MASS RATE OF FLOW METER

Abstract: An angular momentum type flowmeter has an unrestrained rotor (37) and a restrained turbine (40). Sensing means includes a magnetic coupling (47, 53) to generate a signal in a sensing coil (17) when a point on the rotor passes a point on the turbine. The sensing coil coaxially surrounds the turbine. A bucking coil (81) is connected electrically in series with the sensing coil but is magnetically isolated therefrom. A copper sheet formed as a shorted turn (77) is disposed between the sensing coil and the bucking coil to compensate phase shifts in the offsetting voltages produced in the coils by electromagnetic interference.

Description: MASS RATE OF FLOW METER Background of the Invention This invention relates to mass rate of flow meters of the angular momentum type having a swirl generator for imparting swirl to a fluid stream to be measured and a torque balance reaction means for removing the imparted swirl. Such a flowmeter is disclosed in United States Patent No. 4,056,976 (Hildebrand et al). That patent shows a flowmeter including a housing having an axial fluid passage therethrough. A swirl generator imparts a substantially constant angular velocity to an enter¬ ing fluid stream. As the"" " fluid leaves the swirl generator, it passes through an axially spaced, unres- trained rotor. The angular velocity of the rotor accurately represents the angular velocity of the fluid stream as it leaves the rotor to pass through an axially spaced, spring-restrained turbine. The angu¬ lar momentum of the fluid stream angularly displaces the turbine about the axis and against the bias of its restraining spring. Under steady state conditions, this deflection of the turbine is proportional to the mass rate of flow. The rotor carries two circumferentially and longitudinally displaced ' bar ' magnets. The first magnet is disposed on the input end of the rotor and is circumferentially poled. A first sensing coil assembly is radially spaced from the magnet and isolated from the fluid flow. Each time the first magnet passes the first sensing coil, it induces a "start" pulse in the coil that indicates the passage of a predetermined point on the rotor past a predeter¬ mined point on the housing. The second magnet is at the exit end of the rotor and diametrically opposed to the first magnet. A longitudinally extending bar of a highly permeable material, such as soft iron, is mounted on the peri¬ phery of the turbine. The axial spacing between the rotor and the turbine interposes an axial air gap between the bar and the second magnet when they align. A second sensing coil assembly, that is iso¬ lated from the fluid flow, is coaxial with and longi¬ tudinally coextensive with the second magnet and the bar. Each time the second magnet passes the bar, the flux that the bar. couples to the second sensing coil assembly changes and induces a "stop" voltage pulse in the second sensing coil. As des¬ cribed in the foregoing United States Patent No. 4,056,976, timing circuits convert the start and stop pulses from the first and second sensing coil assemblies into an indication of the mass rate of flow through the meter. Flowmeters of the type described in United States Patent No. 4,056,976 are used in aircraft. In such applications, flowmeters are disposed in an environment that is characterised by high levels of electromagnetic interference. For example, alter- nators disposed on the aiTcraft engine and a number of conductors around the engine are sources of electro¬ magnetic interference. It is therefore necessary to construct a flowmeter which is suitably insensitive to this electromagnetic interference. If the flow¬ meter is susceptible to electromagnetic interference, it can produce a signal even while there is no flow and no motion of the rotor. During operation, the rotor undergoes a range of rotational velocities. As the voltage induced in the coils is proportional to that velocity, the magnitude of the voltages that are produced in the sensing coils varies. In one particular application, for example, the voltage amplitudes of the pulses produced over a normally encountered range of fuel flows varies from about 30 millivolts to about 500 millivolts. With such variations it is necessary to reduce the signals that any electromagnetic inter¬ ference might produce to a level below the 30 milli- volt level, that is, that minimum voltage that would normally be generated during operation. The first sensing coil has a longitudinal axis that extends radially with respect to the housing. The coil is a compact coil mounted on the housing of the flowmeter. It may be shielded from electro¬ magnetic interference, as by the use of a highly permeable, magnetic cover that isolates the coil from the effects of electromagnetic interference. However, if that approach is attempted with the second sensing coil, which is wrapped completely around the housing, it does