Abstract:
Position sensors in stick design for installation in hydraulic cylinders, can be maintained and replaced without leakage, with the sensor head housing having a attachable and detachable sensor head cover, which can be opened without removing the remaining housing out of the piston cylinder unit, opening the access to the functional components of the sensor.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims priority to Germany Patent Application No. 102005060676.8 filed 19 Dec. 2005.  
       STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
       [0002]     Not Applicable  
       THE NAMES OF THE PARTIES TO A JOINT RESEARCH AGREEMENT  
       [0003]     Not Applicable  
       INCORPORATION-BY-REFERENCE OF MATERIAL SUBMITTED ON A COMPACT DISC  
       [0004]     Not Applicable  
       BACKGROUND OF THE INVENTION  
       [0005]     The invention regards position sensors in stick form according to a non-contacting functional principle for application in fluids, among other things. Position sensors in stick form measure the position of a position indicator fastened to a component moveable relative to the stick position sensor.  
         [0006]     Such sensors are used, among other things, in an interior of hydraulic or pneumatic cylinders in order to know the exact extension of the piston/cylinder unit, which is of great importance for the control of the machinery and equipment operated therewith.  
         [0007]     The position sensor is thereby located in a tight housing which comprises a long, slender sensor-stick-housing and a connected, shorter sensor sensor-head-housing which has a larger diameter and wherein the processor electronics are located.  
         [0008]     Thereby, the sensor with its sensor-head-housing is located in the piston/cylinder unit in a longitudinal fixed manner so that the slender sensor-stick-housing extends into a typically central bore of the piston or the piston rod where the position indicator is mounted.  
         [0009]     Since the sensor-stick-housing, and thereby the measuring length of the sensor, extends along an entire possible extension length of the piston rod, the current position of the piston rod relative to the cylinder is known at any time.  
         [0010]     Thereby, the housing of the sensor at its exterior side is in direct contact with operating fluid of the piston/cylinder unit and is also exposed to its operating pressure. Especially in hydraulic units with very high pressures, therefore, it is of great importance that the sensor housing is provided stable and tight and that a sufficient sealing between the housing of the sensor and the piston/cylinder unit is provided, mostly through a respective seal at an exterior circumference of the sensor-head-housing relative to the surrounding wall of the pneumatic or hydraulic cylinder.  
         [0011]     As a non-contacting sensor principle thereby differential transformatoric measuring procedures (LVDT&#39;s), non-contacting inductive measuring procedures (LVP&#39;s), inductive potentiometric measuring procedures (DC/DC-sensors), Eddy current procedures and often also magnetic, in particular magnetostrictive functional principles are being used. With the latter ones a permanent magnet is being used as a position indicator with other processes a tube sleeve, a submerged anchor, or a similar component. With PCLD-sensors through a magnet a virtual air gap is being created in a ferromagnetic core.  
         [0012]     As it is well known, magnetostrictive position sensors function as follows:  
         [0013]     A wave conductor typically consists of a tube, a wire, or a band and can also serve as an electric conductor. The wave conductor can also be located in a shape generating linear or circular body made from non-magnetic material, such as plastic or metal, for receiving the wave conductor and holding it in bearings.  
         [0014]     Based on the Wiedemann-Effect an electric impulse fed into the wave conductor generates a mechanic elastic wave when superimposed with a position magnet.  
         [0015]     At a certain location, typically at one end of the wave conductor, in particular, the torsion component of this mechanic/elastic impulse is detected by a detector unit, mostly located in a fixed position relative to the wave conductor. The duration between the triggering of the electrical excitation impulse and the reception of this mechanic-elastic wave, thereby, is a measure of the distance of the slideable position element, e.g. of a position magnet, from the detection device.  
         [0016]     A typical such sensor is described in the U.S. Pat. Nos. 5,590,091 and 5,736,855.  
         [0017]     Subsequently, only magnetostrictive position sensors are referred to without limiting the invention to this position measurement principle.  
