Abstract:
A measuring device configured to detect a metal object includes two emission coils configured to produce superimposed magnetic fields. The measuring device also includes a device configured to determine a differential voltage between the emission coils. The measuring device further includes a control device configured to supply the emission coils with alternating voltages such that the value of an AC voltage component of the differential voltage, which is time synchronized with the alternating voltage, is minimized. The control device is configured to detect the metal object when the ratio of the alternating voltages does not correspond to the ratio of the impedances of the emission coils when the metal object is not there.

Description:
This application is a 35 U.S.C. §371 National Stage Application of PCT/EP2011/053451, filed on Mar. 8, 2011, which claims the benefit of priority to Serial No. DE 10 2010 028 723.7, filed on May 7, 2010 in Germany, the disclosures of which are incorporated herein by reference in their entirety. 
     Certain work on workpieces presents the risk of an article concealed in the workpiece being damaged by the work. By way of example, when drilling into a wall, a water, power or gas line running inside the wall can be damaged. Conversely, it may be desirable to perform the work specifically such that an article concealed in the workpiece is worked on at the same time, for example if the hole from the above example is intended to pass through a steel reinforcement or a supporting structure inside the wall. 
     BACKGROUND 
     For the purpose of sensing such a concealed article, coil-based metal detectors are known in the prior art. Such detectors produce a magnetic field in a measurement region. If there is a metallic article in the measurement region, the article is recognized on the basis of its influence on the magnetic field produced. Frequently, the magnetic field produced is determined by using at least two reception coils which are oriented and connected to one another such that in the absence of a metallic object in the measurement region the measurement signal delivered by both reception coils together is virtually zero (differential measurement). In one variant, a plurality of transmission coils are used to produce the magnetic field, said transmission coils being activated such that the signal measured in the two reception coils is virtually zero regardless of any absence of a metallic object in the measurement region (field-compensated measurement). 
     DE 10 2007 053 881 A1 describes a measurement method for determining the position or the angle of a coil relative to two further coils. To this end, two transmission coils arranged at an angle relative to one another are used to generate a magnetic alternating field. A reception coil is put into the magnetic alternating field and the actuation of the transmission coils is altered such that the same voltage is induced in the reception coil by each of the transmission coils. A ratio of current values supplied to the transmission coils serves as a measure of determination of the position and/or angle of the reception coil relative to the transmission coils. 
     DE 10 2004 047 189 A1 describes a metal detector having printed coils. 
     The disclosure is based on the object of providing simple precise detection for a metallic object. A further object of the disclosure is to specify a method for determining the metallic object. 
     SUMMARY 
     The disclosure achieves these objects by means of a measuring apparatus having the features described below and a method having the features described below. Description below specifies preferred embodiments. 
     According to the disclosure, a measuring apparatus for sensing a metallic object comprises two transmission coils for producing overlaid magnetic fields and a device for determining a differential voltage applied between the transmission coils. In addition, a control device is provided in order to supply the transmission coils with alternating voltages such that an AC voltage component of the differential voltage, which AC voltage component is in sync with the alternating voltages, is minimized in terms of absolute value. The control device is set up to sense the object when the ratio of the alternating voltages does not correspond to the ratio of impedances of the transmission coils in the absence of the metallic object. 
     This is the case when the impedances of the transmission coils are influenced by the metallic object in different ways on the basis of different distances. By dispensing with a receiver coil or a magnetic sensor in the region of the transmission coils, it is possible to reduce the number of coils for the measuring apparatus, which allows the space required for the measuring apparatus to be reduced and manufacturing costs to be saved. The compact design which is thus possible allows a large number of measuring apparatuses to be arranged on a small space, which means that it is possible to increase a spatial resolution of measured values to enter a range that can be presented. 
     Preferably, the alternating voltages are AC voltages, in order to change the magnetic fields of the transmission coils periodically in terms of absolute value and phase. The AC voltages allow synchronous demodulation, which can be used to very effectively reject interfering signals at frequencies that are not equal to the modulation frequency. Furthermore, the AC voltages can produce alternating magnetic fields, in order to induce eddy currents in nonmagnetic materials, such as copper, on the basis of which said materials can then be detected. 
