Sensor unit, locking device, dual detection system and hollow body

Abstract

Sensor unit (100) for dual detection of the positions of two measuring objects (110, 120) that can be moved relative to each other, wherein the sensor unit (100) has the following: a permanent magnet (111) associated with the first measurement object (110) and moving together with the first measurement object (110), a magnetic field changing device (130) associated with the second measurement object (120) and moving together with the second measurement object (120), a multi-directional Hall sensor (140) for detecting a magnetic field of the permanent magnet (111), wherein one of the permanent magnet (111) and multi-directional Hall sensor (140) is fixedly arranged or can be arranged in a fixed position and the other of the permanent magnet (111) and multi-directional Hall sensor (140) is adjustable along a first movement path (B1), wherein the magnetic field changing means (130) is adjustable in an intermediate space (Z) between permanent magnet (111) and multi-directional Hall sensor (140) along a second movement path (B2), wherein the magnetic field changing means (130) is designed as a ferromagnetic shield (130a) whose shielding effect has a course that changes clearly, in particular continuously, in the direction of the second path of motion (B2).

Description

Technical field

[0001] The invention relates to a sensor unit for the dual detection of the position of two objects that can move relative to each other. The invention further relates to a locking device, in particular for the parking lock of an automatic transmission. The invention also relates to a system for the dual detection of the position of two objects that can move relative to each other and a hollow body. State of the art

[0002] Sensor units are used, for example, to detect and monitor the condition or position of moving components in mechanical systems. In many technical applications, it is necessary to precisely detect the condition of a component. Sensor units for the condition detection of locking units in mechanical components, particularly for automatic transmissions, are also known. Although the invention is primarily described here in the context of such locking units, its application is in no way limited to this and can be transferred to various technical systems and mechanisms.

[0003] The known solutions often require separate sensors for each component to be monitored, which increases the design effort and complexity of the system.

[0004] Therefore, attempts have been made in the prior art to simultaneously detect the condition of several components to be monitored using a single sensor unit. In this context, for example, a corresponding sensor unit and a locking unit are known from DE 10 2022 134 624 A1 of the applicant. Description of the invention

[0005] The invention is based on the objective of improving the teaching known from the prior art and, in particular, of providing a sensor unit that offers advantages over the aforementioned prior art. Furthermore, it is an object of the present invention to provide an advantageous locking device, an advantageous system for the dual detection of the position of two objects movable relative to each other, and a hollow body with which the basic idea of ​​the invention can be implemented, for example.

[0006] These problems are solved by the subject matter of the independent claims. Further embodiments of the invention are specified in particular in the dependent claims.

[0007] The solution according to the invention consists in particular of providing a sensor unit for the dual detection of the position of two objects that can be moved relative to each other. The sensor unit comprises a permanent magnet associated with the first object and moving together with it, and a magnetic field-changing device associated with the second object and moving together with it. Furthermore, the sensor unit comprises a multi-directional Hall sensor for detecting the magnetic field of the permanent magnet. One of the permanent magnet and one of the multi-directional Hall sensor are fixed in position, or can be fixed in position, while the other of the permanent magnet and one of the multi-directional Hall sensor are adjustable along a first path of movement.Furthermore, the magnetic field-changing element is adjustable in a space between the permanent magnet and the multi-directional Hall sensor along a second path of movement. The magnetic field-changing element is designed as a ferromagnetic shield, the shielding effect of which exhibits a clearly defined, and in particular continuously, changing profile in the direction of the second path of movement.

[0008] The sensor unit is, in general, an assembly designed to detect the relative positions of at least two moving objects.

[0009] The first object being measured is any object whose position is to be recorded by the sensor unit. A permanent magnet is attached to the first object for this purpose.

[0010] The second object being measured is another object whose position is to be determined by the sensor unit. For this purpose, a magnetic field modifier is attached to the second object. This magnetic field modifier is, generally speaking, any component of any design that modifies the detectable magnetic field of the permanent magnet through its shielding effect.

[0011] In this case, the magnetic field modulator is designed as a ferromagnetic shield, whose function is to shield the magnetic field in such a way that a modified signal detection results. For this purpose, the magnetic field modulator is positioned between the permanent magnet and the multi-directional Hall sensor.

