Inductive proximity switch and method for rapid reduction of the transmission field

Abstract

Inductive proximity switch according to the pulse method with a transmitter control (1), a first transmitter coil (6) and a second transmitter coil (7), which are alternately supplied with current pulses to generate an alternating magnetic field, wherein the current pulses are generated via control pulses generated by the transmitter control (1), a receiver unit (2) for receiving an echo signal caused by the alternating field from an electrically conductive target (3), an evaluation unit (4) and a power supply (5), characterized in that the transmitter coils (6, 7) are each connected to a rectifier (8) and a charging capacitor (9), wherein the currently inactive transmitter coil (6, 7) serves to more quickly dissipate the alternating magnetic field of the active transmitter coil (6, 7).

Description

[0001] The invention relates to an inductive proximity switch using the pulse method according to the features of the preamble of patent claim 1.

[0002] Furthermore, a method for operating the inductive proximity switch according to the invention is claimed.

[0003] Inductive proximity switches are contactless electronic switching devices used primarily in automation technology.

[0004] They contain at least one transmitting coil that generates a magnetic field that can be influenced by a metallic trigger. The influence of the metallic trigger on the magnetic field is evaluated, and when a threshold is exceeded, an electronic switching stage is activated.

[0005] Switchgear of this type is manufactured and distributed in a wide variety of designs, including by the applicant.

[0006] Both the control of the transmitting coil and the evaluation of the influence of the metallic trigger can be carried out in different ways.

[0007] In many cases, the transmitting coil is part of an oscillator that is detuned by the metallic trigger. The amplitude and / or frequency change is evaluated.

[0008] In addition to the widespread sinusoidal control of the transmitting coil and the evaluation of frequency and / or amplitude changes, control with short transmit pulses is also known. In this case, the transmitting coil is not part of an oscillator, but rather is subjected to powerful, usually rectangular voltage or current pulses. The echo pulse triggered by the eddy currents induced in the metallic trigger is evaluated. This evaluation can be performed either directly at the transmitting coil or separately at a magnetically coupled receiving coil.

[0009] In this case, the receiving and transmitting coils form a transformer influenced by the eddy currents in the metallic trigger.

[0010] The range of inductive proximity switches is limited. The interaction with a metallic trigger (target) is limited to the near field in a closed oscillating circuit or coil circuit. Therefore, it decreases approximately three to three and a half times the switching distance.

[0011] The short range achievable with conventional measuring principles, as specified, for example, in the IEC / EN 60947-5-2 standard, is considered a disadvantage.

[0012] One way to improve the range is to increase the transmission current. However, this is subject to strict limitations, as the current consumption of the sensors should not exceed 10 mA for devices with three ports and 2 mA for devices with two ports, or is even specified by the aforementioned standard.

[0013] Although increasing the transmit current can facilitate the separation of useful and interference signals, it is not suitable as a sole measure to significantly increase the switching distance.

[0014] Since the opposing current flow of two transmit coils and the evaluation of the differential signal influenced by the target, as described in DE 197 40 774 A1, have proven advantageous, this principle is to be further developed. The aim is to reduce power consumption and increase the range.

[0015] DE 10 2010 041 288 B3 describes an inductive proximity switch in which a demagnetizing coil ensures rapid decay of the magnetic field, and the induced residual energy of the coil is fed back into the system. This document proposes keeping the transmission pulse as short as possible and measuring the transmission current to switch it off when the desired current value is reached. One disadvantage is that an additional coil is required, which can increase the size and complexity of the sensor.

[0016] DE 10 2009 009 061 A1 describes a method and a sensor device, wherein at least two driver coils are alternately supplied with energy in sequence at the rate of a clock circuit.

[0017] The comparatively high energy consumption caused by the constant alternating current supply to the coils is perceived as a disadvantage.

[0018] Another problem is the superposition of the decaying, strong transmit pulse with the relatively weak echo signal from the target, which makes it difficult to evaluate the high-energy front part of the echo signal. Therefore, we are looking for ways to influence the decay behavior of the transmit coil.

[0019] Furthermore, we are looking for ways to reduce the energy consumption of the sensors with regard to the two-wire devices mentioned above.

[0020] The object of the invention is to provide an inductive proximity switch which can be manufactured simply and cost-effectively, which requires little transmission energy, uses the available transmission energy effectively and in which the eddy current echo can be better separated from the switch-off edge of the transmission current.

[0021] This object is achieved according to the features of patent claim 1. The subclaims relate to the advantageous embodiment of the invention.

[0022] The essential idea of the invention is to shorten the transmission pulses while maintaining the clock frequency and to use the pulse generator's known push-pull output stage to more quickly dissipate the alternating magnetic field (demagnetization) of the currently active transmitting coil. A further inventive concept is to recover this demagnetization energy as electrical energy.

[0023] The invention is explained in more detail below with reference to the embodiment shown in the drawing. Fig. 1 shows the basic circuit diagram of the inductive proximity switch according to the invention. Fig. Figure 2 shows the essential components of the inductive proximity switch according to the invention in detail. Fig. 3 shows a design without a separate receiving coil.

