Inductive proximity switch with increased temperature stability due to phase-insensitive evaluation of the received signal

Abstract

Inductive proximity switch with an oscillator (1) and a transmitting coil (2) for generating an alternating magnetic field that can be influenced by a target (11), a receiving coil pair (3) that is transformer-coupled to the transmitting coil (2) for evaluating the alternating magnetic field that can be influenced by the target (11), a balancing winding (4) for balancing the differential signal of the receiving coil pair (3), and a control unit (5) for outputting a binary switching signal, characterized in that the receiving coil pair (3) is connected to a biased differential rectifier (6) for phase-insensitive rectification, and a differential integrator (7) is provided for comparing the received signals of the receiving coil pair (3), the binary switching signal being the result of the evaluation of the transformer-type coupling factor between the transmitting coil (2) and the receiving coil pair (3).

Description

[0001] The invention relates to a contactless inductive proximity switch according to the preamble of patent claim 1.

[0002] Inductive proximity switches are non-contact electronic switching devices used primarily in automation technology. Inductive proximity switches operating according to the transformer principle have long been known and are produced in large quantities. They are available with both fixed and adjustable switching distances.

[0003] They contain a transmitting coil that generates an electromagnetic magnetic field that can be influenced by a metallic trigger. The influence of the metallic trigger on the magnetic field is evaluated and a switching stage is activated. Such switching devices are manufactured and distributed in a wide variety of designs, including by the applicant.

[0004] The operating principle consists of at least one transmitting and one receiving coil. The two coils are inductively coupled. The measured value is the transformer coupling factor between these two coils. If conductive objects, also known as triggers or control vanes, come close to the transmitting coil, eddy currents are induced, which drain energy from the system. Depending on the specific design, the switching signal is triggered when the amplitude across the oscillator coil or the receiving coil reaches or falls below a certain value. To evaluate the amplitude, the high-frequency signal is rectified, smoothed, and then fed to a comparator.

[0005] Both the control of the transmitting coil and the evaluation of the influence of the metallic trigger can be carried out in different ways.

[0006] In many cases, the transmitting coil is part of an oscillator influenced by the metallic trigger, whose amplitude and / or frequency change is evaluated. However, the interaction with the metallic trigger (target) is limited to the near field. It decreases approximately three times the switching distance. In order to be able to detect even slight interactions with the metallic trigger, it is advantageous to compensate for the signal in the unaffected state and only evaluate the changes caused by the trigger. For this purpose, two receiving coils are preferably operated in a differential circuit. The design is selected so that one of the two coils is more strongly influenced by the target than the other. By zeroing the signal in the unaffected state, an extremely sensitive arrangement is obtained, which is also referred to as a differential transformer (LVDT = linear variable differential transformer).The differential transformer is calibrated so that the signals from the two receiving coils cancel each other out in the unaffected state. The better this calibration, the higher the sensor signal can be amplified without overloading. Since the magnetic field decreases very rapidly with increasing distance, one soon reaches regions where the temperature response, particularly of the copper windings, but also of the other materials and components involved, causes effects on the order of magnitude of the expected sensor signal. Therefore, longer switching distances can only be achieved if the temperature dependence of the arrangement can be compensated over the operating temperature range. Since this balance can be disturbed during production or by the installation situation, subsequent calibration of the differential transformer is desirable, both at the factory and later during operation.

[0007] To increase sensitivity while simultaneously suppressing unwanted influences, DE4031252C1 proposes operating two receiving coils in a direct differential circuit. The two receiving coils are located in the feedback branch of a Meissner oscillator. The oscillator amplitude is evaluated. The switching distance is achieved when the differential alternating voltages of the two coils cancel each other out due to interaction with a metallic trigger. In this case, the oscillator changes its oscillation state abruptly. The arrangement is very sensitive. Although the tolerances of the two receiving coils have a significant influence on the switching distance, no subsequent adjustment is provided.

[0008] DE10003913A1 shows a differential transformer with two receiving coils connected in series in opposite directions. One of the two receiving coils faces the metallic trigger, and the other faces away from the metallic trigger. The differential signal of the two receiving coils is evaluated. To facilitate the adjustment of the coupling factors between the transmitting and receiving coils, two coil pairs, i.e., one transmitting and one receiving coil, can be coupled by transformer. Ideally, only one of the two coil pairs is inductively coupled to the metallic trigger. This compensates not only for the transmitted pulses but also for interfering interference fields. A disadvantage is that not only the amplitude but also the phase shift between the two receiving coils is evaluated. This can be caused by external interference fields, but also by moisture or temperature fluctuations in the coil conductors.These effects can lead to a reduction in sensor sensitivity and a deterioration in noise immunity.

