GB2116703A - Position sensor for flowmeter prover - Google Patents

Abstract

In a flowmeter prover having a measuring piston 30 slidably mounted within a measuring conduit 22 and movable with the flowing fluid, a generally cylindrical rod 32 projects axially from the piston and extends through one end of the conduit. The rod has marks 78, 80, the spacing between which corresponds to the measuring distance. A single fixed sensor 54 is mounted adjacent the surface of the rod and produces an ouput to start or stop a counter 104 when the first or second mark, respectively are at the sensor position. Preferably the rod has a reflective surface and the marks are non- reflective.The sensor comprises a light source and detector with optical fibres leading to a point between two seals 42, 44. <IMAGE>

Description

SPECIFICATION Position sensing apparatus for flowmeter provers This invention relates to position sensing apparatus and more particularly, to position sensing apparatus for use with flowmeter provers and the like.

A wide variety of flowmeter provers have been developed over the years; these devices are used for determining the accuracy of and for calibrating flowmeters. Some of these flowmeter provers employ a measuring conduit containing a movable fluid barrier in the form of a measuring piston. The fluid being measured is caused to flow both through the flowmeter and through the conduit.

During a prover test, the piston is launched into the fluid stream at an upstream end of the conduit, travels with the fluid past two measurement points located along the length of the conduit, and stops at a downstream end of the conduit. The piston is then returned by various means to the upstream position from which it may be launched again for a following test. Generally, an electrical switch actuated by passage of the piston is mounted to the conduit at each of the measurement points to sense when the piston is at the respective point. A comparison of the volume of fluid displaced by the piston in the space between the two measurement points with the measurement by the flowmeter of the same volume of fluid serves to determine the flowmeter accuracy.

One type of flowmeter which may be calibrated using provers of the type described above is designed to produce a series of electrical impulses, each impulse representing a certain volume of fluid.

Flow quantity as measured by the meter is determined by counting the total number of impulses produced as the fluid passes through the meter. The counting of impulses is generally performed by connecting the flow-meter impulses to an electronic counter having gating circuits activated by the electrical switches in the prover. The gating circuits cut off the incoming pulses until the first switch is actuated. The counter then starts and continues counting until the second switch is activated, whereupon the counting is terminated.

The volume of fluid displaced between the two detection points is a known volume which has been precisely measured either by a displacement test or by direct measurement of the measuring conduit diameter and the linear distance between the measurement points.

Following a proving test, a numerical factor defining the number of meter impulses per unit of fluid volume is determined by dividing the number of impulses produced during the proving test by the prover volume. This factor is known as a calibration factor and is expressed in terms of impulses per unit volume. A precise measurement of rate of flow, i.e., volume per unit of time, may be obtained from the prover by dividing the volume between measurement points by the time elapsed during the passage of the measuring piston between the same points.

From the above discussion, it can be seen that the measurement accuracy of the prover is directly related to the accuracy of the apparatus used to sense when the piston is at a measurement point.

This is so because an error in the sensed position of the piston results both in an incorrect number of impulses counted by the counter and in an incorrect measure of the time elapsed during the passage of the measuring piston between the measurement points.

To sense the piston position, some prior art flowmeter provers employ two mechanical limit switches spaced apart along the periphery of the measuring conduit. Each switch includes an actuator which extends into the conduit. A portion of the piston is designed to contact and operate the actuator as the piston moves past the switch. The distance between the actuators defines the measur ing portion of the conduit which in turn defines the known volume used to calibrate the flow meter.

Prover apparatus using mechanical switches actu ated by a measuring piston are described in Pfrehm U.S. Patent No. 3,021,703, issued February 20, 1962, and Fisher petal. U.S. Patent No.3,273,375, issued September 1966.

Still otherflowmeter provers employ magnetically-actuated proximity detectors or magneticallyoperated reed switches spaced apart along the periphery of the measuring conduit to sense the position of the measuring piston. See, for example, Boyle U.S. Patent No. 3,120,118, issued February 4, 1964, which shows the use of magnetic proximity detectors and co-pending U.S. Patent application Serial No. 244,790, filed March 1981,entitled FLOW METER PROVER APPARATUS AND METHOD, assigned to the assignee of the present invention, which shows the use of magnetically operated reed switches.

