Abstract:
A sensor for locating metallic or magnetizable objects comprises two emission coils and a receiving coil which are inductively interconnected. A method for determining the influence of temperature on the sensor includes supplying a first pair of predetermined alternating currents to the emitter coils, and simultaneously sampling current flows which pass through the emitter coils and a first current of the receiver coil. Subsequently, the method includes supplying a second pair of predetermined alternating currents to the emitter coils, and simultaneously sampling current flows which pass through the emitter coils and a second current of the receiver coil. The method further includes determining coupling factors between the emitter coils and the receiver coils based on the determined current flows and voltages, and determining the object based on the coupling factors.

Description:
[0001]    The present invention relates to an object locator, also referred to as a locating device. In particular, the present invention relates to a temperature-compensated object locator and a method for the temperature compensation of an object locator. 
       BACKGROUND INFORMATION  
       [0002]    An object locator for locating a metallic or magnetizable object generates an electromagnetic field by means of a transmitting coil and checks by means of a receiving coil whether the electromagnetic field was changed by the object. 
         [0003]    WO 2010/133328 A1 shows a metal detector having two transmitting coils and one receiving coil. The transmitting coils are supplied with phase-shifted alternating currents, and an output signal of a receiving coil which is inductively coupled to the transmitting coils is evaluated. Currents or voltages of the transmitting coils are changed as a function of the received signal in such a way that the received signal is extinguished. The presence of a metallic object in the area of the transmitting and receiving coils may then be inferred from the ratio of the voltages or currents of the transmitting coils. 
         [0004]    However, a change in a current flowing through the transmitting or receiving coil of an object locator may also be due to a temperature influence. For example, a current or voltage source for the transmitting coil or a measuring amplifier for the receiving coil may be subject to a temperature drift. Geometric ratios of the coils used may also be changed by a change in temperature. 
         [0005]    The drift may be so large that the object can no longer be located with absolute certainty, or is located in a place other than the determined place. 
         [0006]    The object of the present invention is therefore to provide a method for determining a temperature influence on a sensor for locating metallic or magnetic objects, a corresponding computer program product, and a corresponding sensor. Furthermore, the object of the present invention is to provide a measuring device, in particular an object locator, also referred to as a locating device. 
         [0007]    The present invention achieves these objects by means of the subject matter of the independent claims. Subclaims describe preferred specific embodiments. 
       SUMMARY OF THE INVENTION  
       [0008]    A sensor for locating metallic and/or magnetic objects comprises at least two transmitting coils and one receiving coil which are inductively coupled to each other. In one method according to the present invention for determining a temperature influence on the sensor, a first pair of predetermined AC voltages is provided at the transmitting coils, and currents flowing through the transmitting coils and a first voltage of the receiving coil are sampled, in particular they are sampled simultaneously, in particular during the application of the first pair of predetermined AC voltages. Subsequently, a second pair of predetermined AC voltages is provided at the transmitting coils, and the currents flowing through the transmitting coils and a second voltage of the receiving coil are sampled, in particular these currents and the voltage are sampled simultaneously, i.e., in particular during the application of the second pair of predetermined AC voltages. Based on the determined currents and voltages, coupling factors between the transmitting coils and the receiving coil are then determined, and the object is determined based on the coupling factors. A determination of the object is in particular understood to mean the identification of the location or position of the object and possibly also the identification of the type of object. 
         [0009]    The method is based on the idea that both the voltages of the receiving coil and the coupling factors change under the influence of an object in the area of the transmitting coils and the receiving coil. 
         [0010]    If there is a temperature influence which changes the voltages of the receiving coil, the coupling factors remain unchanged. By comparing the determined voltages and the determined coupling factors with reference values which, for example, are determined or fixedly predefined once, the temperature influence may be determined in a simple and reliable manner. Thus, for example, it is possible to determine or compensate for a temperature drift of a voltage or current source for the transmitting coils, or of a measuring amplifier for the receiving coil. The method may thus allow a temperature-compensated object determination. 
