Proximity switch and method for operating one

Abstract

Proximity sensors use coils that are specified for certain installations. However, in some cases, proper operation is only possible if the evaluation unit and the coil are separate units. This is the case, for example, when metal parts in a cable need to be detected, and the cable itself can be of any shape, such as a hose. In such a case, the proposed method allows the use of a coil made from a wire winding, which is then calibrated to an evaluation unit. For this purpose, a resonant circuit is built with the coil and initially tuned using a controller. In the first step, a suitable range is set on the reference oscillator, and then, in the second step, fine-tuning is performed using the adjustable capacitor unit. Subsequently, detection can then be carried out using a static or dynamic measurement method.

Description

[0001] The present invention relates to a proximity switch with a resonant circuit and an evaluation unit electrically connected to the resonant circuit, wherein the evaluation unit comprises a resonant circuit oscillator for generating an oscillation frequency at which the resonant circuit can be operated in the region of its resonant frequency, a reference oscillator for generating a reference frequency, means for detecting a detuning of the resonant circuit due to a deviation of the oscillation frequency from the reference frequency, and means for generating a switching signal in response to an exceedance of a threshold value by such a detuning, as well as a method for operating such a device.

[0002] Such a proximity switch is already known from German patent application DE 10 2014 016 217 B4. This document describes an inductive proximity switch with a resonant circuit oscillator, in which the oscillator frequency and amplitude are monitored. The switching process is triggered by monitoring the resonant circuit oscillator frequency. If the frequency change exceeds a certain value, the output switches. This method has the advantage of exhibiting the same sensitivity with ferromagnetic materials as with non-ferromagnetic materials. The frequency change is small compared to the oscillator frequency. Therefore, to detect even the smallest frequency changes, it has proven useful to compare the oscillator frequency with a stable reference frequency using a mixer and to evaluate the small difference frequency.

[0003] Generally, there are various methods for detecting metallic objects. In inductive proximity switches, the oldest and most widely used method is the monitoring of the oscillation amplitude of a resonant circuit oscillator. The coil is designed as a sensor coil and is oriented towards the metallic object to be detected. When a metal object comes close to the sensor coil, it draws energy from the resonant circuit oscillator, and the oscillation amplitude decreases. If the oscillation amplitude falls below a certain threshold, the switching process occurs.

[0004] Another method is the measurement of the back EMF. For this, the sensor coil is subjected to a pulsed voltage. This causes an increasing coil current, which in turn generates an increasing magnetic field. This increasing magnetic field induces an eddy current in the metal part being detected, which in turn also generates a magnetic field. After switching off the coil current in the sensor coil, the weak magnetic field of the eddy current can be measured. If the measured magnetic field exceeds a certain threshold, the evaluation electronics generate a switching signal.

[0005] Industrial metal detectors typically employ the differential transformer principle with three or more coils. A central coil is energized with an alternating current. The alternating current induced in the two outer coils is evaluated differentially. If the coil arrangement is undisturbed, meaning no metal is present within the coil arrangement, the received amplitudes cancel each other out, resulting in a zero differential signal. If a metal object interferes, a differential voltage is generated, producing a switching signal. A further development of this method is described, for example, in German patent application DE 10 2018 133 507 A1.

[0006] However, there are applications that can only be realized if the sensor coil and the evaluation electronics are two separate units. The sensor coil can be as simple as a piece of wire, perhaps even made by the user and connected to the evaluation electronics, or a flexible coil that the user can wrap around the area to be monitored, such as a hard-to-reach pipe. For these applications, it is particularly important to be able to work with freely shaped and, ideally, differently shaped sensor coils.

[0007] Against this background, the present invention aims to create a proximity switch that can operate with different sensor coils and yet enables accurate and reliable detection, especially of metal parts.

[0008] This problem is solved with a proximity sensor according to the features of independent claim 1, and a method for operating such a proximity sensor according to the features of dependent claim 10. Useful embodiments of both the proximity sensor and the method can be found in the respective subsequent dependent claims.

[0009] The device provided includes a proximity switch with a resonant circuit and an evaluation unit electrically connected to the resonant circuit, the evaluation unit comprising a resonant circuit oscillator for generating an oscillation frequency at which the resonant circuit can be operated in the range of its resonant frequency, a reference oscillator for generating a reference frequency, means for detecting a detuning of the resonant circuit due to a deviation of the oscillation frequency from the reference frequency, and means for generating a switching signal in response to an exceedance of a threshold value by such a detuning.According to the invention, such a proximity switch is characterized in that the resonant circuit comprises a sensor coil detachably connected to the evaluation unit and a capacitor unit whose capacitance is variable, wherein the capacitor unit and the reference oscillator can be adjusted by means of a control unit assigned to the evaluation unit within the framework of a learning process such that the oscillation frequency lies within the range of a predetermined frequency difference to the reference frequency.

