GB2238128A - A roll independent inclinometer - Google Patents

Abstract

A roll independent inclinometer for use in a towed sonar array comprises on outer conducting cylinder (32) and at least one inner conductor (31) having circular symmetry, the cylinder and conductor defining between them a fluid-tight envelope (35) symmetrical about the cylinder axis, there being a fluid (preferably a liquid) partially filling the envelope and in contact with the inner and outer conductors, and electrical circuitry connected to respective terminals (A, B, C) on the inner (31) and outer (32) conductors and responsive to the inclination of the fluid within the envelope. The device may work by change of capacitance using a dielectric fluid, or conductance. <IMAGE>

Description

A Roll Independent Inclinometer The e invention relates to inclinometers and in particular to devices for determining the angle of inclination of an object when required to be insensitive to roll.

One application of inclinometers is in towed linear sonar arrays. As described in GB Patent Application No. 8708953 a towed linear sonar array comprises a number of spaced sonar transducers supported within a flexible hose. The array is neutrally buoyant so as to maintain a constant depth when towed. By suitable processing of the transducer signals, directional sonar beams can be formed. The beam direction, however, is dependent on the disposition of the array which, in general, will not be colinear with the axis of the towing vessel. Correction of the beamforming can be made with knowledge of the array disposition determined by a combination of rollindependent magnetometer and inclinometer measurements.

The object of the present invention is to provide an inclinometer whose measurements are independent of roll angle.

The invention provides a roll-independent inclinometer comprising: a) an outer conducting cylinder; b) an inner conductor having circular symmetry coaxially located with the outer conductor and connected such that the conductors form a flui & ight envelope symmetrical about the cylinder axis; a fluid partially filling the envelope and in contact with the inner and outer conductors; and electrical circuitry connected to respective terminals on the inner and outer conductors and responsive to the inclination of the fluid within the envelope.

Preferably the fluid is a liquid although for some purposes a heavy gas could be used. The fluid may be surgical spirit and preferably the inclinaneter stage is half filled with fluid. Advantageously there is provided a means to fill the inclinometer.

Preferably the liquid does not wet the surface of the envelope. The conductors may be made entirely of metal or could be formed as conducting coatings on formers made, for example, of plastics material.

Preferably an alternating current system is used. In one arrangement the liquid is a dielectric material and the outer conducting cylinder is split by an insulating spacer into two colinear cylinders, each provided with an electrical terminal, and the inner conductor is connected to a reference terminal to form thereby a 3-plate differential complex impedance. In one arrangement the inner conductor may comprise a tube or rod coaxially aligned within the outer conducting cylinder. In a first alternative the inner reference terminal may be connected to a pair of conducting discs, one at each end of the enclosure. Advantageously in this alternative each disc is attached to the surface of an insulating end cap fitted to respective ends of the outer conducting cylinder.In a second alternative the reference terminal may be connected to a conducting disc supported by insulating means centrally within the outer conductor, means being provided for passage of fluid through or around the disc.

In an alternative arrangement the outer conductor is a single cylinder to which a reference electrode is connected and the inner electrode is a rod or cylinder split into two colinear equal lengths and held in position within the outer conductor by means of an insulating spacer.

Two or more stages of inclinometer may be coupled together. In one form the stages may include a ccmnon centre tube or rod. Alternatively a carton outer tube or cylinder may be used. In an alternative multi-stage inclinometer the outer and inner conductors are concentrically arranged tubes forming a first inclinometer stage and a third conductor is located within the inner conductor to form therewith a second envelope partially filled with fluid.

The complex impedance in the above arrangements may be substantially capacitative or a partially conducting liquid may be used to form thereby a resistive potential divider.

In one arrangement a phase sensitive circuit is employed. Advantageously a reference signal is applied to the terminals of the split conducting cylinder to provide at the third terminal an error signal representative of the tilting of the inclinaneter. The representative signal may then be added to the reference signal to provide a signal proportional to the angle of inclination. In one form the reference signal is applied to the split cylinder terminals by a pair of push-pull amplifies and a second pair of push-pull amplifiers is provided to add the reference signal to the output signal from the third terminal. Alternatively a transformer may be used to connect the reference signal to the split cylinder terminals and to connect the output signal from the third terminal to the reference signal.

