WO1993020406A1

WO1993020406A1 - Tilt determination

Info

Publication number: WO1993020406A1
Application number: PCT/GB1993/000683
Authority: WO
Inventors: John Keith Bartington
Original assignee: Wivenhoe Technology Limited
Priority date: 1992-04-04
Filing date: 1993-04-01
Publication date: 1993-10-14
Also published as: AU3897193A; GB9207432D0

Abstract

A method and device for determining an angle of tilt of the device from some datum, or of an acceleration of the device, wherein a housing (10) contains a fluid and a local source (23) of heat or of cooling which, when energised, establishes a non-uniform temperature field within the fluid, primarily by convection and conduction. The non-uniformity in the temperature field is caused by the acceleration (which may be gravity) to which the fluid is subjected. The temperature within the non-uniform field is sensed at least at one point (11) but advantageously at least at two points (11, 12), and by monitoring the sensed temperature, the tilt of the device or the acceleration to which the device is subjected can be determined.

Description

TILT DETERMINATION

This invention relates to methods of and devices for determining the tilt of the device, or determining an acceleration to which the device is subjected, and hence also the tilt or acceleration of an object with which the device is associated.

The term "tilt" is used herein to refer to the angle between a line or plane and a reference line or plane, which may typically be horizontal.

There have been very many proposals for devices able to determine tilt (often referred to as "clinometers"), and these mostly work by sensing the direction in which gravity acts on the device. Such devices generally fall into one of the following categories: (1) Suspended mass devices, where a pendulum is used to determine tilt which may be read directly against a scale or may be sensed electronically from the position of the pendulum;

(2) Buoyancy devices, having a gas bubble trapped within a curved tube (a spirit level); and

(3) Liquid surface devices, where the surface level of a liquid is compared at two or more places - which devices are usually referred to as water gauges or electrolytic cells. Many of the known devices may also be used to determine an acceleration of the device in, or having a component in, the horizontal direction. For example, a pendulum device may be used to determine tilt of the device or the horizontal acceleration of the device since either will cause the pendulum to move away from its datum position.

The known devices described above suffer from a number of disadvantages, especially when high precision is required. Pendulum devices are bulky and require precision mechanical engineering including low-friction bearings which nevertheless introduce some hysteresis into the system. They also require delicate handling unless they are well-damped, but that then results in a loss of response to higher frequencies. Moreover, if an electrical output indicative of tilt is required, additional means have to be provided for pendulum position sensing.

A spirit level requires a precision curved tube if good accuracy is to be obtained, and if an electrical output is required, sensitive methods have to be used to convert the bubble position to an electrical signal. The range of operation is typically restricted to about o

20 and errors are likely to result from residual wetting of the tube. These devices often have long settling times and will not tolerate vibration, for this can cause the bubble to break up or cause foaming of the liquid. Should this occur, the settling time becomes most protracted. Liquid surface devices suffer from similar problems to those of spirit level devices, but can be o designed to measure tilts over a full 360 range.

It is a principal object of the present invention to provide methods of, and devices for, determining the tilt of the device (and so of an object with which the device is associated) from a reference position, or of the substantially horizontal component of acceleration of the device, which device is relatively simple and hence cheap to manufacture, and which at least mitigates if not overcomes many of the disadvantages discussed above of the known tilt-measuring devices.

According to one aspect of the present invention, there is provided a method of determining an angle of tilt from a datum or an acceleration of a device which has an essentially closed housing containing a fluid and in which is mounted at least one body sensitive to the temperature of the fluid in its immediate surroundings, in which method a non-uniform temperature field is produced in said fluid in the housing by effecting a local change in the temperature of the fluid within the housing, and the temperature of the immediate surroundings of said at least one body is monitored, the determination of tilt of the device or of the acceleration thereof being derived from the temperature sensed by the at least one body. In the method of this invention, a non-uniform temperature field is set up in the housing by the combined effects of convection and conduction from the local area the temperature of which is changed. The field is non-uniform because of the effect of the acceleration to which the device is subjected, which acceleration may be due to movement of the device or to gravity or a combination of the two. The non-uniform temperature field may be established by providing a localised source of heat or cooling within the housing, giving rise to temperature gradients set up within the fluid. These gradients can be detected in relative or absolute terms by measurement with one or more temperature sensitive devices.

