Abstract:
The invention relates to a method and a device for determining of at least one end position of a displacement element of an adjustable hydraulic machine, in particular of the axial piston type, shiftable or displaceable between two ends position a by means of a displacement unit between a first end position and a second end position. According to the invention the determination of having reached an end position of displacement takes place by means of a measurement circuit, whose electrical characteristics are changed jumpily when an end position is reached. This is done by connecting a partial branch of the measuring circuit with earth, when a moveable element comes into electric contact with a limit stop assigned to the corresponding end position.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    Applicant hereby claims foreign priority benefits under U.S.C. §119 from German Patent Application No. 10 2015 205 548.5 filed on Mar. 26, 2015, the contents of which are incorporated by reference herein. 
       TECHNICAL FIELD 
       [0002]    The invention relates to a method for detection of at least one end position of displacement element of a hydraulic machine, wherein the displacement element is movable between two end positions. It relates also to a hydraulic machine of the axial piston type having a displacement element displaceable by means of a displacement unit between a first end position and a second end position. 
       BACKGROUND 
       [0003]    The invention is based in particular on an adjustable hydraulic machine of the axial piston type as known exemplarily from DE 10 2005 060 960 B3. This hydraulic machine comprises a cylinder block mounted on a valve segment displaceable in a housing. The valve segment is connected via a rod to a control piston whose movement is electrically controllable by a proportional solenoid. The displacement volume of the hydraulic machine is adjustable by the movement of the valve segment together with the cylinder block. 
         [0004]    DE 101 19 236 C1 discloses a largely similar hydraulic machine, whose movement of the displacement element designed as a valve segment together with the cylinder block is limited by adjustment screws at the housing. The adjustment screws form limit stops for the valve segment which define the end positions of displacement. 
         [0005]    The hydraulic machines according to the state of the art are usually operated by an electronic control unit which is used to set the displacement of the control piston and therefore the displacement volume of the hydraulic machine. Here it should be considered that the range of displacement of the displacement element together with the cylinder block is limited between two end positions. In fact, the actual position of the control piston is detectable by sensors as exemplarily disclosed in DE  101   19   236  Cl, however, there is no reliable and exact indication of an end position of displacement being reached, which can be used by the control unit. Moreover, the use of such sensors incurs increased costs, particularly as these sensors must be calibrated in each hydraulic machine. 
       SUMMARY 
       [0006]    The underlying problem of the invention is to provide a method and a device of the kind mentioned above, whereby, using simple means, it should be possible to accurately and reliably detect when a component of a hydraulic machine, movable between two end positions, has reached one end position. 
         [0007]    The problem with regard to the method is solved when the displacement element reaches a particular end position an electrical contact is established between an electrical component assigned to the corresponding end position and an electrical measuring circuit, and in that the changes brought about in the electrical parameters of the measuring circuit are used for the purpose for detecting that the corresponding end position has been reached. 
         [0008]    The above mentioned problem is solved with regard to a variable hydraulic machine in that the hydraulic machine comprises electrical components of which one is assigned to a first end position, and another one is assigned to the second end position, thereby forming part of an electronic measuring circuit in the corresponding end position of the displacement element. 
         [0009]    A measuring circuit according to the invention is formed by an electric or electronic circuit having a power source, which is connected or connectable via lines or in an electrically conductive manner to electric components, and having evaluation or indication units for detecting, evaluating the currents or voltages in one or more paths of the circuit and outputting and/or indicating them as an output signal. Here, the display unit can be a separate device which is signal-connected to the evaluation unit. Here, the currents in the measuring circuit can be direct or alternating currents. As alternating currents pulse width modulated (PWR) currents are possible, which are used in some hydraulic machines for controlling the solenoid. 
         [0010]    In a preferred embodiment of the method, the electrical components can comprise a resistance and/or a capacity and/or an inductivity. The measuring circuit can be a resonant circuit, for example, whose change of frequency or amplitude is detected when the component is connected to it. 
         [0011]    Particularly preferable the resistance of the measuring circuit or the current flowing in the measuring circuit should be the detected or monitored electric parameter and the electric component should be an ohmic resistor. 
