Inductive proximity switch with reduced temperature dependence

Abstract

Inductive proximity switch (1) with a transmitting coil (2), two anti-serially connected receiving coils (3) for generating a receiving signal and a balancing coil (4) which is coupled to one of the receiving coils (3) in a transformer manner, the balancing coil (4) being connected to a controllable resistor (10), characterized in that the controllable resistor (10) consists of a switch and a resistor and is part of a control loop which has a Schmitt trigger (9) which converts the analog control signal into a pulse-width modulated signal with which the switch is controlled and the receiving signal is controlled to zero on average.

Description

[0001] The invention relates to an inductive proximity switch according to the features of the preamble of patent claim 1 and the method according to claim 2. Inductive proximity switches are used as contactless electronic switching devices, primarily in automation technology.

[0002] 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.

[0003] Switchgear of this type is manufactured and distributed in a wide variety of designs, including by the applicant.

[0004] Both the control of the transmitting coil and the evaluation of the influence of the metallic trigger can be carried out in different ways.

[0005] In many cases, the transmitting coil is part of an oscillator influenced by the metallic trigger, whose amplitude and / or frequency change is evaluated.

[0006] In addition to the widely used sinusoidal control of the transmitting coil and the evaluation of frequency and / or amplitude changes, control with a short rectangular pulse is also known. In this case, the transmitting coil is not part of an oscillator, but rather is subjected to strong voltage or current pulses. The echo triggered by the eddy currents induced in the metallic trigger is evaluated. This evaluation can be performed either directly at the transmitting coil or at a magnetically coupled receiving coil.

[0007] Since this is a closed oscillating circuit, or rather a closed coil circuit, only a small amount of energy is radiated. The interaction with the metallic trigger (target) is limited to the near field. It decreases approximately 3.5 times the switching distance.

[0008] In order to be able to detect small interactions with the metallic trigger, it is advantageous to compensate the signal in the unaffected state and to evaluate only the changes caused by the trigger.

[0009] For this purpose, two receiving coils are preferably operated in a differential circuit. The configuration is chosen so that one of the two coils is more strongly influenced by the target than the other.

[0010] By zeroing in the unaffected state, an extremely sensitive arrangement is obtained, which is also called a differential transformer (LVDT = linear variable differential transformer).

[0011] The differential transformer is adjusted so that the signals from the two receiving coils cancel each other out in the unaffected state.

[0012] The better this adjustment is, the higher the sensor signal can be amplified without overloading. Since the magnetic field decreases very rapidly with increasing distance, one soon reaches ranges where the temperature response, especially 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.

[0013] Since this balance can be disturbed during production or by the installation situation, subsequent adjustment of the differential transformer is desirable both at the factory and during later operation.

[0014] DE 10 2007 014 343 A1 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.

[0015] 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.

[0016] WO 2007 / 012502 A1 proposes a control loop with a controlled system including the sensor. The controlled variable includes, among other things, the amplitude of the sensor circuit.

[0017] The control is carried out on the transmitter side, ie the amplitude is kept constant contrary to the damping disturbance.

[0018] The energy required to regulate the amplitude is supplied via an adjustable resistor. The control variable for this resistor is output as a variable for the instantaneous damping and thus as a measured variable.

[0019] The disadvantage here is that the adjustable resistor, preferably a transistor, has both a temperature response and a non-linear characteristic.

[0020] EP 0 058 076 A2 discloses a metal detector device that uses an oscillator coil and detector coils with compensation signals and phase-sensitive detectors to correct drifts and maintain a balanced state. While such an approach is effective for metal detection, it can be complex, potentially resulting in slower response times and a higher component count. Therefore, it is desirable to provide a simpler and more efficient method that reduces complexity and improves responsiveness.

[0021] EP 0 304 272 A2 discloses an inductive proximity switch in which a measuring coil and a reference coil are connected in series. However, such designs can be sensitive to imbalances between the coils, which can reduce sensitivity and accuracy. Therefore, it is advantageous to provide an inductive proximity sensor that minimizes the effects of coil imbalances.