Generally, the fluid within the housing will be a gas, or mixture of gases, which should be inert to the materials within the housing. It might be possible to use a liquid, particularly a relatively mobile fluid.

Most preferably, the temperature field is established by supplying electricity to at least one electric resistance element located within the housing, so heating the or each element and transferring thermal energy to the fluid. Equally, the temperature may be sensed by monitoring the electrical resistance of a body in the form of a resistive element. These elements may be similar - for example, thermistors.

In one embodiment of this invention, a single elongate body is mounted within the hous ing and electricity is supplied thereto, so as to generate said non-uniform temperature field, the resistance of said single elongate body being monitored to determine a change in the temperature in the immediate surrounds thereo f . Thus , a very s impl e device cou ld be constructed to operate on this method , and give an indication of the tilt of the device , or of its acceleration . A more complex embodiment may have a plurality of electrical resistance bodies within the housing , electricity being supplied to at least one of the bodies in order to heat that body and the resistance of at least one other of said bodies being monitored to s ens e a change in temperature in the immediate surroundings of that body; such a method may be used to monitor tilt about any horizontal axis , or acceleration in any direction having a horizontal component .

The non-uniform temperature field in the housing may be established by furnishing a localised source of heat or of cooling, within the hous ing . Though conveniently an electric heater is employed, other techniques could be used - for example, an inductive heater, an element to which energy is transferred by electro-magnetic radiation , or a coo ling device utilising for instance the Peltier Effect .

According to a second aspect of this invention, there is provided a device for determining the tilt thereof from a reference position or of the horizontal component of acceleration thereof , which device comprises a housing containing a fluid and within which are mounted in close proximity first and second bodies , the first body being arranged to generate a temperature different from the ambient temperature within the housing so as to establish a non-uniform temperature field in the fluid, and the second body being sensitive to the temperature of its immediate surroundings, means to change, the temperature of said first body, and means to monitor the temperature sensed by said second body. The device of this invention is relatively simple in its construction and so may be made relatively small and compact. It may be rugged and inexpensive, and does not need any complex electronics to achieve a useful output signal indicative of tilt.

Accuracy of sensing may be enhanced by having a plurality of second bodies disposed about said first body, the sensed temperature of all of said second bodies being monitored. The temperature sensing elements may take a variety of forms, such as thermistors, thermocouples, optical-fibre temperature sensors, electrically-resistive or semi-conductor filaments or films, or even techniques to determine a dimensional change caused by thermal stress, utilising for instance a piezo-electric material.

Though it will be appreciated that a non-uniform temperature field may be established by providing either a heat source or a cooling source in the housing, reference will hereinafter be made to the preferred arrangement where a heat source, and conveniently an electric resistance heater, is employed.

A device of this invention may have as few as two bodies in close proximity and in this case the electrical output indicative of changing tilt may be expected to follow a sinusoidal profile. However, by having a plurality of bodies arranged around a central body which is supplied with electric current, and monitoring the resistance of the body which is for the time being located generally at the same height as the central body, a more linear output characteristic may be achieved.

The internal shape of the housing should be such that, upon rotation, it will cause minimal disturbance to the temperature field set up in the fluid. Conveniently, the housing is cylindrical or spherical. When in use, the internal surface should have an even temperature distribution so that there is no significant effect on the fluid temperature field. The temperature of an area of the housing surface above the heater may tend to rise, so it is preferred for the material of the housing to have a sufficiently high thermal conductivity to reduce this effect to an insignificant level. The extent of the allowable temperature rise will depend on the housing size and the geometry of the sensor locations .

