Abstract:
The present invention relates to a device for measuring the value of a parameter relating to an electrically conductive object. Such a parameter can be the electric conductivity or the degree in which the material has changed under the influence of external conditions. Alternatively the parameter is the quality of a weld or the thickness of the object.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a device for measuring the value of a parameter relating to an electrically conductive object. Such a parameter can be the electric conductivity or the degree in which the material has changed under the influence of external conditions. Alternatively the parameter is the quality of a weld or the thickness of the object. 
     BACKGROUND OF THE INVENTION 
     International patent application No. 98/02 714, hereby incorporated by reference, relates to measuring the wall thickness of an electrically conductive object. During normal operation, a pulsed eddy current is induced in the object, the decay of the eddy current is determined and a signal is produced representing the decay. The wall thickness can be determined from the signal representing the decay of the eddy current. The wall thickness is determined from the length of a time interval in which the signal decays from a first value to a second, lower, value. Determining the wall thickness involves a numerical operation. 
     U.S. Pat. No. 5,059,902, hereby incorporated by reference, discloses a device for measuring the value of a parameter relating to an electrically conductive object using a signal induced by decaying eddy currents in the object. The device comprises a transmitter coil, means for energizing the transmitter coil, a receiver coil having an output, a master clock, and a signal analysis unit, wherein the master clock controls the predetermined time during which the transmitter coil is energized and a second adjustable timer that controls the predetermined time during which the voltage is measured. 
     It would be advantageous to provide an alternative to the device known from U.S. Pat. No. 5,059,902, wherein the parameters relating to the electrically conductive object are measured with the method disclosed in International patent application, No. 98/02 714, wherein the result can be displayed on a display pertaining to the device, without the need for a numerical analysis. 
     In addition, it would be advantageous to provide a device that is so flexible that it can be used for such different activities as measuring the degree in which the material has changed under the influence of external conditions, determining the quality of a weld or measuring the thickness of the object. 
     SUMMARY OF THE INVENTION 
     The invention relates to a device for measuring a material property of an electrically conductive object and displaying a reading indicative of the material property on a display, which device comprises a transmitter coil, means for energizing the transmitter coil, a receiver coil having an output, a master clock, and a signal analysis unit comprising a hold-and-comparator unit and a digital counter, wherein the master clock comprises a first adjustable timer that controls the predetermined time during which the transmitter coil is energized, a second adjustable timer that controls the moment at which an initial value of the receiver coil signal is measured and held by the hold-and-comparator unit, and a third adjustable timer that controls the moment at which the digital counter starts, wherein the digital counter has a first input for a start-counting signal, a second input for a stop-counting signal and an output connected to the display, and wherein the hold-and-comparator unit has an input that is connected to the output of the receiver coil and an output that is connected to the second input of the digital counter, which hold-and-comparator unit outputs during normal operation a stop-counting signal if the value of the receiver coil signal is less than a scaled initial value of the receiver coil signal. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows schematically a first embodiment of the invention; and 
     FIG. 2 shows schematically a second embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A device for measuring a material property of an electrically conductive object and displaying a reading indicative of the material property on a display according to the present invention comprises a transmitter coil, means for energizing the transmitter coil, a receiver coil having an output, a master clock, and a signal analysis unit comprising a hold-and-comparator unit and a digital counter, wherein the master clock comprises a first adjustable timer that controls the predetermined time during which the transmitter coil is energized, a second adjustable timer that controls the moment at which an initial value of the receiver coil signal is measured and held by the hold-and-comparator unit, and a third adjustable timer that controls the moment at which the digital counter starts, wherein the digital counter has a first input for a start-counting signal, a second input for a stop-counting signal and an output connected to the display, and wherein the hold-and-comparator unit has an input that is connected to the output of the receiver coil and an output that is connected to the second input of the digital counter, which hold-and-comparator unit outputs during normal operation a stop-counting signal if the value of the receiver coil signal is less than a scaled initial value of the receiver coil signal. 
