PRESSURE MEASURING SYSTEM

Abstract: A pressure measuring system wherein light is transmitted from a light source (3) via a first optical fiber (5) to a pressure-­ receiving diaphragm (1) having a reflecting surface (1a) which undergoes a deflection in relation to the pressure being mea­ sured. The diaphragm (3) reflects light transmitted thereto to a second optical fiber (7) which transmits the reflected light to a first photosensitive element (9a). A reference optical fiber (13) is arranged approximately coextensive to the first and second optical fibers (5,7) and transmits light from the light source (3) in round trip fashion along substantially the same path as light transmitted by the first and second optical fibers (5, 7) to a second photosensitive element (9b). The photosensitive ele­ ments (9a, 9b) produce respective electrical signals which are applied to an operational circuit (15) which eliminates varia­ tions in the amount of light due to external disturbances by di­ viding one electrical signal by the other. With such a construc­ tion, it is possible to have a pressure measuring system which is unaffected by electromagnetic induction accompanying large currents and high voltages, which enables more accurate and more sensitive pressure measurements.

Description: This invention relates to a pressure measuring system according to the preamble of claim 1. In a known system employing a pressure gauge of the distortion type, electrical cords for transferring signals are required. These electrical cords are subject to the influences of electromagnetic induction that accompany large currents and high voltages associated with power apparatus and power machines within the plant, and accordingly the output of the prior art system is subject to errors. A pressure measuring system, which is relatively little affected by the influences of the electromagnetic induction, for example, utilizing optical fibers, is known from EP 00 74 055 A2 and from "Navy Technical Disclosure Bulletin", Vol. 4, No. 7, 1979, pages 9 to 12. In such a pressure measuring system, light from a light source is projected onto a pressure-receiving diaphragm mirror via an optical fiber. The reflected light from the mirror is then projected to photosensitive elements through an optical fiber provided for light reception. Received reflected light is then converted to an electrical signal, and pressure is measured based on the change in the reflected light due to deflection in the diaphragm mirror. However, in the system of this kind, use is made of an ordinary light source so that the amount of the received light varies due to the light source itself as well as due to vibrations of the optical fibers caused by external forces. Hence there is the drawback that these variations in the light reveal themselves as apparent variations in pressure. In addition, because of the absence of dust-preventive measures for the installation section of the diaphragm mirror, stains tend to appear on the diaphragm mirror due to attachment of dust or the like. There is another drawback, that the reduction in the amount of the reflected light due to stains also shows itself as an apparent reduction in the pressure. In particular, due to the fact that both the optical fiber coupled to the light source and the optical fiber receiving reflected light are given no tilt with respect to the normal to the reflecting surface, the amount of reflected light received by the optical fiber for light reception is small, and in fact the reflected amount of light is considerably reduced. Moreover, the reduction in the received light amount sets a limitation on the sensitivity of the measurement obtained. US patent No 44 87 206 discloses a fiber optic pressure sensor with temperature compensation and reference. A lense element at the end of the fiber optic collimates the light emanating from the fiber and directs the light in a column towards a closely spaced reflecting diaphragm. Said diaphragm is pressure-responsive and modulates the focal length of the lense diaphragm mirror combination. Three optic fibers transverse the length of the hollow catheter tube, one carrying transmitted light to the tip, one carrying the reflected light from the diaphragm and the third fiber carrying a reflected reference light. The reference detector provides a signal level to control the level of intensity of the light source. The reference light signal is sent to the sensor head down the same fiber as the measuring light signal. It is an object of the invention to provide a pressure measuring system which has a sturdy resistance against, and is unaffected by, electromagnetic induction arising due to large currents and high voltages being in close proximity, and which enables a more accurate and sensitive pressure measurement. This object is solved for a pressure-measuring system according to the preamble of claim 1 in that at least one of said first and second optical fiber means are arranged at an angle set with respect to the normal to said reflecting surface so that light is reflected to said second optical fiber means even where there is no deformation of said diaphragm; a reference optical fiber means arranged substantially coextensive with said first and second optical fiber means for transmitting a beam from the light source in round trip fashion along substantially the same path as travelled by light transmitting by said