Inductive proximity switch with separately controllable compensation coils

Abstract

Inductive proximity switch with a primary coil (2) for generating an alternating magnetic field, with an oscillator (5) for driving the primary coil (2), with a control and evaluation unit (7) which is operatively connected to the oscillator (5) and is configured to detect and evaluate the amplitude and phase of a current in the primary coil (2) and to output a detection signal as a function of the detected current in the primary coil (2), with at least one compensation coil (3) for manipulating the alternating magnetic field generated by the primary coil (2), characterized by that a separate controllable current source (6) is provided for controlling the at least one compensation coil (3), which serves to drive the compensation coil (3) independently of the primary coil (2) in order to compensate for effects of a housing (9) or of installation materials.

Description

[0001] The present invention relates to an inductive proximity switch according to the preamble of claim 1.

[0002] A generic inductive proximity switch comprises the following components: a primary coil for generating an alternating magnetic field, an oscillator for driving the primary coil, a control and evaluation unit that is operatively connected to the oscillator and is configured to detect and evaluate the amplitude and phase of a current in the primary coil and to output a detection signal as a function of the detected current in the primary coil, and at least one compensation coil for manipulating the alternating magnetic field generated by the primary coil.

[0003] A proximity switch with these features is disclosed, for example, in DE 10 2006 053 023 A1. In the proximity switch described therein, a transmitting coil is connected in the opposite direction to the compensation coil, and both the transmitting coil and the compensation coil are operated by a common AC voltage generator. The possibilities for influencing the magnetic field in the immediate vicinity of the proximity switch are therefore limited.

[0004] WO 2014 / 053240 A2 discloses an inductive proximity switch in which a current induced in a receiving coil due to electromagnetic interference from a primary coil is regulated to zero by injecting compensation currents. The measuring principle consists of monitoring the compensation currents in order to detect differences in the presence of objects to be detected.

[0005] In WO 2012 / 104086 A2 a metal detector is described in which two transmitting coils are supplied with suitable currents in order to regulate the currents in receiving coils to zero.

[0006] In "IFM: Training materials efector100, company publication, Training materials Inductive proximity switches (as of March 2003), ifm electronic, October 27, 2006. Essen: ifm electronic, 2006," an inductive proximity switch is described in which the amplitude of a current through the primary coil is evaluated for object detection. Such inductive proximity switches are also referred to as being based on the damping principle.

[0007] One object of the invention can be considered to be to provide a proximity switch in which the alternating magnetic fields can be influenced particularly flexibly.

[0008] This problem is solved by the proximity switch with the features of claim 1.

[0009] The proximity switch of the type described above is further developed according to the invention in that a separate controllable current source is provided for controlling the at least one compensation coil.

[0010] Advantageous further developments of the proximity switch according to the invention are described in the following description, in particular with reference to the dependent claims and the figures.

[0011] The measuring principle of an inductive proximity switch generally consists of a primary coil, also known as a transmitting coil, driven by an oscillator, which emits an alternating magnetic field into a monitoring area. The interaction between this alternating magnetic field and a target object is then measured. A key effect of this process is that the alternating magnetic field induces eddy currents in a metallic target, resulting in losses that draw energy from the oscillator.

[0012] In principle, inductive proximity switches can also provide a distance-dependent signal. However, because this is extremely sensitive to the precise relative positioning between the target and the proximity switch, this is only used in special situations. Typically, inductive proximity switches are used as binary (on / off) switching sensors.

[0013] In the most common evaluation method, which can be used particularly with the proximity switch according to the invention, the damping of the oscillator by the approaching target is evaluated. The proximity switch activates when the damping caused by the approaching target becomes so great that the oscillation amplitude falls below a threshold value. This can be implemented with relatively simple electronic means.

[0014] The oscillator for driving the primary coil typically includes a resonant circuit and a feedback amplifier. In principle, the primary or transmitting coil can also be formed by the inductance of the resonant circuit itself.