[0010] The measurement method described at the outset, involving the measurement of frequency change using a reference oscillator and a mixer, forms the basis of this method, and the method and the proximity switch are thus designed according to the invention. The problem to be solved is that the inductance of the sensor coil, which the user connects to the evaluation unit at their own discretion, is unknown and can vary over a wide range. The inductance is excited to oscillate by means of an adjustable capacitor unit located in the evaluation unit and a resonant circuit oscillator.

[0011] After installing the coil or coil wire in the assembly to be monitored and connecting this sensor coil to the evaluation unit, the user initiates a learning process on the system. A controller within the evaluation unit uses the adjustable capacitor unit to select a capacitance value for the resonant circuit oscillator. It then modifies the controller's adjustable reference oscillator so that the oscillator frequency oscillates at a close, predefined frequency offset from the internal reference oscillator, thus fulfilling the requirement for fast, sensitive oscillator frequency measurement. The controller then sets the threshold for the frequency change to be monitored, so that a switching signal is generated when a frequency change is caused by a metal particle near the sensor coil.Any other metal parts present in the area are also detected during the learning process and ignored as long as they remain unchanged. Only newly added metal parts are recognized.

[0012] This system offers the possibility of implementing applications that are difficult or impossible to achieve with standard inductive proximity switches or industrial metal detectors. It can monitor openings of varying geometries. By using a sensor coil wire, it can be wound around an inaccessible pipe to monitor its interior, or flexible coils can be stretched around an elongated frame, for example, to monitor the surface of a conveyor belt. Many other problems can be solved with this system. After the initial learning process, the system is fully operational, even with attenuation from surrounding metal.

[0013] In a preferred embodiment, the oscillation frequency of the resonant circuit and the reference frequency of the reference oscillator can be applied to a downstream mixer, and an output of the mixer is connected to an input of the control unit. Preferably, a monostable multivibrator (monoflop) and a first RC circuit for generating the switching signal from the signal difference between the two output signals are connected downstream of the mixer. In this way, a superimposed signal can first be generated from the signals of the resonant circuit oscillator and the reference oscillator using the mixer. This superimposed signal is then held by the monostable multivibrator and converted into a voltage signal proportional to the frequency change using the RC circuit.

[0014] Furthermore, a comparator can be connected downstream of the monostable multivibrator (monoflop), by means of which the switching signal of the monostable multivibrator is compared with a reference signal generated from an output signal of the resonant circuit via a second RC circuit. This makes it possible to compare the change in frequency with the original frequency and thus quantify the detuning of the resonant circuit, and consequently determine a switching interval.

[0015] Furthermore, a controller can be added downstream of the second RC circuit to smooth the frequency response of the resonant circuit. This is particularly important because the reference frequency should remain close to the resonant circuit frequency. However, unwanted deviations can negatively impact both the sensitivity and reliability of the system. For example, if the system fails to trigger at too large a distance, it may trigger too frequently at too small a distance.

[0016] In an advantageous embodiment, an OR gate can be connected downstream of the comparator at a first input, wherein the OR gate has a second input which is connected to a switching output of the resonant circuit oscillator. If either the comparator or the resonant circuit oscillator itself indicates, by means of a corresponding signal, that an object has approached, a switching signal is generated by the OR gate in both cases.

[0017] Preferably, the capacitor unit can also be integrated into the evaluation unit, so that the resonant circuit with the sensor coil is located outside the evaluation unit and the capacitor unit is located inside the evaluation unit. Due to the capacitor unit's placement within the evaluation unit, its configuration during the learning process can be easily adjusted via the control unit.

[0018] It can be particularly advantageous if the capacitor unit is a capacitor cascade with individually switchable capacitors, each capacitor in the cascade preferably having twice the capacitance of the preceding capacitor. By using individually switchable capacitors, a capacitance range can be switched through at defined intervals, and the use of capacitors with progressively doubling capacitance provides a simple and efficient way to uniformly cover a large operating range with a small number of capacitors.

[0019] Furthermore, it can be advantageous if the frequency spacing and / or the threshold can be predefined or adjusted via the control unit.

