Low-Frequency Electrostrictive Ceramic Plate Voltage Sensor

1. Field of the Invention

The present invention relates to a low-frequency voltage sensor and more particularly to a stress-driven low-frequency electrostrictive ceramic plate voltage sensor.

2. Description of the Related Art

Field response materials, e.g., magnetostrictive metals and electrostrictive ceramics, are used in sensors.

The measurement of low-frequency magnetic fields utilizing a magnetoelastic amorphous metal low-frequency magnetic field sensors with a magnetostrictive amorphous metal ribbon has been discussed by Mermelstein in the paper entitled A Magnetoelastic Metallic Glass Low-Frequency Magnetometer, IEEE Trans. Magn. Vol. 28, No. 1, pp. 36-56, January 1992. This technique is based upon the magnetoelastic interaction between the magnetization and strain in field-annealed amorphous metal ribbons. The magnetoelastic magnetometer has the ability to measure the low-frequency magnetic field and magnetic field gradiants in the presence of other low-frequency perturbations such as temperature and other stress fluctuations.

The principle of operation of the magnetometer is based upon the field-dependent coupling between the magnetization and strain modes in the magnetostrictive amorphous material. It is constructed by interfacing a resonating piezoelectric plate to a field-annealed magnetostrictive amorphous ribbon with a viscous fluid. The piezoelectric plate serves to generate a well-defined stress wave in the ribbon at acoustic frequencies where the viscous fluid transfers the strain in the ceramic to the ribbon.

Also, electrostrictive ceramics have been demonstrated for use in the detection of low-frequency voltages in fiber optic interferometers, as discussed in the paper Fiber-optic dc and low-frequency electric field sensor, Vohra et al., Opt. Ltrs., Vol. 16 No. 18, pp. 1445-1447, September 1991. However, they require the use of fiber-optic interferometers to measure low-frequency voltages. Presently, this is the only technology available to measure low-frequency voltages using electrostrictive materials.

The object of this invention is to provide a device capable of accurately measuring a low-frequency voltage at a lower cost than currently utilized methods.

A further object is to provide a device for filtering out any extraneous noise present on a measured low-frequency voltage.

These and other objects are achieved by a stress-driven electrostrictive ceramic low-voltage plate voltage sensor that is based on the principle of field-coupling between the strain and polarization in the ceramic material of the sensor. Application of a dithering or driving stress in a piezoelectric tube generates an oscillating polarization in an affixed electrostrictive ceramic wafer material whose amplitude is proportional to an applied low-frequency voltage. The oscillating polarization generates an amplitude modulated displacement current which is measured as a voltage drop across a resistor of known value in an electrical circuit.

FIG. 1 is a schematic of the electrostrictive ceramic sensor.

FIG. 2 is a schematic of the simple electrical circuit for measuring the output of the electrostrictive sensor.

FIG. 3 is a plot of the rms voltage drop across a resistor, R, as a function of bias voltage in the electrical circuit for measuring the output of the electrostrictive sensor.

FIG. 4 is a schematic of a common mode rejection embodiment for noise reduction utilizing the electrostrictive ceramic sensor.

The principle of operation for the strain-sensitive and current-sensitive sensors is based on the field-dependent coupling between polarization and strain modes in the ceramic plate. The application of this principle, in the embodiment described below, may be used in noise-reduction devices or wherever preamplification and voltage detection is required, and for investigation of the electrostrictive properties of other ceramic materials.

With reference now to the drawings in which like numerals represent like elements throughout the views.

The preferred embodiment of a low-frequency electrostrictive ceramic plate voltage sensor 10 is shown in FIG. 1. The electrostrictive ceramic wafer or sensor element 14 may be made of any ferroelectric relaxor wafer material, such as, a barium doped PZT ceramic (lead-zirconium-titanate) or any other electrostrictive material having an electrostrictive coefficient, Q, of between 0.05 farads.sup.2 /V.sup.2 and 0.3 farads.sup.2 /V.sup.2, preferably 0.3 farads.sup.2 /V.sup.2 or as large as possible. A ferroelectric relaxor is any material which can be deformed by an induced electric field thereby producing a voltage, such materials are well known to individuals practicing the art. The electrostrictive ceramic wafer 14, in the preferred embodiment, for example, has a dimension of 7.5 mm.times.10 mm.times.1 mm, with a film of conductive material forming electrodes or electrical connections 22 and 23, such as, gold (Au), deposited on the two facets 17 and 18 of the wafer 14. The wafer 14 is affixed to circular tube 12, having an inner and outer surface, made of a piezoelectric material, such as, PZT 5H, or equivalent, manufactured by Aura Ceramics of New Hope, Minn. The piezoelectric tube 12 is, for example, approximately 50 mm long with an outer diameter of approximately 25.4 mm and a thickness of one mm, however, it may be of any desired interior and exterior dimension or length. The preferred means of affixing the wafer 14 to the piezoelectric tube 12 is a non-conducting epoxy, such as, any "five-minute", or equivalent, commercial grade, non-conducting epoxy, which is well known to those practicing in the art. In the operation of the invention, an oscillating voltage, dither or drive voltage 16 is applied to the opposing end of the piezoelectric tube 12 from the wafer 14, through an electrical connection 19. However, the oscillating stress generating mechanism (piezo-electric driver) does not have to have cylindrical geometry it can be rectangular, oval or any other two wall shape.

