Abstract:
A sensor and a method for detecting objects using an inductive sensor. An electrical alternating field is generated by using an oscillating circuit, wherein the amplitude and/or frequency of a signal of the oscillating circuit changes when an object is present. The signal of the oscillating circuit is rectified. The rectified signal is relayed to a high-pass filter, wherein damping of the oscillating circuit caused by nonmoving objects is filtered by the high-pass filter. The signal is compared with a threshold value, and an output signal is generated when the signal value is above or below the threshold value.

Description:
FIELD OF THE INVENTION 
     The invention relates to an inductive sensor, and a method for detecting objects using an inductive sensor. 
     BACKGROUND OF THE INVENTION 
     Inductive sensors are known. An inductive sensor is described in German Unexamined Patent Application DE 42 28 888 A1, for example. Conventional inductive sensors are used for detecting rotational speeds, for example. An inductive sensor cooperates with a magnet. A voltage is induced in a coil due to change in the magnetic field caused by a magnet moving past. A rotational speed signal, for example, may be picked up in this manner. 
     Another option is to provide a magnet in the sensor itself, the magnet cooperating with ferromagnetic components. A ferromagnetic object may thus likewise be detected via a change in the magnetic field. 
     Inductive sensors are also known from practice which react to metals of any type, and generate a signal when a metallic body is in the vicinity of the sensor. 
     One disadvantage, among others, of such known inductive sensors is that a switching signal is generated in the mere presence of a metallic component. Therefore, such sensors have only limited suitability for detecting dynamic processes. In addition, known inductive sensors are relatively sensitive to interferences. Thus, for example, metal dust or shavings, etc. adhering to the sensor may result in an erroneous detection. 
     OBJECT OF THE INVENTION 
     Accordingly, the object of the invention is to at least reduce the referenced disadvantages of the prior art. 
     It is an object of the invention in particular to provide a sensor in which the detection of dynamic processes is improved. Thus, a particular aim is to reduce interferences caused by static components or other metal parts in the vicinity of the sensor. 
     A further object of the invention is to provide a particularly simple sensor which is also sensitive to relatively small objects. 
     A further object of the invention is to provide a sensor which reacts to any type of metallic object, and which, for example, may be easily exchanged with known inductive sensors which operate on a ferromagnetic principle. 
     A further object of the invention is to provide a sensor which detects dynamic processes independently of direction, and which may be mounted from the outside, i.e., which does not require that objects for detection must be led through the interior of a coil. 
     SUMMARY OF THE INVENTION 
     The invention relates, in the first place, to an inductive sensor which includes an oscillating circuit having a coil. When a metallic object approaches, the field lines and therefore the inductance of the system change. The oscillating circuit is thus detuned, which results in a change, in particular a decrease, in the amplitude or the frequency of the signal of the oscillating circuit. Conclusions may thus be drawn concerning the presence of an object in the detection range of the sensor, and an output signal may be generated. 
     According to the invention, the sensor has means for suppressing changes in the amplitude of a signal of the oscillating circuit caused by static objects. 
     Changes in the amplitude of the oscillating circuit caused by static components may thus be masked during operation of the sensor, thus avoiding, among other things, interference from metallic components present in the vicinity. In addition, for detection of rotational speed, for example, it is ensured that the sensor does not emit an output signal when, for example, the tooth of a pinion is stopped in the detection range of the sensor. 
     Thus, the sensor emits a signal only when an object moves past the sensor, but not when a stationary object is present in the detection range. 
     In one preferred embodiment of the invention, the sensor has a rectifier for rectifying the signal of the oscillating circuit. The rectification of the signal results in improved processability. 
     It is understood that “rectification” does not refer to a sinusoidal signal, for example, or that the signal is completely uniform at constant amplitude. Rather, “rectification” also refers to smoothing of the signal. 
     In one preferred embodiment of the invention, the sensor has a threshold value comparator. 
     It is provided in particular that a rectified signal is further processed using a threshold value comparator, and conclusions are thus drawn concerning the presence of a moving object. 
     In one preferred embodiment of the invention, the threshold value and/or the oscillator current is/are adjusted or modified, thus allowing the sensing distance to be easily set. The oscillator current is preferably set in such a way that a higher resolution of the sensor generally results. 
     An output signal is preferably generated via an output stage when the signal value is below or above the threshold value. 
     In one refinement of the invention, the sensor has means for readjusting the amplitude and/or frequency of the oscillating circuit to a predefined setpoint value for static damping. 
     It is provided in particular that the amplitude and/or frequency of the oscillating circuit is regulated with respect to a predefined setpoint value over a fairly long detection period. This avoids, for example, further reduction in the amplitude when metallic objects deposit in the detection range of the sensor, which could result in impaired sensitivity, or even failure, of the sensor. Such regulation may be carried out, for example, by detecting via a circuit the amplitude over a specified, fairly long period of time, for example at least one minute, when the amplitude is less than a predefined setpoint value over this fairly long period of time; for example, the capacitance of a capacitor of the oscillating circuit is modified in such a way that the amplitude increases once again. 