not produce acceptable results. It does not completely shield the coil from electro¬ magnetic interference. The material that would be needed to provide acceptable results would add weight to the flowmeter which is not desirable in an aircraft application. Another approach that might be used is that of compensating the electromagnetic interference by adding a coil that would produce a voltage of equal, but offsetting, value to that produced in the sensing coil in response to electromagnetic inter¬ ference. Such coils are commonly known as a "bucking" coil. The bucking coil might be disposed either in parallel or in series with the sensing coil. However, it has been found that electromagnetic inter¬ ference apparently induces a current in the housing itself which is composed of an electrically conducting material. This produces a phase shift between the voltages induced in the sensing coil and in the buck¬ ing coil. Thus, while the net effect is to reduce the signals that are induced by electromagnetic inter¬ ference, the minimum levels that are achieved are still greater than acceptable levels. Summary of the Invention According to the present invention there is provided a mass rate of flow meter comprising a hous¬ ing (10) defining a flow path for fluid to be measured, a swirl generator (33), an unrestrained rotor (37) and a restrained turbine (40) axially spaced from one another along an axis within said housing, first sensing means (14,46) to detect the passage of a point on the rotor past a point on said housing, and second sensing means (17,47,53) including magnetic means (47,53) to detect the passage of another point on the rotor past a point on said turbine, said second sensing means including a sensing coil (17) mounted on the housing (10) coaxial with said turbine, charac¬ terised by a bucking coil (81) coaxial with and elec¬ trically connected to said sensing coil (17), and a shorted turn (77) of substantially non-magnetic electrically conductive material disposed between said sensing coil and said bucking coil. In response to electromagnetic interference, the sensing coil and bucking coil produce offsetting interference voltages. Phase shifts in the interfer- ence voltages can be compensated by said shorted turn between the sensing and bucking coils. This structure enables signals induced by electromagnetic interference to be reduced to within acceptable levels. An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings. Brief Description of the Drawings FIG. 1 is a longitudinal section of a mass rate of flow meter embodying this invention; and FIG. 2 is a sectional view of a portion of the flowmeter of FIG. 1. Description of an Illustrative Embodiment FIG. 1 shows a flowmeter that incorporates this invention. It comprises a housing 10 having an inlet port 11 and an outlet port 12 at the ends of the housing 10 which, with other elements of the flowmeter, defines a generally annular passage for a fluid, such as aircraft fuel. The passage is generally disposed along a longitudinal axis 13. A first sens- ing coil assembly 14 generates first timing, or start, pulses and is affixed to the housing 10. The assembly 14 has a longitudinal axis that is perpendicular to the axis 13 and is secured in a shield 15. A second sensing coil * assembly 16 generates second, or stop, timing pulses and is also affixed to the housing 10. The assembly 16 has a longitudinal axis that is coincident with the axis 13 and includes a sensing coil 17 that is located against a flange 20 at the outlet port 12. Conductors from both the first sensing coil assembly 14 and the second sensing coil assembly 16 terminate at a connector assembly (not shown). Both the coil assemblies 14 and 16 are isolated from the flow of fluid through the housing 10. A first, or turbine, inner assembly 22 is radially positioned in the housing 10 by the end flange 20 and is axially positioned by the end flange 20 and a retaining ring 21. The assembly 22 also supports a spring mechanism 23. At the inlet port 11, a second, or rotor, inner assembly includes a flow straightener that comprises a plurality of longi¬ tudinally extending, circumferentially spaced vanes 25. The flow straightener is positioned in a tapered bore 26 and is mounted to one end of a stationary shaft 30A. An .aligned rotatable shaft 30B is suppor¬ ted by the assembly 22 and lies on the longitudinal axis 13. A forward strut element in the rotor assembly comprises a stationary annulus 31 and a plurality of struts 32 that extend inwardly from the annulus 31 and that support a swirl generator 33. The annulus 31 radially positions the rotor assembly and coacts with a retaining ring to axially position the rotor assembly in the housing 10. The swirl generator 33 supports the shaft 30A. A flanged ring 34 is carried on the outer surface of the vanes 25 and G\: supports one end of a variable diameter conduit 35 that includes a plurality of