         [0012]    In implementation of the invention the inventive electric components may be arranged at or in those components of the hydraulic machine which come into electrically conductive contact with the valve segment of the cylinder block or any other components involved in the displacement. Examples of this are minimum or maximum screws known as adjustment screws serving as stoppers for the displacement element, e.g. the cylinder block or its yoke or the valve segment of an axial piston machine of the bent axis type or the swash plate of an axial piston machine of the swash plate type. An arrangement at the displacement unit for the cylinder block is possible as well, for instance in a control piston movable between the two end positions, in which the control piston in its correspondent end position establishes a contact between the components and an electrically conductive part of the hydraulic machine. Needless to say that the inventive arrangement can be used as well merely for the determination of a single end position and is transferrable in an analogues manner for instance to radial piston machines. 
         [0013]    In implementation of the invention it is preferable for the hydraulic machine to comprise a displacement unit for the valve segment of the cylinder block or the swash plate which can be actuated via a solenoid and that the solenoid is part of the measuring circuit. Here it is advantageous if the hydraulic machine is operated by an electronic control unit, via which the current through the solenoid can be set and monitored. 
         [0014]    In a further preferred embodiment of the invention, the hydraulic machine comprises one adjustment screw for determining each end position, the first and the second end position of the displacement element, whereby a first contact of an electric component is arranged isolated from a contact surface of the adjustment screw, the contact surface being contactable by electrically conductive parts of the hydraulic machine when the corresponding end position is reached. When reaching the end position of the displacement element, the first contact of the electric part is connected to earth such that an electric current can flow through the component. This additionally flowing current is provided by the current or power source of the measuring circuit, which is operatively connected with a second contact of the electric component. This current or power source can advantageously be way the current or power source of the solenoid that is present anyway. The occurrence of the additional current, which only starts to flow when the end position is reached, is detected by the evaluation unit of the measuring circuit and is provided as a signal. The evaluation unit of the measuring circuit can be integrated into the electronic control unit of the hydraulic machine. 
         [0015]    In implementation of the invention it is particularly preferable for the second contact of the electric component to be connected in an electrically conductive manner to the solenoid. Hence, this contact comprises the same current or power supply as the solenoid and uses the control and monitoring of the current through the solenoid provided by the hydraulic machine anyway, in that the solenoid is used as a component of the measuring circuit. For completion of the measuring circuit, only a further electric component is necessary then, preferably an ohmic resistor. Here, it has to be considered that the partial current flowing over the electrical component may only be a small fractional part of the current passing through the solenoid. Otherwise the solenoid current would be weakened too much and the force acting on the displacement element of the hydraulic machine would be influenced by it. Therefore, the resistance or the impedance of the electrical component, which can be a resistance, a capacitor or an induction coil, has to be high with respect to the corresponding value of the solenoid. 
         [0016]    Preferably the electrical component is a resistor which is connected in parallel to the solenoid in the measuring circuit if the displacement element is in its corresponding end position. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    In the following, the invention is explained with the help of embodiments depicted in the Figures. In the depicted embodiments the invention is shown only in the example of a hydraulic machine of the bent axis-type. However, the inventive idea also encompasses hydraulic machines of the swash plate-type, whose displacement element is a swash plate. Needless to say that the inventive idea is also applicable in variable radial piston machines, so this type of hydraulic machine is also encompassed by the inventive idea. The Figures show the following: 
           [0018]      FIG. 1  shows a part of a section view of a hydraulic machine according to the state of the art; 
           [0019]      FIG. 2  shows a detail view of one embodiment of the invention; 
           [0020]      FIG. 3  shows a detail view of a further embodiment of the invention; and 
           [0021]      FIG. 4  shows a modification of the embodiment according to  FIG. 2 . 
       
    
    
     DETAILED DESCRIPTION 
       [0022]    In  FIG. 1  shows a detailed view of a variable hydraulic machine  1  of the axial piston type according to the invention in a section view. In a machine housing  2 , of which only a small part is shown, a cylinder block  3  is arranged on a yoke  4  for example being a displacement element  4  for hydraulic machines of the bent axis type. The non-shown pistons in the cylinder block  3  are operated in the known manner by a rotating drive mechanism or are driving a driving shaft, according to whether a pump or motor operation of the hydraulic machine is intended. These details are known to a person skilled in the art, such that further explanations hereto are renounced. 
