GB2032110A - Measuring inclination or acceleration - Google Patents

Abstract

A device for use as an inclinometer or accelerometer comprises a container 1 for a pool 14 of an electrically conducting fluid such as mercury.The inner surface layer 13 of the container is of dielectric material, and a plurality of separate electrical conductors 5, 6, (7, 8) electrically insulated from one another extend over a significant area of the outer surface of the inner layer 13. A common electrode 15 dips into the pool 14 of conducting fluid to maintain electrical contact therewith as the device tilts or is subject to acceleration. Measurements of the capacitance of the individual capacitors formed between the common electrode 15 and the respective separate electrical conductors 5, 6, (7, 8) can be used to provide an indication of the magnitude and/or direction of inclination or acceleration of the device. The measurement provided by the device can be converted to an electrical signal whose frequency is related to the magnitude of the measurement. <IMAGE>

Description

SPECIFICATION Improvements in or relating to inclinometers and/or accelerometers This invention relates to a device sensitive to the direction of the earth's gravitational attraction, and/or to changes in velocity which the device may sustain. In one aspect, the invention can provide a device in the form of a directional inclinometer which can be used to provide an indication of the attitude of the device, and therefore of the attitude of a body upon which the device is mounted, relative to the direction of the earth's gravitational attraction. In another aspect, the invention can provide a device in the form of an accelerometer, which can provide an indication of the magnitude and/or direction of an acceleration sustained by the device, and therefore of the acceleration of a body upon which the device is mounted.

An inclinometer can find application in several fields where it is required that the degree of tilt and direction of tilt be known to very high accuracies, for example in the remote positioning of submerged machines, sea bottom crawling machines, oil platforms, and land-based apparatus. Also in deep drilling operations there is a requirement for an inclinometer which can be carried on the drilling head, so that the direction of advance of the head can be continuously monitored.

In the aforementioned applications, information concerning tilt angle and direction is at present normally obtained by the use of expensive gyroscopic apparatus. Since the environment in which such apparatus is required to operate is frequently hostile or hazardous, it often happens that the apparatus is damaged or lost.

The present invention seeks to make possible the provision of a device for use as an inclinometer and/or an accelerometer which can be of robust construction, provide accurate measurement, and be inexpensive to produce.

According to the present invention, a device for use as an inclinometer or as an accelerometer comprises a container for containing a pool of an electrically conducting fluid, said container having an inner surface layer, a plurality of separate electrical conductors electrically insulated one from another and each extending over a significant area of the said outer surface, and a common electrical conductor so positioned as in use to make electrical contract with the pool of conducting fluid in the container for a range of movement of the fluid over the inner surface layer.

When the fluid is placed in the container, it can act as a common electrode forming a capacitor with each of the said separate electrical conductors. The area of the pool which is effective to form a capacitor with each respective separate conductor will depend upon the angle and direction of tilt to which the device is being subjected, and/or to the magnitude and direction of acceleration to which the device is being subjected, and accordingly measurement of the capacitance of the capacitors so formed can provide a measurement of the aforesaid quantities.

According to a further aspect of the invention therefore the device is provided with means for measuring the capacitance of the capacitors formed between a pool of conducting fluid in the container and each of the said separate electrical conductors.

The measuring means can either provide separate measurements of the capacitance of each capacitor so formed, or can provide measurements related to the sum or difference of the capacitance of selected pairs of capacitors, or related to other useful functions of the individual values of capacitance. The conducting fluid is preferably mercury. In order to render the device less sensitive to vibration or other transient disturbance, the container can also contain a nonconducting fluid in which the conducting fluid is immersed, but is not miscible therewith, so that movements of the conducting fluid are physically damped. A suitable non-conducting fluid for use with mercury is silicon oil.

In a preferred embodiment, the container is substantially completely filled by the conducting and non-conducting fluids, and is sealed.

The invention will now be described by way of example only with reference to the accompanying drawings, of which Figure 1 is a plan view of a device in accordance with the invention and containing a pool of mercury, but without a cover plate, Figure 2 is a sectional view on the line N-S of Figure 1, and Figure 3 is a graphical representation using polar co-ordinates of the capacitance measurements for each of the capacitors formed in the device of Figures 1 and 2, for a tilt of 19.50 over a complete cycle of tilt direction.

