GB2556040A

GB2556040A - Vibrating-element force transducer

Info

Publication number: GB2556040A
Application number: GB1619032.4A
Authority: GB
Inventors: Williams Erik
Original assignee: Geometric Design Ltd
Current assignee: Geometric Design Ltd
Priority date: 2016-11-10
Filing date: 2016-11-10
Publication date: 2018-05-23

Abstract

A vibrating element force transducer comprising an elongate flexural element (2), a driver (4) and a sensor (3) that may be contactless to drive and detect vibration, and an elastically deformable support (1) supporting the flexural element (2) at both ends. Force (12) applied to the support displaces one end of the flexural element relative to the other in a direction proportional to and substantially parallel with the applied force and nonparallel to the longitudinal axis of the flexural element (2). The sensor (3) detects vibration of the element (2) in a harmonic mode higher than the first, this may be higher or lower than the harmonic mode of the flexure and at least part of the support, and these may not overlap. The flexible member may be oriented at an angle of 30-90º relative to the force. The transducer may minimise detection of vibrations lower than the predefined mode. The flexural member may be a beam.

Description

(54) Title of the Invention: Vibrating-element force transducer

Abstract Title: A vibrating element force transducer with a vibrating flexural element (57) A vibrating element force transducer comprising an elongate flexural element (2), a driver (4) and a sensor (3) that may be contactless to drive and detect vibration, and an elastically deformable support (1) supporting the flexural element (2) at both ends. Force (12) applied to the support displaces one end of the flexural element relative to the other in a direction proportional to and substantially parallel with the applied force and nonparallel to the longitudinal axis of the flexural element (2). The sensor (3) detects vibration of the element (2) in a harmonic mode higher than the first, this may be higher or lower than the harmonic mode of the flexure and at least part of the support, and these may not overlap. The flexible member may be oriented at an angle of 30-90° relative to the force. The transducer may minimise detection of vibrations lower than the predefined mode. The flexural member may be a

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VIBRATING-ELEMENT FORCE TRANSDUCER

Technical Field of the Invention

The present invention relates to vibrating-element force transducers.

Background to the Invention

Vibrating-element transducers comprising vibrating strings, tubes or tuning forks are often used in load cells and other force transducing sensors, such as torque gauges, pressure sensors, mass/density sensors, and the like. Resonant frequencies of the vibrating element vary with the load under which it is placed, so that an externally applied force or other loading parameter being measured can be deduced from the detected resonant frequency.

It is desirable to provide robust and inexpensive load sensors which are simple to manufacture and calibrate, reliable to use, sensitive to the loading parameter being measured, insensitive to other factors such as other loading and external sources of vibration, and resistant to damage and contamination. It is also desirable to provide load 15 sensors that achieve accurate measurements within a minimal response time and that consume minimal power.

Objects of the invention include the provision of a vibrating element transducer that addresses these needs.

Summary of the Invention

According to the invention, there is provided a vibrating-element force transducer comprising an elongate flexural element, at least one driver for driving vibration of the flexural element, at least one sensor for detecting vibration of the flexural element, and an elastically deformable support arranged to support the flexural element at opposite ends thereof and to displace one said end of the flexural element relative to the other end in a predefined direction, when a force to be measured is applied to the support, so that the displacement is proportional to the applied force and its direction is substantially parallel to the applied force and nonparallel to a longitudinal axis of the flexural element.

The flexural element’s longitudinal axis may be oriented at an angle, relative to the displacement direction, of at least 30 degrees, preferably at least 45 degrees, more preferably at least 60 degrees, more preferably at least 75 degrees, and most preferably at least 80 degrees. The angle between the flexural element’s longitudinal axis and the displacement direction may be 90 degrees or less, preferably 88 degrees or less, and more preferably 85 degrees or less.

