585,789

PATENT SPECIFICATION

Application Date: Nov. 21, 1941. No. 14981/41.

Complete Specification Left: Nov. 21, 1942.

Complete Specification Accepted: Feb. 25, 1947.

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PROVISIONAL SPECIFICATION

Improvements in or relating to Self-Balancing Bridge Arrangements

We, ALAN DOWER BLUMLEIN, of 37, The Ridings, Ealing, London, W.5, WILLIAM HORACE CONNELL, of 4, Dorset Waye, Hillingdon, Middlesex, and DENNIS GODSON HOLLOWAY, of 38, Australia Avenue, Maidenhead, Berkshire, all British subjects, do hereby declare the nature of this invention to be as follows:-

This invention relates to self-balancing bridge arrangements.

Bridge arrangements have been proposed in which in operation an out-of-balance voltage is developed across the bridge, and this voltage is caused to adjust one of the components of the bridge so as always to tend to maintain the bridge in balance.

If the impedance which is adjusted is of the kind in which adjustment is obtained, for example, by relative movement between two electrodes, and this relative movement is used to indicate the value of the component being measured, the scale law of the indicator may be unsatisfactory if the characteristic of the adjusted component is non-linear since for a given change of impedance the relative movement of the electrode will not be constant.

It is therefore desirable to control the shape of the characteristic of the adjusted component and it is the main object of the present invention to provide means for effectively controlling said characteristic.

According to one feature of the present invention, thee is provided a self adjusting bridge arrangement which is arranged so that out-of-balance voltages or currents developed across said bridge in operation are automatically caused to adjust said bridge towards the balanced condition by simultaneously adjusting at least two components of said bridge in such manner that the impedances of said components are changed in opposite senses.

Preferably, said two components comprise two capacities through which bridge currents are fed in opposite phase and, if desired, according to another feature of the invention said capacities may form a single differential condenser, the moving electrode of which is adjusted by said out-of-balance currents or voltages.

The invention may be conveniently applied to a capacity type height indicator of the kind forming the subject matter of Patent Application No. 547/40 (Serial No. 581,161) and will now be described by way of example as applied to an altimeter of this kind.

The altimeter described in the above mentioned application comprises two electrodes associated with an aircraft of the like which are arranged so that the capacity there between is a function of the height of the aircraft above a conducting surface such as the sea. A self-balancing bridge arrangement is also provided for measuring the capacity between said electrodes, and the height of said aircraft is indicated by the setting of the automatically adjusting element of said bridge.

The element of said bridge which is automatically adjusted may conveniently take the form of the condenser described in the Provisional Specification of Application No. 4478/40, which is cognate with Application No. 547/40 (Serial No. 581,161). Such a condenser comprises a fixed electrode and a movable electrode spaced apart from each other and from further electrodes in the form of metal plates as described in the above-mentioned Provision Specification. The capacity between said electrodes may be automatically adjusted by mounting one of said electrodes on the movement of a moving coil milliammeter, the actuating coil of which is fed with currents proportional to the out-of-balance voltage developed by said bridge.

A variable condenser of this kind is found to have a substantially exponential characteristic, i.e., the capacity between the two electrodes decreases exponentially as the move apart, so that for a given change of capacity, the relative movement between the electrodes decreases as the capacity to which the condenser is adjusted in increased. A condenser having this kind of characteristic is preferred because it gives a substantially linear scale of height in a height indicator of this type.

This kind of characteristic has been found to be inconvenient in some respects. It will be appreciated that part of the capacity between the electrodes attached to the aircraft does not vary with height, and in order to increase sensitivity it has been found desirable to balance out this part of the capacity by means of a subsidiary balancing capacity not controlled by out of balance voltages, but adjusted manually when the aircraft is at a considerable height above the earth. Thus, when the aircraft is at a considerable height above earth, the automatically adjusted condenser will be a t very low capacity setting. In these circumstances, very small changes of the capacity between the electrodes attached to the aircraft will cause relatively large movement so the moving electrode of said condenser, since said condenser is then operating over a portion of its characteristic where a large movement of the moving electrode is required to effect a small change of capacity.