         [0011]    The object may in particular be determined, i.e., identified as or considered to be an object, if the coupling factors differ with respect to the coupling factors of a previous measurement. It is thus possible to distinguish reliably between an influence of the object and a temperature influence on the received signals. 
         [0012]    In one preferred specific embodiment, the previously determined voltages and coupling factors relate to an arrangement in which the inductive coupling between the transmitting coils and the receiving coil is uninfluenced by metallic or magnetizable objects. It is thus possible to ensure that no reference object having predetermined magnetic properties must be used for determining the temperature influence. 
         [0013]    In one specific embodiment, the voltages of the first pair of AC voltages are phase-shifted with respect to each other by less than ±5°, and the voltages of the second pair of AC voltages are also phase-shifted by less than ±5° with respect to each other. 
         [0014]    In one particularly preferred specific embodiment, constants A, B, C1, and C2 are chosen, wherein during the first pair of AC voltages, the voltage of the first transmitting coil is A and the voltage of the transmitting coil is B+C1. During the second pair of AC voltages, the voltage of the first transmitting coil is A+C2, and the voltage of the second transmitting coil is B. The summand C1 is approximately 3% to 7% of A, and the summand C2 is approximately 3% to 7% of B. Furthermore, A/B preferably corresponds to the ratio of the coupling factor of the second transmitting coil to that of the first transmitting coil. 
         [0015]    This choice of voltages and ratio makes it possible to prevent the voltage of the receiving coil from assuming the value zero during both the first and the second pair of AC voltages. The mathematical and process-related treatment of the above-described voltages and currents may thereby be carried out in a simplified manner. As a result, dealing with a special case or discarding an erroneous measurement may be unnecessary. 
         [0016]    In yet another specific embodiment, the coupling factors are determined and offset against each other a first time, in particular scaled and deducted from each other in pairs, and after reconfiguration of the measuring arrangement, in particular after reversing the polarity of the receiving coil, are determined and offset against each other a second time. It is thus possible to further diminish the temperature influence on a measurement result, on which the determination of the object is based, in a simple manner. 
         [0017]    One computer program product according to the present invention includes program code means for carrying out the described method if the computer program product runs on a processing device or is stored on a computer-readable data carrier. 
         [0018]    A sensor according to the present invention for locating a metallic or magnetizable object comprises at least two transmitting coils and one receiving coil which are inductively coupled to each other, a control device for supplying the transmitting coils with AC voltages, and a sampling device for determining currents flowing through the transmitting coils and for determining the received signal, while voltages at the transmitting coils assume first and second predetermined value pairs. In this regard, the processing device is configured to determine coupling factors between the transmitting coils and the receiving coil based on the determined currents and voltages, and to determine the object based on the coupling factors. 
         [0019]    Such a sensor may take into consideration the determined temperature influence during normal operation for detecting an object, in order to compensate for resulting measurement errors which occur. As a result, a temperature-compensated determination of the object may be made possible. 
         [0020]    It is thus advantageously possible to achieve a measuring device, in particular a locating device for detecting objects enclosed in a medium, which is compact, has high performance, and requires no calibration, i.e., no longer has to be calibrated by a user before each individual measurement. 
         [0021]    In one specific embodiment, parts of the sensor are implemented by means of a programmable microcomputer. The sampling device may comprise an analog-digital converter, and selection means may be provided in order to alternatively sample the received signal or one of the coil currents by means of the sampling device. To determine the coil currents, a measuring resistor may be connected in series with each transmitting coil, the voltage dropping across the measuring resistor indicating the current flowing through the transmitting coil. The analog-digital converter may be comprised by the programmable microcomputer, and the selection means may be implemented in a simple manner in order to measure the aforementioned currents or voltages. As a result, circuit complexity of the (possibly internal) peripherals of the programmable microcomputer may be kept low. Accordingly, manufacturing costs of the sensor may be reduced. 
         [0022]    In a similar way, the control device may comprise a digital-analog converter for controlling at least one of the transmitting coils. Two digital-analog converters may also be provided, one being fixedly assigned to each of the transmitting coils. 