first and second fiber means; a second photosensitive element converting light beam transmitted by said reference optical fiber means into an electrical output signal; operational means for eliminating the variations in the amount of light due to external disturbances based on the electrical output signal from said first and said second photosensitive elements and for producing a corresponding output signal; and output adjusting means for establishing a linear relationship between the output signal of the operational means and the measured pressure. It is an advantage of the invention to provide a pressure measuring system which can strongly resist the effects of electromagnetic induction or the like which accompanies large current and high voltage. A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: FIGURE 1 is a simplified block diagram of a first embodiment of the pressure measuring system in accordance with the present invention; FIGURE 2 is a simplified block diagram of a second embodiment of the pressure measuring system in accordance with the present invention; FIGURE 3 is a simplified block diagram of a third embodiment of the pressure measuring system in accordance with the present invention; and FIGURE 4 is a simplified block diagram of a fourth embodiment of the pressure measuring system in accordance with the present invention. Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, and more particularly to Figure 1 thereof, a first embodiment in accordance with the present invention will be next described. As shown in Figure 1, the pressure measuring system of the invention includes a detection section which is attached air-tight to the object 12 to be measured via a sealing material 14, a pressure measuring section 20 which is installed a distance apart from the detection section 10, and which is equipped with a light source, photosensitive elements and processing circuits for outputting the measured value of the pressure, and an optical fiber section which is provided between the detection section 10 and the measuring section 20. Included in the above sections are a pressure-receiving diaphragm 1 which receives pressure P, a light source 3, an optical fiber 5 for light source 3 which transmits the light from the light source 3 to the reflecting surface 1a of the pressure-receiving diaphragm, an optical fiber 7 which transmits the reflected light from the reflecting surface 1a, and a first photosensitive element 9a which converts the reflected light into an electrical signal. In addition, there are provided a reference optical fiber 13 which transmits reference light from the light source 3 back and forth between the light source 3 and the sensor head 11, and a second photosensitive element 9b which converts the reference light into an electrical signal. Furthermore, there is provided an operational circuit 15 which carries out a divisional operation in which the reflected signal output from the first photosensitive element 9a is divided by the reference signal output adjusting circuit 17 which establishes a linear relationship between the output signal from the operational circuit means 15 and the received pressure. The pressure-receiving diaphragm 1 is attached to the installation part 19 and is designed to undergo a deflection in proportion to the pressure P to be measured upon application of such pressure and based thereon change the direction of the reflected light from the reflecting surface 1a. The reflecting surface 1a may have been processed, for example, by buffer polishing. The light source 3 can be an ordinary light source such as an LED. The optical fiber 5 and the optical fiber 7 are installed with a predetermined angle of tilt with respect to the normal to the reflecting surface 1a, and are arranged so as to turn light from one direction into another direction within the sensor head 11. The angular orientation of the fibers 5 and 7 may be constructed such that the above tilt is given only to one or the other or both of the optical fibers 5 and 7. The installation section 19 for the pressure-receiving diaphragm 1 and the sensor head 11 are joined via a sealing section 21, at a position with a predetermined separation between the pressure-receiving diaphragm 1 and the edge surfaces of the optical fiber 5 and the optical fiber 7 in the sensor head 11. This sealing is formed, for instance, by electron beam welding to seal off the space 23 bounded by the reflecting surface 1a and the sensor head 11. The space 23 is connected to a pipe 25 for vacuum pumping. Pipe 25 is blocked up after vacuum pumping is completed to form a pressure measuring system of the absolute pressure type. In pumping the vacuum, a valve, not shown, is installed on the pipe 25. The pipe 25 for vacuum pumping is connected through a vacuum apparatus via this valve. After pumping of the space 23 is completed by the operation of the vacuum apparatus, the pipe 25 for vacuum pumping 25 is plugged by squeezing it between the valve and the sensor head 11. While maintaining the plugging, the pipe 25 is cut-off at a position nearer to the valve than the plugged position and welded at the cut-off section. The reference optical fiber 13 is arranged parallel to the optical fiber 5 and the optical fiber 7 and is substantially coextensive therewith. More particularly, the reference optical fiber 13 is extended along the optical fiber 5, and reaches the sensor