[0012] The multi-directional Hall sensor is a Hall sensor capable of detecting magnetic fields in multiple spatial directions. For this purpose, the multi-directional Hall sensor comprises at least two Hall elements arranged at right angles to each other. The individual Hall elements thus measure different spatial directions, particularly those perpendicular to one another. Preferably, the multi-directional Hall sensor is fixed in position and, most preferably, housed within a casing containing two objects that are movable relative to each other. Accordingly, the permanent magnet, together with the first object being measured, is adjustable along the first path of movement. The first and second paths of movement are preferably parallel to each other.

[0013] The advantage of the sensor unit according to the invention lies particularly in the fact that the shielding effect of the magnetic field-changing means, designed as a ferromagnetic shield, exhibits a clearly defined, and in particular continuously, changing profile in the direction of the second path of movement. Such a clearly changing profile means that the shielding effect along the path of movement follows a defined and measurable rule, so that no indeterminate changes occur and a clear measurement of the shielding effect or the magnetic field strength of the permanent magnet is possible.

[0014] A continuously changing signal pattern is understood to mean a pattern that changes continuously without abrupt changes or interruptions. The effect is that this allows for a uniformly changing influence, particularly shielding, of the permanent magnet's magnetic field. This enables the generation and recording of continuous and finely graduated, clearly distinguishable measurement signals, which in turn lead to precise and complete position determination. Even the smallest changes in the relative position of the objects being measured can thus be detected. However, instead of a continuously changing signal pattern, incremental or other systematically changing patterns could also be implemented, provided that the change in the shielding effect is clearly describable and traceable.

[0015] Overall, the clearly and, in particular, continuously changing course of the shielding effect leads to improved evaluation. The idea according to the invention therefore lies in a clear and, in particular, continuous change in the strength of the magnetic field detected by a single sensor designed as a multi-dimensional Hall sensor. On the one hand, the magnetic field of the permanent magnet acts on the multi-dimensional Hall sensor, which detects the magnetic field and calculates a magnetic field incidence angle from it. The magnetic field incidence angle provides information, in particular, about the position of the permanent magnet relative to the multi-dimensional Hall sensor. Thus, this first effect determines the position of the first object being measured. On the other hand, the magnetic field of the permanent magnet is clearly and, in particular, continuously changed in strength by the magnetic field modulator, depending on the position of the modulator, in particular weakened or strengthened.If the change in magnetic field strength is detected using the multi-directional Hall sensor, the position of the second object can be determined. The angle of incidence of the magnetic field used for the first effect has no relevant influence on the magnetic field strength and therefore on the second effect.

[0016] According to an advantageous embodiment of the invention, the shielding effect is linear, convex, or concave. This configuration allows for the creation of a readily detectable, continuously changing shielding effect.

[0017] In an advantageous embodiment of the invention, the ferromagnetic shield has a magnetically transparent recess. A dimension, in particular the width, of the magnetically transparent recess changes unambiguously in the direction of the second path of movement. Preferably, the width changes continuously. Furthermore, preferably, the dimension, in particular the width, of the recess changes linearly, convexly, or concavely in the direction of the second path of movement. The ferromagnetic shield is thus designed such that it has a magnetically transparent recess that changes in the direction of the second path of movement.

[0018] The shape and size of the recess influence the magnetic field of the permanent magnet by continuously changing the recess's width. Therefore, the recess allows for targeted and continuous modification of the shielding effect. The width is specifically a dimension perpendicular to the second path of movement. For example, the magnetically permeable recess is a cutout or, more generally, an area of ​​the ferromagnetic shield that blocks the magnetic field less strongly than the surrounding material. The recess is designed to influence the magnetic field modification in the direction of the second path of movement. With a linear change in width, the width increases or decreases uniformly along the path of movement. With a convex shape, the width of the recess initially increases slowly along the second path of movement and then increases more rapidly with increasing position.In a concave shape, the width of the recess initially increases rapidly, but then more and more slowly along the second path of movement.

[0019] In summary, the magnetically permeable recess with variable width enables an adjustable and precise change in magnetic field strength. The linear, convex, or concave shape of the width change can be specifically used to meet the requirements for accuracy and sensitivity of the position measurement.

[0020] In an advantageous further development of the invention, the magnetic field modification means is designed as at least one, in particular bent, ferromagnetic sheet.

