Abstract:
A weighing scale that is calibratable without a calibration weight that is separate and distinct from the scale. The scale has a weighing mode and a self-calibration mode and includes a base that supports a load cell, which in turn supports a mass receiver. Electronic circuitry within the scale is configured so that, during the calibration mode, the sprung weight of the base when the scale is inverted and supported by the mass receiver can be used to calibrate the scale. This avoids the need to maintain a calibration weight external to the scale. A process of calibrating the scale includes inverting the scale during the self-calibration process and allowing the circuitry to acquire a calibration parameter that is based on the inverted sprung weight.

Description:
RELATED APPLICATION DATA 
     This application claims the benefit of priority of U.S. Provisional Patent Application Ser. No. 60/978,796, filed Oct. 10, 2007, and titled Self-Calibrating Scale, which is incorporated by reference herein in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention generally relates to the field of weighing scales. In particular, the present invention is directed to a self-calibrating weighing scale and a method of calibrating a weighing scale. 
     BACKGROUND 
     One type of electronic weighing scale utilizes an electrically resistive strain gauge based transducer (or load cell) for determining the weight of a mass being weighed. This type of scale is used in many settings, such as the food service industry where these scales are used, for example, for portion control and for measuring ingredients of food recipes. Portion control is important to many food service organizations, such as franchised restaurants, where the portions of certain ingredients, for example, weight of meat used in a particular sandwich or weight of ice cream used in a certain size cone, provided to a customer must be tightly controlled to maintain profitability. When bakers and cooks follow carefully proportioned recipes, they clearly must use the proper amount of certain ingredients. Sometimes the ingredients can be readily measured by weight. 
     A load cell type electronic weighing scale generally operates by interpolating an electrical resistance signal generated by the load cell when it is placed under load. The interpolation is based upon a calibration curve created by two points of known weight and load cell response. Generally, one calibration point, the zero point, is determined by the load cell output when there is no weight placed upon the scale. The second calibration point is determined by placing an accurately known weight on the scale, often a certified weight, and measuring the load-cell resistance that results. Initial determination of these calibration points is typically done at the time of manufacture of the weighing scale. 
     There are a number of circumstances that can occur during the service life of a load-cell-based scale that will require the scale to be recalibrated. For example, if the scale is dropped or overloaded, the load-cell may be damaged or permanently deformed, altering its strain-resistance response. Modifications or repairs that alter the mass of scale components that rest upon or are supported by the load-cell will also require recalibration. The range of weights that a user may desire to be accurately weighed by the weighing scale may change, and improved accuracy obtained by recalibration of the scale for the exact range of interest. 
     SUMMARY OF THE DISCLOSURE 
     In one implementation, the present disclosure is directed to a weighing scale. The weighing scale includes: a mass receiver for receiving a mass to be weighed by the weighing scale; electronic circuitry configured to provide the weighing scale with a weighing mode and a self-calibration mode, the weighing mode for weighing a mass placed upon the mass receiver; a base supporting the mass receiver when the weighing scale is in the weighing mode, the base contributing to an actual inverted sprung weight of the weighing scale when the weighing scale is inverted and supported by the mass receiver; a load cell located between the mass receiver and the base and in operative communication with the electronic circuitry, the load cell configured to output a weight signal proportional to a force applied to the load cell in each of the weighing mode and the self-calibration mode; and wherein the electronic circuitry is configured to calibrate the weighing scale as a function of the actual inverted sprung weight when the electronic circuitry is in the self-calibration mode. 
     In another implementation, the present disclosure is directed to a method of calibrating a weighing scale. The method includes: switching a weighing scale from a weighing mode to a self-calibration mode in response to self-calibration mode signal triggered by a user; generating a weight signal for an inverted sprung weight of the weighing scale when the weighing scale is in an inverted position relative to a non-inverted position used during the weighing mode; and determining a calibration parameter value as a function of the weight signal. 
