Inductive proximity switch and method for masking the transmitted pulse

Abstract

Inductive proximity switch for detecting an electrically conductive target (3), which operates according to a pulse method, with a transmitting coil (1) for generating a pulsed primary magnetic field and a separate receiving coil (2) for receiving a received signal, characterized in that the receiving coil (2) is connected to a reference potential on one side and is connected via a damping resistor (4) to the input of an integrator circuit (5), at the An output signal is outputted which is forwarded to a control and evaluation unit (7) which generates a switching signal and a controllable switching element (6) is provided which determines the operating state of the integrator circuit (5), wherein the integrator circuit (5) in a first operating state with closed switching element (6) diverts the voltage induced in the receiving coil (2) via the output against the reference potential and in a second operating state with open switching element (6) in conjunction with the damping resistor (4) operates as an inverse integrator, wherein the input of the integrator circuit (5) carries a virtual reference potential in both operating states so that the load on the receiving coil (2) by the damping resistor (4) and the integrator circuit (5) remains constant, wherein the integrator circuit (5) is supplied with a one-sided operating voltage and its operating point in the vicinity of the reference potential is 2% above the reference potential in order to improve the controllability for the received signal.

Description

[0001] The invention relates to an inductive proximity switch according to the features of the preamble of patent claim 1, as well as a method for operating the inductive proximity switch according to the invention according to patent claim 3.

[0002] Inductive proximity switches are contactless electronic switching devices used primarily in automation technology.

[0003] They contain a transmitting coil that generates an electromagnetic magnetic field that can be influenced by a metallic trigger. The magnetic field's influence by the metallic trigger is evaluated, and when a threshold is exceeded, an electronic switching stage is activated.

[0004] Switchgear of this type is manufactured and distributed in a wide variety of designs, including by the applicant.

[0005] In an inductive proximity switch operating according to the pulse principle, a transmitting coil is subjected to powerful, usually rectangular voltage or current pulses (primary signal). The echo pulse triggered by the eddy currents generated in the metallic trigger element (secondary signal) is evaluated. This evaluation can take place either directly at the transmitting coil or at a separate receiving coil.

[0006] In this case, the receiving and transmitting coils also form a transformer influenced by the eddy currents in the metallic trigger.

[0007] DE 199 01 174 C1 shows an inductive sensor with a coil, a pulse generator, a transmitting device for transmitting individual pulses, a receiving device for receiving secondary signals and a sample and hold circuit for generating sample signals of the secondary signals.

[0008] In order to evaluate the electrically conductive objects based on the temporal behavior of their decay signals, the secondary signals generated by them are sampled at at least two different times using the sample and hold circuit mentioned above and integrated over a specified period of time.

[0009] To filter out the much stronger primary signal, the system must wait for the transmit pulse to decay. Therefore, objects with low electrical conductivity are not detected due to the rapid decay of their secondary signals. Another disadvantage is that the amplifier circuit is exposed to the primary signal of the shutdown pulse. Thus, not only the decay of the transmit pulse but also the decay behavior familiar to amplifier circuits with low-pass characteristics must be considered. Otherwise, the large shutdown pulse, as already mentioned above, will be included in the measurement result.

[0010] DE 10 2009 045 460 B3 shows an inductive proximity switch with a single coil that serves as both a transmitting and receiving coil. This proximity switch only evaluates the ratio of two half-waves, which must originate from different excitation pulses. As a result, a large portion of the pulse response remains unused, which is only possible with a sufficiently strong signal. This leads to a correspondingly low sensitivity of the proximity switch.

[0011] Furthermore, an inductively operating sensor is known from DE 198 50 748 A1, which transmits signals to a mixer.

[0012] From DE 102 19 678 C1 an inductive displacement sensor is known in which an electrically decoupled oscillating circuit is arranged on the measuring head, which is excited in phase by a voltage pulse.

[0013] To eliminate the disruptive influences of interfering magnetic fields, in EP 0 936 740 B1, the polarity is reversed with each transmit current pulse, preventing any disruptive current accumulation in the metallic trigger to be detected. Furthermore, the amplifier's complexity is reduced because an alternating voltage is amplified instead of a direct current. This also avoids the drift problems that occur with direct current amplifiers. A further advantage is that there is no need to wait for the desired signal to completely decay. With the help of Zener diodes, the induced current that occurs when the transmit current is switched off is largely diverted to ground.

[0014] However, this cannot be fully achieved due to the residual forward voltage of the Zener diodes. While Zener diodes provide effective overvoltage protection, they are not suitable for ensuring highly sensitive measurements at fluctuating operating temperatures.

[0015] The non-linearities resulting from the suppression of the transmission pulse by a voltage limiter circuit with two anti-parallel connected Zener diodes, as well as the circuit complexity associated with the bridge circuit, are considered to be disadvantages.

