Patent Publication Number: US-2013249539-A1

Title: Detection of a Metal or Magnetic Object

Description:
In certain types of machining workpieces, there is the risk that an object hidden in the workpiece will be damaged by the machining. When drilling into a wall, for example, a water, power or gas line running within the wall can be damaged. In the reverse case, it may be desirable to carry out the machining precisely in such a manner that an object hidden in the workpiece is also machined, for example if the hole from the above example is to run through a reinforcement iron or a bearing construction within the wall. 
     PRIOR ART 
     In the prior art, coil-based metal detectors are known for detecting such a hidden object. Such detectors generate a magnetic field within an area to be measured. If there is a metallic object in the area to be measured, the object is detected due to its influence on the magnetic field generated. Frequently, at least two receiving coils are used for determining the magnetic field generated which are oriented and connected to one another in such a manner that in the absence of a metallic object in the area to be measured, the measurement signal supplied by both receiving coils goes to zero (differential measurement). In one variant, a number of transmitting coils are used for generating the magnetic field which are driven in such a manner that the signal measured in the two receiving coils goes to zero independently of a presence of a metallic object in the area to be measured (field-compensated measurement). 
     DE 10 2007 053 881 A1 describes a measuring method for determining the position or the angle of a coil with respect to two other coils. For this purpose, an alternating magnetic field is generated by means of two transmitting coils arranged at an angle to one another. A receiving coil is brought into the alternating magnetic field and the drive of the transmitting coils is changed in such a manner that the same voltage is induced in the receiving coil by each of the transmitting coils. A ratio of current values supplied by the transmitting coils is used as a measure for a determination of the position and/or angle of the receiving coil with respect to the transmitting coils. 
     DE 10 2004 047 189 A1 describes a metal detector having printed coils. 
     The invention is based on the object of providing a simple and accurate detector for a metallic object. A further object of the invention consists in specifying a method for determining the metallic object. 
     DISCLOSURE OF THE INVENTION 
     The invention achieves these objects by means of a measuring device having the features of claim  1  and of a method having the features of claim  7 . Subclaims specify preferred embodiments. 
     According to the invention, a measuring device for detecting a metallic object comprises a transmitting coil for generating a magnetic field and a compensation network connected to the transmitting coil, wherein a differential voltage is applied at the connection of the transmitting coil with the compensation network. A control device is provided for supplying the transmitting coil and the compensation network with alternating voltages in such a manner that the value of an alternating voltage component, synchronized in timing with the alternating voltage, of the differential voltage is minimized. The control device is configured for detecting the metallic object when the ratio of the alternating voltages does not correspond to the ratio of the currents flowing through the transmitting coil and the compensation network. 
     The metallic object can thus be reliably detected with the aid of only a single transmitting coil. The alternating voltages which are present at the transmitting coil and at the compensation network are thus always controlled in such a manner that the voltages dropped across the transmitting coil and the compensation network correspond to one another even when impedances of the transmitting coil and of the compensation network are not equal. The control signal is interpreted as the actual measurement signal. 
     The alternating voltages are preferably alternating voltages for changing the magnetic fields of the transmitting coils periodically in magnitude and phase. The alternating voltages provide for synchronous demodulation by which means interfering signals having frequencies not equal to the modulation frequency can be very effectively suppressed. In addition, alternating magnetic fields can be generated by the alternating voltages in order to induce eddy currents in nonmagnetic materials such as, e.g., copper due to which currents these can then be detected. 
     The compensation network can comprise at least one complex impedance. The impedances of the transmitting coil and of the compensation network can be equal and interfering influences such as temperature and aging effects can affect the transmitting coil and the compensation network in the same way so that an influence of the interfering influences is compensated for overall. The measuring device can then be calibrated once as part of the production of the measuring device and further calibration by a user can be dispensed with. 
     A connection can be provided between the transmitting coil and the compensation network, the control device being configured for controlling the voltage supply in dependence on a differential voltage present at the connection. Thus, a voltage pointing to a ratio of currents flowing through the transmitting coil and through the compensation network can be easily and precisely determined. 
     The compensation network can have a variable impedance. By this means, a sensitivity of the measuring device can be controllable. The impedance can be discretely or continuously variable and performed especially in dependence on the measurement signal. The compensation network can also be magnetically shielded. 