Prior art apparatus for sensing the position of the measuring piston in flowmeter provers generally possess a number of disadvantages which adversely affect both the accuracy and the repeatability of the prover measurements. For example, the use of two spaced-apart position sensors requires that the spacing between these detectors be maintained to a high degree of accuracy since this spacing is directly related to the measured prover volume. The spacing between the position detectors is affected by several factors which are difficult to control. For example, when one of the position sensors is removed from its mounting for servicing or replacement, the prover may have to be recalibrated after the sensor is replaced as a result of the generally unavoidable change in detector spacing.

When mechanical switches are used as position sensors, it has been found that mechanical wear may cause the point at which the switch actuates to vary with respect to the position of the measuring piston. This variation results in prover measurement error.

The use of magnetically actuated devices as position detectors also has several disadvantages.

For example, it has been found that the response time of such devices varies with the velocity of the measuring piston. In addition, the point at which the detector is actuated is dependent on the strength of a magnetic field which may vary as a function of time and temperature.

Attempts have been made in the prior art to use light beams to sense the position of a piston within a cylinder. For example, Rieke etal. U.S. Patent No.

3,391,269, issued July 9, 1968, discloses the use of a light beam source and a photosensor which are spaced apart on opposite sides of a cylinder used to measure a volume of air. The light beam is directed across the measuring cylinder to impinge on the photosensor. The piston acts to interrupt the light beam as the piston moves through the cylinder. The light source and photosensor must be precisely aligned with each other, and are both exposed to the measuring fluid. This mechanization also requires that the measuring fluid in the cylinder betranspa- rent to light and, hence, is unsuitable for use with flowmeter provers which measure the flow of opaque fluids.

Accordingly, it is an object of the present invention to provide new and improved position sensing apparatus for use with flowmeter provers.

It is another object of the present invention to sense multiple positions of a piston within a conduit using only one position sensing device.

It is yet another object of the present invention to use a light beam to sense the position of a piston within a conduit containing an opaque fluid.

Summary of the invention The foregoing and other objects of the invention are accomplished by providing position sensing apparatus for use with a flowmeter prover having a measuring conduit for containing a quantity of a flowing fluid; a fluid barrier in the form of a piston slidably mounted within the conduit and movable with the flowing fluid, and a generally cylindrical rod which axially projects from the piston and extends through a fluid sealed opening in one end of the conduit. The rod is designed to travel with the piston over a measuring distance corresponding to a fluid measuring portion of the conduit.

The position sensing apparatus includes a position designator located at first and second positions along the length of the rod, where the spacing between the first and second positions corresponds to the measuring distance. A single position designator sensor is mounted in fixed relation to the measuring conduit at a sensor position adjacent the surface of the rod. The sensor is responsive to the position designators for producing an output indication whenever the first and second positions of the rod coincide with the sensor position.

In a preferred embodiment of the invention, the rod is formed to have a surface which reflects light; and the position designator comprises a non- reflective area on the surface of the rod at the first and second positions. The designator sensor is positioned external to the measuring conduit adjacent the surface of the portion of the rod extending through the conduit and includes a reflectivity sensor responsive to the reflectivity of the rod surface. The first and second positions along the length of the rod are chosen so that the first position coincides with the sensor position when the piston is at a first end of the measuring portion of the conduit and so that the second position coincides with the sensor position when the piston is at a second end of the measuring portion of the conduit.The reflectivity sensor is connected to signal conditioning circuitry which produces an output signal whenever the non-reflective areas on the surface of the rod coincide with the sensor position.

These and other objects, features, and advantages of the invention will become apparent from a reading of the specification when taken in conjunction with the drawings in which like reference numerals refer to like elements in the several figures.

Brief description of the drawings Figure 1 is a partially cutaway side view of a flowmeter prover which employs the position sensing apparatus of the present invention; Figure2 is a more detailed partial cutaway side view of the flowmeter prover of Figure 1 showing the location of the reflectivity sensor of the present invention; Figure 3 is an enlarged end view, partially cut away, taken along the line 3-3 of Figure 2, showing the mounting of the reflectivity sensor of the present invention to the prover of Figure 1; Figure 4 is a side view, partially cut away, taken along the line 4-4 of Figure 3, showing further details of the mounting of the reflectivity sensor; Figure 5 is a schematic view of the reflectivity sensor of the present invention; and Figure 6 is a block diagram of the signal conditioning circuitry of the present invention.

