Dynamic Polymer Pressure Transducer With Temperature Compensation

The present invention relates generally to the broad area of pressure probes and, more specifically, to a polymer pressure transducer used to obtain accurate dynamic pressure data in the presence of a changing thermal environment.

The task of obtaining remote pressure determinations without the influence of changing thermal conditions has been alleviated to some degree by prior art techniques. The extent of these prior art techniques is given by the following patents:

U.S. Pat. No. 3,060,748 issued to Schwartz on Oct 30, 1962;

U.S. Pat. No. 3,894,243 issued to Edelman et al on July 2, 1975;

U.S. Pat. No. 3,940,974 issued to Taylor on March 2, 1976;

U.S. Pat. No. 3,970,862 issued to Edelman et al on July 20, 1976; and

U.S. Pat. No. 4,366,714 issued to Adorni on January 4, 1983.

The Taylor device is a temperature sensor which, like the present invention, is formed from polyvinylidene fluoride (PVDF) having both pyroelectric and piezoelectric properties. The two Edelman et al patents show PVDF polymer film transducer sandwich constructions. Adorni discloses a pressure/temperature probe with separate pressure and temperature transducers connected in series to permit temperature correction. Finally, Schwartz shows an accelerometer with temperature correction features. In view of the foregoing discussion, it is apparent that there currently exists the need for a polymer pressure transducer with thermocouple construction that obtains accurate dynamic pressure data in the presence of a changing thermal environment. The present invention is directed towards satisfying that need.

This invention provides a means of obtaining accurate dynamic pressure data in a changing thermal environment through the use of a pressure gage formed from polvinylidene fluoride (PVDF) polymer material. The temperature compensation pressure gage has three major elements:

an active PVDF transducer which obtains remote pressure readings which are uncorrected for thermal effects;

a thermocouple having a short rise time allowing an output thermal signal which dynamically responds to changing thermal conditions; and,

a compensation amplifier circuit receiving the uncorrected pressure readings and the dynamic thermal signal and producing an output signal representing accurate pressure data which is corrected for changing thermal conditions.

It is a principal object of the invention to provide a new and improved means of obtaining accurate pressure data.

It is another object of the invention to provide a means for automatically compensating for changing thermal conditions.

These, together with other objects features and advantages of the invention will become more readily apparent from the following detailed description when taken in conjunction with the accompanying drawing wherein like elements are given like reference numerals throughout.

FIG. 1 is an illustration of the PVDF transducer with the thermocouple; and

FIG. 2 is a block diagram of the temperature compensation circuit as connected to the PVDF transducer and the thermocouple.

This invention provides a means of obtaining accurate dynamic pressure data in a changing thermal environment through the use of a pressure gage formed from polyvinylidene fluoride (PVDF) polymer material. The temperature compensation pressure gage has three major elements:

an active PVDF transducer which obtains remote pressure readings which are uncorrected for thermal effects;

a thermocouple having a short rise time allowing an output thermal signal which dynamically responds to changing thermal conditions; and

a compensation amplifier circuit receiving the uncorrected pressure readings and the dynamic thermal signal and producing an output signal representing accurate pressure data which is corrected for changing thermal conditions.

The PVDF transducer with thermocouple is shown in FIG. 1. It is made from four 12 .mu.m sheets of PVDF 101-104 which have been laminated together using epoxy. The inner two sheets 102 and 103 contain active areas 105 and 106 on which aluminum electrodes have been deposited. The active area can vary in size from 1 mm to several centimeters in diameter. The outer two layers 101 and 104 serve as protection for the inner two so that the gage can be used in environments requiring mechanical ruggedness. A copper-constantan thermocouple junction 107, made with 75 .mu.m (3 mil) wire, is placed between the inner two sheets and within 2 mm of the active transducer area. When laminated together, the thickness of the tranducer is approximatly 100 .mu.m (0.1 mm).

The four sheets 101-104 of the pressure gage illustrated in FIG. 1 are composed of polyvinylidene fluoride (PVDF). It is available as thin sheets or films which are normally inactive and becomes piezoelectrically and pyroelectrically active by poling a region of the material with a large electric field, resulting in its transducer behavior. Therefore, prior to lamination the electroded regions 105 and 106 are made piezoelectrically active by poling them at room temperature with an electric field of 2 megavolts/cm (Mv/cm). The active areas are then laminated face-to-face so that the polarization vectors in each element point in opposite directions. In this bilaminate pattern, the ground electrodes are on the exterior surface and the inner electrodes form the high potential. One of the advantages of this design is that signals generated in the two elements by bending are opposite in polarity and add to zero.

