BACKGROUND OF THE INVENTION The present invention relates generally to pressure sensing devices, and more specifically, to differential/stati pressure sensing devices having high excess pressure protection.

Differential pressure transducers, or transmitters, using a sensor which have only a small displacement over their operating range are well-known in the art as shown in U.S. Patent No. 3,712,143 issued to Heaver et al. In such a transmitter, the sensor is typically a strain-sensitive semiconductor chip with means for applying pressurized fluids to respective sides of the wafer to produce a physical displacement of the wafer in response to a differential pressure of the fluids. Such differential pressure transmitters include a pair of barrier diaphragms being pressurized with a second input fluid having a second pressure

A respective interior chamber on the other side of each of the barrier diaphragms contains a respective substantially incompressible fill fluid. These fill fluids communicate through fluid passages with opposite respective sides of the sensor chip, and the chip is displaced or deflected by an amount which is dependent upon the difference between the two pressures of the fill fluids. The sensor chip in turn, produce an electrical output signal which is dependen upon the displacement and, hence, is representative of a differential pressure of the fill fluids which, in turn, is dependent on the differential pressure of the input fluids.

Such transmitters have usually incorporated pressure overload protection means to prevent the sensor chips from being excessively displaced by overload input differential pressures which could damage the sensor chip. Such overload protection means have been based on providing equal or similar overload protection in both directions of displacement of the sensor wafer.

Another method, as disclosed in U.S. Patent No.. 4,572,000, issued to Kooiman, is the use of sensing diaphrag that are flat when an excessive pressure is applied thereto. Such diaphragms are formed from silicon, quartz, sapphire or other substantially nonhysteretic, brittle, nonmetal.

Yet another method, disclosed in U.S. Patent No. 4,173,149, issued to Critten et al., uses overload diaphragm that are concavo-convex shaped and are stiff over a given working range, but collapse over a relatively short, high pressure range. Each diaphragm collapses against a housing structure that ultimately bears the excess pressure, thus relieving the pressure sensor. Also included are blocks of material having a low coefficient of thermal expansion relat to the housing material to accommodate increased temperature swings which might distort any pressure readings.

SUMMARY OF THE INVENTION According to the invention, a pressure sensing transmitter is provided that has high overpressure protectio The transmitter includes an housing having a first cell, a second cell, a sensing chamber, first and second pressure signal receiving means, and a differential sensor chip for measuring the pressure differential between the first and second pressure signal receiving means. Each cell includes a cylinder with an isolation diaphragm mounted on one face and concave spherical profile on the other face. An overpressure diaphragm is mounted on the main body and nests into the concave spherical profile on the cylinder. The spherical overpressure diaphragms act as moveable barriers between the cells on each side of the device. Each cell further includes channels coupling the overpressure and isolation diaphragms, and further coupling one cell to the first pressure signal receiving means and the other cell to the secc . pressure signal receiving means. Each overpressure diaphragm is positioned such that an overpressure protection cavity is formed between it and th housing. The first cell cavity is further coupled to the second pressure signal receiving means and the second cell

cavity is further coupled to the first pressure signal receiving means. This arrangement allows a predetermined absolute pressure applied against one of the isolation diaphragms, subsequently transmitted to the sensing chamber v one of the pressure signal receiving means, to collapse an appropriate overpressure diaphragm, shifting the excess pressure from the pressure signal means to the housing. The transfer of pressure to the housing relieves the sensor transducers from the excess pressure. The overpressure diaphragm, which is spherical, and the isolation diaphragm, which is convoluted with annulus ridges, exhibit non-linear characteristics that allow for a wide temperature operating band and a standard non-pressurize ambient temperature silicon-oil-filled procedure. In additio the relative small oil volume required for full scale output the differential sensor chip allows for a relatively fast response time.

The invention will now be described, by way of example, with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1A is a cross-sectional view of a differential pressure device according to the invention;

Fig. IB is a partial cutaway of the device along th line B-B of Fig. 1A;

Fig. 1C is a partial cutaway of the device along th line C-C of Fig. 1A;

Fig. 2A is a cross-sectional side view of a differential assembly as found in Fig. 1A; Fig. 2B is a partial cutaway front view of the differential assembly as found in Fig. 2A;

Fig. 2C is a cross-sectional view of the differenti pressure sensor as found in Fig. 2A;

Fig. 2D is a schematic diagram of the differential pressure sensor as found in Fig. 2C;

Fig. 3A is a cross-sectional view of an overpressur diaphragm according to the present invention;

Fig. 3B is a front plan view of the diaphragm in Fi 3A?

