WO1993010428A1

WO1993010428A1 - Force or load sensors

Info

Publication number: WO1993010428A1
Application number: PCT/AU1992/000607
Authority: WO
Inventors: Peter Thomas Goodier
Original assignee: Masstech Scientific Pty. Ltd.
Priority date: 1991-11-12
Filing date: 1992-11-12
Publication date: 1993-05-27

Abstract

A force or load sensor (21) having a vibratory beam (22) whose frquency of vibration varies in accordance with stress induced by an applied load. The sensor (21) includes end portions (24) for mounting the sensor (21) so as to accept the load, the end portions (24) having regions (26) which are fully clamped adjacent the beam (22) to prevent resonance beyond the beam (22). The sensor (21) may incorporate additional beams (27) to carry a proportion of the applied load.

Description

FORCE OR LOAD SENSORS Technical Field

THIS INVENTION relates to force or load sensors and particularly force or load sensors of the vibration type. Description of the Prior Art

Force or load sensors of the vibration type are well known and usually include either a single vibratory beam or a pair of vibratory beams which oscillate or vibrate at a particular measurement frequency related to the stress applied to the sensor by a force or load. Such beams are generally provided with mounting pads to enable the beams to be mounted in measurement apparatus and to enable a load or force to be applied thereto. Prior art vibrating beams or beam transducers use a variety of methods to isolate the mounting pads. These methods of isolation create their own secondary problems by adding other elements to the total structure, generating spurious oscillations that sometimes combine with the main resonance at various stress levels to create activity dips, degrading the quality of the main oscillation. Double ended tuning forks having a pair of vibratory beams are designed to operate with the individual beams 180 degrees out of phase. This creates two essentially opposing wave forms in the end regions, these wave forms being of equal amplitude but opposed in phase. The addition of these two waves in the end regions, theoretically produces a zero sum, and subsequent cancellation of the resonance beyond this point, however, in practice the distribution of these waves in the end regions is subject to variations in both amplitude and phase inhibiting total cancellation and allowing degrees of flow through to occur.

The 180 degrees cancellation effect, however, is not present in a single beam design as single beam systems generate only one reciprocating force which has the effect of trying to reciprocate the end regions. Prior art uses this force by attaching devices to these end regions, to generate an opposing waveform to the major resonance and bring about a cancellation effect similar to the double ended tuning fork design. Some prior art applications suggest that the modified end regions cancel the reciprocating forces, however, investigations indicate that without this force in a single beam system, cancellation of the beam resonance before it reaches the clamping areas compounds the difficulties. Several methods of various complexity have been devised to isolate the main resonance from the end sections. The addition of these isolation methods ncreases the number of elements in the total structure, and therefore increases the potential activity dips. A study of the prior art also shows that success of single beam systems in a commercial sense require materials of very low internal hysterises such as quartz crystal, designed to operate in a region clear of spurious resonances. These conditions restrict the beams frequency range and use the low internal hysterises of the material to compensate for energy loss caused by flow through to mounting areas.

Tests on metal single beams using prior art designs have yielded "Q" values of 200 to 1000. All suffered from varying degrees of activity dips and below a "Q" of 700 displayed a window hysterises effect that showed mark dependence on direction and amount of change of stress in determining resonance. Although tests are continuing to qualify results, initial data suggests that high energy loss combined with lower internal hysterises material, creates a high degree of coupling between elements of the structure, with each individual element, depending on its immediate stress value, influencing the resulting resonance within the bounds of a window rather than a peek resonance. Summary of the Invention The present invention aims to overcome or alleviate the above disadvantages by providing a force or load sensor or transducer of the vibratory beam type in which beam resonance is restricted substantially to the length of the beam or beams of the sensor. It is a further aim to provide a force or load sensor or transducer in which the incidence of activity dips in the beam or beams of the sensor is substantially reduced. The present invention also aims to provide a method for securing or holding a beam type sensor or transducer comprising one or more beams which results in a sensor or transducer which exhibits the desirable characteristics referred to above.

The present invention thus provides in one preferred form a force or load sensor comprising a vibratory beam adapted to vibrate back and forth at a measurement frequency, at least one mounting portion for mounting said sensor to mounting means at which a force or load may be applied to said vibratory beam to stress said vibratory beam and clamping means for securing said mounting portion to said mounting means such as to substantially constrain vibration to said beam, said clamping means establishing in said beam adjacent said mounting portion a nodal point.

The at least one mounting portion is suitably disposed at or adjacent one end of the beam. Preferably, the mounting portion includes an isolating region having a predetermined area, and the clamping means has a clamping surface substantially the same as the predetermined area such that the isolating region is clamped substantially over its area to the mounting means. The isolating region is suitably formed integrally with and disposed adjacent one end of the beam.

Preferably the sensor includes mounting portions at each end thereof by which the sensor may be secured to respective said mounting means at the opposite ends of the sensor.