[0022] DE 10 2010 002 201 A1 discloses an inductive proximity switch in which the control circuit and the receiving circuit are decoupled to reduce unwanted interactions. However, since the control current of the control transistor flows through the receiving coil, temperature dependence can occur. Therefore, it is advantageous to provide an inductive proximity switch that reduces the temperature dependence.

[0023] The object of the invention is to further improve this method. The characteristic curve is to be linearized and the temperature dependence reduced.

[0024] This problem is solved according to the features of patent claims 1 and 2.

[0025] The key idea of the invention is to design the actuator as a switch with a defined internal resistance, which is closed for specific time intervals and then opened again. The resulting pulse-width-modulated switching signal can be directly output as a measured value. In this case, the sensor signal is not permanently zero, but fluctuates around the zero point.

[0026] The key advantage is that the temperature response of the controlled system can be accepted because the manipulated variable, in the form of the duty cycle of a known damping resistor, is measured directly at the actuator. This signal, generated directly at the sensor, is virtually impossible to distort by further processing.

[0027] The invention is explained in more detail below with reference to the drawing.

[0028] They show: Fig. 1 Schematic diagram of an inductive proximity switch according to the invention. Fig. 2 Example with an XOR gate in the actuator, Fig. 3 Example of an embodiment with a multiplexer in the actuator.

[0029] The Fig. 1 shows an inductive proximity switch 1 with a transmitting coil 2, two receiving coils 3, a calibration coil 4 and another winding 5 symbolizing the switching flag or target. The transmitting coil and the two receiving coils form a differential transformer (LVDT).

[0030] The transmitting coil 2 is powered by a high-frequency generator 6. The two anti-serially connected receiving coils 3 are connected to the synchronous rectifier 7. The rectified signal passes through the low-pass filter 8 and controls the Schmitt trigger 9, which switches the controllable resistor 10, consisting of a switch and a resistor Re, on or off.

[0031] The switch remains closed until the signal at synchronous rectifier 7 disappears. After the switch is opened, the signal rises again until the switch is closed again.

[0032] This creates a control loop that regulates the mean value of the sensor signal to zero, thereby generating a pulse width modulated (PWM) signal.

[0033] With optimal calibration, the switch remains continuously open without a target, and in a heavily damped state, it remains virtually closed. The closing time or duty cycle is thus a measure of the damping by the target and can therefore easily be output as a pulse-width-modulated measurement variable. By changing the resistance Re, the measuring range can be switched. Finally, it should be noted that a setup according to the invention can also be implemented with two transmit coils and one receive coil.

[0034] The Fig. Figure 2 shows a detailed example using XOR gates and a junction FET as a switch. Here, a control loop oscillates between overcompensation and undercompensation as a relaxation oscillator. As the damping increases, transistor T1, which acts as a switch, remains closed for a correspondingly longer period. The changing duty cycle serves as the output signal.

[0035] The oscillator equipped with the exclusive-OR gate XOR1 (1 / 4 of the 74HC86) delivers a sine signal whose frequency is determined by the inductance LS and the series-connected capacitors C1 and C2.

[0036] The following relationship applies to the frequency f: f = 1 / (2□□□√□LS □ (C1 □C2) / (C1+C2)). The transmitting coil LS is connected to the two receiving coils L1 and L2 via the transformer coupling factors M1 and M2. They form a differential transformer and are adjusted so that, in the unloaded state, the voltages generated in the receiving windings L1 and L2 are equal.

[0037] The center tap of the two receiving coils is at half the operating voltage. The other ends of the two anti-serially connected receiving coils are connected to the inputs of the operational amplifier OV1. Due to the lack of negative feedback, this amplifier functions as a comparator and pulse shaper.

[0038] The comparator OV2 delivers a square wave that is phase-synchronized to the oscillator. The gate XOR2 acts as a phase comparator. Unequal input levels result in a high signal at the output, and equal input levels result in a low signal. A phase shift of 90° results in a square wave with twice the frequency.

