MASS RATE OF FLOW METER

Abstract: An angular momentum type flowmeter has an unrestrained rotor (37) and a restrained turbine (40). A first sensing coil (14) detects passage of a first magnet (46) on the rotor past the coil. A second sensing coil (17) detects passage of a second magnet (47) on the rotor past a flux linkage blade (53) on the turbine. The second magnet (47) and the end portion (61) of the blade (53) axially overlap to define a radial air gap therebetween which is relatively insensitive to the effects of mechanical vibration and thermal expansion.

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 disclos ' ed 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 angular momentum of the fluid stream angularly dis- places the turbine about the axis and against the bias of its restraining spring. Under steady state conditions, this deflection of the turbine is propor¬ tional 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" voltage pulse in the coil that indicates the passage ' of a predetermined point on the rotor past a predetermined 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. There are, therefore, two separate, magnetic circuits in that flowmeter. The first, defined by the first magnet and the first sensing coil assembly, includes a radial air gap. The second, including the second magnet and the highly permeable bar, v,:?o includes an axial air gap. During use of the flow¬ meter, there are several influences that tend to change the lengths of these air gaps, although to different degrees. Two such influences are vibra- tional forces and thermal expansion. A typical application for such a flowmeter is in an aircraft for measuring the mass rate of flow of fuel to an engine. The engine and air frame are sources of mechanical vibrations. Also, the fuel that passes through the flowmeter is subject to pressure pulsations that introduce other mechanical vibrations. These vibrations have radial and .axial components that are coupled to the flowmeter and produce corresponding radial and axial forces that act on all the elements in the flowmeter. However, with the construction of the flowmeter, including the methods for mounting the rotor and turbine for rotation, the individual elements are more stable in a radial direction than they are in an axial direc- tion and, therefore, are less likely to vibrate in a radial direction. In addition to vibrationally induced forces, the elements also are subjected to the forces of thermal expansion. When an aircraft is flown, the temperature of the fuel varies widely. Any elements that contact the fuel, including the rotor and the turbine of the flowmeter, are therefore subject to these temperature variations and, therefore, to ther¬ mal expansion. Both the rotor and turbine are charac- terised by material dimensions in the radial direction (i.e., radial thickness) that are significantly less than those in the axial direction (i.e., axial length). _Thus, the radial effects of thermal expansion forces are less than the axial effects. During operation of the flowmeter, the radial forces that are produced by virbations and thermal expansion do not vary the length of a radial air gap sufficiently to affect the ability of the first sensing coil assembly to generate detectable pulses on a reliable basis. However, the axial forces that are produced by vibrations and thermal expansion can, in some environments, adversely affect the signal- to-noise ratio of the signal that is produced in the second sensing coil. Moreover, the problems can be increased depending upon the axial motion that is permitted by manufacturing tolerances. It is possible, under some circumstances, for the signal level from the second sensing coil assembly to reach undetectable levels and, under other circumstances, for the noise to reach a level at which it is detected as a signal. Either situation leads to erroneous flowmeter readings. 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, a swirl generator (33) in the housing to impart angular momentum to a fluid stream to be measured, a rotor (37) mounted for unrestrained rotation in the housing downstream from the swirl generator to convey fluid from the swirl generator through the rotor, a restrained tur¬ bine (40) downstream from the rotor to remove angular momentum from fluid discharge from the rotor, first magnetic means (46) carried by said rotor at the upstream end thereof, a first sensing coil (14) dis¬ posed on the housing to sense passage of the first agnetic means past the first sensing coil thereby to produce a first signal, second magnetic means (47) carried by said rotor at the downstream end thereof, magnetic flux linkage means (53) carried by said turbine at a circumferential position thereon, and a second sensing coil (17) disposed on the housing to sense passage of said second magnetic means past said magnetic flux linkage means thereby to produce a second signal, characterised in that said second magnetic means (47) and said magnetic flux linkage means (53) have concentric paths of rotation and axially overlap one another to define a radial air gap (60) therebetween. The provision of said radial air gap exhibits a reduced sensitivity to the effects of thermal expan¬ sion and mechanical vibrations, and thus can lead to more reliable flowmeter readings. 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; FIG. 2 is a sectional view of a portion of the flowmeter of FIG. 1; FIG. 3 is an end view of the turbine of the flowmeter of FIG. 1; and FIG. 4 is a sectional view taken along lines 4-4 in FIG. 3. 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 gener- ally disposed along a longitudinal axis 13. A first sensing 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 per¬ pendicular 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 hous¬ ing 10. A first inner, or turbine, 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 sup- _ ported 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 supports one end of a variable diameter conduit 35 that includes a plurality of spring fingers that encircle the swirl generator 33. The conduit 35 acts as a flow responsive 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). Referring to FIGS. 1 and 2, 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 portion of the turbine, specifi¬ cally the ends of turbine blades 51 on the turbine. Grooves 52 are cut in the outer surface of the exten- sion 50 to carry the bar magnets 47 which are closely spaced to each other. Now referring to FIGS. 1 to 4, 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 53A. An outer band, or shroud, 54 fits over the turbine blades 51, the exciter ' blade 53, and the balancing blade 53A. 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. Referring primarily to FIG. 2, the extension 50 supports the magnets 47 so they rotate in a first circular path. An extension 61 from a main body 62 of the exciter blade axially overlaps the extension 50 and the magnets 47. However, the extension 61 moves in a second circular path that is concentric with the path of the extension 50, so the axially overlapped portions do not interfere during operation. A notch 63 in the exciter blade 53 reduces fluid turbulence that the extension 61 might otherwise introduce if it were not notched. The air gap 60 between the, extension 50 below the magnets 47 and the extension 61 of the exciter blade 53 and another air gap 63 between ring 54 and the housing 10 in the area of the coil 17 both com¬ prise radial air gaps. There is no significant axial air gap. Because of the inherent stability in the length of radial air gaps, the length of the air gaps In the magnetic circuit is less sensitive to fluctuation as a result of mechanical vibrations and thermal expansion. The magnets 47, although within the coil 17, do not induce any significant voltage in the coil winding 17 as the rotation of the magnets 47 does not independently produce flux changes in the annular coil 17. Moreover, the permeable flux collector ring 55 shown in FIGS. 1 and 4 further improves the quality _ of the stop pulses from the coil 17. Specifically, the magnetic circuit associated with the magnets 47 includes the exciter blade 53, the flux collector ring 55, and magnetic shields that are associated with the coil 17. The presence of the flux collector ring 55 about the periphery of the turbine 40 in close association with the coil 17 around its circum¬ ference greatly reduces the reluctance of this flux path. As a result a greater percentage of the total flux from the magnets 47 couples the coil 17, so the quality of the stop pulses is improved. Specific¬ ally the voltage of the stop pulses, even at low rotor speeds, is reliably detectable. The described flowmeter achieves the advantages of this invention because all the magnetic air gaps are radially extending. Other arrangements of exciter blades and magnets could still provide the axially overlapping relationship.

