Abstract:
Methods for eliminating error in magnetic sensors used for measuring a coating thickness caused by static or changing external magnetic fields or temperature. The methods involve measuring an output voltage of a magnetic sensor, corresponding to an internal resistance of the magnetic sensor, in a static or changing magnetic field or external temperature, storing the value of the output voltage, performing mathematical operations with the stored value of the output voltage, and correcting the output voltage of the magnetic sensor to accurately indicate a coating thickness.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application is a divisional application which claims the benefit of application Ser. No. 10/087,216, filed Mar. 4, 2002 now U.S. Pat No. 6,724,187. The disclosure of the prior application is hereby incorporated herein by reference in its entirety. 

   FIELD OF THE INVENTION 
   The invention relates to a method to measure non-magnetic coatings on ferro-magnetic substrates, using a magnetic sensor, where errors due to offset voltage of the magnetic sensor, temperature dependence of the sensor parameters and disturbing magnetic fields are compensated. 
   DESCRIPTION OF THE PRIOR ART 
   Patent EP0028487 discloses a measuring device that uses a Hall-sensor together with a permanent magnet. In this device the flux of external magnetic fields is superimposed to the flux of the internal permanent magnet and can not be separated. Therefore the change of the output signal of the Hall-sensor due to these external fields can not be determined. This results in erroneous measurements. 
   Pat. DE19910411 discloses a method for offset compensation of Hall-sensors. This method eliminates the offset by applying two different currents to the Hall-sensor, measuring the Hall-voltage at each current and calculating the offset from these two measured voltages. However this method does not allow for compensation of external magnetic fields. 
   In Pat. US33359495 a device is described that has a Hall-sensor with a coil arranged around the Hall-sensor. This device uses alternating electromagnetic fields generating eddy currents in the substrate. These eddy currents strongly depend on the conductivity of the substrate. The eddy currents in turn generate a secondary magnetic field opposed and superimposed to the primary field. This results in a change of the output voltage of the Hall-sensor. Since the strength of the eddy currents and consequently the strength of the secondary magnetic field strongly depend on the conductivity, this has a negative influence on the measuring results. An additional error occurs when the substrate is covered with a conductive coating. Also in the conductive coating eddy currents are generated. They have the same effect on the flux through the Hall-sensor as those generated in the substrate. 
   SUMMARY OF THE INVENTION 
   It is an object of the invention to provide simple methods which allow to eliminate the different error sources when using magnetic sensors for the measurement of coating thickness. 
   With the first two methods the influence of external magnetic fields, as they can be present in mechanically or thermally treated substrates, or of time varying fields on the measuring device can be eliminated. Additionally this invention makes use of the advantages of the static magnetic field being insensitive to the conductivity of substrate and metal coatings. 
   A further advantage of this method is the automatic compensation of the temperature dependent offset voltage of magnetic sensors. This offset voltage occurs when applying a current to the sensor, even in the absence of a magnetic field. 
   A further described method of this invention can be used to compensate the influence of temperature on the signal voltage. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram of the measuring device. 
       FIG. 2  is a circuitry block diagram of the magnetic sensor for compensation of the temperature dependence of the signal voltage. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   The measuring device is equipped with a magnetic sensor  10  and a coil  20  disposed in the neighbourhood of the sensor. The control unit  30  contains the sensor electronics  31  to feed the magnetic sensor  10  with a current I and to receive the signal voltage U. The control unit  30  also contains the coil control unit  32  to feed the current I c  through the coil  20 . 
   The compensation of external magnetic fields as well as the offset voltage of the magnetic sensor  10  can be performed by either of the methods described as preferred embodiments hereafter. 
   With the first method the output voltage U in the magnetic sensor  10  is measured first by the magnetic sensor control unit  31  without a current I c  through the coil  20 , when the measuring probe  15  is placed on the object under test  50 . The output voltage U depends on the external magnetic field B ext  through the magnetic sensor  10 . This output voltage U 1 , is transferred from the control unit  30  to the evaluation unit  40 . There it is digitised and stored. With the next step a current I c  is feed through the coil  20 . This current generates a magnetic field B that is superimposed to the external magnetic field B ext . This creates a different magnetic flux through the magnetic sensor  10  and consequently a different output voltage U 2 . This voltage U 2  is transferred from the control unit  30  to the evaluation unit  40 . There it is digitised and stored. A linear relation exists between output voltage U and magnetic flux B through the magnetic sensor which can be written in the following form:
 