The force transducer and/or the at least one sensor may be arranged to detect vibration of the flexural element in a predefined harmonic mode greater than the flexural element’s first harmonic mode, such as its second, third, fourth, fifth, and/or a higher mode. The force transducer and/or the at least one sensor may be arranged to minimise or reject detection of vibrations in a lower harmonic mode than the predefined mode, such as to minimise or reject detection of vibrations in the first, second, third, and/or fourth harmonic modes.

The flexural element may comprise a beam element defining said longitudinal axis, and may consist wholly or substantially of said beam element. The beam element may have a uniform axial cross-section, may have a rectangular axial cross-section, and may be substantially flat. The beam may have a breadth at least five times its depth, preferably at least ten times its depth, more preferably at least twenty times its depth, and most preferably at least forty times its depth. The beam may have a length at least eight times its breadth, preferably at least ten times its breadth, more preferably at least twelve times its breadth, and most preferably at least fourteen times its breadth. The beam may have a length at least 160 times its depth, preferably at least 200 times its depth, more preferably at least 240 times its depth, and most preferably at least 280 times its depth.

The support may comprise a parallel linkage, such as aRoberval-type mechanism, which may comprise flexural hinges. The support and/or the parallel linkage may be integrally formed, such as from a single block of material, and may be formed of a magnetic material. The flexural element may be integrally formed with the parallel linkage and/or the support, or may be rigidly joined thereto, such as by clamping and/or welding its ends. Clamping may be achieved by screws or other suitable fasteners, such as through the ends of the flexural element, or by means of an interference fit.

The force transducer and/or the at least one sensor may be arranged to detect vibration of the flexural element in a predefined harmonic mode that has a higher or a lower frequency than that of a first harmonic mode of a larger structure formed by the flexural element and at least part of the support, such as the structure formed by the flexural element and the parallel linkage. For example, the frequency of the predefined harmonic mode may remain higher, or may remain lower, than that of the larger structure’s first mode within predefined operating limits of the force transducer, such as while the applied force does not exceed a maximum rated force of the force transducer. The predefined harmonic mode may resonate within a frequency range that does not overlap with a frequency range within which the first mode of the larger structure resonates, and may resonate within a frequency range that does not overlap with any frequency range within which a significant harmonic mode of the larger structure resonates. Significant harmonic modes of the larger structure may comprise two or more of its lowest-frequency harmonic modes and/or highest-amplitude harmonic modes.

The at least one sensor may comprise more than one sensor on or adjacent to the beam element, such as at respective antinodes of the predefined harmonic mode.

The/each sensor may be mounted on the beam element, or may be arranged for contactless detection of vibration of the beam element. The transducer may comprise more than one driver on or adjacent to the beam element, such as at respective antinodes of the predefined harmonic mode. The/each driver may be mounted on the beam element, or may be arranged for contactless excitation of the beam element. The vibrating element may be formed of a magnetic material.

The parallel linkage may have a prismatic shape, may have outward-facing surfaces that define a cuboid shape, may have inward-facing surfaces that define a rectangular cut-out through the cuboid shape, and may have inward-facing concave curved surfaces extending the corners of the rectangular cut-out to form an H-shaped or dogbone-shaped cut-out. The parallel linkage may be a rectangular frame, and may define a hollow rectangular prism. The parallel linkage may be elongate with the displacement direction parallel to the minor or intermediate principal axis of the cuboid and/or to a minor axis of the rectangular shape of the prism, frame, and/or cut-out. For example, the minor and/or intermediate principal dimension(s) of the cuboid and/or a minor dimension of the rectangular shape of the prism, frame, and/or cut-out, may be about one half of its length or less, and may be about one third of its length or less.

The support may comprise a pair of columns aligned with the displacement direction, and a pair of cross-members connecting the columns to form the parallel linkage. A cross-sectional area of each column may be greater than a cross-sectional area of each cross-member, such as the cross-sectional areas at respective midsections of the columns and cross-members. For example, a column may be at least twice as thick as a cross-member, preferably at least four times as thick, and more preferably at least five times as thick. Each cross-member may have a predominant thickness, such as the thickness at its midsection and/or along most of its length, that is greater than a minimal thickness near an end of the cross-member. For example, the predominant thickness may be about twice the minimal thickness or more. The support may comprise concave curved surfaces, such as cylindrical surfaces defined by internal radii, at ends of the cross10 members. The curved surfaces may define transverse grooves in each cross-member at which the thickness of the cross-member is minimised to form a flexural hinge therein.