         [0018]     In such position sensors used in piston/cylinder units there are several problem areas.  
         [0019]     One problem area is an increase of overall length of the piston/cylinder unit through the position sensor.  
         [0020]     While the slender sensor-stick-housing extends into an interior of the piston rod, the wider sensor-head-housing requires a respective interior length in the cylinder of the piston/cylinder unit for housing, which increases the overall length of the piston/cylinder unit.  
         [0021]     It is attempted to also shorten the sensor-head-housing in axial direction through miniaturization of the processing electronics housed in the sensor-head-housing. However, with most position sensors only a single cable exit direction is offered for the position signals from the sensor-head-housing, and this is mostly the axial exit of the cable or the connector from the front face of the sensor-stick-housing opposed to the sensor-head-housing.  
         [0022]     However, when depending on the installed solution this front face of the sensor-head-housing is built over the cylinder unit, e.g. through a mounting eyelet required on this side of the piston/cylinder unit, the exit of the cable from the piston/cylinder unit has to be performed towards the side.  
         [0023]     The necessary angulation towards the side of the cable exiting the sensor-head-housing alone, again, requires additional axial volume of the piston/cylinder unit.  
         [0024]     An initially provided exit direction of the cable, or of the connector, perpendicular to the axial direction is disadvantageous, on the other hand, if in the actual installation situation a further axial track of the cable is required and insufficient space is available on the sides.  
         [0025]     Repairs constitute anther problem area.  
         [0026]     Due to the described, often high operating pressures in such piston/cylinder units and a rough operating environment, like for instance, strong vibrations as they often occur in equipment, a failure of the piston sensor can occur so that it needs to be replaced completely, or components of it, possibly a part of the processing electronics or of the wave conductor unit of the magnetostrictive sensor.  
         [0027]     In this case the piston/cylinder unit had to be opened, until presently, and the whole position sensor with its housing had to be removed from the piston/cylinder unit since, especially due to the above mentioned operating conditions, the processing electronics in the sensor-head-housing of the sensor were generally encased solid.  
         [0028]     However, this means that before removing the senso, the operating fluid in the respective piston/cylinder unit has to have ambient pressure, since otherwise large amounts of operating fluid exit into the environment and the connected actuators change positions unintentionally.  
         [0029]     On the other hand, after replacing the position sensor, the respective piston/cylinder unit or the whole operating loop to which it is connected may have to be refilled or at least bled, which entails a considerable effort and poses an additional source for failures if performed incorrectly.  
       BRIEF SUMMARY OF THE INVENTION  
       [0030]     Therefore, it is an object of the present invention to provide a stick-shaped position sensor according to the magnetostrictive operating principle which can be exchanged in a piston/cylinder unit without leakage, thus without opening the loop of the operating fluid, and/or, in addition, requires only little axial space for housing a sensor-head-housing in a cylinder unit.  
         [0031]     Through locating an easily removable sensor-head-cover on the sensor-stick-housing so it faces away from the sensor-head-housing, the functional components of the position sensor initially in the sensor head housing, and after removal of the functional components located there, also in the stick housing, can be reached and extracted completely, without having to remove the housing of the sensor from its installed position.  
         [0032]     Easy disassembly in this context means primarily disengaging a positively locking connection, thus a thread, a lock ring or a similar component and, in particular, not disassembling a connection which is not meant to be disassembled, such as a glue joint, an encasement, a weld, a solder joint, or similar though thereby under certain circumstances the lid would not be destroyed, but the respective connection area.  
         [0033]     When the sensor-head-cover can thus be disassembled without destruction and can be reassembled, therefore, even the same sensor-head-cover can be used again.  
         [0034]     Thereby, it is possible to leave the housing of the position sensor in its sealed assembled position in a piston/cylinder unit, while the position components of the position sensor are tested, repaired, or replaced. Therefore, a draining of the operating medium and a subsequent pressurization and bleeding is not necessary, so that work on the sensor can be performed much quicker.  