[0024] In the Fig. 1 shows the basic circuit diagram of the inductive proximity switch according to the invention.

[0025] The transmitter control 1 generates two asymmetrical control pulses, shifted by 180° from each other, which alternately supply current to the first transmitter coil 6 and the second transmitter coil 7. The resulting alternating magnetic field triggers eddy currents in the target 3, which in turn generate an alternating magnetic field that can be received by the receiver unit 2. The receiver unit 2 has its own coil. As will be shown later, the transmitter coils 6 and 7 can also be used as receiver inductors without departing from the scope of the invention.

[0026] The arrangement represents a high-frequency transformer whose coupling factor is influenced by Target 3.

[0027] The echo signal is sampled by a receiving unit 2 consisting essentially of a sample & hold circuit, amplified by an AC amplifier and fed to an evaluation unit 4 having a microcontroller µC.

[0028] Since the transmitting coils 6 and 7 are wound oppositely, the voltage induced in the second transmitting coil 7 when the first transmitting coil 6 is turned off has a polarity corresponding to the operating voltage, and vice versa. The coils are each connected to ground via a Schottky diode 8 and to a charging capacitor 9, respectively. Thus, the charging capacitor 9 is additionally supplied with current generated from the residual energy of the transmitting coil.

[0029] In the Fig. 2 shows the essential components of an inductive proximity switch according to the invention in detail.

[0030] The clock generator 10, equipped with a 74AC14 Schmitt-trigger inverter, generates a symmetrical square-wave signal with a period length of τ1 = 0.555*(R1*C1). Its output drives the clock input of D-flip-flop 11, which acts as a frequency divider. Its outputs Q and Q drive two additional D-flip-flops, 180° out of phase, serving as pulse controller 12.

[0031] The two N-channel MOSFETs of type BSP295 are switched on alternately via the four Schmitt trigger inverters 13 connected in parallel and the transmitting coils 6 and 7 are supplied with current.

[0032] Due to their inductance, the transmit current increases linearly only with a certain delay. With the help of the shutdown unit 14, which features a current measuring resistor RM and a comparator, the two aforementioned D flip-flops 12 are reset when a certain transmit current is reached. The current measuring resistor RM and the reference voltage Uref determine the maximum transmit current independently of the temperature behavior of the ohmic resistance of the transmit coils. This creates sawtooth-shaped transmit pulses with a defined amplitude and a phase shift of 180°.

[0033] Transistor 15, which is controlled in parallel with the clock input of frequency divider 11 via a differentiator, ensures the suppression of the spike pulses that occur when charging the gate-source capacitance of the aforementioned N-channel MOSFETs. The demagnetization of the transmitting coils 6, 7 and the energy recovery with the Schottky diodes 8 and the charging capacitors 9 were implemented in the Fig. 1 described.

[0034] The sampling time is determined by a controllable delay unit 16. This receives a high / low edge after each transmission pulse via a 74AC32 OR gate. This triggers a monostable multivibrator. Its charging time is controlled by an external pulse-width modulated signal, preferably from a microcontroller, to select the optimal sampling time. Of course, the delay time can be set using a resistor. The width of the sampling window is determined by the short-term pulse generator 17 (monoflop), consisting of a D flip-flop.

[0035] The short-term pulses control the analog switch ADG749 belonging to the sampling unit 18, which samples the signal provided by the OPV1 connected to the receiving coil and operating as a setpoint-actual value comparator and feeds it to the integrator OPV2.

[0036] The target's echo signal appears at its output as a square-wave alternating voltage. With optimal calibration, this signal becomes zero without a target.

[0037] An AC amplifier 19 with measuring range switching is used for level adjustment.

[0038] The Fig. 3 shows a further embodiment with only one transmitting-receiving coil consisting of two halves 6, 7 with grounded center tap.

[0039] The clock generator 10, equipped with a Schmitt-trigger inverter of type 74AC14, generates a symmetrical square wave signal with a period length of τ1 = 0.555∗(R1∗C1).

[0040] Its output drives the D flip-flops used for pulse control 12 directly or via an inverter with a 180° phase shift.

[0041] Their inverting outputs switch on the two PNP output transistors alternately, so that the transmitting coil halves 6 and 7 are supplied with power.

[0042] Due to their inductance, the transmission current increases linearly only with a certain delay. With the help of the shutdown unit 14, which features a current measuring resistor RM and a comparator, the two aforementioned D flip-flops 12 are reset when a certain transmission current is reached. The current measuring resistor RM and the reference voltage Uref determine the maximum transmission current independently of the temperature behavior of the ohmic resistance of the transmission coil 6, 7. This creates sawtooth-shaped transmission pulses with a defined amplitude and a phase shift of 180°.

[0043] The mutual demagnetization of the transmit coil halves 6 and 7, as well as the energy recovery with the Schottky diodes 8 and the charging capacitors 9, has already been described above. However, the two transmit coil halves 6 and 7 are wired in such a way that a negative operating voltage is generated, which can be used to power the operational amplifiers and analog switches (multiplexers). The diodes 8 are each designed as double diodes. If the negative operating voltage, in this case 5V, is exceeded, one of the opposite double diodes opens and diverts the current to the stabilized positive supply voltage. This achieves effective voltage stabilization without additional components.