Claims: CLAIMS : 1. A mass rate of flow meter comprising a housing (10) defining a flow path, a swirl generator (33) in the housing to impart angular momentum to a fluid stream to be measured, a rotor (37) mounted for unrestrained rotation in the housing downstream from the swirl generator to convey fluid from the swirl generator through the rotor, a restrained tur¬ bine (40) downstream from the rotor to remove angular momentum from fluid discharge from the rotor, first magnetic means (46) carried by said rotor at the upstream end thereof, a first sensing coil (14) dis¬ posed on the housing to sense passage of the first magnetic means past the first sensing coil thereby to produce a first signal, second magnetic means (47) carried by said rotor at the downstream end thereof, magnetic flux linkage means (53) carried by said turbine at a circumferential position thereon, and a second sensing coil (17) disposed on the housing to sense passage of said second magnetic means past said magnetic flux linkage means thereby . ' to produce a second signal, characterised in that said second magnetic means (47) and said magnetic flux linkage means (53) have concentric paths of rotation and axially overlap one another to define a radial air gap (60) therebetween. 2. A flowmeter according to claim 1 character¬ ised in that said rotor includes an outer annulus (45) including an integral annular extension (50) extending towards said turbine (40), and wherein said second magnetic means (47) comprises at least one permanent magnet affixed to said annular extension (50). 3. A flowmeter according to claim 1 character¬ ised in that said turbine (40) has a longitudinal slot in an outer surface thereof, said magnetic flux linkage means (53) comprises a member of permeable material disposed in said slot and having an integral extension (61) extending towards said rotor and axially overlapping said second magentic means (47). 4. A flowmeter according to any one of claims 1 to 3 characterised in that said second sensing coil (17) extends circumferentially of said housing (10) and is axially substantially co-extensive with said magnetic flux linkage means (53).

Inventors: ALLEN M
Applicants: GEN ELECTRIC, GENERAL ELECTRIC CO
Application year: 1980
Main classification: G01F1
IPC codes: G01F1