     In still another implementation, the present disclosure is directed to a method of calibrating a weighing scale having a weighing mode and a calibration mode. The method includes: providing the weighing scale; setting the weighing scale to the calibration mode; causing the weighing scale to obtain calibration parameters defining a first calibration point on a calibration curve when the weighing scale is in an upright orientation; inverting the weighing scale from the upright orientation to an inverted orientation; and causing the weighing scale to obtain calibration parameters defining a second calibration point on the calibration curve when the weighing scale is in the inverted orientation. 
     In yet another implementation, the present disclosure is directed to a method of manufacturing a weighing scale. The method includes: providing a base; providing a load cell; providing a mass receiver; providing electronic circuitry for controlling functionality of the weighing scale; assembling the base, the load cell, the weight receiver, and the electronic circuitry into the weighing scale; obtaining a value for an inverted calibration weight, wherein the value is substantially identical to, or identical to, an inverted sprung weight of the weighing scale when the weighing scale is inverted and supported by the mass receiver; and programming the value into the electronic circuitry. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For the purpose of illustrating the invention, the drawings show aspects of one or more embodiments of the invention. However, it should be understood that the present invention is not limited to the precise arrangements and instrumentalities shown in the drawings, wherein: 
         FIG. 1  is a perspective view of an example self-calibrating weighing scale made in accordance with broad concepts of the present disclosure; 
         FIG. 2  is a high-level schematic diagram of the self-calibrating weighing scale of  FIG. 1 ; 
         FIG. 3  is a conventional calibration curve for an electronic weighing scale; 
         FIG. 4  is a perspective view of the self-calibrating weighing scale of  FIG. 1  placed upside down in a step of self-calibration; 
         FIG. 5  is a graph of calibration curves implemented in the self-calibrating weighing scale of  FIG. 1 , including curves with offset corrections; 
         FIG. 6  is a flow diagram of a self-calibration method that can be used with a self-calibrating weighing scale of the present disclosure, such as self-calibrating weighing scale of  FIG. 1 ; and 
         FIG. 7  is a flow diagram of a method of entering offset corrections for self-calibration that can be used with a self-calibrating weighing scale of the present disclosure, such as self-calibrating weighing scale of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
     Referring now to the drawings, wherein the first digit of each element numeral therein denotes the figure number in which the corresponding element is first referenced,  FIGS. 1 and 2  illustrate one embodiment of self-calibrating weighing scale  100  that implements broad concepts embodied in the present disclosure. As described in detail below, weighing scale  100  includes electronic circuitry and programmability that allow it to be self-calibrated, that is, calibrated without the use of any external calibration weights. Self-calibration procedures disclosed herein permit an end user to readily generate a new calibration curve for weighing scale  100 , for example, in the event that some change or incident alters the performance or response of the scale. In this manner, scale  100  can continue to be used accurately after re-calibration, avoiding lengthy and expensive repair procedures which would likely otherwise require the scale to be shipped to a repair center. 
     In this example, weighing scale  100  includes a base  105  and a mass receiver, such as weighing pan  110 , which rests upon a load cell  200  supported by the base. Base  105  includes electronic circuitry  205 , which controls the operation and functionality of weighing scale  100 . Typically, electronic circuitry  205  will include a microprocessor  210 , memory  215 , and an analog-to-digital (A/D) converter  220 . These components, may, but need not, be integrated into a system on chip, such as an application specific integrated circuit. Broadly microprocessor  210  performs various routines and functions needed to provide scale  100  with its functionality. Memory  215  contains, among other things, the routines for performance by microprocessor  210 , as well as any other information needed for the proper functioning of scale  100 , such as scale settings and data needed by the microprocessor at power up. For example, memory  215  contains an interpolation routine  225 , a self-calibration routine  230 , and a data store  235  that contains data used in at least these routines. It is noted that while memory  215  is denoted by a single block in  FIG. 2 , it should be understood that the corresponding actual physical memory may be dispersed throughout circuitry  205  and in various forms, such as BIOS memory, cache memory, ROM and RAM, among others. 