In laboratory shock tube experiments, it has been shown that PVDF can be used as a detector up to pressures 2.5.times.10.sup.9 newtons/m.sup.2 (Pa) (3.6.times.10.sup.5 psi) There appears to be no inherent materials shortcoming which would inhibit the use of PVDF at high pressures.

Since the poled material will respond to both temperature and pressure changes, an uncompensated transducer can be used to measure pressure only under isothermal conditions. In many situations pressure waves are accompanied by adiabatic thermal pulses or thermal energy from another source. In order to measure pressure accurately, it is necessary to correct the electrical signal from the transducer in accordance with the thermal energy which it has absorbed. The measurement system, which is described here, has been developed for the purpose of obtaining accurate dynamic pressure data in the presence of a changing thermal environment. The method of temperature compensation is: (a) to measure the temperature change of the PVDF transducer using a thermocouple; (b) to amplify the thermocouple voltage to equal that generated by the pyroelectric response of the gage; and (c) to add transducer voltage to the amplified thermocouple voltage yielding a corrected gage voltage. This corrected output is solely proportional to the pressure which is applied to the active area of the transducer.

FIG. 2 is an illustration of the temperature compensation method to which the transducer and thermocouple is connected. Temperature compensation is accomplished through the use of five amplifier circuits consisting of: amplifiers A.sub.1, A.sub.2 and A.sub.3, operational amplifiers A.sub.4 and A.sub.5, ten resistors R.sub.2 -R.sub.9, R.sub.in and R.sub.f, and four capacitors C.sub.f, C.sub.2, C.sub.4 and C.sub.5.

In FIG. 2 the PVDF transducer 200 is grounded on the side of negative polarization so that the output voltage of the charge amplifier is negative when a positive pressure is applied, i.e. e.sub.t =-Q/C.sub.f where Q is the charge generated by a positive pressure change P. Since the PVDF transducer 200 is a charge generating device, its signal is converted to a voltage via a feedback capacitor C.sub.f, 201 in a charge amplifier. When the transducer 200 responds to both temperature and pressure simultaneously, the transducer charge, q.sub.t =q.sub.p -q.sub.th, where q.sub.p is the charge proportional to pressure and q.sub.th is the charge proportional to temperature. These charges combine out of phase because a positive pressure and a positive temperature will generate charges of opposite sign. The feedback capacitor 201 converts these charges to corresponding voltages, e.sub.t =q.sub.p /C.sub.f -q.sub.th /C.sub.f =e.sub.p -e.sub.th where e.sub.th is the voltage proportional to the temperature change .DELTA.T, e.sub.p is a voltage proportional to the pressure change .DELTA. P, and e.sub.t is the output (transducer)voltage from the charge amplifier. In terms of pyroelectric and hydrostatic piezoelectric coefficients .lambda. and d.sub.h

where A.sub.e is the area of the electrodes. Equations (1) and (2) define the coefficients .lambda. and d.sub.h.

The time constants, R.sub.f C.sub.f and R.sub.2 C.sub.2, determine the high and low frequency 3 db points and are set at 220 ms and 32 .mu.s respectively; this corresponds to a frequency range from 1 Hz to 10.sup.4 Hz. The amplifier A.sub.2 is a voltage follower with a gain of 1 which serves as a buffer.

Amplifiers A.sub.3 and A.sub.4 determine the amplification factor A.sub.f for the thermocouple signal. The exact value of A.sub.f which is needed for a particular transducer can be obtained by adjusting the gain of A.sub.4. The value of the input amplification, A.sub.3, is set at 400. High and low frequency 3 db points are established using time constants R.sub.4 C.sub.4 and R.sub.5 C.sub.5 where R.sub.4 C.sub.4 =R.sub.2 C.sub.2 and R.sub.5 C.sub.5 =R.sub.f C.sub.f so that the phase shifts for the transducer and thermocouple amplifiers are the same.

The thermocouple junction, which is positioned close to the active area of the transducer, provides a voltage, V.sub.th =K T where K is the thermocouple constant. V.sub.th is amplified to equal e.sub.th by an amplification factor, A.sub.f, so that e.sub.th =A.sub.f V.sub.th. When e.sub.t and A.sub.f V.sub.th are added, we have

and the compensation is achieved.

A.sub.f can be obtained emperically by subjecting the transducer to a temperature change with no accompanying pressure change. In this case, the gage output should be zero. In an experiment, the transducer was immersed in hot or cold water and the value A.sub.f adjusted until the addition, A.sub.f V.sub.th +e.sub.t, is zero.

