Acoustic surface wave oscillator force-sensing devices
United States Patent n 1 Dias et al.  
 [ ACOUSTIC SURFACE WAVE OSCILLATOR FORCE-SENSING DEVICES [75] Inventors: .l. Fleming Dias; Henry E. Karrer,  
 both of Palo Alto, Calif.  
 [73] Assignee: Hewlett-Packard Company, Palo Alto, Calif.  
 [22] Filed: Jan. 8, I974 [21] Appl. No.: 404,829  
 [52] U.S. Cl 331/40; 73/885 R; 73/DlG. 4; 310/85, 310/98; 331/65; 331/107 A [51] Int. Cl. G0lb 7/16; H03b 5/32; H031) 21/00 [58] Field of Search 331/37, 40, 65, 107 A; 73/885 R, 88.5 SD, DIG. 4; 31018.3, 8.5,  
 [56] References Cited UNITED STATES PATENTS 3,350,944 I 1/1967 DeMichele 331/65 X 3,756,081 9/1973 Young 331/65 X Apr. 15, 1975 Whitchousc 331/107 A Zuckcr et al. 331/107 A Primary E.\&#39;aminerSiegfried H. Grimm Attorney, Agent, or FirmF. David LaRiviere; A. C. Smith [57] ABSTRACT An acoustic surface wave oscillator is employed as a force-sensing device. Dual acoustic surface wave oscillators coupled to a single substrate of piezoelectric material which inversely change their respective frequencies in response to a force applied normal to the surface of the substrate comprise a high-sensitivity, temperature-compensated force-sensing device. The outputs of the oscillators are applied to an electronic mixer circuit to produce a difference frequency output signal which is a function of the applied force. Other configurations utilizing the force-sensing properties of acoustic surface wave oscillators are disclosed.  
 11 Claims, 13 Drawing Figures EXTERNAL FORCE FI TEHTEK APR 1 51375 SHUT 1 0f 6 XTERNAL FORCE PLTENTED APR 1 51975 sum 2 or 5 Sample Time In Seconds igure 2 9 w w w w? v 0 553 6:26 m  
 ACOUSTIC SURFACE WAVE OSCILLATOR FORCE-SENSING DEVICES BACKGROUND OF THE INVENTION Acoustic surface wave (ASW) devices such as delay lines and bandpass filters have existed for se\eral years. In all of these devices. an ASW is generated at one location on a substrate. propagates to another location with a certain transit time and is then detected. Since most of the wave energy is confined to within one wave length of the surface of the material on which the wme is propagated. the ASW can be processed and detected on the surface while the substrate material below a depth of about one wave length as well as the under mounting surface is essentially inert. Therefore. ASW devices are easier to design and produce than their bulk wave counterparts.  
  Acoustic surface waves are usually generated and propagated on a piezoelectric substrate. The ASW generator and receiver transducers are commonly vacuum deposited transducers. each consisting of interdigital finger pairs spaced one-half acoustic wave length apart. Such interdigital (ID) transducers are most effective on piezoelectric substrates. Thcsc substrates can be single SUMMARY OF THE INVENTION The preferred embodiment of the present invention utilizes the frequency changes occurring in dual. reciprocally-interacting ASW oscillators coupled to a common piezoelectric substrate to measure the magnitude of an unknown force applied to that substrate while remaining relatively insensitive to ambient temperature change. Accurate. high-resolution measurement of force. weight. pressure. acceleration and voltage may be performed by employing the principles of this invention.  
  An ASW device may be configured as an oscillator by returning the signal at a receiver ID transducer to a generator ID transducer via a feedback amplifier. This configuration has been used to measure the temperature coefficient of delay for lithium niobate delay lines by measuring the oscillator frequency as a function of temperature. Until now. however. the advantages of using ASW oscillators to measure the magnitude of force have not been recognized.  
