Liquid level sender with linear float

Abstract: A fuel level sender including a vertically reciprocal foam float (58) that carries a slideable dual contact member (70) biased by springs (84,85) in the float toward a thick-film resistor plate (36).

Description: The invention relates to a liquid level sender comprising: support means adapted to be suspended within a liquid containing tank; a vertical support rod carried by the support means; an elongated resistance member mounted vertically generally parallel to and spaced from the support rod; a float slidably mounted on the support rod for vertical movement but spaced from the resistance member; a contact member supported for movement on the float toward and away from the resistance member and means continuously biasing the contact member into engagement with the resistance member, the resistance member including a generally vertical sender resistor. Vehicular fuel level senders, as they are commonly called, are actually transducers that convert the level of fuel in the fuel tank into an electric signal to proportionally drive a visually readable electric instrument such as a bi-torque gauge. These fuel senders conventionally include a float carried on the end of a long pivoting arm suspended within the interior of the fuel supply tank, and this arm pivots a short wiper blade having a ball contact slideably engaging wires on a wound wire rheostat that varies in resistance in accordance with the position of the float and hence liquid leve, however not always in directly proportional fashion. The angular position of the pivoting float arm, or more particularly the incremental angle of the arm, is not directly proportional to the vertical incremental change in float position and hence not directly proportional to liquid level. Moreover, in some cases, the instrument itself is not proportional throughout its range and requires some compensation. Another cause for non-linearity is that fuel tanks frequently have non-symmetrical configurations because fuel tanks are frequently designed to fit into restricted envelopes within the vehicle. Irregular tank configurations thus introduce another non-linearity in sender readout and hence require compensation. One way to compensate for these non-linearities is to vary the spacing between the wire turns on wound wire rheostats and more specifically by increasing wire turn spacing, the ratio of resistance change to float arm angle decreases, and conversely it increases with more tightly wound turns. Another common way of varying linearity is to change the individual turn lengths by modifying the shape of the support board on which the wire is wound. One common shape variation includes a tapered section in the form of a frusto-isusceles triangle. This differential wire turn spacing as well as the wire turn length variation has created a problem because the wiper contact as it rides across the wire turns tends to pluck or "banjo" the loosely held wires and not infrequently causes rupture of the wire, which of course results in a complete failure in the rheostat. The non-linearity produces by the pivoting float arm has been eliminated in some cases through the use of a vertically reciprocal float. The vertical float carries contact elements engageable with vertical straight wires, in one case, extending through the float so it produces incremental resistance changes directly proportional to incremental liquid level change and hence eliminates the inaccuracy produced by pivotal float arms. However, because the resistance elements in this design are wires, they have a uniform cross-section through their length and hence cannot compensate for the other non-linearities including fuel tank configuration. For this reason such fuel senders have not found widespread commmercial success. FR-A-22 57 898 relates to a liquid level measuring device comprising a float, an electrical contact, a resistor wire wound around a rod and a guide support rod for a float. The float is in direct contact with the liquid in the tank. As the liquid level varies so does the position of the float. Connected to the float is a U-shaped electrical contact. This contact is spring biased toward the guide support rod and has the resistor wire rod between its legs. Therefore, the legs of the U-shaped contact form sender contacts which are in engagement with the resistor wire wound around the rod. The guide support rod is a metal bar extending parallel to the resistor wire rod. The first function of the guide support rod is to guide the float unit in vertical direction. Apart from this mechanical function, the guide support rod has an electrical function: The U-shaped contact slides on the support rod, which is made of conductive material and connected to ground. DE-A-28 16 337 relates to a device for measuring liquid levels. The measuring device comprises a flotation unit, an electrical contact, a resistance bridge, and a guide support bar. The flotation unit is in direct contact with the liquid in the tank and its vertical position varies as a function of the fluid level. Attached to the flotation unit is a single, slideable electrical contact which engages and measures