Resonant frequency based pressure sensor.

(54) Title: RESONANCE FREQUENCY BASED ON PRESSURE SENSOR. (54) Title: RESONANT FREQUENCY BASED PRESSURE SENSOR.

(57) Summary

A pressure sensor (10) for sensing a pressure of a process fluid includes a sensor body (23) exposed to the pressure of the process fluid. The sensor body 23 is deformed in response to pressure. A diaphragm (16) suspended from the sensor body (23) has a tension that changes in response to deformation of the sensor body (23). A resonant frequency of the diaphragm is measured (16). The measured resonant frequency indicates the inline pressure of the process fluid and the integrity of the insulation fill fluid system. In addition to measuring the resonant frequency, the oscillation mode itself can be used as a diagnostic tool to assess the health of the sensor.

(57) Abstract

A pressure sensor (10) for sensing a pressure of a process fluid ineludes a sensor body (23) exposed to the pressure of the process fluid. The sensor body (23) deforms in response to the pressure. A diaphragm (16) suspended from the sensor body (23) has a tension which changes in response to deformation of the sensor body (23). A resonate frequency of the diaphragm (16) is measured. The measured resonant frequency is indicative of the line pressure of the process fluid and infegrify of the isolation fill fluid system. In addition to measuring the resonant frequency, the oscillation mode itself can be used as a diagnostic tool to assess sensor health.

PRESSURE SENSOR BASED RESONANCE FREQUENCY

BACKGROUND OF THE INVENTION

The present invention relates to pressure transmitters of the type used in industrial process control systems. More specifically, the present invention relates to a pressure sensor for use in a pressure transmitter.

Pressure transmitters are used in industrial process control systems to control the pressures of process fluids. A pressure transmitter includes a pressure sensor that is coupled to a process fluid and provides an output in response to the pressure applied by the process fluid. A well known type of pressure transmitter is the Model 3051 transmitter available from Rosemount Inc. of Eden Prairie, Minnesota. Pressure transmitters are also shown in US Patent No. 5,094,109, for example.

In many facilities, where differential pressure is measured, it is often desirable to obtain line pressure measurements (i.e., the pressure of the process fluid in the pipe or conduit). For example, line pressure can be used to determine the mass flow of process fluid, or for other control applications. However, when a line pressure measurement is required, in addition to the differential pressure measurement, an additional pressure sensor is typically required. This additional pressure sensor requires additional and coupling components for the process fluid. These additional components lead to greater complexity and cost, as well as increasing the probability of failure.

In addition, many pressure sensing technologies are coupled to process fluid through an isolation arrangement that uses an isolation diaphragm exposed to process fluid and an isolation fill fluid that couples the pressure sensor to the isolation diaphragm. This isolation arrangement can potentially be a source of errors, complexity, and potential failure in process devices.

SUMMARY

A pressure sensor for detecting a pressure of a process fluid includes a sensor body exposed to the pressure of the process fluid. The sensor body deforms in response to pressure. A diaphragm suspended by the sensor body has a tension that changes in response to deformation of the sensor body. A resonance frequency of the diaphragm is measured. The measured resonant frequency indicates the pressure of the integrity of the insulation process fluid line filling the fluid system. In addition to measuring the resonance frequency, the oscillation mode itself can be used as a diagnostic tool to assess the condition of the sensor.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a cross sectional perspective view of a pressure sensor in accordance with the present invention.

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a pressure sensor that includes an acoustic source.

Figure 4 is a cross sectional view of a variable process transmitter including a pressure sensor in accordance with the present invention.

Figures 5A-5F illustrate illustrative resonant modes of a central diaphragm in accordance with the invention.

DETAILED DESCRIPTION

The present invention relates to pressure sensors of the type used in pressure transmitters of industrial process control systems. With the present invention, a pressure sensor is provided including a deformable sensor body. A diaphragm is mounted on the

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determined the applied pressure.

Figure 1 is a perspective cross-sectional view of a differential pressure sensor 10 in accordance with an embodiment of the present invention. Pressure sensor 10 is an example of a differential pressure sensor configuration and includes pressure connectors 26 that extend through sensor body 23. The sensor body is formed of mid-cells 46 and 48 and comprises a metal and a glass compound. A cavity 25 within sensor 10 carries a filling fluid. A movable diaphragm 16 extends through cavity 25 and is configured to move in response to an applied differential pressure. The electrodes (capacitor plates) 20A and 20B are arranged in the cavity 25 of the sensor 10. The electrical connections 40 coupled to the electrodes 20 and the diaphragm 16 are used to measure the electrical capacitance between them. This capacitance varies as the diaphragm moves in response to the applied pressure and can be used to determine the applied differential pressure. This differential pressure measurement can be used to determine the flow rate in the pipe or conduit.

In accordance with the present invention, a resonant acoustic transducer 96 (shown in Figure 3) is coupled to the body of deformable pressure sensor 23 of pressure sensor 10 and is configured to resonate diaphragm 16, the frequency of which changes in response to a pressure of the process fluid line. Electrodes 20A and 20B can function as a resonance collector and are described in more detail below.

