Patent Abstract:
An electronic weighing sensor ( 10 ) having a digital signal processing unit ( 19 ), which includes at least one filter ( 12 ) with low-pass characteristic. By means of the filter ( 12 ), the direct-current component (m) is determined from the output signal of the weighing sensor and the weighing result is derived therefrom. A signal, which is dependent on the amplitude of the vibrations, is determined by the digital signal processing unit ( 19 ) and electronic components ( 18 ) change the direct-current component (m) as a function of the magnitude of the vibration-dependent signal. This significantly improves the performance of the weighing sensor ( 10 ) with respect to shocks and vibrations in the installation site of the weighing sensor ( 10 ).

Full Description:
[0001]    The following disclosure is based on German Patent Application No. 100 24 986.8, filed on May 19, 2000, which is incorporated into this application by reference.  
         FIELD OF AND BACKGROUND OF THE INVENTION  
         [0002]    The present invention relates to an electronic weighing sensor having a digital signal processing unit, which includes at least one filter with a low-pass characteristic. By means of the filter, the direct-current component of the output signal of the weighing sensor is determined and the weighing result is derived therefrom.  
           [0003]    Weighing sensors of this type are generally known in the art. The low-pass filter is used to suppress the alternating-current components, which are superimposed on the output signal of the weighing sensor when the installation site of the weighing sensor is subject to shocks or vibrations. Generally, despite this measure, the measuring results of weighing sensors and scales that are based on weighing sensors are clearly more difficult to reproduce if the installation site is subject to shocks or vibrations than if the installation site is steady.  
           [0004]    To improve the performance of weighing sensors and scales at unsteady installation sites, it is known, for instance from U.S. Pat. No. 5,789,713, to provide a second, weighing sensor having a constant load whose output signal is used to derive a correction signal for the actual (measuring) weighing sensor. The mechanical and electronic complexity caused by this second weighing sensor is considerable, however.  
           [0005]    Similarly, German laid-open document DE 40 01 614 A1 teaches the provision of at least one acceleration sensor instead of a second, constantly-loaded weighing sensor, which supplies a correction signal for influencing the measuring result. In both cases, however, the correction signal must be subtracted from the signal of the (measuring) weighing sensor in such manner that the phases of both signals are properly taken into account so that the interference can be corrected. This in-phase subtraction, however, can be achieved within only a limited frequency band. At the upper end of this frequency band, the phase shifts are more and more increased and differ between the measuring path and the correction path. This results, in the worst case, in an addition—and thus an amplification—of the signals rather than a subtraction—and thus a cancellation. This is particularly true for acceleration sensors that are employed as correction generators, since the mechanically very differently structured system of the acceleration sensor has eigenfrequencies that are very different from those of the weighing sensor.  
         OBJECTS OF THE INVENTION  
         [0006]    It is one object of the present invention to improve the performance of a weighing sensor in case of shocks and vibrations in the installation site without requiring a second weighing sensor or an acceleration sensor for correction purposes. Therein, problems due to phase shifts at the edge of the frequency range are to be avoided.  
         SUMMARY OF THE INVENTION  
         [0007]    According to one formulation of the invention, this and other objects are achieved by providing a method for deriving a weighing result, in which a direct-current component of an output signal of an electronic weighing sensor is determined by means of a low-pass filter that is arranged in a digital processing unit. In addition, a further signal is determined by the digital processing unit that is dependent on an amplitude of vibrations of the electronic weighing sensor. The direct-current component is changed by electronic components in accordance with a magnitude of the further signal that is dependent on the amplitude of the vibrations.  
           [0008]    This approach is based on the finding by the inventors that the poor reproducibility of weighing sensors at unsteady installation sites is not only caused by inadequate suppression of the alternating-current component in the output signal of the weighing sensor. Rather, a significant contribution to the poor reproducibility is due to the fact that the direct-current component in the output signal of the weighing sensor is influenced as a function of the amplitude and the frequency range of the alternating-current component. Thus, stronger suppression of the alternating-current component in the output signal of the weighing sensor alone does not sufficiently improve the performance of a weighing sensor that is subject to vibrations. In addition to that, a correction of the direct-current component must be carried out, as proposed by the present invention.  
           [0009]    The causes for this influence on the direct-current component may be illustrated by three examples:  
           [0010]    In the case of weighing sensors that have a non-linear characteristic—as shown in FIG. 1 in exaggerated form—and that are installed at a steady installation site, the load m 1  is associated with the output signal corresponding to point A. In an unsteady site, however, the output signal fluctuates between the extreme points B and C along the non-linear characteristic B-A-C. Depending on the amplitude spectrum of the vibrations, the direct-current component of this output signal is located somewhere between points A and D. As a result, even if the alternating-current component in the output signal is completely suppressed, the direct-current component changes due to the non-linearity of the characteristic curve. This change is suppressed by the electronic components according to the present invention.  