[0009] DE102007014343A1 proposes connecting a receiver coil to trimmable resistors, which are used to adjust the switching distance. These are controlled by an evaluation unit (microcontroller), which is also connected to a temperature sensor, allowing temperature compensation.

[0010] A disadvantage is that temperature compensation involves a relatively lengthy process, from the measurement by the temperature sensor, the calculation of the correction values ​​in the microcontroller, the subsequent digital output of the correction values, and the digital-to-analog conversion to the control of the trimmable resistors. Linearization from, for example, -25°C to +70°C appears problematic given this long chain.

[0011] WO 2007 / 012502 A1 proposes a control loop with a controlled system encompassing the sensor. The controlled variable includes, among other things, the amplitude of the sensor circuit. Control is performed on the transmitter side by keeping the amplitude constant, counter to the "disturbance" caused by the damping.

[0012] The energy required to regulate the amplitude is supplied via an adjustable resistor. The control variable for this resistor is output as the instantaneous damping value and thus as a measured value.

[0013] The disadvantage here is that the adjustable resistor, preferably a transistor, has both a temperature response and a non-linear characteristic.

[0014] In PCT / EP2011 / 067794 (published as WO 2012 / 049205 A1), the manipulated variable, i.e., the resistance, is measured in the form of its control voltage instead of the temperature-dependent control variable. The measurement is performed either analogically or digitally as a pulse width ratio.

[0015] This allows the temperature response of the controlled system to be acceptable because the manipulated variable, in the form of the damping resistor, is measured directly at the actuator. However, it has been shown that phase measurement, while very sensitive, is also susceptible to interference. Therefore, a solution is being sought that avoids this disadvantage—that is, one that is just as sensitive but less susceptible to interference.

[0016] DE 10 2010 042 512 A1 describes a circuit arrangement in which a control loop with a relaxation oscillator and a Schmitt trigger is used to generate a pulse-width modulated signal to measure the attenuation through a target. However, this method can be susceptible to interference that affects the duty cycle of the PWM signal.

[0017] DE 21 21 889 A discloses a circuit arrangement in which a movable shielding plate is used to vary the mutual inductance between coils. However, this mechanical component can impair the reliability and robustness of the arrangement.

[0018] DD 2 24 738 A1 shows an electrical switch for detecting misalignment in conveyor belts, based on a differential transformer with an H-shaped core. However, this solution is highly tailored to this specific application and offers little flexibility for other applications.

[0019] The object of the invention is to further develop the known state of the art. The aim is to improve the temperature response of the differential transformer, increase the stability of the circuit, and reduce its susceptibility to interference.

[0020] This object is achieved according to the characterizing part of patent claim 1. The subclaims relate to the advantageous embodiment of the invention.

[0021] The essential idea of ​​the invention is to dispense with the phase information and only evaluate the amplitudes of the receiving coil pair. According to the invention, the evaluation is carried out based on the difference between their absolute values ​​by connecting the two receiving coils to a differential rectifier without taking their winding sense into account and a phase-insensitive, i.e. phase-neutral rectification of the received signals of the receiving coil pair takes place. This also avoids, among other things, a source of error in manufacturing. In a first advantageous embodiment, the diodes are pre-currented in order to overcome their threshold voltage and thus generate a low differential resistance. In a further advantageous embodiment, the difference signal serves in a manner known per se as a manipulated variable for zeroing in a control loop. The invention is explained in more detail with reference to the drawing.

[0022] The invention relates to an inductive proximity switch with an oscillator and a transmitting coil for generating an alternating magnetic field influenced by a target, a pair of receiving coils coupled to the transmitting coil by a transformer for evaluating the alternating magnetic field influenced by the target, and a tuning winding for tuning the differential signal of the pair of receiving coils. A control unit generates a binary switching signal. The pair of receiving coils is connected to a biased differential rectifier for phase-insensitive rectification. A differential integrator compares the signals from the two receiving coils. The binary switching signal is generated by evaluating the transformer coupling factor between the pair of receiving coils and the transmitting coil.

[0023] In a first advantageous embodiment, the biased differential rectifier is connected to a chopper amplifier for amplifying the DC voltage signal generated at the differential rectifier.

[0024] In a second embodiment, the biased differential rectifier and the differential integrator are components of a control loop for zeroing the differential signal of the receiving coil pair. However, the control loop includes a current-to-voltage converter for generating an output voltage.

