Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01L—MEASURING FORCE, STRESS, TORQUE, WORK, MECHANICAL POWER, MECHANICAL EFFICIENCY, OR FLUID PRESSURE
  • G01L9/00—Measuring steady of quasi-steady pressure of fluid or fluent solid material by electric or magnetic pressure-sensitive elements; Transmitting or indicating the displacement of mechanical pressure-sensitive elements, used to measure the steady or quasi-steady pressure of a fluid or fluent solid material, by electric or magnetic means
  • G01L9/0041—Transmitting or indicating the displacement of flexible diaphragms
  • G01L9/0076—Transmitting or indicating the displacement of flexible diaphragms using photoelectric means
  • G01L9/0077—Transmitting or indicating the displacement of flexible diaphragms using photoelectric means for measuring reflected light
  • G01L9/0079—Transmitting or indicating the displacement of flexible diaphragms using photoelectric means for measuring reflected light with Fabry-Perot arrangements

Definitions

• the present invention relates to the process control industry.
• the invention relates to a pressure sensor in a pressure transmitter.
• Pressure transmitters in process applications measure pressure of a process and responsively communicate the information over a two-wire process application loop, for example a 4-20 mA current loop.
• Pressure sensors in transmitters typically comprise some type of a pressure responsive structure which has a deflectable diaphragm that moves in response to applied pressure . These structures can be used to measure both absolute and differential pressure.
• a differential pressure sensor is a sensor which measures a relatively small pressure differential (such as that generated across an orifice in a flow tube or between two different heights in a fluid filled container) over a relatively wide absolute pressure range.
• two different pressures are applied to opposing sides of the structure causing a relative deformation in the structure which is measured. Measurement of the deformation, for example, can be achieved by measuring a change in electrical capacitance due to movement of capacitor plates carried on the structure, by change in resistance of a resistive strain gauge, etc.
• Typical known pressure sensors used in process applications have unit-to-unit variations in sensitivity to sensed pressure as well as unit-to- unit variations in undesired responses to extraneous parameters such as temperature . This can be a particular problem when the outputs of two absolute or gauge pressure sensors are combined to provide an output representing differential pressure or when the sensor is used over a large pressure range . Additionally, mechanical stress associated with mounting the sensor to the pressure transmitter may result in relatively large errors in pressure measurement .
• the Sittler et al . patent describes a different type of pressure sensor which is made of a brittle material. Capacitive plates are carried in the material and spacing between the capacitive plates changes in response to an applied pressure. This causes the electrical capacitance between the plates to change which can be measured and correlated to the applied pressure.
• a pressure sensor includes a structure that deforms in response to an applied pressure.
• a light source is directed at the structure . This provides a reflection from the structure.
• a sensor is arranged to sense the reflection and provide an output related to the applied pressure.
• Figure 1 is a simplified diagram of a process control or monitoring system.
• Figure 2 is a prospective view of a deformable pressure sensitive body.
• Figure 3 is a diagram illustrating reflected light through the deformable pressure sensitive body.
• Pigure 4 is a diagram that is a defortnable pressure sensitive body.
• Figure 5 is ' a diagram of a pressure transmitter including a deformable pressure sensitive body.
• Figure 6 shows a plan view of another example configuration of a pressure sensor in accordance with the invention.
• Figure 7 shows a plan cross sectional view of another example configuration of a pressure sensor in accordance with the invention.
• a pressure sensor which has a structure which deforms in response to an applied pressure .
• Light is directed at the structure and its reflection is observed and correlated to the applied pressure .
• a single frequency light source is used and the resultant reflection used to determine deflection of a diaphragm.
• the reflected frequency periodically repeats for various deflections of the diaphragm.
• the present invention uses multiple frequencies to determine the deflection of a diaphragm configuration formed by a pair of substrates .
• FIG 1 illustrates an industrial process control or monitoring system 10 of a type in which the pressure sensor of the invention maybe applicable which includes process piping 12.
• a pressure transmitter . 14 is shown coupled to piping 12 and provides a signal to control room 16 over a two wire process control loop 18.