[0015] In proximity switches where the basic measurement signal is derived from the attenuation of an oscillator by an approaching target, the primary coil is not a transmitting coil in the strictest sense. However, the use of the term "transmitting coil" does not imply the presence of a receiving coil. In fact, proximity switches that detect attenuation do not have a receiving coil. Similarly, the use of the term "primary coil" does not necessarily imply the presence of a secondary coil. The coils used for measurement in proximity switches that detect attenuation might more accurately be described as measuring or sensor coils.

[0016] The control and evaluation unit refers to the electronic components used to perform the necessary control and evaluation operations, in particular the evaluation of changes in oscillator characteristics as the target approaches. The control and evaluation unit can consist of analog and / or digital electronic components. The components of the control and evaluation unit that determine the amplitude and phase of the current in the primary coil can also be located within the oscillator or be parts of the oscillator itself.

[0017] The term "detection signal" refers to the signal output by the proximity switch depending on the approach of a target. In special cases, this can also be an analog signal, i.e., distance-dependent. However, the detection signal is usually a binary signal, i.e., an on / off signal, which indicates whether a target is closer to or farther from a specific switching distance of the proximity switch.

[0018] Additional coils that can influence the alternating magnetic field generated by the primary coil are called compensation coils.

[0019] The requirement of a separate, controllable current source means, in general terms, that the current driving the at least one compensation coil is not in a fixed, unchanging, and therefore rigid relationship to the current through the primary coil. This current source should be controllable in the sense that the current through the compensation coil(s) can be variably adjusted.

[0020] The invention has recognized that while the rigid linkage of the drive of the compensation coils with that of the transmitting or primary coil allows for a rough compensation of effects of, for example, a housing or certain installation materials, these undesirable effects can be significantly reduced with a more flexible control of the at least one compensation coil.

[0021] A key idea of ​​the present invention can therefore be considered to be to make the driving of the at least one compensation coil more flexible and, in particular, independent of a supply to the primary coil, and to provide suitable technical means for this purpose in the form of a separate controllable current source.

[0022] A significant advantage of the proximity switch according to the invention is that, compared to devices known from the prior art, it is considerably less susceptible to interference from materials and parts located in its vicinity. Of particular importance is the fact that the proximity switch according to the invention can be individually adjusted and operated in different installation situations.

[0023] In particularly preferred variants of the proximity switch according to the invention, the controllable current sources are configured to control the amplitude and phase position of the respective compensation coil current.

[0024] In principle, the main idea of ​​the present invention is realized when a single compensation coil and an associated separate controllable current source are present. However, the advantages of the invention described above are achieved in a particularly significant way in variants in which several compensation coils, especially two compensation coils, are present.

[0025] In principle, it is possible for several compensation coils to be driven by one and the same controllable current source. Even more possibilities for the individual adaptation of the inductive proximity switch according to the invention are obtained if each of the compensation coils has its own separate controllable current source.

[0026] With regard to the specific geometric design and positioning of the compensation coils relative to the primary coil, there is fundamental freedom of design. This is a particular advantage of the present invention, because the spatial distributions of the alternating magnetic fields can be modeled particularly well due to this freedom of design.

[0027] Cylindrically symmetrical arrangements of the primary coil and the compensation coil(s) have proven particularly advantageous. For example, a compensation coil can surround the primary coil, especially completely. This particularly applies to arrangements in which the primary coil is positioned on an end face of a substantially tubular housing such that a cylindrical axis of the primary coil is coaxial with an axis of the tubular housing, and in which a compensation coil radially surrounds the primary coil thus positioned.

[0028] A compensation coil surrounding the primary coil is particularly effective if the compensation coil has an axial extent that is greater than or equal to the extent of the primary coil or of a coil core of the primary coil in that axial direction.

[0029] Arrangements in which at least one compensation coil is arranged in an axial direction behind the primary coil have also proven to be particularly useful.

[0030] A compensation coil positioned axially behind the primary coil is particularly effective if this compensation coil has a diameter that is larger than or equal to the diameter of the primary coil or the diameter of a coil core of the primary coil.

[0031] In principle, the invention can be implemented in inductive proximity switches without a coil core. However, in particularly advantageous variants, the primary coil has a coil core for shaping the field distribution of the alternating magnetic field and for amplifying it. For example, the coil core can be a cup core of a generally known shape and made of a generally known material, such as ferrite.

