Inductive proximity sensor, sensor system comprising inductive proximity sensor and method for operating such sensor system

Abstract

The invention relates to an inductive proximity sensor (1) comprising:-a sensor coil (2); -a pulse evaluation circuit (3) configured to provide an excitation pulse to the sensor coil (2) and to obtain a resulting voltage response; -a control unit (4) configured to: 0 control the pulse evaluation circuit (3) according to a pulse evaluation process in such a way that the sensor coil (2) is excited by an excitation pulse for a predetermined duration; 0 is detected at a specific first point in time after the excitation pulse is provided, and 0 is provided with an indication as to whether an object (10) to be detected is present in a detection region around the sensor coil (2), a synchronization unit (7) is provided for receiving a synchronization signal, and 0 is provided for the detection of the object (10) to be detected in the detection region around the sensor coil (2). A synchronization signal indicating whether or when the pulse evaluation process in the adjacent proximity sensor is active, and wherein the control unit (4) is configured to initiate the pulse evaluation process in dependence on the synchronization signal.

Description

Technical Field

[0001] The present invention relates to an inductive proximity sensor for measuring whether a conductive object to be detected is present in a detection area by using a pulse evaluation process, and measures for avoiding measurement errors caused by crosstalk of excitation current pulses between sensor coils of two adjacent inductive proximity sensors. Background Art

[0002] An inductive proximity sensor can use a pulse evaluation process to determine whether a conductive object to be detected is present. Here, a current pulse is impressed on the sensor coil, and the voltage response is evaluated. The course of the voltage response depends on the proximity of the conductive object to be detected to the sensor coil. The voltage response changes due to eddy current induction in the object to be detected, such that the analysis of this course - for example, in the form of measuring the amplitude of the voltage response after a predetermined period of time after the coil current has dropped to 0 A - allows the detection of the object to be detected approaching the sensor coil.

[0003] For example, from the documents EP0492029B1 and EP4030623A1, such inductive proximity sensors based on the pulse evaluation principle are known.

[0004] In some applications, it may be necessary to operate several identical inductive proximity sensors close to each other. For example, this can be set up for measuring the speed of an object, detecting multiple objects in a limited space, or implementing redundant measurements. If the sensor coils of the proximity sensors are arranged close to each other, there is a magnetic coupling with a non-negligible coupling coefficient between the sensor coils. Therefore, a time-varying magnetic field caused by one of the sensor coils in the sensor coils may induce an interfering voltage in the sensor coil of another proximity sensor, which may have an impact in the form of an interfering signal when evaluating the voltage response in the sensor coil of the other proximity sensor, and may thus seriously interfere with its signal evaluation.

[0005] In the case of multiple adjacent inductive proximity sensors using pulse evaluation, the instantaneous time offset of the cyclic current pulses and the type of pulse evaluation of the voltage response determine whether the two pulse evaluations of the proximity sensors will interfere with or affect each other. Since the cycle duration of the cyclic current pulses between the proximity sensors varies slightly due to scattering of the clock source in the control unit of the proximity sensors, the phase position of the cyclic current pulses also changes, and usually modulates in the intensity and direction of mutual interference with different cyclic current pulse frequencies. Therefore, by specifying the phase position, it is not possible to automatically exclude the pulse evaluation of the inductive proximity sensor from being interfered with by the operation of adjacent proximity sensors.

[0006] Document DE102011018430A1 discloses an inductive proximity switch, which includes: a circuit that electrically connects a coil to a voltage source, and a capacitor is provided in this circuit to detect the induced voltage through this capacitor; and an evaluation device that generates a switching signal through this evaluation device when the threshold voltage of the capacitor is exceeded. The voltage supply of the coil is shifted by the evaluation device according to the induced voltage of the capacitor measured over time, and the evaluation device detects this voltage immediately before the coil is connected to the voltage source by means of the voltage measured at the capacitor.

[0007] Shortly before the start of a new pulse, each sensor uses a capacitor to detect the pulse voltage. If the capacitor voltage is higher than a specific threshold, it is assumed that interference from an adjacent sensor occurs. To avoid this interference, the start of subsequent pulses is delayed by a specific period of time. This allows the sensors to be synchronized by time delay when the phase position is not important, that is, after one or several cycles, the sensors will no longer interfere with each other. Therefore, the faster of the two sensors adjusts its pulse period to the period of the slower sensor on average.

