Measuring and testing – Fluid pressure gauge – Photoelectric
Reexamination Certificate
1998-09-02
2001-10-23
Oen, William (Department: 2855)
Measuring and testing
Fluid pressure gauge
Photoelectric
Reexamination Certificate
active
06305227
ABSTRACT:
BACKGROUND OF INVENTION
1. Technical Field
The present invention relates to a distributed sensing system; and more particularly, to a distributed sensing system using quartz and optical fiber for sensing a physical property such as pressure in an borehole of an oil well.
2. Description of the Prior Art
A traditional pressure sensor using quartz crystals generates a RF signal whose frequency is proportional to the pressure applied on the pressure sensor. The frequency of the RF signal is measured and converted into a pressure measurement.
To compensate for the temperature effect on the measurement, the pressure sensor is typically made of two quartz structures that have resonance frequencies depending on the temperature and pressure. One of the quartz structures is insulated and is under a constant pressure. The other quartz structure is subject to the pressure at the sensor location. The two resonance signals are mixed. The RF signal whose frequency is a difference between the two resonating frequencies is the output. The difference in resonance frequencies is due to the pressure.
Such a pressure measurement is very accurate and is free of temperature effect. Because the quartz property is very stable over time, the sensor measurement does not drift with time. The quartz pressure sensor is very accurate and reliable for both dynamic and static pressure measurements.
Although the quartz pressure sensor is excellent in applications where a single point or a few point sensing is required, it is, however, very hard to build a distributed pressure sensing system using these pressure sensors. A RF telemetry system requires the capability of generating at a sensor location a carrier RF signal whose frequency does not change with pressure, temperature, and time. The cross talk is eliminated by using a carrier whose frequency is distinct. A digital approach would require attaching electronics to each sensor to digitize the frequency information and feed it into a digital telemetry bus.
Various distributed fiber optic pressure sensing systems have been used to measure acoustic or seismic signals. In particular, systems using fiber Bragg gratings (FBG) have been proposed to perform distributed acoustic sensing. The advantage of the FBG systems is that the sensors are part of the fiber that is also the telemetry system. Such sensing systems are simple, reliable, scalable, and inexpensive. Although good for sensing dynamic signals, simple FBG based systems do not produce reliable measurement of absolute static pressure free of temperature effect over time.
The present invention provides a solution to overcome these disadvantages in the prior art sensing systems.
SUMMARY OF THE INVENTION
The present invention provides a quartz sensing system includes a quartz sensor, an electromechanical converter, an optical source, an optical fiber and a measurement unit.
The quartz sensor responds to a physical property such as pressure in a borehole of an oil well, and further responds to an electrical power signal, for providing a quartz sensor electrical signal containing information about the physical property such as the pressure.
The electromechanical converter responds to the quartz sensor signal, for providing a mechanical force containing information about the sensed voltage or current signal.
In one embodiment, the electromechanical converter includes a piezoelectric or magnetostrictive transducer that responds to the quartz sensor signal, for providing an piezoelectric or magnetostrictive transducer force in the form of an expansion or contraction force containing information about the quartz sensor signal. The piezoelectric or magnetostrictive transducer converts electrical energy into mechanical energy, and vice versa.
In another embodiment, the electromechanical converter includes two acoustic tranmitters, one being connected to the quartz sensor and acting as an acoutic transmitter, and the other being an acoustic receiver. The acoustic transmitter responds to the quartz sensor signal, for providing an acoustic transmitter wave containing information about the quartz sensor signal. The acoustic receiver responds to the acoustic transmitter wave, for providing a receiver force containing information about the acoustic transmitter wave. In effect, the receiver converts acoustic energy into mechanical energy in the form of expansion and contraction.
The optical source provides an optical signal through the optical fiber. The optical signal may be a broadband or narrowband signal depending on whether a wavelength or time division multiplexing signal processing scheme is used.
The optical fiber responds to the electromechanical converter force, for changing an optical parameter or characteristic of the optical source signal depending on the change in length of the optical fiber and providing an electromechanically-converted optical signal containing information about the electromechanical converter force. In effect, the optical fiber converts electromechanical energy from the piezoelectric or magnetostrictive transducer into optical energy by changing a fundamental characteristic or parameter such as the phase or wavelength of the optical signal being transmitted or reflected through the optical fiber. The optical fiber is wrapped around the transducer and affixed thereon, including by bonding or wrapping under tension the optical fiber on the transducer. The optical fiber expands and contracts along with the transducer, which causes the change in the length of the optical fiber, which in turn causes the change in the phase or wavelength of the optical signal. The optical fiber may also have one or more fiber Bragg Gratings therein which change the phase of the optical signal depending on the change in length of the fiber Bragg Grating. Fiber Bragg Grating pairs may also be arranged on a part of the optical fiber not bonded to the transducer when a cavity approach is used with the transducer arranged between the fiber Bragg Grating pairs. In summary, the electrical voltage signal from the quartz sensor causes the transducer to stretch or contract the optical fiber and change in the length of the fiber, which in turn causes a change of the phase or wavelength in the optical signal being transmitted or reflected through the optical fiber.
The measurement unit responds to the electromechanical converter optical signal, for providing a measurement unit signal containing information about the physical property such as the pressure. The measurement unit converts the electromechanically-converted optical signal into the information about the physical property by detecting and processing the change in the optical parameter or characteristic such as the phase or wavelength of the optical signal.
The present invention provides important advantages over the sensing system of the prior art. The quartz sensor is a very stable sensing device. Moreover, since voltage measurements are effectively made at the sensing locations, the sensing system of the present invention does not suffer from the problem of signal attenuation. In operation, the electrical voltage signal is electro-optically converted into an optical phase or wavelength signal that is not adversely affected by amplitude attenuation as long as the amplitude of the light reaching the measurement unit is above some minimum value. Therefore, the voltage signal is effectively digitized at the sensor location without practically any quantization error. The optical detector system of the measurement unit and the property of the piezoelectric or magnetostrictive transducer determine the accuracy of the voltage measurement. Another advantage of the present invention is that there is no need for using sensitive electronics subject in hazardous conditions such as high temperature or pressure in a borehole of an oil well.
The present invention, therefore, can be used in many sensing applications.
The invention will be fully understood when reference is made to the following detailed description taken in conjunction with an accompanying drawing.
REFERENCES
Didden Kevin F.
Hay Arthur D.
Kersey Alan D.
Pruett Phillip E.
Wu Jian-Qun
CiDRA Corporation
Oen William
Ware Fressola Van der Sluys & Adolphson LLP
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