[0021] A ferromagnetic sheet is a material with high magnetic permeability, making it well-suited for shielding magnetic fields. The sheet serves as the main component of the magnetic field modifier and influences the distribution of the magnetic field between the permanent magnet and the Hall sensor. Using a suitable sheet allows for easy integration of the magnetic field modifier into existing systems. Integrating a sheet enables a space-saving design without the need for large additional structures.

[0022] Preferably, the ferromagnetic sheet is not flat, but has a bent or curved geometry. This depends in particular on the design of the second measuring object. In general, however, the shape of the sheet is flexible and adaptable to various requirements of the sensor unit.

[0023] In an advantageous further development, the magnetic field modification device is designed as a sleeve body formed in one piece or in multiple pieces.

[0024] A sleeve body is a component that is at least substantially cylindrical or tubular, designed specifically to enclose the first measuring object. The sleeve body can be constructed either as a single piece, i.e., from a continuous block of material, or as a multi-piece construction, i.e., from several parts assembled together.

[0025] In an advantageous embodiment of the invention, the second measuring object comprises a piston, in particular a hollow one. In particular, the second measuring object is designed at least substantially as a hollow piston.

[0026] The hollow design allows the first object to be positioned inside the second. The multi-directional Hall sensor can then be used to determine the position of both the first and second objects.

[0027] According to an advantageous embodiment of the invention, the at least one ferromagnetic sheet or the sleeve body is arranged on an outside of the piston or an inside of the piston or is overmolded inside the piston.

[0028] In other words, the magnetic field modifying device, designed as a ferromagnetic sheet or as a sleeve body, is either attached to the outer surface of the piston or to the inner surface of the piston, or is arranged within the material that forms the piston.

[0029] The individual arrangements offer specific advantages and can be selected depending on the sensor unit's arrangement options. For an arrangement on the outside of the piston, it may also be preferable to provide a non-magnetic bearing sleeve. The bearing sleeve improves the support and guidance of the second measuring object or the piston.

[0030] According to an advantageous embodiment of the invention, the magnetic field modifying means is designed as a magnetically permeable recess in a wall of the piston. This is an alternative to the embodiment in which the magnetic field modifying means is designed as a sleeve or as a sheet.

[0031] For example, the piston could be made of ferromagnetic material with a corresponding magnetically permeable recess. The aspects described regarding the dimensions, particularly width, and the direction of the shielding effect are applicable to the magnetically permeable recess. In this embodiment, the magnetic field modification device would be integrally formed with the second measuring object, i.e., the piston.

[0032] According to an advantageous embodiment of the invention, the second object being measured is a piston that is movable relative to a housing and, in particular, can be pressurized. Preferably, the first object being measured is a locking unit for locking the position of the piston relative to the housing. The position of the piston and the position of the locking unit can then be detected by means of the sensor unit.

[0033] The piston is generally movable in various positions relative to the housing. The multi-directional Hall sensor is fixedly mounted on the housing. The locking unit is preferably movable between a first position for locking the piston and a second position for releasing the piston.

[0034] The piston's movement generally serves to control or adjust system functions. For example, this could be the parking lock of an automatic transmission. Preferably, the piston is hollow and the locking mechanism is designed to move within the piston.

[0035] More generally, the first object being measured is preferably arranged and movable within the second object being measured. The first and second paths of movement are aligned parallel to each other. Thus, the first and second objects move along paths in the same direction, although the movements are at least essentially independent.

[0036] The solution according to the invention further comprises a locking device, in particular for the parking lock of an automatic transmission, for locking the movement of a piston that is movable by a drive, in particular one that is pressurized with hydraulic pressure. The locking device has a housing through which a longitudinal axis extends. The piston is adjustable along the longitudinal axis between a retracted position and an extended position. The locking device also has a blocking unit that is adjustable along the longitudinal axis between a locking position and a release position. The blocking unit is designed to lock the piston in the locking position and to release it in the release position.

[0037] The key aspect of the locking device lies in a sensor unit, which can be one of the previously described sensor units in a specific application. The sensor unit comprises a permanent magnet, which moves together with the locking unit and is assigned to the first measuring object. It also includes a multi-directional Hall sensor, fixed relative to the housing, for detecting a magnetic field of the permanent magnet. Furthermore, the locking device includes a magnetic field modulator, which moves together with the piston. The piston itself is the second measuring object.