Multi-axis sensing may be obtained by having a plurality of sensing devices within the housing. As an alternative, two simpler but similar devices of this invention may be affixed together but with the principal axes of two devices substantially at right angles . By way of example only, certain specific embodiments of devices and methods of the present invention, for determining tilt or acceleration, will now be described in detail, reference being made to the accompanying drawings, in which :- Figure 1 is a diagrammatic view of a two element device, utilising thermistors;

Figures 2A and 2B show alternative bridge circuits for use with the device of Figure 1;

Figure 3A is a diagrammatic view of a multi- element device, utilising thermistors;

Figure 3B shows diagrammatically the non-uniform temperature field which may be established with the device of Figure 3A;

Figures 4 and 5 are diagrammatic views of two further embodiments of multi-element thermistor devices ; Figure 6 is a diagrammatic view of a two- element device, utilising linear heating elements;

Figure 7 is a diagrammatic view of a four element device, utilising linear heating elements; Figure 8 is a modified bridge circuit for use with the device of Figure 7;

Figure 9 is a diagrammatic view of an alternative two-element device, utilising linear heating elements; Figure 10 is a diagrammatic view of a modified form of the device shown in Figure 3A;

Figure 11 is a diagrammatic view of a simplified form of device of this invention, utilising a single linear heating element;

Figure 12 is a further diagrammatic view of a multi-element device able to sense tilt of the device in any direction, from a datum; and

Figure 13 shows a practical embodiment of device of this invention, corresponding to that shown diagrammatically in Figure 3A. Referring initially to Figure 1, there is shown diagrammatically a device of this invention, for measuring angular movement (i.e. tilt) of the device from an initial position, as shown by arrow R. The device comprises a substantially closed housing 10 which contains a fluid which may be a gas or mixture of gases, the pressure of which need not be atmospheric, to change the characteristics of the device. Mounted in close proximity within the housing are two thermistors 11 and 12, each of which is supported by its two conductor wires 13 and 14, respectively. Each thermistor comprises a bead which exhibits an electric resistance sensitive to temperature, whereby the resistance of the bead changes dependent upon the temperature of the immediate surroundings of the bead. Conversely, the temperature of the bead may be changed by supplying electricity thereto. Figures 2A and 2B show alternative bridge circuits which may be employed with the device of Figure 1, to supply electricity to the two thermistors 11 and 12, and simultaneously to compare the resistances thereof, so as to monitor for a change in the temperature in the immediate surroundings of each bead. The bridge has the two thermistors 11 and 12 mounted in two adjacent arms, the other two arms of the bridge comprising a variable resistance element 15 and a fixed resistance element 16; a current source is impressed across junction 17 and 18, and the voltage between junctions 19 and 20 is monitored, for example by means of an analogue meter.

In use, both thermistors 11 and 12 are heated by the supply of electricity therethrough . With the device in the position shown in Figure 1, and with the device static , a non-uniform temperature field is establ is hed within the gas , by conve ction and conduction . A steady state will be reached , s o permitting the bridge to be balanced by means of the variable resistance element 15 , thus giving a null voltage difference between junctions 19 and 20.

If now the device is tilted as shown by arrow R, one thermistor will be higher than the other; if the device is tilted through 90° , one thermistor will be immediately above the other . The positions of the thermistors in the non-uniform temperature f ield generated thereby will change, so that the effective temperature around each thermistor will also change . The temperature and so the resistance of the higher thermistor will ris e and introduce a potential difference across junctions 19 and 20 of the bridge . The meter connected across junctions 19 and 20 could be calibrated in degrees of tilt from the datum position shown in Figure 1, the output voltage could be used to calculate or otherwise derive the angle of tilt.