Suitably stress reduction means are provided for reducing stress applied to said beam. Preferably, the stress reduction means includes at least one additional beam for carrying a proportion of the load or force applied to the sensor. The at least one additonal beam preferably joins or is formed integrally with a mounting portion and joins the mounting portion at a position spaced from the isolating region.

Most preferably, the at least one additional beam joins or is formed integrally with the opposite mounting portions. Suitably, two additional beams are provided being disposed on opposite sides of the vibratory beam and being joined or formed integrally at opposite ends with the mounting portions respectively.

Preferably, the or each mounting portion includes a mounting aperture, and the clamping means comprise means cooperable with the aperture and disposed on the opposite side of the sensor for clamping the mounting portion therebetween. Suitably, the mounting means includes support means relative to which said clamping means is self-centered. For this purpose, the support means includes an aperture having a V-shaped portion in which the clamping means is self centered. The clamping means may comprise cooperable male and female parts having a cylindrical external configuration.

The clamping means is such as to reflect the wave or vibration back down the beam. Preferably the clamping means are such that the clamping surfaces have sufficient smoothness to effect total clamping of the or each end region of the vibratory beam.

A^* particular advantage of the clamping method described above is that any element connected to the clamping area but not connected to the beam outside the clamping area is essentially isolated from the resonant section. This allows a stress reduction system to be incorporated integrally with the beam or beams without adding further elements to the resonant mass. Such a system attenuates hysterises created by movement in the bonding of the piezo-electric elements and the general lengthening of the beam under stress, by allowing the design of the beam or beams for a high stress to frequency change ratio, then using the stress reducing system to restrict the amount of stress presented to the beam or beams, thereby reducing the consequent hysterises. Brief Description of the Drawings

Reference will now be made to the accompanying drawings which illustrate preferred embodiments of the invention and wherein:- Figs. 1 and 2 illustrate schematically alternative forms of clamping arrangements for a single vibratory beam; Figs. 3 and 4 illustrate schematically alternative vibratory beams according to the invention;

Fig. 5 illustrates a practical embodiment of sensor or transducer beam according to the present invention incorporating a stress reducing system; Fig. 6 is a perspective view showing one end region of the sensor of Fig. 5 clamped to a support block;

Fig. 7 is a perspective view illustrating an alternative manner for clamping the sensor of Fig. 5 to a support block; Figs. 8a, 8b and 8c illustrate a clamping arrangement which introduces twisting into a sensor;

Fig. 9a is an isometric view illustrating a preferred clamping arrangement for the sensor of the invention;

Figs. 9b and 9c are end and plan views of the clamping arrangement of Fig. 9a; and

Figs. 10a, 10b and 10c are isometric plan and side views of a supporting arrangement for a sensor according to the invention. Detailed Description of the Invention Referring firstly to Figs. 1 there is illustrated a single vibratory beam 10 clamped at one end 11 between a pair of opposed clamping elements 12. The beam 10 is clamped only at a relative small portion of the end region resulting in a beam which has a high "Q" with resonance being substantially unaffected by the clamping elements. In the arrangement of Fig. 2 the beam 13 is of increased length with the additional length 14 clamped between the clamping elements 15 but with a freeboard section 16 of substantially the same length as the free portion of the beam of Fig. 1. Normally, the "Q" of the beam 13 is relatively low making it an ineffective sensor.

This is due from a theoretical viewpoint, to the beam of Fig. 2 appearing to be clamped, however, in practical terms there are many points along the clamping area that are either free or partly free, for example, due to microscopic surface imperfections in the mating faces of the clamping elements and beams, these areas being excited into oscillation by flow through from the freeboard section 16. Consequently the beam 13 perceives itself as being longer than just the freeboard section 16, repositioning the nodal points inside the clamping area defined by the clamping elements 15. This creates considerable energy loss and hence low performance. If however the beam 13 is ground and lapped to fine tolerance, and the clamping faces of the clamping elements 15 are also ground and lapped, and then assembled under clean conditions with a high clamping force, the results are entirely different. The full area of the beam 16 inside the clamping region defined by the clamping elements 15 is now totally fixed or clamped, preventing flow through of vibration from the freeboard section 16. The beam 13 now perceives itself to be only the length of the section 16 end as a consequence, the nodal point is positioned at or adjacent the junction between the beam 13 and clamping elements 15 so that the beam 13 has as a consequence a higher "Q".

Provided that the number of elements in the whole beam are reduced and the clamping is sufficient to prevent flow through of resonance, then a usable beam or beams with high " "s will result, with little or no activity dips from spurious resonances.