[0038] The magnetic field modulator is located in a space between the permanent magnet and the multi-directional Hall sensor. Here, too, the magnetic field modulator is designed as a ferromagnetic shield, the shielding effect of which exhibits a clearly defined, and in particular continuously, changing profile in the direction of the second movement path. In general, all of the advantages and aspects described above are transferable to the locking device if the sensor unit is one of those described above.

[0039] The locking device itself is, in general terms, a mechanical device that serves to block or release the movement of the piston in a specific position. In particular, it is used in the parking lock of an automatic transmission to prevent the vehicle from rolling unintentionally.

[0040] The piston is generally a component that moves linearly along its longitudinal axis. This movement is achieved by a drive mechanism, such as a hydraulic system. The housing, in general terms, is a stationary component that encloses the locking device and provides mechanical protection as well as guidance for the moving parts, such as the piston and locking unit.

[0041] In this case, the sensor unit detects the position of the piston, specifically the parking lock piston, via the changing shielding effect depending on the piston's position. The position of the locking unit is determined via the angle of incidence of the magnetic field.

[0042] A more general solution according to the invention further comprises a system for the dual detection of the position of two objects that can be moved relative to each other. This system for dual detection utilizes a single multi-dimensional Hall sensor, whereby the system determines the angle of incidence of a permanent magnet to measure the position of the first object and determines the magnetic field strength of the permanent magnet to measure the position of the second object.

[0043] This is therefore a measurement system that uses a single multi-directional Hall sensor and a combination of two measurement principles, each utilizing different physical quantities of the magnetic field, to determine the positions of two different objects. In particular, the system represents a more general application of the fundamental principle underlying the previously described locking device and sensor units. Accordingly, the individual aspects of the previously described units and devices are transferable to this system and will not be repeated here.

[0044] Furthermore, the invention comprises a hollow body, in particular a piston, for a sensor unit. The hollow body has a central axis and a magnetic field-modifying element designed as a ferromagnetic shield, the shielding effect of which exhibits a clearly defined, and in particular continuously, changing profile along the central axis. The described piston is a preferred component for the second measuring object.

[0045] Accordingly, the details relating to the second measuring object or the piston can be transferred to the hollow body. The piston is fundamentally used in a wide variety of applications. In particular, it is suitable for use in systems for position determination or switching systems. Furthermore, the hollow body according to the invention can be integrated into existing systems, or the existing hollow body can be replaced, so that the existing system can be retrofitted with a suitable sensor unit. Brief description of the drawings

[0046] The various and exemplary features described above can be combined with one another according to the invention, insofar as this is technically feasible. Further features, advantages, and embodiments of the invention will become apparent from the following description of exemplary embodiments and with reference to the figures.

[0047] The figures used to illustrate the exemplary embodiments show: Fig. 1 a schematic representation of a sensor unit according to the invention in an exemplary locking device; Fig. 2 a schematic perspective representation of the components relevant for the sensor unit; Fig. 3 another schematic perspective view in longitudinal section; Fig. 4 an embodiment of a magnetic field modifying device, and Fig. 5 A more detailed view of a magnetic field modifying device designed as a sleeve body. Ways to implement the invention

[0048] In Fig. Figure 1 shows a locking device. This locking device is, for example, a parking lock of an automatic transmission. The function of the locking device will only be briefly discussed below, as it serves merely as an example for implementing the sensor unit according to the invention.

[0049] The locking unit has a housing 150 with a central longitudinal axis L. Inside the housing 150 is a piston 121, which is moved by the pressure of a fluid in an unmarked pressure chamber from the Fig. The retracted position shown in Figure 1 can be moved to an extended position offset along a second movement path B2.

[0050] The position of piston 121 can be locked by means of a locking unit 110a. Detent elements, represented as balls, engage in various detent receptacles to lock the position of piston 121. The locking unit 110a is also movable along the longitudinal axis L, specifically along the first movement path B1. The first movement path B1 and the second movement path B2 run parallel to each other. Accordingly, the locking unit 110a can move between a release position and a linearly offset locking position.

[0051] The electromagnet shown on the left is used to actuate the locking unit 110A. The in Fig. Reference numeral 110a shown in Figure 1 indicates an extension rod of the locking unit 110a. The extension rod connects the actual functional locking unit 110a on the left side to a permanent magnet 111 on the right side. This allows the permanent magnet 111 to be positioned remotely from the rest of the locking unit 110a. A sensor unit 100 can be easily mounted at the position shown.