This condition is undesirable because small changes in the capacity between the wing electrodes due, for example, to changes in the stressing of the aircraft structure in flight, may cause the moving electrode to move to such an extent that it will freeze over at one limit of its travel. Further, as the movement of the moving electrode may also be markedly asymmetrical for equal but opposite small changes of capacity, it becomes difficult to effect the desired manual balance of the bridge as the height indication issued to indicate the condition of balance.

It is found that these difficulties can be overcome if the exponential characteristic of the adjusted capacity is modified so that instead of the slope of the characteristic decreasing steadily towards the lower capacity values, it first decreases, then becomes constant in the neighbourhood of the operating point at which manual balance is effected and thereafter increases.

This form of characteristic may, according to one feature of the invention be obtained by employing n effect a differential condenser and by connecting one fixed electrode to a source of bridge voltage opposite in phase to that which is connected to the other fixed electrode. Thus, in operation, bridge currents in opposite phase flow between each of said fixed electrodes and moving electrode, so that these currents are in opposition in the bridge circuit and the effective characteristic of the adjusted component, namely, the characteristic relating the angular position of the moving electrode and the resultant effective current in the bridge circuit can be arranged to be of the desired form.

In accordance with another feature of the invention, a variable condenser for use in the arrangement above referred to may be of the kind described in the provisional specification of Application No. 4478/40 (Serial No. 581,161) provided with a further fixed electrode similar to the electrode 1 lying between the plates 3 and 4 disposed so that the pointer 2 approaches it as it moves away from said electrode 1. If desired, said further fixed electrode may have a different area from that of the electrode 1. The pointer 2 may conveniently be screwed into an insulating extension of the shaft carrying the moving coil 9 of the milliammeter to which are fed currents derived from the out-of-balance voltages from said bridge.

If desired, the amplitude of the mean voltage applied to the two fixed electrodes of the variable condenser may be different. For example, it may be found convenient to apply to these electrodes voltages having an amplitude of 10 to 1.

Alternatively, the bridge voltage may be applied to the moving electrode and the fixed electrodes may be effectively connected to different points in the ratio arms, for example to different tappings on tightly coupled ratio arms, so that the balance voltages derived from equal currents flowing to each electrode are apposite in sense and different in amplitude.

Although the invention has been described with reference to a self-adjusting bridge forming part of a capacity type height indicator, it will be appreciated that it is generally applicable to self adjusting bridges for the indication of any electrical impedance.

Dated this 14th day of November, 1941.

F. W. CACKETT,

Chartered Patent Agent.

COMPLETE SPECIFICATION

Improvements in or relating to Self-Balancing Bridge Arrangements

We, DOREEN BLUMLEIN, of Lanherne, Lescudjack, Penzance, Cornwall, legal representative of ALAN DOWER BLUMLEIN, deceased, late of 37, The Ridings, Ealing, London, W.5, WILLIAM HORACE CONNELL, of 4, Dorset Waye, Hillingdon, Middlesex, and DENNIS GODSON HOLLOWAY, of 38, Australia Avenue, Maidenhead, Berkshire, all British subjects, do hereby declare the nature of this invention and in what manner the same is to be performed, to be particularly described and ascertained in and by the following statement:-

This invention relates to self-balancing bridge arrangements.

Bridge arrangements have been proposed in which in operation an out-of-balance voltage or current is developed across the bridge, and this voltage or current is caused to adjust one of the components of the bridge so as always to tend to maintain the bridge in balance.

If the component which is adjusted is of the kind in which adjustment is obtained, for example by relative movement between two electrodes, and the relative position of these electrodes is sued to indicate the value of the component being measured, the scale law of the indicator may be unsatisfactory if the characteristic, i.e. the relation between the out-of-balance voltage or current and the admittance of the adjusted component, is non-linear, since for a given change of admittance the relative movement of the electrodes will not be the same at all points in the range of movement.