[0016] The object of the invention is to better separate the transmitted pulse from its response signal, extend the usable sampling period, and improve the sensitivity of the proximity switch for targets with low electrical conductivity. This object is achieved according to the features of patent claim 1 and patent claim 3.

[0017] The subclaims relate to advantageous developments of the invention.

[0018] The essential idea of ​​the invention is to effectively suppress the transmitted pulse using a switched inverse integrator. For this purpose, the receiving coil is connected to a virtual reference potential input of the inverse integrator. This eliminates the need for amplitude limitation on the receiver side.

[0019] A specific temporal section of the pulse response is first integrated, then digitized and finally evaluated.

[0020] The receiver-side impedance is kept constant regardless of the switching state of the inverse integrator in order to avoid nonlinearities and achieve high measurement accuracy.

[0021] For this purpose, the familiar receiver-side amplitude limitation (protective circuit) is completely dispensed with. The received pulse response is derived via the virtual reference potential of the receiving amplifier. This ensures that the receiver-side load remains constant not only during digitization, but throughout the entire measurement period. The decay behavior familiar from conventional amplifier circuits with low-pass characteristics is avoided, allowing measurements to be taken closer in time to the transmitted pulse.

[0022] The invention is explained in more detail below with reference to the drawing.

[0023] They show: Fig. 1: Inductive proximity switch according to the invention with symmetrical operating voltage, Fig. 2: Inductive proximity switch according to the invention with single-sided operating voltage.

[0024] Fig. Figure 1 shows the basic circuit diagram of a proximity switch according to the invention. The transmitting coil 1 is supplied with current by the controllable current source 8 with a pulse generated by a control and evaluation unit 7, a microcontroller. In this case, it is a triangular pulse with a rise time of approximately 20 µs and a fall time of approximately 2 µs.

[0025] The fall time should be kept as short as possible.

[0026] The signal transmission to the receiving coil 2, which is transformer-coupled to the transmitting coil 1, is influenced by the target 3 in a known manner.

[0027] The receiving coil 2 is connected to a reference potential, in this case ground, on one side. The other coil terminal is connected via the damping resistor 4 to the input of the integrator circuit 5, which is virtually grounded. The capacitance of the integrator circuit 5 can be bridged with a controlled switching element 6, so that the inverting input is kept at virtual ground potential. The voltage induced in the receiving coil 2 is dissipated to ground via the damping resistor 4 and the amplifier output.

[0028] Damping resistor 4 not only reduces the tendency to oscillate but also acts as a protective resistor for the amplifier. For a pulse response of, for example, 100 volts, the measurement signal would be limited to 10 mA by a 10 kΩ damping resistor 4, which the operational amplifier output of integrator circuit 5 can easily handle.

[0029] Thus, the input of integrator circuit 5 always remains at reference potential without any further circuitry. In this operating mode, the received signal is not evaluated.

[0030] When the switching element 6 is open, the integrator circuit 5, in conjunction with the damping resistor 4, acts as an inverse integrator. The capacitor C1 now sums the current induced by the induction voltage Ui in the receiving coil 2. The signal Ui generated at the output of the integrator circuit 5 is fed to an analog-to-digital converter belonging to the microcontroller 7. Further evaluation can be performed by comparison with a switching threshold stored in the microcontroller 7. A digital switching signal can be generated via the switching output SA. Of course, an analog signal can also be output via a 4-20 mA current source. Finally, it should be mentioned that the controllable current source 8 and the switching element 6 are also controlled by the microcontroller 7.

[0031] Fig. Figure 2 shows the basic circuit diagram for another embodiment of the inventive proximity switch with single-sided operating voltage. This circuit provides a means of evaluating the pulse response of the transmit pulse induced in the receive coil. This circuit allows monitoring and thus adjustment or regulation of the transmit signal.

[0032] To counteract the overload of the amplifier in the integrator circuit 5, the input signal is reduced by a factor of 100 by the combination of the 10 kΩ damping resistor 4 and the 100 Ω resistor at the non-inverting amplifier input. The reference potential generated by a 4.7 kΩ × 100 Ω voltage divider is only slightly above ground potential in this case. In addition, the controllability of the received signal is almost doubled by the associated asymmetrical operating point setting of the integrator circuit 5. When the switch is closed, the circuit operates as a voltage follower with a reduction ratio of 1 / 100. When switch 6 is open, the circuit operates as an inverting integrator with a significantly higher gain than before. Therefore, the transmit pulse in the Fig. 2 is also shown with a lower amplitude than the received signal. Otherwise, the circuit corresponds to the Fig. 1.