     Preferably, the device for determining the differential voltage is formed from two series-connected nonreactive resistors which are each part of the complex resistance of one of the transmission coils. Discrete resistors can thus be dispensed with, which allows manufacturing costs to be reduced. 
     The transmission coils may be arranged in two spaced, parallel planes, such that the magnetic fields therefrom are oriented parallel. It is then possible to recognize a position of the metallic object relative to the planes from which transmission coil is being supplied with a voltage that is magnified in comparison with the object-free case. Metallic objects which are situated on the other side of the plane of one of the transmission coils can be ignored, in order to avoid a false measurement which is caused by a user of the measuring apparatus, for example. 
     One or both of the transmission coils may be air-core coils and may be in the form of printed coils, particularly in the form of a printed circuit on a circuit board. This allows the measuring device to be designed such that it reacts only very slightly to temperature or ageing influences, which allows calibration to be effected once in the course of manufacture of the measuring apparatus. By virtue of the transmission coils being in the form of printed coils on the circuit board, it is possible for the transmission coils to be produced precisely with little production complexity. In this case, the control device may be designed to be on the same circuit board. By minimizing wiring and component-fitting costs, it is thus possible to save further manufacturing costs. 
     According to a further aspect of the disclosure, a method for sensing a metallic object comprises the steps of producing magnetic fields in an aligned orientation by means of two transmission coils, supplying the transmission coils with alternating voltages and sensing the object when the ratio of the alternating voltages does not correspond to the ratio of the currents flowing through the transmission coils. 
     The disclosure may also be implemented as a computer program product, wherein a computer program product according to the disclosure comprises program code means for carrying out the described method and is able to be executed on a processing device or stored on a computer-readable data storage medium. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The disclosure is described in more detail below with reference to the accompanying figures, in which: 
         FIG. 1  shows a block diagram of a measuring apparatus; 
         FIG. 2  shows an arrangement of coils and a metallic object on the measuring apparatus from  FIG. 1 ; 
         FIG. 3  shows an arrangement of a plurality of pairs of transmission coils for the measuring apparatus from  FIG. 1 ; and 
         FIG. 4  shows a flowchart for a method for the measuring device from  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a block diagram of a measuring apparatus  100 . The measuring apparatus  100  is part of a metal detector  105  for sensing metallic objects, for example made of ferrous material. 
     A clock generator  110  has two outputs at which it provides periodic alternating signals having a phase shift, preferably a 180° phase shift. The alternating signals may comprise square-wave, triangular-waveform or sinusoidal signals, in particular. The outputs of the clock generator are connected to a first controllable amplifier  115  and a second controllable amplifier  120 , respectively. Each of the controllable amplifiers  115 ,  120  has a control input which it uses to receive a signal which controls a gain factor of the controllable amplifier  115 ,  120 . An output of the first controllable amplifier  115  is connected to a first transmission coil  125  and one output of the second controllable amplifier  120  is connected to a second transmission coil  130 . The remaining ends of the transmission coils  125  and  130  are connected to one another via a first and a second resistor  135   a ,  135   b , respectively. In one embodiment, the resistors  135   a ,  135   b  are formed by the nonreactive resistances of the transmission coils  125  and  135 , respectively. 
     Each of the two transmission coils  125  and  130  carries the same respective current at any time. The polarities and amplitudes of the alternating voltages supplied to the two transmission coils  125 ,  130  do not have a separate influence on the magnetic fields produced in the coils  125 ,  130 . As indicated in  FIG. 1  by the dots on the transmission coils  125 ,  130 , the two transmission coils  125 ,  130  are oriented in the same sense. Therefore, the transmission coils set up magnetic fields with an orientation in the same direction; although magnetic fields in opposite directions, as are produced by transmission coils wound in opposite senses, are likewise possible, they are generally less advantageous on account of the minute dipole component of the overlaid magnetic field. However, it is conceivable that this disadvantage is consciously utilized when it is desirable to restrict the detection depth. 
     A connection runs from the interconnected resistors  135   a ,  135   b  to an input amplifier  140 . The input amplifier  140  is shown with a constant gain factor; in other embodiments, however, a gain factor for the input amplifier  140  may also be controllable. By way of example, this allows a spatial resolution and/or sensitivity of the measuring apparatus  100  to be influenceable and, by way of example, to be controllable on the basis of a measured variable. 