The transducer may comprise a mount on the support for supporting the/each sensor and/or the/each driver at positions spaced from the flexural element. The mount may support a pair of electromagnetic coils, which may form a sensor-driver pair. Each coil may be substantially aligned with a respective antinode of the flexural element’s predefined harmonic mode. The coils may each be aligned with respective non-adjacent antinodes of the predefined harmonic mode, such as the antinodes that are closest to opposite ends of the flexural element. The mount may support the coils on respective opposite sides of the flexural element, such as one coil above and one coil below the flexural element. The mount may comprise a pair of arms defining an opening through which the flexural element passes, each arm supporting a respective coil. Each coil may define a respective central axis, which may be oriented perpendicular to the longitudinal axis of the flexural element, and may be oriented neither parallel nor perpendicular to the displacement direction of the transducer, such as inclined from the displacement direction by a few degrees. The mount may be secured to a cross-member of the parallel linkage, may be arranged to rotate when a force to be measured is applied to the support, and may be arranged to space each coil from the flexural element by a separation that remains constant while the support deforms under application of the force to be measured.

Detailed Description of the Invention

In order that the invention may be more clearly understood, embodiments thereof will now be described, by way of example only, with reference to the accompanying drawings, of which:

Figure 1 is a perspective view of part of a vibrating-element force transducer 10 showing a vibrating beam element and a supporting parallel linkage;

Figure 2 is a perspective view of the arrangement of Figure 1 incorporating the mounting structure for a pair of sensor and drive coils;

Figure 3 is a side projection of the arrangement of Figure 2;

Figure 4 is a cross-section view of an encapsulated load cell comprising the force transducer of Figures 1-3; and

Figure 5 is an exaggerated profile of five harmonic modes of the vibrating beam element of Figures 1-4.

Figures 1-3 show a force transducer which may form part of a load cell, weighing scale, torque gauge, strain gauge, or the like. The force transducer has a support structure 20 (1) with a vibrating beam element (2) coupled to excitation and pickup transducers (3,4).

The vibrating beam element (2) has a slender rectangular cross-section and its ends are rigidly joined to the support structure (1).

The support structure is roughly cuboid shaped with a through-thickness generally rectangular cut-out (5) that has its internally radiused corners extended upward and downward, forming a dogbone-like shape that locally thins the respective upper and lower horizontal cross-members (6, 7) to form flexural hinges (8) at the ends of the cross5 members (6, 7). This provides a Roberval-type parallel linkage that keeps the stout end columns (9, 10) of the support structure parallel to one another and parallel to the direction (11) in which, at small deformations, one column displaces relative to the other under an applied vertical load (12). The fixed column is rigidly secured to a base while the free column comprises, or is secured or coupled to, a load platen or other force10 transferring element through which the force to be measured (12) is applied to the deformable support structure (1). For example, in the embodiment shown, the left-most column (10) receives via its upper end (15) a downward force to be measured (12), and the right-most column (9) is secured at its lower end to a fixed base (14). The Robervaltype linkage minimises the transfer of off-axis external forces to the vibrating element 15 (2).

It will be appreciated that the terms ‘vertical’, 'horizontal’, ‘left’, ‘right’, and so on, are provided merely to help orient the reader in relation to the appended figures; in use, such a transducer may be oriented differently, such as when measuring horizontal forces.

