accelerometer

Abstract

An accelerometer for measuring acceleration along a first axis and a second axis orthogonal to the first axis is disclosed, wherein the accelerometer comprises a frame, a movable mass which is movable with respect to the frame, a suspension system arranged to suspend the movable mass in the frame, and an optical fibre fixed to the movable mass and to the frame, wherein the optical fibre comprises at least one first measurement section arranged between the movable mass and the frame for measuring movement of the movable mass along the first axis and at least one second measurement section arranged between the movable mass and the frame for measuring movement of the movable mass along the second axis, wherein the suspension system is arranged to limit movement of the movable mass to movement in only a plane parallel to the first and second axes.

Description

[0001] ACCELEROMETER

[0002] The present invention relates to an accelerometer, in particular a fibre optic accelerometer, for measuring acceleration along a first axis and a second axis orthogonal to the first axis.

[0003] Conventional electrical sensors generate electrical signals that are transported to an interrogator for determining a quantity based on the electrical signals. These electrical sensors are however sensitive to electromagnetic interference, caused by for instance high-voltage electricity, leading to substantial noise in the measurements. In contrast, fibre optic sensors are more suitable for use in such situations, as optical signals are generated and transported through optical fibres that do not conduct electricity, nor do they suffer from electromagnetic interference. Fibre optic sensors are used in a wide variety of sensors measuring strain, pressure, temperature or acceleration. Fibre optic accelerometers are highly suitable for measuring vibrations on, or near, high-voltage equipment, such as vibrations on electric current collectors, so called pantographs, arranged on top of electric trains.

[0004] Fibre optic sensors typically convert the quantity to be measured in such a way that a (tensile) strain variation is imposed on an optical fibre. To determine the imposed strain, a measurement section, such as a Fibre Bragg Grating (FBG), is provided and light with an input spectrum is directed into the fibre. By measuring a change of the spectrum or a reflected light spectrum on the opposite side of the fibre, the imposed strain can be determined.

[0005] In fibre optic accelerometers, typically the same principle is applied. An optical fibre is coupled to a mass that is movably arranged on a frame member such as a housing and, as a result of accelerations, the moving mass introduces a strain in the fibre. The amplitude of the strain is related to the amplitude of the acceleration to be measured.

[0006] A biaxial optical fibre accelerometer for measuring the displacement of an inertial mass relative to a support base in two orthogonal directions for measuring the acceleration of the base in the two directions has been proposed, wherein the mass is supported atop a cylindrical beam attached to the base and functioning as spring to allow the mass to resiliently move in the two directions, wherein the optical fibre is fixed to the top of the movable mass and is arranged for measuring movement of the movable mass in the two directions. A drawback of this biaxial accelerometer is its limited accuracy, in particular at larger accelerations. It is therefore an object of the present invention, amongst other objects, to provide an improved biaxial accelerometer, in particular wherein the above drawback is at least partially alleviated.

[0007] Hereto, an accelerometer for measuring acceleration along a first axis and a second axis having a component orthogonal to the first axis is provided, wherein the accelerometer comprises a frame, a movable mass which is movable with respect to the frame, a suspension system arranged to suspend the movable mass in the frame, and an optical fibre fixed to the movable mass and to the frame, wherein the optical fibre comprises at least one first measurement section arranged between the movable mass and the frame for measuring movement of the movable mass along the first axis and at least one second measurement section arranged between the movable mass and the frame for measuring movement of the movable mass along the second axis, wherein the suspension system is arranged to limit movement of the movable mass to movement in only a plane parallel to the first and second axes.

[0008] It has been found that, in the previously proposed biaxial optical fibre accelerometer described above, due to the way the inertial mass is mounted and due to the arrangement of the optical fibre, the influence of rotational accelerations deteriorates the performance of the accelerometer measuring acceleration in the two orthogonal directions. More specifically, due to crosstalk, a measurement error resulting from the response of the accelerometer to accelerations in a direction other than its measurement direction, part of the output signal generated by the accelerometer originates from accelerations in directions other than the measurement direction, thus decreasing the accuracy of the measurements and thereby the performance of the accelerometer. By, instead, providing and arranging a suspension system in accordance with the present solution, an accelerometer with minimal crosstalk and thus improved accuracy can be obtained. In particular, this is achieved by arranging the suspension system to block any movement of the mass out of said plane, i.e., any movement other than movement in said plane, such that the mass is movable only in said plane.