         [0035]     All functional components of the sensor can be replaced this way, including the whole position sensor, besides its outer housing. Replacing the sensor housing itself is rarely necessary since the housing is typically stable enough so that it is not damaged during normal operation.  
         [0036]     In order for removal of the functional components and of the sensor-head-cover to be performed with as few problems as possible, an extraction in axial direction towards the side facing away from the sensor-stick-housing is provided.  
         [0037]     This is accomplished e.g. through the sensor-head-cover extending into an interior diameter of a wall of the sensor-head-housing and being secured there through positive locking, thus through a thread or a securing element like a lock ring.  
         [0038]     In order to keep the axial space requirement in the installation environment for the sensor-head-housing and, thereby, for the position sensor as small as possible, the connection towards the outside from the sensor-head-housing and from the processing electronics located therein are designed specifically either with a connector located in the sensor-head-housing or with a cable exit for passing through the cable.  
         [0039]     The sensor-head-cover has a high dome, and the cable exit or the connector is located in its side wall. Through positioning not in the enveloping surface of the side wall but through the side wall of a cylindrical shape having an indentation, preferably a flat area formed as a secant, the cable outlet can be located in this indentation or flat area and, thereby, it does not extend exactly radially but partially tangentially out of the dome area.  
         [0040]     Especially because the dome has a smaller exterior diameter than the sensor head, it is possible to run a cable pointing away from the dome in this manner either according to the direction of the cable outlet or of the connector in a radial-tangential manner through the surrounding components to the outside, or to angulate the cable in the area of the flat or indentation into the axial direction and to run it away. Even a partial running of the cable is possible around the dome over a part of the circumference in order to run the cable at a certain location of the circumference axially or radially further to the outside. For the often occurring case of running the cable away in a radial or tangential manner from the sensor-head-housing, thereby, the otherwise required room for angulating the cable is no longer required, so that for the volume requirement only the actual axial extension of the sensor-head-housing, including the dome of the sensor-head-cover, need to be considered.  
         [0041]     The cable outlet can be provided through a connector or a cable pass-through opening, e.g. a cable grommet inserted into a bore of the outer wall or also through an interior thread fabricated into this bore whose diameter is sized relative to the outer circumference of the cable to be run through so that the core diameter of the interior thread is smaller than an exterior diameter of insulation of the cable but larger than an interior diameter of the cable. Thereby, the cable insulation is threaded in the interior thread and fixed in a longitudinal direction.  
         [0042]     In order to make the exit of the cable or socket possible at any desired location on the circumference, the sensor-head-cover is preferably usable in any rotation position relative to the sensor-head-housing and can be fixed there preferably through positive locking between the sensor-head-cover and the sensor-head-housing.  
         [0043]     The housing of the sensor is fixed in the surrounding unit, such as the piston cylinder unit, with a thread and sealed with a gasket. The thread is either located in an outer circumference of the sensor-head-housing or in an outer circumference of the sensor-stick-housing in the section adjacent to the sensor-head-housing, preferably in a diameter area slightly enlarged relative to the sensor-stick-housing. The seal is hereby located on the side of the thread opposite to the sensor-stick-housing in order not to be able to damage the gasket through contact through the interior thread of the surrounding component.  
         [0044]     If an O-ring is used for a gasket, a support ring located axially behind it is to be preferred due to the high occurring pressures. In order to be able to tighten the thread sufficiently, the outer circumference of the sensor-head-housing is partially provided as an external hexagonal shape in order to be able to apply an opened end wrench.  
         [0045]     Also, in order to protect the processing electronics against strong vibrations while still being able to replace the processing electronics and/or the wave conductor unit without having to open the tight installation of the housing of the position sensor in the surrounding assembly group, inside the sensor-head-housing an inside dish can be provided opened toward the sensor-head-cover, e.g. made from plastic, inside which the processing electronics are located and encased relative to the interior dish. The encased electronics can be replaced as a separate component after disconnecting the signal wires and after removing the unit consisting of the processing electronics and the interior dish and the wave conductor unit located behind it can be extracted because of the tight reception of the interior dish in the sensor-head-housing and of the signal connections, e.g. plug connections or threaded connections of the processing electronics, on the one hand, toward the wave conductor unit and, on the other side, toward the cable exit.  
