GB2224598A - Piezoelectric shear mode accelerometer - Google Patents

Abstract

An accelerometer consists of a piezoelectric plate 33 having on its upper face a pair of spaced electrodes 37, 39, having on its lower face a common back electrode 35, and having a mechanical support 41 centrally located beneath the lower face. The polar axis 34 of the piezoelectric material lies in the plane of the plate 33 and orthogonal to the central support 41. The arrangement compensates for spurious pyroelectric signal and is relatively insensitive to the alignment of the polar axis. Accelerometer sensitivity may be enhanced by increasing the inertial mass loading. This may be accomplished by attaching weights or by ensuring that piezoelectric material extends outwardly beyond limits defined by the electrodes. The plate 33 may be of ceramics piezoelectric material, for example, lead zirconate titanate. Alternatively, it may be of single crystal piezoelectric material, for example, lithium niobate, lithium tantalate, barium titanate, lead titanate or lead metaniobate. <IMAGE>

Description

A PIEZOElROTRIC SHEAR MODE ACCELEROMR The present invention concerns improvements in or relating to piezoelectric accelerometers, and in particular piezoelectric shear mode accelerometers. Accelerometers are required for the sensing of vibration or other acceleration, over a wide range of operational conditions. The accelerometers to be described hereinafter use a piezoelectric sensor element, an element that is configured and operated in a compensated shear mode to avoid any pyroelectric signals complicating the response of the accelerometer at low frequencies.

Piezoelectric longitudinal and shear mode accelerometers have been known for a number of years. A typical longitudinal mode piezoelectric accelerometer 1 is shown in schematic cross-section in figure 1 of the drawings. This comprises a plate 3 of piezoelectric material, which plate is poled such that the polar axis (shown by an arrow) is aligned in a direction that is orthogonal to the plane of the plate 3. Opposite principal plane faces of the plate 3 are metallised to provide a pair of electrodes 5, 7 for the extraction of electrical signal. The accelerometer element 3, 5, 7 this provided is mounted upon a mechanical support 9 and is loaded by an inertial mass 11 which is attached to one of the electroded principal plane faces of the plate 3.

Longitudinal mode accelerometers, such as the one shown in figure 1, use a longitudinal piezoelectric coefficient (such as d33) to generate charge, usually through the stresses caused by inertial mass loading 11 acting in a direction parallel to the polar axis of the piezoelectric material. The problems associated with the use of this mode of operation are mainly in the generation of pyroelectric charges when the temperature of the piezoelectric material is changed. These charges appear on the electroded faces 5, 7 orthogonal to the polar axis and thus produce a spurious signal which can be misinterpreted as being due to acceleration.

A typical shear mode piezoelectric accelerometer 13 is shown in schematic cross-section in figure 2 of the drawings. This comprises a pair of plates 15, 17 of piezoelectric material mounted each side of a mechanical support 19. Each plate 15, 17 is poled such that again the polar axis (shown by an arrow) is aligned in a direction that is orthogonal to the plane of the plate 15, 17. An inertial mass 21. 23 is attached to the outermost plane face of each plate 15, 17. Opposite edge planes of each plate 15, 17 are metallised to provide signal electrodes 25, 27 and ground electrodes 29, 31, respectively.

Shear-mode accelerometers, such as the one shown in figure 2, use the piezoelectric charges generated on faces parallel to the polar axis of the piezoelectric material by acceleration-generated shear stresses. The production of such shear stresses by external mass loading is illustrated in figure 3 for a typical piezoelectric ceramic material, using the standard axial definition (inset) given in "IEEE Standard on Piezoelectricity ANSI/IEEE Std. 176-1978 pp. 12-13".

The shear stress T5 (using the reduced tensor notation adopted in the abovementioned Standard) produced by the effects of the acceleration on each loading mass 21, 23 engenders a shear strain S5 which in turn produces an output charge on the electroded edge planes 27, 31 of the piezoelectric material normal to the xl axis via the piezoelectric coefficient d15. Provided that the electroded faces 21, 23 are accurately orthogonal to xi, then the accelerometer 13 will show no pyroelectric sensitivity because the pyroelectric charges are generated only on faces whose normals have a significant component parallel to the polar axis (x3 in figure 3).Piezoelectric accelerometers working on this principle have been constructed and an example is described in detail in United Kingdom Patent No. 1,522,785.

