Patent Publication Number: US-2023139738-A1

Title: Proximity switch and method for operating a proximity switch

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims priority under 35 U.S.C. § 119 to German patent application 10 2021 128 706.5, filed Nov. 4, 2021, the disclosure of which is incorporated by reference herein in its entirety. 
    
    
     TECHNICAL FIELD 
     This disclosure relates to a proximity switch and a method for operating a proximity switch. 
     DESCRIPTION OF RELATED ART 
     Proximity switches, in particular inductive proximity switches, are extensively known in the state of the art. 
     Inductive proximity switches are known in the state of the art, thus for example DE 44 29 314 B4 describes such a proximity switch in which an alternating magnetic field is generated in a coil, with the result that a metal object entering said field influences the oscillation state. The detected change in the oscillation state is evaluated by a circuit, wherein two sensor coils are arranged in the alternating field in a direct differential connection to detect the induced voltages. In the process, the differential voltage of the sensor coils becomes zero in the sensing zone. In the solution according to DE 44 29 314 B4, the wound coils are arranged in three planes or in three corridors. A disadvantage of this solution is that the wound coils have a tendency to age, with the result that the switching distance drifts over time. In particular, the maximum switching distance achievable using wound coils is highly limited. DE 10 2006 053 023 A1 shows an alternative embodiment. A disadvantage of these known solutions is that temperature influences lead to drift or hysteresis. 
     To compensate for the temperature influences, U.S. Pat. No. 8,432,169 B2 therefore proposes providing a current mirror circuit having equilibrium establishment via a trimmer, wherein temperature influences are subsequently taken into account and corrected in a data-based correction. This temperature compensation is highly error-prone and is thus disadvantageous in the case of sensitive detection situations or very high temperature fluctuations. 
     Furthermore, it is known from DE 10 2005 001692 A1 to use a controllable network and to control a transmission function of the controllable network by means of a transconductance amplifier and to control the transmission function of the controllable network depending on the measured temperature by varying the gain of the transconductance amplifier, with the result that temperature-induced variations in the oscillator are compensated for at least in part. Using a controllable differential amplifier as transconductance amplifier is proposed. The solutions are structurally very complex. 
     SUMMARY OF DISCLOSED EMBODIMENTS 
     An object of the disclosed embodiments is to propose an improved proximity switch and an associated method, with which the detection characteristic of a proximity switch in the case of temperature influences is improved. 
     According to the disclosed embodiments, this object is achieved by a proximity sensor and a method according to features described herein. 
     Accordingly, the object is achieved by a proximity switch which is formed in particular as an inductive proximity switch and has a defined detection range. The proximity switch comprises
         an oscillator, which generates an alternating magnetic field, in particular in a self-energized manner, and changes its oscillation state as a result of a target entering the detection range,   at least one oscillator amplifier, and   at least one temperature sensor provided for detecting the temperature of an element of the proximity switch, such as a surface temperature, and/or the ambient temperature, in particular for detecting the temperature of the oscillator amplifier and/or of a coil, wherein       

