Vibrating crystal type force sensing device

Abstract: A force sensing device (40) especially suitable for use in an accelerometer or a pressure transducer, depending upon its particular design, is disclosed herein. This device includes at least one and preferably two adjacent and parallel, elongate, vibratable tines (42) and means (44, 46) for vibrating the tines (42) back and forth in a direction normal to their axes of elongation (48). These tines (42) are supported for vibration by an arrangement (52) including an array of interconnected links (70A-70D) and mounting means (72AB, 72BC, 72CD, 72AD) adapted to be connected to an external member capable of applying a variable external force (Fs) to the support arrangement along an axis (62) normal to the axes (48) of elongation of the tines. The support arrangement (52) is configured so as to apply to the tines (42) its own internal force in response to and varying with the external force (Fs) in a way which causes the tines (42) to vibrate at a frequency that varies with the magnitude of the internal force and therefore the external force (Fs). In this way, the vibrating frequency of either or both tines (42) can be sensed for determining therefrom the magnitude of the external force (Fs) aoplied to the support arrangement (52).

Description: The present invention relates generally to accelerometers, pressure transducers and other such force sensing devices, and more particularly to a specifically designed vibrating crystal type of force sensing device. The utilization of vibrating crystals in force sensing devices such as accelerometers, pressure transducers, and the like is well known in the art. Such prior art devices may be found in, for example, US patent 4,215,570 and 4,372,173 in the name of EerNisse. Force converting mechansisms of other materials are also known. One example is found in GB-A- 2 142 430. Another example is found in Patent Abstracts of Japan, vol. 11, no. 19 (P-537) [2466] of 20th January 1987 in which there is described a force converting mechansim having an integral structure including a vibrator and which displays a substantially linear characteristic between an applied external force and the inherent frequency of the vibrator. It is however, as stated above, to the vibrating crystal type of force sensing device that the present invention relates. Referring specifically to Figure 1, there is illustrated a crystal force transducer 10 which includes a pair of adjacent, parallel, elongate, vibratable tines 14 extending between opposing mounting ends 16. The tines and mounting ends may be integrally formed as a single unit from quartz crystal material, as is well-known in the art. A suitable electrical circuit, for example oscillator circuit 18, in combination with electrodes 20 are used to vibrate the tines back and forth in a direction normal to their centre axis of elongation 22, as indicated by two-headed arrows 24. For reasons to be discussed below, two tines are vibrated at the same frequency and 180° out of phase with one another. Still referring to Figure 1, ends 16 serve not only as a means for supporting the tines 14 in the parallel relationship illustrated but also as a mounting means for connecting the tines to the outside world, specifically an external member diagrammatically represented at 26 and 28. External member 26, 28 may be an accelerometer proof mass or pressure sensing bellows or diaphragm, or any other means that applies a variable external force Fs, e.g. the force to be sensed, through the tines along an axis coinciding with tine axis 22. While the force Fs being sensed is shown placing the tine in compression, it could act on the tines in the opposite direction to place them in tension. In either case, the application of external force Fs to the tines will cause the latter to vibrate at a frequency that varies with the magnitude of Fs. Therefore, it is merely a matter of sensing the vibrating frequency of the tines and calibrating the frequency to read out the magnitude of the force Fs. Suitable read out means 30 may be readily provided to that end. The various components making up the diagrammatically illustrated force transducer 10 including oscillator circuit 18 and read out 30 are well-known in the art and will not be discussed in more detail. To this end, reference is made to the prior art generally and to the above-recited EerNisse patents in particular. It suffices to say here that two parallel tines are used and vibrated in the manner described or at least an attempt is made to vibrate them in the manner described, i.e., at the same frequency and 180° out of phase so that the forces imparted to mounting ends 16 by the vibrating tines tend to balance and cancel one another. If this is successfully achieved, the cancellation of vibration forces at ends 16 is effective, which is a desired objective to achieve. However, there are a number of drawbacks to the transducer design diagrammatically illustrated in Figure 1, as will be discussed below. One drawback with the transducer of Figure 1 is that it is difficult to maintain the vibrating condition of tines 14 such that they reliably remain 180° out of phase with one another. Two primary reasons for this are as follows: manufacturing imperfections resulting in variations in unloaded resonant frequency between the tines; and loading between tines may not be symmetrical. Another drawback associated with the transducer design illustrated in Figure 1 is diagrammatically shown in Figure 2. In theory, in