         [0023]    Yoke  4  is mounted in housing  2  jointly displaceable with cylinder block  3 , wherein the corresponding end positions of displacement are given by adjusting screws  5  or by other fixed or adjustable parts of the hydraulic machine. In  FIG. 1  only one adjusting screw  5  is shown exemplarily. Here, the screw is assigned to a minimum angle of displacement. When reaching this minimum angle, a first end portion  15  of the yoke  4  contacts a front face of the adjustment screw  5  thus forming an limit stop  6 . It is self-evident that such an adjustment screw can also be assigned to the maximum angle of displacement of yoke  4 , against which the second end portion  16  of yoke  4  abuts. In this embodiment yoke  4  plays the role of a displacement element  4 . 
         [0024]    The displacement of yoke  4  together with cylinder block  3  is effected by means of a displacement unit  7 , whose casing  8  is firmly attached to machine housing  2  of hydraulic machine  1 . In casing  8  of displacement unit  7  a displacement spool  9  is arranged movable longitudinally. The displacement of displacement spool  9  is controlled by solenoid  10  acting on a control spool. The control spool (not shown) controls in common way fluid flow rates which act on displacement spool  9  and determine its position. Here, a spring  11  is used as a mechanic return. Displacement spool  9  acts via a rod serving as a carrier for yoke  4  of hydraulic machine  1 . Thus, an axial displacement of displacement spool  9  leads to a correspondent displacement of yoke  4  and therewith of cylinder block  3 . In this manner the displacement or the conveying volume of the cylinders in the cylinder block  3  is adjusted. The control of solenoid spool  10 , which is commonly realized as proportional magnet, i.e. its supply with current is done by a not shown electronic control unit with which the hydraulic machine  1  is operated. 
         [0025]    According to  FIG. 1 , as an example, Casing  8  of displacement unit  7  is provided with an end cap  13  serving as an limit stop  14 , for example for the maximum displacement of the displacement spool  9 . Hence, this limit stop  14  defines the maximum displacement position of yoke  4 . Therefore, limit stop  14  can take over the function of an limit stop  6  formed by displacement screw  5 . Needless to say that for a person skilled in the art, an analog minimum or maximum displacement position of yoke  4  at limit stop  14  is encompassed by the inventive idea as well. 
         [0026]      FIG. 2  shows an inventive hydraulic machine in a partial section view according to  FIG. 1 . In the following Figures all reference signs are maintained for the denomination of equal structural features. The general structure and way of operation of the hydraulic machine  1  corresponds to the details given by means of  FIG. 1 . 
         [0027]    In  FIG. 2  the inventive measuring circuit  20  is shown comprising a current or power source  22 . The power source  22  is part of a non-shown electronic control unit of hydraulic machine  1  and, for example, serves as power supply of solenoid  10 , with which it is operatively connected via lines  23   a ,  23   b . Therefore, solenoid  10  forms part of measuring circuit  20 . One of line  23   a  or  23   b  is connected to earth, which, for example, is formed by the general earth of hydraulic machine  1 . Hereby, one can assume that hydraulic machine  1  consists of metal, hence, being electrical conductive. Earth  26  may be formed by the earth of a not shown electronic control unit for hydraulic machine  1  as well, for example, formed by an inlet pin of a microcontroller for fault monitoring. 