     Base  105  also includes a display  115  and one or more buttons, here, ON/OFF button  120 , UNITS button  125 , and TARE button  130 . Display  115  is configured to display various information to a user, such as weight, units of weight, and other information pertaining to the functionality of weighing scale  100 . Each button  120 ,  125 ,  130  allows a user to select the function(s) denoted on that button, but may also be used to provide addition functionality when pressed in certain combination(s) and/or sequence(s). 
     When an item or other mass to be weighed (not shown) is placed on weighing pan  110 , the force transmitted through the weighing pan (and any corresponding mechanical linkage  207 ) to load cell  200  is converted by the load cell into an analog electrical signal proportional to the weight of the mass. As is well known in the art, in a strain-gauge based load cell this occurs by deformation of one or more strain gauges (not shown), that creates changes in electrical resistance in the strain gauge(s) in an amount proportional to the deforming force. The resistance changes are sensed by circuitry (not shown) within load cell  200  that generates the analog voltage signal proportional to the resistance of the strain gauge(s) at that point in time. This analog signal from load cell  200  is input to A/D converter  220 , which outputs to microprocessor  210  a certain number of digital counts corresponding to the magnitude of the input analog signal. Microprocessor  210  uses the number of counts as input to interpolation routine  225 , which utilizes a current calibration curve (not shown) of weighing scale  100  as stored in data store  235 . The output of interpolation routine  225  is a quantitative weight of the mass, which is displayed on a scale display  115 . While the present example is directed to a strain-gauge-based load cell, in other types of load cells the force-proportional signal results from other mechanisms. For example, in a piezoelectric-element-based load cell, the load-proportional signal results from the deformation of the piezoelectric element and the resulting change in electrical characteristics of that element. Those skilled in the art will understand the different circuitry needed to adapt the broad self-calibrating concepts of the present disclosure to other types of electronic weighing scales. The term “load cell” encompasses a wide array of force-measuring devices that can generate an electronic signal proportional in a predictive manner to the force being measured. Examples include optical devices (e.g., devices that measure deflections using one or more lasers) and acoustic devices (e.g. sonar), as well as piezoelectric devices and semiconductor-based devices and more traditional resistance-type strain gauge devices, among others. 
       FIG. 3  illustrates a graph  300  of a two-point calibration curve  305  for a conventional load-cell-based weighing scale (not shown). In graph  300 , the horizontal axis  310  represents the weight placed on the scale, and the vertical axis  315  represents the digitized signal output by the A/D converter (not shown, but similar to A/D converter  220  of  FIG. 2 ) that corresponds to the analog signal output by the load cell. One of the two points used to establish calibration curve  305  is a zero point  320 , which represents the digitized load-cell signal when no external load is placed upon the weighing pan of the scale. A second calibration point  325  is established by placing a known certified weight (“Wcal”) on the scale and associating that weight with the load-cell digital signal (“CALcounts”). These two calibration points  320 , 325  define calibration curve  305 . Calibration curve  305  provides the basis for an interpolation algorithm when converting a digitized load-cell output signal into a value of weight displayed on the scale. 
     The interplay between scale capacity, calibration weight, acceptable accuracy, and linearity of the strain-electrical resistance curve for a given load-cell, and the impact of these factors on a two-point calibration curve, are well known by those skilled in the art. Useful load-cell behavior is limited by its electro-mechanical properties and the start of inelastic deformation which can damage the load-cell and prevent repeatable performance. Total scale capacity should be less than the weight that would impose sufficient stress to initiate inelastic deformation. The calibration weight, Wcal, should be selected so that the resultant calibration curve spans a nontrivial portion of the total scale capacity. For example, Wcal may be approximately one-third of the total scale capacity. When Wcal is significantly less than the total scale capacity, extrapolation as well as interpolation may be possible with the calibration curve. 