Fig. 4A is a cross-sectional view of a cylinder wit an isolation diaphragm according to the present invention; Fig. 4B is a front plan view of the diaphragm in Fi

4A; and

Fig. 5 is a graph showing volume displacement with respect to pressure according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Fig. 1A is a cross-sectional view of a differential pressure transmitter 10. The transmitter includes a housing which houses a first cell 14 on one side and a second cell 16 on the other. Located in the top portion of housing 12 is a sensor chamber 18 wherein a differential assembly 20 is mounted. Inside the differential assembly 20 is a differenti sensor chip 22 and an absolute pressure chip 24. Each cell i connected to sensor chamber 18 via a port 26 for first cell 1 and a port 28 for second cell 16. First cell 14 is further composed of a cylinder 30, having a spherically concave face on one side of cylinder 30 clear a spherically shaped overpressure diaphragm 32 welded to housing 12, and a convolutely-shaped isolation diaphragm 34 mounted to conform to a like-convoluted face on the other side Cylinder 30 further includes a channel 36 for allowing fluid communication between the two diaphragms and is further connected to port 26. Second cell 16 is identical to first cell 14 with cylinder 38, overpressure diaphragm 40, isolation diaphragm 42, and channel 44 identifying the appropriate parts Each cylinder 30 and 38 is mounted to housing 12 but not touching overpressure diaphragms 32 and 40. In addition, each surface of housing 12 at the boundary is spherically concave, thereby forming overpressure cavities 46 and 48. Eac cavity 46 and 48 allows its respective diaphragm 32 and 40 to conform to the surface of either side as the diaphragm moves under pressure. Each cavity 46 and 48 on the housing side of the diaphragm is in communication with port 28 and 26, respectively, via a relief channel 50 and 52, respectively.

Each diaphragm is welded to its cylinder along its periphery with each cylinder subsequently welded to housing along its periphery. Such welds form air-tight seals that a leak-proof under great pressure and/or temperature extremes that transmitter 10 might encounter. Subsequently, a pair o pressure caps 54 and 56 are either bolted together or welded each cylinder 30 and 38 to protect isolation diaphragms 34 a 42. Each pressure cap 54 and 56 is recessed on its inner fa to allow for a displacement cavity 58 and 60. Each pressure cap 54 and 56 also has an opening to allow the outside environment pressure to act on the outside surface of each isolation diaphragm 34 and 42.

Housing 12, cylinder 30 and 38, differential assem 20, and pressure cap 54 and 56 are made of a high grade, machinable metal such as aluminum or stainless steel, preferably of 316 stainless steel.

Each cell assembly, which includes a cylinder with channels, one diaphragm, and a port, is filled with an incompressible, high pressure and temperature fluid, such as silicon oil. Furthermore, the fluid in each cell assembly i separated from the other cell assembly by the barrier of eac overpressure diaphragm and by the differential pressure sens chip. A first pressure acting on one of the isolation diaphragms is transmitted through the silicon oil to differential sensor chip 22, and a second pressure acting on the other isolation diaphragm is transmitted through the silicon oil to the other side of differential sensor chip 22. The pressure difference of the two pressures acting on sensor chip 22 is converted into an electrical signal which is transmitted to an output device (not shown) attached at a set of differential pressure sensor pins 62.

One of the cell assemblies communicates its pressur to an absolute pressure sensor chip 24. The pressure is converted to an electrical signal and transmitted to a second output device (not shown) attached at a set -of absolute pressure sensor pins 64.

Differential assembly 20, which houses the two sens chips, is secured in place in chamber 18 by a plug ring 66 an

a dummy plug 68. In a specific embodiment, plug ring 66 hol absolute pressure sensor chip 24 with absolute sensor pins 6 extending outward to allow outside connection to the second output device. Both plug ring 66 and dummy plug 68 are secu to housing 12 by welds along both the inner and outer periph to seal both sensor chips from the outside environment and t prevent leakage of the transmission fluid.

Each pressure sensor is a monolithic strain gage w circuits implanted into single crystal silicon. Strain sensitive resistive elements, arranged in the pattern of a four-active arm Wheatstone bridge, are ion implanted in the silicon strain gage (see Fig. 2D) . As pressure is applied t the silicon diaphragm, the piezoresistive influence produces change in resistance in the strain-sensor arms of the bridge Two of the bridge arms are located in a region whe the strain will be tensile; the other two in a region where strain will be compressive. Tension increases the resistanc of an arm; compression reduces it. The overall effect of a pressure-induced strain is a change in resistance. The outp is then conditioned within a set of integral amplifiers (not shown) to produce independent linear-to-static pressure and differential pressure output signals.

A temperature sensor 70 can be located in close proximity to the pressure sensors in chamber 18. Temperature sensor 70 may be used to provide improved correction of therm errors.

Fig. IB illustrates the top of pressure transmitter 10. Two oil fill holes 72 and 74 are located on the top to allow the silicon oil used to be filled into housing 12 in order to fill b^oth cell assemblies. Each fill hole 72 and 74 is partially threaded to allow a set screw to hold a ball sea in place. Such a closure assembly guarantees a hermetic seal that will not allow outside pressure to penetrate into the cells nor to allow any oil to leak past the ball seal. A partial cut-away view shows how port 28 connects to differential assembly 20.