[0008] The disadvantage of such inductive self-synchronization is that the coupling must not be lower than the lowest level of the adjacent proximity sensors to be detected, so a specific minimum distance cannot be exceeded. If the minimum distance is exceeded, or the inductive coupling is briefly weakened due to the approach of the object to be detected, the synchronization of the proximity sensors may be lost, so that in some cases, two adjacent proximity sensors may still affect each other. Therefore, reliable synchronization cannot be guaranteed.

[0009] Alternatively, for each pulse period, the phase positions between the circulating current pulses of multiple proximity sensors may change significantly, so that significantly different interference effects may be detected in each measurement. By using a suitable digital filter, the short-term fluctuations of the voltage response at a specific phase position can be filtered out as anomalies, so that the measurement values with a high degree of interference can be eliminated.

[0010] However, the disadvantage of such a process is that filtering reduces the average measurement rate of the proximity sensors, and the average measurement rate will also fluctuate due to the sporadic elimination of individual measurement values.

[0011] Document EP3531557A1 discloses a proximity sensor for detecting a detection object by using a magnetic field. The proximity sensor includes: a detection coil for generating a magnetic field, an excitation circuit for repeatedly supplying a pulsed excitation current to the detection coil, a detection circuit for detecting the detection object based on the voltage generated across the detection coil during a predetermined time period after the supply of the excitation current is interrupted, and a control circuit for controlling the excitation circuit such that the time for cutting off the supply of the excitation current to the detection coil is aperiodic.

[0012] The object of the present invention is to provide an inductive proximity sensor in which, in a sensor system having a plurality of proximity sensors, pulse evaluation interference caused by the operation of adjacent inductive proximity sensors can be avoided and the measurement rate is not affected. Summary of the Invention

[0013] This object is achieved by an inductive proximity sensor according to claim 1, a sensor system having a plurality of inductive proximity sensors, and a method for operating a sensor system according to the independent claims.

[0014] Further embodiments are specified in the dependent claims.

[0015] According to a first aspect, there is provided an inductive proximity sensor, the inductive proximity sensor comprising:

[0016] - a sensor coil;

[0017] - a pulse evaluation circuit configured to supply an excitation pulse to the sensor coil and obtain a resulting voltage response;

[0018] - a control unit configured to:

[0019] ○ control the pulse evaluation circuit according to a pulse evaluation process such that the sensor coil is excited by an excitation pulse having a predetermined duration;

[0020] ○ detect at least a first measurement voltage at a specific first time point after the excitation pulse has been provided, and

[0021] ○ provide an indication of whether there is an object to be detected in a detection area around the sensor coil,

[0022] wherein a synchronization unit is provided for receiving a synchronization signal indicating whether the pulse evaluation process in an adjacent proximity sensor is active or when it is active, and wherein the control unit is configured to initiate the pulse evaluation process based on the synchronization signal.

[0023] In addition, the control unit may be configured to start the pulse evaluation process only when no pulse evaluation process in adjacent proximity sensors is in an active state.

[0024] The synchronization unit may be configured to signal, under the control of the control unit, the time and duration of the active pulse evaluation process.

[0025] According to an embodiment, the detection of the first measurement voltage may be performed at a specific first time point after the coil current has reached 0 A or after the falling edge of the excitation pulse. The excitation pulse may correspond to a current or voltage square wave pulse of a predetermined duration.

[0026] According to an embodiment, in the case where it is detected that the pulse evaluation process in an adjacent proximity sensor is in an active state, the control unit may be configured to start the pulse evaluation process when the pulse evaluation process in the adjacent proximity sensor has ended.

[0027] In addition, the control unit may be configured to determine the voltage difference between the first measurement voltage and the second measurement voltage and provide this voltage difference as an indication of whether an object to be detected is present in the detection area of the sensor coil. The control unit is configured to detect the second measurement voltage at a specific second time point after the excitation pulse has been provided, for example, after the coil current has reached 0 A or after the falling edge of the excitation pulse and after the first time point.