     In an alternative embodiment, the device for measuring a material property of an electrically conductive object and displaying a reading indicative of the material property on a display according to the present invention comprises a transmitter coil, means for energizing the transmitter coil, a receiver coil having an output, a master clock, and a signal analysis unit comprising a delay line having an adjustable delay and an analog-to-digital converter, wherein the master clock comprises a first adjustable timer that controls the predetermined time during which the transmitter coil is energized, and a second adjustable timer to start the analog-to-digital converter, wherein the acoustic wave device has an input that is connected to the output of the receiver coil and an output for a delayed signal that is connected to a first input of the analog-to-digital converter, and wherein the analog-to-digital converter has a second input that is connected to the output of the receiver coil and an output that is connected to the display. 
     The invention will now be described by way of example in more detail with reference to the accompanying drawings. 
     Reference is now made to FIG.  1 . The device for measuring a material property of an electrically conductive object according to the present invention comprises a transmitter coil  3 , means  4  for energizing the transmitter coil  3 , a receiver coil  5  having an output  10 , a master clock  11   a ,  11   b ,  11   c , a signal analysis unit comprising a hold-and-comparator unit  13  and a digital counter  15 , and a display  18 . 
     The master clock comprises a first adjustable timer  11   a  that controls the predetermined time during which the transmitter coil  3  is energized, a second adjustable timer  11   b  that controls the moment at which an initial value of the receiver coil signal  10  is measured and held by the hold-and-comparator unit  13 , and a third adjustable timer  11   c  that controls the moment at which the digital counter  15  starts. 
     The digital counter  15  has a first input  20  for a start-counting signal  21 , a second input  23  for a stop-counting signal  24  and an output  30  connected to the display  18 . 
     The hold-and-comparator unit  13  has an input  32  that is connected to the output  10  of the receiver coil  5  and an output  35  that is connected to the second input  23  of the digital counter  15 . The hold-and-comparator unit  13  outputs during normal operation a stop-counting signal if the value of the receiver coil signal  10  is less than a scaled initial value of the receiver coil signal  10 . To this end the hold-and-comparator unit  13  comprises a sample-and-hold device  40 , of which the input  32  is connected to the output  10  of the receiver coil  5 . The sample-and hold device  40  measures the value of the signal from the receiver coil  5  at a moment determined by the second adjustable clock  11   b  and holds the initial value for as long as necessary. The output  41  of the sample-and hold device  40  is connected to a variable resistor  42  to scale the initial value, if necessary. The output  43  of the variable resistor  42  is connected to a first input  44  of a comparator  45 . The second input  46  of the comparator  45  is connected to the output  10  of the receiver coil  5 . 
     During normal operation, the first adjustable timer is set to energize the transmitter coil  3  during a predetermined period, for example 0.3 microseconds. At the end of this period the transmitter coil  3  is abruptly de-energized. The second adjustable timer  11   b  is set to measure value of the signal from the receiver coil  5  at a predetermined moment after de-energizing the transmitter coil  3 , for example after 300 microseconds. The variable resistor  42  is set to scale the output of the sample-and-hold device  40  to a predetermined value, for example 20% of the output value. And the third adjustable timer  11   c  is set to start the counter  15  at a predetermined moment after de-energizing the transmitter coil  3 , for example after 400 microseconds. It will be understood that the latter moment is selected after the moment on which the value of the signal is measured. 
     Then the device according to the present invention is positioned near an electrically conducting object (not shown) that is to be inspected, so that the transmitter coil  3  and the receiver coil  5  are close to the near surface of the object, and the measurement is started. 
     Energizing the transmitter coil  3  during a predetermined period and abruptly de-energizing the transmitter coil  3  induces an eddy current in the object. The decay of the eddy current is detected by the receiver coil  5 , and its output is a signal representing the decay of the eddy current. 
     The sample-and-hold device  40  measures the value of the signal at a moment that is determined by the second adjustable timer  11   b  and holds the measured value, and this initial value is scaled down by the variable resistor  42  to the predetermined value. This scaled-down initial value  43  is compared in comparator  45  to the value of the output  10  of the receiver coil  5 . 
     The counter  15  starts counting at a moment after de-energizing the transmitter coil  3  determined by the third adjustable timer  11   c.  The hold-and-comparator unit  13  outputs a stop-counting signal if the value of the receiver coil signal  10  is less than the scaled initial value  43  of the receiver coil signal  10 . 