A calculation of A.sub.f can be carried out by noting that ##EQU1## Substituting T=V.sub.th /K into equation (4) ##EQU2##

The bracketed term is unitless and can be considered to be an amplification factor which, in practice, is attained through amplifiers A.sub.3 and A.sub.4. For typical values of: .lambda.=4 nC/cm .degree.C, A=2 cm.sup.2, C.sub.f =10,000 pf, and K=41 .mu.V/.degree.C (copper-constantan), ##EQU3## The exact value of amplification which is needed for the particular transducer in use can be obtained by adjusting the gain of A.sub.4.

Since positive .DELTA.P will contract the volume of the transducer and positive .DELTA.T will expand the volume, these two excitations will produce opposite electrical charge responses in the transducer. Therefore the corrected signal e.sub.p is obtained by adding e.sub.I and e.sub.th. This is done in the adder A.sub.5. The output of A.sub.5 is the reverse polarity of the input and will give a positive voltage for a positive .DELTA.P applied to the transducer.

The compensation circuit also contains output connections so that the thermocouple signal and the uncorrected transducer signal can be monitored. By inspecting the uncorrected signal, it is possible to obtain information about the pressure pulse at frequencies higher than 10.sup.4 Hz. In the compensating circuit, low noise operational amplifiers are used. This is particularly necessary at the high gain input amplifier A.sub.3 where the noise was measured to be 1.5 .mu.v rms for a bandwidth of 10.sup.4 Hz. With noise at this level it is possible to achieve a sensitivity of .DELTA.T =0.1 .degree.C.

The following components were used in the temperature compensation circuit shown in FIG. 2:

Amplifiers A.sub.1, A.sub.2, A.sub.3, A.sub.4, and A.sub.5 : operational amplifier Resistors and capacitors:

R.sub.in --5 ohms

R.sub.f --22.times.10.sup.6 ohms

R.sub.2 --4.7.times.10.sup.3 ohms

R.sub.3 --560 ohms

R.sub.4 --2.2.times.10.sup.5 ohms

R.sub.5 --22.times.10.sup.3 ohms

R.sub.6 --1.times.10.sup.6 ohm variable

R.sub.7 --5.times.10.sup.3 ohms

R.sub.8 --5.times.10.sup.3 ohms

R.sub.9 --5.times.10.sup.3 ohms variable

C.sub.f --10.sup.4 pf

C.sub.2 --6800 pf

C.sub.4 --150 pf

C.sub.5 --10 .mu.f.

Thermocouple: Copper-Constantan.

The configuration of the FIG. 2 is used because this arrangement will yield nearly coincident thermal response times for both the transducer charge, q(t), and the thermocouple voltage, V(t). If we consider thermal energy associated with a pressure pulse traversing across the thickness of the PVDF film from one side to the other, the conduction of heat can be described approximately by a solution to the heat flow equation in one dimension. The responses q(t) and V(t) will not be exactly the same, however, because V(t) changes directly with T(t), the temperature change, but q(t) follows the integral of temperature over the thickness of the film. Solutions to the heat equation are that both T(t) (or V(t)) and q(t) are expressed as Fourier series both of which contain the same dominant thermal time constant, ##EQU4## where l is the thickness of the transducer and k is its thermal diffusivity. Since the time constant varies with l.sup.2, it is important that thickness of the transducer be the same at the region of piezo-sensitivity and at the location of the thermocouple.

The differences in V(t) and q(t) appear in the coefficients of the Fourier series and in the constant term, but these differences are minimized in the symmetric configuration. For a transducer thickness of 2 mil (50 micrometers) with thermocouple and piezo-sensitive region symmetrically positioned in the center, calculations show that V(t) lags q(t) by 0.4 milliseconds (ms) and that the overall rise time is approximately 9 ms. It should be emphasized that this response time corresponds to the thermal response of the gage and is not indicative of its response to pressure. Response to pressure as fast as 50 nanoseconds (ns) has been observed in shock tests.

The inherent rise time of the thermocouple, independent of its surroundings, was measured using a light flash to deposit thermal energy at the thermocouple junction. The thermocouple signal is not determined by the thermal conductivity of the medium surrounding the junction, but rather by the geometry and electrical characteristics of the junction itself. Using a light pulse with a 20 .mu.s rise time, it was observed that the rise time of the thermocouple was 50 .mu.s. Thus, the thermocouple should respond satisfactorily to a thermal pulse with a rise time longer than 1 ms. In practice, the bilaminate, four layer transducer has a thickness of approximately 100 micrometers (0.10 mm) for which the thermal time constant is approximately 20 ms.

While the invention has been described in its presently preferred embodiment, it is understood that the words which have been used are words of description rather than words of limitation and that changes within the purview of the appended claims may be made without departing from the scope and spirit of the invention in its broader aspects.