The vibrating beam element (2) spans the length of the dogbone-shaped cut-out (5), and is mounted at a slight angle to the horizontal cross-members (6, 7), such as around 5 degrees. This provides a combination of axial (tensile and/or compressive) and transverse (shear) loading to the beam (2) when a compressive vertical load is applied across the end platens (14, 15) of the columns. If the force to be measured (12) is expected to reverse direction, such as where both compressive and tensile loads are to be measured, the beam may therefore be subject to both compressive and tensile axial loading, depending on the direction of the applied force. However, in preferred embodiments where measuring force in opposite directions is not intended, the beam is preferably oriented to receive increasing tensile loading as the force to be measured increases.

The vibrating element may be an integral part of the support structure. For example, in the arrangement of Figures 1-3 the beam (2) and its supporting structure (1) are machined from a single block of material, or could be otherwise integrally formed via a single process, such as laser sintering. Alternatively, the two parts may be separately formed and joined together, such as by welding, clamping, interference fit, or a combination thereof. In such arrangements, the vibrating element may be pre-tensioned so that it remains in tension over the full operating range of the force transducer. Where welding is used to join the beam to the supporting structure, this may advantageously pre-tension the beam by a desired amount.

The elongate cuboid shape of the parallel linkage, in which the vibrating element is oriented at a slight angle from the cuboid’s major principal axis, allows the use of a relatively long and highly sensitive vibrating element within a low-profile support structure that has the form of a cantilever. This form factor may be advantageous, such as where it is desirable to minimise the thickness of a compressive load cell or the height of a weighing scale. In addition, the sloping orientation of the vibrating beam element has been found to amplify the strain experienced by the beam and to boost the sensitivity of the transducer to the applied load.

The excitation and pickup transducers (3, 4) form part of an electrical transducer arrangement coupled to the beam (2) for exciting natural transverse vibration and providing an electrical signal representative of the frequency at which the beam vibrates. Vibration is maintained and detected using a phase-locked loop amplifier of conventional design or by any other suitable means. The applied load can then be correlated with the detected frequency or period of natural vibration, such as via external data capture software, or using a microcontroller or other hardware, whether external or incorporated into the force transducer itself.

The vibration may be driven and/or detected at the fundamental frequency of the 10 vibrating element, i.e. its first harmonic mode (101) as shown in Figure 5. However, in preferred embodiments, the force transduction is based on vibration driven and/or detected in a higher harmonic mode, such as the second (102), third (103), fourth (104), or fifth (105) mode of a vibrating beam element (2). This has been found to provide increased accuracy, repeatability, and sensitivity when compared with measuring the fundamental frequency (101).

The driver and pickup (3,4) are preferably located at or near respective antinodes (110) of the vibrating element’s harmonic mode of interest. For example, if the first harmonic mode (101) is being monitored, the driver and pickup may both be mounted at the midpoint of the vibrating element, such as on opposite faces thereof; if the second mode (102) is to be detected, the vibration transducers may be located at approximately one-quarter and three-quarters of the way along the length of the beam. In the arrangement of Figures 2-4, the driver and pickup (3, 4) are located at respective antinodes of the vibrating element’s third harmonic mode (103), at approximately onesixth and five-sixths of the way along its length. In each embodiment, the stout end columns (9, 10) provide node-determining masses relative to which the element (2) vibrates. For the third and higher-order harmonic modes, more than one sensor and/or more than one driver may be used in certain embodiments, each sensor and/or driver being located at a respective antinode.

A conventional pickup and drive coil arrangement is used to excite natural transverse vibrations of the beam near a particular desired frequency that corresponds to the desired harmonic mode being monitored. The arrangement includes pickup coils and drive coils (3, 4) spaced along the axis of the beam (2), providing an output signal representative of a resonant frequency of the vibrating element. The pickup and drive coils are connected to an output unit, which may be mounted in or on the support structure, or secured to the inside or outside of an external housing in which the support structure is housed.