[0028] The operation of an inductive proximity sensor is generally controlled by a control unit, which may include, for example, a microcontroller. Generally, current pulses are periodically applied to the sensor coil, and after the coil current is turned off, the resulting voltage response is evaluated. The voltage response is evaluated by the following method: measuring a first measurement voltage of the resulting voltage response at a time after the excitation pulse has been provided, such as immediately or shortly after the falling edge of the current pulse or after the coil current has reached 0 A, and optionally measuring a second measurement voltage after a predetermined longer period after the excitation pulse has been provided, such as after the falling edge of the current pulse or after the coil current has reached 0 A, to obtain a reference value to which the first measurement voltage is referenced. The voltage difference between the two measurement voltages then provides a representative value of the voltage response, which can be used to determine whether an object to be detected is present by comparing with a threshold.

[0029] Applying a current pulse to the sensor coil causes a change in the magnetic field close to the sensor coil. If multiple sensor coils are arranged in close proximity to each other such that a current pulse in one sensor coil causes a voltage pulse in an adjacent sensor coil, the evaluation of the voltage response may be affected. To avoid mutual interference during the evaluation of the voltage response in the pulse evaluation process using multiple adjacent inductive proximity sensors, it is necessary to ensure that a measurement voltage is obtained during the voltage measurement process that is not interfered with by the application of a current pulse to the sensor coils of adjacent inductive proximity sensors.

[0030] If, during one of the voltage measurement processes, an adjacent inductive proximity sensor is interfered with by the current pulse along the sensor coil, the voltage measurement result will be distorted. To ensure that the voltage of the voltage response can be measured without being affected by interference, the operations of the inductive proximity switches of the sensor system are synchronized with each other.

[0031] An electrical synchronization circuit can be provided to transmit the synchronization signal. The synchronization signal can use a first voltage level to indicate that no pulse evaluation process is active, and can use a second voltage level to indicate that a pulse evaluation process is active.

[0032] To coordinate the operations of two or more inductive proximity sensors, they can be connected to each other via a synchronization circuit. The synchronization signal is applied to the synchronization circuit by the proximity sensors, which indicates when the pulse evaluation process is currently active or when the pulse evaluation process in the relevant proximity sensor is in progress. If a proximity sensor detects via the synchronization circuit that the pulse evaluation in another proximity sensor is currently active or in progress, the start of the pending pulse evaluation process is delayed, where the start of the pending pulse evaluation process begins with the generation of a current pulse in the sensor coil. This will stop the measurement cycle of the relevant proximity sensor until the synchronization signal indicates that no pulse evaluation process is currently active.

[0033] The synchronization signal can be defined such that: a first voltage level is used to signal that no pulse evaluation process is currently active, and a second voltage level is used to signal that a current pulse evaluation process is active. Thus, applying the first voltage level can signal that: after the voltage response has been evaluated, a measurement can be made by another inductive proximity sensor. The pulse evaluation process can be active when a current pulse is applied and for a predetermined period of time after an excitation pulse has been provided. At least a first measurement voltage and a second measurement voltage (if applicable) are measured during the specified period.

[0034] The synchronization line is connected to all proximity sensors that are close to each other, and the voltage level can be used to indicate whether the pulse evaluation process can be started. For this purpose, a synchronization unit is provided in each of the proximity sensors. When the pulse evaluation process is active, the synchronization unit provides a second voltage level on the synchronization line by means of a corresponding pull-up or pull-down resistor. Or when the pulse evaluation process is not active, the synchronization unit detects the voltage level on the synchronization line as an input signal for the control unit of the relevant proximity sensor.

[0035] In particular, the synchronization process is used to indicate when the pulse evaluation process is active in one of the proximity sensors, thus preventing the pulse evaluation process from being executed in other proximity sensors of the sensor system.

[0036] In normal operation, the control unit of the proximity sensor supplies periodic or cyclic current pulses to the pulse evaluation process. Before applying the current pulse, it is checked whether the voltage level on the synchronization line indicates that no pulse evaluation process is active (at the first voltage level). If no pulse evaluation process is active, the control unit executes the pulse evaluation process and sets the voltage level on the synchronization line so that it indicates that the pulse evaluation process is active (second voltage level). On the other hand, if the second voltage level is detected on the synchronization line before starting the pulse evaluation process, which indicates that the pulse evaluation process is active, the pulse evaluation process to be processed is delayed and waiting is continued until the voltage level on the synchronization line indicates that no pulse evaluation process is in progress (by switching to the first voltage level). For this purpose, the voltage level on the synchronization line can be periodically, especially cyclically, queried in the control unit, or by temporarily extending or shortening the pulse period of the cyclically executed pulse evaluation process, the requested pulse evaluation process is delayed and not executed overlapping with the previously performed pulse evaluation process.