• the output from transmitter 14 is related to pressure of process fluid carried in process pipe 12.
• the two wire process control loop 18 is used for both signaling and providing power to transmitter 14.
• Loop 18 can operate in accordance with any appropriate technique such as known standards including a 4-20 tnA standard, the HART ® communication protocol, FieldBus communications protocols, etc. Additionally, the control loop may operate wirelessly, etc.
• Figure 2 is a perspective view of a deformable pressure sensitive body 50 of a generally transparent or translucent material.
• Deformable pressure sensitive body 50 can be made in accordance with, for example, techniques set forth in US Patent No. 6,484,585 entitled PRESSURE SENSOR FOR A PRESSURE TRANSMITTER issued November 26, 2002 to Sittler et al . and assigned to Rosemount Inc. which is incorporated herein by reference in its entirety.
• deformable pressure sensitive body 50 can be formed of sapphire, silicon, ruby, quartz, diamond and may comprise a single crystal material.
• Body 50 includes a cavity 52 formed therein. As a pressure P is applied to body 50, a spacing d between internal walls 54 of cavity 52 changes.
• Body 50 can be formed of, for example, two or more substrates . of brittle material . which are fusion bonded together by placing the substrates together under pressure and optionally applying heat . Such a configuration reduces the amount of defects in the body 50 and improves the repeatability of the relationship between changes in the spacing d and the applied pressure P.
• Figure 3 is a cross sectional view of deformable body 50 which illustrates operation of the present invention.
• a pressure sensor 58 is formed using a light source 60 and a spectrometer 62.
• Element 62 can be any appropriate detector including a CCD detector sensor.
• Source- 60 is configured to direct a light beam 64 toward body 50.
• a portion 64A of beam 64 is reflected at one interface between wall 54 and cavity 52 while a second portion 64B is _ reflected at the other interface between cavity 52 and wall 54.
• Sensor 62 is positioned to receive light beam 64 including reflected portions 64A and 64B.
• the reflected portions 64A and 64B cause interference there between which is either constructive or destructive of certain wavelengths.
• the interference is a function
• cavity 52 contains a fluid such as oil.
• the thickness of the oil film simultaneously changes.
• reflectance spectroscopy changes in the color of the reflected light can be sensed by sensor 62 and correlated to the applied pressure P.
• a reservoir 55 should be provided
• the cavity 52 can comprise, for example, a vacuum in the constructive/destructive interference between the reflected light as observed.
• the reflected light is monitored using a spectrometer with a defraction grating such as grating 57 shown in Figure 3.
• the grating can be a component within the spectrometer 62. If a spectrometer is not used, for example by using a one dimensional CCD detector, then an external grating can be employed.
• Sensor 62 can comprise a linear CCD.
• the grating 57 provides a disbursed pattern having changing intensity levels. The changing intensity levels are caused by interference in the reflected light . These patterns repeat for various pressures.
• the pressure sensor 58 In typical prior art techniques which use a detraction grating, the pressure sensor 58 must be recharacterized each time the system is used or if power is lost.
• the present invention includes a memory, such as memory 104 (see Figure 5) as configured to store the characteristic of the spectrum of the reflected light across a pressure range .
• the pressure sensor does not require an initial recalibration when the system is powered out .
• the light source 60 can be any appropriate light source including, for example, a laser.
• the light beam 64 can be of any appropriate format and need not be coherent light. Furthermore, the light does not need to be visible light and can be electromagnetic radiation of any appropriate wavelength.
• the senor 62 can be any appropriate sensing technology which is preferably sensitive to light beam 64, for example a one-dimensional linear CCD array.
• An image of the spectrum is projected onto the one-dimensional linear CCD array 62.
• the data corresponding to the pressure is transferred to a memory 104 through an A/D converter 100 (see Figure 5) .
• A/D converter 100 see Figure 5
• controller 103 performs the pressure changes to be detected. If memory 104 comprises a non-volatile memory, even if power is lost, the system still retains the pressure data and spectrum data in memory 104.
• FIG. 4 is a cross sectional view of one embodiment of a pressure sensor module 70 which includes pressure sensor 58.
