GB2224599A - Piezoelectric shear mode accelerometer - Google Patents

Abstract

An accelerometer 49 consists of a plate 37 of piezoelectric material mechanically supported 45 at one end and bearing a pair of spaced electrodes 51, 53 on one of its two principal plane faces. The piezoelectric material is poled such that the polar axis 43 lies orthogonal to the plane of the plate 37. The arrangement of the electrodes 51, 53 and the polar axis 43 is such that this accelerometer 49 is relatively insensitive to the accuracy with which the polar axis 43 is aligned and is relatively insensitive to the generation of any pyroelectric charge resulting from a change in temperature. Sensitivity may be enhanced by increasing inertial mass load - for example, by attaching a loading mass (55, Fig 7) at or near to the free end. In a preferred arrangement, loading plates (59, 61, Fig 8) are mounted, one on each principal plane face. Those latter increase inertial mass loading and reinforce the plate structure to resist bending. The plate may be supported at both ends, to facilitate the wire bonding of electrodes. <IMAGE>

Description

A PIEZOELECTRIC SHEAR MODE ACCELEROIER 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 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 thus 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 Ss 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 doss. Provided that the electroded faces 21, 23 are accurately orthogonal to xl, 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 33 is necessary. If there is a small misalignment E, as shown in figure 4, then there is a component of the polar axis .33 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, where: ip = A a p sin E 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 pyroelecmc 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 sensitivity to acceleration parallel to the transverse axis X3, which is also undesirable.

The embodiments of the invention described below use an 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, which material is poled with polar axis orthogonal to the plane of the plate, which plate is supported at at least one end and bears upon one of its two principal plane- faces a pair of spaced electrodes.

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

The plate of piezoelectric material may be supported at one end only, or it may be supported at both ends. The latter form of support is particularly convenient when mechanical supports are located on the underside of the plate. This can provide support during wire bonding when the electrodes are provided with bonding pads located over (i.e. directly opposite to) the underlying mechanical supports.

It will be observed that accelerometer sensitivity may be enhanced by increasing inertial mass loading.

The inertial mass loading may be increased by attaching a loading mass to the plate at a position remote from any mechanical support. In the case where the plate is supported at one end only, it is optimal to locate the loading mass at the free end of the plate. In the alternative case where the plate is supported at both ends it is optimal to locate the loading mass at the centre of the plate. One or two loading plates may be attached to respective principal plane faces of the plate of piezoelectric material to increase the inertial mass loading. This has the advantage that the structural rigidit of the structure is also increased, resulting in a more robust construction.

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 schematic cross-section view of a model shearmode piezo-electric accelerometer provided to illustrate the operative principle of the present invention; Figure 6 is a perspective view of a practical shear-mode piezoelectric accelerometer constructed in accordance with the present invention;; Figure 7 is a cross-section view of the accelerometer shown in the preceding figure as modified by the attachment of an external loading mass; Figure 8 is a cross-section view of the accelerometer shorn in figure 6 as modified by the provision of reinforcing mass-loading plates; and, Figure 9 is a cross-section view of the accelerometer shorn in figure 6 as modified by the provision of an additional mechanical support.

Embodiments of this invention will now be described and particular reference will be made to figures 5 to 8 of the drawings.

The description that follows is given by way of example only.

A shear mode piezoelectric accelerometer 35 is shown schematically in Figure 5. This accelerometer 35 comprises a plate 37 of piezoelectric material bearing electrodes 39 and 41, one on each end face. The plate 37 is poled such that its polar axis 43 is in the direction orthogonal to the plane of the plate 37. At one end it is bonded to the side of a mechanical support 45. Here an electrical response is produced through a piezoelectric coefficient, such as d15 or d24, that corresponds to a polarisation generated in response to a shear stress. As the piezoelectric plate 37 is supported at one end, it will distort under its own mass when accelerated in the directions indicated 47 in a combination of bending and shearing. It is the latter that generates the required electrical signal.For materials with appropriate crystallographic symmetry, including the widely available poled ferroelectrics, the electrodes 39, 41 in this position do not couple to any pyroelectric coefficient, so the accelerometer 35 is insensitive to temperature fluctuations.

The accelerometer of Figure 5 is difficult to fabricate, especially in miniature devices, as the electrodes 39, 41 are deposited on the ends of the piezoelectric plate. Figure 6 illustrates a practical accelerometer where instead electrodes 51, 53 are located on a principal plane face of the plate 37. This construction is suitable for batch fabrication, for the required electrode pattern can be deposited repeatedly across a wafer of piezoelectric material of required thickness, before the wafer is divided into individual plates 37.

Despite the altered position of the electrodes 51, 53, the accelerometers 35, 49 of Figures 5 and 6 are very similar both mechanically and electrically. The volume of piezoelectric material between the inner edges of the electrodes 51, 53 is active, producing an electric polarisation, in response to the indicated acceleration directions 47, via the same piezoelectric shear coefficient. This polarisation is detected as a voltage difference between the electrodes 51, 53. The symmetry of this structure ensures that there is no pyroelectric response to uniform temperature fluctuations.

Under an acceleration g, the mean shear strain T5, in the active volume is given as Ts = pg(a + c/2) for the dimensions referred to on Figure 6, with a piezoelectric material of density p and pemitivity tensor Ell. Provided the dimensions a and b are significantly larger than the thickness h the device capacitance C is given by: C # #11 hw/c and the charge Q generated by the above stress is given by: Q = dihw.Ts.