It is therefore desirable to control the shape of the characteristic of the adjusted component and it is the object of the present invention to provide means for effectively controlling said characteristic.

According to the present invention there is provided a self-balancing bridge arrangement in which out-of-balance voltages of currents developed in said bridge in operation are caused to adjust a balancing component so as to tend to maintain said bridge balanced, said balancing component having anon-linear characteristic including a port of low slope and in which a further component connected in said bridge so as effectively to oppose said balancing component is arranged to be adjusted by said out-of-balance voltages or currents so that the admittances of said further and said balancing components are changed in opposite sense whereby the characteristic of said balancing component is effectively modified so as to increase the slope of said portion of low slope.

Preferably, said balancing component and said further component are constituted by the capacities between each of the fixed electrodes and the moving electrode respectively of a differential condenser, said moving electrode being arranged to be adjusted in position by said out-of-balance current or voltages.

In order that the nature of the invention may be more fully understood and readily carried into effect it will now be more fully described by way of example as applied to apparatus for indicating the height of an aircraft, with reference to the accompanying drawings, in which:-

Figure 1 shows a general schematic circuit arrangement of the invention,

Figures 2 and 2 show an adjustable capacity for the circuit arrangement of Figure 1 in plan and in side elevation respectively, and

Figure 4 shows the variation of capacity with the angular position of the moving electrode of the variable capacity shown in Figures 2 and 3.

Referring now to Fig. 1, the self-balancing bridge arrangement will be seen to be of the kind forming the subject matter of Patent Application No. 547/40 (Serial No. 581,161), in which a capacity 1 between two electrodes (not shown), which is a function of the height of an aircraft above a conducting surface, such as the sea, is measured by means of a bridge having tightly coupled inductive ratio arms 2 and 3 and an adjustable balancing capacity. As shown in the figure, the generator 4 supplies the alternating bridge current on the one hand through the capacity 1 to be measured and the inductive ratio arm 2, and on the other hand to the balancing capacity comprising the moving electrode 5 and the two fixed electrodes 6, 7 connected respectively to the inductive ratio arm 3 and to a tapping point intermediate the ends of the inductive ratio arm 2. Out-of-balance voltages are picked up on the coupling coil 8 and after amplification by the amplifier indicated schematically by the rectangle 9 control the angular position of the electrode 5 by means of currents passing through the operating coil of a movement, such as a weak current meter movement, attached to electrode 5. The bridge arrangement thus differs from that forming the subject matter of the aforesaid Application in that the automatically adjusted bridge component is in the form effectively of two condensers instead of a single condenser, and, as will no be explained, the introduction of the second balancing capacity and the adjustment of its admittances in the opposite sense to the first capacity give important advantages.

As the variation of capacity 1 with the height of the aircraft is found to be substantially exponential, it is convenient to employ as a balancing capacity a condenser which gives a substantially exponential change of capacity with deflection. Such a condenser may have a characteristic as shown by the dotted curve A in Figure 4 in which the abscissae are proportional to the distance between the electrodes and the ordinates are proportional to the capacity between the electrodes and therefore proportional to the admittances of said capacity at the bridge frequency. It will be seen that as the distance between the electrodes increases, the capacity decreases substantially exponentially, so that the relation between the height of the aircraft and the distance between the electrodes can be made substantially linear, thus giving a convenient scale law to the indicator.

A single balancing capacity having a characteristic of this kind has, however, been found to be inconvenient in practice. It will be appreciated that part of the capacity 1 between the electrodes attached to the aircraft does not vary with height an in order to increase sensitivity it has been found desirable to balance out this constant part of the capacity by means of a subsidiary capacity not controlled by out-of-balance voltages, but adjusted manually when the aircraft is at a considerably height above the earth. Thus, when this subsidiary capacity has been adjusted in these circumstances, the automatically adjusted condenser will be at a very low capacity setting with its electrodes widely separated. In these circumstances, very small changes of the capacity 1 will cause relatively large movements of the moving electrode 5, since the condenser is the operating over the right-hand portion of the characteristic shown in Figure 4 where a large movement of the moving electrode is required to effect a small change of capacity or admittance.