Fig. 3 illustrates a simple vibratory beam 17 which is clamped at 18 at end regions of limited area or size. As before the clamping elements or members and end regions 16 are formed in such a manner that substantial mating occurs therebetween as described above. Additionally the total area of the portion 18 is clamped against movement having no part free to vibrate. This is achieved by having the clamping surfaces of substantially the same configuration as the end portions 18 or larger than those end portions 18.

In the embodiment of Fig. 4, the beam 19 is formed adjacent each end with laterally directed clamping portions 20 which mate in the above described manner with clamping elements or members so as to restrict flow through of vibrations. The beam 19 is long enough so that the nodal points occur at the positions of clamping defined by the clamping portions 20. This beam is thus known as a free-free beam. The clamping portions 20 and the clamp means for holding the clamping portions 20 are formed as above to suppress the passage of oscillations through the clamping portions 20 and clamp means to thereby ensure that the beam has a high "Q" , the clamping portions 20 being fully encompassed by the clamping means leaving no region of the clamping portions 20 free to vibrate.

In the embodiments of Figs. 3 and 4, the beam is reduced to one element to eliminate spurious resonances, and clamping zones reduced to a minimum, effecting total clamping and cut off of oscillation flow through.

In a preferred construction, the beam is formed of flat sheet metal of approximately 1/2mm thickness and 30mm long. The dimensions of the beam however may be considerably varied as may the materials of which it is constructed. All areas outside the major beam are clamped and or welded, soldered, or otherwise fixed in a fashion that prevents flow through of resonance beyond the beam. Another advantage of the sensor or transducer of the invention is that isolation of the required resonant mode of operation is simplified as the only major resonances present are a derivative of the fundamental .

In a modification of the beam of the invention, a weight or weights may be provided intermediate the ends thereof, the weight or weights being coupled to the beam at a nodal point or point of maximum vibration so as to rotate or reciprocate at the measurement frequency. The use of such weights will suppress regeneration of any spurious frequencies or any interference from outside sources. The rotational or reciprocating masses can be of a wide variety of shapes, sizes and angles, however, it is preferred that such masses be located adjacent to the beams to prevent the mass itself vibrating separately. The use of such weights is known from my U.S. Patent No. 4773493 and International Patent Application No. PCT/AU90/00118. It will be appreciated, however, that the beam of the invention need not have such weights.

Whilst the vibratory beam of the invention described above is shown in two different forms it may be of any different forms without departing from the concept of the invention provided that the clamping arrangement of the beam is such as to ensure that oscillation loss does not occur through the clamping arrangements. Fig. 5 illustrates a practical embodiment of the invention comprising a force or load sensor 21 having a main beam 22 which may be provided centrally with weights 23 on either side of the beam to suppress spurious frequencies or interference from outside sources as described above. The beam 22 terminates at either end in mounting end portions 24 which are apertured at 25 for the purposes of receiving clamps as described further below. The end portions 24 incorporate about the apertures integrally formed clamping and isolation areas

26 of generally annular form, including a part annular portion 26* with which the beam 22 joins at each end. The sensor 21 also includes integrally formed load or stress carrying beams

27 which extend substantially parallel to the main beam 22 and join at each end the end portions 24 outwardly from the clamping areas 26. Fig.6 illustrates one arrangement for clamping the end portion 24 of the sensor 21 to a support block 30 in such a manner as to fully secure and hold the clamping and isolation area 26 against vibration. In this arrangement a cap head screw 31 is provided to secure the end 24 to the block 30, the cap head screw 31 having an outer diameter substantially the same as the diameter of the clamping area 26 of the beam 22 so that when tightened the clamping area 26 is fully covered and encompassed by the opposing clamping surface 31' of the screw 31 and secured to the support 30 to be held against vibration. In the alternative arrangement of Fig. 7 a conventional hexagonal screw 32 is employed with in this case a washer 33 of substantially the same diameter as the area 26 being interposed between the head 34 of the bolt 32 and the area 26. Again when the screw 32 is tightened the area 26 will be held against vibration so that any vibration in the beam 22 cannot pass beyond the area 26.

In use, the sensor 21 is clamped in the regions 26 by for example the arrangement of Fig. 6 or Fig. 7 and the force or load to be measured or sensed is applied between the clamped ends of the sensor 21 via the support blocks 30. A vibration is induced in the beam 22 for example by means of a piezo- electric transducer 28 which receives a signal from a signal generator and translates that signal into a vibration. The beam 22 as a result of the applied force or load will vibrate at a measurement frequency proportional to the applied load. The major load, however, will be taken by the beams 27 so that only a proportion of the load is taken by the beam 22. The beams 27, however, being isolated from the beam 22 do not have any effect on the resonance of the beam 22 but only serve to reduce the stress presented to the beam 22. The sensor 21 thus has a considerably increased capacity to take load, this capacity varying with the width or size of the beams 27. The sensor 21, however, need not include the beams 27 so that the full stress can be taken by the beam 22.