         [0023]    Optionally, a first reference voltage source may be provided for supplying the receiving coil with a predetermined voltage. The received signal, which is determinable by means of the receiving coil, may thus be related to the voltage of the first reference voltage source. A second reference voltage source may be provided for supplying each of the transmitting coils with a predetermined voltage. In an additional specific embodiment, the voltage of the first reference voltage source or the voltage of the second reference voltage source may be connected to the analog-digital converter by means of the aforementioned selection means, in order to determine absolute values of the reference voltages. As a result, it may be prevented that a temperature drift of the reference voltage sources is undetected. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES  
         [0024]    The present invention will now be described in greater detail with reference to the attached figures. 
           [0025]      FIG. 1  shows a circuit diagram of a sensor for detecting metallic or magnetizable objects; 
           [0026]      FIG. 2  shows two specific embodiments of coils of the sensor from  FIG. 1 ; and 
           [0027]      FIG. 3  shows a flow chart of a method for determining a temperature influence on the sensor from  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0028]      FIG. 1  shows a circuit diagram of a sensor  100  for detecting metallic or magnetizable objects  105 . The sensor  100  comprises a first transmitting coil  110 , a second transmitting coil  115 , and a receiving coil  120  which are inductively coupled to each other, a preferred structure of the coils  110 ,  115 , and  120  being explained in greater detail with reference to  FIG. 2 . The transmitting coils  110  and  115  are supplied with AC voltages, so that they generate electromagnetic fields which act on the receiving coil  120 . The receiving coil  120  converts the electromagnetic field, which is possibly influenced by the object  105 , into a voltage which is available as a received signal. Based on the received signal, it is possible to infer the presence or absence of the object  105  taking into consideration the geometric arrangement of the coils  110 ,  115 , and  120 . 
         [0029]    For amplification or impedance conversion, a first transmitting amplifier  125  is assigned to the first transmitting coil  110 , a second transmitting amplifier  130  is assigned to the second transmitting coil  115 , and a receiving amplifier  135  is assigned to the receiving coil  120 . In the circuit diagram of  FIG. 1 , a portion of the sensor  100  is preferably designed using digital technology. The digital elements may in particular be comprised by an integrated, programmable microcomputer. 
         [0030]    Alternatively, some or all of the elements may be structured discretely or in an analog manner. The first transmitting amplifier  125  is controlled by a first analog-digital converter  145 , and the second transmitting amplifier  130  is controlled by a second analog-digital converter  150 . The received signal of the receiving amplifier  135  is provided to a digital-analog converter  155 . A processing device  160  is configured to control the analog-digital converters  145  and  150  in such a way that AC voltages are present at the transmitting coils  110  and  115  which generally have different amplitudes and are subject to a small phase shift, generally in the range of below approximately 10°. However, the frequencies and pulse shapes used are the same; a rectangular or an at least approximated sinusoidal shape is used as the preferred pulse shape. 
         [0031]    To determine a temperature influence on the sensor  100 , a first measuring resistor (shunt)  165  is provided in series with the first transmitting coil  110 , and a second measuring resistor  170  is provided in series with the second transmitting coil  115 . The voltages dropping across the measuring resistors  165  and  170  indicate the currents flowing through the transmitting coils  110  and  115 . The measuring resistors  165  and  170  preferably have a low temperature dependency and a low component tolerance. 
         [0032]    Dedicated receiving amplifiers  135  or sampling devices may be assigned to the voltage of the receiving coil  120  and to the currents flowing through the transmitting coils  110  and  115 . In the depicted preferred specific embodiment, instead, selection means  175  are provided which are controllable via the processing device  160 , in order to connect only one of multiple, different signals to the input of the receiving amplifier  135 . The selection means  175  preferably comprise switches or comparable switching elements, of which not more than one is closed at any time. Thus, alternatively, the voltage dropping across the first measuring resistor  165  or the voltage dropping across the second measuring resistor  170  or the measuring signal at the receiving coil  120  may be connected to the input of the receiving amplifier  135 . 