• Pressure sensor module 70 includes a sealed housing 72 which is filled with an isolation fluid 74.
• the body 50 is suspended in the fluid using, for example, a support, or can be mounted to optical fiber 80.
• Isolation fluid can be, for example, an oil or the like which is substantially incompressible.
• An isolation diaphragm 76 extends over an opening in housing 72 such that an applied pressure P applied to isolation diaphragm 76 is transferred across the diaphragm 76 to isolation
• fluid 74 Isolation fluid 74 thereby applies the pressure P to body 50.
• body 50 is illustrated as having two portions, a flat portion 5OA and an etched portion 5OB. The portions 5OA and 5OB are bonded together, for example, using fusion bonding to form the cavity 52.
• An optical fiber 80 extends into housing 72 and has a tip directed at body 50. Optical fiber 80 couples the light beam 64 (see Figure 3) from source/sensor 60/62 to the body 50. In the configuration illustrated in figure 4, the source 60 and the sensor 62 are illustrated as a single component .
• the die of the sensor body must extend outside of the sensor package so electrical connections can be made to the sensing capacitors with the body. This requires that a relatively large seal extend around the die of the sensor body. Further, some sensors have rectangular cross sections which create sharp corners which are difficult to seal . Such seals can be made using a braze . However, the braze can exert large forces 'onto the sensor die and thereby introduce inaccuracies in the measurement .
• the optical fiber 80 has a circular cross section and therefore does not have any sharp corners .
• the fiber can be for example , on the order of 125 ⁇ m in diameter thereby requiring only 0.015 inches circumference to seal where the fiber 80 enters housing 72.
• the seal 81 see change in Fig.
• Isolation diaphragm 76 can be of a thin metal layer and can be fabricated using techniques known in the manufacture of pressure transmitters. While two embodiment shows the sensor body 50 isolated from the process fluid, it is appreciated that other embodiments could provide that the sensor directly contacts the process fluid
• the output 66 is provided to processing electronics 82.
• the maximum deflection distance d which can be accurately sensed is limited to one wavelength of that frequency. This is because the interference pattern will repeat for deflections greater than one wavelength and the processing electronics 82 cannot distinguish the repeating pattern.
• multiple frequencies are used in light beam 64, then a more complex pattern is generated. The more complex pattern can be mapped to deflections of spacing d in body 50 which are greater than one wavelength to thereby allow an extended operating range for the pressure sensor 58.
• light source 60 comprises a multifrequency light source.
• the light source sweeps through a range of about 250 nm to about 700 nm and the intensity of the reflected light is observed.
• the intensity pattern can be characterized and can be correlated with the applied pressure . As this frequency sweep may take a number of seconds, in one configuration the sweep is only performed periodically.
• the applied pressure is determined using the reflectance spectroscopy technique, the pressure can be continued to be monitored by using a single frequency. As discussed above, when using a single frequency, is it not possible to know which of any number of deflection positions caused the reflected light. This is because the reflected light repeats periodically.
• the reflected light may look the same color at multiple spacings between the substrates, for example at spacings of 2500 angstroms, 4000 angstroms, 8500 angstroms and 12000 angstroms.
• the frequency of the reflected light is the same for each of these spacing.
• a single frequency technique can be used to more rapidly update the pressure determination.
• the multifrequency technique can be repeated periodically, or more frequently if the single frequency measurement technique indicates that the deflection between the diaphragms is changing rapidly.
• FIG. 5 is a simplified diagram of transmitter 14 coupled to process piping 12 and including a pressure sensor module 70.
• on the output 66 from source/sensor 60/62 is provided to an analog to digital converter 100.
• the output from analog to digital converter 100 is provided to a controller 102 which can comprise, for example, a microprocessor or the like.
• Controller 102 operates in accordance with programming instructions stored in memory 104 and provides an output 106 to input/output circuitry 108.
• the output 106 is related to the applied pressure P.
• I/O circuitry 108 couples to a two wire process control loop 18 and is configured to transmit an output related to the applied pressure P.
• I/O 108 includes a power output which can be used to power circuitry of transmitter 14..