The e invention will now be described by way of example with reference to the accompanying Drawings of which: Figure 1 is a diagram illustrating a fluid partly filling an inclined cylinder; Figure 2 shows graphically the change in wetted area of the inclinometer tube with inclination for tubes of two different length to radius ratios; Figure 3 is a longitudinal section through an inclinometer according to the invention; Figure 4 is a cross section through the inclinometer of Figure 3; Figure 5 is a DC measuring circuit connected to the inclinometer; Figure 6 shows a phase sensitive AC circuit for measuring the angle of inclination; Figure 7 is an alternative phase sensitive circuit for measuring the angle of inclination;; Figure 8 shows an inclinometer with a conducting rod as the common conductor; Figures 9 and 10 show an alternative inclinometer in longitudinal and transverse cross section having two stages arranged with one stage concentrically within the other; Figure 11 shows one arrangement of multi-stage inclinometer; Figure 12 shows an arrangement similar to Figure 8 with the centre rod conductor replaced by conducting end discs; and Figure 13 shows an arrangement similar to Figure 11 where the conducting end discs are replaced by a conducting centre disc.

In Figure 1 a liquid 10 is shown half filling a cylinder 11 inclined at an angle G > . The cylinder length is 2L and radius r. It can then be shown that the unwetted area A inside the tube is related to the angle e by the equation:

for tan e < r/L.

This can be used to measure the inclination angle e and it can be seen that the dynamic range and sensitivity of the device is dependent upon L, r and e.

For example, a large L/r ratio will give high sensitivity but will truncate the maximum depression angle. This is illustrated in Figure 2 where the change of wetted area in a tube of length 2L is plotted against tilt angle e for L/r = 20 and L/r +10. For Vr =20 the wetted area changes linearly with tilt angle to a limit angle of about 30 when tan e = 1/20.

Figures 3 and 4 show a practical arrangement of inclinometer. Inner and outer brass cylinders 31, 32 are held by insulative SRBF end plugs 33r 34 so as to form a uniform annular space 35 therebetween. The e outer cylinder 32 comprises two equal length cylinders 36, 37 provided with an insulative SRBF spacer/connector between them. Electrical terminals A and B are provided on the outer cylinders 36r 37 and a further terminal C leads through the end cap 33 to the inner cylinder 31. An epoxy resin adhesive is used to seal theSRBF and caps and spacer to the cylinders. The space 35 between the cylinders is half-filled with a dielectric liquid by means of fill plugs 39, 310.

The e inclinometer terminals Ar B and C are connected to an electrical circuit which is balanced at zero angle of inclination when the wetted portions of the cylinders 36 and 37 are equal. Figure 5 shows an electrical circuit operating in a resistive mode. A voltage +Vin is connected to B, a voltage Vin is connected to Cr and A is earthed. The terminals B and C are also connected to the positive and negative inputs of an operational amplifier 50 such that, as an example for conditions of unity gain, the output signal VO is given by: Vo = Vin - (-Vin) = 2vin.

Any differential change in resistance between A and B canpared with A and C resulting from change in inclination angle e results in equal and opposite changes in voltage at the inputs of the amplifier 50. Consequently the output signal VO changes in proportion to e. In practice an AC system is preferred since this should give greater stability and avoids possible electrolytic effects if the liquid used is partially conducting.

