TECHNICAL FIELD

This invention relates in general to transducers and in particular to a capacitance pressure transducer.

BACKGROUND ART

A capacitance pressure transducer provides a capacitance that varies in proportion to a sensed pressure. As shown in Figure 1 , a basic capacitance pressure transducer 1 includes two parallel plates 2 and 3 whose capacitance is determined by plate separation and dielectric material. A variation ir "essure flexes the pressure plate 2 and decreases its separation from the fixed plate 3. The variation in capacitance can be used 3 change the isponse of a parallel circuit or to change the frequency of a resonar circuit. The response or frequency is proportional to the sensed pressure. In certain applications, the capacitance pressure transducer must measure pressure while being subjected to high accelerations and vibrations. Because the pressure plate flexes in response to the accelerations and vibrations, the transducer falsely indicates a change in pressure. To eliminate error due to vibration and acceleration, the fixed plate is replaced with at least one reference plate. The transducer 4 of Figure 2 includes a pressure diaphragm 5 and two reference diaphragms 6 and 7. Electrodes on the pressure diaphragm 5 and one reference diaphragm 6 form a pressure capacitance, and electrodes on the reference diaphragms 6 and 7 form a reference capacitance. This transducer 4 is disclosed in U.S. Patent No. 4,689,999 which is issued to Allied-Signal, Inc., the assignee of the present invention. When the transducer 4 is subjected to

a steady-state acceleration or to a vibratory force, the diaphragms 5-7 move in phase. However, when the transducer 4 is subjected to a pressure, only the pressure diaphragm 5 moves. By analyzing the change in reference capacitance and subtracting it from the change in pressure capacitance, an accurate measurement of change in capacitance due solely to pressure is obtained. A ratio of the pressure and reference capacitances can be obtained from an oscillator disclosed in U.S. Patent No. 4,987,782 which is also issued to Allied-Signal, Inc.

The diaphragms 5-7 and housing 8 of the transducer 4 are made of quartz and are fused together along the dotted lines 9. Resulting is a unitary structure.

However, the quartz has a relatively high thermal elastic coefficient, making it sensitive to temperature variations. The plot of Figure 3 reveals the error as a percentage of full scale pressure for a temperature cycle between 107° (225°F) and -37° (-35°F) ("down'V'up" refers to downside/upside of the temperature cycle). The variations in temperature give false indications of pressure changes because deflections are caused by temperature changes, not pressure changes.

DISCLOSURE OF THE INVENTION

A pressure transducer according to the present invention is less sensitive to temperature variations. This transducer includes a pressure diaphragm and support means that are made of aluminosilicate glass or any other glass having a low thermal elastic coefficient. The pressure diaphragm and support means are bonded together by a hydrate bond or a glass frit. Electrodes on opposing surfaces of the pressure diaphragm and support means cooperate to form a capacitance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a bending beam capacitive pressure transducer according to the prior art;

FIG. 2 is a schematic representation of a quartz capacitive pressure transducer according to the prior art;

FIG. 3 is a plot of transducer error as a function of temperature and pressure for a quartz pressure transducer similar to that shown in Figure 2;

FIG. 4 is a cross-sectional view of a capacitive pressure transducer according to the present invention; and

FIG. 5 is a plot of transducer error as a function of temperature and pressure for the aluminosilicate glass pressure transducer shown in Figure 4.

BEST MODE FOR CARRYING OUT THE INVENTION

Figure 4 shows a capacitive pressure transducer 10 whose pressure diaphragm 12, compensation diaphragm 14 and support 16 are made of aluminosilicate glass such as Corning #1723. The aluminosilicate glass has a low thermal elastic coefficient (TEC) of -30 ppm/°C. As a result of the low TEC, the diaphragms 12 and 14 exhibit little deflection due to changes in temperature, whereby transducer 10 performance is improved. The TEC of the aluminosilicate glass is fairly constant for temperatures below 204°C (400°F). Thus, deflection of each diaphragm 12 and 14 is a linear function of pressure below 204°C (400°F). The softening temperature of most aluminosilicate glass is high, thereby making it less susceptible to creep. The aluminosilicate glass also has a thermal expansion between 40 and 50X10- ⁷ , making it easy to hermetically seal to Kovar alloy.