         [0046]     A mounting of the interior dish with little clearance and thereby without vibration relative to the sensor-head-housing can be performed through axial clamping with the sensor-head-cover to be inserted.  
         [0047]     The cover for closing the head housing can furthermore be used for performing error diagnosis and/or programming of the processing electronics located in the interior of the head housing in a simple manner.  
         [0048]     Thus, the processing electronics in the interior of the head housing can include a light source, such as an LED or an infrared unit, emitting only light of a certain wave length.  
         [0049]     When the lid consists of a material, such as plastic, which is permeable to all light or at least to light of this special wave length, an optical signal emitted by the light source located in the interior of the head housing, e.g. a flash rhythm, can be detected from the outside in a non-contact manner and/or vise versa wherein a respective sensor is located in the interior of the head housing and the respective light source is located externally.  
         [0050]     A simple solution is to provide an opening in the lid at the location of the light source through which the light source and, thereby, the optical signal are visible. Thereby, however the housing is not tight anymore.  
     
    
     DETAILED DESCRIPTION OF THE DRAWINGS  
       [0051]     Embodiments according to the invention are described in more detail.  
         [0052]      FIGS. 1   a  through  1   d  show the position sensors according to the invention in a side view and in an axial face view from a sensor head housing.  
         [0053]      FIG. 2   a  shows the position sensor according to  FIG. 1   a  in a longitudinal sectional view, and  FIGS. 2   b  through  2   g  show enlarged detail views of the sensor head areas.  
         [0054]      FIGS. 3   a  and  3   b  shows the sensor carrier unit as a whole.  
         [0055]      FIGS. 4   a  and  4   b  show a typical installation situation of a stick position sensor.  
         [0056]      FIGS. 5   a  and  5   b  illustrate a position sensor with an alternative lid shape.  
         [0057]      FIGS. 6   a  and  6   b  show an alternate design of the sensor of the present invention.  
         [0058]      FIGS. 7   a  and  7   b  show a further design of the sensor of the present invention.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0059]      FIG. 1   a  shows a position sensor  1 , e.g. a magnetostrictive type sensor, in side view.  
         [0060]     In an interior of a sensor-stick-housing  2 , extending in a longitudinal direction  10 , a measuring device is located. Therein extends a wave conductor unit  24  visible only in a sectional view of  FIG. 2   f  with a central wave conductor  23 .  
         [0061]     At a left end of sensor-stick-housing  2 , a sensor-head-housing  3  is connected in a tight manner having an external diameter  12  several times larger than the diameter of sensor-stick-housing  2 , but only has a fraction of its length. At a transition to sensor-stick-housing  2 , sensor-head-housing  3  comprises an area  16  with an enlarged diameter relative to sensor-stick-housing  2  however significantly smaller than the largest diameter  12  of the otherwise cylindrical sensor-head-housing  3 , which is dish shaped, thus open toward a front side, facing away from sensor-stick-housing  2 .  
         [0062]     Since sensor-stick-housing  2  is closed tight at the right end facing away from sensor-head-housing  3 , e.g. through an end cap, the whole housing of the position sensor is open towards the left end as in  FIGS. 1 and 2 , and closed there by a sensor head cover  8  having a central dome  8   a  protruding beyond the open face of sensor-head-housing  3  as seen in the  FIGS. 1 and 2  towards the left.  
         [0063]     Along sensor-head-housing  2 , an annular position magnet  29  is moved in a radial distance and without contacting. The position of position magnet  29  in longitudinal direction  10  is to be measured by position sensor  1 . An alternate sleeve-shaped position indicator is shown in  FIG. 2   a.    