  Before the present invention. force-sensing devices. which provided pulse-rate or difference frequency output signals. employed either more than one oscillating piezoelectric crystal or one piezoelectric crystal together with elaborate external electronic circuitry to obtain a difference frequency output signal. The crystals were excited to oscillate in thickness shear mode which provide inadequate sensitivity and temperature versus frequency stability. This invention provides a more sensitive force-sensing device with improved temperature compensation and pulse-rate or difference frequency output. The force-sensing devices described herein do not exhibit the undesirable characteristics of devices using thickness shear mode of oscillation in piezoelectric materials. Moreover. when constructed according to the principles of this invention. a single substrate of piezoelectric material can accommodate the operation of two or more ASW oscillator devices.  
 DESCRIPTION OF THE DRAWINGS FIG. I is a schematic diagram of an acoustic surface wave oscillator used as a force-sensing device.  
  FIG. 2 is a plot of the frequency stability ofan acoustic surface wave oscillator.  
  FIG. 3a is a schematic diagram of a dual acoustic surface wave oscillator.  
  FIG. 3b is a one-dimension side view of the dual acoustic surface wave oscillator showing only the arrangement of interdigital transducers and grounding planesv FIG. 31&#39; shows the typical frequency response curve of an interdigital transducer.  
  FIG. 31/ illustrates the use of the response characteristic shown in FIG. St for reducing dual oscillator interaction.  
  FIG. 4 is a schematic diagram of a high-sensitivity. temperature-compensated force-sensing device.  
  FIG. 5 is a schematic diagram of another embodiment of the device of FIG. I.  
  FIG. 6 is a schematic diagram for an acoustic surface wave oscillator used as a pressure-sensing device.  
  FIG. 7a is another embodiment of the pressuresensing device of FIG. 6.  
  FIG. 7b is a plot ofthe output signal of the pressuresensing device of FIG. 7n.  
  FIG. 8 is a schematic view of a force-sensing device employing several acoustic surface wave oscillators constructed according to the principles of this invention.  
  FIG. 9 is a schematic view of another pressuresensing device utilizing the principles of the present invention.  
 DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. I shows a schematic diagram of ASW forcesensing device. which illustrates the basic operation of the several embodiments of this invention. These forcesensing devices translate changes in a dimension and some physical characteristics of the piezoelectric material between the transducers of an ASW oscillator produccd by an applied force into a change in the output signal frequency of the oscillator. Referring to FIG. I. there is shown an ASW force-sensing device comprising substrate 10. on which is deposited a set of two transducers. l2 and I4 respectively. a predetermined distance. I. apart. The input of amplifier 16 is connected to transducer 14 and the output of amplifier I6 is connected to transducer [2 to form an ASW oscillator. Substrate 10 is held in place by mounting I8 so that it elastically deforms the area of piezoelectric material between transducers l2 and 14 in response to applied force I9.  
  Acoustic surface waves travel from generator transducer 12 to receiver transducer I4 with a velocity (1),.) and transit time (1&#39;). They are detected at receiver transducer 14. amplified and fed back to the input by amplifier I6. The amplifier gain need only be sufficient to achieve at least unity loop gain.  
  The oscillator frequency f,, is determined by the phase condition:  
 21m 110,, th  
 where n an integer [determines model r transit time V1).  
 (11,. frequency 2111]..  
 d) phase shift in amplifier and interdigital transducers Equation l 1 describes a system with an infinite number of modes of oscillation. The actual single mode of oscillation is determined by the bandpass characteristics of the ID generator and receiver transducers. I2 and 14 respectively. and the frequency characteristics of amplifier 16. Each ID transducer is similar to a bandpass filter with center frequency f:. determined by the finger spacing which is one-half wave length A. of the desired frequency. where]; 2 v The 34111 bandwidth of an ID transducer-is approximated by UN. where N is the number offinger pairs. The transducer shown in FIG. It: has 5 fingers and 4 finger pairs.  
  The oscillator stability is determined by the phase stability of various components in the loop. The loop must be designed so that the phase slope of the delay line ((ldJ/lflt&#39; 1) is much larger than the phase slopes. ms. ltlir. for other loop components. Therefore, when ddntlw 11d) ldu&#39;. the phase shift in the substrate will dominate and hence control the oscillator stability. The fractional frequency stability of the ASW oscillator used in the force-sensing device of FIG. I is graphically presented in FIG. 2.  