[0032] The coil core is particularly advantageous as it is a shell core whose shape is adapted to the shape of the compensation coils.

[0033] There is generally freedom of design regarding the housing in which the inductive proximity switch according to the invention is accommodated. However, variants in which the primary coil is positioned at one end face of a tubular housing have proven particularly advantageous.

[0034] For example, the primary coil, the compensation coils and the controllable current sources can be arranged in a cuboid or cylindrical, in particular circular cylindrical, housing.

[0035] The influence of the housing on the alternating magnetic field emitted by the primary coil is particularly low with such housing geometries.

[0036] There is also freedom of design with regard to the housing material. It can be made of plastic and / or, in principle, also of metal. A particular advantage of the invention is that the controllable current source for the at least one compensation coil allows for flexible adaptation to different installation situations and housing materials.

[0037] In further particularly preferred embodiments of the invention, the control and evaluation unit, in particular analog and / or digital, is also configured to control the controllable current sources. For example, this control can be software-controlled.

[0038] In this context, it is still preferred if the control and evaluation unit has a programmable logic component, in particular a microcontroller, especially with analog and digital functionality.

[0039] Further features and advantages of the invention are explained below with reference to the accompanying schematic figure. This figure shows: Fig. 1: An embodiment of an inductive proximity switch according to the invention.

[0040] The in Fig. Figure 1, a schematically illustrated embodiment of an inductive proximity switch 10 according to the invention, comprises as essential components a primary coil 2, an oscillator 5, a first compensation coil 3 with an associated first controllable current source 8, and a second compensation coil 4 with an associated second controllable current source 6. These components are housed in a casing 9, which may be made of, for example, metal and / or plastic.

[0041] The primary coil 2, which serves to generate an alternating magnetic field, is housed in a shell core 1. Within the housing 9, which can, for example, have a cubic or cylindrical shape, the shell core 1 with the primary coil 2 is arranged directly against one end face. This allows the alternating magnetic field to escape effectively. The first compensation coil 3 is located radially outside the shell core 1 and completely surrounds it circumferentially. In the axial direction, which is Fig.Since the first compensation coil 3 is designated with reference numeral 11, its dimensions are approximately the same as those of the core 1. However, it should be noted that there is considerable design freedom with regard to the specific geometry and positioning of the compensation coils. For example, the compensation coils do not necessarily have to be formed and positioned flush with the primary coil as shown in the figure.

[0042] The second compensation coil 4 is a flat coil and is located behind the shell core 1 when viewed in the axial direction 11. The second compensation coil 4 essentially covers the entire back side of the shell core 1.

[0043] The oscillator 5 contains electronic means for detecting a current through the primary coil 2. In this respect, components of the oscillator 5 and the control and evaluation unit 7 overlap. The oscillator 5 is connected to the control and evaluation unit 7 to evaluate this current through the primary coil 2.

[0044] In the illustrated embodiment, the first controllable current source 6 and the second controllable current source 8 are connected to the oscillator 5. This means that the frequency of the current through the first compensation coil 3 and the second compensation coil 4 is determined by the oscillator 5.

[0045] The first controllable current source 8 and the second controllable current source 6 also allow for individual adjustment of the amplitude and relative phase of the current by the first compensation coil 3 and the second compensation coil 4, respectively. These further controls can be performed, for example, by the control and evaluation unit 7, as illustrated by the corresponding arrows in the present embodiment. The control and evaluation unit 7 can, for example, be a microcontroller capable of providing analog and digital functionalities. An interface 12 (schematically shown) is provided for outputting a preferably binary switching signal; this interface can, for example, be connected to a bus system.

