Inductive proximity switch with flat receiving coil

Abstract

Inductive proximity switch (10), with a transmitting coil (S1) which generates a primary variable magnetic field (5) covering the monitoring area (20) of the proximity switch (10), at least one receiving coil (E2) which serves to detect a conductive target (6) in the monitoring area (20), and an electronic unit (EE) which is connected to the transmitting coil (S1) and the at least one receiving coil (E2) and which evaluates the magnetic flux change caused by the secondary magnetic field (7) generated by the target (6) in order to detect the target (6) characterized by that the transmitting coil (S1) is a cylindrical coil and the at least one receiving coil (E2) is a flat coil whose coil surface runs parallel to the curved outer surface of the transmitting coil (S1), wherein the at least one receiving coil (E2) does not receive any noticeable magnetic flux of the primary magnetic field (5).

Description

[0001] Inductive proximity switches are used in many areas of automation technology. They are used to detect metallic targets in a monitoring area without contact and to report the detection as a switching signal, for example, to a control system. Key criteria when selecting inductive proximity switches include the switching distance, switching frequency, switching accuracy, and temperature sensitivity.

[0002] These often operate according to the principle of the damped LC oscillator, in which the object to be detected (the target) influences the quality of the oscillator. An evaluation unit evaluates the change in quality and, if necessary, outputs a switching signal.

[0003] An alternative to the oscillator principle is the transformer principle, in which the influence of a target on the coupling factor between a pair of measuring coils consisting of a transmitting coil and a receiving coil is evaluated. A variable magnetic field is generated with the transmitting coil, and the voltage induced on the secondary side is measured in the receiving coil and evaluated after appropriate amplification. When a target is detected, a switching signal can be output.

[0004] Another method for detecting a target is the pulse method. This method involves briefly generating a magnetic field and evaluating the target's response pulse.

[0005] The applicant has been manufacturing such proximity switches for many years and selling them worldwide.

[0006] Inductive proximity switches are known from DE 43 30 140 A1 and DE 197 40 774 A1 which aim to achieve a correction factor of 1 in order to ensure constant switching distances regardless of the properties of the target material.

[0007] DE 103 50 733 A1 discloses an inductive proximity switch comprising two transmitting coils and two receiving coils. Each transmitting coil and receiving coil form a coil pair. A coil pair is arranged in such a way that it is virtually unaffected by the target's eddy current field. This is achieved, for example, by an orthogonal arrangement of the two coil pairs. Alternatively, a shield can be provided between the two coil pairs.

[0008] This proximity switch has a very complex design. A pair of coils serves only for compensation purposes. Furthermore, the coil arrangement requires a lot of space.

[0009] The object of the invention is to provide an inductive proximity switch which is of simple construction, which requires little space and which can be manufactured easily and inexpensively.

[0010] This object is achieved by the features specified in claim 1. Advantageous further developments of the invention are specified in the subclaims.

[0011] The key idea of ​​the invention is that the receiving coil is flat and arranged in such a way that it "sees" the field of the transmitting coil only minimally or not at all. The coil surface extends parallel to the cylindrical surface of the transmitting coil. No additional installation space is required in the direction of the coil axis.

[0012] The receiving coil is advantageously located on the inside of the transmitting coil. Within the coil's interior, the magnetic field runs essentially parallel to the coil axis, so that the planar receiving coil receives no magnetic flux from the transmitting coil. A closer look, taking into account the shape of the magnetic field near the coil wires, reveals that the magnitudes of a single wire section just cancel each other out. The individual field lines always pass through the coil surface twice, so that no net flux is generated here either. Because the receiving coil only detects the target influence, no compensation is necessary, and the signal amplification can be set accordingly high. Temperature influences also have a lesser impact.

[0013] Of course, two receiving coils arranged according to the invention can also be provided, each influenced by the target to a different extent. This allows for the compensation of interference effects that affect both receiving coils in the same way. Sensitivity can thus be further increased.

[0014] The coil arrangement can also be used for the pulse method. Since the receiving coil(s) do not see the transmitted pulse, the evaluation of the received pulse can begin much earlier. There is no need to wait until the transmitted pulse has almost completely decayed for the evaluation. Since the receiving coil does not see the transmitted pulse, the evaluation can begin immediately with the transmitted pulse.

[0015] Since the sensitivity of the coil arrangement according to the invention is very high, the operating frequency can be reduced accordingly. This makes the coil arrangement also suitable for all-metal sensors.

[0016] The invention is explained in more detail below using an embodiment shown in the drawing.

[0017] They show: Fig. 1 Coil arrangement with a transmitting coil and a receiving coil with the magnetic field lines Fig. 2 Flat coil Fig. 3 Top view of the transmitting coil with the receiving coil Fig. 4 Block diagram of an inductive proximity switch

[0018] Fig. Figure 1 shows the field lines of a primary magnetic field 5 generated by a cylindrical transmitting coil S1. To generate eddy currents in a conductive target 6 located in the detection zone 20 of the proximity switch, the magnetic field strength must vary over time, either in a pulsed or oscillating manner.