     The output of the counter  15  is then displayed on the display  18 . 
     In general, the device will first be activated near a part of the object that can serve as a reference, so that any subsequent readings on the display  18  from different parts of the object can be compared to the readings pertaining to the reference. 
     Because four parameters can be set, the duration of the pulse, the moment at which the output from the receiver coil  5  is sampled, the variable resistor  42  and the moment at which the counter  15  starts counting, this device is very flexible. Thus it can be used for a large number of different types of measurements. 
     In this embodiment of the invention the time elapsed before a scaled-down value of the signal from the receiver coil  5  is used to measure a parameter of the object. 
     Reference is now made to FIG. 2 showing schematically an alternative embodiment of the present invention. In this embodiment, the decay of the output signal between two predetermined times is used to measure a parameter of the object. Parts of the device that are similar to the device as discussed with reference to FIG. 1 have the reference numeral used in FIG.  1 . 
     The device for measuring a material property of an electrically conductive object comprises the transmitter coil  3 , means  4  for energizing the transmitter coil  3 , the receiver coil  5  having an output  10 , a master clock  11   a ,  50 , a signal analysis unit comprising a delay line in the form of acoustic wave device  52  having an adjustable delay  53  and an analog-to-digital converter  55 , and the display  18 . 
     The master clock comprises the first adjustable timer  11   a  that controls the predetermined time during which the transmitter coil  3  is energized, and a second adjustable timer  50  to start the analog-to-digital converter  55 . 
     The acoustic wave device  52  has an input  58  that is connected to the output  10  of the receiver coil  5  and an output  59  for a delayed signal that is connected to a first input  60  of the analog-to-digital converter  55 . 
     The analog-to-digital converter  55  has a second input  62  that is connected to the output  10  of the receiver coil  5  and an output  64  that is connected to the display  18 . 
     During normal operation, the first adjustable timer  11   a  is set to energize the transmitter coil  3  during a predetermined period, for example 0.3 microseconds. At the end of this period the transmitter coil  3  is abruptly de-energized. The adjustable delay  53  of the acoustic wave device  52  is set to measure the value of the signal from the receiver coil  5  at a predetermined moment after de-energizing the transmitter coil  3 , for example after 300 microseconds. The second adjustable timer  50  is set to trigger the analog-to-digital converter  55  to determine the difference between its input signals at a predetermined moment after de-energizing the transmitter coil  3 , for example after 400 microseconds. It will be understood that the latter moment is selected after the moment on which the value of the signal is measured. 
     Then, the device according to the present invention is positioned near an object (not shown) that is to be inspected, so that the transmitter coil  3  and the receiver coil  5  are close to the near surface of the object, and the measurement is started. 
     Energizing the transmitter coil  3  during a predetermined period and abruptly de-energizing the transmitter coil  3  induces an eddy current in the object. The decay of the eddy current is detected by the receiver coil  5 , and its output is a signal representing the decay of the eddy current. 
     The acoustic wave device delays outputting the signal  10  for a period determined by the adjustable delay  53 . 
     The analog-to-digital converter  55  measures the difference between the delayed output  59  and the output  10  at a moment determined by the second adjustable timer  50 . 
     The output of the analog-to-digital converter  55  is then displayed on the display  18 . 
     The device will first be activated near a part of the object that can serve as a reference, so that any subsequent readings on the display  18  from different parts of the object can be compared to the readings pertaining to the reference. 
     Because three parameters can be set, the duration of the pulse, the delay of the acoustic wave device, and the moment at which the difference between the value of the delayed signal and the signal is measured, this device is also very flexible. Thus it can be used for a large number of different types of measurements. 
     The delay line is a device that introduces a predetermined delay in the transmission of a signal. Instead of an acoustic wave device, any other known device can be used. 
     The present invention therefore provides simple devices for measuring the value of several parameters relating to an electrically conductive object, that is for different types of measurements. Examples of such parameters are the chromium content of stainless steel, the degree of carbonisation of steel, the present of a sigma phase in steel and the degree of creep in a metal. Other parameters that can be measured are lift-off and thickness.