 U=U   0   +K·B·I   (1)
 
   U 0  is the offset voltage, K is the sensor constant and I is the current through the magnetic sensor  10 . Equation 1 immediately shows that the output voltage is the sum of the offset voltage and the sum of all magnetic flux components B a , B b , . . . through the magnetic sensor  10 :
 
 U=U   0   +U   a   +U   b   + . . . =U   0   +K ·( B   a   +B   b + . . . )· I   (2)
 
   Therefore in the evaluation unit  40  the following difference can be calculated:
 
 U   res =( U   2   −U   1 )=[ U   o   +K· ( B+B   ext )· I]−[U   o   +K·B   ext   ·I]=K·B·I   (3)
 
   The result U res  represents the voltage the magnetic sensor would generate if no external magnetic field were present. Simultaneously the offset voltage U 0  is eliminated. Using the functional relation between output voltage U of the magnetic sensor  10  and the distance of the sensor from the substrate (=coating thickness d=f(U)) the true coating thickness d can be calculated from the result U res . 
   With the second method a certain current I C1  is feed through the coil  20  by the coil control unit  32  when the measuring probe is placed on the object under test. The magnetic field B 1  generated by this current, together with an external magnetic field B ext , generates a magnetic flux through the magnetic sensor  10 , resulting in an output voltage.
 
 U   1   =U   0   +K· ( B   1   +B   ext )·I  (4)
 
   This output voltage U 1  is transferred from the control unit  30  to the evaluation unit  40 . There it is digitised and stored. With the next step a second current I C2  is feed through the coil  20 . In a preferred embodiment the current is selected as I C2 =−I C1 . The absolute value of the magnetic field B 2  is the same as that of B 1 , but with reverse polarity: B 2 =−B 1 . The magnetic field B 2 , together with the external magnetic field B ext  generates a magnetic flux through the magnetic sensor  10 , resulting in an output voltage:
 
 U   2   =U   0   +K ·( B   2   +B   ext )· I=U   0   +K· (− B   1   +B   ext )· I   (5)
 
   This output voltage U 2  is transferred from the control unit  30  to the evaluation unit  40 . There it is digitised and stored. Using equation 2 and the two digitised output voltages U 1  and U 2 , the voltage resulting from the external magnetic field B ext  and the offset voltage U 0  can be eliminated as follows:
 
 U   res   =[U   1   −U   2   ]=K ·[( B   1   +B   ext )−( B   2   +B   ext )]· I=K·[B   1   −B   2   ]·I   (6)
 
   Because of B 2 =−B 1 , =B equation 6 can be rewritten as:
 
 U   res =2 ·K·B ·I   (7)
 
   Using the functional relation between output voltage U of the magnetic sensor  10  and the distance of the sensor from the substrate (=coating thickness d=f(U)) the true coating thickness d can be calculated from the result U res . 
   In a preferred embodiment the influence of time varying magnetic fields, which usually have typical frequencies such as 60 Hz (e.g. those generated by transformers), can be eliminated by repeated changes between those two currents I C1  and I C2 . The resulting output voltage is determined using the following equation:
 
 U   res   ={[U   1   −U   2 ] 1   +[U   1   −U   2 ] 2   + . . . +[U   1   −U   2 ] N   }/N =K ·{[( B   1   +B   ext1 )−( B   2   +B   ext2 )] 1 +[( B   1   +B   ext1 )−( B   2   +B   ext2 )] 2 + . . . +[( B   1   +B   ext1 )−( B   2   +B   ext2 )] N}/N·I = 2  K·[B+{(B   ext1   −B   ext2 ) 1 +( B   ext1   −B   ext2 ) 2 + . . . +( B   ext1   −B   ext2 ) 1N   }/N]·I   (8)
 
with N being the number of repetitions. Those components of external magnetic fields in curved brackets will be averaged to zero, especially when the N pairs of measurements are taken in an interval that which is equal to one or several periods of the time varying magnetic field.
 
