Inductive proximity switch with monolithic coil body block

Abstract

Inductive proximity switch with an oscillator (1) and a transmitting coil (2) for generating an alternating magnetic field, a receiving circuit (3) and a receiving coil (4) and a reference coil (5) operated in differential circuit, for detecting a metallic trigger (6) penetrating the alternating magnetic field, wherein the receiving coil (4) and the reference coil (5) are designed and arranged such that their signals cancel each other out at a certain distance of the trigger (6), wherein all three coils (2, 4, 5) are housed in a common coil former (7), are completely embedded in and enclosed by the coil former material, wherein the coefficient of thermal expansion of the coil former material is less than 10 ppm / K, the common coil former (7) is cylindrical, and the receiving coil (4) is located in the outer region and the reference coil (5) is located in the center of the common coil former (7).

Description

The invention relates to an inductive proximity switch operating in contactless fashion according to the preamble of claim 1.Inductive proximity switches are used as contactless electronic switching devices, especially in automation technology. In particular, inductive proximity switches operating according to the transformer principle are known. They are widely used in industry and are manufactured in large numbers. In order to facilitate the assembly and also the replacement of the devices, they are usually delivered with a permanently set switching distance.An electromagnetic magnetic field that can be influenced by a metallic trigger is generated with a transmitting coil. The influence on the magnetic field by the metallic trigger is electronically evaluated and output as a binary switching signal via a switching stage.Such switching devices are manufactured and sold in a wide variety of designs, among other things, by the applicant.To implement the transformer principle, at least one transmitting coil and one receiving coil inductively coupled to the transmitting coil are necessary. The essential measured variable is the transformer coupling factor between the two coils. The transformer coupling factor of the two coils can be influenced by the metallic trigger. The degree of influence affects the signal at the receiving coil. Depending on the properties of the trigger, phase shifts can also arise, which contribute to the measurement result in different ways according to the evaluation method.When a metallic release, also referred to as a target, switching or control flag, penetrates into the monitoring region of the proximity switch, the transformer coupling factor of the transformer formed by the two coils is influenced, as already explained, and depending on the specific embodiment of the proximity switch, either a switching signal is triggered when the signal at the transmitting coil or at the receiving coil exceeds a specific value, or when it falls below this value.Since the evaluation is usually carried out on the basis of the signal amplitude, the high-frequency signal is rectified, smoothed and fed to a comparator. However, it can also be digitized and processed in a microcontroller.In this case, both the activation of the transmitting coil and the evaluation of the influence of the metallic trigger can be carried out in different ways. In some cases, the transmitting coil is a component of an oscillator that can be influenced by the metallic trigger. However, there are also transmitting coils externally controlled by a high-frequency generator.The interaction with the metallic trigger is, however, limited to the near field. Therefore, it decreases approximately 3 times the power of the switching distance. In order to be able to detect even slight interactions with the metallic trigger, it is advantageous to compensate the signal in the unaffected state and to evaluate only the changes caused by the trigger. For this purpose, preferably two receiving coils are operated in differential circuit. The structure is chosen so that one of the two coils is more strongly influenced by the trigger than the other. By zeroing in the unaffected state, a very sensitive differential coil arrangement is obtained. This is adjusted in such a way that the signals of the two receiving coils cancel each other out in the unaffected state or in a specific state. The better this adjustment is achieved, the higher the sensor signal can be amplified without any overload of the amplifier occurring.Since the magnetic field and thus also the interaction of the coil arrangement with the metallic release decreases rapidly with increasing distance, temperature influences, in particular changes in position of the copper windings, but also the temperature response of the other materials and components involved, can cause signal changes which are of the same order of magnitude as the sensor signal to be expected. Therefore, higher switching intervals can only be achieved if the temperature dependence of the arrangement can be compensated over the entire operating temperature range. This equilibrium can be disturbed already by pouring the devices during production, but also by the installation situation at the place of use.Factory trimming of the differential coil assembly during manufacture can eliminate the problem only for a narrow temperature range. In order to increase the sensitivity and at the same time to suppress undesired influences, DE 41 02 542 A1 proposes operating two receiving coils in direct differential connection. One coil serves as the actual receiving coil and the other as the reference coil less, ideally unaffected by the trigger. The two receiving coils are located here in the feedback branch of a Meissner oscillator. The oscillator amplitude is evaluated. The switching distance is reached when the differential alternating voltages of the two coils cancel one another out due to the interaction with a metallic trigger. In this case, the oscillator changes its oscillation state abruptly. The arrangement is therefore very sensitive, but is also correspondingly susceptible to faults.Therefore, DE 100 12 830 A1 proposes applying the oscillator frequency to the signal to be evaluated in order to filter out the interference signals. Furthermore, it