Fig. 1C is a partial cut-away side view of pressure transmitter 10 without a pressure cap. The cut-away portion

exposes how relief channel 52 connects port 26 to overpressu cavity 48. Relief channel 50 similarly connects port 28 to overpressure cavity 46. Also shown is how the convolutions isolation diaphragm 34 form concentric rings or annuluses. A cross-sectional side view of differential assemb

20 is shown in Fig. 2A with a partial cut-away front view sh in Fig. 2B. Differential sensor chip 22 is mounted on a die which is further mounted on assembly 20. Die 74 and sensor chip 22 form a barrier between the fluid chamber connection cell 14 and cell 16 (Fig. 2C) . A set of electrical leads 76 are bonded on differential sensor chip 22 and are connected sensor pins 62. Preferably, leads 76 are made of a highly conductive metal such as gold. An electrical insulation shie 78 is mounted on the front of assembly 20 to protect sensor chip 22 and leads 76. Fluid openings 80 are spaced about the outer periphery of shield 78 to allow the oil to act on senso chip 22 as pressure is transmitted therethrough.

In a specific embodiment, a low pressure access por 82 allows pressure transmitted through isolation diaphragm 42 and port 28 via the silicon oil to act on one side of sensor chip 22. A high pressure access side 84 allows pressure transmitted through isolation diaphragm 34 and port 26 via th silicon oil to act on the opposite side of sensor chip 22.

Overpressure diaphragm 32 is illustrated in cross- section in Fig. 3A and in front plan in Fig. 3B. Preferably, overpressure diaphragm 32 is made from a hard metal alloy, su as Alloy Steel A286. After overpressure diaphragm 32 is welded to housing 12, it is solution annealed at 1650 degrees for two (2) hours, then aged at 1350 degrees F for twelve (12 hours. Overpressure diaphragm 32 has a thickness of 0.013 inches, a diameter of 1.88 inches, and a spherical curve of a radius of 12.75 inches such that the deflection between the outside edge and the center is approximately 0.03 inches. At location on its periphery, a groove 86 is provided that allow the silicon oil trapped under the surface of the diaphragm to flow out to the other side of the cell to provide a void spac once the critical overpressure is reached. This design, therefore, allows the chip to withstand a very high

differential pressure. With the use of the extra void space created within cell 30 or 38 when the spherical diaphragm collapses, the isolation diaphragm will be supported by the cell because the oil trapped between the isolation diaphragm and the convoluted cylinder surface is to be used to fill th void space just created. Such a design allows the invention measure and bear high pressures reaching 2250 psia (pounds p square inch absolute) . This is limited only to the pressure that the mechanical housing assembly can withstand, and as such, the diaphragm is designed to sustain more than 1500 ps in a preferred mode. It would now be within the ability of skilled in the art to make an isolation diaphragm according the present invention.

A specific embodiment of cell 16 is illustrated in cross-sectional side view in Fig. 4A. Shown are cylinder 38, isolation diaphragm 34, channel 44 and port 28. Each isolat diaphragm 34 and 42 is made of a high quality, stiff metal, such as 316L stainless steel. Highlighted on isolation diaphragm 34 are convolutions 88. The outer most convolution 88 is a relatively large annulus (Fig. 4B) that is designed give diaphragm 34 a spring-rate that allows it to deflect as the outside pressure acts upon it. Its preferred diameter is 1.955 inches. The three smaller convolutions 88 are concentr with the large annulus and are added to stiffen the center of diaphragm 34 to a degree that allows great deflection accurac over a large pressure and temperature zone. Their preferred dimensions are 0.680 inches. The accuracy is designed to be within +/~0.25% over the temperature range of -20 to +250 degrees F and with pressures exceeding 1500 psi. The four diaphragm design in pressure transmitter 1 allows differential pressure sensor chip 22 to be protected from the high overpressure generated by a wide environmental temperature change and/or high line pressure. The overprotection action works as follows: An outside pressure acts on isolation diaphragm 34; this pressure is transferred pressure sensor chip 22 by means of the silicone oil in cell except overpressure cavity 46. overpressure diaphragm 32 responds non-linearly to the pressure according to the chart

Fig. 5. When the critical buckling pressure is reached, overpressure diaphragm 32 collapses. This collapsing of diaphragm 32 transfers the oil behind it to cell 16, allowin isolation diaphragm 34 to rest on cylinder 30. The pressure acting on isolation diaphragm 34 is absorbed by cylinder 36, thus protecting differential sensor chip 20. Upon removal o the excess pressure from overpressure diaphragm 32, diaphrag 32 returns to its original position as does isolation diaphra 34. Should the excess pressure be exerted on isolation diaphragm 42, overpressure diaphragm 40 would collapse in a similar fashion to protect differential sensor chip 20 from t extreme environmental pressure.

As can be appreciated from the present invention, a relatively small oil volume is required for full scale output by differential sensor chip 20 (VI) as shown in Fig. 5. The critical pressure differential P _critical ranges from 8 to 15 ps depending upon the design and diameter of the convolutions. increasing the thickness of the overpressure diaphragm and increasing its spherical depth, the diaphragm can withstand a high a pressure as 500 psi before yielding to relieve the excess pressure.

The invention has now been explained with reference to specific embodiments. Other embodiments will be apparent those of ordinary skill in the art. It is therefore not intended that the invention be limited except as indicated by the appended claims.