[0044] The sampling time is determined by a controllable delay unit 16. This receives a high / low edge after each transmission pulse via a 74AC32 OR gate. This triggers a monostable multivibrator. Its charging time is controlled by an external pulse-width modulated signal, preferably from a microcontroller, to select the optimal sampling time. Of course, the delay time can be set using a resistor. The width of the sampling window is determined by the short-term pulse generator 17 (monoflop), consisting of a D flip-flop.

[0045] The short-term pulses control the 74HC4053 analog switch belonging to the sampling unit 18. This sampler samples the signal received by the second transmitting coil 7, which in this case also serves as a receiving coil, and provided by the OPV1, which acts as a setpoint / actual value comparator.

[0046] At the output of the integrator belonging to the scanner 18, the echo signal of the target appears as a rectangular alternating voltage.

[0047] An AC amplifier 19 with measuring range switching is used for level adjustment. The measuring range switching can be performed externally via a 74HC4053 analog switch to prevent overloading of a subsequent analog-to-digital converter.

[0048] The signal generated at the output of the AC amplifier 19 receives an offset corresponding to half the operating voltage via a voltage divider, which is superimposed by twice the amplitude of the amplified sampling signal. The offset also serves to prepare the analog-to-digital conversion.

[0049] Power supply 5 is not explicitly shown in this figure. The respective supply voltages are obvious to a specialist or can be determined from the labeling.

[0050] The inductive proximity switch according to the invention operates according to the pulse method, whereby two transmitting coils or transmitting coil halves 6, 7 are alternately supplied with current pulses to generate an alternating magnetic field.

[0051] These current pulses are generated by a transmitter controller 1. The current intensity is measured. When a preset value is reached, the transmitter output stage is switched off, producing sawtooth pulses with a constant amplitude.

[0052] A receiving unit 2 is used to receive the echo signal generated by an electrically conductive target 3 through the alternating electromagnetic field. An evaluation unit 4 is used to determine the presence of the target 3 based on the received echo signal and to generate a binary switching signal.

[0053] The transmitting coils, or transmitting coil halves 6, 7, are connected to a rectifier 8 and a charging capacitor 9 in such a way that the currently inactive transmitting coil 6, 7 contributes to the faster reduction of the alternating magnetic field (demagnetization) of the active transmitting coil 6, 7, and the residual energy thus obtained from the transmitting coil can be returned to the proximity switch's power supply 5 as electrical energy. This allows the transmitting pulse to decay more quickly, and the measurement of the echo signal evoked by the target 3 can begin at an earlier point in time.

[0054] In an advantageous embodiment, this electrical energy is used to generate a negative supply voltage for the operational amplifiers. Providing a negative supply voltage for the operational amplifiers improves their controllability without the use of additional materials, e.g., through a DC / DC converter.

[0055] If the negative operating voltage is exceeded, the current is diverted via a diode to the stabilized positive supply voltage. This achieves effective voltage stabilization without the need for additional components.

[0056] Furthermore, a method for operating an inductive proximity switch is claimed, in which the two transmitting coils 6, 7 are alternately supplied with energy and the inactive transmitting coil is used for faster reduction of the alternating magnetic field of the active transmitting coil (demagnetization).

[0057] The method according to the invention not only uses the transmission energy more efficiently, but also improves the measurement accuracy, since the echo signal caused by the target can be recorded over a longer period of time. List of abbreviations 1 transmitter control 2 receiving unit 3 Target 4 Evaluation unit 5 Power supply 6 First transmitting coil or transmitting coil half 7 Second transmitting coil, or transmitting coil half 8 rectifiers for energy recovery (Schottky diodes) 9 Charging capacitor 10 clocks 11 frequency dividers 12 Pulse control 13 Schmitt trigger inverters 14 Shutdown unit 15 transistor for needle pulse suppression 16 Delay unit 17 Short-term pulse generator 18 scanning unit 19 AC amplifiers

Claims

[1] Inductive proximity switch according to the pulse method with a transmitter control (1), a first transmitter coil (6) and a second transmitter coil (7), which are alternately supplied with current pulses to generate an alternating magnetic field, wherein the current pulses are generated via control pulses generated by the transmitter control (1), a receiver unit (2) for receiving an echo signal caused by the alternating field from an electrically conductive target (3), an evaluation unit (4) and a power supply (5), characterized by that the transmitting coils (6,7) are each connected to a rectifier (8) and a charging capacitor (9), wherein the currently inactive transmitting coil (6,7) serves to more quickly reduce the alternating magnetic field of the active transmitting coil (6,7). [2] Inductive proximity switch according to claim 1, characterized bythat the charging capacitors (9) are connected to the power supply (5) and the energy obtained when the alternating magnetic field of the active transmitting coil (6,7) is reduced is fed to the power supply (5). [3] Method for operating an inductive proximity switch according to one of the preceding claims, characterized by that the two transmitting coils (6, 7) are alternately supplied with energy and the inactive transmitting coil (6, 7) is used to more quickly reduce the alternating magnetic field of the active transmitting coil (6, 7).

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