spring fingers encircling the swirl generator 33 and that acts as a flow respon¬ sive valve. A second ring 36 clamps the conduit 35 and the ring 34 to the vanes 25. This ring 36 also coacts with the housing 10 to radially position the shaft 30A. A rotor 37 is journalled on shaft 30A and is supported and positioned thereon by thrust bearings 41 and 42. A turbine 40 is axially spaced from rotor 37 and is secured to rotatable shaft 30B. Shaft 30B is journalled for rotation in assembly 22 by thrust bearings 43 and 44. A flat band, helical spring (not shown) in the spring mechanism 23 is clamped between the turbine shaft 30B and assembly 22 to restrain rotation of the turbine 40 about the axis 13. An outer annulus 45 on the rotor 37 supports a group of permanent bar magnets 46 in the periphery of the rotor 37. Each time the magnets 46 rotate past the sensing coil assembly 14, a start pulse is induced in " the coil assembly 14 that indicates the passage of a predetermined point on the rotor 37 (i.e., the location of the magnets 46) past a predetermined point on the housing 10 (i.e., the location of the coil assembly 14) . Another group of permanent bar magnets 47 also mounts to the outer annulus 45 of the rotor 37. More specifically, the annulus 45 has an annular extension 50 that extends toward and overlaps a por¬ tion of the turbine, specifically the ends of turbine blades 51 on the turbine. Grooves 52 are cut in the outer surface of the extension 50 to carry the bar magnets 47 which are closely spaced to each other. In addition to the turbine blades 51, the turbine 40 carries an exciter blade 53 of a permeable material and a diametrically opposed, non-permeable, balancing blade (not shown). An outer ring band, or shroud, 54 fits over the turbine blade 51, the exciter blade 53, and the balancing blade. The band 54 engages a flux collecting ring 55 of a permeable material between the band 54 and a radial extension 56 on the turbine 40. The ring 55 bears against a tab 57 from the exciter blade 53 and a similar tab from the balancing blade. Each time the magnets 47 pass the exciter blade 53, flux linkages are coupled to the coil 17 through the exciter blade 53 and the flux collection ring 55 and induce an electrical stop pulse in the sensing coil 17 that indicates the passage of another predetermined point on the rotor 37 (i.e., the loca¬ tion of the magnet 47) past a predetermined point on the turbine (i.e., the position of the exciter blade 53). The time between the start and stop pulses is representative of mass flow rate. As shown in FIG. 1, the sensing coil assembly 16 has a significant axial length and surrounds the housing 10. The sensing coil 17 is therefore very susceptible to electromagnetic interference. However, FIG. 2 discloses a cross-section taken through the upper portion of the sensing coil assembly 16 and the housing 10 that is , constructed to reduce the effects of electromagnetic interference to acceptable levels. The sensing coil assembly 16 is positioned on the housing 10, as shown in FIG. 1, to be axially coextensive with at least a portion of the turbine 40. As shown in FIG. 2, the housing 10 has formed therein an end wall 60 provided on the flange 20, and an end wall 61 that is spaced from the end wall 60 toward the input port 11. As shown in FIG. 2, the end walls 60 and 61 are positioned respectively at the left and right ends of the sensing coil assembly 16. These end walls and the cylindrical surface of the housing 10 that extends between them provide a mech- anical support for the sensing coil 17 in both radial and axial directions. Still referring to FIG. 2, the sensing coil assembly 16 includes magnetic shields 62 and 63, affixed to the end walls 60 and 61 respectively. Annular insulating pieces 64 and 65 are positioned against the magnetic shields 62 and 63 thereby to provide, with the shields 62 and 63, an insulated, magnetic shield. Insulation 66 also is disposed on the periphery of the housing 10 between the annular insulating members 64 and 65 so there is provided an insulated core for the sensing coil 17. The sensing coil 17 is wound on this insulated core. Additional insulation 75 is wrapped about the sensing coil 17 and a magnetic shield 76 is placed over the insulation 75. The magnetic shields 62 and 63 on the end walls 60 and 61 and the magnetic shield 76 thereby provide a magnetically shielded, electrically isolated sensing coil 17 that is in the magnetic circuit with the exciter blade 53 but that is isolated magnetically from any elements that lie radially outside the magnetic shield 76. A sheet of copper wraps about the magnetic shield 76 to form a shorted turn 77. In one example, the sheet extends between the insulated end walls 60 and 61; an axially extending seam is soldered. As described later, this shorted turn 77 serves as a phase correcting means. A layer 80 of insulating tape is wrapped about the shorted turn 77, and