Abstract:
In a method and apparatus for determining the moisture content of a material with a measuring line extending through the material and having a proximal end connected to a signal generator for supplying a pulse signal to the measuring line, which pulse signal is reflected and returned back to the proximal end of the measuring line to which a comparator is connected for measuring the travel time of the signal supplied, and the reflected signal returned, to the proximal end of the measuring line and its amplitude is with compared with an amplitude corresponding to a threshold, an adjustable threshold value generator and controller are provided generating a threshold value signal which depends on the amplitude of the reflected signal.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The invention relates to a method and apparatus for determining the moisture content of a material by way of measuring the dielectric constant of the material, wherein a pulsed signal is applied to a proximate end of a measuring line extending through the material and is reflected at the distal end of the measuring line and the reflected signal is detected at the proximate end and compared with a threshold value and also the time delay between the signal application and the return of the reflected signal is evaluated.  
         [0002]     Such a method and apparatus is disclosed for example in EP 0 478 815, which is assigned to the assignee of the present application and is marketed by the Assignee with considerable success. In this known method, a square signal is applied to a measuring cable. The pulse direction of the signal is greater than twice the travel time of the signal in the measuring cable. At the input of the measuring cable or, respectively, the output of the measuring signal generator the respective amplitudes are therefore superimposed thereby generating a summing signal formed by the input signal and the signal reflected from the distal end of the cable.  
         [0003]     Although this known method provides very good results, it does have the disadvantage that the amplitude of the reflected signal affects the measuring accuracy. This is disturbing since the amplitude of the reflected signal depends on the electric conductivity and, consequently, the moisture content of the material. Furthermore, the ambient temperature around the measuring cable affects the amplitude of the reflected signal.  
         [0004]     U.S. Pat. No. 6,477,474 B2 discloses a method for determining the dielectric constant of a material wherein, by means of an impulse generator, a pulse signal is supplied to a measuring cable and the travel time up to the reception of the reflected signal is determined and the dielectric constant is determined therefrom. A threshold value control circuit is provided by which a threshold value can be adjusted to a fixed threshold value or it can be adjusted depending on the reflected signal. However, definite criteria for the adjustment of the threshold value are not available from this publication.  
         [0005]     Furthermore, U.S. Pat. No. 5,610,611 discloses an impulse reflectometer wherein an automatic threshold value adjustment occurs depending on the size of the reflected signal.  
         [0006]     It is the object of the present invention to provide a method and apparatus for determining the moisture content of a material by which the accuracy of the measurement is improved.  
       SUMMARY OF THE INVENTION  
       [0007]     In a method and apparatus for determining the moisture content of a material with a measuring line extending through the material and having a proximal end connected to a signal generator for supplying a pulse signal to the measuring line, which pulse signal is reflected and returned back to the proximal end of the measuring line to which a comparator is connected for measuring the travel time of the signal supplied, and the reflected signal returned, to the proximal end of the measuring line and its amplitude is with compared with an amplitude corresponding to a threshold, an adjustable threshold value generator and controller are provided generating a threshold value signal which depends on the amplitude of the reflected signal.  
         [0008]     It has been recognized that, upon arrival of the reflected signal at the proximal end of the measuring line or, respectively, the output of a corresponding signal generator, the voltage does not increase vertically but that the voltage increase of the summing signal formed from the signal supplied to the measuring line and the signal reflected from the distal end of the measuring line occurs within a certain time period which must not be neglected. The rate of increase of voltage depends on the height of the amplitude of the reflected signal. With a high amplitude, the voltage slope is steeper so that the threshold value of the comparator is reached more rapidly. Consequently, with a fixed threshold value, the point in time of switching of the comparator, that is the measured point in time of return of the reflected signal depends on the height of the amplitude of the reflected signal.  
         [0009]     If, with a predetermined amplitude of the reflected signal, the level of the threshold value corresponds to about 80% of the amplitude of the reflected signal (over to the amplitude of the input signal), the comparator switches at a certain point in time. If the amplitude of the reflected signal changes, for example under the influence of the temperature or moisture, the level of the threshold value is no longer 80% of the amplitude of the reflected signal (over the amplitude of the input signal). The comparator then switches at a different point in time. This is disadvantageous: It introduces some inaccuracy, since the point in time of switching should depend only on the travel time of the signal.  