     The output of the input amplifier  140  is connected to a synchronous demodulator  145 . The synchronous demodulator  145  is also connected to the clock generator  110  and receives from the latter a clock signal which indicates the phase of the signals provided at the outputs of the clock generator  110 . In a simple embodiment, in which the signals provided by the clock generator  110  are symmetrical square-wave signals, one of the output signals can be used as clock signal. The synchronous demodulator  145  essentially takes the clock signal provided by the clock generator  110  as a basis for connecting the measurement signal received from the input amplifier  140  alternately to its upper or lower output. 
     The two outputs of the synchronous demodulator  145  are connected to an integrator (integrating comparator)  150 , which in this case is shown as an operational amplifier connected up to two resistors and two capacitors. Other embodiments are likewise possible, for example as an active low-pass filter. A digital implementation subsequent to the synchronous demodulator is also conceivable, in the case of which the signal at the output of the synchronous demodulator is subjected to analog-to-digital conversion at one or more time(s) within a half-cycle and is then compared with the corresponding value from the next half-cycle. The difference is integrated and, by way of example, converted back to an analog signal and used for controlling the amplifiers. While the synchronous demodulator  145  provides the measurement signal received from the input amplifier  140  at the lower of its outputs, the integrator  150  integrates this signal over time and provides the result at its output. While the synchronous demodulator  145  provides the measurement signal received from the input amplifier  140  at its upper output, this signal is integrated by the integrator  150  in inverted form over time and the result is provided at the output of the integrator  150 . The voltage at the output of the integrator is the integral of the difference between the low-pass-filtered outputs of the synchronous demodulator. 
     The differential voltage that can be tapped off between the resistors  135   a ,  135   b  can be used to sense an impedance difference between the transmission coils  125  and  130 . The impedance of each of the transmission coils  125 ,  130  is dependent on a distance of a metallic object from the transmission coil  125 ,  130 . If the impedances of the transmission coils  125 ,  130  are of the same magnitude, the signals provided at the outputs of the synchronous demodulator  145  are also of equal magnitude on average over time, and the output of the integrator  150  provides a signal which is virtually zero (ground). If the impedances of the transmission coils  125 ,  130  differ, however, then the signals provided at the outputs of the synchronous demodulator  145  are no longer equal on average, and the output of the integrator  150  provides a positive or negative signal. 
     The signal provided by the integrator  150  is provided via a connection  155  for further processing. In addition, a microcomputer  165  may be connected to the control inputs of the controllable amplifiers  115 ,  120 . The microcomputer  165  compares the provided signal with a threshold value and outputs at an output  170  a signal which indicates the metallic object. The signal can be presented to a user of the metal detector  105  visibly and/or audibly. 
     Furthermore, the microcomputer  165  can perform further processing of the signals tapped off from the control inputs of the controllable amplifiers  115 ,  120  and can take said signals as a basis for controlling parameters of the measuring apparatus  100 . By way of example, a frequency or signal shape of the alternating voltages at the outputs of the clock generator  110  can be varied or a sensitivity of the reception amplifier  140  can be changed. In a further embodiment, further instances of the elements shown from the measuring apparatus  100  are implemented by the microcomputer  165 , for example the clock generator  110 , the synchronous demodulator  145  or the integrator  150 . 
     The same signal from the integrator  150  is also used to control the gain factors of the controllable amplifiers  115  and  120 , wherein the second controllable amplifier  120  is connected directly to the output of the integrator  150  and the first controllable amplifier  115  is connected to the output of the integrator  150  by means of an inverter  160 . The inverter  160  prompts inversion of the signal with which it is provided such that on the basis of the output signal from the integrator  150  the gain factor of the first controllable amplifier  115  increases to the extent that the gain factor of the second controllable amplifier  120  decreases, or vice versa. It is also conceivable for only the gain factor of one of the two controllable amplifiers  115 ,  120  to be controlled, while the gain factor of the second controllable amplifier  115 ,  120  is kept at a fixed value. 
     In comparison with a measuring apparatus having only one transmission coil, a temperature influence on the impedance of the transmission coils  125 ,  130  is compensated for by appropriate changes in both transmission coils  125 ,  130  in the case of the present measuring apparatus. Furthermore, a suitable geometric arrangement of the transmission coils  125 ,  130  allows an increased directional effect of the measuring apparatus  100  to be attained. 