is proposed to subtract the remaining offset of the measurement signal from the measurement signal by adding the inverted oscillator signal. A disadvantage is the limitation of the maximum achievable switching frequency by the chopping and re-oscillation of the oscillator.As shown in DE 100 12 830 A1, the differential coil arrangement is generally constructed symmetrically, i.e. the reference coil has the same diameter and also the same distance from the transmitting coil as the actual receiving coil. This is because only in this way can the thermal stability required for highly sensitive devices be achieved. At different distances from the transmitting coil, the inductive coupling factors are functions of the temperature because of the thermal expansion coefficient of the carrier materials, which would be difficult to correct. As stated above, the position changes of the copper windings due to their thermal expansion must be taken into account.The symmetrical differential coil arrangement is also problematic, however, because the decoupling of the reference coil is only insufficiently successful. The reference coil is shielded from the trigger only insufficiently, above all because of its diameter, by the transmitting coil. The remaining inductive coupling of the reference coil to the trigger necessarily affects the measurement signal.For this reason, DE 103 50 733 B4 proposes an arrangement having two transformers (coil pairs) decoupled from one another. The influence of the trigger on the reference coil is thus largely excluded. However, a second transmitting coil is now required. Disadvantageous here are the material outlay for the additional transmitting coil and the space requirement for the two coil pairs decoupled from one another, i.e. preferably offset by 90° with respect to one another.It is known to produce coil arrangements for proximity switches by injection molding with plastic, as shown in DE 10 2006 012 792 A1. However, such plastic structures can lead to thermal instabilities due to the material properties.In order to increase the robustness, DE 10 2012 203 449 A1 proposes melting the coils into glass and thereby adjusting the thermal expansion coefficients of the materials. However, this method is expensive from a manufacturing standpoint.Although LTCC ceramic technology is already used for a coil in DE 10 2011 088 752 A1, this serves primarily for electromagnetic shielding there and not for the targeted improvement of the stability in the event of temperature fluctuations.The object of the invention is to overcome the disadvantages of the prior art and to provide a compact, long-term stable and temperature-insensitive inductive proximity switch.This object is achieved according to the invention by the features specified in patent claim 1. Advantageous embodiments of the invention are specified in the dependent claims.The essential inventive concept consists in accommodating all three coils of the differential coil arrangement in a monolithic coil former block with high dimensional accuracy and a low coefficient of thermal expansion. The coil body block advantageously consists of an LTCC glass ceramic, low temperature cofired ceramics, which has a thermal expansion coefficient of 6-8 ppm / K and the desired high dimensional stability.The known printed circuit board coils based on the printed circuit board material FR4 do not achieve the necessary thermal stability. The sensor coil structure is therefore embodied according to the invention as a multilayer ceramic in LTCC technology. In this case, the coils are printed layer by layer on the non-fired (green) ceramic by the screen printing method. The conductor tracks are preferably made of copper, but can also consist of silver. After stacking and pressing, the multilayer structure is fired at about 900° C. in a process furnace. If necessary, capacitors, shielding grids and / or a metal structure for predamping the reference coil can also be introduced into the ceramic body. The structure according to the invention also improves the thermal coupling of the coils. An advantage of the LTCC ceramic over other ceramics is its low dielectric losses. The permittivity of the ceramic is about 7.The invention is explained in more detail with reference to the drawings. The following are shown: FIG. 1 : shows a proximity switch according to the invention with a current mirror oscillator, FIG. 2 : shows a coil body according to the invention with the receiving coil at the edge, FIG. 3 : shows a cylindrical proximity switch according to the invention in longitudinal section.FIG. 1 shows the essential circuit elements of the inductive proximity switch according to the invention in a greatly simplified illustration. The generator 1 is designed as a current mirror oscillator. The high amplification is advantageous, which leads to rapid oscillation and prevents the oscillation break-off in the event of severe damping. The amplitude of the oscillator signal can be matched to the resistor Ra. A further advantage is that this circuit makes do without center tapping of the oscillator coil simultaneously serving as the transmitting coil 2.The antiseries-connected receiving coils 4 and 5 are connected to a transconductance mixer 10, the emitter branch of which is supplied with the oscillator signal. This arrangement, in particular the coupling of the transmitting coil 2 to the slope mixer 10, is shown in a greatly simplified manner. A Gilbert cell is certainly more suitable here. All three coils are enclosed in a monolithic ceramic block, the coil body 7. The design is selected such that the difference signal of the two receiving coils 4 and 5 is zero in the unaffected state. The low coefficient of thermal expansion of the coil body material of typically 8 ppm / K ensures the necessary thermal stability of the arrangement.The coil body 7 is advantageously made of LTCC ceramic and, in the embodiment shown, contains, in addition to the three coils, the resonant circuit capacitor 8 and a pre-attenuation surface 9 for the reference coil 5.The pre-attenuation surface 9 can also be structured. It serves for defined pre-attenuation of the reference coil 5, which ideally cannot be influenced by the metallic trigger 6, and thus the influence of the installation position