Instead of measuring fluid resistance the inclinometer circuitry can be arranged to measure capacitative changes, including complex impedance changes when the liquid is partially capacitative and partially resistive, caused by tilting of the liquid. A phase sensitive circuit shown in Figure6 employs a pair of push-pull amplifiers 60, 61 to provide an AC drive to inclinometer terminals A and B from a reference AC source "N" (64). The output signal from terminal C gives the AC error signal resulting from an inclination.Push-pull amplifiers 62 and 63 sum the error signal and the reference signal "N" (64) to give output signals which are rectified by diodes 65, 66 to give a direct current proportional to the inclination angle 6. A capacitor 67 connected to earth and a low pass filter 68 connected in line with the signal remove unwanted residual alternating voltage components from the signal which is measured at output 69.

Figure 7 shows an alternative phase sensitive circuit employing a transformer. The circuit is similar in operation to the Figure 6 circuit.

An AC reference source 70 provides a drive source via coil 71 connected between the inclinometer terminals A and B. The error signal resulting from tilting of the inclinometer produces signals which are added to the reference signal 72 in output coil 73 to produce signals representing the inclination angle 6. These signals are rectified by diodes 74, 75 and the rectified signal voltage V is measured across an output resistor 76 connected between the diodes and the terminal C. A high pass filter 77 is connected across the resistor 76 to remove residual AC components.

Assuming that the inclinometer has the following parameters: L = 50mum, r = 5mm and dielectric thickness = liirn liquid dielectric constant = 3 then the capacitance at zero displacement is 20.423 x 10-12f and the maximum change in capacitance is 4.54 x 10-12f. Where the applied voltage is this leads to a voltage range of (0.29 - 0.64)VA at a frequency of 100 kHz and a desired inclination resolution of 0.10 corresponds to a voltage measurement of 6.65 x 10-3 VA.

The effect of acceleration or deceleration will be to cause the dielectric fluid to surge. This can be minimised by the use of bulkheads or filters in the dielectric gap if capacitative. Alternatively an integrator can be included in the output stage to smooth any short term perturbations.

An alternative arrangement shown in Figure 8 has a centre conducting rod 80 coaxially located within an outer cylindrical conductor 81 split into two equal length cylinders 82, 83 by insulating spacer 84. Insulating end caps 85, 86 seal the cylinder 81 to form a liquid-tight enclosure, half filled with liquid. A connector 87 in the end-cap 85 provides an electrical connection to the rod 80. The spacer 84 has perforated 88 to enable the liquid to flow when the inclinaneter is tilted. The Figure 8 arrangement could be modified by providing two longitudinally aligned centre conductors within a single outer conducting cylinder.

A modification of the Figure 3 arrangement is shown in Figures 9 and 10 with like integers represented by like reference numerals. A further cylindrical conductor 90, split into two equal length cylinders 91, 92 is located within the inner conductor 31 by means of insulating connector 93, similar to the outer connector 38, and suitably recessed end caps 94 and 95. The space 10 between the conducting cylinders 31 and 32 and also the space 96 are halffilled with liquid. As in the arrangements already described electrical connections to the inner tubes are made by means of lead-through connectors provided in the end caps 94, 95. This arrangement provides two concentric differential sensors, the outputs of which can be combined to increase the sensitivity of the inclinometer.

Sensors used in a high sensitivity mode having long length to radius ratios, can exhibit instability particularly after liquid displacement. By employing a series or gauged set of devices 110 as shown in Figure 11 the settling time is reduced because the volume of fluid per device is reduced.

The devices 110 are connected in parallel as indicated by connecting wires 111, 112 such that the the sensitivity will be the sum of the sensitivities of the individual devices 110. Terminals A, B and C then connect the wires 111,112 and centre rod conductor 113 to an electrical circuit as shown in one of Figures 5 - 7. This multi-stage inclinometer could be constructed in an alternative arrangement with a single continuous outer electrode forming each stage reference electrode and with split tube or rod inner electrodes supported within each stage by insulating spacers.

In practice the rod or tube surfaces over which the inclinometer liquid flows should be highly polished to reduce resistance to flow and reduce residual wetting. The liquid also should be selected to have suitable electrical properties and low wetting characteristics. Liquid viscosity will also effect operation of the inclinometer since increased viscosity will increase the damping of the instrument and resistance to vibrational effects while reducing the speed of response. Bulkheads, which may be used to increase the current path between adjacent split conductors, may also be used to slow liquid flow as necessary.