Description of the preferred embodiment Figure lisa partial cutaway view of a flowmeter prover 10 which employs the position sensing apparatus of the present invention. The prover 10 includes a cylindrical outer housing 12. One end of the housing 12, called the upstream end, is enclosed bya generally hemispherical end cap 13 having an inlet pipe 14 projecting therefrom. Afluid outlet pipe 16 is provided adjacentthe opposite, or downstream end of the housing 12.

A housing end plate 18 is fastened to a flange 20 which is affixed to the downstream end of the housing 12; the plate 18 acts to fluid seal this end of the housing 12. A cylindrical measuring conduit 22, open at both ends, is coaxially mounted within the housing 12. The downstream end of the conduit 22 mates with the end plate 18 to support the conduit 22. The conduit 22 is further supported near its upstream end by an annular ring 24 affixed to the inside wall of the housing 12. The ring 24 also serves as a fluid seal which divides the housing 12 into an upstream prover chamber 48 and a downstream annular cavity 50. The conduit 22 is provided with several rows of circular ports formed in the wall thereof. These ports include upstream ports 26 and downstream ports 28.

A movable fluid barrier in the form of a piston 30 is slidably mounted within the measuring conduit 22 in a manner which forms a fluid seal between the piston 30 and the inner wall of the conduit 22. One end of an actuating rod 32 is centrally fastened to the downstream side of the piston 30. The other (downstream) end of the rod 32 extends through an opening 34 in the end plate 18 and into a cylindrical pressure housing 36.

One end of the housing 36 terminates in a flange 38 which is fastened to the outside surface of the end plate 18 using bolts 40. A pair of spaced-apart O-ring seals 42 and 44 are mounted within the flange 38, surround the rod 32, and act as fluid seals for the conduit 22 and the pressure housing 36, respectively.

The operation of the prover 10 described thus far is as follows. Prior to the initiation of a prover run, the piston 30 is moved to the upstream end of the conduit 22. This is accomplished by supplying hydraulic fluid under pressure to the interior of the housing 36 through inlet tube 46. The hydraulic pressure exerts a force against the rod 32, forcing it, together with the piston 30 to move upstream within the conduit 22.

The fluid to be measured flows through a flowmeter 52, which is to be calibrated, enters the prover 10 through the inlet pipe 14, and fills the upstream prover chamber 48. With the piston 30 at the upstream end of the conduit 22, the fluid is able to flow around the piston 30, through the upstream ports 26, and into the interior of the conduit 22. The fluid fills the interior of the conduit 22, exits the conduit by way of the downstream ports 28, fills the downstream annular cavity 50, and exits the prover 10 by way of the outlet pipe 16.

A prover run is started by releasing the hydraulic pressure in housing 36, which allows the piston 30 to begin moving in a downstream direction in response to the force of the moving fluid. As the piston 30 moves downstream, it covers the upstream ports 26 so that fluid cannot flow around the piston 30. Fluid flowing against the piston 30 causes it to move with the fluid flow. The prover run is completed when the piston 30 moves past the downstream ports 28 at which time the fluid is able to flow through the ports 28 (bypassing the piston 30) and to exit the prover by way of the outlet pipe 16.

The volume of fluid displaced by the downstream motion of the piston 30 within a measuring portion of the conduit 22 is used to calibrate the flowmeter 52. The measuring portion of the conduit 22 lies between the upstream and downstream ports 26 and 28, respectively.

The measuring portion of the conduit 22 is defined by designating first and second positions of the piston 30 within the conduit 22 which correspond to the upstream and downstream ends, respectively, of the designated measuring portion of the conduit 22.

To precisely calibrate the flowmeter 52, it is necessary to accurately and repeatably detect when the piston 30 is at the first and second designated positions.

The position sensing apparatus of the present invention is used to sense the position of the piston 30 within the conduit 22 and includes a reflectivity sensor 54 mounted to the side of the flange 38 as shown in Figures 1,2, and 3. The sensor 54 includes a housing 56 and a cylindrical sensing probe 58 which projects therefrom. The probe 58 fits within a radial hole 60 provided in the flange 38. The hole 60 extends from the side oftheflange 38 to a coaxial opening 62 in the flange 38 through which the rod 32 extends. As shown in Figure 2, the hole 60 is positioned between the two spaced-apart O-ring seals 42 and 44. When installed within the hole 60, a tip portion 76 of the probe 58 is located closely adjacent the surface of the rod 32.