         [0064]      FIG. 2   b  shows an interior layout of position sensor  1  in an enlarged longitudinal cross section, especially in the head area, which is of particular interest here.  
         [0065]     Initially, a housing of the position sensor is manufactured by inserting stick-sensor-housing  2  into an opening of a bottom of the dish-shaped sensor-head-housing  3  and connecting it on an exterior side of sensor-stick-housing  2 , or its front face with sensor head housing  3  through at least one annular circumferential weld  21  in a tight manner. Sensor stick housing  2  is also sealed tight at the other end through a cover lid, which is also welded on.  
         [0066]     The functional elements of position sensor  1  are installed into this housing, wherein initially a positioning sleeve  27  is placed onto a mouth of a pass-through in the bottom of sensor-head-housing  3  in axial direction in a positively locking manner. It is pressed onto the bottom of an indentation of sensor-head-housing  3  and fixed by inserting a dish-shaped printed circuit board  26  over an exterior circumference of positioning sleeve  27 , onto which processing electronics  20  are built. Positioning sleeve  27  is axially fixed through engaging an outer rim of printed circuit board  26  with a spacer sleeve  28  in an axial manner, which on the other hand is engaged by a sensor-head-cover  8  inserted into a free space of sensor-head-housing  3  and fixed through a lock ring  9 . Through a respective annular shoulder in sensor-head-housing  3 , positioning sleeve  27  is also radially fixed in sensor-head-housing  3 .  
         [0067]     Before inserting sensor head cover  8 , a wave conductor unit  23 , which extends substantially over a whole length within sensor-stick-housing  2 , is moved forward through positioning sleeve  27  into sensor-stick-housing  2  until a shoulder of the rearward, slightly expanded end of a sensor carrier unit  24  touches at a front annular face of positioning sleeve  27 . Detector unit  22  is located within sensor carrier unit  24 .  
         [0068]     In this final position, sensor carrier unit  24 , in whose longitudinal middle the indicated wave conductor  23  extends, protrudes in the direction of sensor-head-cover  8  beyond printed circuit board  20  of processing electronics  20  into a dome  8   a  of sensor-head-cover  8 . Through a long overhang over a plane of processing electronics  20  a dead zone of the position sensor is reduced, where no position determination is possible. At the right end, facing away from sensor head housing  3 , a damper can be seen at sensor carrier unit  24 , damping an electro mechanic wave arriving in wave conductor  23 .  
         [0069]      FIGS. 2   b  furthermore shows a gasket  7 ″ located in an exterior circumference of sensor-head-cover  8  through which a penetration of dust into an area of the processing electronics is to be prevented.  
         [0070]     However, it can be difficult to avoid turning head cover  8  in sensor-head-housing  3  with this mounting of cover  8  in the head housing through a radially abutting gasket  7 ″, and through axially positively securing through locking ring  9 . This can have a disadvantageous effects on the cable connections.  
         [0071]     Alternatively,  FIG. 2   f  shows a mounting method wherein an outer edge  8   c  of cover  8  pointing into an interior space of sensor-head-housing  3  is beveled so that in the non-beveled, internally circumferential abutting shoulder of sensor-head-housing  3  an annular free space remains which is triangular in cross section. As shown in the expanded illustration of  FIG. 2   f , in this free space a gasket  7 ″, preferably an O-ring made from elastic material, is housed and sized so that it is wedged at the respective axial shoulder of sensor-head-housing  3  in a free space  41 , when cover  8  comes into axial contact, thereby imparting axial and radial forces onto cover  8 .  
         [0072]     Through the radial support of the gasket or the O-ring at sensor-head-housing  3  and also at cover  8 , penetration of dirt into the interior and thereby into processing electronics  20  is avoided. Through axial compression, cover  8  is pressed against an interior side of a lock ring  9 , and thereby through force engagement is prevented from turning relative to lock ring  9 , and also turning of lock ring  9  relative to sensor-head-housing  3  is reliably prevented, whereby the strength of this force engaging connection depends on the degree of compression and elasticity of gasket  7 ″.  