  Since 1 and d1 in Equation l I) are constants. the oscillator frequency is a function ofr Hi)... If v. is constant for a given elastic medium. the frequency of the output signal of the oscillator changes as the length I between ID transducers I2 and 14. is changed (for example by applying an external force 19 to elongate the substrate). However. 11,. is not constant when the substrate is subjected to stress. Applied force 19 causes variations in elastic constants and density to occur in pieloelectric substrate [0 which in turn affects the surface wave velocity v, between ID transducers l2 and 14. Since the frequency of the oscillator changes as a function of both v, and l in response to applied force. the frequency of the output signal of the force-sensing device shown in FIG. I varies as a function of applied force l9. Similar stresses also arise in piezoelectric substrate II] in response to a change in temperature thereof. Therefore. ASW oscillator frequency f,, is a function of v as given by;  
 where A. is the wave length selected by the ID transduc ers and v. is a function of substrate temperature. I. and stress. a. In the mounting configuration of the embodiment shown in FIG. I. the substrate forms a cantilevcred beam whose axis is parallel to the path of ASW propagation. Other variations of substrate mounting and transducer orientation are anticipated by this invcntion and are described later in this application.  
  The principal parameters for the force-sensing device described above are its sensitivity and stability. The normalized stress sensitivity may be expressed as:  
  ft/r i S: Arr (psi where rr is the stress along the path of acoustic surface wave propagation (not necessarily co-linear with u This stress sensitivity is a property of the material and is caused by changes in elastic constants. density and length as a fuunction of stress.  
  The force sensing device stability is dependent upon the stability of the ASW oscillator and may be ex pressed as the fractional frequency stability. (Af/f.,) Therefore. the transducer resolution is the product:  
 l A I i, s A 1/! I S rrlpsi) FIG. 3a is a schematic diagram of a pair of ASW oscillators coupled to a common substrate of piezoelectric material 30. Each oscillator comprises an amplifier 32 and 34. which are each connected to a separate set of ID transducers. 36 and 38, 37 and 39. respectively. deposited a predetermined distance. I. apart on each side of the common substrate. The operation of both oscillators is the same as the operation of the one described in connection with the force-sensing device of FIG. 1.  
  Even though most of the ASW energy penetrates to a depth of only about one wave length. there is still some second-order signal cross-talk or interaction between the transducers of one oscillator with those of the other oscillator. Two techniques are successful for reducing such cross-talk. Referring to FIG. 3b. the first technique for reducing cross-talk requires that each transducer set be offset on its respective surface of substrate 30 along the direction of wave propagation such that no transducer is directly opposite another transducer disposed on the other side. Grounding plane 2 is then disposed on one surface of the substrate in a position substantially aligned with transducer 3 on the opposing side of the substrate. Grounding planes 4. 6 and 8 are similarly disposed on the opposite side from the corresponding transducers I. 7 and 5, respectively. For  
 this embodiment ofthe invention. since the signal path for spurious signals from transducer 1 to grounding plane 4 is shorter and to a lower potential generally than from transducer 1 to transducer 3, signal interaction between transducer I and 3 is reduced. Similarly. cross-talk between transducers 5 and 7 is also reduced.  
  The second technique employs the frequency selectivity of ID transducers. Referring again to FIG. 3a. the bandwidth of ID transducer is inversely proportional to the number and spacing ofthe fingers 35 comprising it. FIG. 30 shows the frequency response curve of such transducers. which is characterized by the relation I sin .v/x. Zeroes in the response curve occur at mathematical w,,/N where w is the center frequency of the transducer and N is the number of periodic sections in the transducer. The periodic section of an ID transducer contains one wave length of the signal generated or detected by the transducer i.e. 3 fingers). No acoustic surface waves are propagated at frequencies corresponding to 0),, I run/N.  
  Referring now to FIG. 3d. if the center frequency of the transducers ofone transducer set l referred to as the top) is (1),. then zeroes occur at w, w,/N. If the center frequency ofthe transducers ofthe other transducer set (referred to as the bottom) is u, then zeroes occur at (0,, i (u /N. In accordance with the principles of the present invention. when w, E (1),, (u /N and an, E w,  
 w,/N. cross-talk between transducer sets at the frequencies of greatest wave energy is significantly reduced.  
