Ltd., TOOTAL BROADHURST LEE CO., Ltd., TAYLOR, M. K., and NIELD, L. C. Sept. 19, 1944, No. 37235/46. Divided out of 591,147. [Class 38 (iv)] [Also in Group XXXVI] Humidity; variables in general.-An apparatus for controlling a physical state such as wetness of a material, comprises a main testing condenser, between the plates of which the material is disposed, which condenser is incorporated in the tuned circuit of an amplifier so that the resonant frequency of the circuit varies in accordance with changes in the physical state, a variable oscillator for providing an input to the amplifier, control circuits having material testing capacitative components in which the material comprises a dielectric whereby the resonant frequencies of the control circuits also undergo variations in accordance with changes in the physical state of the material, adjusting means controlled by the variations and which vary the frequency of oscillations of the variable oscillator to maintain automatically the tuned circuit in a critical condition of resonance, and means for deriving the value of the resonant amplitude of oscillation in the timed circuit at different degrees of amplitude occasioned by different dielectric losses due to differences in the physical state. The web 70 of the material to be tested passes between upper and lower plates 71, 72 of a main testing condenser and between two additional upper plates 74, 75 and an 'extension of the lower plate 72. The lower plate is earthed-and the upper plate 71 is connected to a pentode 20, which is connected to an oscillator 76. The plates 74, 75 are connected to circuits 77, 78 and 79, 80, which are tuned to frequencies slightly above and slightly below that of the oscillator. The tuned circuits are coupled by coils 81, 82 to an automatic frequency control 83 connected to a reactance valve circuit 85 which is connected to the oscillator 76. The anode of the pentode 20 is connected to the positive H.T. through a load inductance 22. The anode of the valve 20 is also connected through a condenser 28 to the anode of a diode rectifier 29. The anode of this valve is connected to earth through a load resistance 30 and also to the control grid of a triode D.C. amplifier 31 through a resistance 32. The anode of the valve 31 is connected to the H.T. supply through a load resistance 34 and also through a milliammeter 35 to the sliding contact of a potentiometer 36. The inductance 22 and a decoupling condenser 37 form one branch of an A.C. parallel circuit, of which the other branch is formed by the series connected condensers 24 and 71, 72. Oscillations generated by the oscillator are amplified by the pentode 20 and rectified by the diode 29. The D.C. potential developed across the resistance 30 is applied to the control grid of the valve 31, the reading of the milliammeter 35 being proportional to the current through this valve. A sample of absorbent material of known moisture content is placed between the plates of the condenser 25 and the circuit tuned to resonance. The minimum reading of the milliammeter 35 is noted, or the milliammeter may be adjusted to zero by varying the potentiometer 36. The milliammeter may thus be calibrated in terms of moisture content. The sample to be tested is then placed between the plates of the condenser 25 and the frequency is readjusted until the oscillator and the pentode amplifier with the tuned circuit are in tune, giving the minimum reading of the milliammeter for that particular sample. The frequency in the three tuned circuits is that determined by the oscillator 76 and with the desired degree of wetness of the web 70 is mid-way between the resonant frequencies of the two control circuits 77, 78 and 79, 80, no control potential being developed. If the wetness of the material, and consequently its dielectric constant changes, the resonant frequencies of both control' circuits become displaced up or down the frequency scale whilst maintaining approximately the same separation, so that the frequency injected into these circuits from the main circuit no longer lies midway between their resonant frequencies. A control potential is therefore developed, causing the reaction valve 85 to alter the tuning of the oscillator 76 so as to tend to adjust its frequency to the new resonant frequency of the main oscillatory circuit caused by the altered capacitance of the condenser 71, 72. A servomechanism may be controlled so as to restore the moisture content when a change occurs, as by varying the rate of heating or the speed of the web. Specifications 275,741, 337,785, [both in Class 37], 396,098, [Group XXXVI]. and 558,491 are referred to.