[0052] In addition to the permanent magnet 111, the sensor unit 100 includes a multi-directional Hall sensor 140. The multi-directional Hall sensor 140 is fixedly mounted in the housing 150. The multi-directional Hall sensor 140 allows, on the one hand, the determination of the position of the blocking unit 110A as the first object 110. On the other hand, it allows the determination of the position of the piston 121 as the second object 120. Thus, the sensor unit 100 is designed for the dual detection of the position of two objects 110 and 120 that can move relative to each other. The detection of the first object 110 is achieved via the permanent magnet 111 located on the first object 110. In particular, its magnetic incidence angle is determined. The measurement of the second object 120 is carried out using a magnetic field changing device 130 that moves together with the second object 120. In particular, the magnetic field strength of the permanent magnet 111 is determined.

[0053] In Fig. 2 and Fig. Figure 3 shows perspective views of the sensor unit 100, which further illustrate its function. The magnetic field modifying device 130 is shown more clearly in these views.

[0054] The first measuring object 110, equipped with the permanent magnet 111, is located within the second measuring object 120, represented as piston 121. The magnetic field modifier 130 is designed as a ferromagnetic shield 130a. Accordingly, the magnetic field modifier 130 has a shielding effect with respect to the permanent magnet 111. This shielding effect modifies the magnetic field strength of the permanent magnet 111 as detected by the multi-directional Hall sensor 140. For this purpose, the magnetic field modifier 130 is arranged in a space Z between the permanent magnet 111 and the multi-directional Hall sensor 140.

[0055] As in Fig. 2 and Fig. As can be seen in Figure 3, the magnetic field-changing means 130 is not uniformly formed along the longitudinal axis L, but has a recess 131 that changes in the longitudinal direction L or along the second path of movement B2. In particular, the recess 131 is a magnetically permeable recess 131 within the ferromagnetic shield 130a. This is therefore an area in which the shielding effect of the ferromagnetic shield 130a is either absent or at least reduced.

[0056] As in Fig. As can be clearly seen in Figure 5, the window-like recess 131 has a continuously changing shape. Consequently, the shielding effect of the magnetic field-changing agent 130 also exhibits a continuously changing shape when moving along the path of motion B2. The in Fig. The depicted path 5 is particularly linear. Specifically, the width B of the recess 131 decreases linearly along the second path of movement B2. However, it would also be conceivable that this path is concave or convex.

[0057] Due to the constantly changing pattern and the corresponding shielding effect, a good detection of differences in magnetic field strength is possible. As in Fig.As shown in Figure 5, the magnetic field modifier 130 can, for example, be a sleeve body 130c formed in one piece or in multiple pieces. For instance, the sleeve body 130c could be formed from a bent ferromagnetic sheet 130b. However, this is merely an example. Alternatively, the magnetic field modifier 130 could be integrated directly into the second object 120. For example, the magnetic field modifier 130 could be formed as a magnetically permeable recess 131 in a wall of the second object 121.

[0058] It should be noted that the features of the invention described with reference to individual embodiments or variants, such as the type and design of the individual components as well as their precise dimensions and spatial arrangement, may also be present in other embodiments, unless otherwise specified or is precluded for technical reasons. Furthermore, not all features of such combined features of individual embodiments need necessarily be implemented in a given embodiment. Reference symbol list 100 sensor units 110 First object to be measured 110a Blocking unit 111 Permanent magnet 120 Second measuring object 121 pistons 130 magnetic field modifying agents 130a Ferromagnetic shielding 130b Ferromagnetic sheet 130c shell body 131 Magnetically permeable recess 140 multi-room direction Hall sensor 150 cases B1 First movement path B2 Second movement path Z space L Longitudinal axis b width QUOTES INCLUDED IN THE DESCRIPTION

[0000] This list of documents cited by the applicant was automatically generated and is included solely for the reader's convenience. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions. Cited patent literature

[0000] DE 10 2022 134 624 A1

[0004]