order for tines 14 to respond to external Fs in the desired manner, they should be equally supported in their no load (no force Fs) condition. If one of the tines is placed in greater load (compression or tension) than the other, the output reading of the overall transducer will be adversely affected. Thus, theoretically, the two tines should be supported between member 26, 28 so that the load forces applied by the member are uniformly distributed between the two tines. In reality, it is quite possible for the load forces to be imbalanced, as indicated by arrows 32 in Figure 2. This places a twisting force on the crystal transducer, as diagrammatically illustrated by arrows 34. This, in turn, places one of the tines, for example, the right-hand tine illustrated in Figure 2 in compression, as indicated by arrows 36, while the other tine is placed in tension, as indicated by arrows 38, thus adversely affecting the transducer's output. Still another drawback inherent in the design of transducer 10 resides in the specific orientation of tines 14 with respect to sensing force Fs. Specifically, because the sensing force is coincident with the tines' common axis 22, as illustrated in Figure 1, there is a 1:1 relationship between the external force and the arrangement of tines. Stated another way, for a given external force Fs, one-half of that force is applied to one of the tines and one-half of the force is applied to the other tine. The design does not allow for either amplification of the external signal or for deamplification of the signal, each of which may be advantageous depending upon the intended use of the transducer, as will be discussed hereinafter. In view of the foregoing, it is one aim of the present invention to provide a vibratory type of force sensing device of the general type described above but designed to eliminate the previously discussed drawbacks associated with the prior art design. Another, more particular aim of the present invention is to provide a vibratory type of force sensing device which includes dual vibrating tines and a specifically designed arrangement for supporting the tines such that the latter are reliably balanced in their motion. Another particular aim of the present invention is to provide a vibratory type of force sensing device of the last-mentioned type in which its support arrangement is specifically designed to reduce sensitivity caused by a twist of the type described above in conjunction with Figure 2. Still another particular aim of the present invention is to provide a vibratory type of force sensing device of the last-mentioned type in which the support arrangement is configured such that the external force being sensed can be either enlarged (amplified) or decreased (deamplified) in magnitude as it is applied to the vibrating tines, whereby the device can be custom designed for its intended use as, for example, an accelerometer by amplifying the external sensing force or, for example, a high pressure transducer by deamplifying the external force. A further aim of the present invention is to provide a vibratory type of force sensing device of the last-mentioned type or one utilizing a single tine in which under certain circumstances the crystal transducer can be provided with enhanced sensitive axis compliance and thereby made structurally more reliable. These aims are achieved with a device as defined in the claims. The foregoing aims and other aims and features of the present invention will become more apparent from the detailed description to follow in conjunction with the drawings wherein: FIGURE 1 is a diagrammatic illustration, partially in plan view, of a vibratory type of force sensing device typical in the prior art; FIGURE 2 is a diagrammatic illustration of a part of the device of Figure 1, specifically illustrating certain operational aspects of the device; FIGURE 3 is a diagrammatic illustration, partially in plan view, of a vibratory type of force sensing device designed in accordance with the present invention; FIGURE 4 diagrammatically illustrates an operational feature of the device of Figure 3; FIGURE 5 diagrammatically illustrates another operational feature of the device of Figure 3; FIGURE 6 diagrammatically illustrates a pair of vibrating tines and associated support arrangement for use as a preferred embodiment of an acceleration transducer as it would appear if integrally formed as a single unit from quartz crystal material, which integral unit forms part of overall device of the type illustrated in Figure 3; and FIGURE 7 is a diagrammatic illustration, partially in plan view, of a vibratory force sensing device designed in accordance with a second embodiment of the present invention. FIGURE 8 diagrammatically illustrates a pair of vibrating tines and associated support arrangement for use as a preferred embodiment of a pressure transducer as it would appear if integrally formed as a single unit from quartz crystal material, which integral unit forms part of overall device of the type illustrated in Figure 3. The dimensions in this figure are indicated in inches, whereby 1 inch corresponds to 25,4 mm. Having discussed Figures 1 and 2 previously, attention is immediately directed to Figure 3 which, as indicated above, diagrammatically illustrates a vibratory force sensing device designed in accordance with the present invention. This device, which is generally indicated by the reference numeral 40, includes a pair of parallel, adjacent elongated vibratable tines which, for purposes