     According to a further aspect of the invention, a method for detecting a metallic object comprises the steps of supplying a transmitting coil and a compensation network connected to the transmitting coil with alternating voltages, of determining a differential voltage present at the connection of the transmitting coil with the compensation network, wherein the supplying of the transmitting coil and of the compensation network with alternating voltages is carried out in such a manner that the value of an alternating voltage component, synchronized in timing with the alternating voltages, of the differential voltage is minimized and of detecting the object when the ratio of the alternating voltages does not correspond to the ratio of the currents flowing through the transmitting coil and the compensation network. 
     The invention can also be designed as a computer program product, wherein a computer program product according to the invention comprises program code means for performing the method described and can run on a processing device or be stored on a computer-readable data medium. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       In the text which follows, the invention will be described in greater detail with reference to the attached figures, in which: 
         FIG. 1  shows a block diagram of a measuring device; 
         FIG. 2  shows a detailed view of the measuring device from  FIG. 1 ; 
         FIG. 3  shows an arrangement of a number of transmitting coils for the measuring device from  FIG. 1 ; and 
         FIG. 4  shows a flowchart of a method for the measuring device from  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       FIG. 1  shows a block diagram of a measuring device  100 . The measuring device  100  is a part of a metal detector  105  for detecting metallic objects, for example of ferrous material. 
     A clock generator  110  has two outputs at which it provides phase-shifted periodic alternating signals, preferably phase-shifted by 180°. The alternating signals can comprise in particular rectangular, triangular or sinusoidal signals. The outputs of the clock generator are connected to a first controllable amplifier  115  and to a second controllable amplifier  120 , respectively. Each of the controllable amplifiers  115 ,  120  has a control input via which it receives a signal which controls a gain factor of the controllable amplifier  115 ,  120 . One output of the first controllable amplifier  115  is connected to a transmitting coil  125  and one output of the second controllable amplifier  120  is connected to a compensation network  130 . The compensation network  130  provides an impedance which is within the range of that of the transmitting coil  125 . In some embodiments, the compensation network  130  can have a variable impedance. 
     In the text which follows, an embodiment is described in which the impedances of the transmitting coil  125  and of the compensation network  130  correspond to one another. However, this is generally not required for the operation of the measuring device  100 . 
     A second terminal of the transmitting coil  125  is connected to a shunt resistor  138  leading to ground and to a resistor  135   a  leading to an input amplifier  140 . The compensation network  130  has a ground terminal and is connected to the input amplifier  140  via a resistor  135   b . The voltage dropped across the shunt resistor  138  is proportional to the current flowing through the transmitting coil  125 . The currents flowing via the resistors  135   a  and  135   b  produce a total current via a third resistor  139  to ground. This current is proportional to the sum of the voltage dropped across the shunt resistor  138  and of the voltage dropped in the compensation network  130 . The voltage dropped across the resistor  139  to ground is thus proportional to the sum of the voltage dropped across the shunt resistor  138  and the voltage dropped in the compensation network  130 . This is present at the input of the input amplifier  140 . 
     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 it a clock signal which points to the phase angle 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 symmetric rectangular signals, one of the output signals can be used as clock signal. The synchronous demodulator  145  essentially switches the signal received from the input amplifier  140  alternatingly through at its top and lower output, respectively, on the basis of the clock signal provided by the clock generator  110 . 
     The two outputs of the synchronous demodulator  145  are connected to an integrator (integrating comparator)  150  which is represented here as an operational amplifier provided with two resistors and two capacitors. Other embodiments are also possible, for example as active low-pass filter. A digital embodiment of the integrator  150  following the synchronous demodulator  145  is also conceivable in which the signal at the output of the synchronous demodulator  145  is analog/digital converted at one or several times within a halfwave and is then compared with the corresponding value from the next halfwave. The difference is integrated and, e.g. changed again into an analog signal and used for controlling the amplifier. 
     Whilst the synchronous demodulator  145  provides the measurement signal received by 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. Whilst the synchronous demodulator  145  provides the measurement signal received from the input amplifier  140  at its upper output, it is integrated inverted over time by the integrator  150  and the result is provided at the output of the integrator  150 . The voltage at the output of the integrator  150  is the integral of the difference of the low-pass-filtered outputs of the synchronous demodulator  145 . 
