POSITIVE DISPLACEMENT FLOW SENSOR

Abstract: A positive displacement flow sensor includes two piston assemblies (52, 54) reciprocated in separate chambers (22, 24) in a housing (20) and interconnected by a single synchronizing link (70). A sensing unit (92) detects the movement of the link and produces an output indicating flow of fluid through sensor.

Description: POSITIVE DISPLACEMENT FLOW SENSOR DESCRIPTION Reference to Related Application The present invention is a Continuation-In-Part application of PCT Application No. PCT/US84/00290 filed February 27, 1984. OMPI Technical Field The present invention relates generally to flow sensing devices and, more particularly, to a positive displacement flow sensing device that accurately measures low flow rates. Background Prior Art Flow meters for measuring flow rates or totalized flow of a variety of liquids have been known for years. One such type of flow meter that is presently being marketed by several companies is known as a positive displacement flow rate transducer or sensor which can accurately measure a wide range of flow rates. This transducer or sensor consists of a housing that has two elongated cylindrical intersecting chambers with four pistons at the outer ends of the respective chambers. The pistons are interconnected with a crank shaft through four separate linkages such that movement of the respective pistons by fluid flow through the transducer moves the pistons, which in turn give an indication of the flow rate. While this type of sensing device has enjoyed a remarkable degree of commercial success, the cost for producing such units is fairly high and efforts are constantly being made to reduce the cost so that the price can be competitive with other less expensive types O PI of sensing devices that do not have the accuracy of a unit of this type. A substantial expansion of the market area could be achieved if the device would cost significantly less and still have the same accuracy. For example, the need has developed in recent years because of the energy crisis for measuring various fuels on vehicles, particularly such agricultural vehicles as tractors that operate on diesel fuel. Other areas of interest in controlling and accurately measuring flows would be in the agricultural area, such as fertilizers and insecticides, that farmers usually use in large quantities for various crops. While positive displacement flow meters have been used extensively at higher flow rates, the efficiency of such units decreases as the flow rate decreases, and these units have heretofore been impractical when the flow rate is substantially less than one gallon per minute. Thus, a need remains for a positive displacement flow meter that can accurately measure variants in flow rate at very low flow and still maintain accuracy when the flow rate is substantially less than one gallon per minute. Summary of the Invention According to the present invention, a simplified positive displacement flow sensor has been developed that can be manufactured at a fraction of the cost of conventional commercially-available constructions. •SΛr- K WHO The positive displacement flow sensor of the present invention consists of a housing having a pair of elongated chambers or bores that are vertically offset from each other and are interconnected through a connecting chamber with first and second pistons respectively reciprocated within the respective chambers or bores. The pistons are interconnected by a single synchronizing link which substantially reduces the number of components necessary for producing such unit. An inlet is connected to the connecting chamber and various conduits within the housing connect the respective ends of the chambers to each other to provide a flow path through the meter to an outlet. The single connecting link is moved in response to reciprocation of the respective pistons within the chambers and the movement is translated as an output signal to indicate flow rate through the sensor. More specifically, a given point, the mid-point between opposite ends of the synchronizing link, moves around a generally circular path and an element or pin is designed to move along the same circular path and is connected to a disc rotated about a fixed axis which can then be utilized for producing an output signal as an indication of the flow rate. According to one aspect of the present, invention, it has been determined that there is a critical relation between size of the piston or the bore in which it recriprocates and the stroke of the piston. O PI This ratio is to some measure dependent upon the coefficient of friction of the bearing members which support the movable pistons in the stationary housing. Brief Description of Several Views of Drawings FIG. 1 is a perspective view of the sensing device constructed in accordance with the present invention; FIG. 2 is an exploded diagramatic view of the components and conduits located within the housing of the sensing device; FIG. 3 is a fragmentary plan view of the sensing mechanism; FIG. 4 is a cross-sectional view of the internal construction of the sensing device; ' ■ FIGS. 5 and 6 are views similar to FIG. 4 showing the various positions of the pistons. Detailed Description While this invention is susceptible of embodiment in many different forms, there is shown in the drawings and will herein be described in detail a preferred embodiment of the invention with the understanding that the present disclosure is to be considered as an exemplification of the principles of the invention and is not intended to limit the broad aspect of the invention to the embodiment illustrated. FIG. 1 of the drawings discloses a flow sensor generally designated by the reference numeral 10, consisting of a flow rate transducer body 12 and an output or indicator unit 14. The sensor or transducer body consists of a housing 20 that includes first and second chambers or bores 22 and 24. The respective chambers or bores 22 