Claims

[1] Sensor unit (100) for dual detection of the positions of two objects (110, 120) that can be moved relative to each other, wherein the sensor unit (100) has the following: a permanent magnet (111) associated with the first measurement object (110) and moving together with the first measurement object (110), a magnetic field changing device (130) associated with the second measurement object (120) and moving together with the second measurement object (120), a multi-directional Hall sensor (140) for detecting a magnetic field of the permanent magnet (111), wherein one of the permanent magnet (111) and multi-directional Hall sensor (140) is fixedly arranged or can be arranged in a fixed position and the other of the permanent magnet (111) and multi-directional Hall sensor (140) is adjustable along a first movement path (B1), wherein the magnetic field changing means (130) is adjustable in an intermediate space (Z) between permanent magnet (111) and multi-directional Hall sensor (140) along a second movement path (B2), wherein the magnetic field changing means (130) is designed as a ferromagnetic shield (130a) whose shielding effect has a course that changes clearly, in particular continuously, in the direction of the second path of motion (B2). [2] Sensor unit (100) according to claim 1 characterized by that the shielding effect is linear, convex or concave. [3] Sensor unit (100) according to claim 1 or 2 characterized by , that the ferromagnetic shield (130a) has a magnetically permeable recess (131), wherein a dimension, in particular width (b), of the magnetically permeable recess (131) changes uniquely and preferably continuously in the direction of the second path of movement (B2). [4] Sensor unit (100) according to claim 3 characterized by , that the dimension, in particular width (b), of the recess (131) changes linearly, convexly or concavely in the direction of the second path of movement (B2). [5] Sensor unit (100) according to one of the preceding claims characterized by , that the magnetic field changing means (130) is designed as at least one, in particular bent, ferromagnetic sheet (130b). [6] Sensor unit (100) according to one of the preceding claims characterized by , that the magnetic field modifying means (130) is designed as a sleeve body (130c) formed in one piece or in multiple pieces. [7] Sensor unit (100) according to one of the preceding claims characterized by, that the second measuring object (120) has a piston (121), in particular a hollow one. [8] Sensor unit (100) according to one of claims 5 to 7 characterized by , that the at least one ferromagnetic sheet (130b) or the sleeve body (130c) is arranged overmolded on an outside of the piston (121), an inside of the piston (121) or inside the piston (121). [9] Sensor unit (100) according to claim 7 characterized by , that the magnetic field modifying agent (130c) is designed as a magnetically permeable recess in a wall of the piston (121). [10] Sensor unit (100) according to one of the preceding claims characterized by , that the second measuring object (120) is a piston (121) that is movable relative to a housing (150), in particular one that can be pressurized, and the first measuring object (110) is a blocking unit (110a) for blocking the position of the piston (121) relative to the housing (150). [11] Sensor unit (100) according to one of the preceding claims characterized by , that the first measurement object (110) moves within the second measurement object (120) and that the first and second motion paths (B1, B2) are aligned parallel to each other. [12] Locking device, in particular for the parking lock of an automatic transmission, for locking the movement of a piston (121) that can be moved by a drive, in particular one that can be pressurized with hydraulic pressure, wherein the locking device has a housing (150) through which a longitudinal axis (L) extends, and the piston (121) is adjustable along the longitudinal axis (L) between a retracted retraction position and an extended extension position, wherein the locking device has a blocking unit (110a) adjustable along the longitudinal axis between a locking position and a release position, which is designed to lock the piston position of the piston (121) in the locking position and to release it in the release position, wherein the locking device comprises a sensor unit (100), in particular according to one of the preceding claims, comprising the following: one of the blocking unit (110a) and moving together with the blocking unit (110a), a multi-directional Hall sensor (140) arranged in a fixed position relative to the housing (150) for detecting a magnetic field of the permanent magnet (111), a magnetic field changing means (130) associated with the piston (121) and moving together with the piston (121), which is arranged in an intermediate space (Z) between the permanent magnet (111) and the multi-directional Hall sensor (140), wherein the magnetic field changing means (130) is designed as a ferromagnetic shield (130a) whose shielding effect has a course that changes clearly, in particular continuously, in the direction of the second path of motion (B2). [13] System for dual detection of the positions of two objects that can be moved relative to each other (110, 120) using a single multi-directional Hall sensor (140), wherein the system determines the magnetic angle of incidence of a permanent magnet (111) for measuring a position of the first object (110), and wherein the system determines the magnetic field strength of the permanent magnet (111) for measuring a position of the second object (120). [14] Hollow body, in particular piston (120) for a sensor unit (100), in particular according to one of claims 1 to 11, wherein the hollow body has a central axis, wherein the hollow body has a magnetic field changing means (130) designed as a ferromagnetic shield (130a), the shielding effect of which has a course that changes clearly, in particular continuously, along the central axis.

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