Patent Publication Number: US-2021190055-A1

Title: Axial Piston Pump

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to EP Application Serial No. 19218312.7, filed Dec. 19, 2019, titled “AXIAL PISTON PUMP,” the content of which is incorporated by reference herein in its entirety. 
     FIELD OF THE INVENTION 
     The present invention relates to an axial piston pump, on the swash plate of which a magnet encoder is arranged. 
     BACKGROUND OF THE INVENTION 
     Axial piston pumps are devices used in hydraulics to convert mechanical energy into hydraulic energy. They are used in industrial applications, such as in heavy machinery and plastic machinery, as well as in mobile work machines. A distinction is thus made between inclined-axis pumps and swash plate pumps. Axial piston pumps can have a constant or variable delivery volume and a constant or variable delivery direction. 
     A further field of application of axial piston pumps is the use in hydrostatic transmissions in vehicles. Here, power (rotational speed and torque) is transmitted via an oil pressure of a maximum of 500 bar. The oil flow is continuously controllable, resulting in a continuously adjustable transmission with very high power density. Typical fields of application are excavators, wheel loaders, tractors, snow groomers, combine harvesters and many other slow-moving vehicles. To monitor the operation of an axial piston pump, a magnet can be attached to a movable element of the axial piston pump. The movement of the magnet can then be monitored by a magnetic field sensor which is attached to a housing of the axial piston pump. 
     EP 0 343 581 A1 describes the attachment of a number of magnets which are arranged in a line on a piston of the axial piston pump in a direction identical to the reciprocal movement of the piston. The magnetic field sensor then generates pulse signals, whose number correlates to a number of magnets opposite the sensor. The number of pulses generated by the sensor can be counted using a counting device. However, this makes it necessary to firmly install the magnets in the piston. 
     Another possibility for monitoring an axial piston pump by means of a magnetic field sensor is to attach a magnetic encoder to its swash plate. In this way, the angular position of the swash plate can be determined. However, when the ferromagnetic pistons of the axial piston pump move, they function as a source of interference to the magnetic field of the magnetic encoder. Maladjustments of such a magnetic encoder lead to considerable non-linearities. 
     DE 40 15 006 A1 describes a swash plate compressor having a magnetic field sensor which detects movements of a permanent magnet connected to the swash plate. A disturbing influence of a magnetic leakage flow of a coupling coil mounted on the compressor on the measurement result of a delivery rate detector is suppressed by suitable feeding of the coupling coil. 
     DE 20 2009 008 372 U1 describes a magnetic encoder having two frontally polarised permanent magnets which are arranged on a base plate made of a magnetically conductive steel sheet. 
     From DE 10 2008 052 804 A1, a magnetic rotary encoder is known, which consists of a plastic-bonded anisotropic hard ferrite. It has a preferred magnetic direction which runs in an arched direction from one half to the other half of the front side of the rotary encoder which points to a magnetic sensor fixed on the front of the rotary encoder. 
     US 2002/0118011 A1 describes a position sensor for determining a linear or radial position. This sensor comprises two metal plates with two magnets each, wherein a Hall-effect sensor is arranged between the metal plates. 
     DE 34 23 722 A1 describes an inductive proximity switch having a sensor field in front of its active surface for a permeable trigger and having an oscillator whose oscillating circuit has a pot-shaped ferrite shell core. The ferrite shell core is short-circuited by a yoke having a saturation-sensitive position, which is arranged between two bar magnets magnetically connected in series. 
     An object of the invention is to provide a way of monitoring an axial piston pump by means of a component which can be subsequently attached to it without the monitoring being disturbed by the movement of the ferromagnetic pistons of the axial piston pump. In addition, the component should be largely insensitive to maladjustments. 
     SUMMARY 
     This object is solved by an axial piston pump, on whose swash plate a magnetic encoder is arranged. This has at least two permanent magnets and a plate consisting of a ferromagnetic material. In particular, the ferromagnetic material can be steel. The permanent magnets are arranged on the plate in such a way that each faces a magnetic pole towards the plate and this pole is at least partially covered by the plate. Preferably, at least 80% of the surface of each pole is covered by the plate. More preferably, each pole is completely covered by the plate. In this case, the dimensions of the plate correspond at least to the surface area spanned by the permanent magnets. However, the plate can also be larger. 
     It has been shown that, when using at least two magnets, preferably exactly two magnets, their arrangement on the plate prevents a magnetic field sensor from being significantly affected by the movement of the pistons of an axial piston pump. At the same time, such a magnetic encoder can be designed to be compact such that only a small amount of installation space is used in the axial piston pump and its weight does not cause any imbalance on the swash plate. 