[0025] In a third embodiment, the biased differential rectifier and the differential integrator are also components of a control loop for zeroing the differential signal of the receiving coil pair. However, the control loop includes a constant current source for determining the effective AC resistance of the tuning winding through a voltage measurement.

[0026] In a fourth embodiment, the control loop for zeroing the difference signal of the receiving coil pair has a trigger for generating a pulse width modulated signal.

[0027] They show: Fig. 1: The most important components of a proximity switch according to the invention, Fig. 2: The controlled zero adjustment and evaluation with a voltage divider, Fig. 3: The controlled zero adjustment and evaluation with current-voltage converter, Fig. 4: The controlled zero adjustment and evaluation with a current source, Fig. 5: The proximity switch according to the invention with a chopper amplifier, Fig. 6: The inductive proximity switch with PWM output according to the invention.

[0028] The Fig. Figure 1 shows the essential components of an inductive proximity switch according to the invention. It shows a differential transformer (LVDT) with a transmitting coil 2, a pair of receiving coils 3, and a tuning winding 4 with a tuning resistor Ra, as well as a target 11 symbolically represented as a coupling winding with a load resistor. The oscillator 1, equipped with the integrated circuit 74HCU04, 74AC04 / 74HC04, generates a sine signal in the frequency range 50 kHz to 1 MHz with an amplitude of a few volts at the transmitting coil 2. The differential transformer is arranged in the housing such that one of the two receiving coils 3 can be more strongly influenced by the target 11 than the receiving coil located on the side of the differential transformer facing away from the target 11 and inside the housing, which is more strongly coupled to the tuning winding 4.The differential transformer is adjusted so that the signals of the two receiving coils 3 almost cancel each other out in the unaffected state, but the inward-facing receiving coil 3 still delivers a slightly higher signal.

[0029] Transmitting coil 2 also acts as a frequency-determining inductance. According to Thomson's formula, the frequency is determined from this inductance and the two capacitors connected in series.

[0030] The receiving coil pair 3 and the tuning winding 4 can also be located on the same coil core or ferromagnetic carrier. The target 11 is symbolically represented by a winding and the load resistance Rx. It should be noted here that the invention can be implemented with two separate transformers, but also without a coil core using magnetically coupled air-core coils.

[0031] The transmitting coil 2 forms the primary coil and the receiving coil pair 3 form the secondary coils of a differential transformer (LVDT), wherein the coupling factors of the primary coil to the first secondary coil can be influenced by the target 11 and to the second secondary coil by the tuning winding 4.

[0032] The receiving coil pair 3 is connected at one end to a reference voltage of 1.5 volts. This overcomes the threshold voltage of the two rectifiers 6. The base resistors, designated RL, have the same value. They determine the quiescent current (bias current) of the diodes, the input resistance of the differential rectifier 6, and thus the load on the differential transformer. For pure voltage measurements, they should be between 100 kOhm and 1 MOhm. Instead of the load resistors RL, constant current sources with a theoretically infinite input resistance can be advantageously used. For better thermal coupling, their active semiconductor components should be housed in a single package (double transistor or operational amplifier).

[0033] The capacitors, also located at the base of the diodes, ensure integration and smoothing, thus ensuring phase-independent evaluation of the received signal. This reduces the circuit's temperature drift and makes it less sensitive to humidity.

[0034] The resulting DC voltage signal is first fed to a differential amplifier and then to the differential integrator 7. Its output is connected to the microcontroller µC belonging to the control unit 5. This is where the analog-to-digital conversion takes place. A pulse-width modulated signal for offset and drift correction can be output via pin Ro to the operational amplifier designated OPV. For this purpose, the pulse-width modulated (PWM) signal is converted into a DC voltage by the low-pass filter TP.

[0035] The microcontroller can generate a binary switching signal, output an analog measured value to a 4-20 mA current interface, or forward a digital value to a higher-level control unit via a bus interface. According to the invention, the phase evaluation of the differential signal is omitted because the phase-insensitive evaluation of the coupling factors of the differential transformer described above exhibits significantly lower temperature drift. The cause is apparently phase shifts between the transmitting winding and the two receiving windings, which are caused by the load during voltage measurement, but also by capacitance changes in the coils caused, for example, by moisture. These influences are significantly reduced by the present invention.

[0036] Differential rectifier 6 is equipped with Schottky diodes. They are housed in a common housing and are biased with the same current and loaded with similar resistors RL. Thus, only the signal amplitudes of the receiving coil pair are compared, which facilitates calibration during production.

[0037] The Fig. Figure 2 shows the inductive proximity switch according to the invention with a control loop for controlling the received signal. Control is advantageously carried out to zero, i.e., the load on the differential transformer caused by the target 11 is compensated by the adjustment winding 4, and the controlled variable is output as a measured value.