[0009] At least one, preferably each, of the measurement sections may comprise a Fibre-Bragg Grating (FBG). FBGs are sensitive to strain, which promotes a shift of a reflected Bragg wavelength, which can be directly related to the imposed external acceleration.

[0010] The first measurement section preferably extends along, and parallel to, the first axis and the second measurement section preferably extends along, and parallel to, the second axis. Aligning the measurement sections of the optical fibre with the respective axes the optical fibre can enable an optimal strain induction in the measurement sections such that the sensitivity of the accelerometer can be increased.

[0011] Movement of the mass in said plane may promote the stretching or compression of a measurement section. The measurement sections of the optical fibre are preferably arranged for measuring a strain of the optical fibre. The strain induced in the optical fibre results in a signal or signal change, from which an acceleration can be derived. In the accelerometer, this strain is induced by a relative motion between the points where the fibre is respectively connected to the mass and the frame. Accelerations imposed on the accelerometer in a direction perpendicular to said plane preferably do not, or hardly, lead to accelerations and motions of the moving mass, such that an accelerometer with minimal crosstalk can be obtained for accurate and reliable acceleration measurements.

[0012] According to a preferred embodiment of the accelerometer, the optical fibre comprises: a pair of first measurement sections respectively arranged between the movable mass and the frame for measuring movement of the movable mass along the first axis; and / or a pair of second measurement sections respectively arranged between the movable mass and the frame for measuring movement of the movable mass along the second axis.

[0013] Arranging an additional measurement section to measure movement of the movable mass along the same axis enables at least partial compensation for the effect of strain variations along the fibre. Preferably, measurement sections of the same pair are respectively arranged on opposite sides of the movable mass. This way, movement of the mass in said plane along one of the axes promotes respectively the stretching and the compression of the measurement sections of the respective pair, and as the measurement sections are particularly sensitive to strain, that pair of measurement sections can accurately measure accelerations in directions along the axis. When the mass moves in a direction along the first axis, one of the pair of first measurement sections is stretched and the other of the pair of first measurement sections is compressed. Similarly, when the mass moves in a direction along the second axis, one of the pair of second measurement sections is stretched and the other of the pair of second measurement sections is compressed. The displacement of the mass along an axis can then be accurately determined based on the difference in strain in the respective two measurement sections.

[0014] To accurately measure contractions of a measurement section, it is preferred if the optical fibre is mounted with a certain amount of pre-stress in that measurement section such that the measurement section is already slightly elongated in the equilibrium position of the movable mass, whereby a certain initial tensile strain is introduced in the measurement section such that, as the measurement section shortens due to movement of the mass, the strain in the measurement section decreases. Thus, according to a further preferred embodiment of the accelerometer, at least one, preferably each, of the measurement sections is pretensioned. The measurement section is preferably pretensioned such that, during use of the accelerometer under regular circumstances, the strain in the measurement section never reaches zero such that slack in the measurement section, which could result in unreliable measurements, can be prevented. That is, the measurement sections are pretensioned not only to ensure accurate measurements from the stretching and compression of the measurement sections due to movement of the mass, but in particular also to ensure that the fibre does not bend at the location of the measurement section due to compression of the measurement section during use.

[0015] It is further preferred if the measurement sections of the same pair extend coaxially. This way, interference by rotational accelerations within said plane, for instance about a rotation axis perpendicular to said plane, can be efficiently minimised as the respective effects of a rotational acceleration on the two measurement sections of the same pair cancel each other out. This effect can also be achieved by the coaxial arrangement of the measurement sections of the same pair without the suspension system being arranged to limit movement of the movable mass to movement in only a plane parallel to the first and second axes. For example in the previously proposed biaxial optical fibre accelerometer, measurement sections of a same pair arranged on opposite sides of the movable mass may be arranged to extend coaxially. As such, according to another aspect, an accelerometer for measuring acceleration along a first axis and a second axis having a component orthogonal to the first axis is provided, wherein the accelerometer comprises a frame, a movable mass which is movable with respect to the frame, a support system arranged to support the movable mass in the frame and to guide movement of the movable mass along the first axis and the second axis, and an optical fibre fixed to the movable mass and to the frame, wherein the optical fibre comprises a pair of first measurement sections respectively arranged between the movable mass and the frame on opposite sides of the movable mass for measuring movement of the movable mass along the first axis and at least one second measurement section arranged between the movable mass and the frame for measuring movement of the movable mass along the second axis, wherein the first measurement sections extend coaxially.