  FIG. 4 shows the preferred embodiment of the highscnsitivity. ternperature-compensated stress sensor. which is similar in configuration to the cantilever orientation of the embodiment shown in FIG. I. A substrate of quartz having a top surface 41 and bottom surface 43 is rigidly held at one end by mounting 42 and force is applied at the other end. The substrate 40 is polished on both sides. and one set of two ID transducers are deposited on each side. Each transducer set is deposited so that ASW propagation is parallel to the axis of the cantilever beam. and offset in the direction of propagation with respect to one another as described for the dual oscillator of FIG. 3a. The transducer set consisting of transducers 44 and 46 is connected to amplifier 48 forming a first ASW oscillator. and the transducer set comprising transducers 45 and 47 to amplifier 49 forming a second ASW oscillator. A grounding plane is deposited on the opposite side of the substrate from and corresponding to each of the ID transducers. The purpose ofthe grounding planes is to diminish the signal interaction between the transducers on opposite sides of the substrate. as explained above. The grounding planes causc negligible perturbation to ASW propagation between transducers of a set because the metal is very thin relative to the wave length of the signal propagated.  
  With force so applied. a tensile stress on the top surface 4] causes the frequency of the first oscillator to decrease; i.e.f Af,. and a corresponding compressive stress on the bottom surface 43 increases the frequency of the second oscillator to f; if By appropriate transducer designs.j&#34; may be set equal to f, in the absence of force 50. Also. ifthe ID transducers were symmetrically placed. Af. A1}. However. since the transducer sets are disposed asymmetrically in order to reducc crosstalk. Af, A]; in this embodiment. Of course a change in temperature affects the frequencies of both oscillators by identical amounts. i.e. Afl. Hence. the outputs of the two oscillators are:  
 for fl ll fie fl f A If these outputs are mixed and the difference frequency determined with mixer 51. we get a signal whose frequency is proportional to the stress as given below:  
  III II The above described embodiment provides approximately twice the stress sensitivity of the embodiment of FIG. I. In addition. the sensor is compensated for variations in ambient temperature.  
  Referring now to FIG. 5. another embodiment of a force-sensing device employing an ASW oscillator is shown. The beam of piezoelectric substrate in this arrangement is clamped at both ends and the force 59 to be measured is applied at the midpoint between the ID transducers 52 and 53. Transducers 52 and 53 are connected to amplifier 54.  
  Transducer set orientation relative to the crystallographic oreientation of the substrate strongly influ ences the propagation as well as the frequency versus temperature characteristics of the ASW. Furthermore.  
 transducer set orientation relative to the point ofapplication and direction of the force applied to the substrate is an important consideration in the design of a force-sensing device employing an ASW oscillator. For example. the transducer set in the embodiment of FIG. 5 could be reoriented orthogonally to the axis of the beam. In such orientation. however. the sensitivity to applied force is considerably less since changes in elastic constants and density of the piezoelectric material are caused by cross-coupling of stresses within the crystal lattice of the substrate. and there is virtually no change in the distance. 1. between transducers 52 and 53.  
  FIG. 6 shows an application of the stress sensitivity of an ASW oscillator for measuring pressures. The sensor consists of a thin active diaphragm of piezoelectric material 60 and a corresponding ring-shaped mounting body 6]. The underside of the diaphragm is provided with ID transducers 62 and 63. shield lines 64 and acoustic absorbers 65. Diaphragm 60 is bonded to body 61. which is also made of piezoelectric material having the same crystallographic orientation as the diaphragm. Shield lines 64 are provided to reduce the electromagnetic transmission between the two transducers. The acoustic absorbers 65 reduce spurious effects due to reflected waves from the boundaries. The output of amplifier 66 is connected to generator ID transducer 62 and the input to receiver ID transducer 63 to form an ASW oscillator. The pressure sensor can be adapted for measuring absolute pressures in which case the cavity formed by diaphragm 60 covering mounting body 6I is evacuated and sealed. Measurement of gage pressures is facilitated by providing a small porthole in the body.  