The source of the oscillating voltage may be a signal generator, such as a HP 3526A Frequency Synthesizer, manufactured by Hewlett-Packard Co. of Santa Clara, Calif. Application of the oscillating voltage causes the dimensions of the outside diameter of the piezoelectric tube 12 to change in dimension during oscillation. Such changes in dimensions are transferred to the electrostrictive ceramic wafer 14, thereby generating a dynamic longitudinal stress, .sigma.(.omega.). Eq. (1) shows that an oscillating longitudinal stress in the electrostrictive ceramic wafer 14 at a frequency, .omega., generates a polarization, P(.omega.), normal to the wafer 14 which is given by the formula P(.omega.)=-[a.chi./C].sigma.(.omega.). ##EQU1## where a=a.sub.O +2CQ.chi.V.sup.s >/h is an effective field-dependent piezoelectric constant, P is the polarization, E is the electric field, .sigma. is the stress, e is the strain, C is the longitudinal elastic modulus, <V.sub.s > is the low-frequency signal voltage which is assumed to be very small, Q is the electrostriction coefficient, and h is the plate thickness. The off-diagonal matrix elements in Eq. (1) determine the polarization response to stress driving the strain response to voltage driving.

A phenomenological potential energy density function for the electrostrictive ceramic wafer 14 is given by the equation:

where .chi. is the dielectric susceptibility, and a.sub.O is the piezoelectric coefficient. The electric field, E, and the stress, .sigma., in the electrostrictive ceramic 14 are given by the partial derivatives of U with respect to the polarization, P, and strain, e. This energy density function represents a spatial average for a sample that is predominately electrostrictive with some regions exhibiting piezoelectricity. Therefore, the piezoelectric coupling is assumed to be weak. Eq. (2) will lead to the susceptibility for the coupled polarization and strain as shown in Eq. (1).

The stress applied by the piezoelectric tube 12 may be approximated by the formula .sigma.(.omega.)=C'dV.sub.d (.omega.)/h', where C' is the Young's modulus of the piezoelectric tube 12, d is the piezoelectric coefficient, h' is the piezoelectric tube 12 thickness and V.sub.d (.omega.) is the dither voltage 16. The rms stress, .sigma..sub.rms, for a device utilizing an electrostrictive ceramic wafer 14 made of barium-lead-zirconium-titanate (PZT) material is estimated to be 5.1.times.10.sup.4 N/m.sup.2 for the parameter values listed in Table 1, where h' is the wall thickness of the piezoelectric tube 12, h, the thickness of the barium-PZT ceramic material, .epsilon..sub.O the permativity of free space, d, the piezoelectric coefficient, C the longitudinal elastic modulus of barium-PZT ceramic wafer 14, .omega..sub.O /2.pi. the frequency of the dithering voltage 16, C' the Young's modulus of the piezoelectric tube 12, R the resistance of a resistor, R,(described below), and V.sub.d the drive voltage 16 applied by dithering to the piezoelectric tube 12. The fluctuating polarization in the electrostrictive ceramic wafer 14 generates a displacement current, i(.omega.)=j.omega.AP(.omega.), where A is the electrostrictive ceramic wafer 14 surface area, P is polarization and j=.sqroot.-1. This displacement current is measured with the circuit 20 shown in FIG. 2.

The output of the electrostrictive ceramic wafer 14 is measured by the voltage drop, V(.omega.), across a resistor, R, in the circuit shown in FIG. 2, as determined by the equation ##EQU2## where the piezoelectric offset voltage is V.sub.O =a.sub.O h/2CQ.chi.. A lock-in amplifier 25, such as, a Model 5210, manufactured by EG&G of Princeton, N.J., measures the rms carrier amplitude of the voltage drop across resistor R, which is an amplitude modulated carrier whose displaced amplitude is proportional to the signal voltage, <V.sub.s >. Further, the lock-in amplifier 25 is used to demodulate the signal voltage,<V.sub.s >, producing the output 27 given by the formula V.sub.rms =.eta..vertline.<V.sub.s >+V.sub.O .vertline., where the sensitivity is defined by as .eta.=2.omega.AR.chi..sup.2 Q.sigma..sub.rms /h.