As shown in Figures 1 and 2, a directional inclinometer comprises a cylindrical casing 1 which can be of brass or other suitable material, which is closed by means of a screw threaded brass cover plate 2 (figure 2), the closure being sealed by means of a neoprene O-ring 3. The inner surface of the casing 1 is covered by an insulating layer 4 of Tufnol (Trade Mark) which is also of cylindrical form. Within the insulating layer 4 there are provided four separate electrical conductors in the form of four brass segments 5, 6, 7, 8 labelled for convenient ease of reference respectively as NE, SE, SW and NW. The brass segments are separated from one another by means of layers 9, 10, 11, 12 of insulating material, which lie flush with the inner surface thereof.These layers 9, 10, 11 and 12 can also be of Tufnol and if desired can be formed integrally with the layer 4. The brass segments together form a hemispherical cup. The inner surface of the cup is coated with a layer 1 3 of insulating material, and polytetrafluoroethylene (PTFE) is eminently suitable. The cup constitutes a container for a pool of mercury 14.

A common electrical conductor 1 5 passes through an insulating support 16 in the cover 2, and is positioned symmetrically in the container, extending almost to the bottom thereof so as almost to contact the layer 1 3. The conductor 1 5 is of platinum or other conducting material compatible with mercury. Since platinum has low mechanical strength it may be necessary to provide reinforcement for the conductor 1 5, for example in the form of a glass tube fixed to the cover and extending almost to the free end of the conductor 1 5. Bending by action of the mercury can thus be avoided. External electrical contact with the conductor 1 5 can be made by means of a connector 1 7 mounted on the cover 2 and insulated therefrom.

As shown in Figure 2 only, external electrical contact with each of the brass segments 5, 6, 7, 8 is provided for by means of individual electrical conductors 1 8 (which can be for example of copper and covered to prevent contamination of the mercury) and connectors 1 9 mounted in insulators 20 in the cover.

In use as an inclinometer the embodiment described is mounted on the body whose angle and direction of inclination is required to be measured relative to the earth's gravitational field.

As the device is tilted, the capacitance of each capacitor formed between the pool of mercury 14 and the individual segments 5, 6 7, 8 will vary according to the angle and direction of tilt. The value of the capacitance of each capacitor so formed can be measured at any instant by way of the connectors 1 7 and 1 9. Any convenient means can be used for such measurements as will be apparent to those skilled in the art, eg by use of an operational amplifier. One convenient method where an indication is required at a remote location involves the generation of a cyclically varying electrical signal whose frequency is proportional to the magnitude of the capacitance in each case. This can easily be achieved, eg by making each capacitor form part of a "voltage ramp" circuit, or an oscillator circuit.An indication of the magnitude of each capacitance can then readily be transmitted to a remote location without the use of connecting wires to each capacitor. Such methods are useful, for example, in connection with operation of deep drilling apparatus, or on the sea bed.

The device can be calibrated before use, so that capacitance measurements can be directly related to the parameters which it is desired to measure.

In Figure 3 there is shown plotted on polar coordinates, the value of capacitance for each of the capacitors formed between the mercury pool 14 and the brass conductors, 5, 6, 7, 8 as the inclinometer shown in Figures 1 and 2 is turned through a complete rotational cycle at an angle of tilt of 1 930 to the vertical as defined by the direction of gravity. In order to calibrate the inclinometer completely, similar data would need to be collected for the full desired range of tilt angles.

The information contained in Figure 3 can readily be converted to cartesian form. Thus it is possible to derive measures X and Y representative of tilt components in the respective directions W and N shown in the figures, as follows X=(NE+SE) - (NW+SW) Y=(NW+NE) - (SW+SE) where NE, SE, SW and NW represent respectively the capacitance readings for the segments 5,6,7 and 8.

Thus, the capacitance measurements for the individual segments can be combined electrically to provide directly measures corresponding to X and Y giving tilt information related to cartesian co-ordinates.

