Patent Grant
7020354
This invention relates in general to the field of pressure sensors and in particular to the field of fiber optic pressure sensors.
Active sound control systems often require, in addition to actuator and electronic control components, specialized sensor devices. The requirements associated with such sensors to a large part are determined by the particular active control approach employed, as well as the kind of performance expected of the sound controlling system. One type of sensor is the microphone, which measures sound or the existence of atmospheric pressure waves in a particular area of interest.
These sensors are often used as components of instrumentation packages employed on missiles, satellites or other rocket payloads, aircraft or other mechanisms in which atmospheric pressure waves, acceleration or vibration can be quite severe. When in employed in such environments, weight becomes a critical variable. A 10 gram weight differential at rest becomes a 150 gram weight difference during a 15 G acceleration event.
While various electro magnetic microphones offer high levels of sensitivity and good performance in terms of size and efficiency, these electromagnetic devices are often very sensitive to EMI and aren't always physically robust. The hardening of these devices to EMI almost invariably causes an increase in physical size, weight, sensitivity and expense.
The main emphasis has been given to phase modulated interferometric fiber optic sensors for detecting pressure and strain. Though interferometric type sensors offer very high sensitivity, polarization effects and high cost have delayed the implementation of these sensors to commercial applications. Frequency modulated sensors utilizing fiber gratings are very promising for monitoring strain in different location along one fiber.
One solution is to employ microphones that are not electrical or semiconductor in nature. The use of fiber optics for sensors in general, and for sound detection in particular, are one option available to designers which simultaneously solves the EMI sensitivity, size and weight problems inherent in electrical sensors.
Since fiber optics use light rather than electricity, a fiber optic microphone is generally insensitive to EMI, or EMF and is therefore more efficient in an environment that has a large amount of electro magnetic energy. As a result, fiber optic microphones can be located adjacent to or actually attached to electronic devices that generate large electro magnetic fields without negative affects to either the microphone or the area of interest.
Intensity modulated fiber optic sensors, while often less sensitive than comparable interferometric sensors, are simpler and less expensive.
Therefore it is an object of this invention to offer an intensity modulated fiber optic pressure sensor that is small and lightweight.
It is yet a further object to offer a pressure sensor that is EMI insensitive.
It is a further object of this invention to offer a pressure sensor that features high levels of sensitivity.
It is a yet another object offer a pressure sensor that is physically robust.
It is a further object of this invention to offer a pressure sensor that is economical.
A fiber optic pressure sensor includes a cartridge housing having an end that is exposed to the atmosphere, and a thin flexible membrane covering the exposed end of the cartridge housing such that the flexible membrane has an exposed side and a protected side. A fiber bundle is disposed within the cartridge housing, featuring a transmitting fiber having a first and second ends. The first end has a polished finish and the second end is coupled to a light source. The fiber bundle also features a multitude of receiving fibers disposed around the transmitting fiber with each receiving fiber having first and second ends where the first ends are also polished. The fiber bundle is disposed within the cartridge housing such that the first end of the transmitting fiber and the first end of each receiving fiber is adjacent to the protected side of the flexible membrane with free space between the first fiber end and the protected side of the flexible membrane. A light sensing means is coupled to second end of said receiving fibers wherein light launched into the transmitting fiber propagates emerges at the polished end, propagates a very short distance in air, and is reflected by the flexible membrane into the receiving fibers, propagates therethrough, and is detected by light sensing means. Upon a change in the atmospheric pressure, pressure waves cause the flexible membrane to distort causing a change in the amount of light reflected by the protected side of the flexible membrane. The intensity of the light coupled into the sensing fibers is modulated in relation to the intensity of pressure wave.
Referring now to the figures where like numbers denote like elements,
Upon a change in the atmospheric pressure, pressure waves cause the flexible membrane 140 to distort causing a change in the amount of light reflected by the protected side of the flexible membrane into the receiving fibers 120. Thus the intensity of the light coupled into the receiving fibers 120 modulates in relation to the intensity of pressure wave causing the flexible membrane 140 to distort. These pressure waves can be the result of a change in the environment such as a sound, if the sensor is configure to operate as a microphone. Other modes for the pressure sensor are also possible such as an altimeter mode that senses a change in atmospheric pressure, motion sensor or etc. Other sensor modes based on an event that results in a pressure wave are also possible.
The type of fiber employed in the example embodiment shown in
Generally multimode fibers with a combination of a thick core and thin clad fiber are preferred. Light incident on clad is lost, thus the core needs to be as close in proximity to the outer perimeter of the clad is possible to ensure efficient light coupling in the core. Thus light coupling within the fiber is maximized with a thick core thin clad structure. This however, does not limit the use of fibers in this device to multimode fibers with thick core thin cladding structures. Varying degrees of effectiveness and light coupling are possible with other fiber configurations.