[0016] According to a further preferred embodiment of the accelerometer, the movable mass is provided with a first channel extending parallel to the first axis and / or a second channel extending parallel to the second axis, wherein the optical fibre is arranged in the first and / or second channel. One or both channels may be grooves. By arranging the optical fibre in the channels, the measurement sections can be efficiently maintained in their respective positions. According to a further preferred embodiment of the accelerometer, the suspension system comprises a set of three mutually spaced elongate suspension elements extending substantially perpendicular to said plane and connecting the movable mass to the frame for limiting movement of the movable mass to movement in said plane. The elongate suspension elements are rigid elements also referred to as rods or stakes, which limit movement in their longitudinal direction due to their axial stiffness. With such a configuration of the suspension system, wherein the elongate suspension elements are spaced far apart, movement of the movable mass can be efficiently limited to movement in only a plane parallel to the first and second axes. Furthermore, by providing a total of three of such elongate suspension elements, it can be ensured that the suspension system is stable as well as statically determinate. Preferably, the set of three elongate suspension elements is arranged around a central axis extending through the movable mass and perpendicular to said plane. In other words, the set of three elongate suspension elements preferably surrounding the point where the optical fibre is connected to the movable mass, as seen in said plane.

[0017] According to a further preferred embodiment of the accelerometer, the frame comprises an annular frame member coplanar with said plane and surrounding the movable mass. This way, the measurement sections can be conveniently arranged between the movable mass and the frame at respective locations around the movable mass. The movable mass is then preferably provided with at least one protruding member, such as a flange or three arms, protruding radially outwards from the movable mass and underneath the annular frame member, wherein the three elongate suspension elements respectively connect the at least one protruding member to the annular frame member. This way, the movable mass can be connected to the frame in a suspended manner by the suspension system such that movement of the mass is effectively limited to movement in said plane. The three elongate suspension elements may be firmly connected by a full moment connection at one or both ends.

[0018] Moreover, it is preferred if the resonance frequency of the accelerometer is sufficiently high. Thereto, the one or more protruding members of the movable mass are preferably elongate. As the protruding members or arms are elongate, it is possible to connect the elongate suspension elements to the distal ends of the arms at such a distance from the main mass body, or centre of mass of the movable mass, that particularly the resonance frequency of rotation of the movable mass about an axis perpendicular to said plane, e.g. the central axis, is sufficiently high. The length of the protruding members and the location of connecting the elongate elements thereto are thus preferably selected in such a way that the resonance frequency of the movable mass is optimised. That is, by elongating the protruding members of the movable mass to which the elongate elements are connected, the resonance frequency of rotation of the movable mass about the central axis can be increased, in addition to the stability of the movable mass in said plane being enhanced, such that the accelerometer is further improved. It may be preferred if the resonance frequency is at least 0.1 kHz, more specifically between 0.2 and 10 kHz. For instance, for a particular accelerometer it may be preferred if the resonance frequency of the accelerometer is between 200 and 400 Hz, more preferably between 250 and 300 Hz, for example about 270 Hz. For accelerometers for other applications, it may be preferred if the resonance frequency of the accelerometer is between 1 and 2 kHz, for example about 1.3 kHz, or between 2 and 4 kHz, more preferably between 2.5 and 3 kHz, for example 2.7 kHz.

[0019] According to a further preferred embodiment, the accelerometer, in particular the frame or frame member, further comprises a fibre guide arranged to guide and hold the optical fibre between the first measurement section and the second measurement section. The fibre guide thereby holds the optical fibre in a guided manner as it extends from the first measurement section to the second measurement section. For example, the frame may comprise a fibre guiding portion, for instance in the form of a block, around which a section of the optical fibre between the first measurement section and the second measurement section, as seen along the fibre, is arranged. The fibre guiding portion then preferably comprises a curved fibre holding surface along said section between the first measurement section and the second measurement section for contacting and holding the fibre along said section of the fibre when said section is placed in a loop around the fibre guide.

[0020] Preferably, the fibre guide is arranged to keep the at least one pretensioned measurement section under tension.

[0021] According to a further preferred embodiment, the accelerometer further comprises a blocking member surrounding the movable mass to define an area of said plane within which the movable mass is movable, wherein the blocking member is arranged to limit movement of the movable mass to movement within said area. Preferably, the blocking member is coupled to the frame.