           [0011]    In the case of weighing sensors that operate based on the principle of electromagnetic force compensation, as illustrated schematically in FIG. 2, a coil  2 , through which a current flows, is located in the magnetic field of a permanent magnet  1 . The current flowing through the coil  2  is regulated by a position indicator  3  and by a downstream regulation amplifier  4  in such a manner that the electromagnetically generated force is precisely equal to force F to be measured. The magnitude of this current is measured at a measurement resistor  5  and is supplied to an output  6  as an output signal of the weighing sensor. As is well known, the quantitative relation between the magnetic field B, the current I and the generated force F is:  
             F≈B·I   (1)  
           [0012]    To achieve optimal efficiency, the coil  2  is positioned in such a way that it is located at the point of the maximum magnetic field of the permanent magnet  1 . If the coil  2  is caused to oscillate due to vibrations in the base, the coil  2  moves sometimes outside the magnetic field maximum and into a region with a lower magnetic field. On temporal average, the magnetic field B in equation (1) is thus lower than in a steady installation site where coil  2  is always located within the magnetic field maximum. Consequently, according to equation (1), a greater average current I is required to generate the same force F. Thus, in this example too, the direct-current component changes when there are shocks/vibrations so that, in addition to suppressing the alternating-current component, the direct-current component must also be corrected in order to obtain a stable and exact result.  
           [0013]    If the characteristic curve of the position indicator  3  in the above-described weighing sensor according to FIG. 2 is asymmetrically non-linear, the average transient position is shifted when there are vibrations, as illustrated above by means of FIG. 1. On the one hand, this change in the average transient position causes a change in the effective magnetic field at the position of the coil, as shown in the second example. On the other hand, a slight deflection in the parallel guidance  7  of the scale tray  8  and coil  2  is caused by this change. If the parallel guidance  7  is realized by spring-type hinges, this causes a vertical spring force, which changes the direct-current component in the weighing signal.  
           [0014]    Other causes, which are not further described here, may also distort the direct-current component in the weighing signal, e.g., non-linear amplifiers or non-linear transmission levers. Of course, all the described effects are small and, therefore, have consequences at high resolutions of the weighing sensor only.  
           [0015]    Advantageous embodiments are set forth in the dependent claims.  
           [0016]    The use of a high-pass filter for determining the vibration-dependent alternating-current components in the output signal of a weighing sensor and the control of a display element or a print lock (in case the alternating-current component exceeds a predefined limit value) is generally known in the art of scale technology. Such structures and methods are described, for instance, in German Patent DE 23 23 200. In the prior art, however, no influence is effected on the direct-current component of the output signal of the weighing sensor. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]    The invention and further advantageous refinements of the invention according to the features of the dependent claims are explained in more detail below with the aid of diagrammatic, exemplary embodiments in the drawings, in which:  
         [0018]    [0018]FIG. 1 shows a non-linear characteristic curve of a weighing sensor;  
         [0019]    [0019]FIG. 2 shows a weighing sensor that operates in accordance with the principle of electromagnetic force compensation;  
         [0020]    [0020]FIG. 3 is a block diagram of a first embodiment of the weighing sensor according to the present invention;  
         [0021]    [0021]FIG. 4 is a block diagram of a second embodiment of the weighing sensor according to the present invention;  
         [0022]    [0022]FIG. 5 is a block diagram of a third embodiment of the weighing sensor; according to the present invention; and  
         [0023]    [0023]FIG. 6 is a block diagram of a fourth embodiment of the weighing sensor according to the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0024]    [0024]FIGS. 1 and 2 have already been described in the “Summary of the Invention”.  