     The four calibration parameters required for calibration curve  305  are the two pairs of values for first calibration point  320  and second calibration point  325 . The values of the two parameters for first calibration point  320  may be obtained merely by accepting the load-cell digital output count value when no weight is placed on the scale. Ideally, this would be a zero count and a zero weight. Normally, however, the load-cell is generating an analog output signal, and thus an associated digital output count, because of the force caused by the weight of the sprung-weight, primarily the empty weighing pan and any corresponding linkage between the pan and the load cell. These can be mathematically redefined as zero count and zero weight by accounting for the weight of the weighing pan as a tare weight. The second calibration point requires knowing the digital output count (CALcounts) with a known weight Wcal. This second calibration point is conventionally established with a certified weight that is independent of the scale, and it is initially determined as part of the manufacturing process and quality control procedures. Often scale users, however, do not have a certified weight available and/or may have difficulty obtaining one. 
     In contrast to conventional weighing scale calibration that utilizes an external certified or other known weight, a self-calibrating process and weighing scale of the present disclosure allows a scale user to calibrate the scale without the need for any external weights. Rather, the self-calibration process utilizes the sprung weight of the weighing scale when the scale is inverted and supported by its weighing pan. As used herein and in the appended claims, this sprung weight is denoted “inverted calibration weight,” and is equal to the total weight of the scale components that cause strain in the load cell when the scale is inverted and placed on a solid level surface so that the scale&#39;s base is essentially supported on the surface by the weighing pan (or pan support if the pan is removable and is removed for the inversion). An example of a self-calibrating process is described below. 
     Referring to  FIG. 4 , in the context of weighing scale  100  of  FIGS. 1 and 2 , during the calibration process the scale is inverted, or turned upside down, and weighing pan  110  is placed on a solid level surface  400 . Thus, the mass (weight) of base  105  and any sprung components of load cell  200 , is supported by the load-cell when scale  100  is inverted. It is recognized that this self-calibration procedure requires that the inverted calibration weight Wcal inverted  be accurately known to weighing scale  100 , for example by programming the value into memory  215 . This may be handled in any of a number of ways. For example, if the manufacturing processes used result in very little variability of the inverted calibration weight from one scale to the next, an average value of the scales&#39; weights may be programmed into every scale. However, if there is a relatively large amount of variability in the inverted calibration weight as from one scale to another, the manufacturing process may involve adding a corrective weight to each scale as appropriate based on a weighing of that scale&#39;s base at an appropriate point in the manufacturing process. This way, all scales may be programmed with the same inverted calibration weight. As yet another example, the inverted calibration weight may be determined for each scale and that particular weight programmed into the memory of that scale.  FIG. 4  also illustrates the presence of an indicator, here LED  404 , that electronic circuitry  205  ( FIG. 2 ) uses to communicate information about the status of the self-calibration mode to the user. 
     Reference should now be made to  FIG. 5 , which shows a graph  500  containing several calibration curves  505 ,  510 ,  515  useful in illustrating calibration functionality of a weighing scale made in accordance with broad concepts of the present disclosure. Regardless of how the inverted calibration weight is determined and programmed into weighing scale  100 , the weight of a mass placed on the weighing pan of a weighing scale of the present disclosure, such as weighing pan  110  of scale  100  ( FIGS. 1 and 2 ), is ideally determined from calibration curve  505 . Since the “zero” point  520  of calibration curve  505  is truly at (0,0), the slope of this curve (which is straight line in the region of operation) is simply Wcal inverted /Calcount. Consequently, when there is no variance in Wcal inverted , the weight of the mass placed on weighing pan  110  is determined from the following formula:
 
Weight=digital output count× W cal inverted /Calcount  {1}
 
     However, it is recognized that the actual sprung weight of scale  100  when it is inverted, i.e., Wsprung inverted , is subject to variance over the service life of the scale due to wear, damage, repairs, etc. Therefore, a self-calibration process of the present disclosure can be subject to inaccuracies in obtaining the parameters of the second calibration point  525 . In such cases, the value of inverted calibration weight Wcal inverted  programmed into memory  215  ( FIG. 2 ) may no longer be a suitable calibration weight. Consequently, the corresponding digital output count of the load-cell under influence of actual inverted sprung weight Wsprung inverted  of scale  100  will not be equal to calibration digital count Calcount as long as the difference between actual inverted sprung weight Wsprung inverted  and inverted calibration weight Wcal inverted  is greater than the sensitivity of A/D converter  220 . To account for such differences, the actual digital calibration count when actual inverted sprung weight Wsprung inverted  is greater than programmed inverted calibration weight Wcal inverted  is designated Calcounts + , and the actual digital calibration count when the actual inverted sprung weight Wsprung inverted  is less than programmed inverted calibration weight Wcal inverted  is designated Calcounts − . 