     When exactly two permanent magnets are used, the two permanent magnets are preferably arranged in such a way that one permanent magnet faces its south pole towards the plate and the other permanent magnet faces its north pole towards the plate. This enables a simple differentiation of both permanent magnets by the sensor, whereby non-linearities occurring due to a possible maladjustment of the magnetic encoder can be easily compensated. 
     In order to minimise the influence of the pistons of the axial piston pump on the magnetic field sensor, it is preferable that a distance between the permanent magnets and the plate is a maximum of 500 μm, more preferably a maximum of 100 μm. This distance can be, for example, a constructional air gap between the permanent magnets and the plate, or it can be filled by a non-ferromagnetic filling material, which may also serve to bond the plate to the permanent magnets. Particularly preferably, both permanent magnets are preferably placed on the plate without gaps. 
     The thickness of the plate is preferably in the range of from 0.5 mm to 1.2 mm, more preferably in the range of from 0.6 mm to 1.0 mm and most preferably in the range of from 0.7 mm to 0.9 mm. If the plate is thinner, the magnetic field sensor is more strongly influenced by the movement of the pistons of the axial piston pump. A thicker plate no longer causes a substantial improvement in shielding against the interference effects of the piston movement. Instead, it merely results in an enlargement of the design and increases the weight of the magnetic encoder. 
     To avoid short-circuiting the permanent magnets, it is preferable that all surfaces of the permanent magnets which do not contact the plate are surrounded by at least one non-ferromagnetic material. In this respect, the plate can also have curvatures, as long as this does not cause it to extend between the permanent magnets. The non-ferromagnetic material is in particular a plastic. 
     In a particularly simple embodiment of the axial piston pump, the permanent magnets are attached to the plate without the aid of an additional component. In order to surround them with a non-ferromagnetic material, which is not just air, they can then be over-moulded with a casting compound, for example. However, the magnetic encoder preferably has a housing made of a non-ferromagnetic material. This has two recesses in which the permanent magnets are arranged. The recesses are covered by the plate. This allows an easy production of the magnetic encoder by inserting the permanent magnets into the recesses of housing and then closing them by attaching the plate to the housing. In principle, the housing can also have further production-related recesses which do not serve to accommodate permanent magnets. 
     In order to attach the magnetic encoder to the swash plate, it is preferable that the housing has at least one fastening element. This fastening element is designed to be fastened to the swash plate. In order to prevent maladjustment of the magnetic encoder, it is preferred that the housing has several fastening elements. The at least one fastening element is arranged on the same side of the housing as the plate. Since the fastening element is turned towards the swash plate when the magnetic encoder is attached to the swash plate, this causes the plate to face towards the piston of the axial piston pump such that it can shield the permanent magnets from the pistons of the axial piston pump. 
     The axial piston pump has several pistons. The magnetic encoder is arranged on a swash plate of the axial piston pump. Furthermore, the axial piston pump has a magnetic field sensor, in particular a Hall-effect sensor. This is arranged in such a way that it faces towards the magnetic encoder. If the angle of the swash plate changes in relation to a pump shaft, this causes a movement of the magnetic encoder relative to the magnetic field sensor, which can be detected by means of the magnetic field sensor. From this movement, a conclusion on the angle of the swash plate and thus on the operating condition of the axial piston pump is possible. 
     The plate is preferably arranged between the magnetic field sensor and the pistons of the axial piston pump in such a way that the plate faces towards the piston. The permanent magnets then face towards the magnetic field sensor. This allows the plate to shield the permanent magnets and the magnetic field sensor from the pistons. 
     In one position of the swash plate, the plate of the magnetic field sensor is preferably arranged in parallel to the pistons. This can in particular be a rest position of the swash plate, in which it is at a known predetermined angle to the pump shaft. This arrangement of the magnetic field sensor allows a very simple and reliable determination of the angle of the swash plate. Furthermore, it is preferable that the longitudinal axes of both permanent magnets are arranged, in one position of the swash plate, parallel to the pistons. This position is in particular a rest position of the swash plate. This arrangement of the permanent magnets minimises the influence of the pistons on the position determination of the magnetic encoder by the magnetic field sensor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments of the invention are depicted in the drawings and explained in more detail in the following description. 
         FIGS. 1 a  and 1 b    each show an isometric view of a housing of a magnetic encoder according to an exemplary embodiment of the invention. 
         FIG. 2  shows an isometric view of a magnetic encoder according to an exemplary embodiment of the invention having a removed plate. 