This condition is undesirable because small changes in capacity 1 are liable to occur due to the stressing of the aircraft structure in-flight, so that even when the aircraft is flying at a constant height the moving electrode may oscillate with a relatively large amplitude and may even freeze over at one limit of its travel. Further, as the movement of the moving electrode may also be markedly asymmetrical for equal but opposite small changes of capacity, it become difficult to effect the desired manual adjustment of the subsidiary balancing capacity.

It is found that these difficulties can be overcome by introducing a further adjustable bridge component in such a manner that the out-of-balance voltages or currents adjust the two bridge components as to cause their admittances to change in opposite senses. Thus referring again to Figure 4, the dotted line B shows the variant of the capacity or admittance between the electrodes 5 and 7 as the distance between these electrodes is decreased by the movement of the electrode 5 away from the electrode 6.

Due to the connection of the electrode 7 to the opposite bridge arm 2 from that to which the electrode 6 is connected the two bridge components comprising the condensers 5, 7 and 5, 6 respectively are effectively in opposition so far as the bridge is concerned. Further, as the current flowing through the admittance between the electrodes 5, 7 only flows through a portion of the arm 2, its effect upon the bridge balance is reduced and it becomes equivalent to a smaller admittance connected to the outer end of the arm 2. The admittance between electrodes 5 and 7 therefore increase negatively according to an exponential law as the admittance between electrodes 5 and 6 decreases although its maximum value is considerably less than the maximum value of the admittance between the electrodes 5 and 6. Thus, the resultant characteristic can be represented by the full line C which is the resultant of curves A and B, and it will be observed that instead of the slope of the characteristic decreasing continuously towards a lows lope at one limit as in the case of curve A, the slope of the resultant curve C continuously decreases to a given value greater than the lowest slope of curve A and thereafter increases again. Thus, the oscillations of the moving electrode above referred to are greatly reduced due to the greater slope of the characteristic C in the neighbourhood of the zero effective admittance or capacity value and also the movements of the pointer are caused to be more symmetrical.

The differential condenser 5, 6, 7 shown in the circuit arrangement of Figure 1 may conveniently take the form shown in Figures 2 and 3, which is a modification of the variable condenser described in the specification of the aforesaid patent application. This variable condenser comprises a weak current meter movement shown diagrammatically at 11 and having an insulating pillar 12 fixed to the moving element of the meter 11 and carrying a light conducting pointer 13. The pointer 13 projects through the radial slot 14 in the earthed conducting structure 15 comprising two parallel conducting metal plates of the form shown in plan in Figure 2 joined together on their corresponding straight and inner semi-circular edges by means of a conducting metal strip containing the radial slot 14. Two electrodes 16, 17, each in the form of a small slotted rectangular block, are supported in an insulated manner within the structure 15, the slot in each electrode being so shaped and positioned that the moving pointer 13 can enter it without electrical contact with the electrode. The provision of these slots avoids the possibility of metallic contact between the moving electrode 13 and either of the fixed electrodes 16 or 17 which might disturb the bridge arrangement and give rise to "hunting". This variable condenser will be seen to differ from that shown in the specification of the aforesaid application chiefly due to the fact that two fixed electrodes are provided instead of one and, as explained in the aforementioned specification, the scope of the characteristic may be modified by varying the spacing between the members of the earthed structure 15, as, for example, by the provision of slots to permit suitable distortion of these plates.