[0020] It is understood that the inductance of the sensor coil cannot be chosen to cover completely freely selectable ranges. Therefore, it is preferred that the sensor coil be a coil or coil wire with an inductance in the range of 0.5 µH to 10 µH, preferably in the range of 1 µH to 4 µH.

[0021] The learning process can be initiated by a switching signal, triggered by a key press, an interface signal, or by connecting the sensor coil to the evaluation unit via a separate sensor coil connection. This allows the learning process to be restarted both in a specific situation, such as after the user has set up the proximity switch, and remotely from a central location if environmental conditions change. If separate sensor coil connections are provided on the evaluation unit, the device can either be learned and then put into operation when the sensor coil is connected, or connected to the evaluation unit via a different connection after the learning process.

[0022] It can also be advantageous to include a first counter during the learning process that counts the oscillation pulses of the resonant circuit for a certain period of time, thus determining the current oscillation frequency. The reference frequency is then set via the reference oscillator if the oscillation frequency falls within a suitable range for a potential reference frequency. This first counter roughly measures the current oscillator frequency at the beginning of the learning process. This provides the control unit with the information to configure the reference oscillator and the capacitor cascade so that a precise adjustment can subsequently be made in small steps. However, for operation, the mixer and the second counter are still used to detect the subtle frequency shift caused by an approaching object.

[0023] Basically, there are two possible approaches for calibrating to a previously unknown sensor coil. One method involves first setting the reference oscillator to a new value during the learning process, then adjusting the capacitor value by successively changing it, and finally exiting the learning process. This is the preferred approach, as the spacing of the individual capacitors within a capacitor cascade is best suited for this purpose. However, the reverse approach is also possible: first setting the capacitor value to a new value during the learning process, and then adjusting the reference oscillator value by successively changing it. In both cases, it is advisable to keep the reference oscillator and capacitor values ​​constant at least until the sensor coil is changed again after exiting the learning process.

[0024] Finally, the evaluation unit can be configured to switch between static and dynamic object detection. In static object detection, a switching signal is triggered by a continuously applied difference between the reference frequency and the oscillation frequency, while in dynamic object detection, a switching signal is triggered by a momentary difference between the reference frequency and the oscillation frequency. For monitoring moving metal parts, the dynamic operating mode, which offers higher sensitivity, can be selected.

[0025] The invention described above will be explained in more detail below using an exemplary embodiment.

[0026] They show Fig. 1 a proximity switch according to the state of the art in which the resonant angular frequency is compared with a reference frequency in an equivalent circuit diagram, Fig. 2a any coil connected to an evaluation unit in schematic representation, Fig. 2b a free-form sensor coil connected to an evaluation unit in schematic representation, Fig. 2c a coil wire wound around a tube, which is connected to an evaluation unit in a schematic representation, as well as Fig. 3 the proximity switch according to Fig. 1 with modifications according to the invention and summaries in an equivalent circuit diagram.

[0027] Fig. Figure 1 shows a schematic circuit diagram of an electrical circuit forming an inductive proximity switch, as known from the prior art. The proximity switch detects the approach of an object 30. The distance 301 to be measured is variable and logically decreases during the approach. This approach is to be detected by the proximity switch, and in the event of a near-limit approach, the evaluation unit implemented in the circuit should react, at least by incrementing the counter, and possibly also by triggering an alarm.

[0028] The circuit begins with a resonant circuit oscillator 223, which excites a resonant circuit 10 at its natural resonant frequency. A sensor coil 101 and a capacitor unit 102 are connected in parallel within the resonant circuit 10. A reference oscillator 220 is arranged in parallel with the resonant circuit 10. A mixer 21 is connected to the resonant circuit 10 and the reference oscillator 220. A monostable multivibrator 224 and a first RC circuit 225 are connected downstream of the mixer 21. A signal shaped by the series-connected components of the mixer 21, monostable multivibrator 224, and the first RC circuit 225 is compared by means of a downstream comparator 227 with a second signal, which is generated at the output of the resonant circuit 10 by a second RC circuit 226 and is influenced by a controller 228. The comparator 227 is connected to a subsequent OR gate 229.The inputs of the OR gate 229 receive the output signals of the resonant circuit oscillator 223 and the comparator 227.