In certain arrangements, other driver and sensor elements, such as piezoelectric elements, may be mounted directly on the beam. In other arrangements, the driver and sensor coils are supported in spaced relation to the vibrating element for non-contact detection of its vibrations. In the preferred embodiment of Figures 1-5, the coils (3, 4) are supported by a mounting bracket (20) having a pair of arms (23, 24) that wrap around the vibrating beam (2). Each coil (3, 4) is mounted on a respective arm (23, 24) of the bracket (20). The bracket is secured to a cross-member (6) of the parallel linkage and is arranged to tilt with the beam under an applied load (12), so that the mean (i.e. frequency-independent) spacing between the beam (2) and each coil (3, 4) remains essentially unchanged as the beam and parallel linkage deform under the applied load.

The sensor and driver coils are solenoids comprising ferrite or magnetised cores. A least one of the solenoids may comprise dual ferrite/magnetic cores, such as spaced parallel ferrite/magnetic rods within respective series-connected coils or within a single coil with a non-cylindrical winding. In preferred embodiments, at least one of the sensor and drive coils has an oval (stadium-shaped) winding and a pair of cylindrical ferromagnetic cores that are spaced apart. For example, in the arrangement of Figure 2, the sensor coil (3) has an oval (stadium-shaped) winding and a pair of cylindrical ferromagnetic cores (30)—only one of which is visible in Figure 2—whilst the drive coil (4) has a single core with a cylindrical annular winding. Although the windings are not 10 illustrated, their shape and position can be readily inferred from the shape of the bobbins (33, 34) that support the cores and windings. Such an arrangement has been found to improve sensitivity and reduce noise in the detected vibration signal.

As best illustrated in Figure 3, the respective driver and sensor coils (3, 4) are positioned near opposite faces of the beam (2) and near opposite ends thereof. Thus, for coils and a vibrating beam element of any given size and sensitivity, the sloping angle of the beam and the diagonally opposed placement of the coils enable provision of a lowprofile structure suitable for use in an extremely compact load cell. Positioning the sensor and driver coils (3, 4) near opposite ends of the beam minimises cross-coupling effects.

In operation, the force to be measured (12) is applied vertically to the load plate (15, 16) at one end of the vibrating element (2), parallel to the end column (10). The vibrating element is excited into natural transverse vibration in the desired harmonic mode by the drive coil(s). The mode is maintained by feedback from the pickup coil(s) to the drive coil(s) via the circuitry of the output unit, and an output signal is provided representing the resonant frequency of the desired harmonic mode.

The drive and pickup arrangement may comprise solenoid coils (as described above), other electromagnetic or capacitive elements, piezoelectric devices, optical sensors, other suitable vibration driving and/or sensing means, or a combination thereof. The desired frequency range of the selected harmonic mode to be monitored is isolated by means of filtering circuits and digital processing. A phase-locked loop is used to track the relevant modal frequency, eliminating any out-of-phase components in the captured vibration signal.

Compensation for the effects of temperature on the natural frequency of the vibrating element is achieved by measuring its temperature and adjusting the detected natural frequency according to the vibrating element’s temperature coefficient of frequency. The calibration coefficients used may be stored within circuitry of the output module, such as within a memory register of a microcontroller. To minimise thermal effects, the vibrating element is preferably manufactured from a metal alloy of low thermal expansion coefficient, such as the nickel-iron alloy known as Invar or the nickel-iron-chromium alloy known as Ni-SPAN C 902 (a trade mark of Special Metals

Corporation, USA). Alternatively, other alloys may be used, such as stainless steel. For electromagnetic drive and pickup coils, the vibrating element must comprise a magnetic material (i.e. of sufficient magnetic permeability).

To protect the vibrating element and vibration transducers from damage and 20 contamination, the supporting structure (1) is enclosed in a protective housing. Preferably, the housing and supporting structure form a sealed enclosure within which the vibrating element and vibration transducers are fully encapsulated. Such an arrangement is illustrated in Figure 4, in which opposite ends of respective columns (9, 10) are secured to a load plate (16) and a base plate (14), between which a protective shell (40) encloses the parallel linkage (6, 7, 8, 9, 10) and the vibrating element (2). A flexible membrane seal (42) is provided between the outer shell (40) and the part of the supporting structure or load plate (15, 16) that emerges from the shell (40). An output module (44) comprising amplifier circuitry is also enclosed within the shell, for which a suitable mounting bracket or other fixtures may be provided. A cable gland or connector (46) and a seal (48) between the base and outer shell maintain a sealed unit.