Although the capacities between the moving electrode 13 and each of the fixed electrodes 16, 17 respectively, are similar they can be made effectively dissimilar in order to give characteristics comparable to A and B as shown in Figure 4 by feeding bridge voltages of different amplitudes to the electrodes 6, 7, or alternatively, feeding the bridge currents to the moving electrodes 13 and passing the currents received by the electrodes 6, 7, through different numbers of turns of the ratio arms of the bridge. Thus, if an adjustable condenser of the kind shown in Figures 2 and 3 is used in the circuit arrangement to Figure 1, the fixed electrode 7 may be connected to a tapping in the inductive ratio arm 2, such that the bridge current flowing from the electrode 7 only passes through 1/10th as many turns on the ratio arm 2 as the number of turns in the ratio arm 3 though which flows the bridge current from electrode 6. Alternatively, the effectiveness of the capacity between the electrode 5 and 6 or 7 may be controlled by introducing a capacity potentiometer between the electrode and the bridge so that only a fraction of the current reaching the electrode 6 or 7 flows into the appropriate ratio arm of the bridge. Further the bridge voltage applied to the moving electrode 5 need not be same as that applied to the capacity 1 to be measured and may be reduced to any desired extent, for example, by means of a step-down transformer or a capacity potentiometer.

If desired, the electrodes 16, 17 may be made different in size so as to give the desired different variations of capacity or admittance without requirement the application of different bridge voltages or the use of different tappings on the ratio arms of the bridge.

It will be understood that although in the arrangement above referred to the two adjustable components of the bridge have taken the form of a single differential condenser, separate components may be used provided that they are arranged to be controlled by the out-of-balance voltages or currents to as to adjust the admittances of the separate components in opposite senses. Further, if desired, one or more of the adjustable components may be variable inductances or resistances, the nature of the element being chosen to be appropriate to the nature of the components to be measured.

It will be appreciated that, as is usual with alternating current bridges, auto-transformer couplings may be replaced by transformer couplings and the source of bridge current and the source of out-of-balance voltage may be interchanged. Although the invention has been describe with reference to a self-balancing bridge arrangement embodied in a height indicator, it will be appreciated that it is generally applicable to self-adjusting bridges for the measurement and indication of any electrical impedances.

Having now particularly described and ascertained the nature of our said invention and in what manner the same is to be performed, we declare that what we claim is:-

  1. A self-balancing bridge arrangement in which out-of-balance voltages or currents developed in said bridge in operation are caused to adjust a balancing component so as to tend to maintain said bridge balanced, said balancing component having a non-liner characteristic including a portion of low slope and in which a further component connected in said bridge so as effectively to oppose said balancing component is arranged to be adjusted by said out-of-balance voltages or currents so that the admittances of said further and said balancing components are changed in opposite senses whereby the characteristic of said balancing component is effectively modified so as to increase the slope of said portion of low slope.
  2. A self-balancing bridge arrangement according to Claim 1, in which said characteristic is substantially exponential with said portion of lower slope at one limit thereof, the arrangement being such that as the admittance of said balancing component decrease exponentially the admittance of said further component increases exponentially and vice versa.
  3. A self-balancing bridge arrangement according to Claim 1 or 2 in which said balancing component and said further component are constituted by the capacities between each of the fixed electrodes and the moving electrode respectively of a differential condenser, said moving electrode being arranged to be adjusted in position by said out-of balance current or voltages.
  4. A self-balancing bridge arrangement according Claim 3 and substantially as described with reference to Figure 1 of the accompanying drawings.
  5. A self-balancing bridge arrangement according to Claim 3 or 4 in which said differential condenser is substantially as described with reference to Figures 2 and 3 of the accompanying drawings.
  6. Apparatus for indicating the distance from a conducting surface of an object, such as an aircraft, as claimed in Application No. 547/40 (Serial No. 581,161), comprising a self-balancing bridge arrangement as claimed in any one of Claims 2-5 inclusive.

Dated this 20th day of November, 1942.

F. W. CACKETT,

Chartered Patent Agent.

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Leamington Spa: Printed for His Majesty’s Stationery Office, by the Courier Press. — 1947.

Published at The Patent Office, 25, Southampton Buildings, London, W.C.2, from which copies, price 1s. 0d. each (inland) 1s. 1d. (abroad) may be obtained.