     The signal provided by the integrator  150  is provided for further processing via a terminal  155 . In addition, a microcomputer  175  can be connected to the control inputs of the controllable amplifiers  115 ,  120 . The microcomputer  175  compares the provided signal with a threshold value and outputs at an output  180  a signal which points to the metallic object. The signal can be offered visually and/or audibly to a user of the metal detector  105 . 
     In addition, the microcomputer  175  can carry out further processing of the signals picked up from the control inputs of the controllable amplifiers  115 ,  120  and, in dependence on these, control parameters of the measuring device  100 . For 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 receiving amplifier  140  can be changed. In a further embodiment, other ones of the elements shown of the measuring device  100  are implemented by the microcomputer  175 , for instance the clock generator  110 , the synchronous demodulator  145  or the integrator  150 . 
     The same signal of the integrator  150  is also used for controlling the gain factors of the controllable amplifiers  115  and  120 , the second controllable amplifier  120  being connected directly to the output of the integrator  150  and the first controllable amplifier  115  being connected to the output of the integrator  150  by means of an inverter  160 . The inverter  160  inverts the signal provided to it in such a manner that the gain factor of the first controllable amplifier  115  increases in dependence on the output signal of the integrator  150  to the same extent to which the gain factor of the second controllable amplifier  120  decreases, or conversely, respectively. It is also conceivable that only the gain factor of one of the two controllable amplifiers  115 ,  120  is controlled whilst the gain factor of the second controllable amplifier  115 ,  120  is kept at a fixed value. 
     If there is no metallic object  170  in the area of the magnetic field generated by the transmitting coil  125 , the impedances of the transmitting coil  125  and of the compensation network  130  are equally large and a voltage of zero is present between the resistors  135   a  and  135   b.  If necessary, the measuring device  100  must be calibrated for this condition before a metallic object  170  is brought within range of the transmitting coil  125 . 
     If the metallic object  170  is within range of the transmitting coil  125 , this changes the impedance of the transmitting coil  125  and thus the current flowing through the transmitting coil  125 . Correspondingly, the alternating voltage component, which is synchronized in timing, of the voltage present between resistors  135   a,    135   b  is not equal to zero and the signal present at the output of the integrator  150  changes by an amount with respect to zero. The controllable amplifiers  115  and  120  are thereupon changed inversely in their gain factors in such a manner that the voltages which are present at the transmitting coil  125  and at the compensation network  130  are changed in such a manner that the alternating voltage component, which is synchronized in timing, of the voltage present between resistors  135   a  and  135   b  is reduced back to zero. The presence of the metallic object  170  can be detected by comparing the output voltage of the integrator  150  with zero. 
     In the case of different impedances of the transmitting coil  125  and of the compensation network  130 , the signal output at terminal  155  is not zero but another predetermined value in the case where there is no object. The comparison of the control value as described above then takes place with respect to the predetermined value. A determination of the predetermined value can be determined in the context of a calibration in that the signal at terminal  155  is determined in absence of the metallic object. 
     The compensation network  130  is advantageously designed in such a manner, if possible, that it is subject to temperature and aging effects which correspond to those of the transmitting coil  125  in order to, by influencing the elements  125 ,  130  in the same sense, compensate for an influence on the measuring device  100  by temperature and aging overall. In this case, a calibration of the measuring device  100  can be performed once during the production of the measuring device  100  and does not need to be repeated by a user contemporaneously with a measurement to be performed. 
       FIG. 2  shows an expanded representation of the compensation network  130 . In a simple embodiment, the compensation network  130  only comprises a complex voltage divider  210  which comprises complex impedances  220  and  230 . The complex impedances  220 ,  230  are selected in such a manner that, together, they form an impedance which corresponds to the impedance of the transmitting coil  125  when there is no metallic object to be detected in the area of the magnetic field generated by the transmitting coil  125 . The voltage, divided to ground by means of impedances  220  and  230 , of the second controllable amplifier  120  is coupled out by means of the second resistor  135   b  and conducted to the input amplifier  140  as is described above with reference to  FIG. 1 . 