The present invention relates to a novel form of piezoelectric accelerometer which possesses a number of advantages over the type of accelerometer shown schematically in figures 2 and 3. In particular, a disadvantage of the accelerometer shown in figures 2 and 3 is that precise alignment of the shear element faces with respect to the polar axis is necessary. If there is a small misalignment e, as shown in figure 4, then there is a component of the polar axis onto the electroded edge planes and the accelerometer 13 will be sensitive to changes in temperature due to the pyroelectric effect.Such sensitivity will produce output currents, ip, where: ip A dT psine dt where A = area of one electroded face; = = rate of change of element temperature with time; and, dt p = pyroelectric coefficient of the piezoelectric material.

Even a 1 degree misorientation e of the polar axis will lead to a pyroelectric sensitivity which is about 2% of the value obtained with an equivalent longitudinal mode device.

Also, in the design shown in figure 2, external loading masses 21, 23 are of necessity required. This can result in a bulky construction. Furthermore the accelerometer 13 will exhibit a sensitivit) to acceleration parallel to the transverse axis x3, which is also undesirable.

The embodiments of the invention described below use a compensating electrode structure which is provided to obviate spurious pyroelectric effects and to diminish sensitivity to acceleration in unwanted directions.

In accordance with the invention thus there is provided a piezoelectric shear mode accelerometer comprising: a plate of piezoelectric material mounted upon a centrally located mechanical support, which plate is poled such that the polar axis thereof lies in the plane of the plate and in a direction orthogonal to the mechanical support; a common electrode disposed upon one principal plane face of the plate; and, a pair of spaced electrodes, disposed upon another opposite principal plane face of the plate, located symmetrically and to each side of the mechanical support.

In the accelerometer defined above, signal is extracted from the pair of electrodes located upon a common principal plane face of the plate. This signal is relatively insensitive to the accumulation of pyroelectric induced charge and is relatively insensitive to acceleration in a direction parallel to the polar axis. In particular, it exhibits a reduced sensitivity to the accuracy with which the polar axis is aligned relative to the principal plane faces of the plate and thus is commensurated with a relaxation in manufacturing tolerance, resulting thus in improvements in yield.

It will be observed that accelerometer sensitivity may be enhanced by increasing inertial mass loading. This latter can be implemented by attaching external inertial masses each side of the mechanical support. Alternatively, and to equivalent effect, the dimensions of the plate may be chosen such that portions of the plate extend beyond that active part of the accelerometer defined by the electrode metallisation.

The plate may be of ceramics piezoelectric material - for example one of the lead zirconate titanate family of ceramics materials. Alternatively, it may be of single crystal piezoelectric material for example one of the lithium niobate, lithium tantalate, barium titanate, lead titanate or lead metaniobate piezoelectric materials.

In the drawings accompanying this specification: Figure 1 is a cross-section view of a known longitudinal mode piezoelectric accelerometer; Figure 2 is a cross-section view of a known shear-mode piezoelectric accelerometer; Figure 3 is a cross-section view showing part of the accelerometer of the preceding figure and is to illustrate distortion occurring under acceleration; Figure 4 is a schematic cross-section of a piezo-electric element of the accelerometer shown in figure 2, showing a misoriented polar axis; Figure 5 is a cross-section view of a shear-mode piezo-electric accelerometer constructed in accordance with the present invention; Figure 6 is a plan-view of the accelerometer shown in the preceding figure; Figure 7 is a circuit diagram of a WET pre-amplifier used in conjunction with the accelerometer of the preceding two figures;; Figure 8 is a cross-section view of the accelerometer shown in preceding figures 5 and 6, as modified by attached weights; and, Figure 9 is a cross-section view of an alternative modification of the accelerometer shown in preceding figures 5 and 6.

So that this invention might be better understood, embodiments thereof will now be described and particular reference will be made to figures 5 to 9 of the drawings. The description that follows is given by way of example only.