[0029] The resonant circuit oscillator 223 excites the resonant circuit 10 at a given frequency and simultaneously detects the amplitude of the resonant circuit 10. If the resonant circuit oscillator 223 cannot maintain the amplitude of the resonant circuit 10 constant, it generates a vibration detection signal, which is then applied to the OR gate 229. Therefore, either such a vibration detection signal from the resonant circuit oscillator 223 or a sufficiently large change in the oscillation frequency f1 of the resonant circuit 10 compared to the reference frequency f2 of the reference oscillator 220 is sufficient for a switching signal.

[0030] To determine the latter change, the mixer 21 generates a mixing frequency from the frequencies f1 of the resonant circuit 10 and f2 of the reference oscillator 220, which corresponds to the resonant frequency change of the resonant circuit 10. The resonant frequency change is converted into an analog voltage proportional to the frequency change by means of the downstream monostable multivibrator 224 and the first RC circuit 225.

[0031] The comparator 227 then compares this voltage, which corresponds to the change in the resonant frequency, with a reference voltage corresponding to the oscillation frequency f1. The reference voltage can also be adjusted via the regulator 228 to set the switching distance 301 of the proximity switch. If the measured voltage is greater than the reference voltage, the comparator 227 outputs a switching signal to the downstream OR gate 229.

[0032] Fig. 2a, Fig. 2b and Fig. Figure 2c shows various sensor coils 101, each connected to an evaluation unit 20. In principle, such a coil can take on various forms, and it makes no difference whether a sensor coil 101 is shaped as shown in Fig. 2a wound in a circle, as in Fig. 2b free-form or as in Fig. 2c is wound around a tube 40 in the form of a coil wire. In each of these cases, the sensor coil 101 is connected to the evaluation unit 20 via sensor coil connections 201, and this unit outputs a switching signal should a detected object 30 approach – possibly within the tube 40.

[0033] Fig. 3 shows further training of the Fig. 1 known circuit, wherein the monostable multivibrator (224), the RC elements (225, 226), the comparator (227), and the OR gate (229) are combined in the evaluation unit (20), preferably in software, in a controller 22, which, however, with the exception of the following limitations, functions analogously to the known circuit. The corresponding functions operate in the same way, but can be implemented in software. The controller initially has at least four inputs, namely a first counter 221, a second counter 222, the connection of the reference oscillator 220, and a connection to the OR gate. For clarity, the mixer 21, as well as the capacitor unit 102, which is variable by means of a capacitor cascade 103, are implemented outside the controller unit 22.

[0034] In the simplest case, a learning process takes place immediately when the sensor coil 101 is connected to a sensor coil connection 201. However, it can also be started at any time by a signal, such as by pressing a button or via an interface signal.

[0035] Once the sensor coil 101 is connected to the evaluation unit 10 and in its final installation position, so that the environment around the sensor coil 101 no longer changes, the learning process can begin. For this, the first counter 221 counts the pulses for a certain period of time and thus determines the current oscillation frequency f1. If the oscillation frequency f1 is within a suitable range for a possible reference frequency f2, which is fed to the mixer 21, the reference frequency f2 is set accordingly via the reference oscillator 220. Fine-tuning is then carried out by changing the oscillator frequency by adding further capacitors 102 from the capacitor cascade 103 until the system is balanced. The sensor is now balanced, and the second counter 222 takes over the measurement. If objects 30 now approach the sensor within the switching distance 301, they are counted and trigger a switching signal, as explained in the section on... Fig. 1 described.

[0036] The above description thus describes a proximity switch that can operate with different sensor coils and still enables accurate and reliable detection, especially of metal parts. REFERENCE MARK LIST 10 Oscillating circuit 101 Sensor coil 102 Capacitor unit 103 Capacitor cascade 20 evaluation units 201 Sensor coil connection 202 Switching output 21 mixers 22 Control unit 220 Reference oscillator 221 first counter 222 second counter 223 Resonant circuit oscillator 224 Monoflop 225 first RC link 226 second RC link 227 Comparator 228 regulators 229 OR gates 30 Item 301 distance f1 Oscillation frequency f2 Reference frequency