       FIG. 2  shows an arrangement  200  of the transmission coils  125 ,  130  relative to a metallic object  210  in order to explain the measurement principle of the measuring apparatus  100  from  FIG. 1 . The transmission coils  125  and  130  are oriented relative to one another such that the directions of their main magnetic fields are in alignment with one another, with the transmission coils  125 ,  130  being at a certain distance. In the case of transmission coils  125 ,  130  which have a diameter substantially greater than their length, for example when the transmission coils  125 ,  130  are in the form of printed coils, the transmission coils  125 ,  130  may be situated in mutually parallel planes, for example on opposite surfaces of a circuit board in the example of the printed coils. 
     As described above with reference to  FIG. 1 , the transmission coils  125 ,  130  are arranged and connected to one another such that they each generate alternating magnetic fields on the basis of the signals provided by the clock generator  110 . 
     A metallic object  210  is situated in the region of the magnetic fields from the transmission coils  125  and  130 , said metallic object  210  being at a shorter distance from the first transmission coil  125  than from the second transmission coil  130 . The magnetic field from the first transmission coil  125  is thus influenced by the metallic object  210  to a greater extent than the magnetic field from the second transmission coil  130 . Accordingly, the impedance of the first transmission coil  125  differs from the impedance of the second transmission coil  130 . If the impedances of the transmission coils  125  and  130  are of different magnitude, the voltage between the resistors  135   a  and  135   b  has an AC voltage component that is different than zero and that is in sync with the alternating voltages of the controllable amplifiers  115 ,  120 . This synchronous AC voltage component is determined by the synchronous demodulator  145  and the downstream integrator  150 . The absolute value of the synchronous AC voltage component of the differential voltage is dependent on the imbalance in the impedances of the transmission coils  125 ,  130 . The output of the integrator  150  therefore produces a signal which is dependent on the imbalance in the magnetic fields. The phase of the synchronous AC voltage component of the differential voltage differs by 180° depending on whether the metallic object  210  is closer to the first transmission coil  125  than to the second transmission coil  130 , as shown, or whether the metallic object  210  is closer to the second transmission coil  130  than to the first transmission coil  125 . 
     Depending on the output voltage from the integrator  150 , the gain factors of the controllable amplifiers  115 ,  120  are altered in different directions, with the result that the transmission coils  125 ,  130  are supplied with voltages of different magnitude. It is also possible for only the gain factor of one of the controllable amplifiers  125 ,  130  to be altered, while the gain factor of the second controllable amplifier  125 ,  130  is kept at a fixed value. For the arrangement shown in  FIG. 1 , each of the two transmission coils  125 ,  130  carries the same current at any time. On account of the different impedances of the transmission coils  125 ,  130  when an object  220  is present, however, the voltage drop across the transmission coils  125  and  130  is of different magnitude and a synchronous AC voltage component of the differential voltage that is different than zero is obtained. 
     The presence of the metallic object  210  in the magnetic fields can be sensed by virtue of the deviation in the control signal applied to the connection  155  from zero. In one embodiment, metallic objects are sensed only on the basis of a predetermined arithmetic sign of the control signal. Thus, objects on one side of the transmission coils  125 ,  130  are ignored, these possibly being caused by a user of the measuring apparatus, for example. 