The fiber optic pressure sensor's cartridge housing is constructed of a rigid and lightweight material. A rigid structure provides the sensor with a robust design. Another advantage to the rigid structure is that the sensor is very resistant to vibration, G forces and other structural forces with could impact sensor performance.
The cartridge housing features means to adjust the distance between the polished end of the transmitting fiber and the flexible membrane.
In the preferred embodiment the system employs a screw type configuration, with a locking nut and an adhesive material applied to the nut to maintain a consistent distance. This configuration also provides the user the ability to adjust the distance from the fiber bundle to the flexible membrane by very small increments, and to lock or otherwise maintain a consistent distance in a dynamic environment.
While other attachment and adjustment methods may be applied to the sensor as an adjustment means, to extract optimum performance from the sensor the mechanism must be resistant to vibration and temperature variations. If the distance from the fiber bundle to the membrane is not maintained precisely, the accuracy and sensitivity of the sensor package will suffer.
The cartridge housing features means to adjust the tension of the flexible membrane drawn across the exposed end of the cartridge housing. This may be accomplished via a screw type adjustment or some other means of adjustment.
The flexible membrane also features a coating on the protected side to enhance its light reflecting properties. In a preferred embodiment the flexible membrane is constructed of mylar and features at least one surface coated with thin aluminum film. The aluminum film increases the light reflecting properties of the flexible membrane, however other coatings may be applied to increase the membrane's sensitivity, or toughness.
The fiber optic pressure sensor fiber bundle features a single multimode transmitting fiber and a multitude of receiving fibers all encased in a protective tubing. In the example embodiment shown in
The light sensing means is at least one silicon PIN diode. LEDs represent a very efficient way to launch light into the fiber. LED are generally low cost and feature low noise operation in a fiber system. LEDs are also tend to be very stable over extended periods of time. Laser diodes are also applicable, although they increase the expense and complexity of the system. Current laser diodes, also tend to introduce additional noise to the sensor package. One suitable LED for use as a light source is an Optek OPF370A LED emitting light at 850 nm.
In another embodiment of the fiber optic pressure sensor the fiber optic pressure sensor features a cartridge housing having an end that is exposed to the atmosphere, a thin flexible membrane covering the exposed end of the cartridge housing such that the flexible membrane has an exposed side and a protected side. A fiber bundle is disposed within the cartridge housing, featuring a transmitting fiber having a first and second ends.
The first end has a polished finish and the second end is coupled to a light source. The fiber bundle also features a receiving fiber disposed adjacent to the transmitting fiber. The receiving fiber has a first and second end where the first end is also polished. The fiber bundle is disposed within the cartridge housing such that the first end of the transmitting fiber and the first end of the receiving fiber is adjacent to the protected side of the flexible membrane with free space between the first fiber end and the protected side of the flexible membrane. A light sensing means is coupled to second end of the receiving fiber wherein light launched into the transmitting fiber propagates emerges at the polished end, propagates a very short distance in air, and is reflected by the flexible membrane into the receiving fiber, propagates through the receiving fiber, and is detected by light sensing means. Upon a change in the atmospheric pressure, pressure waves cause the flexible membrane to distort causing a change in the amount of light reflected by the protected side of the flexible membrane into the receiving fiber. Thus the intensity of the light coupled into the sensing fiber modulates in relation to the intensity of pressure wave causing the flexible membrane to distort.
Epoxy is applied to the seven fibers so the fibers form a symmetric bundle close to the tubing end with the transmitting fiber at the center and cured. After curing, the fiber bundle can be cut close to the tubing end and the probe end can be polished. The probe, which includes the bundle and the protecting stainless steel tube, is housed within the plastic cartridge housing. The fiber bundle 230 is disposed within the cartridge housing 150 such that the first end of the transmitting fiber and the first end of each receiving fiber is adjacent to the protected side of the flexible membrane 240 with free space between the first fiber end and the protected side of the flexible membrane. The flexible membrane 240 is a 1.27×10−3 cm mylar (polyester) layer, having one surface that is coated with a thin aluminum film. The tension on the mylar membrane is adjusted to achieve the desired acoustic bandwidth. The membrane- probe separation is also adjusted to achieve the desired bandwidth sensitivity. A somewhat broad dynamic sensivity maximum was found for a probe—membrane separation between 180 and 250 μm.