[0037] In addition, it can be set that the control unit is configured to start the pulse evaluation process if the synchronization signal indicates that the pulse evaluation process in an adjacent proximity sensor has been active for more than a predetermined period of time.

[0038] In particular, the control unit can thus determine that the pulse evaluation process is active via the synchronization line within a predetermined period of time, and if this period is exceeded, the pulse evaluation process can be started regardless of the voltage level on the synchronization line to prevent the measurement cycle of the proximity sensor from being permanently deactivated and no longer being able to perform measurements.

[0039] Furthermore, the control unit may be configured to set the voltage level of the synchronization signal to a second voltage level when or before applying the excitation pulse, and / or to set the voltage level of the synchronization signal to a first voltage level after having detected at least the first measured voltage or the second measured voltage (if applicable).

[0040] The synchronization signal may specify the clock for the cyclic operation of the pulse evaluation process. The pulse evaluation process may then be started based on this clock, so that each proximity sensor in the proximity sensor system starts the pulse evaluation process at a separate phase position relative to the clock signal. When the sensor system is started, the synchronization line may then be used to assign a cyclic frequency and individual phase positions to the proximity sensors.

[0041] According to a further aspect, there is provided a sensor system comprising a plurality of the above-described proximity sensors, wherein the proximity sensors are connected by a synchronization line.

[0042] According to a further aspect, there is provided a method for operating an inductive proximity sensor having a sensor coil, the method comprising the steps of:

[0043] - providing an excitation pulse to be applied to the sensor coil for a predetermined time duration according to a pulse evaluation process;

[0044] - measuring at least one measured voltage within the voltage response after providing the excitation pulse;

[0045] - providing an indication of whether an object to be detected is present within the detection area of the sensor coil;

[0046] wherein the state of an adjacent proximity sensor is received, which state indicates whether or when the pulse evaluation process is active in the adjacent proximity sensor, and wherein the pulse evaluation process is started only when no pulse evaluation process is active in the adjacent proximity sensor. Description of the Drawings

[0047] The embodiments will be explained in more detail below with reference to the drawings, wherein:

[0048] Figure 1 shows a schematic view of an inductive proximity sensor comprising a synchronization unit;

[0049] Figure 2 shows a signal timing diagram of a pulse control voltage, a switching signal, a coil current, a coil voltage, and a voltage at an analog-to-digital converter;

[0050] Figure 3 shows a flowchart for explaining the process performed in the control unit of a proximity sensor; and

[0051] Figure 4 shows a signal timing diagram of the synchronous line potential, the coil currents of two proximity sensors, and the voltages of two analog-to-digital converters of the proximity sensors; and

[0052] Figure 5 shows a schematic diagram of an inductive proximity sensor, which includes a synchronization unit in which the measurement of the second measurement voltage is suppressed. Detailed implementation mode

[0053] Figure 1 is a schematic diagram of an inductive proximity sensor 1 according to an embodiment of the present invention.

[0054] The proximity sensor 1 includes a sensor coil 2, and the sensor coil 2 includes an inductance L C and a parasitic resistance R C , and the sensor coil 2 is electrically connected to a pulse evaluation circuit 3. The pulse evaluation circuit 3 has a passive network 31 connected in parallel with the sensor coil 2.

[0055] The sensor coil 2 is connected in series with a switchable current source 32 to cyclically apply current pulses to the sensor coil 2 under the control of a pulse control signal PWM_Puls from a control unit 4.

[0056] The passive network 31 includes a discharge resistor Rp, an RC low-pass resistor R TP , C TP and a diode D. Shortly after the current source 32 is turned off, the voltage response is initially dominated by the self-inductive pulse of the coil. The function of the passive network 31 is to limit the self-inductive pulse in terms of its level to protect subsequent circuit elements.