[0019] The eddy currents in the target 6 generate a secondary magnetic field 7. Due to the cylindrical symmetry, the magnetic fields each have only one axial B z and a radial B r Component.

[0020] According to the invention, a flat receiving coil E2 is provided, which is arranged such that it does not receive any magnetic flux from the primary magnetic field 5. In the illustrated case, the surface of the receiving coil E2 runs parallel to the cylindrical surface of the transmitting coil S1. A flexible coil, which can be designed, for example, as a flexible circuit board, is conceivable for this purpose.

[0021] The flexible circuit board conforms to the inside of the transmitting coil S1. The surface of the receiving coil E2 is curved accordingly. The z-component of the B-field of the transmitting coil S1 runs tangentially to the curved coil surface and therefore does not generate an induced voltage. The same applies to the r-component of the magnetic field present near the coil wires, since the field lines pass back and forth across the coil surface and therefore do not generate a net flux or induced voltage.

[0022] In Fig. Figure 3 shows a schematic view of the transmitting coil S1 with the receiving coil E2. In principle, the receiving coil E2 can also cover an angular range larger than 180°.

[0023] In Fig.Figure 4 shows a block diagram of an inductive proximity switch 10 according to the invention. Both the transmitting coil S1 and the receiving coil are connected to an electronic unit EE, which controls a switching output SA. A switching signal is forwarded via the switching output SA, for example, to a controller (e.g., PLC). The proximity switch 10, which is designed as a 3-wire device, has an M12 thread.

[0024] The electronics unit EE can, for example, include a microcontroller in which digital signal processing takes place. The known signal evaluation methods for inductive proximity switches can be used for signal evaluation; therefore, these will not be discussed in detail. Proximity switch 10 triggers according to the set switching threshold.

[0025] The operation of the invention is explained in more detail below. As the conductive target 6 approaches the monitoring area 20, eddy currents are generated in the target 6 due to the primary magnetic field 5, which increase sharply as the distance decreases. The eddy currents in the target 6 generate a secondary magnetic field 7.

[0026] Due to its spatial arrangement, the receiving coil E2 does not see the primary magnetic field 5 or only sees it minimally.

[0027] However, since the secondary magnetic field 7 exhibits a noticeable r-component at the location of the receiving coil E2, this generates an induction voltage in the receiving coil E2. This induction voltage is amplified in the amplifier V and subsequently evaluated in the electronics unit EE. This induction voltage is caused only by the target 6 and is unaffected by the primary magnetic field 5. Therefore, the amplification factor of the amplifier V can be selected accordingly high. Compensation for the influence of the primary magnetic field by differential circuitry is not necessary.

[0028] Of course, two receiving coils E2 arranged according to the invention can also be provided. These could, for example, be arranged one behind the other. As a result, they will be influenced to different degrees by the target 6 due to the different distances. This allows for the compensation of interference effects that affect both receiving coils in the same way. Sensitivity can thus be further increased.

[0029] The coil arrangement according to the invention can also be used for the pulse method. Since the receiving coil E2 or the receiving coils E2, E2' do not see the pulsed magnetic field (transmission pulse) generated by the transmitting coil S1, the evaluation of the voltage induced in the receiving coil E2 (received pulse) can begin much earlier. It is no longer necessary to wait until the transmitted pulse has almost completely decayed for the evaluation. Since the receiving coil E2 does not see the pulsed magnetic field of the transmitting coil S1, the signal evaluation can begin immediately with the transmitted pulse.

[0030] Since the sensitivity of the coil arrangement according to the invention is very high, the operating frequency can be reduced accordingly. This makes the coil arrangement also suitable for all-metal sensors.

Claims

[1] Inductive proximity switch (10), with a transmitting coil (S1) which generates a primary variable magnetic field (5) covering the monitoring area (20) of the proximity switch (10), at least one receiving coil (E2) which serves to detect a conductive target (6) in the monitoring area (20), and an electronic unit (EE) which is connected to the transmitting coil (S1) and the at least one receiving coil (E2) and which evaluates the magnetic flux change caused by the secondary magnetic field (7) generated by the target (6) in order to detect the target (6) characterized by , that the transmitting coil (S1) is a cylindrical coil and the at least one receiving coil (E2) is a flat coil whose coil surface runs parallel to the curved outer surface of the transmitting coil (S1), wherein the at least one receiving coil (E2) does not receive any noticeable magnetic flux of the primary magnetic field (5). [2] Inductive proximity switch (10) according to claim 1, characterized by that the at least one receiving coil (E2) rests against the cylindrical inner side of the transmitting coil (S1). [3] Inductive proximity switch (10) according to one of the preceding claims, characterized by that two receiving coils are provided which are influenced to different degrees by the secondary magnetic field (7).

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