         [0010]     As the threshold value is adjustable depending on the amplitude of the reflected signal, the point of switching of the comparator can be adapted advantageously to the reflected signal. Differences in the size of the amplitude of the reflected signal can be equalized wherein it is of no importance on the basis of which disturbing influences the amplitude of the reflected signal has changed. As a result, in a highly advantageous manner, also influences caused by temperature variations of the material of the measuring line and the evaluation device are almost completely eliminated. The time of switching of the comparator depends only on the travel time of the signal which, with a travel time measurement, has a substantial influence on the accuracy.  
         [0011]     It has been found to be highly advantageous to determine the voltage change of the signal generator, that is, the slope of the voltage change for forming the threshold value. It has been found that the measuring accuracy is highest if the switching time of the comparator is always at a point where the slope of the voltage change of the summing signal over time has always the same predetermined inclination.  
         [0012]     In this connection, it is very advantageous if the threshold value of the comparator corresponds to the voltage of the summing signal determined from the input signal and the reflected signal at which the voltage change of the summing signal is about one third to two thirds, particularly two fifths to four fifths and preferably one half of the average of the inclination of the increasing flank of the signal supplied to the measuring line. The average inclination of the increasing flank of the voltage of the signal supplied to the measuring line is formed from the difference between 90% of the amplitude of the input signal and 10% of the amplitude of the input signal and the associated points in time.  
         [0013]     It is in this connection very advantageous to form the threshold value from the sum of the amplitude of the input signal and a fraction of the amplitude of the reflected signal as it is provided for in a particular embodiment of the invention. It has been found to be very advantageous if the fraction of the amplitude of the reflected signal is about 50 to 95%, particularly 65 to 85% and preferably 75%.  
         [0014]     Since for forming the threshold value, a fraction of the amplitude of the reflected signal is used, the point in time of switching of the comparator is always about in the same area of the flank of the voltage change at the proximal end of the measuring line or, respectively, the output of the signal generator. This is very advantageous with regard to the accuracy of the measuring results.  
         [0015]     For determining the rate of the voltage change over time, first, an empirically determined threshold value can be set. With this threshold value, a first measurement is performed. The travel time of the signal up to the return of the reflected signal at the proximal end of the measuring line determined in this way or, respectively, the second point in time determined in the way is recoded as the first measuring value. Then the threshold value can be slightly increased and a second measurement performed. The difference between the two measured times is divided by the difference between the two threshold values.  
         [0016]     In this way, the inverse value of the slope or the amplitude change over time is obtained. If a comparison with a predetermined value shows that the slope is too large, the procedure is repeated with higher threshold values. If the slope is too small, a lower threshold value is used. The threshold values are changed until the slope is within the tolerance limits for the predetermined value.  
         [0017]     With the threshold value determined in this way, it is ensured that the point in time of switching of the comparator is always in the same area of the curve showing the voltage change over time independently of whether the reflected signal has a large amplitude or a small amplitude. A range has been found to be very advantageous in which the reflected signal has reached about 75% of its amplitude top.  
         [0018]     Very good measuring results were obtained with an inverse value of the quotient of about 3 nanoseconds per volt to 8 nanoseconds per volt, particularly 4 nanoseconds per volt to 6 nanoseconds per volt and preferably 5 nanoseconds per volt.  
         [0019]     In another embodiment of the invention, a reference line is provided to the proximal end of which a pulsed signal is supplied. A second comparator is present which at the proximal end of the reference line receives the signal reflected at the distal end of the line and compares the reflected signal with a reference threshold value.  
         [0020]     Since the signals are supplied to the reference line by a separate signal generator, the internal resistance of the signal generator for the measuring line is smaller which is advantageous with respect to the accuracy of the measurement. Furthermore, the reference line and the signal line are completely uncoupled so that they do not influence each other. Also, the respective circuit components with an ASIC can be spatially separated whereby a very good HF uncoupling is achieved.  
         [0021]     Further details, features, and advantages of the invention will become more readily apparent from the following description of a particular embodiment thereof described below with reference to the accompanying drawings. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0022]      FIG. 1  shows an arrangement for performing the method according to the invention,  
         [0023]      FIG. 2  shows another arrangement for performing the method according to the invention,  
         [0024]      FIG. 3  shows a voltage curve for a signal entering the measuring line,  
         [0025]      FIG. 4   a  to  FIG. 4   c  show idealized signal curves with a reflection having a negative amplitude and a reflection having a positive amplitude in the signal line, and  
         [0026]      FIGS. 5   a  to  5   c  show idealized signal curves with a reflection with negative amplitude on the signal line.  