[0057] The sensor coil 2 acts as a probe for the proximity sensor 1 and generates a magnetic field. According to the pulse evaluation process, current pulses are impressed on the sensor coil 2 such that the voltage response of the induced voltage depends on whether there is an object 10 to be detected in the detection area, and the voltage response of the induced voltage is determined by self-inductance and the conductivity and permeability of the object 10 to be detected.

[0058] In addition, an offset voltage source 33 of the pulse evaluation circuit 3 can be connected to the sensor coil 2 to apply a voltage offset V offs to the resulting voltage response, so that the voltage response is within a suitable voltage measurement range.

[0059] The measured voltage U_ADC1 of the voltage response can be amplified by the amplifier circuit 34 (the amplifier circuit 34 may include an operational amplifier 341) at certain time points and can be measured by the analog-to-digital converter in the control unit 4. For this purpose, the control unit 4 may include a microcontroller in which an analog-to-digital converter is integrated to provide the measured voltage in digital form.

[0060] To avoid saturation of the operational amplifier of the amplifier circuit 34 and the associated infinitely long recovery time, the voltage response of the sensor coil 2 can be temporarily disconnected from the amplifier input by the analog switch 35. If the analog switch 35 is opened at the start of the current pulse and closed again after a period of time (for example, between 10 μs and 50 μs) after the coil current is disconnected, the self-induction pulse, that is, the voltage level of the voltage response has decayed to such an extent that amplifier saturation is excluded. The control unit 4 also controls the analog switch 35 through the corresponding switch signal PWM_Shutter.

[0061] The evaluation of the voltage response is also controlled by the control unit 4 by measuring two measured voltages with a predetermined time interval.

[0062] Figure 2 A signal timing diagram is schematically shown to illustrate the execution of the pulse evaluation process. It can be seen that the coil current I provided by the control unit 4 using the current pulse signal PWM_Puls C The current pulse signal of causes the current pulse to be applied to the sensor coil 2. Figure 2 The current pulse is shown in c. Due to the falling edge of the current pulse I C That is, the current drops to 0 amperes, voltage induction occurs in the sensor coil, and this voltage induction slowly dissipates through the passive network 31.

[0063] To avoid overvoltage in subsequent circuit components, the analog switch is controlled using the switch signal PWM_Shutter ( Figure 2 b) such that the analog switch 35 is turned on only after reaching 0 amperes or within a short period of time after the falling edge of the current pulse, so that the voltage response of the sensor coil 2 is applied to the amplifier circuit 34 only when it has decayed to a certain extent. Figure 2 The voltage response is shown in the process of e.

[0064] The voltage measurement of the measured voltage is performed at predetermined time points t1 and t2, and these two time points can be defined according to the time of the falling edge of the current pulse signal PWM_Puls. Therefore, the voltage difference between the measured voltages allows the measurement of an indication related to the mutual inductance of the object to be detected, which acts on the sensor coil 2 and depends on whether the object 10 to be detected is present.

[0065] The first measured voltage U1 at time point t1 approximately corresponds to the level of the voltage response, and the second measured voltage U2 at time point t2 corresponds to a reference voltage that serves as a reference or benchmark voltage for the first measured voltage that has been measured first.

[0066] The voltage difference can be provided as an output signal through a suitable interface 6. Alternatively, the result of a threshold comparison of the voltage difference can be performed using a predetermined threshold of the voltage difference, and the result of the threshold comparison can be provided through interface 5. Then, the result of the threshold comparison corresponds to an indication of the presence or absence of the object 10 to be detected.

[0067] A synchronization unit 7 is provided to synchronize the operation of the proximity sensor 1 with the operation of adjacent proximity sensors 1 arranged adjacent thereto, in particular to eliminate interference effects during measurement using a pulse evaluation process. The synchronization unit 7 is connected to one or more proximity sensors 1 through a synchronization line 8. Thus, the synchronization line 8 interconnects a plurality of proximity sensors 1, and each proximity sensor 1 is provided with a synchronization unit.

[0068] The synchronization unit 7 includes a pull-up resistor R PU , to generate an impressed first voltage potential VDD on the synchronization line 8. The synchronization line 8 is connected to the control unit 4 via a protection resistor in applicable cases using a synchronization input Sync_In.