the resistance of the resistance bridge. The electrical contact/flotation unit is attached to the guide support rod. The rod is a metal tube positioned vertically in the tank. Its purposes are to mechanically guide the contact/flotation unit and to electrically connect the contact/flotation unit to ground. DE-A-20 44 103 relates to a liquid level measuring device which comprises a guide support rod, a flotation device, a single electrical contact, and an elongated resistance member. The flotation device rides on the surface of the liquid, moving vertically as a function of the liquid level. The flotation device is slideably mounted on the guide support member, acting as a suction tube. Attached to the flotation device is a single electrical contact. The electrical contact is spring biased forward, making contact with the resistance bridge. The electrical contact measures the resistance of the resistance member. The resistance member is prefereably composed of a wire wound rheostat. The resistance of the resistance member varies as a function of the liquid level in the tank. The guide rod mechanically guides the flotation device. All three documents cited above disclose a support rod, which has to perform two functions, i.e. to mechanically guide the float and to electrically connect a moving contact member to ground. EP-A-0 055 312, FR-A-25 78 049 and EP-A-0 038 894 disclose a fuel sender with a vertical resistance plate, which carries resistors on opposite sides thereof. Contact members, which are mounted on a vertically moving float, are biased into engagement with the resistors on both sides of the plate. There is no separate support rod to guide the float. DE-A-19 25 020 discloses a fuel sender. A ring-shaped contact member is arranged around a cylindrical guide rod within a cylindrical tube. The tube is hermetically sealed and the float outside the tube drives the contact member inside the tube. It is a primary object of the present invention to ameliorate the problems noted above and to provide a liquid level sender with wider possibilities to compensate for non-linearities, particularly those produced by irregular tank shapes. The float should slide without stick-slip problems on the support rod. Furthermore, good electrical contact and even contact wear should be possible with simple contact members. According to the invention, these objects are achieved by the features given in claim 1. The liquid level sender according to the invention has a vertical support rod, which has to perform a mechanical function, i.e. to guide the float, only. As it has no electrical function it may be designed of any suitable material, which provides good sliding properties for the float. The coplanar arrangement of ground and sender resistors ensures good electrical contact and even contact wear. Particularly, if thick film resistors are used, the coplanar arrangement of ground and sender resistors is advantageous because they may be fixed to the same side of a backing plate in a single production step. The improved compact fuel level sender may include a vertically reciprocating closed-cell foam float that carries a transversely slideable dual contact member biased by springs toward an adjacent stationary thick-film resistor. The thick-film resistor plate carries a pair of deposited thick-film resistors that can be of any desired configuration to compensate for non-linearities produced by irregular fuel tank configuration as well as other causes. The float itself is reciprocably mounted on a support rod in an enclosing tube-like housing, and the rod permits the float to pivot somewhat in a horizontal plane. The dual contact member is also permitted slight pivotal movement also in a horizontal plane with respect to the float so that as the springs bias the contact member into engagement with the thick-film resistors, this limited pivoting action balances the forces urging the contact member toward the resistor plate. The resistor plate is generally flat in configuration and has a ceramic substrate upon which generally vertical, spaces ground and sender resistors are deposited by electro-deposition, sputtering, or silk screening. The contact member is generally U-shaped in configuration with spaced sender and ground contacts carried on a front portion thereof with its side legs slideably mounted in the float. Two springs, one generally aligned with each of these contacts and mounted within the float, engage the U-shaped contact member urging it outwardly from the float toward the resistor plate. Variations in thickness in the thick-film resistors, or the resistor plate itself, slight errors in contact protrusion from the contact member, and other inaccuracies in the manufacture of the float, the contact assembly, the springs and the resistors do not affect the distribution of forces on the contacts and the resistor plate, because the dual spring arrangement spaced on opposite sides of the axis of the float support rod, tend to "rock" the float about the post and balance the spring forces acting on the contact member. Other objects and advantages of the present invention will appear from the following detailed description. Fig. 1 