     the oscillation amplifier is formed so as to be controllable in an open-loop and closed-loop manner and has at least one amplifier stage, ideally two amplifier stages. 
     Here, by coil is meant the transmission coil and/or at least one receiver coil of the oscillator. 
     Here, the oscillator can be controlled in an open-loop and/or closed-loop manner on the basis of the temperature determined by the temperature sensor, with the result that temperature-induced influences and variations in the oscillator properties can be compensated for at least in part or in full. Also comprised is a microprocessor, which is connected to a storage medium, for controlling the gain of the oscillator amplifier. Compensation data are or can be stored on the storage medium and can be electronically detected and processed by the microprocessor. 
     Here, compensation data or compensation values mean any temperature-dependent (digital or analogue) compensation and correction values, characteristic curves, algorithms and/or dependency data that describe a functional relationship between the proximity sensor, the oscillator, individual coils or components of the oscillator depending on the temperature and associated control data for compensation. The compensation data can be ascertained and/or supplemented in the laboratory or during ongoing operation of the proximity sensor. 
     At times, the terms “compensation data”, “compensation values” and/or “control data” will be used synonymously herein. 
     The voltage and/or data transmission can be effected on the known systems such as Ethernet, Single Pair Ethernet (SPE), IO Link, PoDL or any other suitable communication technology and/or protocol. 
     For the temperature-dependent open-loop and/or closed-loop control, in the present case at least one amplifier stage of the oscillation amplifier has a controllable temperature compensation circuit which is formed to influence the oscillation behaviour of the oscillator, in particular to compensate for temperature influences and prevent hysteresis, on the basis of a data receipt (compensation value) from the microprocessor and/or from the storage medium and depending on the temperature ascertained by the temperature sensor. 
     Here, it is crucial that the oscillation behaviour of the oscillator is influenced directly via the control of the oscillation amplifier, in particular by controlled negative current feedback taking place. 
     The amplifier stages have suitable resistors for adjusting the operating point, and at least one resistor for the negative current feedback, for the thermal operating-point stabilization of the amplifier stage. This resistor has an influence in particular on the current flow through the transistor and hereby also an influence on the amplitude of the measurement signal. The temperature-dependent compensation of the circuit components takes place via the emitter branch by adjusting and controlling the earth-referenced resistance in a targeted manner. 
     In an improved embodiment variant, the temperature compensation circuit is arranged in an emitter branch of the at least one amplifier stage. 
     Advantageously, the oscillation amplifier has two amplifier stages, wherein the temperature compensation circuit is incorporated in at least one of them. Here, the emitter branch of the amplifier stage means in particular that no further current branch-off, circuit and/or functional element is provided thereafter upstream of the earthing or earth line. 
     In an advantageous embodiment, the oscillation amplifier has two amplifier stages, wherein the temperature compensation circuit is incorporated in the first amplifier stage. 
     The two amplifier stages are advantageously provided in order to be able to achieve the highest possible overall gain and to influence the phase position of the oscillator such that it can start to oscillate, because each of the transistor stages causes a 180° phase rotation. 
     Here, first amplifier stage means the amplifier stage in which the phase position undergoes the first 180° rotation as a result of the transistor, and the second amplifier stage means the amplifier stage in which the phase position undergoes a further 180° rotation, with the result that thereafter the phase has been rotated by 360° in relation to the starting position. 
     A further improvement can be that the oscillation amplifier has two amplifier stages, wherein one temperature compensation circuit is incorporated in each of the first amplifier stage and the second amplifier stage. These two temperature compensation circuits can be formed structurally different or also identical and ideally can be activated and deactivated. If the temperature compensation circuits are formed structurally different, it is advantageous if they are activated or deactivated depending on the detected temperature range. 
     For example, one temperature compensation circuit can be formed to operate more sensitively (more quickly, more precisely) in a low temperature range, with the result that it is activated in the corresponding temperature range. Analogously, the other temperature compensation circuit is formed for a high or higher temperature range and is activated for that purpose as needed. 
     For normal industrial applications, incorporation in the first amplifier stage is sufficient. For the use of the proximity switch in a wider temperature range of, for example, −40° C. to 100° C., a second temperature compensation circuit can be provided for better control, e.g. for sensors in the food industry or applications which are subject to extreme temperature fluctuations. 
     For example, the temperature sensor is a contact sensor which is secured to or on the surface of the oscillation amplifier and/or at least one amplifier stage. The temperature sensor is ideally a semiconductor sensor or an arrangement of one or more diodes or also a temperature-dependent resistor. In the normal scenario, this sensor is positioned between the coil and the oscillator amplifier in order to detect the temperature thereof. 
     In another alternative embodiment, the temperature compensation circuit comprises and/or substantially consists of a digital potentiometer, which is formed in particular as a controllable resistor and is formed in particular to receive digital control commands from the microcontroller. In this case, the temperature compensation circuit on the voltage branch can have a branch-off to earth, in which a resistor is arranged. This branch-off on the voltage side of the digital potentiometer stabilizes the digital potentiometer. 
     The resistor connected in parallel with the digital potentiometer is used so as to be able to adapt the change range of the temperature compensation circuit. In addition, this makes it possible to configure the change in the compensation value to be more sensitive. 
     In a second alternative embodiment of the proximity switch, the temperature compensation circuit has an operational amplifier, which in particular is a voltage follower, which is also used to decouple the microprocessor from the oscillation amplifier. In this case, the temperature compensation circuit is formed to receive a pulse-width modulation (PWM) as compensation value. As an improvement, a resistor and/or an earthed, branching-off line branch, in which a capacitor is also arranged, can be provided in the control line, the RC path to the operational amplifier. 
     The PWM resolution represents a measure for the sensitivity of the control and can advantageously lie in the range of an 8-bit to 32-bit resolution. In addition, a low-pass filter can advantageously be provided, consisting for example of a capacitor and a resistor, which converts the PWM signal into an equivalent DC voltage. 
     In a third alternative embodiment of the proximity switch, the temperature compensation circuit has an operational amplifier, wherein the temperature compensation circuit is formed to receive and process an analogue value as compensation values. Advantageously, a digital-to-analogue converter can be arranged in the control line, the path to the operational amplifier. The operational amplifier acts as a voltage follower and serves to decouple the microprocessor from the oscillation amplifier. 
     In the process, the quantization of the digital-to-analogue converter determines the sensitivity of the temperature control, wherein 8-bit to 32-bit is generally sufficient. 
     The disclosed embodiments also encompass a method for operating a proximity switch, in particular an inductive proximity switch, with an oscillator, for detecting a target, wherein the oscillator contains at least one oscillator amplifier. In this case:
         the oscillator generates an alternating magnetic field, which changes its oscillation state because of a target entering its detection range,   the temperature of an element of the proximity sensor and/or the ambient temperature is/are detected at least intermittently by means of at least one temperature sensor,   the measured values, and/or data derived therefrom, of the temperature sensor are sent to a microprocessor and/or to a storage medium connected thereto, wherein temperature algorithms and/or temperature-dependency data are stored on the storage medium. Here, on the basis of the temperature determined by the at least one temperature probe and the temperature algorithms and/or temperature-dependency data stored on the storage medium, the microprocessor ascertains control data and sends it to at least one temperature compensation circuit of the oscillation amplifier, with the result that temperature-induced influences on the behaviour of the oscillator are compensated for at least in part, ideally are compensated for in full.       