         [0073]     In the same way, a dish  28  receiving processing electronics  20  and printed circuit board  26  in an interior of head housing  3  can be secured against rotation in sensor-head-housing  3 . The dish on the other hand is supported at its free end at a bottom side of cover  8 , again, axially preloaded through a gasket  7 ″ compressed between the bottom of sensor-head-housing  3  and an outer edge of the bottom of interior dish  19  into a triangular free space  41 .  
         [0074]     On a large cylindrical outer circumference of sensor-head-housing  3  a first annular groove close to sensor-stick-housing  2  can be seen, wherein a seal  7  is shown as an O-ring sealing relative to the surrounding component. In an axially connecting direction towards a free end of sensor-head-housing  2 , a support ring  17  is located in the same groove, greatly increasing the load bearing capability of O-ring  7  and preventing a squeezing out of the groove under pressure.  
         [0075]     In a second annular groove located towards the free end in  FIG. 2   b  further to the left, an additional supplementary O-ring can be located in an outer circumference as shown in the lower half of the picture.  
         [0076]     This layout of the outer circumference and sealing relative to the environment is also chosen in the solution according to  FIG. 2   f.    
         [0077]     On the other hand, in a solution according to  FIG. 2   g , outer circumferential seal  7  is located in an outer circumference of an enlarged diameter area  16  at a transition between sensor-head-housing  3  and sensor-stick-housing  2 .  
         [0078]     An upper half of the picture shows how an exterior hexagonal shape  18  can be provided on an exterior circumference of sensor-head-housing  3  for engaging an opened end wrench in order to be able to thread or tighten it.  
         [0079]     Thereby, a significant feature is the shape of sensor-head-cover  8  and the manner of running cable  6  out of sensor-head-housing  3  in order to be able to conduct data derived by processing electronics  20  and to process itoutside of the sensor. For this, exit of cable  6  through a cable exit  4  is illustrated. However, a connector or a connector socket could be located in sensor head cover  8  at the same location and in the same orientation as cable exit  4 . An illustration of a further path of cable  6  in an interior of sensor-head-housing  3  was left Out in order to make  FIGS. 2   b ,  3  and  4  clear.  
         [0080]     Also  FIG. 2   b  in connection with a face view of  FIG. 1   b  shows sensor head cap  8  with its exterior circumference located tight in an interior circumference of dish shaped sensor-head-housing  3 . A cap shaped dome  8   a  thereby extends out of the interior of sensor-head-housing  3  with a reduced exterior diameter relative to an outer rim of sensor-head-cover  8  toward the left, thus out of an opening of sensor-head-housing  3 . Into dome  8   a  protrudes, on the one hand on the interior side, sensor carrier unit  24  with its left end and, on the other hand in side wall  8   b  of dome  8   a , is located cable exit  4  for cable  6 .  
         [0081]     As  FIG. 1   b  shows, a cylindrical enveloping surface of side wall  8   b  has an indentation  13  toward the inside, e.g. shaped as a secant, through which a flat area  14  is formed, displaced from an outer circumference of dome  8   a  toward the inside, wherein cable outlet  4  is located. Cable  6  thereby is not completely radial at the location of the cable outlet, but slightly tangential to the enveloping surface of dome  8   a , wherein indentation  13  is preferably sized so that in its area the cable exiting from cable outlet  4  within indentation  13  can be bent into the desired path without nicking, either into a further axial path or into a radial or tangential path.  
         [0082]     The  FIGS. 1   c  and  1   d , on the other hand, show a solution wherein at the same location at dome  8   a , a connector  5  is located as a cable exit for inserting a connector, which is not shown.  
         [0083]     The  FIGS. 6 and 7  show further embodiments of the shape of dome  8   a  in lid  8  which are different than those seen in  FIG. 1 .  
         [0084]     While the dome in side view, thus seen perpendicular to the longitudinal direction  10  in the  FIGS. 1   a  and  1   b  is substantially rectangular, dome  8   a  in the solution according to  FIGS. 6 and 7  in side view is beveled at an outer circumferential annular edge.  