[0022] By limiting movement of the mass to movement within a certain area of said plane, bending or damaging of the measurement sections due to excessive displacement of the mass can be prevented. The blocking member thereby reduces the fragility of the accelerometer and, e.g., enables the accelerometer to be conveniently transported. Preferably, the blocking member is releasably fastened to the frame. This way, the blocking member can be efficiently removed after transport. Preferably, said area and a cross-section of the movable mass as seen in said plane are circular, wherein the radius of said area exceeds the radius of the cross-section of the movable mass.

[0023] Furthermore, the blocking member is preferably arranged to limit movement of the movable mass from an equilibrium position thereof to a displacement of less than 1 millimetre, preferably less than 0.5 millimetre, more preferably 0.2 millimetre or less, along the first axis and / or along the second axis. Thus, the radius of said area may exceed the radius of the cross-section of the movable mass by less than 1 millimetre, preferably less than 0.5 millimetre, more preferably 0.2 millimetre or less.

[0024] In the following, the present invention is further elucidated with reference to the attached drawings, wherein:

[0025] Figure 1 represents an isometric view of an accelerometer according to the invention; Figure 2 represents a top view of the accelerometer shown in Figure 1 without its base; Figure 3 represents a side view of the accelerometer shown in Figure 2 with a limiter; Figure 4 represents a top view of the accelerometer with the limiter as shown in Figure 3.

[0026] In Figures 1-4, a biaxial accelerometer 1 for measuring acceleration along a first measurement axis Mi and a second measurement axis Mz orthogonal to the first measurement axis Mi (indicated in Figure 2) is shown. The accelerometer 1 comprises a base 2 and a frame 3, comprising an annular frame member 30 and three frame support legs 31 fixed to the base 2 and arranged to support the annular frame member 30 above the base 2. The accelerometer 1 further comprises a movable mass 4 which is surrounded by the annular frame member 30 and which is movable with respect to the frame 3. The movable mass 4 comprises a circular main mass body 40 and three arms 41 protruding radially outwards from the main mass body 40 and underneath the annular frame member 30. The accelerometer 1 further comprises a set of three mutually parallel spaced apart stakes 5 arranged around a central axis C and respectively connecting the ends of the three arms 41 of the movable mass 4 to the annular frame member 30, such that the movable mass 4 is connected to the frame 3 in a suspended manner in such a way that movement of the mass 4 is limited to movement in only a plane P (represented by a dashed line in Figure 3) parallel to the measurement axes Mi, Mj and perpendicular to the stakes 5. The accelerometer 1 further comprises an optical fibre 6, comprising a pair of coaxially extending first measurement sections 61, 63 respectively arranged between the main mass body 40 and the annular frame member 30 on opposite sides of the main mass body 40 and aligned with the first measurement axis Mi for measuring movement of the movable mass 4 along the first measurement axis Mi, and a pair of coaxially extending second measurement sections 62, 64 respectively arranged between the main mass body 40 and the annular frame member 30 on opposite sides of the main mass body 40 and aligned with the second measurement axis Mj for measuring movement of the mass 4 along the second measurement axis M2. The base 2 is provided with a connector receiving portion 20 for installing an optical connector (not shown) therein for connecting the fibre 6 to further sensor components.

[0027] The upper surface of the main mass body 40 is provided with grooves 42 respectively aligned with the measurement axes Mi, M2. Inner intermediate sections 65, 67 of the optical fibre 6 between the measurement sections 61, 62, 63, 64 are held in the grooves 42. The annular frame member 30 comprises a pair of fibre guiding portions 32, each with a curved outer surface 33, around which outer intermediate sections 66, 68 of the optical fibre 6 are arranged in a loop to be held in a guided manner as the outer intermediate sections 66, 68 extend from a first measurement section 61, 63 along the curved outer surface 33 to a second measurement section 62, 64 and vice versa. This way, the fibre guide portions 32 are arranged to keep the measurement sections 61, 62, 63, 64, which have been pretensioned during installation, under tension.

[0028] Each of the measurement sections 61, 62, 63, 64 comprises a Fibre-Bragg Grating arranged for measuring a strain of the measurement section 61, 62, 63, 64 or a variation thereof as a result of movement of the mass 4 relative to the frame 3 in said plane P. The base 2 is provided with mounting holes 21 for mounting the base 2 to an object (not shown) whose acceleration is to be measured. Upon acceleration of said object in a direction along, e.g., the first measurement axis Mi, the mass 4 moves relative to the frame 3 in the opposite direction along the first measurement axis Mi such that one of the first measurement sections 61 is stretched, whereby the strain therein increases, and the other of the first measurement sections 63 is shortened, whereby the strain therein decreases relative to the initial strain that resulting from the pretension. The displacement of the mass 4 relative to the frame 3 along the first measurement axis Mi can then be accurately determined based on the difference in strain in the respective two measurement sections 61, 63.