The resultant voltage signal V is given by: V = Q/C # (d15/#11) #gc(a + c/2) For low frequency operation there is a trade off between size and both charge and voltage response. For some applications, the operating bandwidth is fixed. The bandwidth of this accelerometer 49 is restricted to frequencies below the frequency f0, of the first bending mode resonance. By modelling the structure as a thin cantilever beam clamped at one end, f0 can be estimated as: for O.O67.(h/(c + a)2).(Yip)112; where Y is the Young's modulus of the piezoelectric, provide that the width w is sufficiently less than the total unsupported length (c + a).

The voltage signal V can be rewritten as: V = (disleii).(pg/2).((c + a)2 - a2)* i.e. V = (d15/#11). (#g/2).(0.067(h/f0)(Y/#)1/2 a2), Assuming a - 2h, the thickness can be selected to maximise the response: h = (0.0084/fo).(ylp)ll2 and correspondingly V - 1.4 x 10'4.(dls/E11).(Yg/f02) This analysis shows, that by reducing all the dimensions of the accelerometer 49 by the same proportion, it is possible to gain a very wide bandwidth at the cost of reduced response.

Response to accelerations perpendicular to the above directions 47 couple via piezoelectric coefficients dll and dl6. For many piezoelectric materials, both of these coefficients are necessarily zero, resulting in a very low cross-axis sensitivity.

There are a number of modifications that may be made to the basic structure of figure 6. The addition of an inertial mass 55 to any face of the free end of the accelerometer 49, e.g. Figure 7 will boost the shear stress. This has similar effects to increasing the plate dimensions, in that it increases the response and lowers the bandwidth, but results in a more compact structure. Where a less compact structure can be tolerated, the plate 37 may be dimensioned such that a portion 57 of the piezoelectric material extends outwardly beyond the active volume of the plate 37 defined by the electrodes 51 and 53.

The inertial mass can be increased by providing two electrically-insulated plates 59, 61 preferably of similar thickness one on each plane face of the piezoelectric plate 37 (Figure 8). Many piezoelectric materials either are fragile or show degraded electrical properties under tensile stress. As this accelerometer 49 will bend as well as shear under acceleration, it is vulnerable under very high levels of acceleration, as the bending generates tensile stresses.

However, the peak tensile stress occurs at one of the outer faces, while the shear stress is largest at the centre. The two plates 59, 61 therefore act both as inertial mass and as strengthening reinforcement against failure via bending, without degrading the shear response.

The structures of Figures 6 to 8 all require a contact to be made to the unsupported, i.e. free, end of the piezoelectric plate 37. The structure of Figure 9 avoids this by using two mechanical supports 45, 59 with wire bond connections made to electrode bonding pads 51, 61 located immediately above each support 45, 59. This is particularly advantageous for wirebonding small devices. Each of the two forms of supplementary inertial mass 55 and 59, 61 described above can be used in conjunction with this modified structure.

The properties of several different piezoelectric materials are given in Table 1: TABLE I

Material #11/#0 d15 Y # F1 F2 RV fo (pCN-1) (1010Nm-2) (103kgm-3) (V82m-3) (109VM-1) (mVgn-1) (kHz) PZT-4 1475 496 11.5 6.8 258 4.37 10.1 15 PZT-5H 3130 741 11.7 6.8 182 3.13 7.1 15 PZT-7H 840 368 17.5 6.8 336 8.66 13.2 19 x-cut LiNbO3 84.61 74 24.5 4.7 464 24.2 18.2 27 x-cut Li-TaO3 51 26 27.6 7.45 429 15.8 16.8 23 The materials can be compared using two figures-of-merit:: F1 =d1 & 11 and F2 = d15Y/e11 F1 is proportional to the response of a structure similar to figure 6, assuming the dimensions are fixed. F2 is proportional to the response of the same structure, for assuming the frequency, fO is fixed. The table includes the appropriate operational parameters, calculated for h f 0.5mm, a = imam, b = c = lmm.

The PZT group of materials are all piezoelectric ceramics while LiNbO3 and LiTaO3 are single crystals. X-cut LiNbO3 shows the highest figures-of-merit, both F1 and F2, and correspondingly the strongest calculated voltage response, Rv. Its permittivity Ell is much lower than that of the PZT ceramics, so for the smallest devices its capacitance will be very low. For such devices the PZT series of materials allow a compromise between increasing capacitance and decreasing voltage response, in the sequence PZT-7H, PZT-4, PZT-5H.

Claims (9)

1. What I/We claim is: 1. A piezoelectric shear mode accelerometer comprising: a plate of piezoelectric material, which material is poled with polar axis orthogonal to the plane of the plate, which plate is supported at at least one end and bears upon one of its two principal plane faces a pair of spaced electrodes.
2. 2. An accelerometer, as claimed in claim 1, including a single mechanical support located at one end of the plate.
3. 3. An accelerometer, as claimed in claim 2 wherein that one of the pair of electrodes that lies more remote from the single mechanical support is positioned at a location spaced inwardly from the other and free end of the plate.
4. 4. An accelerometer, as claimed in claim 1, including a pair of mechanical supports, which mechanical supports are located one at each end of the plate.
5. 5. An accelerometer, as claimed in claim 4, wherein the mechanical supports are located adjacent to the other one of the two principal plane faces of the plate, and, the electrodes include bonding pads that are located in positions directly opposite to the mechanical supports.
6. 6. An accelerometer, as claimed in any one of the preceding claims, including an external loading mass attached to the plate at a position remote from any mechanical support.
7. 7. An accelerometer, as claimed in any one of the preceding claims, including a mass loading plate bonded to a principal plane face of the piezoelectric plate.
8. 8. An accelerometer, as claimed in claim 7, including a pair of mass loading plates, each plate being bonded to a respective one of the two principal plane faces of the piezoelectric plate.
9. 9. A piezoelectric shear mode accelerometer constructed, adapted and arrange to operate substantially as described hereinbefore with reference to and as shown in any one of the figures 6 to 9 of the drawings.