     In a further embodiment of the compensation network  130 , yet another complex impedance  240  is provided which can be connected in parallel with the complex impedance  230  by means of a switch  250 . By operating the switch  250 , it is possible to switch between two different impedances of the compensation network  130 . Correspondingly, further impedances can also be achieved by connecting yet other and/or further complex impedances like the complex impedance  240  in parallel or in series. 
     In an exemplary implementation, the switch  250  is driven by a threshold switch  260  which comprises a comparator (operational amplifier)  270  and two resistors  280  and  290 . The resistors  280  and  290  form a voltage divider between a supply voltage U of the measuring device  100  and ground. The divided voltage is connected to a non-inverting input of the comparator  270 . An inverting input of the comparator  270  is connected to the output of the integrator  150  or terminal  155 , respectively. If the voltage provided by the integrator  150  exceeds the voltage provided by the voltage divider  280 ,  290 , the comparator  270  operates the switch  250  and, by doing so, changes the impedance of the compensation network  130 . 
       FIG. 3  shows an arrangement  300  having a number of pairs of transmitting coils and compensation networks for the measuring device  100  from  FIG. 1 . In addition to the arrangement described with reference to  FIG. 1 , of the transmitting coil  125  and the compensation network  130  with the resistors  135   a,    135   b,  a further transmitting coil  325  and a further compensation network  230  having further resistors  335   a,    335   b  are provided in corresponding interconnection. Resistors leading from one of the terminals of the transmitting coils  125 ,  325  to ground corresponding to the shunt resistor  138  in  FIG. 1  are not shown. The resistor  139  from  FIG. 1 , connected to the input amplifier  140 , is also not shown in  FIG. 3 . The coils  125  and  325  can be constructed as printed circuits (“printed coils”) on a circuit board. Other elements of the measuring device  100  can also be arranged on the same circuit board. 
     Two switches  310  and  320  coupled to one another in each case selectively connect terminals of the transmitting coil  125  and of the compensation network  130  or of the transmitting coil  325  and of the compensation network  330  to the outputs of the controllable amplifier  115 ,  120  from  FIG. 1 . The connections between resistors  135   a,    135   b ,  335   a  and  335   b,  corresponding to one another, are connected to one another and lead to the input amplifier  140 . 
     In a further embodiment, only one compensation network  130  is provided which is interconnected with various transmitting coils  125 ,  325 . In this case, the switch  320  selectively connects the terminal not connected to the compensation network  130  of the second resistor  135   b  to one of the first resistors  135   a,    335   a.  Elements  335   b  and  330  are omitted. 
     If the differential voltage at the input amplifier  140  changes when the switches  115  and  120  are switched over, it is possible, on the basis of the geometric arrangement of the coils  125 ,  325 , to infer a direction in which the metallic object  210  is located, for example by triangulation. Similarly, it is conceivable to infer a distance of the metallic object. The determination of direction can be refined by further transmitting coils. If a large number of transmitting coils arranged sufficiently close to one another is used, a resolution of the measuring device  100  can be increased up into a pictorial domain. 
       FIG. 4  shows a diagrammatic flowchart of a method  400  for detecting a metallic object  210  corresponding to the measuring device  100  from  FIGS. 1 and 2 . In a step  410 , the transmitting coil  125  and the compensation network  130  are in each case supplied with alternating voltages, the voltages being phase-shifted with respect to one another, preferably phase-shifted by 180°. 
     In a subsequent step  420 , a differential voltage is determined which appears at the resistors  135   a  and  135   b  and which points to a ratio of a current flowing through the transmitting coil  325  with respect to a current flowing through the compensation network  330 . Subsequently, the differential voltage is demodulated synchronously with a phase of the alternating voltages in a step  430  and the result is integrated. 
     In a step  440 , the controllable amplifiers  115  and  120  are driven inversely on the basis of the integrated result until the alternating voltage component, synchronized in timing, of the differential signal has approached zero again. 
     Finally, it is compared in a step  450  whether the integrated result deviates from zero by more than a predetermined amount and in this case the metallic object  170  is detected. Optionally, a visual and/or audible reference to the metallic object  170  can be output to a user.