The rod 32 is fabricated of a corrosion resistant material having a low coefficient of thermal expansion such as INVAR-36 steel supplied by Carpenter Technology Corporation. Except for two nonreflective marks 78 and 80, the entire surface of the rod 32 is made light-reflective by, for example, polishing it to a surface finish of about sixteen microinches RMS. The non-reflective marks are provided at first and second positions, respectively, along the length of the rod 32. In the preferred embodiment, the non-reflective marks are formed by providing a circumferential groove in the surface of the rod at each of the two positions. The grooves are then filled with a non-reflective material such as a black epoxy similar to epoxy type 2850 FT supplied by Emerson & Cuming, Canton, Massachusetts.

The location along the length of the rod 32 of the first and second non-reflective marks is determined as follows. Referring to Figures 1 and 2, the location of the first non-reflective mark 78 is chosen so that when the piston 30 is at a position within the conduit 22 corresponding to the upstream end of the measuring portion of the conduit 22, the mark 78 is aligned with the tip 76 of the probe 58. The location of the second non-reflective mark 80 is chosen so that when the piston 30 is at a position within the conduit 22 corresponding to the downstream end of the measuring portion of the conduit 22, the mark 80 is aligned with the tip 76. In the preferred embodiment, the width of the marks 78 and 80 is about ten thousandths of an inch.

The reflectivity sensor 54 is used to detect changes in the reflectivity of the surface of the rod 32 in the following manner. Referring to Figure 5, the sensor 54 includes a light source 64, such as a tungsten filament lamp, and a photosensor 66, such as a phototransistor, both of which are mounted within an enclosure 68. Light from the source 64 is coupled to one end of a first group of parallel optical fibers 70. The photosensor 66 is optically coupled to one end of a second group of parallel optical fibers 72.

The first and second groups of fibers 70 and 72 are bundled together so that individual fibers in each of the first and second groups are randomly distributed within a bundle 74. The bundle of fibers 74 is inserted through a hollow tube 57, such as a stainless steel tube, which extends from the enclosure 68 and forms the sensing probe 58. The fibers 74, which in the preferred embodiment are glass fibers, are secured within the tube 57 using a high temperature resin such as an epoxy resin. The ends of the fibers within the bundle 74 terminate at the tip end 76 of the probe 58. The tip 76 forms the sensing area of the sensor 54, and the probe 58 adjacent the tip 76 is reduced in diameter at 59 to establish a small reflectivity sensing area at the tip 76. In the preferred embodiment, the diameter of the tip 76 is about fifteen thousandths of an inch.

When the light source 64 is illuminated, light is coupled by way of the randomly distributed fibers 70 to the tip 76 which is located closely adjacent the surface of the rod 32. When the light reflective surface of the rod 32 appears in front of the tip 76, some of the light emanating therefrom is reflected back to the tip 76 along a path substantially parallel to the path of the emanating light as shown in Figure 5. This reflected light is coupled to the photosensor 66 by way of the randomly distributed fibers 72.

When either of the non-reflective marks 78 or 80 on the surface of the rod 32 is aligned with the tip 76, substantially none of the light is reflected back to the photosensor 66.

The photosensor 66 is connected to signal conditioning circuitry described below and provides a sensor signal proportional to the reflectivity of the portion of the surface of the rod 32 aligned with the tip 76. A typical reflectivity sensor 54 of the type described above for use in the present invention is a series S58 fiber optic scanner supplied by SKAN-A MATIC Corporation, Elbridge, New York.

Figures 3 and 4 show in detail the way in which the reflectivity sensor 54 is mounted to the prover 10.

The enclosure 68 containing the light source 64 and the photosensor 66 is fastened within the housing 56 using a bracket 98 and screws 94 and 96. The probe 58 projects through an opening in the housing 56 and extends into the hole 60 in the flange 38. The housing 56 is in turn fastened to the side of the flange 38 using bolts 100.

The hole 60 includes a reduced diameter section 61 in the area adjacent the coaxial opening 62. The section 61 is formed to closely engage the reduced diameter portion 59 of the probe 58, and acts to hold the tip 76 in place adjacent the surface of the rod 32.

