Abstract:
The position indicator for a stick shaped sensor is received in a particularly annular sliding element, comprised of wear resistant, slide capable material as e.g. plastic, with an extended guidance length, which additionally comprises axial flow channels for the pressure medium in order to avoid local pressure buildup.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to German Patent Application No. 10 2006 031562.6 filed 7 Jul. 2006 and to German Patent Application No. 10 2006 047966.1 filed 10 Oct. 2006. 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable 
     THE NAMES OF THE PARTIES TO A JOINT RESEARCH AGREEMENT 
     Not Applicable 
     INCORPORATION-BY-REFERENCE OF MATERIAL SUBMITTED ON A COMPACT DISC 
     Not Applicable 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to position or velocity sensors. 
     2. Description of the Related Art 
     Position and velocity sensors are often based on the effect of external magnetic fields, e.g. according to the magneto-inductive or the magnetostrictive functional principle. 
     In such sensors, the current position of the component to be monitored is determined by detecting a position indicator mounted to the component with respect to its position relative to the remainder of the sensor, thus the sensor unit. 
     Depending on the functional principle, the position indicator can be a metal part or a magnet which can either be disposed and moved on one side next to the sensor unit or can enclose it in an annular manner as a bushing. Hereinafter, a magnetostrictive sensor is referred to in an exemplary manner in order to designate the components in a simplified manner, without restricting the invention, in which the sensor unit is designated as wave conductor unit and the position indicator is designated as magnet or ring magnet. 
     Thus, a typical application for this is a hydraulic cylinder or pneumatic cylinder in order to determine the position of the piston in the cylinder. 
     The wave conductor unit is thus mounted to the head of the cylinder in a solid manner and protrudes into a center bore in the interior of the piston. The position indicting ring magnet is thus mounted in the forward, expanded end of the center bore in the piston rod. 
     Though the interior diameter of the annular magnet is selected larger than the exterior diameter of the wave conductor unit, both often slide on each other during their relative motion, since the wave conductor unit is fixed only on one side, and can be laterally deflected in a substantial manner due to lengths of up to 6 m, this can occur through oscillations or jolts during the operation, or also simply due to its own weight, in case of a non-vertical, but slanted or horizontal installation. 
     In case of a friction induced wear on the ring magnets, which can lead to the destruction of the magnet and thus to the non-functionality of the entire sensor and necessitating a replacement of the magnet, the ensuing costs through shutdown times of the respective machinery are typically higher than the cost of the sensors itself. 
     Often a canting of the magnet, relative to the wave conductor unit, and an associated kinking and destruction of the wave conductor unit occurs which is also caused by a previous undue wear at the interior circumference of the magnet or by an insufficient axial guidance length. 
     Also, the bushings made from aluminum or brass, which were used so far for receiving the magnet, have only partially solved these problems, since their friction coefficients were not optimum, neither were their guidance length. 
     In addition, there is the possibility that the wear properties, the occurring abrasion material, the canting of the annular magnet, and the kinking of the wave conductor unit are facilitated by the locally occurring pressure conditions within the cylinder which occur though the fact that the magnet acts as a throttle location for the hydraulic medium. 
     Thus, for functionality reasons, hydraulic fluid does not necessarily have to penetrate into the central bore for the wave conductor unit in the piston rod beyond the ring magnet, however, the ring magnet never seals this space reliably, since it would have to withstand the high working pressure in the piston. 
     On the other hand, the radial clearance is too small for the ring magnet not having to be considered as a throttle body for the fluid flow into this space. 
     BRIEF SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a sliding element for receiving a position indicator of an e.g. magnetostrictive sensor or to provide a sensor equipped therewith which is simple and cost effective to manufacture and which avoids the disadvantages of the state of the art. 
     In the following description of the invention, the position indicator is referred to as a magnet in an exemplary manner, and the sensor unit is referred to as a wave conductor unit, and thus, a magnetostrictive sensor is referred to, without limiting the invention thereupon, which is applicable for all sensors mentioned above. 
     Since the sliding element in which the magnet is disposed has sufficient sliding capability relative to the wave conductor unit and high abrasion resistance, no destruction occurs through wear due to abrasion material. 
     Since the radial clearance between the interior circumference of the bushing and the outer circumference of the wave conductor unit is kept very small while the axially supported length between the two is large, there can be no canting when sliding the magnet. Thus, the sliding element is preferably made from a material with a large sliding coefficient, in particular, from a plastic like for example PA or PEEK, which additionally has a small friction coefficient relative to the wave conductor unit, which is mostly made from steel or titanium, and good cold flow properties and small water absorption. 
     Thus, the magnet is disposed close to the interior diameter at the sliding element so that only a thin radial inner wall separates it from the interior diameter of the bushing so that it is disposed at an optimum distance from the wave conductor unit. 
     Thus, the axial extension of the sliding element should be at least twice the diameter of the sensor element, better the quadruple diameter of the sensor element. However, in order to avoid canting, the radial clearance between the two of them should be +/−0.5 at the most, at least +/−0.1 mm of the diameter of the wave conductor unit. 
     Also, the beveling or strong rounding of the ends of the interior circumferential surface contributes to this avoidance. 
     In order to provide the unsupported length as long as possible, the interior diameter of the sliding element relative to the exterior diameter is even axially extended so that the snap ring holding the sliding element in place is disposed in an indentation in the outer circumference of the sliding element. 
     The axial flow channels through the sliding element, provided as bores or grooves in the outer circumference, allow for a pressure equilibration between both sides and thus the avoidance of local pressure buildup. The frontal flow grooves serve the same purpose, connecting the orifices of these flow channels with the interior diameter and the dead-end hole therein of the receiving piston rod. 