on the switching distance of the proximity switch can also be reduced.The difference signal of the receiving coils 4 and 5 is fed to the slope mixer 10, which operates as an analog multiplier. It multiplies the received signal by the oscillator signal, which here also serves as a transmit signal.By the in-phase evaluation, disturbances are largely masked out. However, phase shifts caused by the trigger 6 also enter into the result.The pulsating DC voltage signal produced at the multiplier 10 is smoothed and fed to a trigger or comparator which compares the signal with a threshold value and generates a binary switching signal as a function of the attenuation state of the coil arrangement. According to the invention, the evaluation circuit 3 can also contain, instead of the multiplier 10, an integrator or a correlator which is advantageously stored as software in a microcontroller. The switching output A can itself have the functions usual in proximity switches, such as electronic fuse and / or overvoltage protection.The invention is of course not limited to the arrangement shown. The ceramic coil according to the invention can also be a component of a three-point oscillator. It also does not necessarily have to belong to a frequency-determining resonant circuit, but can be supplied with sine, triangular or rectangular pulses of any desired frequency and pulse shape by a high-frequency generator 1.FIG. 2 shows a ceramic coil body 7 according to the invention. The outer coil contacts are only schematically shown. The number of coil layers is not representative.In order to keep the transmission current as low as possible, the transmitting coil designated at its contacts as 2 has more turns than the receiving coil 4 and the reference coil 5. Because of the better contact with the trigger 6, the receiving coil 4 is arranged at the edge of the coil body 7 at a certain distance from the transmitting coil 2. The resonant circuit capacitor 8, not shown here, is advantageously arranged on the rear side of the transmitting coil 2. The pre-attenuation surface 9 is also not shown. According to the invention, it can lie on both sides of the reference coil 5, that is to say also on the side of the reference coil 5 facing the release.All three coils are contacted via the rear side of the coil body 7. FIG. 3 shows a longitudinal section of a proximity switch according to the invention in a cylindrical design. The circuit elements shown in FIG. 1 are shown here in an even more simplified manner.The front surface 11 can consist of metal, preferably stainless steel, but also of plastic or ceramic. Its edge region is used as completely as possible as a receiving surface and is therefore filled by the receiving coil 4.The apparatus comprises a plug 12 with threaded connection M8 x 1 and is provided with an external thread M12 x 1 to facilitate assembly. The other switching elements have already been explained. The evaluation circuit 3 here comprises a preamplifier, a multiplier 10, an integrator and a Schmitt trigger for generating the binary switching signal. The power supply and the switching stage usually equipped with a current limiting device or a short circuit protection device are not shown.The invention relates to an inductive proximity switch having an oscillator 1 and a transmitting coil 2 for generating an alternating magnetic field, a receiving circuit 3 having two receiving coils 4 and 5 operated in differential connection for detecting a metallic trigger 6 penetrating into the alternating magnetic field, wherein the receiving coils 4 and 5 are arranged and constructed such that they can be influenced differently by the trigger 6 and the induced receiving voltages cancel each other out at a desired (switching) distance of the trigger 6. This can also be the case in the absence of the trigger 6. All three coils are accommodated in a common coil body 7 and are completely embedded in the coil body material. It has proven advantageous to arrange the receiving coil 4 at the edge and the reference coil 5 in the center of the common coil body 7.The coil body material has a thermal expansion coefficient of less than 10 ppm / K. Typically, 8 ppm / K. The permittivity ε relativ of the bobbin material is less than 7.The coil body 7 advantageously consists of a multilayer ceramic body made of LTCC or HTCC ceramic, the abbreviation HTCC standing for "High Temperature Cofired Ceramics".The two receiving coils 4 and 5 can be interleaved with the turns of the transmitting coil 2, i.e. the coils can penetrate each other.The distances of the two receiving coils 4 and 5 from the transmitting coil 2 and also their diameters can be different. Because of its smaller diameter, the reference coil 5 requires a higher number of turns than the receiving coil 4; since the reference coil 5 has a smaller diameter and is arranged in the region of the proximity switch close to the axis, it is significantly less influenceable by the trigger than the actual receiving coil 4.In a further advantageous embodiment of the invention, the reference coil 5 is completely surrounded by the transmitting coil 2. Thus, the reference coil 5 is better decoupled from the trigger 6 and the sensitivity of the arrangement is increased. This arrangement has a positive effect on the temperature characteristic. Advantageously, the receiving coil 4 and the reference coil 5 have the same transformer coupling factor to the transmitting coil 2. In some cases, the same inductive resistor is also advantageous.The thermal conductivity of the coil body material is at least 3 W / (m*K). A capacitor 8 (resonant circuit capacitor) and a pre-attenuation surface 9 for pre-attenuation of the reference coil 5 can be embedded in the coil body 7. The receiving coil 2 can be arranged at a distance from the transmitting coil 2.List of reference characters1 Oscillator, high-frequency generator 2 transmitting coil 3 receiving circuit 4 receiving coil (1st receiving coil) 5 reference coil (2nd receiving coil) 6 metallic trigger, target 7 coil body with transmitting coil 2 and receiving coils 4 and 5 8 capacitor, resonant circuit capacitor 9 pre-attenuation surface 10 multiplier 11 front surface 12 plug with threaded connection M8 x 1 A switching output Ub operating voltage Ra balancing resistor