         [0025]    The block diagram of FIG. 3 shows a weighing sensor  10  to whose output  11  both a low-pass filter  12  and a high-pass filter  13  are connected. At its output, the low-pass filter  12  supplies a signal m, which is proportional to the direct-current component of the output signal of the weighing sensor  10 . Thus, the signal m is essentially proportional to the loading of the weighing sensor  10 . The high-pass filter  13 , at its output, supplies a signal that is proportional to the alternating-current component of the output signal of the weighing sensor  10 . Thus, this signal is proportional to the vibrations of the weighing sensor at constant loading of the weighing sensor  10 . This signal is rectified and slightly smoothed (block  14 , wherein the diode and the capacitor in the drawing symbolize this generally and should not be construed as being limited to one-way point rectification) and is thus proportional to the average alternating-current component of the output signal of the weighing sensor  10 . On the one hand, the signal is then supplied to a threshold comparator  15 , as known in the prior art. If the signal level is below the threshold, the threshold comparator  15  causes the gram symbol “g” in a display  16  to light up. On the other hand, the signal is supplied to an arithmetic logic unit  18  via a squaring element  17 . In the arithmetic logic unit  18 , a corrected output signal m′ is calculated from the (uncorrected) input signal m (of low-pass filter  12 ) and a correction signal x 2  of the squaring element  17  according to e.g. the following formula:  
           m′=m+ ( a·m+b )· x   2   (2)  
         [0026]    If the installation site is steady, i.e., x 2 =0, then m′=m. If the installation site is unsteady, an addend that is proportional to the square of the alternating-current component x is added to the direct-current component m of the output signal of the weighing sensor  10 . The factor “b” corrects for load-independent effects, while the factor “a” corrects for effects that increase with the loading of the weighing sensor. The quantity and sign of “a” and “b” depend on the physical causes of the effect and thus the behavior of the individual weighing sensor. In the case of the weighing sensor having a non-linear characteristic according to Example 1 and FIG. 1 of the “Summary of the Invention”, “a” and “b” depend on the sign and the curvature of the characteristic curve. In the example shown in FIG. 1, “a”=0 and “b” is negative, provided that the curvature of the characteristic curve is constant, i.e., independent of the loading of the weighing sensor  10 . In the weighing sensor according to Example 2 and FIG. 2 of the “Summary of the Invention”, which is based on the principle of electromagnetic force compensation and where the effect is caused by a decrease in the magnetic field, “b”=0 and “a” is negative. In the third example of the non-linear position indicator, both “a” and “b” are not equal to 0. The quantity of “a” is determined by the decrease in the magnetic field, whereas the quantity and sign of “b” are determined by the non-linearity of the position indicator and by the magnitude of the spring constant of the parallel guidance.  
         [0027]    The quantity of the correction factors “a” and “b” must be determined and defined for each weighing sensor type in accordance with its individual behavior. It should be noted that x 2  in equation (2) and in FIG. 3, i.e., the squared alternating-current component, is ordinarily an appropriate parameter to use. However, in some special cases, a deviating dependence on the magnitude of the alternating-current component may occur, in which case a different equation would be used to perform the correction.  
         [0028]    The corrected value m′ is then forwarded by the arithmetic logic unit  18  to the display unit  16 .  
         [0029]    The above description omits all the other standard computational processes with the output signal of the weighing sensor, e.g., balancing/taring and calibrating, but they are of course carried out.  
         [0030]    The components of the circuit described individually above are typically realized by a single digital signal processing unit  19  in the form of a microprocessor. The filters  12  and  13  are advantageously implemented as digital filters and are realized within the digital signal processing unit  19 . In FIG. 3, all of the components that can be realized by the microprocessor of the digital signal processing unit  19  are boxed with a dashed line.  
         [0031]    If the filters  12  and  13  are digital filters, the output signal of the weighing sensor  10  must of course also be digital or must be digitized by an analog/digital converter, which would have to be seen as part of the weighing sensor  10  in FIG. 3. The filters  12  and  13  (and the rectification and smoothing  14 ) can of course also be analog filters, which are directly connected to the analog output signal of the weighing sensor  10 . The squaring element  17  and the arithmetic logic unit  18  can also be analog, in which case the corrected value m′ would have to be digitized only in front of the digital display unit  16 . Today, however, at least the arithmetic logic unit  18  will typically be digital, so that a respective analog/digital converter must be installed behind the analog low-pass filter  12  and behind the analog smoothing component  14  or behind the squaring element  17 .  
         [0032]    [0032]FIG. 4 shows a second embodiment of the weighing sensor according to the present invention. A weighing sensor  9  operates according to the generally known principle of electromagnetic force compensation, which was briefly described above: A coil  2  is located in the air gap of a permanent magnet  1 . A current I, which flows through the coil  2 , is regulated by means of a position indicator  3  and a regulating amplifier  4  such that the electromagnetically generated force is precisely as large as the force generated by the object being weighed on a scale tray  8 . Therein, the scale tray  8  is guided by a parallel guidance  7 . At a measurement resistor  5 , the load-proportional output voltage is tapped, digitized (analog/digital converter  20 ) and supplied to a digital low-pass filter  22 . The vibration-dependent signal is tapped directly at the position indicator  3  via a capacitor  23 . This signal is rectified, smoothed (block  24 ) and digitized (analog/digital converter  25 ) and represents the correction signal x. In an arithmetic logic unit  28 , the correction is carried out in accordance with equation (2), wherein x 2  is being formed in the arithmetic logic unit too.  