[0029] As shown in Figures 3 and 4, an annular plate 7 is fastened to the lower side of the annular frame member 30 by means of three screws 8 in threaded holes 34 in the annular frame member 30, and surrounds the main mass body 40. The inner radius of the annular plate 7 exceeds the outer radius of the main mass body 40 to form a gap 9 therebetween having a width W of 0.1 millimetre (indicated in close-up view A). The annular plate 7 thereby defines an area within which the mass 4 is movable and limits movement of the mass 4 to movement within that area.

[0030] The drawings and the above description serve to illustrate specific embodiments of the invention and do not limit the scope of protection defined by the following claims.

Claims

CLAIMS1. Accelerometer for measuring acceleration along a first axis and a second axis orthogonal to the first axis, wherein the accelerometer comprises:- a frame;- a movable mass which is movable with respect to the frame;- a suspension system arranged to suspend the movable mass in the frame;- an optical fibre fixed to the movable mass and to the frame, wherein the optical fibre comprises at least one first measurement section arranged between the movable mass and the frame for measuring movement of the movable mass along the first axis and at least one second measurement section arranged between the movable mass and the frame for measuring movement of the movable mass along the second axis, wherein the suspension system is arranged to limit movement of the movable mass to movement in only a plane parallel to the first and second axes.

2. Accelerometer according to claim 1, wherein the optical fibre comprises:- a pair of first measurement sections respectively arranged between the movable mass and the frame for measuring movement of the movable mass along the first axis; and / or- a pair of second measurement sections respectively arranged between the movable mass and the frame for measuring movement of the movable mass along the second axis.

3. Accelerometer according to claim 2, wherein measurement sections of the same pair are respectively arranged on opposite sides of the movable mass.

4. Accelerometer according to claim 3, wherein the measurement sections of the same pair extend coaxially.

5. Accelerometer according to any of the preceding claims, wherein the first measurement section extends along, and parallel to, the first axis and / or the second measurement section extends along, and parallel to, the second axis.

6. Accelerometer according to any of the preceding claims, wherein the movable mass is provided with a first channel extending parallel to the first axis and / or a second channel extending parallel to the second axis, wherein the optical fibre is arranged in the first and / or second channel.

7. Accelerometer according to any of the preceding claims, wherein the suspension system comprises a set of three mutually spaced elongate suspension elements extending substantially perpendicular to said plane and connecting the movable mass to the frame for limiting movement of the movable mass to movement in said plane.

8. Accelerometer according to claim 7, wherein the set of three elongate suspension elements is arranged around a central axis extending through the movable mass and perpendicular to said plane.

9. Accelerometer according to claim 8, wherein the frame comprises an annular frame member coplanar with said plane and surrounding the movable mass, wherein the movable mass is provided with at least one protruding member protruding radially outwards from the movable mass and underneath the annular frame member, wherein the three elongate suspension elements respectively connect the at least one protruding member to the annular frame member.

10. Accelerometer according to any of the preceding claims, wherein at least one, preferably each, of the measurement sections is pretensioned.

11. Accelerometer according to any of the preceding claims, further comprising a fibre guide arranged to guide and hold the optical fibre between the first measurement section and the second measurement section.

12. Accelerometer according to claims 10 and 11, wherein the fibre guide is arranged to keep the at least one pretensioned measurement section under tension.

13. Accelerometer according to any of the preceding claims, further comprising a blocking member surrounding the movable mass to define an area of said plane within which the movable mass is movable, wherein the blocking member is arranged to limit movement of the movable mass to movement within said area.

14. Accelerometer according to claim 13, wherein said area and a cross-section of the movable mass as seen in said plane are circular, wherein the radius of said area exceeds the radius of the cross-section of the movable mass.

15. Accelerometer according to claim 13 or 14, wherein the blocking member is arranged to limit movement of the movable mass from an equilibrium position thereof to a displacement of less than 1 millimetre, preferably less than 0.5 millimetre, more preferably 0.2 millimetre or less, along the first axis and / or along the second axis.

16. Accelerometer according to claim 13, 14 or 15, wherein the blocking member is releasably fastened to the frame.

17. Accelerometer according to any of the preceding claims, wherein at least one, preferably each, of the measurement sections comprises a Fibre -Bragg Grating (FBG).

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