[0069] The synchronization units 7 of the respective proximity sensors 1 are connected to the control unit 4 and can generate a second voltage potential on the synchronization line 8 through a transistor T under the control of a synchronization output Sync_Out, where the synchronization line 8 is connected to a lower second voltage potential GND through the transistor. Thus, the synchronization unit 7 includes a driver for applying a second voltage level to the synchronization line 8 under the control of a synchronization signal at the synchronization output Sync_Out. If the transistor T is closed, the second voltage potential is applied to the synchronization line. If the transistor T is opened, the pull-up resistor RPU pulls up the voltage level of the synchronization line 8 to the first voltage potential.

[0070] The control unit 4 can detect the voltage level on the synchronization line 8 via the synchronization input Sync_In to correspondingly control the execution of the pulse evaluation process.

[0071] The synchronization line 8 of the proximity sensor can also be connected separately to the higher-level controller. In this case, the controller can selectively start and stop the pulse evaluation process of the proximity sensor by specifically selecting the appropriate voltage level of the synchronization line 8, and thus allow the operation of adjacent proximity sensors to be undisturbed. For this purpose, the controller can evaluate the synchronization signal on the one hand to determine which proximity sensor is currently actively performing the pulse evaluation process. For example, this can be done by encoding via the magnitude of the second voltage level. On the other hand, the controller can actively prevent the start of the pulse evaluation process because the control results in the second voltage level on the synchronization line 8.

[0072] Figure 3 The flowchart shows a method for operating a sensor system including a plurality of inductive proximity sensors 1. In Figure 4 The signal-time diagram in further explains this process. The progress of this process is controlled by the control unit 4 of the proximity sensor 1 and typically occurs regularly according to the predetermined operating cycle frequency of the pulse evaluation process in each control unit in the control unit 4. Here, the pulse evaluation process is carried out without overlap but with a time delay. The process starts with the analog switch 35 being opened.

[0073] In step 1, it is checked by the control of the internal clock whether the pulse evaluation process is to be actively performed. If this is the case (alternatively: yes), the process continues with step S2. Otherwise (alternatively: no), the process returns to step S1.

[0074] In step S2, first the voltage level of the voltage on the synchronization line 8 is queried, and then the applied voltage level is checked. If it is determined in step S2 that the second voltage level is applied, which indicates that the pulse evaluation process of another proximity sensor in the proximity sensor 1 is still active (alternatively: yes), the process returns to step S2 and waits until the voltage level on the synchronization line 8 rises to the first voltage level again. If it is determined that the first voltage level has been reached and thus no other pulse evaluation process is active (alternatively: no), the process continues with step S3.

[0075] In step S3, the synchronization output Sync_Out of the control unit 4 is activated, thereby closing the transistor T and reducing the voltage potential on the synchronization line 8 to the second voltage level. This signals to the other proximity sensors 1 on the synchronization line 8 that the pulse evaluation process cannot be used for measurement.

[0076] At the same time or shortly thereafter (e.g., between 1 μs and 100 μs), in step S4, an electric current pulse is applied by applying a current pulse signal PWM_Puls, and the pulse evaluation process is started, and the current pulse signal PWM_Puls is generated for a predetermined period of time, which applies the current pulse to the sensor coil 2 via the switched current source 32. This is shown in the figures in Figure 4 b and Figure 4 c for the proximity sensor 1. The current pulse has a predetermined duration T Puls , and this duration can be significantly reduced, i.e., less than 20% or 10% of the cycle duration Tp of the cyclic pulse evaluation process.

[0077] If the duration of the current pulse has elapsed, i.e., there is a falling slope of the coil current, for example, or the coil current has reached 0 A, then with the aid of the switching signal PWM_Shutter, after a short first period of time, e.g., between 10 μs and 50 μs, in step S5, the previously opened analog switch 35 is closed to apply the voltage of the voltage response that has decayed to a certain extent to the amplifier circuit 34.

[0078] In step S6, at a time point t1 after the current pulse has been applied and reached 0 amperes, after a predetermined second duration of time, the first measured voltage U_ADC11 is measured, and at a later time point t2 defined by a predetermined third period after the current pulse has reached 0 amperes, the second measured voltage U_ADC12 is measured. This measurement is performed by the analog-to-digital converter of the control unit 4. In an analogous manner, the first measured voltage and the second measured voltages U_ADC21, U_ADC22 of another proximity sensor are obtained.