is a front view of the present tubular liquid level sender assembly supported within a fragmented fuel tank; Fig. 2 is a rear view of the present liquid level sender tube assembly supported within a fragmented fuel tank; Fig. 3 is a top view of the tank illustrated in Fig. 1, showing the relative location of the sender, its support tube and a fuel pump; Fig. 4 is an enlarged fragmentary longitudinal section of the liquid level sender assembly illustrated in Fig. 1; Fig. 5 is a top view of the liquid level sender assembly illustrated in Fig. 4; Fig. 6 is a bottom view of the liquid level sender assembly illustrated in Fig. 4; Fig. 7 is a cross-section of the liquid level sender assembly taken generally along line 7-7 of Fig. 4; Fig. 8 is a cross-section of the liquid level sender assembly taken generally along line 8-8 of Fig. 4; Fig. 9 is a longitudinal section of the present liquid level sender assembly taken generally along line 9-9 of Fig. 8; Fig. 10 is a longitudinal section of the liquid level sender assembly taken generally along line 10-10 of Fig. 9, showing the resistor plate; Fig. 11 is a fragmentary longitudinal section illustrating the front of the float and contact member taken generally along line 11-11 of Fig. 4; Fig. 12 is a cross-section illustrating the contact member mounting in the float taken generally along line 12-12 of Fig. 4; Fig. 13 is a fragmentary longitudinal section illustrating the spring and contact member mounting in the float taken generally along line 13-13 of Fig. 12; and Fig. 14 is a longitudinal section of the liquid level sender assembly similar to Fig. 9 with the float in its raised position. Referring to the drawings and particularly Figs. 1 to 3, a compact unitary tube-like liquid level sender assembly 10 is illustrated according to the present invention shown mounted in a fuel tank 12 by a support rod 13 and a clamp assembly 14. The liquid level assembly 10 is supported separately from a fuel pump assembly 16 having its own support rod 17. Support rods 13 and 17 as well as top plate portion 18 of the fuel level assembly 10 project through a large opening 19 in tank top wall 20, closed by a circular stepped closure plate 21 sealed around the tank opening 19 by a gasket 23. Closure plate 21 is fastened to top wall 20 by a plurality of fasteners 24 illustrated more clearly in Fig. 3. Because the liquid level sender assembly 10 is cylindrical in configuration and has a relatively small diameter, it is much more easily mounted within the tank 12 than prior liquid level senders that have exposed pivotally mounted floats. The top plate portion 18 has ground and sender terminals 26 and 27 projecting upwardly therefrom that are adapted to receive conventional connectors attached to conductors that extend into the vehicle's electrical system and provide an output proportional to the liquid level in tank 12. Referring to remaining Figs. 4 to 14 for a more detailed description of the internal parts and operation of liquid level sender assembly 10, it is seen to include a cylindrical metallic tube 30, roll staked over both a cylindrical bottom plate 31 and the multiple plate top plate portion 18 that hold the assembly 10 together as a unit. The support rod 32 is carried by support means which comprise top plate portion 18, tube 30 and bottom plate 31. Rod 32 has knurled lower end 38 press-fitted in a bore in bottom plate 31 and an upper end 39 held in position by a snap ring 41 on the top of top plate portion 18. As seen more clearly in Figs. 8, 9, 10 and 12, teh resistance member 36 includes a rectangular aluminum backing plate 43 having upper outwardly extending tabs 45 and 46 and a lower downwardly projecting tab 48 that extend through complementary apertures in tube 30 and bottom plate 31 to lock the resistance member 36 in position in tube 30. The forward face of backing plate 43 has a ceramic layer 50 bonded thereto that is generally vertical thick-film ground and sender resistors 51 and 53. Resistors 51 and 53 are deposited and bonded onto the ceramic substrate 50 by conventional techniques such as electro-deposition, sputtering, or silk screening. Sender resistor 53 is connected to sender terminal 27 by a parallel adjacent resistor portion 55 and connector not shown in detail in the drawings and similarly ground resistor 51 is connected to ground terminal 26 by a suitable conductor. While the resistors 51 and 53 are shown having uniform areas along their length, it should be understood that one or both of these resistors can be irregular in longitudinal configuration to compensate for non-linearities produced elsewhere in the system, including non-linearities in the instrument itself and those produced by irregular tank configuration. The support rod 32 may be nickel plated steel or solid nylon to provide a sliding surface fo the float assembly 34, illustrated clearly in Figs. 9, 11, 12, 13 and 14. As seen clearly in Fig. 13, float assembly 34 includes a semiannular closed cell foam float 58 preferably constructed or molded from a one-part foam such as one of the polyethylene foams and one of these found suitable in "Hycar". The float 58 includes a semiannular rear surface 60 extending