head 11. The reference optical fiber 13 reaches to the inside of the sensor head 11 and is folded back and extended along the optical fiber 7 to reach the second photosensitive element 9b. The first and second photosensitive elements 9a and 9b are constructed in a similar manner and each converts light into a corresponding electrical signal. In other words, elements 9a and 9b convert the light received from the optical fiber 7 and the reference optical fiber 13 to electrical signals respectively corresponding to the amount of light received and apply these electrical signals to the operational circuit 15. Next, operation of the first embodiment will be described. The light rays from the light source 3 are, irradiated on the reflecting surface 1a by means of the optical fiber 5. The reflected light from the reflecting surface 1a is led by the optical fiber 7 to the first photosensitive element 9a where it is converted to a corresponding electrical snginal output. Here, the pressure-receiving diaphragm 1 is deflected in proportion to the pressure P received, which varies the amount of reflected light to be incident from the reflecting surface 1a on the optical fiber 7. On the other hand, light rays from the light source 3 travel round trip through the reference optical fiber 13, approximately parallel to the optical fiber 5 and the optical fiber 7 and is led to a second photosensitive element 9b. The light received by the second photosensitive element 9b. The light received by the second photosensitive element 9b is converted to a corresponding reference electrical signal output. The reflected signal output and the reference signal output produced by the first photosensitive element 9a and the second photosensitive element 9b, respectively, are input to the operational circuit 15 where division of the reflected signal output by the referenc signal output is carried out, and the result is output. Then, the output from the operational circuit 15 is input to an output adjusting circuit 17 where a linear relationship between the output signal from the operational means 15 is established. Now, if the amount of light beam when there are no vibrations of the optical fibers 5, 7 ans 13 due to external forces, variations in the amount of light from the light source 3, and so forth, that is, when there are no external disturbances, is called I, the amount of the reflected light received by the optical fiber 7 for light reception from the reflecting surface 1a of the pressure-receiving diaphragm 1 which is in proportion to the pressure P it receives may be represented, for instance, by I. Here, is the rate of change in the amount of received reflected light which is proportional to the pressure P. Then, if the disturbance coefficient when vibrations exist in the optical fibers 5, 7, and 13 due to external force, and so forth, that is, when external disturbances exist, is called , the amount of reflected light from the reflecting surface 1a which is distorted in proportion to the pressure P will become I. The amount of reference light which passes through the reference optical fiber 13 in this case will be I. Here, the first photosensitive elment 9a receives an amount, I, of light from the optical fiber 7, and converts it into an electrical signal, for instance, E (where E I) which is input to the operational circuit 15. The second photosensitive element 9b receives an amount, I, of light from the reference optical fiber 13, and converts it similarly to an electrical signal E which is input to the operational circuit 15. At the operational circuit 15, the following division is performed using these signals αE and E. aE/E = α. Since the result of the divisional operation in the operational circuit 15 is the change in the amount of light received by the pressure-receiving diaphragm 1 in proportion to the pressure P, as was mentioned earlier, the result shows that the elements in the change of the light amount due to external disturbances have been eliminated. Based on the valve of thus found, its linear relationship with the pressure P received is established in an output adjusting circuit 17. In this manner, it becomes possible to eliminate apparent pressure variations due to variations in the amount of light from the light source 3 as may be caused by vibrations of the optical fibers due to external forces, and so on, thereby enabling one to obtain more accurate pressure measurements. In addition, since the optical fiber 5 and the optical fiber 7 in the sensor head 11 are constructed to have a predetermined angle with respect to the normal to the reflecting surface 1a, the amount of reflected light received by the optical fiber 7 is increased in comparison to the prior art pressure measuring system which has an angle of 0 o with respect to the normal to the reflecting surface 1a. Consequently, even when a reduction in the amount of reflected light occurs due to stains on the reflecting surface 1a, or the like, the rate of the reduced amount in relation to the total amount of light is low, so that the present system is less susceptible to influences due to reduction in the light amount, and hence it is possible to carry out a more accurate pressure measurement. Moreover, in the present embodiment the space 23 formed between the reflecting surface 1a and the sensor head 11 is kept in a vacuum state. Therefore, there will be few stains on the reflecting surface 