Figures 12 and 13 show arrangements of the inclinometer in which the inner electrode does not extend along the length of the liquid enclosure. The Figure 12 inclinometer has an outer conducting cylinder 120 split into two halves 121,122 separated and supported in recesses in a centre insulating spacer 123 provided with holes 124 for passage of a liquid 125. End caps 126,127 recessed to close the inclinometer in liquid-tight manner, have conducting discs 128,129 on their inner surfaces spaced from and concentric with the outer cylinder 120. Comnon reference electrical connection A is made to the end-cap discs 128,129 and the connections B and C are made to 121 and 122. In this instance the characteristics are dependent upon the contact area of the end cap conductors.

Figure 13 shows an arrangement also having a split outer cylindrical conductor 120 but with electrical connections A and B made to the respective halves 121 and 122, The two halves 121,122 are joined by annular insulating spacer 130 and the ends of the cylinder 120 are sealed by insulating caps or plugs 131,132. A conducting disc 133 with central hole 134 for passage of liquid 135 is located centrally within the spacer 130 and has a connection for the common electrical connection C. Alternatively the centre or inner electrode could be formed as a solid disc supported by a web within the insulating spacer so as to form annular fluid passages around the disc.

The invention was tested using DC and AC power supplies in the following configurations: Example 1 : An inclinometer was made up with an inner conducting cylinder divided into two within a single outer conductor and with the following para meters: Outer tube (continuous): 20mm x 180cm aluminium alloy; Split inner tube: 10mm brass; Conducting liquid: surgical spirit (o = 6.8 x 106 ohm cm).

Example 2: A second inclinometer was made up as shown in Figure 8 with the following parameters: Outer tube (split): 2 x 20mn x 9cm aluminium alloy; Inner conductor (continuous): 3tmn phosphor bronze red; Conducting liquid: surgical spirit as in Example 1.

Example 3: A further inclinometer was made with the following paramenters: Outer tube (split): 2 x lOrnrn x 37itwn brass; Inner conductor (continuous): 1.5man phosphor bronze rod; Conducting liquid: surgical spirit as in Example 1.

The results shcwed that the inclinometer works with both AC and DC sources.

The linearity of the devices suffered towards the end of the expected linear range, when the fluid rises near to filling the internal bore at the lower end.

This non-linearityr however, can easily be overcome by using a RDX look-up table, for example. The effect of liquid surge was found to be larger in Example 1, possible due to the larger total wetted area in this inclinometer.

The effect of surge could be reduced by slowing down the measurement rate to allow the inclinometer fluid to settle down.

The e zero (level) offset experienced after severe perturbations of the internal fluid was of the order of 2%-5%. This phenomenon was corrected, within the limits of the measuring equipnent to hand, by switching off the supply and then restoring it. It is believed that this is due to the fact that the liquid acts as a partial dielectric and as such exhibits strain when a voltage is applied. In an unpolarised dielectric it is immaterial that the applied voltage is alternating; the direction of the strain remains the same.

This has the effect of reducing the surface tension of the fluid and it will then cling, to some extent, to either electrode causing small displacements of the fluid.

A practical solution to this problem can consist of one of the following: a. Low frequency amplitude modulated alternating current.

b. Pulsed alternating current.

c. Swept frequency alternating current (zero to max).

d. Very low frequency alternating current.

Claims (26)

Claims

1. A roll-independent inclinometer comprising: a) an outer conducting cylinder; b) an inner conductor having circular asymmetry coaxially located with the outer conductor and connected such that the conductors form a fluid-tight envelope symmetrical about the cylinder axis; a fluid partially filling the envelope and in contact with the inner and outer conductors; and electrical circuitry connected to respective terminals on the inner and outer conductors and responsive to the inclination of the fluid within the envelope.