somewhat more than 180°, a flat top surface 61 and a flat parallel lower surface 62 and a vertical flat forward surface 63. Float 58 has a through-bore 65 offset on a radius extending from the geometric axis of tube 30. Bore 65 receives opposed nylon bushings 67 and 68 shown clearly in Fig. 13 that provide bearing surfaces for slideable mounting the float on rod 32. This mounting permits the float to pivot or swing about rod 32. The float 58 slideably carries a generally U-shaped contact member 70. Contact member 70 includes a forward bite portion 71 and backwardly extending leg portions 72 and 73 slideable in rearwardly extending complementary recesses 75 and 76 in the front wall 63 and rear wall 60 of the float 58. The recesses 75 and 76 while generally guiding the contact member 70 for linear reciprocating motion toward and away from the resistance member 36, nevertheless permit the contact member 70 to pivot slightly in the plate of Fig. 12 because the contact member has a loose fit therein, and of course the float 58 is free to pivot about support rod 32. The forward portion 71 of the contact member 70 carries spaces ground and sender contacts 78 and 79 preferably staked to the contact member, that have enlarged rear bosses 81 and 82 and they in turn provide forward seats for coil compression springs 84 and 85 mounted in parallel bores 87 and 88 in the float that serve to bias the contact member away from float forward wall 63 toward the resistor plate assembly 36 so that contact 78 firmly engages the ground resistor 51 and contact 79 firmly engages the sender resistor 53. Because the float 58 is free to pivot about the axis of rod 32 and spring 84 acts on one side of the rod axis and spring 85 acts on the other side of the rod axis, the forces acting on contacts 78 and 79 are always equal regardless of irregularities in the resistor assembly 36, the contact assembly 35, and the float assembly 34. This assures even contact wear and also assures good electrical contact at all times. As shown in comparing Figs. 9 and 14 and the resistor configuration illustrated in Fig. 10, it can be readily seen that as the float assembly 34 moves upwardly, sender resistance can increase or decrease.

Claims: A liquid level sender comprising: support means (18, 30, 31) adapted to be suspended within a liquid containing tank; a vertical support rod (32) carried by the support means (18, 30, 31); an elongated resistance member (36) mounted vertically generally parallel to and spaced from the support rod (32); a float (58) slidably mounted on the support rod (32) for vertical movement but spaced from the resistance member (36); a contact member (70) supported for movement on the float (58) toward and away from the resistance member (36) and means (84, 85) continuously biasing the contact member (70) into engagement with the resistance member (36), the resistance member (36) including a generally vertical sender resistor (53) characterized in that the resistance member (36) further includes a generally vertical ground resistor (51), said sender and ground resistors (53, 51) being substantially coplanar, and said contact member (70) having a sender contact (79) in engagement with the sender resistor (53) and a ground contact (78) in engagement with the ground resistor (51). A liquid level sender as defined in claim 1 including means (84, 85) for biasing the contact member (70) to balance the forces on the sender and ground contacts (78, 79). A liquid level sender as defined in claim 2, wherein the biasing means includes a first spring (85) in general alignment with the sender contact (79) and biasing the contact member toward the resistance member and a second spring (84) in general alignment with the ground contact (78) and also biasing the contact member (70) toward the resistance member (36). A liquid level sender as defined in claim 1, wherein said contact member (70) has limited pivotal movement generally about the axis of the support rod (32) to assist in assuring balanced forces on the contacts (78, 79) regardless of contact or resistor sizing errors. A liquid level sender as defined in claim 1, wherein the resistance member (36) includes a backing plate (43) with both the sender and ground resistor (53, 51) being fixed to a front surface thereof. A liquid level sender as defined in claim 1, wherein the resistance member (36) includes a backing plate (43) with a thick-film sender resistor (53) and a thick-film ground resistor (51) thereon. A liquid level sender as defined in claim 1, wherein the float is a foam float. A liquid level sender as defined in claim 7, wherein the float (58) is closed cell polymer foam. A liquid level sender as defined in claim 1, wherein the support rod (32) is fixed within a cylindrical support tube (30). A liquid level sender as defined in claim 1, wherein the float (58) has limited pivotal movement about the support rod (32). A liquid level sender as defined in claim 1, wherein the contact member (70) is generally U-shaped.

Inventors: WEAVER FRANK C, WEAVER, FRANK C.
Applicants: STEWART WARNER CORP, STEWART WARNER CORPORATION
Application year: 1989
Main classification: G01F23
IPC codes: G01F23