     If the true inverted sprung weight Wsprung inverted  is not equal to inverted calibration weight Wcal inverted , the calibration curve generated by a self-calibration method of the present disclosure will generate errors in the accuracy of the weight readings output by weighing scale  100 . These errors are perhaps best illustrated by calibration curves  510 ,  515 . The desired calibration curve is curve  505 . However, when inverted sprung weight Wsprung inverted  is greater than inverted calibration weight Wcal inverted , the calibration curve used by weighing scale  100 , because during the calibration process it uses the preprogrammed inverted calibration weight Wcal inverted , is curve  510 , which has a slope (with zero point being actually at (0,0)) of Wcal inverted /Calcount + . Consequently, the weight output by weighing scale  100  for a given mass placed upon weighing pan  110  will be determined by the formula:
 
Weight=digital output count× W cal inverted /Calcount +   {2}
 
This is so because weighing scale  100  will use point  530  in determining the calibration curve, i.e., curve  510 , based on the assumed (preprogrammed) inverted calibration weight Wcal inverted  and the actual digital count Calcount +  that is based on the greater true inverted sprung weight Wsprung inverted . Similarly, when inverted sprung weight Wsprung inverted  is less than inverted calibration weight Wcal inverted , the calibration curve used by weighing scale  100  is curve  515 , which has a slope of Wcal inverted /Calcount −  and is based on the point  535 . In this case, weighing scale  100  will determine the weight to output for a given mass placed upon weighing pan  110  using the following formula:
 
Weight=digital output count× W cal inverted /Calcount −   {3}
 
Clearly, the weights output by weighing scale  100  based on formulas {2} and {3} above will be inaccurate, with the inaccuracy increasing with the increasing weight of the mass being weighed.
 
     To account for these errors, a self-calibrating weighing scale, such as scale  100  of  FIGS. 1 and 2 , may be configured to address and overcome this problem. In one example, weighing scale  100  allows a user to enter into the scale a weight-offset value that the scale will add or subtract, as appropriate, from actual inverted sprung weight Wsprung inverted  during the self-calibration process when the inverted sprung weight Wsprung inverted  is known to be greater or less than preprogrammed inverted calibration weight Wcal inverted  so that the calibration process will end up with the appropriate calibration curve, curve  505  in  FIG. 5 , or a curve much closer to this curve than would result without the use of the weight offset. In essence, this permits a more accurate approximation of inverted calibration weight Wcal inverted  to be used for the second calibration point. The function of the weight offset feature is illustrated in  FIG. 5 . 
     As seen by (incorrect) calibration curve  510  in  FIG. 5 , when the actual inverted sprung weight Wsprunginverted is greater than preprogrammed inverted calibration weight Wcalinverted, it is necessary to adjust the inverted calibration weight Wcalinverted by a positive weight-offset value so that the point used by weighing scale  100  in determining calibration curve  505  (or the slope of this curve) is point  540 , which lies on calibration curve  505  and has the coordinates (Wcalinverted+offset, Calcount+). This will have the effect of shifting the slope of calibration curve  510  back in the direction of the original and correct calibration curve  505 . After weighing scale  100  has been calibrated using a positive offset value, it will essentially determine the weight to output for a given mass using the formula:
 
Weight=digital output count×[( W cal inverted +Offset)/Calcount + ]  {4}
 
Mathematically, this is equivalent to dividing the uncorrected calibration line slope by a correction factor of (Wcal inverted  Offset)/Wcal inverted .