     In a further embodiment shown in  FIG. 2 , a third resistor  135   c  and a fourth resistor  135   d  are provided, each of which route a connection of the transmission coils  125 ,  130  which is connected to one of the resistors  135   a  and  135   b  to ground. A fifth resistor  135   e  is routed from the input of the input amplifier  140  to ground. In contrast with the embodiment shown in  FIG. 1 , the embodiment shown in  FIG. 2  allows each of the two transmission coils to carry different currents at any time. The ground-referenced polarities and amplitudes of the alternating voltages applied to the two transmission coils  125 ,  130  therefore have a separate influence on each of the two magnetic fields produced in the coils  125 ,  130 . The following variations of the measuring apparatus  100  can thus be implemented:
         the controllable amplifiers  115  and  120  deliver opposite voltages, referenced to ground. In the object-free case, the voltages have identical amplitudes. In the presence of a metallic object  210 , the amplitudes of the applied voltages differ. Preferably, magnetic fields in the same direction are used, but magnetic fields in opposite directions are also conceivable.   the controllable amplifiers  115  and  120  deliver voltages in the same direction, referenced to ground, and the windings of the transmission coils  125 ,  130  are in the same sense, with the result that magnetic fields in the same direction are produced. In this case, the voltages which are applied to the two transmission coils  125 ,  130  during one half-cycle already have different amplitudes in the object-free case. However, in the subsequent half-cycle, these amplitudes appear on the respective other transmission coil  125 ,  130  in the object-free case. In the presence of a metallic object  210 , on the other hand, the amplitudes on the respective other transmission coil  125 ,  130  also differ in successive half-cycles.   the controllable amplifiers  115  and  120  deliver voltages in the same direction, referenced to ground, and the windings of the transmission coils  125 ,  130  are in opposite senses, with the result that magnetic fields in opposite directions are produced. For the amplitudes of the voltages, the comments regarding the preceding variation apply.       

       FIG. 3  shows an arrangement  300  with a plurality of pairs of transmission coils for the measuring apparatus  100  from  FIG. 1 . In addition to the arrangement of the transmission coils  125 ,  130  with the resistors  135   a ,  135   b  which is described with reference to  FIG. 1 , appropriately connected further transmission coils  325 ,  330  with further resistors  335   a ,  335   b  are provided. Two switches  310  and  320  that are coupled to one another connect respective connections of the transmission coils  125 ,  130  or of the transmission coils  325 ,  330  selectively to the outputs of the controllable amplifiers  115 ,  120  from  FIG. 1 . The connections between mutually corresponding resistors  135   a ,  135   b ,  335   a  and  335   b  are connected to one another and are routed to the input amplifier  140 . 
     The coil pairs  125 ,  130  and  325 ,  330  may be arranged in one plane or may be situated in different planes. Particularly coils that are situated next to one another may be in the form of printed coils. If the differential voltage on the input amplifier  140  changes when the switches  115  and  120  are changed over, it is possible to infer a direction in which the metallic object  210  is situated on the basis of the geometric arrangement of the coil pairs  125 ,  130  and  325 ,  330 , for example by means of triangulation. Similarly, it is possible to infer a distance of the object. The direction-finding can be refined by further coil pairs. If a large number of sufficiently closely arranged transmission coils are used, it is possible to increase a resolution of the measuring apparatus  100  until it enters a graphical range. 
       FIG. 4  shows a schematic flowchart of a method  400  for sensing a metallic object  210  in line with the measuring apparatus  100  from  FIGS. 1 and 2 . In a step  410 , the transmission coils  125 ,  130  are used to produce magnetic alternating fields oriented in the same direction. In a subsequent step  420 , the transmission coils  125 ,  130  are supplied with phase shifted alternating voltages from the clock generator  110  by controlling the gain factors of the amplifiers  115 ,  120 , specifically such that the AC voltage component of the differential voltage—which component is in sync with the alternating voltages—is minimized in terms of absolute value. In a final step  430 , the metallic object  210  is sensed when the ratio of the alternating voltages does not correspond to the ratio of the currents flowing through the transmission coils  125 ,  130 . 
     In the measuring apparatus shown in  FIG. 1 , the currents through the two transmission coils  125 ,  130  are always the same, and the ratio of the currents is therefore 1. The differential voltage at the input of the input amplifier  140 , to be more precise the synchronous AC voltage component of the differential voltage, is regulated to zero by using the controllable amplifiers  115 ,  120  to apply different voltages to the two transmission coils  125 ,  130 . In this case, the ratio of the voltages across the transmission coils  125 ,  130  differs from 1, and hence from the ratio of the currents through the transmission coils  125 ,  130 . The ratio of the voltages corresponds to the control signal at the output of the integrator  150 . The control signal at the connection  155  is thus not equal to zero precisely when the ratio of the voltages across the transmission coils  125 ,  130  is not equal to 1, which is caused by different impedances of the transmission coils  125  and  130 . If the impedances of the transmission coils  125 ,  130  are the same in the object-free case, the signal applied to the connection  155  indicates the object  210  if the signal is not equal to zero.