  The effect of force on the elastic constants and density of diaphragm-shaped piezoelectric material is differcnt from previously discussed configurations by virtue of crystal lattice interaction. By cutting longitudinal dimples 67 and 68 parallel and along each side of the path of ASW propagation. the sensitivity of this device may be increased nearly two-fold. The dimples tend to confine the stress in the diaphragm to the area between transducers 62 and 63 in the direction of propagation of the ASW&#39;s.  
  An extension of this concept is shown in FIG. 7a where the longitudinal dimples 67 and 68 have been cut all the way through substrate 71 to form slots 72 and 73. An activator 74 is inserted between substrate 7] and thin diaphragm 75 to transmit the force of the pressure load on diaphragm 75 to the piezoelectric substrate. This activator has an archway 76 that allows substantially unrestricted propagation of an ASW between transducers 77 and 78. Thin diaphragm 75 is bonded to activator 74. and the cylindrical body 79 which is an integral part of diaphragm 75. is also bonded onto the substrate 7]. Amplifier is connected to transducers 77 and 78 to form an ASW oscillator.  
  In operation. the pressure acting on diaphragm causes the area of substrate 71 between the slots to bend in response to the travel of activator 75 without interferring with the propagation of an ASW. The changes in oscillator frequency caused thereby are a function of the pressure acting on diaphragm 55. FIG. 7b shows a plot of frequency versus pressure for the configuration of FIG. 7:1 for which the sensitivity is on the order of 1500 Hz/psi for an oscillator operating at approximately 4l MHz.  
  More than two ASW oscillators may be incorporated onto a multi-surface elastic material for the sensing of both magnitude and direction of force. Referring to FIG. 8. such a device designed in accordance with the principles of the present invention is shown. It consists ofa columnar substrate 80 having an upper end bonded to a mounting surface 8] and a lower end bonded to a weight 82. Each of the four surfaces of column 80 incorporate ASW oscillators which are located in this diagram by reference to their respective substrate surfaces 83. 84 85 and 86. As mounting surface 8] is rotated through angles :8 and/or id). the surfaces of substrate 80 become variously stressed since weight 82 will tend to stay at rest. For example. for a rotation through the +H angle. surface 85 is under compression and surface 83 is in tension. The central portion of surfaces 84 and 86 are essentially unchanged. while the edgeportion nearest surface 85 is in compression and the edge-portion nearest surface 83 is in tension. By appropriate electronic resolution of the change in frequency of the output signal of each ASW oscillator. the amount of inclination and its direction can be determined More surfaces with ASW oscillators can be added to substrate 80 for greater accuracy and resolution.  
  FIG. 9 shows yet another pressure-sensing device. A cylindrical piece of piezoelectric material is cored out so that the cylindrical surface 97 now constitutes the pressure diaphragm. The two end caps 91 and 92 are bonded to each end of the hollowed cylinder. End cap 9| has a pressure entry port 96. Variation in pressure stresses the cylindrical surface. The transducers 93 and 94 are placed in a direction that is most efficient for propagation. depending on the orientation of the crystal axes. These transducers are then connected to a wide band amplifier 95 to produce an ASW oscillator whose frequency is a function of the pressure applied to the inside of the cylinder 90 via entry port 96.  
 What is claimed is:  
  l. A dual acoustic surface wave oscillator comprising:  
 a substrate of piezoelectric material having a first sur face and a second surface:  
 two transducers disposed at predetermined distance apart on the first surface of the piezoelectric material forming a first transducer set for propagating a first acoustic surface wave thereon;  
 two transducers disposed a predetermined distance apart on the second surface of the piezoelectric material forming a second transducer set for propa gating a second acoustic surface wave thereon;  
 a first amplifier connected to the first transducer set forming a first acoustic surface wave oscillator having an output terminal for coupling the output signal therefrom.  
 a second amplifier connected to the second transducer set forming a second acoustic surface wave oscillator having an output terminal for coupling the output signal therefrom; and  
 mixing means. connected to the output terminals of the first and second acoustic surface wave oscillators. for combining the output signals therefrom to form a resultant output signal having a frequency representing the difference in the frequency of the first and second acoustic surface wave oscillators.  