 
     Similarly, when the actual inverted sprung weight Wsprung inverted  is less than preprogrammed inverted calibration weight Wcal inverted , it is necessary to adjust the inverted calibration weight Wcal inverted  by a negative weight-offset value so that the point used by weighing scale  100  in determining calibration curve  505  (or the slope of this curve) is point  545 , which lies on calibration curve  505  and has the coordinates (Wcal inverted +offset, Calcount + ). This will have the effect of shifting the slope of calibration curve  515  in the direction of the original and correct calibration curve  505 . After weighing scale  100  has been calibrated using a negative offset value, it will essentially determine the weight to output for a given mass using the formula:
 
Weight=digital output count×[( W cal inverted −Offset)/Calcount − ]  {5}
 
Mathematically, this is equivalent to dividing the uncorrected calibration line slope by a correction factor of (Wcal inverted  Offset)/Wcal inverted .
 
     As just seen, adding the appropriate offset will decrease the slope of the calibration curve, and subtracting the offset will increase the slope. If the offset is precisely equal to the difference between actual inverted sprung weight Wsprung inverted  and the programmed inverted calibration weight Wcal inverted , use of the offset as just described will correct the calibration curve slope exactly. The difference between actual inverted sprung weight Wsprung inverted  and programmed inverted calibration weight Wcal inverted  may be ascertained, at least approximately, prior to self-calibration in any suitable manner. For example, if the scale is missing a footpad, the offset may be approximately equal to the weight of a footpad, which might be attainable from the manufacturer or weighing one of the remaining footpads. In another example, if a piece is broken off of the housing, an item of estimated similar size, material, etc. can be obtained and weighed, for example, on another, properly calibrated, weighing scale. 
     Following is an example that should solidify the usefulness of a self-calibrating scale made in accordance with broad concepts of the present disclosure, such as scale  100  of  FIGS. 1 and 2 . Initially, weighing scale  100  is calibrated using a calibration weight, which in this case is inverted calibration weight Wcal inverted , and A/D converter  220  produces a digital count of Calcount with this calibration weight. This calibration could be done either using the self-calibrating procedure or an external weight equal to inverted calibration weight Wcal inverted . This data corresponds to second calibration point  525  on graph  500  of  FIG. 5 . 
     If, at some later time, weighing scale  100  is dropped, resulting in a nonfunctional (relative to the weighing functionality of the scale) piece of base  105  breaking off and creating some concern about the accuracy of the scale, a user may wish to recalibrate the scale. The missing piece of base  105  will not impact first calibration point  520 . If load-cell  200  is undamaged or unaffected, it is presumed that the after-drop true calibration curve should be equal to original calibration curve  505 . 
     During the self-calibration process, the new second calibration point must be determined. If weighing scale  100  is placed upside down on its weighing pan  110 , inverted sprung weight Wsprung inverted  of the scale is no longer going to be equal to inverted calibration weight Wcal inverted  because of the missing piece of base  105 . However, the self-calibration procedure presumes that the calibration weight at that stage of the procedure is still inverted calibration weight Wcal inverted . The digital output of A/D converter  220 , though, will be Calcount, reflecting the smaller digital count generated by the smaller force (i.e., actual inverted sprung weight Wsprung inverted  that does not include the missing piece of base  105 ) on load-cell  200 . Without correction, the calibration procedure will select point  535  (Wcal inverted , Calcount − ) as the second calibration point. The scale would thus err on the low side, outputting weight values that are smaller than the actual weights being weighed. 
     To correct this error, a user must estimate or ascertain the actual weight change of the scale due to the broken housing. This will be the offset used to shift the second calibration point to point  545  ((Wcal inverted −Offset)/Calcount − ). This places the second calibration point back onto original calibration curve  505 , where it should be. 