Since differential pressure is applied to sensor body 23 through pressure connections 26, in addition to movement of diaphragm 16, the general shape of sensor body 23 also changes in response to line pressure. This deformation in the shape of the sensor body changes the resonance frequency of the diaphragm 16. The resonance frequency of the diaphragm can be measured according to any appropriate technique. For example, an acoustic signal transducer (source) can be used to resonate diaphragm 16. The resonant frequency of the diaphragm can then be sent by measuring changes in capacitance between electrodes 20A and 20B and the diaphragm.

16.

The following equation is used to predict the resonant frequency of a prestressed membrane:

_ 4, Γσ ~

Ja ~ 2 and pA (Eq. I) where:

F_n = natural membrane (Hz)

Áij constant value based on resonance mode based on nodal radii (i) and nodal diameters (j) σ = central membrane tension (0.07 kg / cm²) p = membrane property of the material as a function of the force of mass, volume and gravity (10686.9 gm / cm³)

A = effective area of ​​the resonance membrane

Equation 1 describes a relationship that can be used to calculate the center diaphragm voltage by measuring the frequency at which the diaphragm resonates. Simplifying equation 1 shows that:

/ η ~ λ / σ (Eq. 2) which establishes that the resonance frequency of the central diaphragm is proportional to the tension of the square root of the central diaphragm 16.

Figure 2 is a simplified cross-sectional view of sensor 10 showing deformation of sensor body 10 in response to applied line pressure from the process fluid. In response to an applied pressure, the cavity depth increases, which causes cells 46 and a medium 48 to radically deflect inward. This results in a reduction of the tension (stress) of the central diaphragm 16. As shown in Figure 2, the depth cavity (Zo) increases (ΔΖ) with increasing line pressure. The deflection follows Hook's law and is directly proportional to the line pressure, P, that is:

Z = Zq + k_zP (Eq. 3) where k₂ is the spring of constant proportionality between the line pressure and the depth of the cavity. Similarly, the radius (r) of the sensor in the center of the diaphragm (CD) contracts (Ar) with applied line pressure. This deviation is linear with the line pressure (p):

r = r₀-k_orP where k_r Proportionality between (Eq. 4) is the spring pressure of line constant and radial changes. Because of this, the DC voltage is also a linear function of the line pressure:

o = oo - k_toP (Eq. 5) where k_or is the constant source of proportionality between pressure and center line diaphragm stress. Since the DC stress is biaxial, the stress can be converted to the stress as follows:

E £ σ = 1-v (Eq. 6) where ε = tension = r,

Young, and v = Poisson's ratio for CD. linear proportionality, you can write:

E = Module

Due to this (Eq. 7)

Sympathetic resonance is a harmonic phenomenon in which a passive body responds to external vibrations for which it is harmonically similar. By using sympathetic resonance, energy can be transferred and stored between resonance systems. With the present invention, the central diaphragm 16 of the sensor body is sympathetically resonating, for example, by an acoustic source. The central diaphragm is placed into acoustic or mechanical resonance and the resonance frequency is measured to determine the line pressure. The resonance frequency can also be used to diagnose the integrity of the core membrane, as well as the isolation diaphragms and pressure connectors that are filled with oil and is used to isolate the sensor from the process fluid.

The center diaphragm will resonate at a specific frequency based on its tension. Factors influencing center diaphragm tension include line pressure, differential pressure, and temperature. Since differential pressure and temperature are measured in the device, its contribution to changes in diaphragm tension can be characterized and thus offset its effects. With this, only the line pressure remains as an unknown and its frequency contribution value can be calculated as illustrated by equation 8:

ί! J *⁼ / measured ~ í Di * ¿/ temperature (Eq. 8)

The differential pressure will add tension (and increase the resonance frequency) as the center diaphragm is displaced from its neutral axis. The temperature sensor will either increase or decrease the tension in the central diaphragm as the materials expand or contract.

For a practical sensor based on a resonant diaphragm, the question of damping means becomes important. When the diaphragm is surrounded by a liquid, such as insulation oil in a typical application, the resonance behavior of the diaphragm will be very damped. This occurs because the oil, for example, must be physically displaced in order for the diaphragm to vibrate. This problem can be mitigated by various means: One is to use the sensor in a gas medium that has a reduced effect on diaphragm attenuation. In some applications, however, this is not feasible and a liquid, typically oil, must be in contact with the diaphragm.

To avoid this, a second approach can be employed. Higher order modes of membrane vibration tend to have many undulations in the stretched diaphragm membrane and typically have lower amplitudes of displacement. This reduces the net volume shift and consequently the damping in the way shown in Figure 5C is less severe than what occurs for the mode shown in Figure 5A.

A third method is even more effective in driving only the so-called azimuthal asymmetric modes shown in Figures 5D-F. These particular modes have the advantage of not displacing any net volume because upward displacement is counteracted by equal downward displacement.