     
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0027]     As shown in  FIG. 1 , a measuring line  7   a ,  7   b  is connected to a signal generator  1  by way of a connecting line  7 ′. The output of the signal generator  1  or, respectively, the proximal end of the measuring line  7   a ,  7   b  is connected to a first input of a first comparator  2 . A second input of the first comparator  2  is connected to an output of an adjustable threshold value generator  9 . The output of the first comparator  2  is connected to a variable logic circuit  8 . Furthermore, a control output of the signal generator  1  is connected to the variable logic circuit  8 .  
         [0028]     The signal generator  1  is further connected to the proximal end of a reference line  13 . The respective output of the signal generator  1  or, respectively, the proximal end of the reference line  13  is connected to the input of a second comparator  11 . A second input of the second comparator  11  is connected to a reference threshold value generator  12 . The output of the second comparator  11  is connected to the variable logic circuit  8 . The output of the variable logic circuit  8  is connected to an input of an AND gate  4 . A second input of the AND gate  4  is connected to an oscillator  3 . An output of the AND gate  4  is connected to the counter  5 . The output of the counter  5  is connected to a control unit  10  and to an evaluation and display unit  6 . The output of the control unit  10  is connected to a control input of the variable threshold value generator  9 .  
         [0029]     The operation of the arrangement will be described below on the basis of the voltage curve shown in  FIG. 3 , which was recorded at the output of the signal generator  1  or respectively the input of the connecting line  7 ′.  
         [0030]     First a measuring signal is supplied by the signal generator  1  to the connecting line  7 ′. The measuring signal has the same pattern as shown by the first increase of the voltage curve given in  FIG. 3 . At the time t 0 , the signal increase begins. At the time t 0 ′ 10% (U 0 ′) of the maximum amplitude of square signal has been reached. At the time t 0 ″ 90% (U 0 ″) of the maximum amplitude (U s ) has been reached. After reaching the maximum amplitude (U s ), the voltage level of the output signal of the signal generator  1  remains constant for the duration of the travel time measuring period.  
         [0031]     In the voltage curve as shown in  FIG. 3 , the signal supplied to the connecting line  7 ′ is reflected upon transition from the connecting line  7 ′ to the measuring line  7   a ,  7   b  with a negative amplitude. The reflected signal arrives at the output of the signal generator  1  at the time t 1  whereby the voltage of the summing signal present at the output of the signal generator is slightly reduced (U 1 ).  
         [0032]     Corresponding to the moisture content of a particular material to be examined, the signal is delayed in the measuring line  7   a ,  7   b  and at the end of the measuring line  7   a ,  7   b , is reflected time-delayed with a positive amplitude. The reflected signal arrives about at the time t 2  at the output of the signal generator  1 . Since the flank of the summing signal formed at the output of the signal generator  1  is not exactly vertical, the time at which the reflected signal arrives can be determined generally only quite inaccurately, particularly since the amplitude of the reflected signal depends on the electric conductivity and therefore the moisture content of the material.  
         [0033]     At the point in time of arrival of the reflected signal is determined in accordance with the invention when the summing signal obtained at the output of the signal generator  1  corresponds to a threshold value which is adjusted depending on the amplitude of the reflected signal. For determining the threshold value a first search threshold value U 2 ′ which corresponds about to the amplitude of the measuring signal (U s ) supplied by the signal generator  1  to the connecting line  7 ′ plus 40% of the amplitude of the signal reflected in the measuring line  7   a ,  7   b  is first provided at a first comparator  2  by means of the control device  10  and the threshold value generator  9 . With this first search threshold value U 2 ′, a first travel time measurement is performed wherein the voltage of the signal value at the time t 2 ′ obtained at the output of the signal generator  1  corresponds to the first search threshold value U 2 ′.  
         [0034]     Upon arrival of the signal reflected in the measuring line  7   a ,  7   b  at the respective output of the signal generator  1  or, respectively, at the input of the first comparator  2 , the signal present at the input of the first comparator  2  exceeds the threshold value present at the second input of the first comparator  2 . As a result, the signal  2   b  present at the output of the first comparator  2 , which up to this point was positive, becomes zero (see  FIG. 4   b ).  
         [0035]     At the same time, when the measuring signal is applied by the signal generator  1  to the reference line  13 . The measuring signal is reflected in the reference line  13  with a positive amplitude. After the signal reflected in the reference line has arrived at the respective output of the second comparator  11 , the signal  11   a  present at the input of the second comparator  11  exceeds the reference threshold value present at the second input of the second comparator  11 . As a result, the signal  11   b  provided at the output of the second comparator  11 , which so far was positive, becomes zer 0  (see  FIG. 4   a ). In the variable logic circuit  8  the output signal  11   b  of the second comparator  11  is combined with the output signal  2   b  of the first comparator  2  in such a way that the signal  8   a  provided at the output of the logic circuit  8  at the time when the second comparator switches, becomes positive and becomes again zero when the first comparator  2  switches (see  FIG. 4   c ).  