     The inductive sensor is preferably designed for continuous measurement. 
     The resolution of the sensor is therefore essentially determined by the frequency of the oscillating circuit. In one preferred embodiment of the invention, the undamped oscillating circuit has a frequency of 10 kHz to 1000 kHz, preferably 300 kHz to 700 kHz. The sensor is therefore suitable for successively detecting objects at relatively short intervals. 
     The means for suppressing changes in the amplitude of the signal of the oscillating circuit caused by static objects preferably include at least one capacitor. This design allows a particularly simple circuit. Alternatively, electronic filtering, also using a microcontroller, for example, may be carried out. 
     In one preferred embodiment of the invention, the capacitor forms a high-pass filter to which the rectified signal of the oscillating circuit is relayed. 
     An object moving past causes a change in amplitude of the oscillating circuit, and therefore also in the magnitude of the rectified signal. Using a capacitor, it is possible to mask, for example, frequencies of less than 0.5 Hz, preferably less than 1 Hz, and particularly preferably less than 2 Hz, in a particularly simple manner. 
     Thus, the signal passes into a threshold value comparator for further processing only when a dynamic process is involved. 
     The sensor is preferably designed in such a way that the detection range is located outside the coil. 
     It is provided in particular that a coil is installed in a sensor housing having an essentially cylindrical design, the detection range being located in the front region, outside the sensor housing. 
     Such a sensor may be installed from the outside in a particularly simple manner, and is therefore particularly suited for exchanging with known inductive sensors which operate according to any previously known principle. 
     The coil axis is preferably situated on a center axis of the cylindrical sensor housing. 
     In one refinement of the invention, the sensor or the sensor housing has a thread for easily screwing on. 
     In one preferred embodiment of the invention, all electronic components necessary for operation of the sensor are situated in the sensor housing, in particular on a printed circuit board, which, viewed outwardly from the detection range, is located behind the coil. 
     The invention further relates to a method for detecting objects using an inductive sensor. 
     An electrical or magnetic field is generated using an oscillating circuit. The oscillating circuit is detuned by metallic objects in the detection range, thus changing the amplitude or frequency of the oscillating circuit. 
     The signal of the oscillating circuit is rectified and relayed to a high-pass filter. 
     Thus, the signal passes through the high-pass filter only when the signal changes in a predefined minimum frequency. 
     Only signals which reflect a dynamic process reach a threshold value comparator. 
     By means of the threshold value comparator, an output signal is generated in a known manner, for example via an output stage, when the signal value is below or above the threshold value. 
     The static portions of the signal of the oscillating circuit are preferably filtered out using at least one capacitor and/or microcontroller. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is explained in greater detail below with reference to the drawings in  FIGS. 1 through 5 . 
         FIG. 1  schematically shows the principle of known inductive sensors; 
         FIGS. 2 and 2   a  schematically show in greater detail the principle of a sensor according to the invention which detects only dynamic processes; 
         FIG. 3  shows an example of a wiring diagram of a sensor according to the invention; and 
         FIGS. 4 and 5  show an inductive sensor. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  schematically illustrates the function of an inductive sensor which also detects static processes. The statically undamped, statically damped, and dynamically damped states are illustrated for comparison. 
     The first column illustrates the respective signal of the oscillating circuit  10 . 
     In the top row the inductive sensor is statically undamped, and the signal of the oscillating circuit  10  is at a maximum value. 
     The signal of the oscillating circuit  10  is converted to a rectified signal  11  and relayed to a threshold value comparator. The signal  16  present at the threshold value comparator thus corresponds to the rectified signal  11 . 
     In the first row, third column, the rectified signal  16  which has been relayed to the threshold [value] comparator is above the set threshold value  13 . The sensor is designed in such a way that an output signal is generated only when the signal value is below the threshold value  13 . Accordingly, as shown in the top row at the far right, for an undamped sensor no output signal is emitted by the output stage. 
     The second row shows a statically damped sensor. As a result of the detuning of the oscillating circuit, the amplitude in the first column, second row is less than in the first row. 
     It is apparent that the rectified signal  11  is also lower than in the first row. 
     For the threshold value comparator, the rectified signal  16  present at the threshold [value] comparator is lower than the threshold value  13 . 
     The output stage accordingly emits an output signal  14 . 
     The sensor then delivers an output signal, although only one stationary component is present in the detection range. This may, for example, easily result in interferences, for example because the metallic components are too close to the sensor. For example, in certain applications it is no longer possible to detect the rotational speed due to the fact that the sensor continuously delivers an output signal. 
     The dynamic damping of the sensor is illustrated in the third row. For a dynamic change, the amplitude of the signal of the oscillating circuit  10  continuously changes as the metallic object passes through the detection range of the sensor. 
     Accordingly, the amplitude of the rectified signal  11  also changes. In the idealized form shown here, this is illustrated as a square wave signal. 
     The rectified signal  16  present at the threshold value comparator is then periodically below the threshold value  13 , and the output stage therefore emits a periodic output signal  14 . 