         [0085]      FIG. 6  shows a variant with a connector socket  6 ′ as cable outlet  4 ; while in  FIG. 7   b  cable  6  is run out of an opening in dome  8   a  without a connector.  
         [0086]     The design of the sensor-head-housing with dome  8   a  thereby entails, besides gaining space for installing the connector or cable, a reduced installation space requirement in axial direction for the head area of the position sensor in e.g. a piston/cylinder unit  30 , as shown in  FIG. 4 .  
         [0087]     Piston/cylinder unit  30 , of which only one end is shown in  FIG. 4 , comprises a cylinder  32  shaped as a tubular section closed on one side through a face plate  34 . At an interior circumference of cylinder  32 , a piston  33 ′ abuts tight but moveable, forming a thickened end of a piston rod  33  located in cylinder  32 . Face plate  34  has a central pass-through opening  34   a.    
         [0088]     Position sensor  1  is located with thicker sensor-head-housing  3  on/or in face plate  34  and extends with slender sensor-stick-housing  2  through pass-through opening  34   a  and piston  33 ′ into a central dead end bore of piston rod  33 : having a slightly larger diameter, so that no contact can occur between piston rod  33  and sensor-stick-housing  2 . The element acting as a position indicator for the position of piston  33 ′, e.g. the annular position magnet  29 , is inserted into piston  33 ′ and/or the piston rod.  
         [0089]     Due to the position of a slideable piston  33 ′ shown in  FIG. 4 , between the piston and the cylinder in this case only a small amount of operating fluid  31  is enclosed, wherein, however, it is apparent that this operating fluid is in direct contact with the housing of position sensor  1 .  
         [0090]     Most piston cylinder units  30  have to be coupled at their two longitudinal ends with abutting components and, for this purpose, have to comprise a respective mounting element on both ends, e.g. a depicted mounting eyelet  37 .  
         [0091]     Mounting eyelet  37  cannot be directly mounted to face plate  34  or provided in one piece together with it, since on the side of face plate  34  facing away from piston  33 ′, sensor-head-housing  3  of position sensor  1  is located and mounted, and it has to be disassembled.  
         [0092]     A front cover  35  is placed therefore onto a free front face of face plate  34  and mounted through longitudinal threading  36 . On the one hand, sensor-head-housing  3  is protected and covered. On the other hand, at the free end of this front cover  35  the necessary mounting element can be mounted, such as mounting eyelet  37 .  
         [0093]     Thereby,  FIG. 4   a  shows that front cover  35  can reach close to the front end of sensor-head-housing  3 , thus sensor head cover  8 , since no additional axial space is required for cable  6  to axially run out of the sensor head cover. Cable  6  can either be run out through face cover  35  radially between threads  36  or can be run out in an axial manner, wherein for deflecting into axial direction indentation  13  in dome  8   a  is sufficient, as shown in  FIG. 4   b.    
         [0094]     In comparison,  FIG. 4   b  shows the advantage of a beveled circumferential edge of dome  8   a  of lid  8 .  
         [0095]     Thereby, an interior contour in a superimposed front cover  35  between an interior wall and a bottom can be beveled, which greatly improves the torsion resistance of front cover  35  relative to a rectangular position of  FIG. 4   a  in this location, since in such a beveled interior edge the occurrence and inception of a fatigue fracture is much less likely than with a sharp rectangular interior circumferential edge, as in the front cover  35  according to  FIG. 4   a . Analogously, front cover  35  can have reduced dimensions with a beveled interior edge according to  FIG. 4   b.    
         [0096]     On the other hand,  FIG. 4  shows that after loosening threaded connection  36  and removing front cover  35 , the functional components of functional positional sensor  1  are accessible and can also be replaced without having to loosen the tight connection between the housing, ⅔ of the sensor and the face plate  34 , and thereby from piston cylinder unit  30  in its entirety.  