         [0033]    In the depicted preferred specific embodiment, an end of the receiving coil  120  which is not connected to the input amplifier  135  is connected to a first reference voltage source  180 , and ends of the transmitting coils  110  and  115  which are not connected to the transmitting amplifiers  125  and  130  are connected to a second reference voltage source  185 . 
         [0034]    The reference voltage sources  180  and  185  each provide a predetermined DC voltage in order to be able to operate the coils  110 ,  115 , and  120  at a predetermined operating point. In the depicted specific embodiment, the selection means  175  are also configured alternatively to connect the second reference voltage source  185  to the receiving amplifier  135 . In one enhancement, the selection means  175  may also be configured to connect the first reference voltage source  180  to the receiving amplifier  135 . 
         [0035]    The processing device  160  is configured to apply first AC voltages to the transmitting coils  110  and  115  by means of the output amplifiers  125 ,  130  and then to determine the input signal of the receiving coil  120  and the currents flowing through the transmitting coils  110  and  115  by means of the selection means  175  and the input amplifier  135 . The AC voltages present at the transmitting coils  110  and  115  preferably have predetermined amplitude ratios and a predetermined phase shift. This approach is subsequently repeated with second AC voltages which are different from the first AC voltages. 
         [0036]    A first coupling factor K1 between the first transmitting coil  110  and the receiving coil  120 , and a second coupling factor K2 between the second transmitting coil  115  and the receiving coil  120 , are then determined based on the determined currents and voltages. At least coupling factors, preferably coupling factors and received signals of a preceding determination, are stored in a memory  190 . The stored values may be due to the design of the sensor  100  and fixedly predefined. In another specific embodiment, the values are determined once, for example, in the described manner, while no object  105  is in the inductive area of influence of the coils  110  to  120 ; then, the defined values are stored in the memory  190 . 
         [0037]    If the received signals of the receiving coil  120  are different from the stored received signals, it may be possible to attribute this to an object  105  or a temperature influence on portions of the sensor  100 . If the determined coupling factors K1 and K2 do not differ from the stored coupling factors, the sensor  100  is subject to a temperature influence. The determined temperature influence may be output via an interface  195 . Otherwise, if the coupling factors K1, K2 are different with respect to the previous measurement, an object  105  is present in the area of the coils  110  to  120 . The size, type, and position of the object  105  may then be determined more exactly based on the determined received signals and coupling factors with reference to the geometry of the transmitting coils  110  and  115  and the receiving coil  120 . The result may then also be output via the interface  195 . 
         [0038]    For the measurements of the currents and voltages at the coils  110  to  120 , the following equations apply: 
         [0000]        U   sec (1)= K 1 ·I 1(1)+ K 2 ·I 2(1)   (Equation 1)
 
         [0000]        U   sec (2)= K 2· I 1(2)+ K 2 ·I 2(2)   (Equation 2)
 
         [0039]    where: 
         [0040]    U sec (1) is the voltage of the receiving coil ( 120 ) (received signal) during the first AC voltage, 
         [0041]    U sec (2) is the voltage of the receiving coil ( 120 ) (received signal) during the second AC voltage, 
         [0042]    I1(1) is the current flowing through the first transmitting coil  110  during the first AC voltage, 
         [0043]    I2(1) is the current flowing through the second transmitting coil  110  during the first AC voltage, 
         [0044]    I1(2) is the current flowing through the first transmitting coil  110  during the second AC voltage, 
         [0045]    I2(2) is the current flowing through the second transmitting coil  110  during the second AC voltage, 
         [0046]    K1 is the coupling factor between the first transmitting coil ( 110 ) and the receiving coil ( 120 ), and 
         [0047]    K2 is the coupling factor between the second transmitting coil ( 110 ) and the receiving coil ( 120 ). 
         [0048]    The equation system of equations 1 and 2 is solved according to the coupling factors K1 and K2, and the coupling factors K1 and K2 are determined based on the determined voltages and currents. 