     While the magnet is almost automatically received next to one of the axial ends of the sliding element in a one-piece embodiment of the sliding element, in a two-part embodiment, shaped as two axially connecting parts, the magnet can be disposed in an annular groove of one of the front faces, forming the contact surface with the other partial element, and thus, e.g. in the middle longitudinal areas of the sliding element, additionally well protected against any kind of damages. 
     For this purpose, the magnet is covered on both sides or on one side by flux guidance pieces, which can also be provided mostly annular, bonded or encased in the respective annular groove, or only held through a respective protrusion of the other partial element in a form locking manner. 
     The two partial elements are radially centered relative to each other through an annular flange at the outer circumference of the one partial element which engages into a respective indentation of the other partial element. 
     In the rear face of the sliding element, furthermore, an elastomer element, e.g. an O-ring, is half received in an annular groove which serves as a buffer ring when the sliding element stops at the bottom of the receiving dead-end hole. 
     The two partial elements are preferably glued or screwed together, and the entire sliding element is preferably held in the provided indentation through a snap ring. 
    
    
     
       BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS 
       Embodiments according to the invention are subsequently described in more detail in an exemplary manner. 
         FIGS. 1   a  and  1   b  show the application in a hydraulic piston. 
         FIG. 2  shows another embodiment of the sliding element of the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  shows a magnetostrictive sensor  2  according to the invention in a longitudinal sectional view which is installed in a hydraulic piston cylinder unit, with its tubular wave conductor unit  2   a  and an axially connecting enlarged head housing  2   b.    
     Thus, sensor  2  with head housing  2   b  is placed externally onto the hydraulic piston bottom so that tubular wave conductor unit  2   a  extends through a cylinder bottom  21   a  in longitudinal direction, centrally in an interior of cylinder  21 , being received in a center dead-end bore  22  of a piston rod  20 , when piston rod  20  is inserted into cylinder  21 . 
     A ring magnet  3  is mounted in an expanded orifice of dead-end bore  22  and thus moves together with piston rod  20 , whose position is to be monitored. 
     Thereby, it immediately becomes apparent that the piston/cylinder unit has to be disassembled when an exchange of ring magnet  3  becomes necessary and, for this purpose, before that the hydraulic medium has to be drained, subsequently filled again, and bled which causes expenses and lost time. 
     In the expanded partial view of  FIG. 1   b  the sliding element is more visible. 
     Movement of the magnet out of the dead-end hole is avoided through a snap ring  5  which engages into an annular groove of the piston rod in a form locking manner. 
     An O-ring, half received in an annular groove, serves as a buffer ring  4 , axially between the sliding element  1 , and the bottom of the indentation, and thus of piston rod  20 . 
     The interior diameter  7  of sliding element  1  is made from a plastic which is abrasion resistant, slide capable and, in particular, self-lubricating, temperature resistant and chemically resistant against the hydraulic medium. Interior diameter  7  is provided axially longer than an external diameter  11  in order to provide the guidance length relative to wave conductor unit  2   a  as long as possible. 
     Axial flow channels  12  run through the guidance bushing  1  from the one front face to the other front face, and they are positioned so that buffer ring  4 , as well as the snap ring  5 , at least do not cover their orifices completely, preferably, not at all. 
     Additionally, flow grooves  6 , which radially extend over the orifice of each flow channel  12  and reach to interior diameter  7  and thus to dead-end bore  22 , are provided in the front face of sliding element  1  which faces piston rod  20 . 
     The annular magnet  3  is housed in an indentation  13  which is open on the front side and separated from interior diameter  7  by a narrow inner wall and which is provided in the front face of sliding element  1 , facing towards buffer ring  4 . 
     Within flow channel  12 , flow grooves  6  extend radially over indentations  13  so that ring magnet  3  does not axially extend into the area of flow grooves  6 . 
     The axial ends of interior diameter  7  are provided as lead in radius  8 . 
       FIG. 2  shows a second embodiment of the sliding element  1 ′ as a separate component. 
     Contrary to  FIG. 1 , the sliding element is thus formed by two partial elements  1   a, b  which connect to each other in axial direction, and wherein a partial element  1   a  has an annular flange  14  at its outer circumference, and the other one has an annular indentation  15  at the respective location, which center both partial elements  1   a, b  radially relative to each other. 
     Flow channels  12  extend through the two partial elements in an aligned manner, wherein the alignment is achievable in the simplest manner, by some of flow channels  12  being used for bolting the partial elements together or securing them relative to each other through alignment pins. 
     The annular indentation  13  in which annular magnet  3  is located, is located in the bushing  1   b  so that the ring magnet is disposed approximately in the center of the axial extension of interior diameter  7  of bushing  1 . 
     The interior wall between indentation  13  and interior diameter  7  is thereby sized as thin as possible in order to bring ring magnet  3  as close as possible to interior diameter  7  and, thus, to wave conductor unit  2   a.    
     In this case, ring magnet  3  is flanked by two axially adjacent annular flux conductor pieces  16 , and this entire magnet assembly is fixed in indentation  13  through glue or an encasement material  9 . 
     As an additional difference, buffer ring  4  is positioned radially within the orifices of flow channels  12  in the solution seen in  FIG. 2 . 
     The enveloping flow of hydraulic fluid is made possible, however, through the orifices of flow channels  12  located therein, being connected through one of the radially extending flow grooves  6  in the front face with interior diameter  7 , which are provided deeper than the groove for the buffer ring  4  preferably also extending to the outside to the outer diameter  11 .