• the sensor body is formed of at least a partially transparent material so that the light can enter the material and be reflected.
• Figure 6 shows another example configuration 120 of the present invention in which the pressure sensor is formed by directly coupling an optical fiber 122 to a deformable sensor body 124.
• the optical fiber 122 comprises a cladding 123 and a core 125.
• the deformable sensor body comprises, preferably, a transparent brittle material which is bonded to the end of fiber 122.
• the first and second layers discussed above are formed by layer 126 of sensor body 124 and by the face 128 provided at the end of optical fiber 122.
• the body 124 in one particular configuration, can comprise an etched sapphire wafer bonded hermetically directly to the tip of fiber 122.
• Figure 7 shows another example configuration 127 of the present invention in which the deformable sensor body 124 is formed using a thin sapphire diaphragm 130 that is fused to a thin sapphire disk 129 having a hole 131 formed in the center.
• Sapphire disk 129 provides a spacer between the surface of the thin sapphire diaphragm 130 in the end of the optical fiber 122.
• the diaphragm 130 and disk 129 can be any appropriate configuration and are not limited to the round configuration discussed herein.
• the disk 129 is fused on the sapphire fiber 122. Any appropriate materials can be used including quartz and the fiber can comprise a standard optical fiber. This configuration allows improved control over the thickness, parallel orientation and surface finish quality of the diaphragm. This also more easily allows the diaphragm and the fiber tip to be more nearly parallel.
• the hole or gap 131 is also more easily controlled and fabricated.
• the diaphragm 130 itself can be separately finished as desired.
• the invention relates to a known effect which occurs, for example, with soap bubbles or on a thin oil film on water.
• This effect is used for the determination of the film thickness .
• many colors are visible on a soap bubble which change according to the layer thickness, e.g. when a soap bubble is blown up.
• These "colors at thin layers” are • based on the interference phenomenon, i.e. on the superposition of light waves, which have been reflected at the front and back side of the layer (at two boundaries with different optical densities) .
• the undisturbed superposition of the two reflected light rays 1 and 2 leads to periodical amplifications -and extinction in the spectrum of a white continuum light source (such as a halogen spectral lamp as a pseudo white-light source) .
• a white continuum light source such as a halogen spectral lamp as a pseudo white-light source
• the senor is illuminated through, for example, a fiber optics cable with a coupler, which is connected to the spectrometer and a halogen lamp.
• the reflected interference spectrum is guided back to the spectrometer, where at it is analyzed and the cavity length d change is computed.
• the multifrequency technique of the present invention can be implemented as a supplement to a single frequency technique.
• a multifrequency measurement technique is employed to determine the true distance between the substrates. Subsequent measurements can then be performed using the single frequency technique.
• a multifrequency measurement can be repeated periodically or more frequently, if the single frequency measurement technique indicates that the spacing between the substrates is rapidly changing.
• the multifrequency measurement technique of the present invention can be implemented using a multifrequency source in which multiple frequencies are simultaneously provided, or a multifrequency source that moves through a frequency over time .
• the frequency range can be a continuous range or it can be discreet steps . Any appropriate film material may be used including oil, air, other gas or liquid, etc.
• the present invention includes the use of reflectance spectroscopy to determine the variation of a spectrum (spectrum shift/moving) to measure the variation of a cavity that is related to variation of a pressure.

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• Physics & Mathematics (AREA)
• General Physics & Mathematics (AREA)
• Measuring Fluid Pressure (AREA)

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• 2006
  • 2006-05-23 US US11/439,097 patent/US7409867B2/en active Active
• 2007
  • 2007-05-18 WO PCT/US2007/012050 patent/WO2007139745A2/en not_active Ceased
  • 2007-05-18 CN CN2007800186102A patent/CN101449139B/zh active Active
  • 2007-05-18 EP EP07777186A patent/EP2029990B1/en active Active
  • 2007-05-18 JP JP2009512074A patent/JP5562635B2/ja not_active Expired - Fee Related
  • 2007-05-18 CA CA002652179A patent/CA2652179A1/en not_active Abandoned

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