         [0033]    The corrected value m′ is displayed in a display unit  26 . This embodiment is essentially distinguished from the first embodiment according to FIG. 3 in that the vibration-dependent signal is tapped directly by the position indicator  3  and is not derived from the output signal of the weighing sensor  9 / 10 . Thereby, the frequency-dependent influence of the regulating amplifier  4  is avoided. In addition, the vibration-dependent signal is tapped directly where the cause of the effect in weighing sensors according to the principle of electromagnetic force compensation lies: at the position indicator  3 , which detects the position of coil  2  and thus directly the deviation of the coil position from the maximum magnetic field. For instance, any resonance sharpness that occurs on the coil  2  at certain frequencies is detected by the position indicator  3  and is not partly suppressed by the PID regulation behavior of the regulation amplifier  4 . Only then is the resonance sharpness detected in the output signal of the weighing sensor  9 .  
         [0034]    [0034]FIG. 5 shows a third embodiment of the weighing sensor according to the present invention. The output signal of the weighing sensor  10 , which is again assumed to be digital, is supplied to the low-pass filter  12  and to three bandpass filters  30 ,  31  and  32 . Each bandpass filter is adjusted to one of the resonance frequencies of the weighing sensor. Each bandpass filter filters out this frequency range from the output signal of the weighing sensor  10 . A rectifier and smoothing module  34  is connected downstream from each bandpass filter. Subsequently, a respective squaring element  37  is connected to each rectifier and smoothing module  34 . The output signal of these squaring elements  37  is the vibration-dependent signal with reference to the corresponding frequency range. In subsequent multiplying units  38 , each of these signals x 2  is multiplied by the coefficient (a·m+b) according to equation (2). Therein, the coefficients (a·m+b) are provided by a memory and multiplication unit  39  labeled “weighting.” In the three bandpass filters shown, three coefficients “a” and three coefficients “b” are also stored in the unit  39 . The results of the multiplying units  38  are added up in an adder  40 . The sum is then added to the signal m in an adder  41 . The circuit according to FIG. 5 operates exactly the same way as the circuit according to FIG. 3 except that the circuit according to FIG. 5 can carry out the correction separately for three different frequency ranges. Thus, the circuit according to FIG. 5 corrects different behaviors of the weighing sensor  10  in the different frequency ranges. The number of three bandpass filters and thus three frequency ranges is of course only an example; any other number is possible. Likewise, equation (2) is only an exemplary linkage of m′ and m. In the general case, the following equation is valid:  
               m   ′     =     m   +       ∑   j            f   i          (       a   i     ,     b   i     ,   m   ,     x   i       )                   (   3   )                               
 
         [0035]    where f 1  is any function and the index i relates to the different frequency ranges. In the example of FIG. 5, therefore, i=1 . . . 3. The coefficients a i  and b i  represent weighing sensor-specific constants. The dependence of the individual x i  will in many cases be squared—as shown in equation (2)—but any other dependence is also possible.  
         [0036]    [0036]FIG. 6 shows a fourth embodiment of the weighing sensor according to the present invention. The output signal of an actual weighing sensor  50  is supplied to a low-pass filter  52  and yields signal m. The vibration-dependent signal is derived from an acceleration sensor  51 , which is fixed to the base point of the weighing sensor  50 . Depending on the structure of the acceleration sensor  51 , the output signal is either directly supplied to a rectifier and smoothing module  54  (if the acceleration sensor, because of its structure, detects only alternating accelerations anyway) or via an interposed high-pass filter  53  (if the acceleration sensor  51  also detects direct-current components, or if its frequency behavior significantly deviates from the required frequency behavior). The correction signal x thus obtained changes the direct-current component m from the low-pass filter  52  in the arithmetic logic unit  58 , as described above. The result m′ is displayed in a display unit  56 . What has been described above in the other embodiments regarding analog or digital filters and the correction equation applies to this embodiment as well.  
         [0037]    The above description of the preferred embodiments has been given by way of example. From the disclosure given, those skilled in the art will not only understand the present invention and its attendant advantages, but will also find apparent various changes and modifications to the structures and methods disclosed. It is sought, therefore, to cover all such changes and modifications as fall within the spirit and scope of the invention, as defined by the appended claims, and equivalents thereof.

Technology Classification (CPC): 6