[0039] Gate XOR2 receives both the oscillator signal and the receive signal. When these signals are phase-synchronized, the output of gate XOR2 goes low. When they are shifted by 180°, it goes high. The receive coils L1 and L2 are connected so that, when the attenuator is in the attenuated state, a high signal appears at the output of comparator OV1.

[0040] OV3 acts as a differential integrator. It integrates the incoming pulses until the switching threshold of the subsequent Schmitt trigger XOR3 is reached.

[0041] With weak damping this takes longer than with strong damping.

[0042] When the switching threshold of trigger XOR3 is reached, it opens the N-channel J-FET T1 of type U310, thus ensuring strong attenuation of the receiving coil L2. The resulting overcompensation produces a phase shift at the input of OV1. Since the oscillator signal and the received signal are now in phase, an L appears at the output of XOR2. The differential integrator is discharged, and the Schmitt trigger XOR3 goes to L. Transistor T1 then blocks, and the charging process begins again. As already mentioned above, the control loop oscillates as a relaxation oscillator. A sawtooth voltage is generated at the depletion-mode FET T1, the frequency of which is determined by the RC element on the integrator OV3 and the hysteresis of the Schmitt trigger XOR3. The duty cycle is a measure of the attenuation. Assuming L1 = L2, transistor T1 is always conductive until the product of its duty cycle with the resistance Re is equal to the symbolic damping resistance Rx.

[0043] The damping by the target, represented by resistor Rx, is only compensated for on average. The control signal for transistor T1 can be output as a pulse-width-modulated square wave.

[0044] The Fig. Figure 3 shows an example using analog multiplexers of the type 74HC4053. The sinusoidal transmit signal is received by the two-channel analog multiplexer MUX1. The above formula applies to the frequency f.

[0045] The square wave signal at the multiplexer output controls the multiplexer MUX2, which, in conjunction with the differential integrator OV2, acts as a phase-sensitive rectifier. The conditions at the differential transformer correspond to those in the Fig. 2. The operational amplifier OV1 works as a comparator.

[0046] As soon as the equilibrium is disturbed by a reduction in the resistance Rx connected to the symbolic target winding 5, the output of the differential integrator OV2 produces a DC voltage. This is fed to the Schmitt trigger OV3, which switches the multiplexer MUX3 when its switching threshold is exceeded. If the output of MUX3 is connected to ground, energy is removed from the differential transformer via the resistor Re, as described above.

[0047] When the attenuation symbolically represented here by Rx is compensated by the target, the differential integrator OV2 no longer supplies a signal, the Schmitt trigger OV3 falls back and the multiplexer MUX3 opens again.

[0048] Therefore, the control is not continuous here either, but rather either overcompensated or undercompensated. The balanced state is only achieved on average. The control signal for the multiplexer MUX3 is also output as a pulse-width modulated square wave. Since the three required multiplexers only require one 74HC4053, they are absolutely identical and always at the same temperature. List of reference symbols 1 inductive proximity switch 2 transmitting coil 3 receiving coils 4 Adjustment coil, adjustment winding 5 Target winding, symbolizes the target 6 high-frequency generator 7 synchronous rectifiers 8 low-pass filter 9 Schmitt triggers 10 Controllable resistance (resistor + switch)

Claims

[1] Inductive proximity switch (1) with a transmitting coil (2), two anti-serially connected receiving coils (3) for generating a receiving signal and a balancing coil (4) which is transformer-coupled to one of the receiving coils (3), wherein the balancing coil (4) is connected to a controllable resistor (10), characterized by that the controllable resistor (10) consists of a switch and a resistor and is part of a control loop which has a Schmitt trigger (9) which converts the analog control signal into a pulse-width modulated signal with which the switch is controlled and the received signal is controlled on average to zero. [2] Method for operating an inductive proximity switch according to claim 1 characterized bythat the controllable resistor (10) is periodically switched on and off via a Schmitt trigger (9) and the received signal is thus regulated to zero on average and the pulse width ratio is output as a measuring signal.

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