         [0097]     It is only necessary to pull off sensor cover  8  in order to get into the interior of sensor-head-housing  3  and thereby to the processing electronics housed therein (not shown in  FIG. 4 ) or also after its removal, to be able to pull wave conductor unit  23  out of sensor-stick-housing  2 .  
         [0098]     Such a complete sensor unit, comprising sensor carrier unit  24  including wave conductor  23  located therein, detection unit  22  and interior dish  19  with processing electronics  20  housed therein, which is not shown,  FIG. 3   a  also shows in a longitudinal sectional view, wherein in this Solution, the interior dish  19  is connected with all other mentioned components in a positively manner into a unit, which can be handled in its entirety.  
         [0099]     On the other hand,  FIG. 3   b  shows a sensor unit according to another measuring principle in which a coil  42  extends along sensor carrier  24  in measuring direction instead of a wave conductor. Also a magnet  29  or another element is being used as position generator.  
         [0100]     In case of  FIG. 4 , the exterior circumference of the sensor-head-housing and the mounting and sealing relative to the piston cylinder unit are designed differently from  FIGS. 1 and 2 , thus analogous to  FIG. 2   d.    
         [0101]     The area  16  with an enlarged diameter relative to sensor-stick-housing  2  at the transition between sensor-stick-housing  2  and sensor-head-housing  3  serves only for mechanical central alignment in pass-through opening  34   a  of face plate  34  and does not have an external thread.  
         [0102]     The mechanical fixation between sensor-head-housing  3  and face plate  34  is performed through a thread  15  at an outer circumference of sensor-head-housing  3  close to its free end and through a respective interior thread in face plate  34 . The sealing between both components is performed via a seal  7  with an adjacent support ring  17  in a respective annular groove in the outer circumference of sensor-head-housing  3  on the side facing sensor-stick-housing  2 , which is supported by an interior diameter of face plate  34  which is reduced relative to thread  15 .  
         [0103]      FIGS. 2   e  and  2   g  also show a radial seal ring  7 , however, positioned in a groove radially open to the outside not of the large exterior diameter  12 , but of the enlarged diameter area  16  at a transition between sensors-stick-housing  2  and sensor-head-housing  3 .  
         [0104]     Instead of the radial seal ring  7 , a seal  7 *, as shown in  FIG. 2   c , can be located at a shoulder of the sensor head housing  3  in a groove open in an axial direction toward sensor-stick-housing  2 , supported against a respective frontal shoulder of front plate  34 , e.g. when tightening thread  15 .  
         [0105]     Furthermore,  FIG. 5   a  shows another design of the head area in a longitudinal sectional view, differing from the analogous depiction of  FIG. 2   b  with regard to the design of the cover.  
         [0106]     The lid cover does not have a dome, but it is a flat cover with a pass-through opening in the middle through which in a customary manner, using a protective cable grommet made from rubber or plastic. Cable  6  extends from the inside to the outside.  
         [0107]     Furthermore, in  FIG. 5   a  at a location in the interior of the head housing, a LED  38  is shown in the processing electronics, located under a respective LED opening  39  in the cover  8 , so that the lighting or non-lighting of this LED  38  can be seen from the outside through the opening  39 .  
         [0108]     Alternatively, to this non-tight variant with opening  39 , an infrared unit  40  can be located in the interior of the head housing  3 , including an infrared sensor and/or a infrared source. The cover  8  is made from a material which is permeable for infrared light. Thereby, the transmission of optical signals through infrared light through the otherwise tight closing cover is possible for error diagnosis and programming of processing electronics  20  in the interior of the head housing  3 .  
         [0109]     This design, without dome  8   a  in lid  8 , is suitable for very small processing electronics  20  or for housing of the processing electronics outside head housing  3 . Also, with this design a leak free repair and a replacement of the sensor unit is possible through the removable lid  8 .  
         [0110]      FIG. 5   b  shows the installation situation of this variant into a piston cylinder unit according to  FIG. 4 .