As a simple illustration, when the tilt direction is O" (ie direction N), the following values of X and Y as defined above are obtained from Figure 3.

X,=(230+115)-(230+115)=0 Y,=(230+230)-(115+115)=230 Similarly for tilt direction 450 (Figure 3) X45=(265+ 70) - (170+1 00)=1 65 Y45=(265+265) -(100+170)=165 From this simple iliustration, it will be seen that the values derived have the following relationship, X45=Y45=Yo sin 450 which of course is as expected.

Modification to the embodiment particularly described will be apparent to those skilled in the art. For example, the inclinometer described has a hemispherical container, which is able to provide constant sensitivity through a large angle of tilt. If greater sensitivity through a smaller tilt angle is required, then a shallower container would be appropriate. Whilst four is a very convenient number of segments, there may be instances where either less or more segments will be appropriate. The segments could be of conducting material other than brass, especially where mercury is the conducting fluid and if it is found that brass tends to contaminate the mercury. The segments 5, 6, 7, 8 need not necessarily be of massive construction, and indeed might with advantage be provided as thin sheets or sprayed layers of conducting material. The inner layer 1 3 might also advantageously be applied as a thin coating on the inner surface of the segments.

Whilst the combination of PTFE forming the layer 13 with mercury as the conducting fluid provides good sensitivity in view of the low frictional resistance to movement of the mercury pool 14, and the high inertia of the mercury pool, other combinations are possible.

It is also appreciated by the Applicant that in many instances it will be advantageous to make external electrical connections to the common conductor 1 5 and the separate segments 5, 6, 7 and 8 through the casing 1 rather than through the cover 2, as unnecessary contact leading to contamination of the conducting fluid can thus be avoided. This can be important where mercury is the conducting fluid.

Whilst the specific embodiment has been described as an inclinometer, it will be apparent that it can also be used, with or without modification, as a directional accelerometer provided that the appropriate calibration is performed.

These and other modifications which will be apparent to those skilled in the art, are to be considered as forming part of the invention.

Claims (16)

1. A device for use as an inclinometer or as an accelerometer comprising a container for containing a pool of an electrically conducting fluid, said container having an inner surface layer of a dielectric material, and adjacent the outer surface of said layer, a plurality of separate electrical conductors electrically insulated one from another and each extending over a significant area of the said outer surface, and a common electrical conductor so positioned as in use to make electrical contact with the pool of conducting fluid in the container for a range of movement of the fluid over the inner surface layer.

2. A device according to claim 1 including means for measuring the capacitance of the capacitors formed between a pool of conducting fluid in the container and each of the said separate electrical conductors.

3. A device according to claim 2 wherein the measuring means is capable of providing separate measurements of the capacitance of each of the said capacitors.

4. A device according to claim 2 wherein the measuring means is capable of providing measurements related to the sum or difference of the capacitance of selected pairs of capacitors.

5. A device according t6 any one of claims 2 to 4 wherein the measuring means is capable of providing a measurement in the form of an electric signal.

6. A device according to claim 5 wherein the electric signal varies cyclically with a frequency which is proportional to the capacitance being measured.

7. A device according to any one preceding claim wherein the container is of part spherical form.

8. A device according to any one preceding claim wherein the said dielectric material is of polytetrafluoroethylene.

9. A device according to any one preceding claim and containing the said pool of electrically conducting fluid.

10. A device according to claim 9 wherein the said electrically conducting fluid is mercury.

11. A device according to claim 9 or 10 wherein the device also contains a nonconducting fluid, said non-conducting fluid not being miscible with the said conducting fluid.

12. A device according to claim 11 wherein the non-conducting fluid is a silicone oil.

13. A device according to claim 11 or 12 wherein the container is substantially completely filled by the said conducting and non-conducting fluids, and is sealed.

14. A device according to any one preceding claim wherein the common conductor is of platinum.

1 5. A device according to claim 14 wherein the common conductor is provided with physical reinforcement of a non-conducting material substantially impervious to mercury.

16. A device according to claim 1 5 wherein the non-conducting material substantially impervious to mercury is glass.

1 7. A device substantially as hereinbefore described with reference to Figures 1 and 2.