Instead of being used to determine the angle of tilt of the device, it could be used to determine the horizontal component of acceleration of the device. The non-uaiformity of the temperature field will depend upon the acceleration to which the device is subjected; if the device is subjected to an acceleration having a horizontal component as well as gravity, the field will be distorted in the direction of that acceleration, so changing the temperature distribution within the housing.

Figure 3A shows a second embodiment of device similar to that of Figure 1, and the same reference characters are used for the same components or components having a similar function; consistent reference characters are used throughout the drawings and the respective components will not be described in detail for each embodiment of device of this invention. In the device of Figure 3A, the separation of the thermistors 11 and 12 is increased slightly and a further thermistor 23 is mounted between the thermistors 11 and 12 SJ that all the thermistors have their centres substantially on the same axis, the further thermistor 23 being supported by its conducting wires 24 and 25. In this embodiment, electricity is supplied to the further thermistor 23 in order to raise the temperature thereof and so to generate a non- uniform temperature field within the housing 10, and the resistance of the thermistors 11 and 12 is compared for example by means of a bridge circuit similar to that of Figures 2A or 2B.

Figure 3B shows by way of isothermals t the resultant temperature distribution of the non-uniform temperature field established within the device of Figure 3A. As will be appreciated, tilting the device from the illustrated position will result in the two thermistors 11 and 12 lying in regions of the field at different temperatures, so giving rise to a voltage across the bridge from which the tilt may be determined.

In the embodiment of Figure 3A, the thermistors 11 and 12 may be replaced by other kinds of temperature sensors, such as optical fibre sensors known per se, by thermo-couples, semi-conductor devices or even devices using the deflection of a light beam due to refraction through the temperature gradient of the gas . Dimensional change could also be detected, typically by the use of piezo-electric materials or by optical measurement. The further thermistor 23 may be replaced by another kind of heating element - for example, the element could be inductively heated or heated by chemical reaction, by light, or other electro-magnetic energy focused or impinging on a target within the housing. Yet another possibility would be to deliver energy to an element by an optical fibre or similar wave guide. Alternatively, the thermistor 23 could be replaced by a cooling element.

Figure 4 shows an arrangement similar to Figure 3A, but including three pairs 11 and 12, 26 and 27, 28 and 29, of temperature sensing elements, disposed symmetrically around the further thermistor 23. The temperature of each element is sensed by a suitable circuit, and an analysis of all the sensed temperatures is made in order to determine the tilt of the device in the direction of arrow R, from some datum position. As in the arrangement of Figure 3A, temperature sensing elements other than thermistors may be employed.

Figure 5 shows a modification of the arrangement of Figure 3A, where the thermistors 11 and 12 are both disposed higher than the further element 23, so that both will lie in a different part of the non-uniform temperature field established by element 23, when the device is in the datum position illustrated. This configuration may give greater sensitivity for small angular movements from the datum position.

Figure 6 shows an arrangement similar to that of Figure 1, but here the thermistors 11 and 12 are replaced by linear heaters 30 and 31, arranged to lie in the same plane and to extend parallel to one another in a closely-spaced disposition. The heaters comprise metallic resistance wire, which may be relatively fine to have a low thermal capacity; for example, tungsten resistance wire may be employed. Each wire may be in the form of a coiled helix of relatively small diameter, and arranged with the helix axis straight, or the coiled wire may itself then be coiled helically, with the axis of that helix straight. The heaters 30 and 31 are supported by their respective conductor wires 32 and 33, as in the embodiment of Figure 1. A bridge circuit such as that of Figure 2A or Figure 2B may be employed both to supply current to the two heaters 30 and 31 and to compare the resistances thereof.

This embodiment operates in a closely similar manner to that of Figure 1, described above. However, in view of the elongate heaters 30 and 31, a greater volume of a non-uniform temperature field may be established in the gas within the housing, so enhancing the sensitivity of the device. Figure 7 shows a modification of the embodiment of Figure 6, and employing four resistance heaters 34, 35, 36 and 37 arranged in two pairs in mutually perpendicular planes. These heaters may be similar to heaters 30 and 31 (Figure 5) and be connected in a bridge configuration as shown in Figure 8, and including a variable potentiometer 38; the voltage is sensed between the slider of that potentiometer and junction 39.