         [0049]    In one preferred specific embodiment, the AC voltages of the first transmitting coil  110  and the second transmitting coil  115  each have a phase difference Δ of less than approximately ±5° during the first or second AC voltages. The amplitudes of the voltages at the transmitting coils  110  and  115  are preferably chosen as a function of the coupling factors K1 and K2. For this purpose, constants A and B are formed, and during the first AC voltage, the voltage of the first transmitting coil  110  is (A) and the voltage of the second transmitting coil  115  is (B+C1). During the second AC voltage, the voltage of the first transmitting coil  110  is (A+C2) and the voltage of the second transmitting coil  115  is (B). The summands C1 and C2 are preferably equal. C1 is preferably approximately 3% to 7% of A; C2 is preferably approximately 3% to 7% of B. The ratio A/B preferably corresponds to the ratio K2/K1, the reciprocal value of the coupling factors K1 and K2. 
         [0050]    In an additional specific embodiment, the received signal, i.e., the voltage of the receiving coil  120 , is determined under different polarities of the receiving coil  120 , while the AC voltages at the transmitting coils  110  and  115  remain unchanged. The voltage of the first transmitting coil  110  is thus A and the voltage of the second transmitting coil  115  is B; a phase difference Δ between the voltages is in the range of approximately ±5°. A, B, and Δ are chosen in such a way that the received signal does not change when reversing the polarity of the receiving coil  110 . 
         [0051]      FIG. 2  shows two specific embodiments of coils  110  to  120  of the sensor  100  from  FIG. 1 . The coils are distributed over two planes, which are offset in parallel with the observation plane. In the exemplary embodiment shown above from  FIG. 2A , the first transmitting coil  110  is in a first plane and the second transmitting coil  115  is in a second plane. The receiving coil  120  comprises a first D-shaped portion  205  which is in the first plane and a second D-shaped portion  210  which is in the second plane. Directions along which the portions  205  and  210  or the transmitting coils  110  and  115  are adjacent to each other form an angle α between them, which is preferably not equal to 90°. 
         [0052]      FIG. 2B  shows one specific embodiment which resembles that of  FIG. 2A , in which, however, the portions  205  and  210  of the receiving coil  120  have different numbers of turns. The specific embodiments of  FIGS. 2A and 2B  are preferably suitable for use on the sensor  100  from  FIG. 1 . 
         [0053]    However, other specific embodiments or arrangements of the coils  110 ,  115 , and  120  are possible, if the transmitting coils  110  and  115  are inductively coupled to the receiving coil  120  and there is an electromagnetic area of influence in which a generated electromagnetic field is able to be influenced by an object  105 . 
         [0054]      FIG. 3  shows a flow chart of a method for determining a temperature influence on a sensor for locating metallic or magnetic objects like that of  FIG. 1 . The method  300  is in particular configured for running on the processing device  160 . 
         [0055]    In a first step  305 , a first AC voltage is applied to the first transmitting coil  110 . In a step  310 , a second AC voltage is applied to the second transmitting coil  115 . Subsequently, in a step  315 , the current I1(1) flowing through first transmitting coil  110  is determined; in a step  320 , the current I2(1) flowing through the second transmitting coil  115  is determined; and in a step  325 , the voltage at the receiving coil  120  is determined. 
         [0056]    In a similar manner, in a step  330 , a third AC voltage is applied to the first transmitting coil  110 , and in a step  335 , a fourth AC voltage is applied to the second transmitting coil  115 . Afterwards, in a step  340 , the current I1(2) flowing through the first transmitting coil  110  is determined; in a step  345 , the current I2(2) flowing through the second transmitting coil  115  is determined; and in a step  350 , the voltage of the receiving coil  120  is determined. In a variant of the method  300 , the determination of the currents I1(1), I2(1), I1(2), and I2(2) may also be carried out less frequently than the determination of the voltage at the receiving coil  120 . 