         [0028]    The other part of measuring circuit  20  is built according to the invention by a further line  23   c  and, for example, by resistors  27   a  and  27   b  being connectable to ground  26  parallel to solenoid  10 . Resistor  27   a  is assigned to adjustment screw  5 ; Resistor  27   b  to yoke  4 . Resistor  27   a  and the corresponding line  23  are mounted electrically isolated within adjustment screw  5 , wherein part  29  of line  23   c  ends at front face  17  of adjustment screw  5  serving as an limit stop  6 . This part  29  of line  23   c  forms a contact area  30  on front face  17  being electrically isolated from the rest of front face  17 . The first end portion  15  of yoke  4  assigned to adjustment screw  5  also shows a contact surface capable to establish electrically conductive contact with contact area  30  of adjustment screw  5 . This case occurs if yoke  4  is at the opposite limit stop, for instance in the position of the lowest or biggest displacement, i.e. if end portion  15  of yoke  4  contacts limit stop  6  arranged on front face  17  of adjustment screw  5 . In this position a partial current of current or power source  22  is connected via lines  23   c ,  29  and the resistors  27   a  and  27   c  to earth  26 . Resistor  27   c  however can be formed by the reduced conductivity of the material of hydraulic machine  1  such that a separate discrete resistor  27   c  can be omitted. 
         [0029]    If the electrical contact between the contact areas of yoke  4  and the adjustment screw  5  is closed, as descript already above, a partial current of measuring circuit  20  flows through solenoid  10  and a further partial current flows through the branch of measuring circuit  20  which is built by the lines  23   c  and  29  and the resistors  27   a  and  27   b  to earth  26 . This can be identified by a jumpy change in the resistance of measuring circuit  20  in the moment the contact is closed. This change of resistance can be seen for example by a change of current or voltage in the measuring circuit and can be detected by the electronic control unit of the hydraulic machine. Thus, its occurrence is a secure indication that yoke  4  has reached one of its end positions of displacement. 
         [0030]    By the selection of the resistors  27   a  and  27   b  it should be considered that the partial current flowing over the same is significantly lower than the current flowing over solenoid  10 . Otherwise, with the closed contact between contact area  30  of adjustment screw  5  and the yoke  4 , a non-admissible high reduction of current flowing through the solenoid  10  would occur and would lead to a jumpy and undesired change of the displacement angle of yoke  4 . 
         [0031]    In implementation of the invention it is further possible to substitute resistors  27   a  and  27   b  by capacitors and/or inductivities. Thereby, the change of parameters of the measuring circuit  20  occurring by contact of the adjustment screw  5  with yoke  4  can, for example, detected by a change in frequency or amplitude in the resonant circuit. 
         [0032]    In  FIG. 3  a further embodiment of the invention is shown, wherein similar parts are denominated with the same reference signs as in  FIGS. 1 and 2 . As described above, one branch of the measuring circuit  20  is built by magnet coil  10  and lines  23   a  and  23   b  connected to the current or voltage source  22 . The other branch of measuring circuit  20  leads over line  23   c  and over resistors  28   a  and  28   b  to earth  26 . Here, the resistor  28   a  is arranged in end cap  13  of displacement element  7 , wherein its outlet contact is conducted electrically isolated to a contact area  18  of limit stop  14  arranged in end cap  13 . As shown in  FIG. 3 , contact area  19  faces contact area  18  in the end position of displacement spool  9 , wherein both contact areas  18 ,  19  are connected electrically conductive. From contact area  19  in displacement spool  9  an isolated line  31  leads over resistance  28   b  to earth  26 . In this embodiment displacement spool  9  functions as displacement element  4 . As displacement spool  9  is coupled via rod  12  with yoke  4 , the end position of displacement spool  9  corresponds in a functional way to the end position of yoke  4 , and, hence, to the one of cylinder block  3 . 
         [0033]    In  FIG. 3  limit stop  14  is depicted as a fixed stopper arranged in end cap  13  or formed by the same. Naturally, this stopper  14  can be adjustable by implementation of the invention. This eventually permits a vernier adjustment of the designated end position of displacement unit  4  and, thus, of cylinder block  3 . As can be seen easily by a person skilled in the art, at this limit stop  14  the maximum as well as the minimum angle of displacement of the axial piston machine, i.e. the maximum or the minimum displacement or conveying volume is reached. 