a bucking coil 81 is formed on this layer 80. The number of turns is less than the number of turns in the sensing coil 17. Specifi¬ cally, the coil is wound proportionally to provide an offsetting voltage in response to electromagnetic interference that is equal to and opposite the voltage induced in the sensing coil 17. In one particular embodiment the ratio of the turns in the sensing coil 17 to the turns on the bucking coil 81 is about 55:9. Corresponding leads connect the sensing coil 17 and the bucking coil 81 in electrical series. Insula¬ tion 83 over the entire axial length of the bucking coil 81 also secures the remaining leads 17A and 81A from the sensing and bucking coils. Another magnetic shield 84 is placed about the insulation 83. The leads 17A and 81A pass from the interior portions of the coil assembly 16 and are directed back axially along the magnetic shield 84 toward the end wall 60. A piece of shrink tubing 85 over a portion of the entire assembly adjacent to the end wall 61 and the end wall 61 itself positions and supports the leads 17A and 81A. The leads 17A and 81A are directed _r back toward the end wall 61. A' second piece of shrink tubing 86 extends axially from the end wall 60 toward the end wall 61 until it overlaps the tubing 85 and secures the leads 17A and 81A. As previously indicated, the number of turns in the sensing coil 17 and the bucking coil 81 are selected to provide a minimum response to electromag¬ netic interference. Currents that are induced in the shorted turn 77 by electromagnetic interference act with the currents that are induced in the housing 10 to shift the phase of the signals induced in the bucking coil 81. The resulting relative phase shift between the signals induced in the sensing coil 17 and the bucking coil 81 provides significantly better compensation with the shorted turn 77. The shielding about the sensing coil 17 minimizes any magnetic flux that is coupled to the bucking coil 81. As a result, any flux that does reach the bucking coil has only a minimal effect and does not adversely affect the signal level that the sensing coil 17 generates. A test " of the sensitivity of a flowmeter to electromagnetic interference requires a conductor to be wound tightly around the flowmeter and then to be energized with a 20 ampere, 400 Hz current. During one test it was found that the voltage at one measurement point in a circuit that was produced by the voltage induced in the sensing coil 17 had a peak voltage of 16 millivolts. This was well within the acceptable limit of 60 millivolts peak for that measurement point and significantly below the 700 millivolt peak interference voltages that were detec¬ ted when a similar flowmeter of a prior design was tested. During another' test, the voltage across the leads 17A and 81A was found to be at a peak value of 13 millivolts which is well within the allowable limits of 20 millivolts at that position.

Claims: CLAIMS : 1. A mass rate of flow meter comprising a housing (10) defining a flow path for fluid to be measured, a swirl generator (33), an unrestrained rotor (37) and a restrained turbine (40) axially spaced from one another along an axis within said housing, first sensing means (14,46) to detect the passage of a point on the rotor past a point on said housing, and second sensing means (17,47,53) including magnetic means (47,53) to detect the passage of another point on the rotor past a point on said tur¬ bine, said second sensing means including a sensing coil (17) mounted on the housing (10) coaxial with said turbine, characterised by a bucking coil (81) coaxial with and electrically connected to said sens- ing coil (17), and a shorted turn (77) of substant¬ ially non-magnetic electrically conductive material disposed between said sensing coil and said bucking coil. 2. A flowmeter according to claim 1 character- ised in that said bucking coil (81) is connected electrically in series with said sensing coil (17). 3. A flowmeter according to claim 2 character¬ ised by a magnetic shield (76) disposed to magneti¬ cally isolate said sensing coil from said bucking coil. 4. A flowmeter according to claim 3 character¬ ised in that said bucking coil (81) is disposed rad¬ ially outside said sensing coil (17), and in that said magnetic shield (76) extends about the periphery of said sensing coil. 5. A flowmeter according to claim 4 character¬ ised in that the ratio of turns in said sensing coil (17) to the turns in said bucking coil (81) is greater than one. 6. A flowmeter according to claim 5 character¬ ised in that said shorted turn (77) comprises a copper sheet. 7. A flowmeter according to claim 6 character¬ ised in that said copper sheet (77) is electrically insulated from said sensing coil (17) and said bucking coil (81).

Inventors: PFUNTNER R, WYLER J
Applicants: GEN ELECTRIC, GENERAL ELECTRIC CO
Application year: 1980
Main classification: G01F1
IPC codes: G01F1