[0033] The mode of operation is explained again below. The inductive proximity switch according to the invention operates according to the pulse principle. The transmitting coil 1 generates a pulsed primary magnetic field. In the receiving coil 2, a voltage signal is induced by the pulsed primary magnetic field. The voltage signal depends on the pulsed primary magnetic field and the eddy currents induced in an electrically conductive target 3, which generate a secondary magnetic field. The receiving coil 2 is connected via a damping resistor 4 to the input of an integrator circuit 5, at whose output an output signal is output. The output signal is forwarded to a control and evaluation unit 7, which generates a switching signal.

[0034] The operating state of the integrator circuit 5 is determined by a controllable switching element 6. In a first operating state with the switching element 6 closed, the integrator circuit 5 diverts the voltage induced in the receiving coil 2 to a reference potential via its output.

[0035] This operating state is selected at the beginning of the induced impulse response to suppress the primary impulse. Only when the primary magnetic field has largely decayed is the switching element 6 opened to receive the impulse response of target 3.

[0036] In the second operating state with the switching element 6 open, the integrator circuit 5 works in conjunction with the damping resistor 4 as an inverting integrator.

[0037] In both operating states, the input of the integrator circuit 5 carries a virtual reference potential so that the load on the receiving coil 2 through the damping resistor 4 and the subsequent integrator circuit remains constant regardless of the operating state.

[0038] In an advantageous embodiment, the integrator 5 is supplied with a single-ended operating voltage, and its operating point is set close to the reference potential, preferably 2% above it. This improves the controllability of the pulse signal.

[0039] This arrangement is particularly advantageously combined with a step-down circuit to reduce the amplitude of the received signal. Here, the integrator circuit 5 operates as a voltage follower with a gain of 1 during the transmission pulse with the switching element 6 closed, and as an inverse integrator with a very high gain when receiving the echo signal with the switching element 6 open.

[0040] According to the method according to the invention for operating an inductive proximity switch, the transmitting coil 1 is energized with a transmitting pulse, a temporal section of the received signal is integrated by the integrator circuit 5, then digitized and evaluated.

[0041] The integrator circuit 5 has two operating states. In a first operating state, it acts as a voltage follower with a gain of 1, diverting the received signal to a virtual reference potential to protect the circuit from the relatively strong echo pulse of the transmitted pulse. This prevents damage to the sensitive input circuit without the need for additional components such as Zener diodes or similar.

[0042] In a second operating state, the integrator circuit 5, in conjunction with the damping resistor 4, forms an inverse integrator with significantly higher gain to detect the echo signal influenced by the target 3. In this way, the receiving coil 2 always remains connected to the same virtual reference potential, regardless of the operating state of the integrator circuit 5. Reference drawing list 1 transmitting coil 2 receiving coil 3 Target 4 Damping resistor 5 Integrator circuit 6 Switching element 7 Control and evaluation unit, microcontroller 8 Controllable power source SA switching output

Claims

[1] Inductive proximity switch for detecting an electrically conductive target (3), which operates according to a pulse method, with a transmitting coil (1) for generating a pulsed primary magnetic field and a separate receiving coil (2) for receiving a received signal, characterized by that the receiving coil (2) is connected to a reference potential on one side and is connected via a damping resistor (4) to the input of an integrator circuit (5), at whose An output signal is outputted which is forwarded to a control and evaluation unit (7) which generates a switching signal and a controllable switching element (6) is provided which determines the operating state of the integrator circuit (5), wherein the integrator circuit (5) in a first operating state with closed switching element (6) diverts the voltage induced in the receiving coil (2) via the output against the reference potential and in a second operating state with open switching element (6) in conjunction with the damping resistor (4) operates as an inverse integrator, wherein the input of the integrator circuit (5) carries a virtual reference potential in both operating states so that the load on the receiving coil (2) by the damping resistor (4) and the integrator circuit (5) remains constant, wherein the integrator circuit (5) is supplied with a one-sided operating voltage and its operating point in the vicinity of the reference potential is 2% above the reference potential in order to improve the controllability for the received signal. [2] Inductive proximity switch according to claim 1, characterized by that a circuit is present to reduce the amplitude of the received signal. [3] Method for operating an inductive proximity switch according to one of the preceding claims, wherein the transmitting coil (1) is energized with a transmitting pulse, a temporal section of the receiving signal is integrated by the integrator circuit (5), this receiving signal is subsequently digitized and evaluated, wherein the integrator circuit (5) has two operating states, wherein in a first operating state it derives the receiving signal against a virtual reference potential in order to protect the integrator circuit (5) from the echo pulse of the transmitting pulse and in a second operating state it operates as an inverse integrator in order to detect the receiving signal influenced by the target (3), so that the receiving coil (2) is always connected to the same virtual reference potential regardless of the operating state of the integrator circuit (5).

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