 2. The dual acoustic surface wave oscillator of claim wherein:  
 the piezoelectric material has plane parallel surfaces on which the transducer sets are disposed: and  
 the transducers include interdigital fingers.  
 3. The dual acoustic surface wave oscillator of claim wherein:  
 each transducer set is offset on its respective surface along the direction of wave propagation such that no transducer is directly opposite another transducer of the other set;  
 a first set of two grounding planes is disposed on the second surface of the piezoelectric material in a position directly opposite the first set of two transducers for reducing signal interaction between the first and second set of transducers: and  
 a second set of two grounding planes is disposed on the first surface of the piezoelectric material in a position directly opposite the second set of two transducers to reduce signal interaction between the second and first set of transducers.  
 4. The dual acoustic surface wave oscillator of claim wherein:  
 the transducers ofthe set disposed on the first surface ofthe substrate for propagating an electromagnetic signal thereon have a frequency response generally characterized by the mathematical relation Y sin .v/v; and  
 the transducers of the set disposed on the second surface of the substrate for propagating an electromagnetic signal thereon have a frequency response generally characterized by the mathematical relation Y sin .r/.v where v amplitude and .v frequency&#39;.  
 said first surface transducer set having a frequency of maximum amplitude equal to a frequency of minimum amplitude of the second surface transducer set for reducing signal interaction between transducer sets of the first and second surfaces of the substrate.  
 5. A force-sensing device employing an acoustic surface wave oscillator comprising:  
 a substrate of piezoelectric material having a surface on which transducers may be disposed;  
 mounting means connected to the substrate for rigidly holding the substrate to respond to a force applied thereto;  
 two transducers disposed a predetermined distance apart forming a transducer set on the surface of the substrate for propagating an acoustic surface wave thereon:  
 an amplifier connected to the transducer set to form an acoustic surface wave oscillator having an output signal frequency that varies in response to changes in the physical characteristics of the substrate and the distance between the transducers of the transducer set; and  
 output means connected to an output of the acoustic surface wave oscillator for receiving an output signal therefrom;  
 said physical characteristics of the substrate and distance between transducers of the transducer set changing in response to the force applied to the substrate.  
 6. The acoustic surface wave force-sensing device of claim 5 wherein:  
 the piezoelectric substrate is generally elongated in shape in which the planar surfaces have a longitudinal dimension greater than a width dimension;  
 the elongated piezoelectric substrate is oriented in the mounting means such that one end is rigidly clamped to form a cantilever beam having an axis along the longitudinal dimension of the substrate: and  
 the transducer set is disposed to propagate the acoustic surface wave along a path substantially corresponding to the axis of the cantilever beam.  
  7. the dual acoustic surface wave oscillator described in claim 1 employed as a temperature-compensated force-sensing device wherein:  
 the substrate is mounted in mounting means to respond to a component of force applied normal thereto; and  
 the second acoustic surface wave oscillator has an output signal frequency that varies inversely in rcsponse to the same component of force applied normal to the substrate when compared to the output signal frequency of the first acoustic surface wave oscillator;  
 said output signal frequency of the second acoustic wave oscillator being substantially equal to the output signal frequency of the first oscillator when no component of force is applied to the substrate.  
 8. The temperature-compensated force-sensing deice as described in claim 7 wherein the:  
 resultant output signal has a frequency directly related to the magnitude ofthe force component applied to the substrate.  
  9. The temperaturc-compensated force-sensing device as described in claim 8 wherein the resultant output signal is a signal whose frequency is the algebraic difference between the frequencies of the output signals of the two acoustic wave oscillatorsv 10. The temperature-compensated force-sensing device ofclaim 7 wherein the difference frequency ofthe output signal of the mixing means is substantially zero with no component of force applied normal to a planar surface of the piezoelectric substrate,  
  11. The tcmperature-compensated force sensing device of claim 7 wherein:  
 the output signal frequency of the first oscillator increases in response to compression stress of the first surface:  
 the output signal frequency of the second oscillator decreases in response to tensile stress of the second surface; and  
 the difference frequency output signal of the mixing means is directly related to the magnitude of the force applied normal to a planar surface of the piezoelectric substrate,