     In an improved method, the proximity switch is formed according to one of the aforementioned embodiments. 
     A further improvement of the method is that the temperature sensor detects the temperature of the coil and of the oscillation amplifier or of at least one amplifier stage of the oscillation amplifier, wherein by “coil” is meant the transmission coil and/or at least one receiver coil of the oscillator the individual or joint temperature of which is detected. 
     The method can additionally be improved if the microprocessor sends the control data to the temperature compensation circuit of the operational amplifier and/or of the at least one amplifier stage the temperature of which has been detected by the temperature sensor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further details and advantages of the disclosed embodiments will now be explained in more detail on the basis of example embodiments represented in the drawings. There are shown in: 
         FIG.  1    is a circuit diagram illustrating the proximity switch according to an embodiment, 
         FIG.  2    illustrates certain elements of the circuit diagram according to  FIG.  1    for an alternative embodiment of the proximity switch, 
         FIG.  3    is a circuit diagram illustrating a first embodiment of a temperature compensation circuit, 
         FIG.  4    is a circuit diagram illustrating a second embodiment of a temperature compensation circuit as an improvement of the design according to  FIG.  3   , 
         FIG.  5    is a circuit diagram illustrating a third embodiment of a temperature compensation circuit, and 
         FIG.  6    is a circuit diagram illustrating a fourth embodiment of a temperature compensation circuit. 
     
    
    