         [0036]     At the output of the AND gate  4 , there are consequently the impulses of the oscillator  3  present during the time between the arrival of the signal reflected in the reference line  13  and the arrival of the signal reflected in the measuring line  7   a ,  7   b . These impulses are counted in the counter  5 . The count value represents a measure for the travel time of the measuring signal in the measuring line  7   a ,  7   b  and is a measure for the moisture content of the material in which the measuring line  7   a ,  7   b  is disposed. The output value of the counter  5  is provided to the display unit  6 .  
         [0037]     Furthermore, the output value of the counter  5  is applied to the control device  10  which adjusts the threshold value generator  9 . With the first travel time measurement, the first search threshold value is obtained.  
         [0038]     After completion of the first travel time measurement, at the first comparator  2  a second search threshold value U 2 ″ is set which corresponds to the value of the amplitude of the measuring signal supplied by the signal generator  1  to the connecting line  7 ′ plus twenty five % of the amplitude of the signal reflected in the measuring line  7   a ,  7   b . For forming the second search threshold value U 2 ″, the first search threshold value U 2 ′ was consequently reduced by fifteen percent of the amplitude of the signal reflected in the measuring line  7   a ,  7   b . With the second search threshold value U 2 ″, a second travel time measurement is performed, wherein the voltage of the signal present at the output of the signal generator  1  at the point in time t 2 ″ corresponds to the second search threshold value U 2 ″. By means of two measurement values obtained in this way, the inclination of a straight line G is calculated which is defined by the two measurement values U 2 ′,t 2 /U 2 ″,t 2 ″ of the voltage curve of the summing signal present at the output of the signal generator  1 .  
         [0039]     If the inclination or slope of the straight line G has less than a predetermined value, the measuring cycle described above is repeated with new first and second search threshold values wherein the new first search threshold value is selected so as to be sixteen percent of the amplitude of the signal reflected in the measuring line  7   a ,  7   b  below the search threshold value selected in the previous measuring cycle. The new first search threshold value of the second measuring cycle therefore corresponds to the amplitude of the measuring signal supplied to the connecting line  7 ′ plus twenty four percent of the amplitude of the signal reflected in the measuring line  7   a ,  7   b . The new second search threshold valve is again by fifteen percent of the amplitude of the signal reflected in the measuring line  7   a ,  7   b  lower than the new first search threshold value. By way of the new travel time measurements obtained with the two new actual search threshold values the slope of a straight line is again determined which is defined by the two points determined by the two measurement values of the summing signal at the output of the signal generator  1 .  
         [0040]     If the slope obtained in this way is still smaller than the predetermined slope a third measurement cycle is performed, wherein the first search threshold value is again reduced by sixteen percent of the amplitude of the signal reflected in the measuring line  7   a ,  7   b . The measuring cycle is repeated with a search threshold value reduced by sixteen percent of the amplitude of the signal reflected in the measuring line  7   a ,  7   b , until the calculated slope of the straight line defined by the two points determined by the two measurement values is greater than the predetermined slope.  
         [0041]     When the calculated slope is greater than the predetermined slope, that is, when in the comparison the sign is reversed, the last first search threshold value is increased by one half of the reduction which was previously employed up to the sign reversal that is, in the present case by ½ of sixteen %, that is 8% of the amplitude of the signal reflected in the measuring line  7   a ,  7   b  and a new measuring cycle is performed.  
         [0042]     Generally, it can be said: As long as a comparison does not result in a sign reversal, the first search threshold value is changed by the same amount by which it was changed in the previous measuring cycle. When the comparison shows a sign reversal, the search threshold value is changed by half the amount by which it was changed in the preceding measurement cycle and in the opposite sense. The second search threshold value is always by fifteen percent of the amplitude of the signal reflected in the measurement line  7   a ,  7   b  smaller than the first search threshold value.  
         [0043]     The iterative approximation to the predetermined slope as described above is performed until the first search threshold value has been changed by only one percent of the amplitude of the signal reflected on the measuring line  7   a ,  7   b  and a new change with reversed sign would be necessary.  
         [0044]     The first search threshold value determined in this way is then finally reduced by twenty percent of the amplitude of the signal reflected in the measuring line  7   a ,  7   b . This value then forms the threshold value on the basis of which the moisture content of the material is determined.  