and 24 extend perpendicular to each other and through the entire housing from one sidewall to an opposing sidewall. The chambers are preferably enclosed with cover plates 26 on opposite ends of the chamber. As illustrated in FIG. 2, elongated chamber 22 is located above chamber 24 so that opposite ends of the respective chambers are isolated from each other. However, the chambers are interconnected in the area of ''the intersection by a generally vertical connecting chamber 30, shown in FIG. 4. The connecting chamber has a fluid inlet conduit 32 connected adjacent the lower end to supply fluid to the connecting chamber 30. The parameters of the connecting chamber 30 will be described in further detail later. The positive displacement flow sensor also incorporates a plurality of conduits that are formed within the housing to interconnect various portions of the chambers or bores and to also interconnect the bores with the outlet. Thus, as illustrated in FIG. 2, outlet 34 is connected by branch conduits 36, 38, 40 and 42 through two intermediate portions of opposite ends of the respective chambers 22 and 24. Also, each end of each chamber is connected to an intermediate portion of an adjacent chamber 1 through conduits 44, 46, 48 and 50. In ~ actuality, all of the conduits are openings machined into the respective portions of housing 20. r Elongated bores 22 and 24 respectively have first and second piston means or assemblies 52 and 54 which are identical in construction. As illustrated in FIG. 2, each piston means 52, 54 consists of a piston rod 60 having two pairs of spaced pistons 62 and 64 at opposite ends. The spacing of the pistons on the piston rods is such that during reciprocation of the piston rod, the piston 64 will alternately open and close annular chambers 66 and 68, located adjacent each end of each bore and will place the respective ~ conduits in communication with the connecting chamber, as well as the outer ends of the first and second bores, as will be described later. A single synchronizing link 70 interconnects the two piston rods 60 of the first and second piston rod assemblies 52 and 54 to produce synchronized movement between the two through the pressure of the fluid, as will be described later. Each end of synchronizing link 70 has an opening 72, and openings 72 respectively receive stub shafts 74 and 76, which are fixedly secured to piston rods 60. Alternatively, stub shafts 74 and 76 could be fixed to link 70 and rotatable in bearings carried by piston rods 60. Before describing the operation of the unit so far described, a brief description of the sensing mechanism will be made. The sensing mechanism is generally 5 indicated in FIGS. 2 and 3 and includes an extension 82 of stub shaft 76, a second link 84 supported on the upper end of extension 82 which in turn has a pin or element 86 extending upwardly therefrom and rotatably connected to a -"•*- disc 88 that is rotated about a fixed axis 90. A sensor 92, located adjacent the periphery of disc 88, is utilized to sense the movement or rotation of the disc which may have slots, optical graticule, gear teeth or any other 15 identification thereon which can be used to count the extent of radial movement of the disc. The output of sensor 92 may be converted in any suitable manner and displayed on indicator unit 14. 20 The operation of the positive displacement flow sensor will now be described and the novel attributes of the present invention will be described in assocation therewith. 25 Initially, pressurized fluid is supplied through fluid conduit 32 to pressurize the internal or connecting chamber 30, the periphery of which is defined by the pistons 64 on the respective piston rods 60. With the 0 piston assemblies 52, 54 in the position illustrated in FIG. 4, pressurized fluid will be delivered from connecting or center chamber 30 through conduit 50 to the right-hand end of piston assembly 52, while the left-hand OMPI end of chamber 22 will be connected through return line 46 and conduit 40 to outlet conduit 34. This will cause the piston assembly 52 to be forced towards the left by the pressure of the fluid in the right-hand end of chamber 22. During this movement, piston assembly 54 is correspondingly moved through the interconnecting or synchronizing link 70. Continued movement of piston assembly 52 will ultimately place the center chamber 30 in communication with conduit 48 and apply pressure to the upper end of piston assembly 54. Thus, the pressure of the incoming fluid on respective ends of both piston rods will cause the piston assembly 52 to move to the left and the piston assembly 54 to move downwardly, which in turn will cause a generally clockwise rotation of the synchronizing link 70. Continued downward movement of piston assembly 54 will ultimately block communication between conduit 50 and center chamber 30. Prior to this time, initial leftward movement of the piston assembly 52 will open communication between conduits 38 and 44 so that the lower end of chamber 24 is in communication with the outlet 34. Continued movement will continue in a reciprocating fashion so that the conduits 50, 48, 46 and 44 are alternately pressurized, while conduits 46 and 40, 44 and 38, 50 and 36 or 48 and 42 are in communication with the outlet. The continued reciprocating movement produces an unusual movement of the synchronizing link which will move or generally rotate in a clockwise direction as the pistons are reciprocating. It has been observed that only one point on.?the synchronizing link will rotate in a true circular path, and this point is located mid-way between the two shafts 74 d 76 at each end of the link. This mid-point is located on the axis X in FIG. 2, which is spaced from and parallel to the axis of pin 76 for a purpose that will be described later. Also, this mid-point moves in a counterclockwise direction about the center of the circular path and the diameter of the circular path is equal to