     DETAILED DESCRIPTION 
       FIG.  1    shows a circuitry or a circuit diagram for the proximity switch  1  according to the disclosed embodiments. The oscillator  2  or transmission coil  3  is connected via the line  6  to a voltage source via the line  6 . In the present case, the transmission coil  3  interacts in a known manner with receiver coils  7 . The construction and mode of operation thereof will not be explained further in the present case, in particular not the usual electronic elements for balancing the switching distance. 
     The oscillator  2  comprises an oscillation amplifier  10 , which comprises a first amplifier stage  11  and a second amplifier stage  12 . In addition, the circuitry of the proximity switch  1  has a, schematically represented, microprocessor  15 , which is connected to an output driver  20  at least in a data-carrying manner, preferably is connected in a data- and current-carrying manner via a current-modulated line. 
     A temperature compensation circuit  13 , which is described in more detail in  FIGS.  2  to  6    below, is provided in the emitter branch  17  of the first amplifier stage  11 . The emitter branch  18  of the second amplifier stage  12  is formed without a temperature compensation circuit. The other (micro-)electronic components can be identified by their respective symbols and are not described in more detail. 
     In the lower region of  FIG.  1   , a temperature sensor  5  is also represented, which is connected to the microprocessor  15  via the line  8  and is also independently connected to a voltage source. In the microprocessor  15 , the measured values received from the temperature sensor  5  are detected, evaluated and compared with the compensation values, temperature algorithms and/or temperature-dependency data stored in the storage medium  16 . The microprocessor  15  then sends control data to the temperature compensation circuit  13  via the control line  9 . 
     In the embodiment according to  FIG.  2   , the circuit according to  FIG.  1    is represented in simplified form, and identical reference numbers have the same meaning and therefore in some cases are not named and/or explained again. In the example shown, the oscillation amplifier  10  has the temperature compensation circuit  13  in the first amplifier stage  11 . In addition, a temperature compensation circuit  14  is also arranged in the second amplifier stage  12  in the emitter branch of the second amplifier stage  12 . 
     In the present case, these temperature compensation circuits  13 ,  14  are structurally identical and formed in accordance with an embodiment according to  FIGS.  3  to  6   . The two temperature compensation circuits  13 ,  14  receive the control data for compensating for the temperature influences from the microprocessor  15  or from the storage medium  16 . The control data are supplied to the two temperature compensation circuits  13 ,  14  via the control lines  9 . 
     In each case in the embodiment example according to  FIG.  3    and in the further  FIGS.  4  to  6   , only the first amplifier stage  11  and the temperature compensation circuit  13  arranged in the emitter branch  17  is, wherein comparable or identical elements and/or circuits of the temperature compensation circuit can also be provided in each case in the second amplifier stage  12  and/or in the two emitter branches  17 ,  18 . Compared with  FIGS.  1  and  2   , the circuit is represented in an even more simplified form. The first embodiment of the temperature compensation circuit  13 , shown in  FIG.  3   , is formed as a digital potentiometer  21 . For example, a resistor that can be controlled in a stepless manner (rheostat) and/or another switchable micro-electronic component form(s) the core piece in this case. 
     In the second embodiment of the temperature compensation circuit  13  according to  FIG.  4   , a digital potentiometer  21  is provided, analogously to  FIG.  3   , wherein an earthed line branch  19 , in parallel therewith, with a resistor  29  is additionally arranged for adjusting the sensitivity of the digital potentiometer  21 . 
       FIG.  5    shows a third embodiment of the temperature compensation circuit  13 , in which an operational amplifier  22  is comprised as a voltage follower and for decoupling the microprocessor  15  from the oscillator amplifier  10 . In this case, the operational amplifier  22  is formed to receive a pulse-width modulation (PWM value) as the control data and/or compensation values via the control line  9 , the RC path. 
     For the purpose of attenuation and signal smoothing, a resistor  26  can be arranged in the control line  9 , as also shown in the example according to  FIG.  5   , and a line branch-off  24  in which a capacitor  25  is arranged can be arranged between the resistor  9  and the operational amplifier  22 . Together with the resistor  26 , the capacitor  25  forms a low-pass filter which converts the PWM signal into an equivalent DC voltage. 
     The PWM resolution represents a measure for the sensitivity of the control and is at a 12-bit resolution in the example shown. 
     Lastly,  FIG.  6    shows the fourth embodiment of the temperature compensation circuit  13 , in which, analogously to the embodiment according to  FIG.  5   , an operational amplifier  27  is provided as a voltage follower and for decoupling the microprocessor  15  from the oscillator amplifier  10 . The latter is formed to receive and process analogue values as control data or compensation values. In the example shown, digital control data are sent to the operational amplifier  27  via the control line  9 , the RC path, by the microprocessor  15 , with the result that a digital-to-analogue converter  23  is arranged in the control line  9 . The digital-to-analogue converter  23  can also be arranged upstream of the microprocessor  15  or interact with the storage medium independently and prepare the storage data in a suitable way for the microprocessor  15  and/or the operational amplifier  27 . 
     In this embodiment variant, the quantization of the digital-to-analogue converter determines the sensitiveness of the temperature control, wherein a  12 -bit resolution has also been effected in this example. The advantage of using digital-to-analogue converters is that they are already contained in many microprocessors. Furthermore, compared with the variant shown in  FIG.  5   , no low-pass filter is required, which reduces component costs. 
     It is immediately apparent to a person skilled in the art that the above embodiments have been described independently as subject matters but can be combined in parts according to requirements. 
     LIST OF REFERENCE NUMBERS 
       1  Proximity switch 
       2  Oscillator 
       3  Transmission coil 
       5  Temperature sensor 
       6  Line 
       7  Receiver coils 
       8  Line 
       9  Control line, RC path 
       10  Oscillator amplifier 
       11  Amplifier stage 
       12  Amplifier stage 
       13  Temperature compensation circuit 
       14  Temperature compensation circuit 
       15  Microprocessor 
       16  Storage medium 
       17  Emitter branch 
       18  Emitter branch 
       19  Line branch 
       20  Output driver 
       21  Digital potentiometer 
       22  Operational amplifier 
       23  Digital-to-analogue converter (DAC) 
       24  Line branch 
       25  Capacitor 
       26  Resistor 
       27  Operational amplifier 
       29  Resistor 
       30  Detection range