[0079] In step S7, after the second measured voltage 8 is measured, the synchronization line 8 is immediately released again, and the synchronization output Sync_Out of the control unit 4 is deactivated. This turns on the transistor T and thus raises the voltage potential on the synchronization line 8 to the first voltage level. Then the pulse evaluation process is completed, and another proximity sensor on the synchronization line 8 can perform the pulse evaluation process.

[0080] In step S8, the voltage difference between the first measured voltage and the second measured voltage can serve as the output signal, or it can also be used to generate the output signal described above. In particular, this (based on the threshold comparison of the differential voltage) can provide an indication of whether there is an object 10 to be detected within the detection area of the sensor coil 2.

[0081] The number of proximity sensors in the sensor system can also be more than two, where, however, the sum of the durations of the pulse evaluation processes of all proximity sensors must be less than the total cycle duration T of each proximity sensorP 。

[0082] If it is determined in step S2 that a second voltage level exists on the synchronization line 8 for a predetermined period of time, such as two cycle durations T P , then the measurement can be started regardless of the voltage level on the synchronization line 8. This excludes the case where the voltage level on the synchronization line 8 remains at the second voltage level due to an error.

[0083] Figure 5 A schematic diagram of an inductive proximity sensor with a synchronization unit 7 is shown, in which the measurement of the second measurement voltage is suppressed. For this purpose, a short - circuit switch 36 is provided, which is arranged in parallel with the passive network 31 and shorts the voltage drop across the network 31 under the control of the reset signal PWM_reset of the control unit 4. Since the resistance R P is usually selected to be significantly lower than R TP , the closing of the short - circuit switch 36 has no effect on the energy of the sensor coil 2. The short - circuit effect of the network 31 is to suppress the signal of the sensor coil 2 so that the offset voltage of the offset voltage source 33 and any additional offset voltage of the amplifier circuit can be measured without being affected by the voltage of the sensor coil. Therefore, the second measurement voltage can be measured at any time, and this second measurement voltage should correspond to the voltage of the offset voltage source 33 plus any offset voltage of the subsequent circuit components 34 and 35, without being affected by any coupling interference (e.g., affected by the interference field of an adjacent sensor).

[0084] Therefore, the determination of the voltage difference can only be carried out at one time point - namely the first time point t1. This effectively halves the interference probability and also makes the pulse evaluation process significantly more flexible and shorter, because using the second measurement voltage to determine the pulse offset does not absolutely require waiting for the end of the voltage response. Therefore, the voltage level on the synchronization line 8 can also be set to the first voltage level immediately after the measurement of the first measurement voltage, so that other proximity sensors of the sensor system can start the pulse evaluation process.

[0085] The purpose of the synchronization signal is to simply signal when the pulse evaluation process is active in one of the proximity sensors connected to the synchronization line. In addition to the voltage level, the signal can also be sent by applying or transmitting a PWM signal, an oscillation signal or a digital signal, which contain information about the time when the pulse evaluation process starts, the duration of the pulse evaluation process and / or the identification of the proximity sensor in which the pulse evaluation process is currently active. This allows the implementation of a predefined measurement sequence, for example, especially when more than two sensors are connected to the synchronization line, which is advantageous for accurately evaluating the object speed, for example.

[0086] Measurements can also be carried out at different measurement rates in multiple proximity sensors to meet different requirements regarding measurement applications.

[0087] In addition, the synchronization signal can also have more than two voltage levels to be able to distinguish different phases of the pulse evaluation process, such as the phase in which the coil current increases, the phase in which the coil current is constant (not equal to zero), the phase in which the coil current decreases, and the phase in which the measured value is recorded. For example, this enables synchronization with the phase position, where the first proximity sensor detects the measured value while the coil current of the adjacent second proximity sensor is constant but not equal to zero, so there is no induced interference voltage in the sensor coil of the first proximity sensor. Therefore, the maximum number of proximity sensors that can be synchronized via the synchronization line within a given pulse period can be increased.