2. A roll-independent inclinometer as claimed in claim 1 wherein the fluid is a liquid.

3. A roll-independent inclinometer as claimed in claim 2 wherein the liquid is selected such that weting of the enclosure is a minimum.

4. A roll-independent inclinometer as claimed in claim 2 or 3 wherein the fluid is surgical spirit.

5. A roll-independent inclinometer as claimed in any one preceding claim wherein the inclinometer stage is half filled with fluid.

6. A roll-independent inclinometer as claimed in any one preceding claim wherein there is provided a means to fill the inclinometer.

7. A roll-independent inclinometer as claimed in any one preceding claim wherein the conductors are formed as conducting coatings on formers.

8. A roll-independent inclinometer as claimed in claim7 wherein the formers are made of plastics material.

9. A roll-independent inclinometer as claimed in any one preceding claim wherein an alternating current system is used.

10. A roll-independent inclinometer as claimed in claim 9 wherein the liquid is a dielectric material.

11. A roll-independent inclinometer as claimed in any one preceding claim wherein the outer conducting cylinder is split by an insulating spacer into two colinear cylinders, each provided with an electrical terminal, and the inner conductor is connected to a reference terminal to form thereby a 3-plate differential complex impedance.

12. A roll-independent inclinometer as claimed in claim 11 wherein the inner conductor comprises a tube or rod coaxially aligned within the outer conducting cylinder.

13. A roll-independent inclinometer as claimed in any one of claims 1 to 10 wherein an inner reference terminal is connected to a pair of conducting discs, one at each end of the enclosure.

14. A roll-independent inclinometer as claimed in claim 13 wherein each disc is attached to the surface of an insulating end cap fitted to respective ends of the outer conducting cylinder.

15. A roll-independent inclinometer as claimed in any one of claims 1 to 10 wherein a reference terminal is connected to a conducting disc supported by insulating means centrally within the outer conductor, means being provided for passage of fluid through or around the disc.

16. A roll-independent inclinometer as claimed in any one of claims 1 to 10 wherein the outer conductor is a single cylinder to which a reference electrode is connected and the inner electrode is a rod or cylinder split into two colinear equal lengths and held in position within the outer conductor by means of an insulating spacer.

17. A roll-independent inclinoneter as claimed in any one preceding claim wherein two or more stages of inclinometer are coupled together.

18. A roll-independent inclinometer as claimed in claim 17 wherein the stages include a cogmon centre tube or rod.

19. A roll-independent inclinometer as claimed in claim 17 wherein a common outer tube or cylinder is used.

19. A roll-independent inclinometer as claimed in claim 17 wherein the outer and inner conductors are concentrically arranged tubes forming a first inclinometer stage and a third conductor is located within the inner conductor to form therewith a second envelope partially filled with fluid.

20. A roll-independent inclinometer as claimed in any one preceding claim wherein the liquid is substantially or partially capacitative and alternating current electrical circuitry is connected to form thereby a complex impedance potential divider.

21. A roll-independent inclinometer as claimed in any one of claims 1 to 19 wherein the liquid is at least partially conducting and electrical circuitry is connected to form thereby a resistive potential divider.

22. A roll-independent inclinometer as claimed in any one preceding claim wherein a phase sensitive circuit is employed.

23. A roll-independent inclinometer having a split conducting cylinder as claimed in any one preceding claim wherein a reference signal is applied to the terminals of the split conducting cylinder to provide at the third terminal an error signal representative of the tilting of the inclinometer.

24. A roll-independent inclinometer as claimed in claim 23 wherein the representative signal is added to the reference signal to provide a signal proportional to the angle of inclination.

25. A roll-independent inclinometer as claimed in claim 24 wherein the reference signal is applied to the split cylinder terminals by a pair of pushpull amplifiers and a second pair of push-pull amplifiers is provided to add the reference signal to the output signal from the third terminal.

26. A roll-independent inclinometer as claimed in claim 24 wherein a transformer is used to connect the reference signal to the split cylinder terminals and to connect the output signal from the third terminal to the reference signal.