In the preferred embodiment, the distance between the tip 76 and the surface of the rod 32 is about six thousandths of an inch.

An opening 101 is provided in the housing 56, as shown in Figure 4, to accommodate a cable 102 used to provide electrical connections between elements in the enclosure 68 and signal conditioning circuitry 81 as indicated in Figures 1 and 2.

The conditioning circuitry 81 is connected to the light source 64 and the photosensor 66 as shown in the block diagram of Figure 6. The circuitry 81 includes a power supply 82, an amplifier 84, a level detector 86, and a pulse generator 88. The power supply furnishes power to illuminate the light source 64 and to operate the circuits 84, 86, and 88. The photosensor 66 is connected to provide the sensor signal described above to the input of the amplifier 84 which amplifies this signal and provides the amp!ified signal to the input of the level detector 86.

In response to the amplified sensor signal, the level detector 86 provides a trigger signal to the input of the pulse generator 88 whenever the photosensor 66 detects that the surface of the rod 32 in front of the tip 76 has changed from a reflective surface to a non-reflective surface. In response to the trigger signal, the pulse generator 88 provides on a line 92 a pulse having a waveform 90. In a preferred embodiment of the invention, the pulse 90 has a width of about one hundred microseconds.

The operation of the position sensing apparatus of the present invention described above may be understood by referring to Figures 1 and 2. The piston 30 is positioned at the upstream end of the conduit 22 prior to the start of a prover run and in this position a reflective portion of the surface of the rod 32 to the right of the mark 78 appears in front of the tip 76. The O-ring seals 42 and 44 on opposite sides of the tip 76 act to wipe the surface of the rod 32 so that the portion of the rod 32 between the seals 42 and 44 is free both of measuring fluid from the conduit 22 and of hydraulic fluid from the housing 36. Accordingly, these fluids, which in many instances are opaque, do not interfere with the operation of the reflectivity sensor 54.

During a prover run, the piston 30 starts moving downstream; and when it passes the upstream ports 26, the mark 78 passes in front of the tip 76. The position of the piston 30 at this instant corresponds to the upstream end of the measuring portion of the conduit 22. The width of the mark 78 is made substantially equal to the diameter of the sensing portion of the tip 76 so that when the mark 78 is aligned with the tip 76, substantially all of the light from the source 64 is prevented from being reflected back to the photosensor 66. This event causes the pulse generator 88 to generate a first pulse 90 on the line 92. The random distribution of the fibers 70 and 72 within the bundle 74 ensures a relatively even distribution of light-emanating and light-sensorfibers over the area of the tip 76.Therefore, the mark 78 must be substantially aligned with the tip 76 to block the reflected light.

The line 92 is connected to an electronic counter 104 as shown in Figures 1 and 2. Impulses produced by the flowmeter 52 in response to the flow of the measuring fluid are also provided to the counter 104 using cable 106. The first pulse 90 generated when the mark 78 is aligned with the tip 76 is used to start the counter 104 counting the impulses from the flowmeter 52.

When the piston 30 reaches a downstream position near the position of the downstream ports 28, the mark 80 passes in front of the tip 76. The position of the piston 30 at this instant corresponds to the downstream end of the measuring portion of the conduit 22. When the mark 80 is aligned with the tip 76, the pulse generator 88 generates a second pulse 90 on the line 92. The second pulse 90 is used to stop the counter 104 from counting the impulses from the flowmeter 52. The number of impulses counted by the counter 104 during the prover run is divided by the known volume of fluid displaced by the piston 30 over the measuring portion of the conduit 22 to arrive at the flowmeter 52 calibration factor.

The spacing between the marks 78 and 80 defines the length of the measuring portion of the conduit 22 and, hence, is directly related to the known volume of displaced fluid. Accordingly, accurate determination of the known volume is achieved by accurate spacing of the marks 78 and 80 on the rod 32. In the preferred embodiment, the marks 78 and 80 are formed by cutting grooves in the rod 32 as mentioned above. By mounting the rod 32 in a high precision engine lathe equipped with a laser interferometer, it has been found that the spacing between the grooves may be controlled to an accuracy of about thirty millionths of an inch. Forming the rod 32 of a material having a low coefficient of thermal expansion insures that the spacing between the marks 78 and 80 does not change substantially with changes in the temperature of the rod 32.The surface of the non-reflective material used to fill the grooves is made flush with the surface of the rod 32 to ensure the integrity of the fluid seals provided by the O-rings 42 and 44.

