Patent GB-581161-A (GB)
Title: Improvements in methods and apparatus for the measurement of small capacities and applications thereof
IPC: G01R17/00
Filed: 1940-01-10
Abstract: 581,161. Indicating proximity of earth ; capacitance measurements; condensers. BLUMLEIN, A. D. Jan. 10, 1940, Nos. 547, 4478, 10483 and 10485. [Class 37] [Also in Group XXXVIII] In apparatus for measuring the distance of an object such as an aircraft from a conducting surface such as the earth by measuring a capacitance which is a function of the distance to be measured, means are provided for eliminating casual changes of capacitance between the electrodes and the body of the aircraft. As shown in Fig. 2 (10485/40), electrodes 3, 4, in the form of conducting plates, are fixed to the under surface of the fuselage of an aircraft 2 and insulated therefrom. The capacitances C3, G4 between-these electrodes and earth-and the capacitance C5 between the aircraft 2 and earth are variable with the height of the aircraft above the ground,. and this height may be determined by measuring the capacitance of the network containing the capacitances C3, C4, C5. The effect of the capacitances Cl, C2, between the electrodes 3, 4 and the aircraft 2 is eliminated by the employment of a bridge having tightly coupled inductive radio arms L1, L2, the inductance L1 shunting the capacitance C2. An A.C. generator 5 is connected in shunt with the capacitance C1, an adjustable capacitance Cv forming the third arm of the bridge. The capacitance Co between the electrodes 3, 4 may be balanced out by means of a subsidiary capacitance connected similarly to Cv. Errors due to couplings between the leads to the electrodes are eliminated by making the connection between the generator 5 and the aircraft 2 in the neighbourhood of the electrode 3. An inductance may be connected in parallel with the capacitance C1 and is of such a value as to tune the capacitance C1 and form a rejcetor circuit. Similar arrangements may be made with regard to the capacitance C2. In order to eliminate leakage errors due to spray reaching the electrodes, a conductance balance is provided to enable the bridge to be balanced for the conductance component of the capacitance to be measured. This may comprise an adjustable resistance in parallel with the condenser Cv. Also the generator 5 may have a low impedance so as to render the voltage set up across the electrode 3 less dependent on leakance variation. The generator 5 may be a valve oscillator as described in Specification 563,464, [Group XL], coupled to the bridge circuit by a push-pull amplifier with negative feed back so that it has a low output impedance. The capacitance C2 is preferably shunted by a relatively low resistance to reduce the effect of variations of leakance consequent on leakage between the electrode 4 and.aircraft 2. This low resistance may be provided by means of a negative feedback as described in Specification 528,179, [Group XL]. Subsidiary electrodes may be provided on the lowest part of the aircraft, such as the undercarriage or floats, and may be connected to the long electrodes 3, 4. The electrodes 3, 4 and also the subsidiary electrodes may be formed by a sprayed metal coating on a sheet of flexible insulating material attached to the under surface of the aircraft a considerable margin being left unsprayed to reduce leakage. The electrodes may be mounted beneath the nose and tail of the aircraft. In order to enable a larger balancing condenser Cv to be employed, the voltage applied thereto may be less than that applied tothe electrode 3. Such an arrangement is shown in Fig. 1 (Comp.) in which the apparatus comprises three units, each consisting of a screened box earthed to the body of the aircraft and housing parts of the bridge. The control screened box S, situated in a position accessible to the aircraft crew, contains the generator 5, balancing capacity Cv, and the balance indicator 6. The screened box 7 is located in the neighbourhood of the electrode 3 and contains a transformer 8 and tapped choke 9. The screened box 10 is located in the neighbourhood of the electrode 4 and contains the closely coupled inductances L1, L2 and a transformer 11. Screened cables 12, 13, 14, 15 couple the apparatus in the screening boxes 7, 10 with that in the screening box S. The generator 5 is connected between the electrode 3 and the metallic backing 16 of the insulator carrying it through the step-up transformer 8. Across the secondary winding of this transformer is connected an inductance 9, a fraction of the voltage of which is tapped off and supplied to one side of the balancing condenser Cv. The other side of this condenser is connected to the outer end of the inductance L2. The outer end of the inductance L1. which has many more turns than the inductance L2, is connected to the electrode 4, and the common point of the inductances L1, L2 is connected to the metallic backing 17 of the insulator carrying the electrode 4. The balance is indicated by telephones 6 which are connected