1a. This prevents a reduction in the amount of reflected light due to stains, enabling one to have more accurate pressure measurements. In this case, it is noted that the measuring system will function as a pressure measuring system of the absolute pressure type since the space 23 is kept under vacuum. Figure 2 shows a second embodiment of the present invention, which unlike the case of the first embodiment is constructed such that the reference optical fiber 13 is disconnected in the sensor head 11, and the reflected light from a wallsurface 27 formed in the sensor head 11 is led to the second photosensitive element 9b. The wall surface 27 is given such a mirror surface processing as aluminum evaporation. Therefore, in addition to obtaining the improved pressure measurement as in the first embodiment provides easy assembling of the system. The remaining construction is substantially identical to that of the first embodiment, so that further explanation is omitted. Figure 3 relates to a third embodiment of the present invention which shows a further simplification of the invention. In this embodiment, the reference optical fiber 13 is constructed by a single line, and light reflected from the wall 27 in the sensor head 11 is arranged to be branched off by a coupler 29 which is provided at the midpoint in the optical fiber for reference 13, to be led to the second photosensitive element 9b. Therefore, in addition to the fact that the improved pressure measurement of the first embodiment is obtainable, the third embodiment further is easy to construct and makes possible compacting of the system. The manufacture of the system due to this embodiment may further be simplified since the coupler 29 may be constructed as a half mirror, or instead of giving a mirror surface finishing to the wall surface 27, the end surface 13a of the reference optical fiber 13 in the sensor head 11 may be given a mirror surface finishing by means of aluminum evaporation or the like. The remaining construction is substantially identical to the first embodiment and further explanation is therefore omitted. Figure 4 shows a fourth embodiment of the present invention which is a further simplification of the third embodiment. In contrast to the third embodiment, in this embodiment, two couplers 31 and 33 are placed in the optical fiber 5. Light is branched to the optical fiber 7 from the coupler, 31, while branching to the first photosensitive element 9a is made through the other coupler 33. Therefore, in addition to the fact that effects similar to the third embodiment are obtained, a further compacting of the construction of the system is achieved by the fourth embodiment. The remaining construction is substantially identical to the third embodiment and further explanation is therefore omitted. It should be noted that the present invention is not limited to the embodiments described in the foregoing. For example, the light source may be replaced by a laser light source. In addition, the optical fiber 5, the optical fiber 7, and reference optical fiber 13 may be constructed of fiber bundles. Further, as the operational circuit 15, use may be made of a multiplicative operational circuit, an additional operational circuit, or a subtractional operational circuit. Still further, stains on the reflecting surface may be prevented by sealing an inert gas in the space 23. In such a case, the pressure measuring system will function as a differential pressure-type system. In this manner, the present invention may be embodied in various modified forms. In accordance with the first embodiment of the present invention, optical fibers are used for signal transmission so that the system of the invention is insensitive to electromagnetic inductive noise accompanying a large current and a high voltage, which enables accurate pressure measurements. Moreover, when the reflected signal output, which is obtained by converting the reflected light from the reflecting surface of the pressure-receiving diaphragm into an electrical signal, is divided, for example, by the reference signal output derived from reference light which travelled along substantially the same path between the light source and the photosensitive element, it becomes possible to remove the variations in the amount of light from the light source, the variations in the amount of light generated by the vibrations in the optical fibers due to external forces, and so forth. Therefore, apparent pressure variations due to variations in the amount of light can be removed, permitting more accurate pressure measurements. Furthermore, in the second embodiment of the present invention, the optical fiber 5 and the optical fiber 7 are given an angle with respect to the normal to the reflecting surface in order to direct light at least from one fiber to the other. Thus, the amount of reflected light received by the optical fiber 7 will be increased, and it is made less susceptible to the influence of the reduction in the amount of the reflected light due to stains on the reflecting surface or the like, enabling one to obtain more accurate measurements. Moreover, more sensitive measurements of pressure become possible due to increase in the amount of the reflected light. Obviously, numerous additional modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims, the invention may be precticed otherwise than as specifically described herein.