one-half of the piston assembly stroke for the respective assemblies 52 and 54. Thus, by locating the pin or element 86 on the axis X and having ''the axis Y for pin 90 located on the center of the circular path, the movement of the link or mid- point of the link will be directly translated into rotational movement of the disc 88, resulting in a true accurate output. It should be noted that the synchronizing link 70 actually rotates in a clockwise direction while its midpoint moves in a counterclockwise direction in a true circular path. According to one aspect of the invention, it has been determined that there is a critical relationship between the diameter of the bores for the pistons and the stroke of the pistons within the bores. This critical relation becomes even more important when operating at a very low flow rate. While not O PI limited to any particular range of flow rates, the present invention is particularly suited for operating at minimum flow rates of less than 0.01 gallons/minute, and preferably flow rates in the range of about 0.005 to about 5 gallons/minute. The critical ratio of bore diameter to piston stroke has been determined to be preferably less than 3:1 and also preferably greater than 1:1. The preferred ratio is also believed to be to some degree dependent upon the coefficient of friction, as will be explained later. In terms of bore diameters, it was determined that a bore diameter of about one inch was a desirable diameter in terms of manufacturing simplicity for the unit and " • standardization of components. However, it is presupposed that other bore diameters, either somewhat larger or somewhat smaller, could work equally as well. Utilizing a diameter of 1.000 inches for the bore, various stroke lengths were utilized with varying results. More specifically, a piston stroke of about 0.750 inches was found to work well. However, for small flow rate unit, it was known that the piston stroke is preferably as small as possible to maintain the size of the unit at a minimum and at the same time provide a stroke of sufficient length to insure smooth transition of directional changes of movement for the piston while still being capable of operating at very low flow rates. - 3RE OMPI Actual tests have shown that the stroke of the piston is preferably less than the diameter of the bore, but also has a lower practical limit to have smooth transitions during direction changes for the pistons. Thus, it was determined that the practical lower limit for the stroke length has to be greater than 1 the diameter, and more specifically greater than 1/3 the bore diameter. In terms of physical dimensions, it is believed that the piston stroke must be more than i inch to have a satisfactory operating unit. The preferred bore-to-stroke ratio is preferably less than about three and greater than about one and more preferably is less than about 2.5 and greater than about 1.5. " As indicated above, the friction coefficient also has a very significant effect on the operation of the unit. It has been determined that the following formulation can be used as a guide in selecting parameters: f bore \ K *=(stroke)x coefficient of friction Actual experiments have shown that the value of K must be less than 0.90 in order to have a workable unit and that this value is preferably less than 0.80 and more preferably less than about 0.70. Of course, all of the above limitations are based upon the assumption that all of the parts are manufactured with minimal acceptable tolerances based upon sound engineering practices. Additional parameters 4 that have an effect on the operation of the "** unit are the clearance between the piston shafts and bearings, as well as the offset between the spaced bearings for the respective shafts. As can be appreciated from the above description, the present invention provides an extremely simplified mechanism for interconnecting two piston assemblies in such a fashion that the unit is small and compact, can be mass-produced at minimum cost and still provide very ac'curate measurements at extremely low flow rates. One additional significant advantage of the present construction is that the displacement of the piston assemblies 52 arid 54 can be changed by. varying the length of the synchronizing link which in turn will vary the output count of the unit for a given unit of measured fluid. Of course, numerous different types of indicator units may be used, such as magnetic units which allow the fluid and electrical portions of the unit to be isolated from each other.

Claims: CLAIMS 1. A positive displacement single- link flow meter including a housing having interconnecting axially-offset bores interconnected by a connecting chamber with a pair of spaced pistons connected to a shaft and reciprocated in each of said bores along a piston stroke and a synchronizing link interconnecting said shafts in said connecting chamber, the improvement of said bores having a predetermined diameter which has a ratio with respect to said piston stroke that is greater than about one and less than about three. 2. A single-link flow meter as defined in Claim 1, in which said ratio is " *• between about 2.5 and 1.5. 3. A single-link flow meter as defined in Claim 2, in which said piston stroke is greater than one-fourth of said diameter. 4. • A single-link flow meter as defined in Claim 3, in which said diameter is about one inch and said piston stroke is greater than one-fourth inch. 5. A single-link flow meter as defined in Claim 1, in which said shafts are supported in bearings in said housing with said bearings having a predetermined coefficient of friction and wherein the bore-diameter-to- piston-stroke ratio times said predetermined coefficient of friction has a value of less than about 0.90.

Inventors: BARTLING LOREN E, HOWE LAWRENCE R, BARTLING, LOREN, E., HOWE, LAWRENCE, R.
Applicants: FLO TECH INC, FLO-TECH, INC.
Application year: 1984
Main classification: G01F3
IPC codes: G01F3