       FIG. 6  illustrates an example self-calibration method  600  that can be used to calibrate an electronic weighing scale that incorporates self-calibration capability. Weighing scale  100  of  FIGS. 1 and 2  is used for illustrating self-calibration method  600  for convenience, since it has such capability. With weighing scale  100  in its normal non-inverted position, at step  605  a user places the scale into a self-calibration mode, which initiates self-calibration routine  230  stored in memory  215 . This may be done in any of a number of ways, for example, by the user pushing TARE and UNITS buttons  130 ,  125  simultaneously with one another and then turning weighing scale  100  on with “ON/OFF” button  120 . When this is done, weighing scale  100  may indicate it is in the self-calibration mode, for example, by display  115  displaying “Cal”. 
     With weighing scale  100  still in its non-inverted position and with no external weight on weighing pan  110 , at step  610  the first calibration point (here zero point  520  ( FIG. 5 )) is determined and set. In one embodiment, weighing scale  100  is configured to set zero point  520  in response to the user pressing TARE button  130 . In this example, when weighing scale  100  has set zero point  520 , the scale may be programmed so that display  115  displays “Cal 0” and LED  404  ( FIG. 4 ) on the bottom of the scale turns on. At step  615 , the user waits for weighing scale  100  to set zero point  520 . Leaving the still-empty scale  100  undisturbed, self-calibration routine  230  automatically stores the parameters of zero point  520  in data store  235  of memory  215 . After a predetermined period of time has passed, in this example self-calibration routine  230  causes display  115  to display “SC” to indicate weighing scale  100  has entered the part of the self-calibration routine in which the second calibration point is set. In the case, as in this example, wherein inverted sprung weight Wsprung inverted  of weighing scale  100  is assumed to be identical to inverted calibration weight Wcal inverted , this second calibration point corresponds to point  525  on curve  505  of  FIG. 5 . 
     At step  620 , weighing scale  100  is turned over and weighing pan  110  placed face down on a firm, level surface. At this point, calibration routine  230  may be programmed so that display  115  flashes the displayed “SC” to indicate that the scale is obtaining the parameters for the second calibration point. In this case, inverted sprung weight Wsprung inverted  of weighing scale  100  is presumed to be equal to inverted calibration weight Wcal inverted , and the digital signal count from A/D converter  220  corresponds to the output of load cell  200  with inverted sprung weight Wsprung inverted  suspended by the load cell. Self-calibration routine  230  may be programmed to flash LED  404  on the bottom of weighing scale  100  (that is now visible while the scale is inverted) so as to provide an accessible visual signal that the scale is self-calibrating. This flashing may occur at step  625 , wherein the user waits for self-calibration routine  230  to store the parameters of the second calibration point. Self-calibration routine  230  may be further programmed to stop the flashing of LED  404  and display “Cal F” on display  115  to indicate that the self-calibration routine has captured the necessary parameters and the self-calibration is finished. At step  630 , the user may cause weighing scale  100  to exit the self-calibration mode and place the scale into weighing mode. This may be done in any of a number of ways, including the user pressing “TARE” button  130 . Weighing scale  100  may then be placed upright. After self-calibration method  600  has been performed, weighing scale  100  will be functioning in its weighing mode with calibration curve  505  that is based upon zero point  520  and point  525 . 
     As discussed above, if actual inverted scale sprung weight Wsprung inverted  is not equivalent to preprogrammed inverted calibration weight Wcal inverted , calibration curve  505  will be incorrect.  FIG. 7  illustrates an example method  700  for entering a weight correction (or “offset”) to the second calibration point of the calibration curve that enables the user to essentially force, in this example, weighing scale  100  ( FIGS. 1 and 2 ) to create a new calibration curve that more closely, or exactly, matches original calibration curve  505 . In method  700 , at step  705  weighing scale  100  is placed in self-calibration mode. This may be accomplished in the manner discussed relative to step  605  of self-calibration method  600  of  FIG. 6 , for example, by holding down TARE and UNITS buttons  130 ,  125  simultaneously with one another and then pushing ON/OFF button  120 . Display  115  may display “Cal”, as before. At step  710 , the user causes weighing scale  100  to enter a weight-offset entry sub-mode, which, like other self-calibration functionality in example method  700  and example method  600  of  FIG. 6 , is controlled by self-calibration routine  230  stored in memory  215 . In this example, the user enters the weight-offset-entry sub-mode by pressing UNITS button  125  four times in uninterrupted succession. When weighing scale  100  has successfully entered the weight-offset-entry sub-mode, in this example self-calibration routine  230  causes display  115  to change the display of “Cal” to “OFFS”. 