[0046] The compensation coils in 3 and 4 serve to locally displace the alternating magnetic field generated by the primary coil 2. The electrical parameters of the primary coil current, in particular amplitude and phase, are measured. Controlled by the adjustable current sources 6 and 8 respectively, the compensation coils 3 and 4 generate local compensation fields. The ability to adjust the amplitude and phase of the current sources 6 and 8 allows for extremely flexible adaptations with regard to adjacent mounting materials, for example, at the edge of the housing.A particular advantage of the invention is that the aforementioned installation materials at the edges of the housing, which would be detected by the primary coil 2 without compensation, can be particularly well hidden and are therefore no longer detected by the evaluation unit 7 or only detected to a very minor extent, without impairing the detection sensitivity for the targets of interest.

[0047] The present invention provides a novel inductive proximity switch in which significant improvements are achieved with regard to the suppression of objects and materials in the vicinity of the proximity switch, thanks to the fundamental concept of individually adjusting the current of the compensation coils, which is particularly independent of a primary current through the transmitting coil. This increases the reliability of the inductive proximity switch. Reference symbol list 1 coil core 2 Primary coil 3 compensation coil 4 compensation coil 5 Oscillator 6 controllable power sources 7 Control and evaluation unit 8 controllable power sources 9 cases 10 Inductive proximity switches 11 Axial direction 12 Interface

Claims

[1] Inductive proximity switch with a primary coil (2) for generating an alternating magnetic field, with an oscillator (5) for driving the primary coil (2), with a control and evaluation unit (7) which is operatively connected to the oscillator (5) and is configured to detect and evaluate the amplitude and phase of a current in the primary coil (2) and to output a detection signal as a function of the detected current in the primary coil (2), with at least one compensation coil (3) for manipulating the alternating magnetic field generated by the primary coil (2), characterized by , that a separate controllable current source (6) is provided for controlling the at least one compensation coil (3), which serves to drive the compensation coil (3) independently of the primary coil (2) in order to compensate for effects of a housing (9) or of installation materials. [2] Inductive proximity switch according to claim 1, characterized by , that several compensation coils (3, 4) are present. [3] Inductive proximity switch according to claim 2, characterized by that each of the compensation coils (3, 4) has its own separate controllable power source (6, 8). [4] Inductive proximity switch according to one of claims 1 to 3, characterized by , that the at least one controllable current source (6, 8) is set up to control an amplitude and / or a phase angle of the compensation coil current. [5] Inductive proximity switch according to any one of claims 1 to 4, characterized by , that a compensation coil (3) surrounds the primary coil (2), in particular completely. [6] Inductive proximity switch according to claim 5, characterized by, that the compensation coil (3) surrounding the primary coil (2) has an axial extent which is greater than or equal to the extent of the primary coil (2) or of a coil core (1) of the primary coil (2) in that axial direction. [7] Inductive proximity switch according to any one of claims 1 to 6, characterized by , that a compensation coil (4) is arranged in an axial direction behind the primary coil (2). [8] Inductive proximity switch according to claim 7, characterized by , that the compensation coil (4) arranged behind the primary coil (2) has a diameter that is larger than or equal to a diameter of the primary coil (2) or the diameter of a coil core (1) of the primary coil (2). [9] Inductive proximity switch according to any one of claims 1 to 8, characterized by , that the primary coil (2) has a coil core (1). [10] Inductive proximity switch according to claim 9, characterized by, that the coil core (1) is a shell core. [11] Inductive proximity switch according to any one of claims 1 to 10, characterized by , that the housing (9) is a cuboid or cylindrical, in particular circular cylindrical, housing (9) and that the primary coil (2), the compensation coils (3, 4) and the controllable current sources (6, 8) are arranged in the cuboid or cylindrical, in particular circular cylindrical, housing (9). [12] Inductive proximity switch according to any one of claims 1 to 11, characterized by , the housing (9) is a tubular housing (9) and that the primary coil (2) is positioned at one end face of the tubular housing (9). [13] Inductive proximity switch according to claim 12, characterized by , that the housing (9) is made of plastic and / or metal. [14] Inductive proximity switch according to any one of claims 1 to 13, characterized by, that the control and evaluation unit (7), in particular analog and / or digital, is also set up to control the controllable power sources (6, 8). [15] Inductive proximity switch according to claim 14, characterized by , that the control and evaluation unit (7) has a programmable logic component, in particular a microcontroller, especially with analog and digital functionality.

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