The basic structure of this novel accelerometer is shown schematically in figure 5. It consists of a plate 33 of piezoelectric material, cut so that the polar axis 34 is parallel to its plane. The whole of the back plane face is covered with a conductive common electrode 35, which is neither connected to earth nor to a device output, but is left 'floating'. There are two electrodes 37, 39 of equal area on the upper plane face, one 37 earthed as shown and the other 39 connected to give the output signal. The whole piezoelectric plate 33 is fastened symmetrically to a mechanical support 41 as shown (by glueing using, for example, epoxy resin or by soldering if the support 41 is pre-metallised).In this configuration, acceleration in the direction shown 43 will produce equal and opposite shear stresses in the two arms of the piezoelectric plate 33 by self mass loading and hence equal and opposite voltages on the two top electrodes 37, 39. The parametric responses of this accelerometer under an acceleration are given approximately as follows: Capacitance = C = #0 #11 W (L + 1) ; 2t 2 Charge response = RQ = pL2Wa dl5; ; and, 2 Voltage response = Rv = p L t a (1 + t/2L) #11 where p = density of the piezoelectric material; Eli = dielectric constant of the piezoelectric material, parallel to the crystallographic xl axis; and d15 = charge piezoelectric coefficient with shear (for a fall definition of the coefficients Eij, dij, gij, see the Standard referred to above).

The first bending-mode resonant frequency would be at a frequency fO, given approximately by:

where Y = Young's modulus of the piezoelectric material.

It can be seen from figure 5 that small misorientations of the polar axis 34 will lead to equal pyroelectric signals at each top electrode 37, 39 if the temperature of the piezoelectric material is changed Hence, the accelerometer is inherently insensitive to pyroelectric effects. Furthermore, the compensated structure also makes it very insensitive to acceleration parallel to the polar axis 34 or parallel to the long axis of the support 41.

The peak stresses in the piezoelectric element are approximately as follows: Peak shear stress = Tps = 3p La ; 2 (This occurs at points A in figure 5) Peak bending stress = TPB = 3p L2 a; t (This occurs at points B in figure 5).

The properties of several different piezoelectric materials are given in Table 1, together with the appropriate operational parameters, calculated for t = lmm, L = W = 3mm. The PZT group of materials are all piezoelectric ceramics while LiNbO3 and LiTaO3 are single cmTstals, The highest voltage responsivity is exhibited by x-cut LiNbO3 while the highest charge response is exhibited by PZT-SH.

The resonant mode bending frequencies for these accelerometers are all very high (in excess of 30kHz).

TABLE I

Material #11/20 d15 g15 Y p C RQ RV TPE fo (pCN-1) (10-3VmN-1 (1010Nm-2) (103kgm-3) (pF) (pC8n-1) (mVgn-1) (MPa at 100g) (kHz) PZT-4 1475 496 39.4 11.5 6.8 68.5 0.45 6.8 0.18 30 PZT-5H 3130 741 26.8 11.7 6.8 145.5 0.65 4.6 0.18 31 PZT-7H 840 368 49.5 17.5 6.8 39 0.35 8.5 0.18 38 x-cut LiNbO3 84.61 74 99 24.5 4.7 3.85 0.0045 11.7 0.12 53 x-cut LiTmO3 51 26 56 27.5 7.45 2.37 0.025 10.5 0.2 45 The selection of the appropriate material for this accelerometer is determine by the mode of operation and the electronic amplifier which will be used. Piezoelectric accelerometers of the type described here exhibit high output impedance and thus if used in voltage mode need to be interfaced to a high input impedance amplifier, such as a field effect transistor. A simple unity-gain source-follower amplifier circuit is shown in figure 6. C represents the capacitance of the accelerometer element and RG is a gate bias resistor whose value is selected to bias a FET transistor 45 in the onstate by the gate leakage current.For a typical low-noise JFET (such as a Texas Instruments BF800) the gate leakage current is approximately lpA. The value of RG thus needs to be about 3SlOl2Q.

This sets the RGC time constant. For frequencies well above I/RGC, the voltage output VO will be proportional to the acceleration, and very nearly equal to RV above. Consideration of the primary noise sources in the circuit shown in figure 6 permits the calculation of a noise equivalent acceleration (n.e.a).The primary sources of noise at frequency f are: (i) FET Current Noise, giving a noise voltage of Nj = iA/27tf(CA + C); (ii) Dielectric Noise: ND = (4kTc)CtanS)l/2; cD(CA+C) (iii) FET Voltage Noise: eA where o = 2xf; CA = amplifier capacitance; k = Boltzmann's constant; T = absolute temperature; and, tan 6 = dielectric loss tangent of the piezoelectric material.