     At step  715 , the user chooses the sign of the weight offset. If actual inverted scale sprung weight Wsprung inverted  is smaller than preprogrammed inverted calibration weight Wcal inverted , for example, if a piece is missing from base  105  of weighing scale  100 , the user would select a negative offset. On the other hand, if actual inverted scale sprung weight Wsprung inverted  is greater than preprogrammed inverted calibration weight Wcal inverted , for example, a repair has been made to base  105  using a part that is heavier than an original part it replaces, the user would select a positive offset. As discussed above in connection with  FIG. 5 , a negative weight offset essentially moves point  535  to the left to a location closer to, or on, calibration curve  505 , such as to point  545 , which precisely falls on calibration curve  505 . Conversely, a positive weight offset essentially moves point  530  to the right to a location closer to, or on, calibration curve  505 , such as to point  540 , which precisely falls on calibration curve  505 . In the current example, the sign of the offset is selected in a toggling manner by pressing TARE button  130  as needed until display  115  displays either “+0.0” or “−0.0”. Those skilled in the art will recognize that there are many other ways of allowing a user to select the sign of the weight offset. 
     At step  720 , the user sets the magnitude of the estimated or known weight-offset value. In one example, weighing scale  100  is configured so that a press of UNITS button  125  will cause display  115  to flash the left-hand side (LHS) digit, and pressing of TARE button  130  will increment the value of the LHS digit. Similarly, another press of UNITS button  125  will cause display  115  to switch the flashing digit to the right-hand-side (RHS) digit. Pushing of TARE button  130  while the RHS digit is flashing will increment the value of this digit. If one or more additional digits are provided on either side of the decimal point, the setting of such digit(s) may proceed in a similar manner, such as the digit-by-digit manner just described. Alternatively, if there is more than one digit on either side of the decimal point, those digits may be grouped together in a rolling manner, such that the least significant digit first cycles through 0-9 and then causes the next-to-least significant digit to increment, and so on. Those skilled in the art will understand that there are many other ways to input a weight value, such as through a numeric keypad, if provided. 
     At step  725 , weighing scale  100  saves the just-inputted weight offset value and its sign. At step  730 , the user causes weighing scale  100  to exit the weight-offset-entry sub-mode and place the scale back in weighing mode. In the present example, weighing scale  100  is configured to exit the weight-offset-entry sub-mode at this point in self-calibration routine  230  by a simultaneous pressing of TARE and UNITS buttons  130 ,  125 . Weighing scale  100  is further configured in this example so that display  115  briefly displays “End” to signify the scale has left the weight-offset-entry sub-mode. After exiting the weight-offset-entry sub-mode, weighing scale  100  will then be operating with a calibration curve defined by first calibration point  520  (0,0) and a second calibration point, for example, either point  540  ((Wcal inverted +Offset)/Calcount + ) or point  545  ((Wcal inverted −Offset)/Calcount − ) or a point as close as practicable to these points. The weight offset value used should, of course, be chosen to make the expression (Wcal inverted ±Offset) come as close to programmed inverted calibration weight Wcal inverted  as possible. 
     Exemplary embodiments have been disclosed above and illustrated in the accompanying drawings. It will be understood by those skilled in the art that various changes, omissions and additions may be made to that which is specifically disclosed herein without departing from the spirit and scope of the present invention.