   A further method in accordance with this invention allows to compensate the temperature dependence of the output voltage U. The temperature in the magnetic sensor  10  or the temperature change with respect to a reference temperature can be determined by measuring the internal resistance  11  of the magnetic sensor  10 . This can be used to determine a compensation factor to correct the output voltage for temperature changes. 
   Starting from equation 1 the temperature gradient of the output voltage can be calculated as follows (a change in offset voltage can be neglected as effect of second order):
 
 dU/dT=B·[dK   H   /dT·I+K·dI/dT]=B·K·I·[α+β]   (9)
 
using the following abbreviations:
 
 dK/dT=K ( T   0 )·α,  dI/dT=I ( T   0 )·β
 
K(T 0 ) und I(T 0 ) are related to reference temperature T 0 .
 
   α and β are the temperature coefficients of the output voltage U and the sensor resistance  11  respectively. Since the coefficients α and β are known for each individual type of magnetic sensor, these parameters can be implemented directly in the control unit or used as parameters in a digital control unit. 
   The temperature dependant output voltage can be determined using the following equation:
 
 U ( T )= U ( T   0 )+ dU/dT ·( T−T   0 )= B·K ( T   0 )· I ( T   0 )·{1+[α+β]·( T−T   0 )}  (10)
 
   Equation 10 shows that the output voltage, measured at a temperature T, needs to be corrected by a factor {1+[α+β]·(T−T 0 )} as given in equation 10, in order to reduce the output voltage to the correct value at reference temperature T 0 . To do so the evaluation unit  40  has to calculate the factor in curved brackets which is transferred via the control unit  30  to the control of the magnetic sensor  31  to adjust the current I through the magnetic sensor by this factor. 
   To determine the temperature T the voltage drop across the sensor resistance  11  is measured by the sensor control unit  31  by feeding a constant current through this resistance  11 . First this is done at temperature T 0  as a reference, and then at each measurement of the output voltage U. To calculate the actual temperature T or the temperature deviation ΔT from the reference temperature T 0  the following equation is used:
 
 R ( T )= R ( T   0 )·[1+β·( T−T   0 )]= R ( T   0 )·[1 +β·ΔT]   (11)
 
β is the temperature coefficient of the sensor resistance  11  given above.
 
   Then the temperature difference relative to T 0  is calculated as:
 
 ΔT =(1/β)· R ( T )/ R ( T   0 )  (12)
 
   The procedure to determine a temperature compensated coating thickness is as follows: 
   First with a reference measurement the sensor resistance R(T 0 ) is determined in the control unit  30 . This value is digitised and store in the evaluation unit  40 . Each time a coating thickness measurement is taken the value R(T) is determined first, transferred from the control unit  30  to the evaluation unit  40 , where it is digitised and stored. This value is then used to calculate the temperature difference ΔT according to equation 12. Subsequently, using ΔT, the correction factor {1+[α+β]·(T−T 0 )} is calculated in the evaluation unit  40  and transferred to the control unit  30 . Then this parameter is used in the magnetic sensor control unit  31  to adjust the current I such, that the output voltage conforms to the respective voltage at reference temperature T 0 . 
   Alternatively to adjusting the current I through the resistor  11  of the magnetic sensor  10  upon temperature change the correction of the output voltage U at the measured temperature T can also be done digitally in the evaluation unit  40  using equation 10 and 12. 
   Of course it is obvious for a person skilled in the art to combine the method for temperature compensation of the output voltage with one of the methods for compensation of external magnetic fields.