Each of the arrangements and embodiments described above may incorporate any combination of one another’s features. The above embodiments are described by way of example only. Many variations are possible without departing from the scope of the invention as defined in the appended claims.

Claims

1. A vibrating-element force transducer comprising an elongate flexural element, at least one driver for driving vibration of the flexural element, at least one sensor for detecting vibration of the flexural element, and an elastically deformable

5 support arranged to support the flexural element at opposite ends thereof and to displace one said end of the flexural element relative to the other end in a predefined direction, when a force to be measured is applied to the support, so that the displacement is proportional and substantially parallel to the applied force and nonparallel to a longitudinal axis of the flexural element, wherein the

10 force transducer is arranged to detect vibration of the flexural element in a predefined harmonic mode higher than the flexural element’s first harmonic mode.

2. The force transducer according to claim 1 wherein the flexural element’s longitudinal axis is oriented at an angle, relative to the displacement direction, of

15 between 30 degrees and 90 degrees.

3. The force transducer according to claim 1 or claim 2 arranged to minimise or reject detection of vibrations in a lower harmonic mode than the predefined mode.

4. The force transducer according to any preceding claim wherein the flexural

20 element comprises a beam element defining said longitudinal axis.

5. The force transducer according to any preceding claim wherein the predefined harmonic mode has a lower frequency than that of a first harmonic mode of a larger resonant structure comprising the flexural element and at least part of the support.

6. The force transducer according to any one of claims 1 to 4 wherein the predefined harmonic mode has a higher frequency than that of a harmonic mode of a larger

5 resonant structure comprising the flexural element and at least part of the support.

7. The force transducer according to any preceding claim wherein the predefined harmonic mode resonates within a frequency range that does not overlap with a frequency range within which the first harmonic mode of a larger resonant structure resonates, the larger structure comprising the flexural element and at

10 least part of the support.

8. The force transducer according to claim 6 wherein the predefined harmonic mode resonates within a frequency range that does not overlap with any frequency range within which a significant harmonic mode of the larger structure resonates.

9. The force transducer according to any preceding claim wherein the at least one

15 sensor is arranged for contactless detection of the flexural element’s vibration.

10. The force transducer according to any preceding claim wherein the at least one driver is arranged for contactless excitation of the flexural element.

11. The force transducer according to any preceding claim wherein the sensor and the driver are positioned on or adjacent to opposite faces of the flexural element.

20

12. The force transducer according to any preceding claim comprising a mount on the support for supporting each of the at least one sensor and/or driver at positions spaced from the flexural element.

13. The force transducer according to claim 12 wherein the mount is arranged to rotate under the application of the force to be measured.

14. The force transducer according to claim 13 wherein the support comprises a parallel linkage and the mount is secured to a transverse arm of the parallel

5 linkage that rotates under the application of the force to be measured.

15. The force transducer according to any preceding claim wherein the support comprises a parallel linkage and the flexural element is integrally formed with the parallel linkage.

16. The force transducer according to claim 14 or claim 15 wherein outward-facing

10 surfaces of the parallel linkage substantially define a cuboid shape.

17. The force transducer according to claim 16 wherein the parallel linkage is elongate, with the displacement direction parallel to the minor or intermediate principal axis of the cuboid shape.

18. The force transducer according to claim 16 or claim 17 wherein the intermediate

15 principal dimension of the cuboid shape is less than half its length.

19. The force transducer according to any preceding claim comprising a housing that encloses the vibrating element and at least part of the support.

20. A vibrating-element force transducer substantially as herein described with reference to the appended figures.

Intellectual

Property

Office

Application No: GB1619032.4 Examiner: Mr Michael Knight