         [0034]    The embodiment according to  FIG. 4  corresponds in structure and function to the one of  FIG. 2 . However, it differs therefrom by the arrangement of a light emitting diode (LED)  32  or another lighting device arranged in line  23   c  connecting the power source  22  of measuring circuit  20  with electrical component  27   a . Light emitting diode  32  lights if a contact between electrical component  27   a  and ground  26  is established and indicates that displacement element  4  has reached the end position. Hence, light emitting diode  32  serves as an optical indicator. It is obvious that line  23 , only shown schematically in  FIGS. 1 to 4 , can be conducted through the working machine such that light emitting diode  32  may be arranged for example in an indication area of the electronic control unit of the hydraulic machine. Commonly, the electronic control unit is a separate assembling group of the hydraulic machine being connected with the same via electric lines. For the present explanation of the inventive idea and for simplification reasons only, it is assumed that the electronic control unit is arranged on the hydraulic machine. 
         [0035]    The way of operation of this embodiment of the invention is as follows: In a position of displacement spool  9  between the end positions of its displacement no electric conductive contact between the contact areas  18  and  19  in the end cap  13  and in the displacement spool  9  is established. Thus, no current flows through line  23   c  and through resistors  28   a  and  28   b . However, if the displacement spool reaches the end position, which is shown in  FIG. 3 , the contact areas  18  and  19  at limit stop  14  and at displacement spool  9  contact each other such that a current via resistors  28   a  and  28   b  flows to earth  26 . The caused jumpy change of the situation in the measuring circuit  20  serves for detecting that the displacement spool  9  has reached the end position and, thus, also yoke  4  of the hydraulic machine  1  coupled with displacement spool  9 . 
         [0036]    In modification of the invention a modification of the conditions in measuring circuit  20  can consist also therein that at the beginning of a take-off of displacement element  4  from its end position, current through the electrical component or through the electrical components stops flowing, as the contact to earth is interrupted. The jumpy reduction of current or of the total resistance in the measuring circuit  20  then constitutes an indication in such regard that the designated end position is left. Such a signal can be used also by the electronic control unit of hydraulic machine  1 . This case, for example, occurs if the current in the solenoid is raised continuously in a ramp-like manner beginning at zero current. As the displacement spool commonly starts with a movement only, if the solenoid current reaches a predetermined minimum value, this value has to be reached in order that the displacement spool  9  or the displacement element  4  takes off from the limit stop (contact) and, thereby, interrupting the partial current in that branch of measuring circuit  20 . Thus, the inventive method is suitable to indicate the reaching as well as the departure from an end position of displacement elements  4 ,  9 . 
         [0037]    As displacement spool  9  preferably consists of metal and is connected electrically conductive via casing  8  or other parts with the overall earth of hydraulic machine  1 , in realization of the invention, it is possible to waive resistance  28   b  and to bring contact area  18  at stopper  14  directly into contact with the facing front face of displacement spool  9 . In this case the modification of displacement unit  7  is limited to the end cap  13  only, which has to be provided with a resistor  28   a , a contact area  18  at stopper  14  and with a line  23   c . The corresponding electrically conductive contact areas  18  and  19  are preferably designed such that they are elevated with regard to the surrounding surface. This guarantees a reliable electric contact if the two contact areas facing each other came into contact. 
         [0038]    In implementation of the invention, at least for a person skilled in the relevant art, it is possible without more to combine the two embodiments mentioned above in one hydraulic machine  1 . Here, for example, one of both end positions of displacement of displacement element  4 , e.g. the yoke  4 , in which end portion  15  of yoke  4  abuts against front face  17  of adjustment screw  5  (c.f.  FIG. 2 ), is detected by means of a measuring circuit  20 , for example the one according to  FIG. 2 . The other end position of displacement shown in  FIG. 3 , corresponding to the other extreme displacement of yoke  4 , can be determined, for example, by means of a measuring circuit  20  according to  FIG. 3 . 
         [0039]    Thus, the invention provides with a simple and reliable method for detecting that a displacement element of an adjustable hydraulic machine, e.g. of an axial piston type, has reached an end position of displacement, as well provides with a design of a hydraulic machine suitable for carrying out the method. 
         [0040]    While the present disclosure has been illustrated and described with respect to a particular embodiment thereof, it should be appreciated by those of ordinary skill in the art that various modifications to this disclosure may be made without departing from the spirit and scope of the present disclosure.