Claims: A pressure measuring system for measuring pressure at the section to be measured, comprising: a light source (3); a first optical fiber means (5) for transmitting light from the light source (3); a pressure-receiving diaphragm (1) having a reflecting surface (1a) for reflecting light transmitted by the first optical fiber means (5), wherein said diaphragm (1) is mounted for undergoing deflection in relation to the pressure to be measured; a second fiber means (7) for receiving light reflected off the diaphragm (1) and for transmitting the received reflected light from the pressure-receiving diaphragm (1); a photosensitive element (9a) for converting the reflected light transmitted by the second optical fiber means (7) into an electrical signal; output means (15) for producing an output signal corresponding to the measured pressure based at least in part on said electrical signal; characterized in that: at least one of said first and second optical fiber means (5, 7) are arranged at an angle set with respect to the normal to said reflecting surface (1a) so that light is reflected to said second optical fiber means (7) even where there is no deformation of said diaphragm (1); a reference optical fiber means (13) arranged substantially coextensive with said first and second optical fiber means (5, 7) for transmitting a beam from the light source (3) in round trip fashion along substantially the same path as travelled by light transmitting by said first and second optical fiber means (5, 7); a second photosensitive element (9b) for converting the light beam transmitted by said reference optical fiber means (13) into an electrical output signal; operational means (15) for eliminating the variations in the amount of light due to external disturbances based on the electrical output signal from said first and said second photosensitive elements (9a, 9b) and for producing a corresponding output signal; and output adjusting means (17) for establishing a linear relationship between the output signal of the operational means (15) and the measured pressure. A pressure measuring system according to claim 1, characterized by: said reference optical fiber means (13) comprising a single optical fiber having an end section (13a) in the vicinity of said diaphragm; means (27, 13a) for reflecting light transmitted by said reference optical fiber means (13) from said light source (3) to said end section back towards said light source (3) and a coupler means (29) inserted in said reference optical fiber means (13) for branching light reflected back by said reflecting means (13a, 27) in said reference optical fiber (13) to said second photosensitive element (9b), said second photosensitive element (9b) producing an electrical output signal which is coupled to said output means (15), said output means (15) producing said output signal based on the electrical output signals from both said photosensitive elements (9a, 9b). A pressure measuring system as claimed in claim 1, characterized by: a sensor head (10) having a first section for joining an end section of the first optical fiber means (15) and an end section of the second optical fiber means (7) with a space in-between such that said end faces face the reflecting surface (1a) of said pressure-receiving diaphragm (1), said sensor head (10) further having a second section for turning light transmitted by said reference optical fiber means (13). A pressure measuring system as claimed in claim 3, characterized in that said first section comprises means for imparting an angle with respect to a normal to said reflecting surface (1a) to at least one of optical fiber means (5, 7) such that light is reflected to said second optical fiber means (7) even when there is no deformation of said diaphragm (1). A pressure measuring system as claimed in claim 4, characterized in that said space (23) in-between said end sections of said first and second optical fiber means (5, 7) is kept in a vacuum state. A pressure measuring system as claimed in claim 3, characterized in that said second section of said sensor head (10) is a wall surface (27) of said sensor head having a mirror surface finishing. A pressure measuring system as claimed in claim 6, characterized in that said reference optical fiber means (13) comprises: an optical fiber; and a coupler (29) installed in said reference optical fiber means (13) for branching light reflected from said second section to the second photosensitive element (9b). A pressure measuring system as claimed in claim 3, characterized by: two couplers (31, 33) installed in said first and second optical fiber means (5, 7), one of the couplers (31) being installed in the vicinity of the sensor head in order to branch off light between the first and second optical fiber means (5, 7) at the sensor head, the other coupler (33) provided for branching the reflected light to the first photosensitive element (9a). A pressure measuring system as claimed in claim 7, characterized by: two couplers (31, 33) installed in said first and second optical fiber means (5, 7), one of the couplers (31) being installed in the vicinity of the sensor head in order to branch off light between the first and second optical fiber means (5, 7) at the sensor head, the other coupler (33) provided for branching the reflected light to the first photosensitive element (9a). A pressure measuring system as claimed in claim 3, characterized in that said space (23) in-between said end sections of said first and second optical fiber means (5, 7) is filled with an inert gas.

Inventors: IWAMOTO, KATSUHARU, KAWAKAMI, OSAMU, MIYAGAWA, HIDEO, TAKIJI, ISHIMURA, TASHIRO, IKUO
Applicants: KABUSHIKI KAISHA TOSHIBA
Application year: 1985
Main classification: G01L9
IPC codes: G01L9