of description, are assumed to extend in the plane of the paper. The device also includes a suitable oscillator circuit 44 connected to the tines through suitable electrodes 46 for vibrating the tines back and forth in the plane of the paper and in a direction normal to their common center axis 48, as indicated by the two-way arrows 50. Circuit 44 is specifically designed to vibrate tines 42 at the same predetermined frequency, out of phase with one another by 180°, under no load conditions, that is, conditions in which the overall device is not subjected to any external sensing forces Fs other than possibly a no load null force and, ideally, also under load conditions. The vibrating tines just described are supported for vibration by means of a support arrangement which is designed in accordance with the present invention and which is generally designated by the reference numeral 52. While the support arrangement will be described in more detail hereinafter, for the moment it suffices to say that the arrangement includes mounting means 54 and 56 which lie in a common plane (in the paper) and which are placed against, physically connected to or otherwise coupled with an external member diagrammatically represented by dotted lines at 58 and 60. Like previously described member 26, 28 (see Figure 1), member 58, 60 is capable of responding to a variable external force Fs (the force to be sensed) to support arrangement 52, but does so along an axis 62 normal to the elongation axes of the tines 42 and therefore normal to common tine axis 48, rather than parallel as in the case of transducer 10. In fact, in the particular embodiment illustrated in Figure 3, axis 62 actually bisects the two tines 42. The member 58, 60 may be, for example, an accelerometer proof mass or pressure sensing bellows or diaphragm or other such arrangement. As Will be described in more detail hereinafter, support arrangement 52 is specifically configured so as to apply to each of the tines 42 its own internal force in response to and varying proportionately with external force Fs. These internal forces will place the tines in further tension if applied to support arrangement 52 in the manner illustrated or in further compression if applied in the opposite direction. In either case, the presence of these internal forces will cause the tines to vibrate at a frequency that varies with the magnitude of these internal forces and there-fore the magnitude of external sensed force Fs. Thus, overall device 40 is provided with a read out circuit 64 connected to the output of oscillator circuit 44 for sensing the vibrating frequency of one or both tines and for determining therefrom the magnitude of the external force Fs at any given time during operation of device 40. Before proceeding with a detailed description of support arrangement 52 which forms the basis of the present invention, it is important to make certain comments relating to the device as described thus far. First, it is to be understood that the tines per se, for the purpose intended, can be readily provided by one with ordinary skill in the art along with oscillating circuit 44, its associated electrodes 46 and read out circuit 64. Specifically, the present invention does not reside in the specific selection of tines 42, either in the material utilized or their size and shape, and it does not reside in the particular way in which the tines are made to vibrate under no load conditions or the specific circuitry for accomplishing this. Rather, the present invention resides in the particular way in which the tines are supported with respect to the force Fs being sensed and in the particular way in which the support arrangement responds to the external forces being sensed to impart internal forces to the vibrating tines in order to cause the tines to vibrate at a frequency that varies with these internal forces. Still referring to Figure 3, attention is now directed to the structural details of support arrangement 52. As shown there, this arrangement includes four substantially rigid, identical,elongate links 70A, 70B, 70C and 70D, and connecting means 72AB, 72BC, 72CD and 72AD interconnecting the links to one another and to opposite sides of the tines 42 so as to substantially form a parallelogram. As seen in Figure 3, the elongation axes of the tines 42, actually their common center axis 48, extends between opposing connecting means 72AB and 72CD and the force sensing axis 62 along which external force Fs is applied extends between opposing connecting means 72BC and 72AD, both of which also serve as the previously recited mounting means 56. The four links and connecting means are suitably selected and may be readily provided so as to form a five bar linkage assembly as illustrated in and to be described below in conjunction with Figure 4. Referring to Figure 4, the five bar linkage formed by structural arrangement 52 and tines 42 is diagrammatically illustrated with the two tines 42 being shown as a single cross link. For purposes of description, the five links shown in Figure 4 will be designated L1, L2, L3, L4 and L5 and the corners will be designated C1, C2, C3, C4. As is well-known, in a five bar linkage assembly, if a force, for example Fs, is applied downward at the top corner C4, as shown, the force Fx in cross link L5 is a function of force Fs in accordance with the following equation: Fx = Fs/tan ϑ As shown in Figure 4, ϑ is one-half the angle defined by corner C1 or C3, that is, that angle subtended by L1 and L5; L2 