For a BF800 JFET, iA = 3 X 10-16 AHz-1/2 CA = 1pF eA = (1.2 X 10-16 + ##########)1/2VHz-1/2 Table 2 give the values of 1/RGC, and noise equivalent accelerations for the detector elements listed in Table 1 when used with a BF800 amplifier for signal bandwidths in the ranges 25Hz to lkHz and 0.01Hz to 100Hz. The accelerometers show very acceptable sensitivities, with the best overall performance being given by the PZT-4 piezoelectric ceramic.

TABLE 2

aterial C tan # RV nea (10-6gn) nea (10-3gn) 1/RGC* (pF) (mV gn-1) 25-1000Hz 0.01-100Hz (Hz) PZT-4 68.5 0.004 6.8 130 1.04 0.005 ZT-5H 145.5 0.02 4.6 217 0.83 0.002 ZT-7A 39 0.004 8.5 133 1.45 0.009 iNbO3 3.85 0.001 11.7 259 10.6 0.09 LiTaO3 2.37 0.001 10.5 438 19.2 0.08 *RG = 3 X 1012n Either PZT-4 or PZT-SH should be able to achieve a threshold resolution of Img in this form of circuit in the 0.01Hz to lû0Hz bandwidth, with a sensitivity of +68mV at accelerations of flOg,.

As a further refinement to the above design, the sensitivity can be increased by mass loading the extremities of the accelerometer, either by using small external masses 47, 49 bonded to each extremity or by leaving electrically inert volumes of the plate at each end as shown in the figures 8 and 9 respectively. Either design would require only minor changes in dimensions and would result in little increase in complexity.

Two experimental design devices, based upon the struct=--s shown in figures 5 and 9, have been constructed and assembled onto T05 headers with values of RG = 2 X 1011. The circuit in figure 7 was used for each with no shunt capacitor. In each case the active material was a "hard" piezoelectric ceramic of the type known as PZT4 (a material well-known to those skilled in the art) for which it has been calculated that gls = 11 X 10-3VmST-l. The first device design used dimensions L = t = lmm, W = 2mm. For this device, a response of 380pLV(rms)/g(pk) was measured, with a capacitance of 25pF and a noise at 10Hz of 0.55pVHz-l/2 (with a 1 frequency dependence).

f All parameters are in good agreement with theory. The second design device was of similar design, but incorporated a loading mass of electrically inactive piezoelectric ceramic at each end. Again, good agreement with theory was obtained with a response of 1.2mY (rms)/g(pk) and a 10Hz noise of 0.45VHz-1/2. Both design devices showed a frequency response which was essentially flat from 10Hz (the lowest available measurement frequency) to lkHz, with no sign of any structural resonances, as would be expected from the simple design.

Claims (7)

1. IlWe claim: 1. A piezoelectric shear mode accelerometer comprising: a plate of piezoelectric material mounted upon a centrally located mechanical support, which plate is poled such that the polar axis thereof lies in the plane of the plate and in a direction orthogonal to the mechanical support; a common electrode disposed upon one principal plane face of the plate; and, a pair of spaced electrodes, disposed upon another opposite principal plane face of the plate, located symmetrically and to each side of the mechanical support.
2. 2. An accelerometer, as claimed in claim 1, wherein a loading mass is attached to the plate each side of the mechanical support.
3. 3. An accelerometer, as claimed in either claims 1 or 2, wherein the piezoelectric material of the plate extends outwardly beyond each of the pair of spaced electrodes.
4. 4. An accelerometer, as claimed in any one of the preceding claims, when mounted within a package integrally with a junction field effect transistor and a gate bias resistor included therein.
5. 5. An accelerometer, as claimed in any one of the preceding claims, wherein the plate is of a piezoelectric material selected from the lead zirconate titanate family of piezoelectric ceramics.
6. 6. An accelerometer, as claimed in any one of preceding claims 1 to 4, wherein the plate is of a single crystal piezoelectric material selected from lithium niobate, lithium tantalate, barium titanate, lead titanate, or lead metaniobate.
7. 7. A piezoelectric shear mode accelerometer constructed adapted and arranged to perform substantially as described hereinbefore with reference to and as shown in figures 5 to 9 of the drawings.