FIG. 3 shows a typical plot of the signal voltage as a function of applied dc voltage 26 for the low-frequency electrostrictive ceramic plate voltage sensor 10 with the parameters as shown in Table 1. The dashed line is a nonlinear least squares fit to the trial function V.sub.rms =.eta..vertline.<V.sub.s >-V.sub.O .vertline., where .eta. is the sensitivity and V.sub.O is the offset field attributed to the piezoelectric effect. The fitting routine yields a .eta.=0.68 mV/V and v.sub.O =2.6 V. It is seen that the signal voltage exhibits good linearity in the indicated voltage range. The slope of the plot yields a sensitivity of 0.68 mV/V in good agreement with the calculated value of 0.51 mV/V based on the material and device parameters listed in Table 1. The offset voltage is found to be 2.6 V, which yields a piezoelectric coefficient, a.sub.O, of 3.4.times.10.sup.6 V/m.

The minimum detectable field of the low-frequency electrostrictive ceramic plate voltage sensor 10 is determined by measuring the signal voltage power spectral density (PSD) corresponding to the amplitude output of the lock-in amplifier 25. The signal voltage PSD is measured using a dynamic signal analyzer, such as, a HP 3562A, manufactured by Hewlett-Packard Co. of Santa Clara, Calif., in the 0 to 10 Hz bandwidth. The voltage equivalent noise is given by the formula V.sub.s,rms =V.sub.rms /.eta.G, where G=10.sup.3 is the lock-in amplifier 25 gain. This expression is evaluated to yield a minimum detectable field of 1.4 mV/.sqroot.Hz for .eta.=0.68 mV/V under the aforementioned conditions shown in Table 1.

A typical application of the low-frequency electrostrictive ceramic plate voltage sensor 10 is shown in FIG. 4. A noise reduction circuit 30 to enhance the signal-to-noise ratio of a detector (not shown) applies the low-frequency electrostrictive ceramic plate voltage sensor 10 to commonly known rejection techniques to provide a relatively noise-free output voltage.

A voltage, V.sub.s, to be measured is input a measurement module 32 comprised of a low-frequency electrostrictive ceramic plate voltage sensor 20. The voltage, V.sub.s, which normally contains two elements of noise--thermal and electromagnetic noise--is measured by an electrostrictive ceramic sensor circuit 20, as described above, and applied to a first differential amplifier 52 operating as a subtractor.

As the temperature of the environment changes, a thermal noise is present in the voltage source, V.sub.s, and is measured in a reference module 34. This is accomplished by applying a known reference voltage 26 to a electrostrictive ceramic plate voltage sensor circuit 20, as described above. The measurement of the reference voltage 26 containing the quantum of the thermal noise present is then applied to a second differential amplifier.

Any electromagnetic interference (EMI) present upon the input voltage, V.sub.s, is detected by a capacitor 42 in an electromagnetic interference detector module 36. This capacitor 42 senses any electromagnetic field present from an external electromagnetic source 46 and generates a voltage signal 44. This voltage signal 44 is applied to the second differential amplifier 48, which acts as an adder, to combine the EMI noise with the voltage generated by the thermal noise in the reference module 34. If there is no EMI present this figure will be zero.

The output 64 of the second differential amplifier 48 is applied to the first differential amplifier 62, where the combined reference voltage/EMI module voltage 64, is subtracted from the measured input voltage 58 and a filtered voltage is provided as an output 54.

The noise floor of the electrostrictive low-frequency ceramic plate voltage sensor 10 is determined by the large temperature sensitivity of the residual piezoelectric coefficient a.sub.O particular to the composition used and a careful selection of the composition can remove this effect resulting in an accompanying reduction of noise floor. The sensor 10 is operated at a relatively low driving stress, .sigma..sub.rms, which is limited by the Joule heating of the piezoelectric tube. Application of large driving stresses will increase the sensitivity, thereby reducing the minimum detectable voltage. Reduction of the plate thickness of the ceramic wafer 14 will further increase the sensitivity (see Eq. (3)), thereby improving the sensitivity and minimum detectable voltage. This is accomplished at a lower cost than the existing methods utilizing fiber-optic interferometers.

This specification sets forth the preferred embodiments of the low-frequency electrostrictive ceramic plate voltage sensor. For devices other than that specified in the preferred embodiment, other individuals skilled in the art may be required to engineer the specific values of the components, such as, the differential amplifier to minimize the noise, the dithering voltage, etc., to meet the requirements of the voltage range sought to be detected in their particular application. There are numerous trade-offs between the dimensions of the piezoelectric tube, the dithering voltage, the resonance of the piezoelectric tube, etc., which are generated by the geometry of the piezoelectric tube; and the composition of the electrostrictive ceramic wafer that can be made. The main consideration in the construction of this invention is the maximization of the sensitivity of the device. It is also realized that individuals skilled in the art may visualize many other methods for achieving the same result, but the scope of the invention is as set forth in the claims.