Therefore, for minimum resonance regulation when the diaphragm is in contact with a liquid, the highest order modes azimuthal asymmetric should be considered.

As a diagnosis, the particular mode that is excited can also change if some aspect of the sensor has changed, and therefore if detected, would indicate a potential sensor failure.

Figure 3 is a cross sectional view of the pressure sensor cell 10. In Figure 3, the electrodes 20 are illustrated as central electrodes 20A and ring electrodes 20B. These electrodes are coupled to electrical connections 40. An acoustic transducer 96 is illustrated mounted in one of cells 46 and a half is used to apply an acoustic signal to the center of diaphragm 16. Acoustic transducer 96 couples to cables 98 and is driven at one frequency, or swept through a range of frequencies, in order to excite the central diaphragm in resonance. This resonance can be detected by measuring variations in capacitance between electrodes 20A / B and central diaphragm 16. Although the capacitance of being used to detect deflection of the central membrane 16 due to resonance is illustrated in this example, other techniques may also be used. These include acoustic techniques, optical, mechanical, or other sensors.

FIG. 4 is a cross sectional view of a transmitter 100 including a pressure sensor 102 in accordance with an embodiment of the present invention with an acoustic transducer 96. Transmitter 100 is known in the industry for having a Coplanar ™ platform and diaphragms for Insulation 106 and 108 are generally aligned in the same plane. The flange 111 is coupled to the transmitter 100 through pressure pins 110 P_x And p₂ thus pair of isolation diaphragms 106 and 108. Gaskets 109 provide a seal between flange 111 and isolation diaphragm 106, 108. A substantially incompressible fluid is made in pressure connectors 120 that are coupled to pressure sensor 102. Similar To pressure sensor 10, sensor 102 has a sensor body that is formed from two media cells 112, 114, respectively, filled with glassware 116, 118. Electrical conductors 124 pairs of capacitor plates (not shown) that are made on brittle material sensor surfaces 116, 118. A diaphragm 122 deforms in response to applied pressures P_x And p₂ causing a capacitive change, which is detected by a transmitter circuit 123 that provides an output related to pressures P_x And p₂ through a process control loop. The process control loops are can be in accordance with any appropriate standard including two process control wire loop, such as 4-20 mA, HART ® or FieldBus circuits, based on wireless loop control loops, etc. Furthermore, the process control loop may comprise a wireless control loop in which wireless communication techniques are used to transmit data.

In addition to determining the line pressure based on the resonance of the central diaphragm as discussed above, the resonance frequency and mode type can also be used to determine the condition of the central diaphragm as well as the filling system. of oil. Transmitter circuit 123 provides a diagnostic circuit and acoustic transducer pairs 96 through wires 98. Circuitry 123 is configured to energize transducer 96 and sensing in response to the resonant frequency of diaphragm 122 as discussed above. Circuitry 123 may provide a diagnostic output, for example, through the transmitter output. Damage to the central diaphragm, or the appearance of oil leaks, will lead to changes in the resonance frequency of the central diaphragm. Although resonance measurement is illustrated based on changes in capacitance, other measurement techniques may be employed such as the use of acoustic, optical, mechanical, or other sensors. The measured resonance frequency can be compensated based on the measured differential pressure and temperature as desired to improve the accuracy of the measurements. If temperature compensation is desired, a temperature sensor 130 can be thermally coupled to pressure sensor 102 as illustrated in Figure 4. Temperature sensor 130 can be in accordance with any suitable sensor technology and coupled to the circuitry. 123. Changes in the resonance frequency of the central diaphragm 122 may indicate physical damage such as a hole, a perforated or torn diaphragm, or other damage to the transmitter or membrane components. Loss of oil pressure on one or more sides of the diaphragm will also cause a change in the resonance frequency. In one configuration, differential pressure measurements can also be obtained using a prestressed membrane, an acoustic transducer (source), and an acoustic pickup. The measurement of the resonance of the insulation diaphragm can be used to determine the integrity of the insulation membrane and indicates the line pressure. Diaphragm resonance can also be induced by electrostatic techniques. In another example, the power source used to place the diaphragm in center resonance is located outside the transmitter. For example, a test device may be configured to mate with the transmitter and transmit acoustic energy into the transmitter, thereby resonating the diaphragm.

Although the above description has discussed the sensor body as glass and metal composites, other materials can be used that have desirable characteristics. Examples include plastic materials or the like. Any suitable technology for resonance detection can be used as capacitance, strain gauge, optical techniques, silicon techniques, etc. Other sensors, multiple can be used for security, redundancy, self-validation or the like. As used herein, the resonance frequency sensor may comprise any suitable sensor technology that is used to measure or detect the resonance frequency of the central diaphragm. In the figures shown in this document, the resonance frequency sensor is illustrated as an acoustic source and an independent displacement sensor that measures the displacement of the central diaphragm · based on the electrical capacitance. However, the present invention is not limited to this particular resonance frequency sensor.