and L5; L3 and L5; or L4 and L5. It is important to note the particular relationship between external force Fs, internal force Fx and angle ϑ. More specifically, when ϑ is equal to 45°, there is a 1:1 relationship between Fs and Fx. However, for angles less than 45°, Fx is greater than Fs (i.e., amplified) and for angles greater than 45°, Fx is less than Fs (i.e., deamplified). Thus, in providing the internal force Fx through its cross link L5, the five bar linkage assembly can be configured either to serve as a means of amplifying the externally applied signal Fs or deamplifying it. This is an important feature of support arrangement 52, as will be discussed immediately below. As indicated previously, force sensing device 40 may be specifically designed for use as, for example, an accelerometer or it may be specifically designed for use as, for example, a pressure transducer. In the former case, the full scale sensing forces are typically small, for example on the order of 0.0091 kg (0.02 lb). In the latter case, the sensing forces would be comparatively large, for example on the order of 0.182 kg (0.40) lb. Thus, if the device is designed for use as an accelerometer, its five bar linkage should be configured to include a relatively small angle ϑ. Of course, because device 10 includes two identical parallel tines taking up the cross force, each tine would be subjected to one-half of the amplified internal force. Nevertheless, each tine would be subjected to a force substantially greater than the externally applied force and therefore would provide a more sensitive output. Another advantage in providing an amplified cross force Fx is that the larger force places a greater strain on the tines than would be the case if the cross force were equal to the externally applied force. As a result, the overall structure deflects through a greater distance than would otherwise be the case and therefore overload caging is easier and there is greater structural integrity. In the case of the deamplification of the applied force, a more rigid (less compliant) crystal will provide better restraint, for example for a pressure sensing diaphragm, to reduce hysteretic strain in such a sensing element. It should be apparent from Figures 1 and 2 that the typical prior art arrangement diagrammatically illustrated there does not have either of the advantages discussed immediately above, that is, either enhanced sensitivity or structural integrity because the forces actually applied to the tines cannot be greater than the external force Fs applied thereto. In fact, the force applied to each tine is, at most, approximately one-half of the external force. Obviously, this does not enhance the sensitivity of the overall device and it certainly does not enhance the structural integrity of the tines. The discussions immediately above related to the use of device 10 as an accelerometer or other such means taking advantage of the amplification capability of support structure 52. In the case of the pressure transducer, for example, by making the angle ϑ large, the internal force applied to each tine 42 can be reduced substantially as compared to the external force Fs. In this type of arrangement, induced compliance is not as critical as in the case of an accelerometer because the forces in question are relatively large and the tines themselves can be initially made with more structural integrity while maintaining the desired sensitivity. In the case of an accelerometer where the anticipated external forces are quite small, to preserve the desired sensitivity, the linkage must be initially made much more compliant. Another advantage in the five bar linkage configuration of support arrangement 52 resides in its ability to more reliably maintain the out of phase frequency mode of tines 42 during operation of the overall device. As indicated previously, from the standpoint of accuracy, it is important to balance the tines 42 forces as much as possible. This is accomplished by making the tines as identical as possible, and exciting them with the oscillator to vibrate as close to 180° out of phase as possible. The support arrangement 52 does more to accomplish this than the arrangement illustrated in Figures 1 and 2. One reason for this is associated with added compliance which separates the natural frequencies of the in phase and out of phase, in plane modes. Still another advantage to applicant's specifically configured support arrangement is illustrated diagrammatically in Figure 5. As seen there, the support arrangement is being subjected to the same end twist shown in Figure 2, as represented by arrows 34. In the case of device 10, the end twist placed one of the tines in compression while the other tine was placed in tension, thereby making it difficult to vibrate the two tines at the same frequency and thereby adversely affecting the ability to vibrate tines 14 out of phase. On the other hand, as illustrated in Figure 5, the very same end twisting on support arrangement 52 places both of the tines in substantially the same amount of compression. Hence, both tines are placed in compression and since they are placed in approximately the same amount of compression, these loads due to end twisting can be readily nulled out by means of circuit 44 and read out 64 or computerized calibration or modeling. Thus, the effects of end twisting can be readily compensated for in device 40. Turning now to Figure 6, a five bar linkage assembly 76 serving as a combination support arrangement 52/tines 