     The basic principle of the invention is explained with reference to  FIG. 2 . Corresponding to the illustration in  FIG. 1 , the three states statically undamped, statically damped, and dynamically damped are shown in rows one beneath the other. 
     The first row essentially corresponds to a conventional sensor. The rectified signal  16  present at the threshold value comparator is already above the threshold value  13 , so that the output stage emits no output signal. 
     As previously illustrated in  FIG. 1 , for a statically damped sensor the amplitude of the signal of the oscillating circuit  10  is lower, so that the rectified signal  11  also has a lower value. As the result of dynamic recognition  15 , for example in the form of a high-pass filter which allows the rectified signal to pass through only when it changes at a predefined frequency, the rectified signal  11  is blocked in such a way that the signal  16  present at the threshold value comparator essentially corresponds to the signal of the undamped sensor. Accordingly, in the second row the rectified signal  16  in the threshold value comparator is above the threshold value  13 , so that the output stage emits no output signal. 
     The accuracy of the sensor may thus be greatly improved, and in particular it is possible to set the sensing distance more precisely. 
     The dynamic damping illustrated in row  3  once again essentially corresponds to the illustration in  FIG. 1 . The value of the rectified signal  11  varies, as the result of which the filter for static processes, in the present case referred to as dynamic recognition  15  and which is preferably designed as a high-pass filter, allows the signal  11  to pass through. The signal  16  present at the threshold value comparator is then periodically below the threshold value, and the output stage emits a periodic output signal  14 . 
       FIG. 2 a   , similar to  FIG. 2 , shows a sensor according to the invention which is dynamically damped; in this illustration an irregular rectified signal  11  is present due to objects moving past at various speeds. 
     The processing takes place analogously to  FIG. 2 . 
     It is apparent from this illustration that the output signal  14  has a pulse width which is limited by the high-pass filter or the means for suppressing the changes in the amplitude and/or frequency caused by static objects. 
     If a moving object to be detected passes into the detection range of the sensor, provided that the object moves past over a fairly long period of time, only the entry of the object into the detection range is detected, since further damping is likewise suppressed by the high-pass filter. The pulse width is therefore limited. 
       FIG. 3  schematically shows a wiring diagram of a sensor according to the invention. The important functional components of the switching circuit are divided into groups for purposes of explanation. 
     The switching circuit includes an oscillating circuit  20  which is connected to a coil and which supplies a preferably sinusoidal output signal. If a metallic object (not illustrated) approaches, the oscillating circuit  20  is detuned, thus reducing the amplitude of the signal, which is relayed to a rectifier  21 . 
     The oscillating circuit includes the two transistors  27  and  28 , which are used to avoid sensitivity to temperature. The connections to the coil are denoted by reference characters A and M. 
     In this exemplary embodiment, the rectifier  21  is designed as a filter circuit having capacitors  22  and  23 , so that the signal does not undergo ideal rectification and instead is merely smoothed. 
     The signal rectified in this manner is relayed via a capacitor  24  to a threshold value comparator  25 . 
     The capacitor  24  acts as a high-pass filter, so that a modified signal is present at the threshold value comparator  25  only when the magnitude of the rectified signal changes at a minimum frequency. Static damping of the sensor is suppressed in this manner. 
     When the signal value is below a threshold value, the threshold value comparator  25  relays the signal to an output stage  26 , which emits an appropriate output signal. 
     The design of a sensor is explained in greater detail with reference to  FIGS. 4 and 5 . 
     As illustrated in  FIG. 4 , the inductive sensor  1  includes a housing  2  having a thread  3 . The inductive sensor  1  may thus be easily mounted from the outside at a suitable location, and is particularly suited for exchanging with known inductive sensors based on the magnetic principle. The sensor  1  includes a connecting line  4  by means of which the sensing distance may also be adjusted. 
       FIG. 5  shows an inductive sensor  1  according to the invention, with the housing open. 
     A coil  5  having a ferrite core  6  is present in the front region of the sensor. The detection range is therefore at the front, outside the coil  5 . The plane defined by the coil  5  is essentially perpendicular to the axis of the sensor  1 . 
     A printed circuit board  7  on which the sensor electronics system is situated is located behind the coil  5 . 
     The sensor according to the invention is particularly suited for improved recognition of rather small moving masses, is independent of direction, and may be mounted from the outside. 
     It is understood that the invention is not limited to a combination of the features described above, but, rather, that all features may be combined by one skilled in the art if this is expedient. 
     LIST OF REFERENCE NUMERALS 
     
         
           1  Inductive sensor 
           2  Housing 
           3  Thread 
           4  Connecting line 
           5  Coil 
           6  Ferrite core 
           7  Printed circuit board 
           10  Signal of the oscillating circuit 
           11  Rectified signal 
           13  Threshold value 
           14  Output signal 
           15  Dynamic recognition 
           16  Signal present at the threshold value comparator 
           20  Oscillating circuit 
           21  Rectifier 
           22  Capacitor 
           23  Capacitor 
           24  Capacitor 
           25  Threshold value comparator 
           26  Output stage 
           27  Transistor 
           28  Transistor