Each diaphragm 12 and 14 is preferably constructed as a disc whose thickness and diameter is such that its first mode of vibration is far in excess of any exciting frequency to which the transducer (10) is subjected. First and second electrodes 18 and 20 are deposited on upper and lower surfaces of the pressure diaphragm 12. The second electrode 20

is employed in a capacitor, but the first electrode 18 is not. Instead, the first electrode 18 prevents the pressure diaphragm 12 from bending during temperature variations. If only one surface of the pressure diaphragm 12 is plated with a deposited metal electrode, the diaphragm 12 will behave like a bimetallic strip and bend slightly when heated or cooled. Thus, the first electrode 18 prevents thermal curvature. It also can serve as a ground plane around the pressure capacitor.

Third and fourth electrodes 22 and 24 are deposited on the upper and lower surfaces of the compensation diaphragm 14. A conductor 26 extending through a passageway in the reference diaphragm interconnects the third and fourth electrodes 22 and 24.

The electrodes 18-24 and conductor 26 are preferably made of a noble metal such as platinum or gold. A thin film of noble metal is sputtered onto the aluminosilicate glass to a thickness between one thousand and two thousand angstroms. For best adhesion, reactive metals such as chrome and titanium are used under the noble metal films. Geometry of the electrodes 18-24 is circular. All electrodes 18-24 must have the same physical characteristics. If, for example, the first and second electrodes 18 and 20 are thicker than the third and fourth electrodes 22 and 24, the diaphragms 12 and 14 will bend differently. But because the electrodes 18-24 are thin, the bending is minimal.

The pressure and compensation diaphragms 12 and 14 are bonded together such that the second and third electrodes 20 and 22 form a capacitor. A cavity is etched into the compensation diaphragm to a depth that establishes the spacing between the second and third electrodes 20 and 22. The bond is formed by hydrate bonding or glass fritting, which processes are well known to those skilled in the art of pressure sensors.

The lower surface of the compensation diaphragm 14 is hydrate bonded or glass fritted to the upper surface of the support 16. Etched into the support 16 is a cavity, which allows the compensation diaphragm 14 to deflect when the transducer 10 is accelerated and/or vibrated. The support

16 should be roughly five times a^ ⁴ hick as the pressure diaphragm 12. It is mounted onto a pressure vess or printed wiring board by a silastic material such as RTV silicone or . iioether. The support 16 serves the function of sealing terminals A ana u and buffering the stresses associated with mounting the transducer 10.

When pressure is applied to the pressure diaphragm 12, it deflects towards the compensation diaphragm 14. This deflection decreases the distance between the second and third electrodes 20 and 22, thereby changing the capacitance formed by the second and third electrodes 20 and 22. However, when the transducer 10 is vibrated or accelerated, the pressure and compensation diaphragms 12 and 14 move in unison. Because the second and third electrodes 20 and 22 also move in unison, the capacitance does not change.

To provide differential pressure measurements, the support 16 can be provided with a seal-off tube (not shown) which allows a reference pressure to be applied to the lower surface of the pressure diaphragm 12. The pr^nary pressure is applied to the upper surface of the pressure diaphragm 12.

Terminals A and B are made of a thermally matched metal. Terminal A is connected to the second electrode 20 by a thin wire 28 extending through the support 16, compensation diaphragm 14 and third and fourth electrodes 22 and 24. A first passageway 30 accommodates this wire 28. Terminal B is connected to the fourth electrode 24 (and third electrode 22) by a thin wire 32 extending through the support 16. A second passageway 34 accommodates this wire 32. The wires 28 and 32 are extremely fine, such that they do not interfere with movement of the pressure and reference diaphragms 12 and 14. During evacuation, the terminals A and B are sealed to the support 16 by an indium-based solder in a heated vacuum chamber. A transducer 10 made of Corning #1723 was fabricated by hydrate bonding and thereafter tested (see Figure 5). The temperature sensitivity

of this transducer 10 was found to be less than 10% of that of an equivalent quartz transducer.

The transducer 10 can be made of any glass having a TEC between +30 ppm/°C. One such glass is borosilicate glass such as Schott BK7. Borosilicate crowns have TECs between 10 and 30 ppm/°C and a Young's modulus that is fairly constant below 204°C (400°F). However, borosilicate glass has a lower softening temperature and higher thermal expansion than aluminosilicate glass.

Although the present invention was described in connection with the transducer shown in Figure 4, its teachings can be applied to the capacitive pressure transducers shown in Figures 1 and 2 as well as any other known capacitive pressure sensor. Other sensors might include vibrating diaphragms or beams, wherein the sensing element depends upon Young's modulus or the bulk modulus of elasticity.