[0038] The effective trimming resistor Ra, and thus also the load through the target 11, is measured at a voltage divider consisting of the series resistor Rb and the desired value Ra, where the output voltage Uout is: Uout = Ub*Ra / (Ra+Rb). For this purpose, the trimming resistor Ra is replaced by a transistor, in this case an N-channel MOSFET. Control is achieved via the biased differential rectifier 6 and the differential integrator 7.

[0039] The Fig. 3 shows a similar, but significantly more sensitive circuit than Fig. 2. The current flowing through the tuning winding is fed to a current-to-voltage converter 9 for signal amplification. The resistor Rb in the negative feedback path of the operational amplifier determines the relationship between input current and output voltage. Uout = Uref*(Rb / Ra+1).

[0040] The output voltage Uout is fed to a microcontroller (not shown here) belonging to control unit 5. Here, the signal is either digitized and / or compared with a comparator threshold.

[0041] The Fig. Figure 4 shows the inductive proximity switch according to the invention, also with a control loop. However, the transistor acting as the actuator is supplied by a current source 10. Thus, the output voltage Uout is a measure of the effective resistance of the balancing winding 4 and is fed to the control unit 5. This circuit is also more sensitive than the one shown in Fig. 2, because the current source generates more voltage across Ra than the series resistor Rb. The following applies: Uout=Iconst *Ra.

[0042] The Fig. Figure 5 shows an inductive proximity switch according to the invention with a chopper amplifier 8 instead of the Fig. 1. The clock signal is generated as a TTL signal by the 74HCU04 oscillator gate. The advantage of this arrangement is that it completely avoids the temperature-related drift that usually occurs in DC amplifiers. For this purpose, the signal is first converted (chopped) into an AC voltage, amplified as such, and then converted back into a DC voltage. Such amplifiers are offered as integrated circuits under the designation "zero-drift OP-AMP" (OPA2335, ADA4528, LTC2054). These are usually equipped with an internal generator and operate using spread spectrum technology. Of course, this circuit or the components mentioned can be combined with the Fig. 2, Fig. 3, and Fig. 4 circuits shown can be combined.

[0043] The Fig.Figure 6 shows an inductive proximity switch according to the invention with a 74HC14 Schmitt trigger in the control loop. Due to the trigger's hysteresis, the MOSFET acts as a switch. It is either blocked or saturated. This creates a pulse-width modulated output signal whose duty cycle represents the measured variable. This eliminates the need for analog-to-digital conversion when reading the signal into a microcontroller. List of reference symbols 1 oscillator, high-frequency generator 2 transmitting coil 3 pairs of receiving coils 4 Adjustment winding 5 Control unit 6 Pre-current differential rectifier 7 Differential integrator 8 chopper amplifiers 9 current-voltage converters 10 Constant current source 11 Target

Claims

[1] Inductive proximity switch with an oscillator (1) and a transmitting coil (2) for generating an alternating magnetic field which can be influenced by a target (11), a pair of receiving coils (3) which is coupled to the transmitting coil (2) by a transformer for evaluating the alternating magnetic field which can be influenced by the target (11), a balancing winding (4) for balancing the difference signal of the pair of receiving coils (3) and a control unit (5) for outputting a binary switching signal, characterized by that the receiving coil pair (3) is connected to a biased differential rectifier (6) for phase-insensitive rectification and a differential integrator (7) is provided for comparing the received signals of the receiving coil pair (3), wherein the binary switching signal is the result of the evaluation of the transformer coupling factor between the transmitting coil (2) and the receiving coil pair (3). [2] Inductive proximity switch according to claim 1, characterized bythat the biased differential rectifier (6) is connected to a chopper amplifier (8). [3] Inductive proximity switch according to claim 1, characterized by that the adjustment winding (4), the biased differential rectifier (6) and the differential integrator (7) are components of a control loop for zeroing the differential signal of the receiving coil pair (3) and the control loop has a current-voltage converter (9) for generating an output voltage. [4] Inductive proximity switch according to claim 1, characterized by that the balancing winding (4), the biased differential rectifier (6) and the differential integrator (7) are components of a control loop for zeroing the differential signal of the receiving coil pair (3) and the control loop has a constant current source (10) for determining the effective AC resistance of the balancing winding (4) by means of a voltage measurement. [5] Inductive proximity switch according to claim 3 or 4, characterized by that the control loop for zeroing the difference signal of the receiving coil pair (3) has a trigger (12) for generating a pulse width modulated signal.

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