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 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 point on the turbine. The first coil (14) is wound on a copper coil form (60) to shift the phase of signals from the first coil (14) to compensate the phase shift which occurs in signals from the second coil (17) at low rotor speeds.

Description: MASS RATE OF FLOW METER Background of the Invention This invention relates to mass rate of flow ireters 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 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 c:,'V 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" pulse in the second sensing coil. As described 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. In this type of flowmeter, the unrestrained rotor rotates at different angular velocities at different fluid flow rates. In some applications, the range is from one revolution per second up to and exceeding six revolutions per second. Rotor speeds below four revolutions per second are consid- ered to be low rotor speeds. During various tests, it has been found that at low rotor speeds the indi¬ cated rate of flow is less than the actual rate of flow through the flowmeter. This error, in part, arises because the rela¬ tive timing of the start and stop pulses produced in the sensing coils is dependent, in part, upon rotor speed. This dependence can adversely affect the flowmeter because the timing circuits respond to negative-going zero crossings of the start and stop pulses. Specifically, it has been found that flux changes that produce the stop pulses also induce eddy currents in the aluminium housing. These eddy currents shift the phase of the stop pulses without a corresponding shift in the phase of the start pulses. Consequently, the measured time interval between the start and stop pulses is dependent upon both fuel flow and rotor speed. As a result, at low flow rates the flowmeter can indicate a flow rate that is less than the actual flow rate. 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 meas¬ ured, 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) to detect the passage of another point on the rotor past a point on said turbine, said first sensing means including magne ic means (46) on the rotor and a coil (64) disposed on the housing about ' an axis (72) transverse to said housing axis (13), characterised in that said coil (64) is disposed on a substantially non- magnetic, electrically conductive support means (60) positioned on said housing (10) about said coil axis (72). Said coil support means acts as a shorted turn and can thus shift the phase of pulse signals generated by said first sensing means to compensate the phase shift which occurs in pulse signals gener¬ ated by said second sensing means a * t low rotor speeds. An embodiment of the present 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 a sensor of the flowmeter of FIG. 1; FIG. 3 is a cross-sectional view of a coil form that is depicted in FIG. 2; FIG. 4 is a top view of the coil form of FIG. 3; FIG. 5 is a sectional view taken along lines 5-5 in FIG. 2; and FIG. 6 is a graph that depicts the improved operating results achieved with the flowmeter of FIGS. 1 to 5. 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 inner, or rotor, 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 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 the 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 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. _- " y 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. FIG. 2 depicts a portion of the rotor 37 and the housing 10 at the first sensing coil assembly 14. The magnets 46 on the annulus 45 are in their position passing adjacent to the sensing coil assembly 14. The shield 15 is composed of highly magnetically permeable material and mounts around the coil assembly 14 thereby to magnetically shield the coil assembly from electromagnetic interference. It is affixed to the housing 10. There is also mounted to the housing 10 a coil form assembly 60 including a coil form core 61 and end plates 62 and 63. A sensing coil 64 wraps around the coil form core 61 between the end plates 62 and 63. Referring to FIGS. 3 and 4, the coil form core 61 has an oblong shape, thereby to define an oblong slot 65 vertically through the coil form core 61. The walls of the coil form core 61 have upper and lower recesses 66 and 67 that accept and position the end plates.._62 and 63. As shown more specifically in FIG. 4, the end plate 62 is generally rectangular with radiused corners 70. As is most readily seen in FIG. 3, the end plates 62 and 63 thereby nest onto the coil form core 61. Referring to FIGS. 2 and 5, the coil assembly 14 additionally includes a magnetic core 71 that reduces the reluctance of the magnetic current assoc- iated with the magnets 46. This magnetic core 71 is L-shaped. One leg 71A is disposed in the oblong slot 65 through the hollow coil form core 61. The free end of the leg 71A abuts the housing 10. The other leg 71B extends between and is in contact with the lower end plate 63 and the magnetic shield 15. The coil form core 61 and end plates 62 and 63 are composed of copper. Thus they constitute a coil form assembly 60 that electrically is a shorted turn. Referring again to FIG. 2, the coil form assem¬ bly 60 is positioned with a longitudinal axis 72 through the coil form 60 lying in a plane that is transverse to the longitudinal axis 13 and that inter- sects the magnets 46 that are centred longitudinally on the axis 72. During operation of the flowmeter, eddy cur¬ rents induced in the housing 10 produce a phase shift in the signals from the coil assembly 16 at low rotor speeds. Without the copper coil form assembly 60 shown in FIG. 2, the dashed Graph A in FIG. 6 results. As can be seen, the timing error increases as the rotor speed decreases. This means that the flowmeter indicates a flow rate that is less than the actual flow rate. Graph B in FIG. 6 depicts the error that exists when the copper coil form assembly 60 of FIGS. 3 to 5 is utilized. It can be seen that the addition of a coil form assembly 60 constructed in accordance with this invention significantly reduces the error rate over a rotor speed range from four revolutions per second to one revolution per second. These particular graphs are representative of a test that is produced with a 1,200 turn coil 64. In Graph A, the coil was wound on a conventional coil form assembly of an insulation material and Graph B depicts the test results when the coil 16 is wrapped about a 65 mil thick copper core 61 having 15 mil thick end plates 62 and 63. Experimentation has shown that the phase shift is more responsive to changes in the thickness of the copper core 61 than it is to the end plates 62 and 63. Thus addit¬ ional corrections can be achieved by changing the thickness of the core. The coil form assembly 60 thus introduces correcting relative phase shifts into the timing signals, especially ~ at low rotor speeds. As a result, the flowmeter produces more accurate indications of flow rate over a wider range of operating condi¬ tions. -^3RE / C .7 \ l - '

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) to detect the passage of another point on the rotor past a point on said turbine, said first sensing means includ¬ ing magnetic means (46) on the rotor and a coil (64) disposed on the housing about an axis (72) transverse to said housing axis (13), characterised in that said coil (64) is disposed on a substantially non¬ magnetic, electrically conductive support means (60) positioned on said housing (10) about said coil axis (72). 2. A flowmeter according to claim 1 character- ised in that said support means (60) includes a hollow core (61) formed cf a substantially non-magnetic electrically conductive material, and an end plate (62,63) affixed to each end of said core and formed of a substantially non-magnetic electrically conduc- tive material, said coil (64) being wound around said core between said end plates. 3. A flowmeter according to claim 2 character¬ ised in that said substantially non-magnetic electri¬ cally conductive material is copper. 4. A flowmeter according to claim 2 character¬ ised in that said core (61) is elongate in cross- section, and said end plates (62,63) are planar rec¬ tangular plates with radiused corners. 5. A flowmeter according to claim 2 character¬ ised by a magnetic core (71) of permeable material disposed in- said hollow core (61). 6. A flowmeter according to any one of claims 1 to 5 including a magnetically permeable shield (15) mounted to said housing and extending externally around said coil (64) and said support means (60). ____ ' -

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