Claims

Proximity switch with a resonant circuit (10) and an evaluation unit (20) electrically connected to the resonant circuit (10), wherein the evaluation unit (20) comprises a resonant circuit oscillator (223) for generating an oscillation frequency (f1) at which the resonant circuit (10) can be operated in the region of its resonant frequency, a reference oscillator (220) for generating a reference frequency (f2), means for detecting a detuning of the resonant circuit (10) due to a deviation of the oscillation frequency (f1) from the reference frequency (f2), and means for generating a switching signal in response to an exceedance of a threshold value by such a detuning, characterized in that the resonant circuit (10) comprises a sensor coil (101) detachably connected to the evaluation unit (20) and a capacitor unit (102) whose capacitance is variable.wherein the capacitor unit (102) and the reference oscillator (220) can be adjusted by means of a control unit (22) assigned to the evaluation unit (20) within a learning process such that the oscillation frequency (f1) lies within the range of a predetermined frequency difference to the reference frequency (f2). Proximity switch according to claim 1, characterized in that the oscillation frequency (f1) of the resonant circuit (10) and the reference frequency (f2) of the reference oscillator (220) are applied to a downstream mixer (21) and an output of the mixer (21) is connected to an input of the control unit (22), wherein a monostable multivibrator (224) and a first RC element (225) for generating the switching signal from an output signal of the mixer (21) are preferably connected downstream of the mixer (21). Proximity switch according to claim 2, characterized in that a comparator (227) is connected downstream of the monostable multivibrator (224), by means of which the switching signal of the monostable multivibrator (224) is compared with a comparison signal generated via a second RC element (226) from an output signal of the resonant circuit (10). Proximity switch according to claim 3, characterized in that a controller (228) is connected downstream of the second RC element (226). Proximity switch according to one of claims 3 or 4, characterized in that an OR gate (229) is connected downstream of the comparator (227) at a first input, wherein the OR gate (229) has a second input which is connected to a switching output of the resonant circuit oscillator (223). Proximity switch according to one of the preceding claims, characterized in that the capacitor unit (102) is structurally integrated with the evaluation unit (20). Proximity switch according to one of the preceding claims, characterized in that the capacitor unit (102) is a capacitor cascade (103) with individually switchable capacitors, wherein each capacitor of the capacitor cascade (103) preferably always has a capacitance twice as large as a previous capacitor. Proximity switch according to one of the preceding claims, characterized in that the frequency spacing and / or the threshold value can be predefined or adjusted via the control unit (22). Proximity switch according to one of the preceding claims, characterized in that the sensor coil (101) is a coil or coil wire with an inductance in a range of 0.5 µH to 10 µH, preferably in a range of 1 µH to 4 µH. Method for operating a proximity switch with a resonant circuit (10) and an evaluation unit (20) electrically connected to the resonant circuit (10), wherein the evaluation unit (20) comprises a resonant circuit oscillator (223) for generating an oscillation frequency (f1) at which the resonant circuit (10) is operated in the region of its resonant frequency and a reference oscillator (220) for generating a reference frequency (f2), wherein a detuning of the resonant circuit (10) due to a deviation of the oscillation frequency (f1) from the reference frequency (f2) is detected and a switching signal is generated in response to an exceedance of a threshold value by such detuning, characterized in that the resonant circuit (10) comprises a sensor coil (101) detachably connected to the evaluation unit (20) and a capacitor unit (102) whose capacitance is variable.wherein the capacitor unit (102) and the reference oscillator (220) are set by means of a control unit (22) assigned to the evaluation unit (20) as part of a learning process such that the oscillation frequency (f1) lies within the range of a predetermined frequency difference to the reference frequency (f2). Method according to claim 10, characterized in that the learning process is started by a switching signal which is triggered by a key input, an interface signal or by the connection of the sensor coil (101) to the evaluation unit (20) via a separate sensor coil connection (201). Method according to one of claims 10 or 11, characterized in that, during the learning process, a first counter (221) counts the pulses of the oscillation of the resonant circuit (10) for a period of time and thus determines the current oscillation frequency (f1), wherein the reference frequency (f2) is set via the reference oscillator (220) when the oscillation frequency (f1) is in a suitable range for a possible reference frequency (f2). Method according to one of claims 10 to 12, characterized in that, as part of the learning process, the reference oscillator (220) is first set to a new value and then adjusted by successively changing the capacitor unit (102) and finally the learning process is exited. Method according to one of claims 10 or 11, characterized in that, as part of the learning process, the capacitor unit (102) is first set to a new value and then adjusted by successively changing the reference oscillator (220). Method according to one of claims 10 to 13, characterized in that after exiting the learning process the reference oscillator (220) and the capacitor unit (102) are kept constant. Method according to one of claims 10 to 14, characterized in that the evaluation unit (20) is switchable between static and dynamic object recognition, wherein in the case of static object recognition a switching signal is triggered due to a permanently applied difference value between oscillation frequency (f1) and reference frequency (f2) and in the case of dynamic object recognition a switching signal is triggered due to a temporarily occurring difference value between oscillation frequency (f1) and reference frequency (f2).

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