         [0045]     By means of the reference line  13  and the second comparator  11  provided therefor, the start of the travel time measurement can be changed in an advantageous way. The travel time measurement may be started for example only when the signal reflected in the reference line  13  arrives at the output of the signal generator  1 . In this way, a change of the voltage curve caused particularly by a reflection with negative amplitude can be eliminated at the output of the signal generator  1  as it is apparent from  FIGS. 4   a  to  4   c.    
         [0046]     In  FIG. 4   a , the signal values  11   a  obtained at the input of the reference line  13  or respectively at the corresponding output of the signal generator  1 , which are also present at the input of the second comparator  11  and the signal values  11   b  present at the output of the second comparator  11  are schematically shown.  FIG. 4   b  shows the signal values  2   a  present at the input of the connecting line  7 ′ or, respectively, the corresponding output of the signal generator  1 , which are also present at the input of the first comparator  2 , as well as the signal values  2   b  present at the output of the first comparator  2 .  FIG. 4   c  shows schematically the signal values  8   a  present at the output of the logic circuit  8 .  
         [0047]     As shown in  FIGS. 4   a  and  4   b , a reference signal is supplied to the reference line  13  at the time t 0  when a measuring signal is supplied to the connecting line  7 ′. The signal reflected in the reference line  13  arrives at the output of the signal generator  1  at the time t 2 . As a result, the second comparator  11  switches. The signal  11   b  present until then at the output of the second comparator  11  then becomes zero. In this way, the output signal  8   a  of the logic circuit becomes positive as shown in  FIG. 4   c . With the positive output signal  8   a  of the logic circuit  8 , the travel time measurement is started at the time t 2 .  
         [0048]     When at the time t 3 , the signal reflected in the measuring line  7   a ,  7   b  arrives at the respective output of the signal generator  1  with a positive amplitude, the first comparator  2  switches. The signal  2   b  present until then at the output of the first comparator  2  then becomes zero. As a result, the output signal  8   a  of the logic circuit  8  becomes zero whereby the travel time measurement is ended.  
         [0049]     The travel time is therefore measured only between the time points T 2  and t 3  as apparent from  FIG. 4   c . Disturbances arriving with a negative reflection at the time t 1  as shown in  FIG. 4   b  have therefore no effect.  
         [0050]      FIG. 5  shows schematically the signal values  11   a ′ which are present at the output of the reference line  13 , or at the respective output of the signal generator  1  and also at the input of the second comparator  11  and the signal values  11   b ′ present at the output of the second comparator  11 .  FIG. 5   b  shows the signal values  2   a ′, which are present at the input of the connecting line  7  or the respective output of the signal generator  1  and also at the input of the first comparator  2  and the signal values  2   b ′ present at the output of the first comparator  2 .  FIG. 5   c  shows schematically the signal values  8   a ′ present at the output of the logic circuit  8 .  
         [0051]     As shown in  FIGS. 5   a  to  5   c , the reference line  13  may also be so selected that the signal reflected in the reference line  13  arrives at the respective output of the signal generator  1  already when the signal reflected at the transition from the connecting line  7 ′ to the measuring line  7   a ,  7   b  with negative amplitude has not yet arrived at the respective output of the signal generator  1 . Consequently, the travel time measurement can be started at a time t 2 ′ when the signal  2   a ′ supplied to the connecting line  7 ′ or, respectively, the measurement line  7   a ,  7   b  has reached the maximum amplitude. In this way, a threshold value can be set at the first comparator  2 , which is below the amplitude of the measuring signal supplied to the connecting line  7 ′. This has the advantage that a reflection with a negative amplitude can be measured.  
         [0052]     As a result, the arrangement according to the invention can be used for measuring filling degrees. It has been found that if the measuring line  7   a ,  7   b  is for example partially immersed into a liquid a reflection with negative amplitude occurs at the surface of the liquid.  
         [0053]     In the embodiment shown in  FIG. 2 , a separate signal generator  14  is provided for the reference line  13 . Otherwise the embodiment of  FIG. 2  is essentially the same as that shown in  FIG. 1 . Identical elements are therefore indicated by the same reference numerals.  
         [0054]     Because of the separate signal generator  14  for the reference line  13 , the internal resistance of the signal generator  1  may be lower which is advantageous with regard to the measurement accuracy. Furthermore, there is no feedback from reference line  13  to the measuring line  7   a ,  7   b , which is also advantageous with respect to the measuring accuracy. In addition, the respective circuit parts can be spatially separate whereby a good HF uncoupling can be achieved.