         [0057]    Subsequently, in a step  355 , coupling factors K1 and K2 are determined, as described above in greater detail with reference to equations 1 and 2. In an additional specific embodiment, the steps  305  to  355  may subsequently also be run through again, with the difference that the polarity of the receiving coil  120  is reversed. In this case, the voltages at the transmitting coils  110  and  115  preferably remain unchanged, so that the steps  315  and  320 , in which the transmitting currents of the transmitting coils  110  and  115  are determined, preferably have to be run through only once. Reversing the polarity may be carried out by means of the selection means  175 , or a dedicated polarity reversal device may be provided for the receiving coil  120 , for example, a relay or a bridge circuit. In one specific embodiment, each end of the receiving coil  120  may alternatively be connected to the input amplifier  135  or the first reference voltage source  175  by means of the polarity reversal device. It may also be provided to isolate the receiving coil  120  on one or both sides from the rest of the circuit. Subsequently, the determined coupling factors K1 and K2 of both measurements are scaled and subtracted from each other in pairs: 
         [0000]        K 1 ′=K 1(1)− K 1(2)   (Equation 3)
 
         [0000]        K 2 ′=K 2(1)− K 2(2)   (Equation 4)
 
         [0058]    where: 
         [0059]    K1(1) is the first coupling factor K1 between the first transmitting coil  110  and the receiving coil  120  after the first iteration, 
         [0060]    K1(2) is the first coupling factor K1 between the first transmitting coil  110  and the receiving coil  120  after the second iteration, 
         [0061]    K2(1) is the second coupling factor K2 between the second transmitting coil  115  and the receiving coil  120  after the first iteration, 
         [0062]    K2(2) is the second coupling factor K2 between the second transmitting coil  115  and the receiving coil  120  after the second iteration. 
         [0063]    Before subtracting, the coupling factors K1 and K2 may also be provided with scaling. Scaling and subtraction in pairs may possibly also be carried out separately for the real and imaginary parts of the coupling factors. The required scaling factors may be stored in the nonvolatile memory of the processing unit or calculated by the processing unit based on measurements. 
         [0064]    In particular, if a voltage amplifier is used for measuring the voltage induced in the receiving coil  120 , this voltage may be a function of the polarity with which the receiving coil  120  is connected to the receiving amplifier  135 . This behavior always appears if the input impedance of the receiving amplifier  135  is different from the impedance of the second reference voltage source  185 . By reversing the polarity of the receiving coil  120 , determining the coupling factors K1 and K2 for both polarities, and subtracting the coupling factors K1 and K2 from each other in pairs, the dependency of the voltage induced in the receiving coil  120  on the polarity may be eliminated. Thus, it is also possible to eliminate unavoidable temperature influences on the induced voltage, which arise due to temperature-related changes in the impedances of the input amplifier  135  or the second reference voltage source  185 . 
         [0065]    When subtracting the coupling factors K1 and K2 determined under differing polarities, interference, the sign of which is a function of the polarity the polarity of the receiving coil  120 , doubles in magnitude. This in particular relates to induction-related interference. On the other hand, interference, the sign of which sign which is invariant with respect to the polarity under which the coupling factors K1 and K2 were determined, is eliminated. This in particular includes capacitance-related interference. By subtracting, the coupling factors K1′ and K2′ are better able to indicate the object  105  instead of a temperature influence. 
         [0066]    In a step  360 , it is determined that the received signals of the receiving coil  120  are different from the stored values which, for example, may be obtained from the memory  190  in  FIG. 1 . Afterwards, in a step  365 , it is determined whether the determined coupling factors K1 and K2 differ from the stored coupling factors. If this is the case, in a step  370 , the presence of an object  105  in the area of the coils  110 ,  115 , and  120  is inferred. Otherwise, in a step  375 , a temperature influence on the sensor  100  is determined. 
         [0067]    The method  300  may subsequently return to step  305  and be performed again. A temperature influence determined in a step  375  may be taken in consideration during a later determination of the object  105  in a step  375 , so that the temperature influence on the sensor  100  is compensated for, and the signal provided at the interface  195  indicating the object  105  is adjusted for the temperature influence.