In the embodiment shown in Figure 9, there are two resistance heaters 40 and 41 disposed in a V-shaped formation with conductors 42, 43 and 44 being connected to the ends of the two wires and to the common junction therebetween, these conductors also serving to support the heaters 40 and 41. This arrangement may be used with a bridge circuit as shown in Figure 2A or Figure 2B. The arrangement will give enhanced sensitivity at angles of tilt in the range of ± 45°, from the illustrated position

Figure 10 shows a modification of the device of Figure 3A, where the further element 23 is disposed centrally within a tube 45, the conducting wires (not shown) to that element extending through the walls of the tube. The temperature sensing thermistors 11 and 12 are disposed at the ends of the tube. With the device in the position shown in Figure 10, though there will be a non-uniform temperature field, there will be essentially no convection current when the further thermistor 23 is heated. Very small angles of tilt will however produce a significant convection current along the tube from the lower end to the higher end thereof, so establishing a non-uniform field having a significantly higher temperature at the upper end of the tube. This arrangement thus gives very high sensitivity to relatively small angles of tilt.

Figure 11 shows a simple form of device of this invention, utilising only a single heating element 46 in the form of an elongate linear metallic resistance wire. This wire is supported within the housing 10 by conductors 47 and 48 connected respectively to the two ends of that wire. A circuit is arranged to supply current to that wire, and at the same time to determine the resistance of the wire by monitoring both the voltage across the conductors 47. and 48 and the current passing therethrough.

When the device is in the illustrated datum position, and current is passed through the element 46, a non-uniform temperature field will be generated within the housing 10. On tilting the housing in the direction of arrow R, one end of the wire will be higher than the other end of that wire and the higher parts of the wire will lie at least partially in the gas heated by the lower parts of the wire. In the limiting position, the wire will be vertical and lie wholly in gas heated by lower parts of the wire. Thus, the temperature of the wire will rise whenever the device is tilted from the position illustrated in Figure 11 and that results in a change in the resistance of the wire, which can be determined and used to derive an indication of the tilt of the device from the datum position.

Figure 12 shows a complex arrangement having a spherical housing 50 in which is mounted a central heating element 51, together with a plurality of temperature sensing elements 52 disposed symmetrically around the heating element 51. The temperature sensing elements 52 could be thermistors, or other forms of temperature sensing element, as has been discussed hereinbefore. Each of the elements 51 and 52 are supported by wires (not shown) as in the previously described embodiments.

In use, current is supplied to the heating element 51 in order to generate a non-uniform temperature field in the gas contained within the housing 50. The sensing elements 52 lie in that field, and sense the temperature thereof; by monitoring the sensed temperatures, the tilt of the device about any horizontal axis (shown diagrammatically by the arrows R and S) can be derived. Figure 13 shows a practical realisation of the arrangement shown diagrammatically in Figure 3A. This device comprises a cylindrical housing 60 having end walls 61 and in which is contained an inert gas, such as argon. A thermistor 62 is supported by its conductors 63 centrally on the two end walls 61, so that the thermistor 62 is disposed mid-way between those end walls, on the axis of the cylindrical housing. Appropriate seals 64 are formed around the conductors 63, where these pass through the end walls. Two temperature sensing thermistors 65 are supported by arms 66 so as to lie one to each side of the thermistor 62, with all three thermistors on the same diametral line. The arms 66 are mounted on diametrically opposed positions on the cylindrical wall of the housing and are sealed thereto.

The operation of the device shown in Figure 13 is as has been described above with reference to Figures 3A and 3B; it will not therefore be described again here.

