Bi-directional flow transducer
BACKGROUND OF THE INVENTION 
     This invention relates to a fluid flow transducer of the bi-directional type. 
     It is becoming a more common practice to electronically control the operation of hydraulic circuits, and in the control of hydraulic circuits, it is often desirable to sense the rate of flow of hydraulic fluid through some portion of the circuit. Thus, it is desirable to have a fluid flow sensor which generates an electrical signal which represents the fluid flow and which has a linear relationship with respect to the rate of fluid flow. Finally, it is desirable to have an inexpensive electro-hydraulic fluid flow sensor which senses fluid flow in both forward and reverse directions. 
     SUMMARY OF THE INVENTION 
     An advantage of the present invention is that it provides an electro-hydraulic fluid flow transducer which responds to bi-directional fluid flow. 
     Another advantage of the present invention is that it provides an electro-hydraulic flow transducer with a linear response. 
     These and other advantages are achieved by the present invention which provides a flow sensor housing with a bore which slidably receives a movable hollow cylindrical magnetic core. A rod is mounted coaxially within the core and is fixed with respect to the housing. The core and rod define a variable annular flow restriction therebetween. A pair of springs act on opposite ends of the core and are biased to maintain the core in a neutral position. Abutments are operable to uncouple the core from one of the springs when the core is displaced to compress the other spring. The displacement of the core is linear with respect to fluid flow in either direction and an LVDT displacement transducer generates an electrical output signal representing the core displacement. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The FIGURE is a cross-sectional side view of applicant&#39;s flow transducer. 
    
    
     DETAILED DESCRIPTION 
     A bi-directional fluid flow transducer 10 includes a housing 12 of a non-magnetic material such as a non-magnetic-type stainless steel. Housing 12 has a stepped bore 14 defining a shoulder 16 and a port 18. One end of the housing 12 threadably receives an end fitting 20 which includes a port 22 and shoulder 24. A fluid-tight seal between fitting 20 and housing 12 is maintained by means of O-ring seal 26, locknut 28 and washer 30. Fluid may flow between ports 18 and 22 and through the bore 14 in either direction. 
     An axially symmetrical valve member or rod 32 is positioned centrally and coaxially in the bore 14. Rod 32 includes a pair of smaller diameter end portions 34 and 36 separated from a larger non-uniform diameter central portion 38 by a pair or annular and outwardly facing shoulders 40 and 42. The central rod portion 38 includes an outwardly extending ridge 39 which tapers smoothly and symmetrically without any discontinuities from a maximum diameter at its center to a uniform diameter portion which terminates at the shoulders 40 and 42. 
     The rod 32 is supported by a pair of identical supports 50, each with a hollow cylindrical central portion 52 which receives a corresponding one of the smaller diameter rod portions 34 or 36. Each support 50 also includes a flange portion 54 which engages the wall of bore 14 and which engages a corresponding one of the shoulders 24 or 16. Each flange portion 54 includes a frustoconical portion through which extends six (6) uniformly spaced bores 56 with axes oriented at an angle of 30 degrees with respect to the longitudinal axis of the rod 32. (Two of the six bores 56 are visible in the single FIGURE). A pair of apertured spacers 60 are positioned coaxially on the smaller diameter rod portions 34 and 36 between the supports 50 and corresponding shoulders 40 and 42. Each spacer 60 includes three radially extending arms 61, two of which are visible in the FIGURE. Annular shims are inserted onto the rod portions 34 and 36 between the spacers 60 and the shoulders 40 and 42 so that the rod 32, the spacers 60 and the supports 50 are rigidly fixed axially with respect to the housing 12 between shoulders 24 and 16. 
     An axially symmetrical sleeve or magnetic core 66 coaxially surrounds the rod 32 and is movably mounted in bore 14. Core 66 includes a pair of larger outer diameter end flanges 68 and 70 on either end of a cylindrical central portion 72 with a smaller outer diameter. End flanges 68 and 70 slidably engage the wall of bore 14. An annular ridge 74 of triangular cross-section extends radially inwardly from the central sleeve portion 72. Ridge 74 cooperates with the ridge 39 of rod 32 to define a variable annular restriction to fluid flow through bore 14 depending upon the axial position of the core 66 relative to the rod 32. A small annular clearance exists between ridges 74 and 39 when the core 66 is in the neutral position shown in the FIGURE. Ridges 74 and 39 are shaped so that the displacement of core 66 will vary linearly as a function of the rate of fluid flow. 
     Flow transducer 10 also includes a pair of identical cylindrical guides 80. Each guide 80 includes a smaller diameter cylindrical portion 82 slidably received by a corresponding end of the core 66 and an annular end face 84 which is engageable with the arms of a corresponding one of the spacers 60. Each guide 80 also includes a larger diameter portion 86 which receives a corresponding one of a pair of centering springs 88 and 90. Each guide 80 also includes an annular shoulder 92 which is engageable with a corresponding end of core 66 and an annular shoulder 94 which engages a corresponding one of the springs 88 and 90. The other ends of springs 88 and 90 engage corresponding ones of the supports 50. The springs 88 and 90 are both biased to urge the core 66 via guides 80 to a centered or neutral position with respect to the rod 32 and housing 12 (as seen in FIG. 1) in the absence of fluid flow through bore 14. Both springs 88 and 90 are slightly preloaded when the core 66 is in the neutral position so that the springs 88 and 90 may center the core 66 in spite of frictional losses. 
     The axial position of core 66 may be sensed by a conventional linear visible differential transformer (LVDT) which may include a cylindrical LVDT coil assembly 100, such as made by Schaevitz Engineering of Camden, New Jersey and which may be coaxially mounted on the exterior of the non-magnetic housing 12. A conventional electrical circuit, such as described on pages 17-55 and 17-56 of D. G. Fink&#39;s Electronics Engineers Handbook, McGraw-Hill, 1975, may be connected to the coil assembly 100 to generate an electrical output signal which represents the axial position of the core 66 relative to the coil assembly 100. 
     Mode of Operation 
     When no fluid is flowing through the bore 14, the springs 88 and 90 maintain the core 66 in the neutral position shown in the FIGURE. However, fluid flow through bore 14 causes the core 66 to be displaced from the neutral position in the direction of the fluid flow and by an amount proportional to the rate of fluid flow. For example, if fluid is flowing to the right, viewing the FIGURE, then the core 66 will be displaced to the right, compressing spring 90 until the flow forces are balanced by the increased resistance of spring 90. When core 66 moves rightwardly from its neutral position, the spacer 60 engages the end face 84 of the left-hand guide 80 and prevents guide 80 from moving to the right along with core 66. This engagement uncouples the spring 88 from the core 66. In this manner, when the core is displaced left or right from the neutral position, only one of the springs 88 or 90 exerts any force on the core 66. The core assembly 100 and LVDT circuit (not shown) operate in a conventional manner to produce output signals which represent the direction and magnitude of the core displacement, and thus represent the direction and magnitude of the fluid flow through bore 14. Increased efficiency and sensitivity is achieved by having the cylindrical core 66, rather than the rod 32, be the movable flow responsive member. This increased efficiency and sensitivity results from the close proximity and the resulting increased magnetic flux linkages between the movable core 66 and the fixed LVDT coil assembly 100.