through a transformer 11 across the inductance L1. To reduce errors due to capacity couplings with the body of the aircraft, the balancing condenser Cv is connected to the inductances L2, 9 over balanced cables. A conductor is run in parallel with each conductor connected to the condenser Cv and excited with an equal and opposite voltage derived from a tapping on the inductance L2 and from a tapping beyond the earth tapping on the inductance 9. Electrostatic screens may be provided between the windings of the transformers 8, 11 and connected to the body of the aircraft. A second balancing condenser 18 may be connected as shown for balancing out the capacitance Co between the electrodes 3, 4. Alternatively, two such balancing condensers of large size may be connected in opposition, one to the inductance L2 and the other to the inductance L1. The transformer 8 may be arranged to tune the capacitance C1 and the inductance 9 to resonance so as to decrease the current demand. The balancing condenser Cv may be constructed as shown in Fig. 1 (4478/40). A fixed electrode 1 and a movable electrode 2, each formed of thin wire or strip, are spaced apart from each other and from further electrodes 3. These are in the form of semi-circular metal plates joined along their diametral edges by a metal plate 5 having a longitudinal slot through which the electrode 2 projects. The plate 5 and the electrodes may be pressed from a single metal sheet. One electrode 3 is bent to modify the law of the condenser. The electrode 2 is attached to the moving coil of a milliammeter 8 and may be moved in relation to the electrode 1 by adjusting the current through the moving coil 9. The spacing between the electrodes 1, 2 is greater than that between either of these electrodes and the further electrodes 3 over the majority of the range of adjustment. By earthing the electrodes 3 the capacitance between the electrodes is reduced. The electrodes 3 may be provided with radial or other slots in order to modify the law of the condenser. Such a condenser may be employed in a self-balancing bridge as shown in Fig. 1 (10483/40). This comprises the two tightly coupled radio arms 1, 2 corresponding to L1, L2, Fig. 2 (10485/40), the electrode capacitance 3 to be measured, and the adjustable condenser 4 corresponding to Cv, Fig. 2 (10485/40). Out of balance currents induce alternating voltages in a coil 6 coupled to one or both of the'inductive ratio arms 1, 2. These voltages are amplified by valves 7, 9 and fed through a transformer 12, 12a and phase adjusting device 13 to a load resistance 14. The valve 7 may be provided with a feedback.path comprising a blocking condenser 10 and a high resistance 11 so as to provide a low resistance shunt across the ratio arms to limit the effect of leakage between the electrode connected to the inductance 1 and the metal portions of the aircraft. The voltage across the resistance 14 is applied in parallel to two diode rectifiers 15, 16. Voltages derived from the oscillator 5 are applied in push-pull to the diodes 15, 16 via transformers 17, 17a. The diodes 15, 16 are provided with output resistances 18, 19 the common point of which is connected to the end B of the resistance 14. The resistances 18. 19 are shunted by condensers 20,21 which bye-pass alternating currents having frequencies equal to or higher than that of the generator 5. The rectified voltages across the resistances 18, 19 are applied to the control electrode of a cathode follower valve 22 via a network comprising resistances 23, 24 and condensers 25, 26. The output of the valve 22 is fed through filters to the actuating winding 27 of the adjustable condenser 4. The phase adjuster 13 is so adjusted that when the bridge is balanced the voltage across the resistances 14 is in quadrature with the voltage set up in the winding 17a by the oscillator 5. Unbalance of the bridge causes a phase change which alters the amplitude of the voltage applied to the diodes 15, 16 and changes the current fed to the coil 27 so that the adjus table condenser 4 is varied in the correct sense to restore the balance of the bridge. This current may also be fed to a meter calibrated in terms of capacitance so that the capacitance 3, which is a function of the height of the aircraft, is continuously indicated. The network 23, 24, 25, 26 corrects phase changes due to the inertia and spring control of the condenser 4 and prevents consequent hunting and instability. The amplifier may be provided with a high impedance anode load to limit increase in anode current due to accidental unbalance. The winding 27 and the windings of the indicating meters in series therewith are preferably fed from a pair of pushpull cathode follower valves arranged to limit their output by negative feedback. The condenser 4 may be adjusted to balance the bridge when the aircraft is at the desired height above the sea, and small out of balance voltages due to small changes in height utilised to operate an indicator or control. A small change in the adjustable con