In normal clamping arrangements a twisting is introduced into the sensor and thus the beam prior to a load being applied to the beam. This twisting is apparent in Figs. 8a, 8b and 8c where a force or load sensor 35 is clamped at each end by means of a conventional clamping system 36 incorporating clamping screws 31 or 32. Tightening of the clamping screws 31 or 32 will as illustrated introduce a twisting into the sensor 35 in accordance with the direction in which the screws 31 or 32 are tightened.

To overcome this problem the clamping arrangement as illustrated in Figs. 9 and 10 is employed. In this arrangement a force or load sensor 21 of the type shown in Fig. 5 is clamped at each end by means of an isolation clamp assembly 37, each comprising a male member 38 having a threaded portion 39 which is receivable through the aperture 25 in the sensor 21 to engage with an internally threaded female member 40. The members 39 and 40 are tightened so as to firmly clamp therebetween an end of the sensor 21. The members 39 and 40 have a cylindrical outer form so as to faciliate mounting of the members to a support as described further below. It will be appreciated that other clamping arrangements may be employed in lieu of the assemblies 37. For example a pair of internally threaded female members may be provided on a threaded shaft which passes through the apertures 25. Support blocks 41 are provided (see Fig. 10) to support the isolation clamp assemblies 37 and the sensor 21. Each support block 41 is partially split as at 42 into two arms 43 to receive the sensor 21. Alternatively the support blocks 41 for ease of assembly may comprise two separate arms 43. The arms 43 are provided with aligned openings 44 for receipt of the clamp assemblies 37. The openings 44 are such as to enable self centering of the clamp assemblies 37 within the blocks 41. so that upon a load being applied, no distortion as a result of the connection between the sensor 21 and blocks 41 occurs. Self centering may be achieved by forming the openings 44 as large diameter holes or having the openings on one side of a V-shape as at 45 so that the clamp assemblies 37 may co-operate therewith in a self-centering manner by virtue of the cylindrical outer surfaces of the clamp members locating in the Vs.. If desired means may be provided for urging the clamp assemblies 36 against the V-shaped regions 45 of the openings 44. It will thus be apparent that in this arrangement, a load applied between the supports blocks 41 will be applied directly to the beam of the sensor 21 through the clamping assemblies without any twisting being induced into the sensor 21.

Whilst the above has been given by way of illustrative embodiment of the invention, all such modifications and variations thereto as would be apparent to persons skilled in the art are deemed to fall within the broad scope and ambit of the invention as herein defined in the appended claims.

Claims

1. A force or load sensor comprising: a vibratory beam adapted to vibrate back and forth at a measurement frequency, at least one mounting portion for mounting said sensor to mounting means at which a force or load may be applied to said vibratory beam to stress said vibratory beam; and clamping means for securing said mounting portion to said mounting means such as to substantially constrain vibration to said beam, said clamping means establishing in said beam adjacent said mounting portion a nodal point.

2. A force or load sensor according to Claim 1 wherein: said at least one mounting portion is disposed at or adjacent one end of said beam.

3. A force or load sensor according to Claim 1 wherein: said mounting portion includes an isolating region having a predetermined area; and said clamping means has a clamping surface substantially the same as said predetermined area such that said isolating region is clamped substantially over its area to said mounting means.

4. A force or load sensor according to Claim 3 wherein: said isolating region is formed integrally with and disposed adjacent one end of said beam.

5. A force or load sensor according to Claim 3 wherein: said sensor includes mounting portions at each end thereof by which said sensor may be secured to respective said mounting means at the opposite ends of said sensor.

6. A force or load sensor according to Claim 5 and including: stress reduction means for reducing stress applied to said beam.

7. A force or load sensor according to Claim 6 wherein: said stress reduction means includes at least one additional beam for carrying a proportion of said load or force.

8. A force or load sensor according to Claim 7 wherein: said at least one additonal beam joins or is formed integrally with a said mounting portion and joins said mounting portion at a position spaced from said isolating region.

9. A force or load sensor according to Claim 8 wherein : said at least one additional beam joins or is formed integrally with the opposite said mounting portions.

10. A force or load sensor according to Claim 9 and including: two said additional beams disposed on opposite sides of said vibratory beam, said additonal beams being joined or formed integrally at opposite ends with said mounting portions respectively.

11. A force or load sensor according to Claim 1 wherein: said mounting portion includes a mounting aperture; and said clamping means comprise means co-operable with said aperture and disposed on the opposite side of said sensor for clamping said mounting portion therebetween.

12. A force or load sensor according to Claim 11 wherein said mounting means includes: support means relative to which said clamping means is self-centered.

13. A force or load sensor according to Claim 12 wherein: said support means includes an aperture having a V- shaped portion in which said clamping means is self centered.

14. A force or load sensor according to Claim 13 wherein: said clamping means comprises co-operable male and female parts having a cylindrical external configuration.