Claims

Inductive proximity switch having an oscillator (1) and a transmitting coil (2) for generating an alternating magnetic field, a receiving circuit (3) and a receiving coil (4) and a reference coil (5), which are operated in differential circuit, for detecting a metallic trigger (6) penetrating into the alternating magnetic field, wherein the receiving coil (4) and the reference coil (5) are designed and arranged such that their signals cancel one another at a specific distance of the trigger (6), wherein all three coils (2, 4, 5) are accommodated in a common coil former (7), are completely embedded in the coil former material and are surrounded by it, wherein the coefficient of thermal expansion of the coil former material is less than 10 ppm / K, the common coil former (7) is cylindrical, and the receiving coil (4) is arranged in the casing region and the reference coil (5) is arranged in the center of the common coil former (7).Inductive proximity switch according to Claim 1, characterized in that the coil body (7) is designed as a multilayer LTCC or HTCC ceramic.Inductive proximity switch according to one of Claims 1 to 2, characterized in that the receiving coil (4) and the reference coil (5) have the same transformer coupling factor with respect to the transmitting coil (2).Inductive proximity switch according to one of Claims 1 to 3, characterized in that a capacitor (8) is embedded in the coil body (7).Inductive proximity switch according to one of the preceding claims, characterized in that the coil body (7) has a metallic pre-attenuation surface (9) for the electrical pre-attenuation of the reference coil (5).

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