Claims

1. An inductive proximity sensor (1), the inductive proximity sensor comprising: - a sensor coil (2); - a pulse evaluation circuit (3), the pulse evaluation circuit being configured to provide an excitation pulse to the sensor coil (2) and to obtain a resulting voltage response; - a control unit (4), the control unit being configured to: ○ Control the pulse evaluation circuit (3) according to the pulse evaluation process, such that the sensor coil (2) is excited by an excitation pulse of a predetermined duration period; ○ Detecting at least first measurement voltages (U_ADC11, U_ADC12) at a specific first time point after providing the excitation pulse, and ○ Based on the first measured voltages (U_ADC11, U_ADC12), an indication is provided as to whether an object (10) to be detected is present within the detection area around the sensor coil (2). characterized in that: a synchronization unit (7) is provided for receiving a synchronization signal, the synchronization signal indicating whether or when a pulse evaluation process is active in an adjacent proximity sensor, and wherein the control unit (4) is configured to initiate the pulse evaluation process based on the synchronization signal.

2. The inductive proximity sensor (1) according to claim 1, wherein, The control unit (4) is configured to initiate the pulse evaluation process only when no pulse evaluation process is active in an adjacent proximity sensor.

3. The inductive proximity sensor (1) according to claim 1 or 3, wherein, The synchronization unit (7) is configured to emit a signal regarding the time and duration of an active pulse evaluation process under the control of the control unit (4).

4. An inductive proximity sensor (1) according to one of claims 1 to 3, wherein, The control unit (4) is configured to: in the case where a pulse evaluation process in an adjacent proximity sensor is detected as being active, initiate the pulse evaluation process when the pulse evaluation process in the adjacent proximity sensor has ended.

5. The inductive proximity sensor (1) according to one of claims 1 to 5, wherein, An electrical synchronization line (8) is provided for transmitting the synchronization signal.

6. The inductive proximity sensor (1) according to one of claims 1 to 5, wherein, The synchronization signal indicates that no pulse evaluation process is active by a first voltage level, and the synchronization signal indicates that a pulse evaluation process is active by a second voltage level.

7. The inductive proximity sensor (1) according to claim 6, wherein, The control unit (4) is configured to: apply the voltage level of the synchronization signal as the second voltage level at or before the time when the excitation pulse is provided; and / or, apply the voltage level of the synchronization signal as the first voltage level after at least the first measurement voltages (U_ADC11, U_ADC12) have been detected.

8. An inductive proximity sensor (1) according to one of claims 1 to 7, wherein, The control unit (4) is configured to: initiate the pulse evaluation process when the synchronization signal indicates that a pulse evaluation process in an adjacent proximity sensor (1) has been active for more than a predetermined period of time.

9. An inductive proximity sensor (1) according to one of claims 1 to 8, wherein, The control unit (4) is configured to determine the voltage difference between a first measurement voltage (U_ADC11, U_ADC12) and a second measurement voltage (U_ADC21, U_ADC22), and use the voltage difference as an indication of whether an object to be detected (10) is present in the detection area of the sensor coil (2), wherein the control unit (4) is configured to detect the second measurement voltage (U_ADC21, U_ADC22) at a specific second time point, the second time point being after the excitation pulse is provided and after the first time point.

10. The inductive proximity sensor (1) according to claim 9, wherein, A short - circuit switch (36) is provided to designate the second measurement voltage (U_ADC21, U_ADC22) as a reference voltage when it makes a measurement.

11. A sensor system, the sensor system comprising a plurality of proximity sensors (1) according to one of claims 1 to 10, wherein, The proximity sensors (1) are connected to each other via a synchronization line (8).

12. A method for operating an inductive proximity sensor (1), the inductive proximity sensor (1) comprising a sensor coil (2), the method comprising the following steps: - Provide (S4) an excitation pulse to be applied to the sensor coil (2) for a predetermined period of time according to the pulse evaluation process; - Measure (S6) at least one measured voltage (U_ADC11, U_ADC12) within the voltage response after providing the excitation pulse; - Provide an indication of whether an object (10) to be detected exists within the detection area of the sensor coil (2) based on the at least one measured voltage (U_ADC11, U_ADC12); It is characterized in that: Receive the status of adjacent proximity sensors, where the status indicates whether the pulse evaluation process is active or when it is active in the adjacent proximity sensors, and wherein the pulse evaluation process is only started (S2) when no pulse evaluation process is active in the adjacent proximity sensors.

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