42 is shown. The tines, links and interconnecting means making up this five bar linkage are illustrated as they would appear integrally formed as a contemplated, preferred working embodiment of a single unit of quartz crystal material serving as an acceleration transducer. This unit is drawn in proportion throughout and therefore all of its dimensions can be determined from the figure. The mounting pads are shown at 54′ and 56′ and the links at 70′A-70′D. The tines are shown at 42′. The overall thickness of the unit is 0.1524 mm (.006 inch), although it is thinner at points 100 which serve as flexures for the links. Figure 8 illustrates a similar assembly as it would appear integrally formed from quartz crystal material as a preferred working embodiment serving as a pressure transducer. Actual contemplated dimensions are shown on the figure. Having described overall dual tine device 40 and its various advantages, attention is directed to Figure 7. This figure illustrates a device 80 which may be substantially identical to device 40 in all respects, except two. First, device 80 includes a single cross tine 82 in place of dual tines 42. Second, in the case of device 40, the tines and links are placed in a common plane, specifically the plane of the paper, and the tines are vibrated back and forth in the same common plane. In the case of device 80, the links and tine lie in the plane of the paper but applicant has found that the single tine 82 can also be vibrated back and forth in a plane through the tine and normal to the common plane of the paper. Device 80 includes many of the advantages attributed to device 40. Specifically, it can provide the same amplification or deamplification between its externally applied sensing force Fs and the internal force applied to its tine. Also it can be made substantially insensitive to end twisting. However, like any single tine device, its vibration forces are not balanced. It does allow for balance via counterweights 83 and 84 action and via the compliance of the arms 85 which reduce the strain in the attachment pads for a given tine vibratory amplitude.

Claims: A force sensing device (40) comprising at least one elongate, vibratable tine (42), means (46) for vibrating said tine or tines (42), an arrangement (52) for supporting said tine or tines (42) for vibration including a plurality of links (70A,70B,70C,70D) formed integrally with said tine or tines (42) of a single unit of quartz crystal material and mounting means (54,56) adapted for connection to an external member (58,60) and capable of applying a variable external force to the supporting arrangement (52), the supporting arrangement (52) being configured so as to apply to said tine or tines (42) an internal force in response to and varying with said external force which causes the tine or tines (42) to vibrate at a frequency that varies with the magnitude of said external force, and means for sensing the vibrational frequency of said tine or tines (42) and for determining therefrom the magnitude of said external force, characterised in that said mounting means (54,56) is adapted to apply said external force to the supporting arrangement (52) along an axis transverse to the length of said tine or tines (42). A device (40) in accordance with claim 1, wherein said tine or tines (42) is vibrated in a direction normal to its or their length or lengths. A device (40) in accordance with claim 1 or claim 2, wherein said tine or tines (42) is vibrated in a direction normal to the axis along which said external force is applied. A device (40) in accordance with any preceding claim, wherein said mounting means (54,56) is adapted to apply said external force to the supporting arrangement (52) along an axis which both bisects said tine or tines (42) and extends in a direction normal to its or their length or lengths. A device (40) in accordance with any preceding claim, wherein said supporting arrangement (52) includes four links (70A,70B,70C,70D) of substantially equal length and means (72AB,72BC,72CD,72AD) interconnecting the links (70A,70B,70C,70D) to one another and to opposite ends of said tine or tines (42) so as to form a parallelogram such that the tine or tines (42) extend along an axis joining a first pair of opposing corners of the parallelegram and the external force is applied along an axis that extends between a second pair of opposing corners of the parallelogram. A device (40) in accordance with claim 5, wherein the corners of the parallelogram between which the tine or tines (42) extend are each less than 90°. A device (40) in accordance with claim 5, wherein the corners of the parallelogram between which the tine or tines (42) extend are each greater than 90°. A device (40) in accordance with any preceding claim, and including a second substantially identical vibratable tine (42) supported by said arrangement (52) for vibration adjacent to and parallel with said first mentioned tine (42) and vibrated by said vibrating means (46) such that its vibrational frequency varies with the magnitude of said external force. A device (40) in accordance with claim 8, wherein said vibrating means (46) vibrates said tines (42) at the same frequency but 180° out of phase with one another in a direction normal to their lengths and parallel with the axis along which said external force is applied.

Inventors: HANSON RICHARD A, HANSON, RICHARD A.
Applicants: HANSON RICHARD A, HANSON, RICHARD A.
Application year: 1988
Main classification: G01L1
IPC codes: G01L1, G01P15