         FIG. 3  shows an isometric view of a magnetic encoder according to an exemplary embodiment of the invention. 
         FIG. 4  shows a schematic sectional view of a magnetic encoder according to an exemplary embodiment of the invention. 
         FIG. 5  shows a schematic view of elements of an axial piston pump. 
         FIG. 6  shows, in a schematic sectional view, the arrangement of a magnetic encoder in an axial piston pump in an exemplary embodiment of the invention. 
         FIG. 7  shows a schematic sectional view of a magnetic encoder which is not in accordance with the invention. 
         FIGS. 8 a  and 8 b    show, in diagrams, changes in linearity in the event of maladjustment of a magnetic encoder on a swash plate. 
         FIG. 9  shows an isometric view of another magnetic encoder which is not in accordance with the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In  FIGS. 1 a  and 1 b   , a housing  1  of a magnetic encoder according to an exemplary embodiment of the invention is depicted. This housing  1  consists of zinc and can be produced by means of die casting. It is divided into a first section  11  and a second section  12 , which are integrally connected to each other. The housing  1  is shown in  FIG. 1 a    from its upper side and in  FIG. 1 b    from its underside. The first section  11  is thinner than the second section  12 , wherein the second section  12  protrudes towards the upper side of the housing  1  in relation to the first section  11 . The first section  11  has two fastening elements  111 ,  112  in the form of pins on the underside. These serve to precisely adjust the position of the magnetic encoder on a swash plate. The second section  12  is open towards the underside. It has two substantially cuboid recesses  121 ,  122 , whose longitudinal axes run in parallel and extend towards the first section  11 . These two recesses  121 ,  122  are designed to accommodate permanent magnets. A third recess  123 , which is not provided to accommodate a permanent magnet, is arranged between the first two recesses  121 ,  122 . It is used for manufacturing the housing  1  in a die casting process. Four pins  124  to  127  are arranged in the second section and extend away from the underside. They serve to connect the housing  1  to a plate which closes the first two recesses  121 ,  122 . 
     In  FIG. 2 , it is depicted how two permanent magnets  2 ,  3  can be arranged in the two recesses  121 ,  122  of the housing  1 . Furthermore, a plate  4 , which consists of steel, is depicted. This has four openings whose positions correspond to the positions of the pins  124  to  127 . The housing  1 , the two permanent magnets  2 ,  3  and the plate  4  together form a magnetic encoder  5  according to an exemplary embodiment of the invention. 
     In  FIG. 3 , the magnetic encoder  5  is depicted in its assembled state. The plate  4  now covers all recesses  121  to  123  in the second section  12  of the housing  1 . It is pressed with the pins  124  to  127  in such a way that it rests without gaps on the two permanent magnets  2 ,  3 . Here, it completely covers the two permanent magnets  2 ,  3  and also extends beyond the rectangular surface area spanned by the permanent magnets  2 ,  3 . 
     A sectional view of the magnetic encoder  5  transverse to the longitudinal axis of the permanent magnets  2 ,  3  is depicted in  FIG. 4 . The permanent magnets  2 ,  3  are each polarised in such a way that one of their poles faces towards the plate  4  and contacts it, whereas the other pole faces away from the plate  4  and contacts the housing  1 . Here, the south pole  21  of the first permanent magnet  2  faces towards the plate  4  and its north pole  22  faces away from it. The north pole  31  of the second permanent magnet  3  faces towards the plate  4  and its south pole  32  faces away from it. Except of the underside of the magnetic encoder  5 , on which the permanent magnets  2 ,  3  contact the plate  4 , the permanent magnets  2 ,  3  are surrounded by zinc of the housing  1 . 
       FIG. 5  shows the construction of a conventional axial piston pump  6 , which is designed as a swash plate pump. A rotatable drum  62  is arranged on a control disk  61 , which can be set in rotation via a pump shaft  63 . This has six pistons  64 , which are each mounted on an inclined swash plate  65 . 
       FIG. 6  shows how the magnetic encoder  5  can be arranged on the swash plate  65  according to the above-described exemplary embodiment. It is fastened to the swash plate  65  with its fastening elements  111 ,  112  in such a way that the underside of its second section  12  and thus the plate  4  faces towards the pistons  64 . The pump housing  66 , which encloses the elements of the axial piston pump  6 , has an opening in the area of the magnetic encoder  5 , through which a magnetic field sensor  7  in the form of a Hall-effect sensor is guided. This allows movements of the permanent magnets  2 ,  3  in the magnetic encoder  5  to be detected and the angle of the swash plate  65  in relation to the pump shaft  63  to thus be deduced. The pistons  64 , which consist of ferromagnetic steel, only minimally disturb the magnetic field of the two permanent magnets  2 ,  3  during their movement, since they are shielded from the pistons  64  by the plate  4 . In addition, the longitudinal axes of the permanent magnets  2 ,  3  in the magnetic encoder  5  run in parallel to the longitudinal axes of the pistons  64 . 