It should be noted that while the volume of displaced fluid in the conduit 22 is proportional to the spacing between the marks 78 and 80, the volume is substantially unaffected both by smalls variations in the placement of the marks 78 and 80 in relation to the ends of the rod 32 and by small variations in the position of the probe 58 in relation to the housing 12. Such variations act to shift the measuring portion of the conduit 22 in relation to the ends of the conduit 22. It will be appreciated that as long as the measuring portion of the conduit 22 remains between the upstream and downstream ports 26 and 28, respectively, small variations in the absolute position of the measuring portion between these ports has a negligible effect on the volume of displaced fluid.

From the above discussion, it can be seen that if the sensor 54 is removed for servicing or replacement and if upon its replacement the tip 76 has been shifted slightly in position, the accuracy of the prover 10 remains substantially unaffected. Thus, no recalibration of the prover 10 is required in such instance.

While the invention is thus disclosed and the presently preferred embodiment described in detail, it is not intended that the invention be limited to the embodiment shown. Instead, many modifications will occur to those skilled in the art which-lie within the spirit and scope of the invention. It is thus intended that the invention be limited only by the appended claims.

Claims (8)

1. Position sensing apparatus for use with a flowmeter prover having a measuring conduit for containing a quantity of flowing fluid; a fluid barrier slidably mounted within the conduit and movable with the flowing fluid; and a generally cylindrical rod which axially projects from the fluid barrier and extends through a fluid sealed opening in one end of the conduit, and where the rod travels with the fluid barrier over a measuring distance corresponding to a fluid measuring portion of the conduit; the apparatus comprising: designator means for designating first and second positions along the length of the rod, where the spacing between the first and second positions corresponds to the measuring distance; and single designator sensing means mounted in fixed relation to the measuring conduit at a sensor position adjacent the rod and responsive to the designator means for producing an output signal whenever the first and second positions of the rod coincide with the sensor position.

2. The apparatus of claim 1 in which the rod is formed to have a surface which reflects tight, and the designator means includes meansforforming nonreflective areas on the surface of the rod at the first and second positions.

3. The apparatus of claim 2 in which the single designator sensing means includes reflectivity sensor means responsive to changes in the reflectivity of the surface of the rod for producing an output signal whenever the non-reflective areas on the surface of the rod coincide with the sensor position.

4. The apparatus of claims 1 or 3 in which the sensor position is external to the measuring conduit and adjacent the surface of the portion of the rod extending through the conduit; and the first and second positions along the length of the rod are chosen so that the first position coincides with the sensor position when the fluid barrier is at a first end of the measuring portion of the conduit, and so that the second position coincides with the sensor position when the fluid barrier is at a second end of the measuring portion oftheconduit.

5. The apparatus of claim 2 in which the rod has a polished surface which is reflective; and the means for forming non-reflective areas includes means for providing a circumferential groove in the surface of the rod at the first and second positions, and means for filling the groove with a non-reflective material.

6. The apparatus of claim 3 in which the reflectivity sensor means includes a light source and a photosensor, means for directing light from the light source to reflect from the surface of the rod in proportion to the reflectivity of the rod surface, and means for directing the reflected light to the photosensor.

7. Position sensing apparatus for use with a flowmeter prover having a measuring conduit for containing a quantity of flowing fluid; a fluid barrier slidably mounted within the conduit and movable with the flowing fluid; a generally cylindrical rod which axially projects from the fluid barrier and extends through an opening in one end of the conduit, and where the rod travels with the fluid barrier over a measuring distance corresponding to a fluid measuring portion of the conduit; and first and second fluid seals spaced apart along the length of the rod to fluidically isolate the portion of the rod between the two seals; the apparatus comprising: designator means for designating first and second positions along the length of the rod, where the spacing between the first and second positions corresponds to the measuring distance; and single designator sensing means mounted in fixed relation to the measuring conduit at a sensor position adjacent the rod and between the first and second seals and responsive to the designator means for producing an output signal whenever the first and second positions of the rod coincide with the sensor position.

8. A position sensing apparatus, substantially as herein described with reference to the accompanying drawings.