I n al l the above embodiments , the us e of thermistors has been described, but the temperature- sensing thermistors could be replaced by thermocouples or other temperature sensing devices . Equally, the heating elements could be replaced by cooling elements, which still generate a non-uniform temperature distribution within the housing.

Claims

1. A method of determining an angle of tilt from a datum or an acceleration of a device which has an essentially closed housing containing a fluid and in which is . mounted at least one body sensitive to the temperature of the fluid in its immediate surroundings, in which method a non-uniform temperature field is produced in said fluid in the housing by effecting a local change in the temperature of the fluid within the housing, and the temperature of the immediate surroundings of said at least one body is monitored, .rhe determination of tilt of the device or of the acceleration thereof being derived from the temperature sensed by the at least one body.

2. A method according to Claim 1, in which the non- uniform temperature is established by supplying electricity to an electric resistance element located within the housing, so heating that element and transferring thermal energy to the fluid.

3. A- method according to Claim 1 or Claim 2, in which the electrical resistance of the body is monitored in order to determine a change in temperature of the immediate surroundings thereof.

4. A method according to Claim 2 or Claim 3, in which a single elongate body is mounted within the housing and electricity is supplied thereto to establish said non-uniform temperature field, the resistance of said single elongate body being monitored to determine a change in the temperature in the immediate surroundings thereof.

5. A method according to any of Claims 1 to 3, in which there is a plurality of electrical resistance bodies within the housing, electricity being supplied to at least one of the bodies in order to heat that body and the resistance of at least one other of said bodies being monitored to sense temperature in the immediate surroundings of that body.

6. A device for determining the tilt thereof from a reference position or of the horizontal component of acceleration thereof, which device comprises a housing containing a fluid and within which are mounted in close proximity first and second bodies, the first body being arranged to generate a temperature different from the ambient temperature within the housing so as to establish a non-uniform temperature field in the fluid, and the second body being sensitive to the temperature of its immediate surroundings, means to change the temperature of said first body, and means to monitor the temperature sensed by said second body.

7. A device according to Claim 6, wherein there is a plurality of second bodies disposed about said first body, the sensed temperature of all of said second bodies being monitored.

8. A device according to Claim 6 or Claim 7, wherein at least the first body comprises an electrical resistance element the temperature of which will be raised by the passage of electric current therethrough.

9. A device according to Claim 8, wherein each of the first and second bodies comprises a metallic resistance wire.

10. A device according to Claim 9, wherein each body is in the form of a linear element, all of the elements being arranged in a substantially parallel, closely spaced disposition.

11. A device according to Claim 9, wherein there are two linear elements lying in the same plane and disposed in a generally V-shaped arrangement.

12. A device according to Claim 8, wherein each of the first, and second bodies comprises a thermistor.

13. A device according to any of Claims 8 to 12 and employing two said bodies, wherein the current supplying means and the resistance determining means are together comprised by a bridge circuit, with the two bodies connected into two adjacent arms of the circuit.

14. A device according to any of Claims 8 to 13, wherein there are three closely-spaced bodies in a substantially common plane, current being supplied to the central body to raise the temperature thereof and the resistance of the outer two bodies being monitored.

15. A device according to Claim 14, wherein a tube surrounds the central of said three bodies and the other two bodies are disposed co-axially with said tube, at or adjacent the ends thereof.

16. A device according to any of Claims 6 to 15, in combination with a second similar device affixed to the first-said device but with the principal axes of the two devices substantially at right angles.

17. A device according to any of Claims 6 to 8, wherein each of said second bodies comprises a thermocouple junction.

18. A device according to Claim 6 or Claim 7, wherein there is a plurality of second bodies arranged around a central first body which central first body is supplied with electric current to raise the temperature thereof, said plurality of second bodies being arranged in pairs around the central first body, each second body being a thermocouple and for each pair of second bodies, one second body serves as a hot junction and the other second body as an associated cold junction.