     As a comparative example,  FIG. 7  schematically depicts the construction of a magnetic encoder  8  which is not in accordance with the invention. This has a single permanent magnet  81  having two poles  811 ,  812 . Here, the north pole  811  is located on one end of the longitudinal axis of the permanent magnet  81  and its south pole  822  is located on the other end of its longitudinal axis. While one side surface of the permanent magnet  81  is exposed, its other side surfaces are surrounded by a non-ferromagnetic plastic  82 . This in turn is surrounded by a metal housing  83 . Orthogonally to the longitudinal axis of the permanent magnet  81 , this is distanced by 4 mm from the metal housing  83  by means of the plastic material  82 . 
     Changes in the linearity L of the magnetic encoder  5  according to the invention and the magnetic encoder  8  not according to the invention when arranged in an axial piston pump  6  in the manner depicted in  FIG. 6  are depicted in  FIGS. 8 a  and 8 b    for different maladjustments of the magnetic encoders  5 ,  8 . Here,  FIG. 8 a    shows the influence of maladjustments d x  along the plane of the magnetic encoder  5 ,  8 , and  FIG. 8 b    shows the influence of maladjustments d z  of the magnetic encoder  5 ,  8  along an axis between the piston  64  and the magnetic field sensor  7 . Since the magnetic encoder  5  according to the invention has two permanent magnets  2 ,  3 , the changes in the linearity L 2 , L 3  are set out for the two permanent magnets  2 ,  3 , while for the magnetic encoder  8  which is not in accordance with the invention, only the change in linearity L 81  of its individual permanent magnet  81  is depicted. It can be seen that a radial maladjustment d x  of the magnetic encoder  5  according to the invention leads to considerably fewer non-linearities than with the magnetic encoder  8  not in accordance with the invention. In addition, the magnetic encoder  5  according to the invention is substantially insensitive to an offset between the piston  64  and the magnetic field sensor  7 , while the magnetic encoder  8  not in accordance with the invention also reacts to this with considerable non-linearities. 
     As a further comparative example,  FIG. 9  depicts the design of a magnetic encoder  9  not in accordance with the invention according to DE 20 2009 008 372 U1. This has two parallel permanent magnets  91 ,  92  each having two poles  911 ,  912 ,  921 ,  922 . Here, the north pole  911  of the first permanent magnet  91  is located on a first end of the longitudinal axis of the first permanent magnet  91 , and its south pole  912  is located on the second end of its longitudinal axis. The south pole  921  of the second permanent magnet  92  is located on the first end of the longitudinal axis of the second permanent magnet  921  and its north pole  922  is located on the second end of its longitudinal axis. Thus both permanent magnets  91 ,  92  are oppositely polarised. The permanent magnets  91 ,  92  are arranged on a plate  93  made of sheet steel. 
     In order to compare the magnetic encoder  5  according to the invention with the magnetic encoder  9  according to DE 20 2009 008 372 U1, simulations of two magnetic encoders were carried out, which each have two cuboid permanent magnets having a length of 16.25 mm in the x-direction, a width of 6.6 mm in the y-direction and a height of 4.5 mm in the z-direction. These are arranged at a distance of 6.5 mm from each other in the y-direction and each have a nominal remanence of 1.1 T. A polarisation of the permanent magnets was assumed in the z-direction for the magnetic encoder according to the invention, and in the x-direction for the magnetic encoder not according to the invention, wherein the two permanent magnets were polarised in opposite directions. At a distance of 6 mm in the z-direction, it resulted in a magnetic flux density of 50 mT for the magnetic encoder according to the invention, and a magnetic flux density of 1.1 μT for the magnetic encoder not according to the invention. A distance of 6 mm is structurally necessary in the arrangement according to  FIG. 6  between the magnetic encoder  5  and the magnetic field sensor  7 . For the function of the magnetic field sensor  7 , a magnetic flux density in the range of from 30 mT to 90 mT at the magnetic field sensor  7  is recommended. This shows that the magnetic encoder  9  not in accordance with the invention, unlike the magnetic encoder  5  according to the invention, is not suitable for monitoring an axial piston pump  6  by attaching it to a swash plate  65 .