Upon a change in the atmospheric pressure, pressure waves cause the flexible membrane to distort causing a change in the amount of light reflected by the protected side of the flexible membrane into the receiving fibers. Thus the intensity of the light coupled into the receiving fibers modulates in relation to the intensity of pressure wave causing the flexible membrane to distort.
In yet another embodiment a single fiber may be used as the transmitting and receiving fiber. In this embodiment the light source and the means for sensing the received light are both coupled to the fiber end. The light is launched from the fiber into the flexible membrane and is reflected back into the same fiber.
In the single fiber embodiment, it is suitable to use a multimode optical fiber with a 200 μm glass core, and 230 μm plastic clad, a 500 μm Tefzel plastic coating, and a numerical aperture of approximately 0.37. The probe housing is a stainless steel tube of 902 μm outer diameter and 584 μm inner diameter. To form the probe, the fiber is inserted in the tubing with its coating and epoxy is applied on the fiber. After the epoxy has cured, the fiber is cut closely to the tubing and the end of the fiber is polished well. A 50/50 (at 850 nm), low loss, optical coupler, such as one manufactured by Gould Electronics, can be used in the one fiber probe.
The one fiber probe can also be dynamically displaced against the mirror by vibrating the PZT transducer electrically. The displacement amplitude of the vibrating mirror can be obtained from the output of a small reference accelerometer (for example, the Endevco 2250A) mounted close to the mirror. Results are shown in
Thus, the increased sensitivity difference and the high cost of the multimode coupler used in the one fiber probe make a seven-fiber probe sensor better for some applications than a one fiber probe sensor, even though the one fiber probe sensor uses only one fiber instead of seven. In other applications, for example, in remote sensing applications in which longer fiber lengths are needed, a one-fiber probe sensor can be a better choice.
Based on the results shown in
Typically, a good PIN detector can detect a fraction of a picowatt ac signal, the minimum detectable displacement limited by the detector noise is: minimum detectable displacement>0.01 A.
Pressure can be detected by replacing the mirror used in the fiber probe with a reflecting surface such as a reflecting membrane. The membrane used in a one or seven fiber pressure sensor such as the one shown in
The cartridge provides mechanisms for applying the desired membrane tension and for clamping the probe in place at the optimum membrane-probe end distance. The optimum membrane-probe end disatnce can be found in practice by adjusting the distance, monitoring the detected light from the receiving fibers, and taking into account the
The pressure response of the seven fiber probe microphone illustrated in
The noise equivalent power (NEP) of a good detector is approximately 0.17 pW/Hzl/2 (e.g., the EG&G model HUV-1100 PIN detector with a preamplifier), which corresponds to a minimum detectable pressure of 41 dB re 1 μPa/Hz1/2. Typically, however, noise from other sources can limit the minimum detectable pressure, so the minimum detectable pressure will be higher than 41 dB it would be higher than 41 dB re 1 μPa/Hz1/2.
In the above-mentioned experiment, the light source is the OPF 370A Optek LED driven by a LD-3620 Lightwave Technology Power supply and the detector is a PDA 55 PhorLabs PIN whose output is stored directly into 3582A HP spectrum analyzer. The minimum detected pressure for this arrangement was found to be 84 dB re 1 μPa/Hz1/2.
The minimum detectable pressure and bandwidth of a microphone with a diagragm can be determined by the following equations, in which T is the tension, pm is the density, and r is the radius of the diaphragm:
For conventional microphones such as the 4938 B&K: x/p=r2/(4T) , where x is the displacement of the diaphragm generated by an applied pressure p.
For the fiber microphone: x/p=r2/(8T).
By comparing these equations, it is apparent that the sensitivity of the fiber microphone is twice as high as that of conventional microphones. Further, the microphone sensitivity is directly proportional to the radius squared. Thus, the minimum detectable pressures for the fiber microphone and the 4938 microphone are expected to be about the same for diaphragms of the same radius. Thd microphone sensitivity is also inversely proportional to the diaphragm tension. On the other hand, the first resonance of the microphone, which determines the microphone bandwidth, is inversely proportional to the radius and proportional to the square root of the tension: fR=(2.4/(2πr))*(T/(pmt))1/2.
In general, the radius and tension of the diaphragm can be chosen to satisfy the required minimum detection pressure, bandwidth, and size of the microphone.
Although this invention has been described in relation to the exemplary embodiments thereof, it is well understood by those skilled in the art that other variations and modifications can be affected on the preferred embodiment without departing from scope and spirit of the invention as set forth in the claims.
The present application claims the benefit of the priority filing date of provisional application No. 60/383,577 filed on 28 May 2002, hereby incorporated, in its entirety, by reference.
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