METHOD FOR SCALE CALIBRATION AND WEIGHING

Abstract: A scale utilizes a plurality of load cells (50) positioned beneath a weighing platform for determining the weight of items placed upon the platform. During a weighing operation, each load cell (50) generates a voltage proportional to the load applied to the cell (50). The voltages thus generated are transmitted through a multiplexer (116) and converted to digital values by an analog-to-digital converter (118). These digital values are employed by a microprocessor (120) to determine the weight of the item on the platform. The scale is calibrated by weighing a test weight in a plurality of different locations on the platform, and utilizing the individual digital values thus obtained for the load cells in each such weighing operation in a plurality of simultaneous equations, which are then solved by the microprocessor (120) to provide constant values respectively associated with the load cells (50). These constant values are used by the microprocessor (120) in subsequent weighing operations.

Description: The present invention is directed to a method of scale calibration and weighing, using an electronic scale. Weighing scales are widely used in supermarkets or the like for weighing produce or other merchandise items and must meet stringent requirements as to performance and cost. The scales must be accurate enough to satisfy public weights and measures authorities. These requirements are usually met at the time that the scale is manufactured by an initial adjustment. As part of these requirements, the scale must display a predetermined reference value, normally zero, when no merchandise item is being weighed so that the change in the output display produced by a merchandise item being weighed will accurately represent the true weight of the item. Through use, the zero reference of a scale may change due to a number of causes, including the following: accumulation of foreign matter on the scale platform or its supports; a change in the position of the scale platform; the effect of temperature on the load cells and electronic components; and hysteresis, especially after weighing of a heavy object. In certain types of electronic scales which employ a plurality, typically four, of strain gauges, as a means of determining the weight, these strain gauges are usually configured in a Wheatstone bridge circuit for increased accuracy because their fractional change in resistance when strained by a force is very small (for metal-wire strain gauges, typically of the order of 0.1%). The signal outputs from the strain gauges in the Wheatstone bridge circuit are summed algebraically to provide a signal representative of the weight being determined. In such precision scale applications, the strain gauges must have precisely equal sensitivities, i.e. the same fractional change in resistance when strained by the same force centered at the same relative position. In at least one type of scale, this achievement of equal sensitivities is accomplished by precisely sanding the strain gauges, which is a slow, expensive, and labour-intensive method. WO-A-87/07370 discloses a method of recalibrating an electronic scale, employing adjustable 16 bit registers, which is disadvantageously complex. It is an object of the present invention to provide a simple method for calibrating an electronic scale. According to the invention there is provided a method of calibrating an electronic scale having a plurality of load cells for supporting a weighing platform on which an item to be weighed may be placed and each of which is arranged to generate a voltage proportional to the load applied to the cell, a corresponding plurality of amplifiers respectively coupled to said load cells, a multiplexer coupled to said amplifiers for sequentially receiving voltages from said amplifiers, an analog-to-digital converter coupled to said multiplexer for converting voltages representative of load cell outputs received from said multiplexer to digital values, microprocessor means coupled to said analog-to-digital converter for receiving and utilizing said digital values, and a display controlled by said microprocessor means, said method including the following steps: (a) performing a test operation without an item on said weighing platform to determine whether said scale is properly calibrated to zero weight; (b) if said scale is not properly calibrated to zero weight, operating a calibration switch to initiate a calibration operation; c) displaying on said display an instruction to the scale operator to place a predetermined test weight at predetermined positions on the weighing platform; (d) performing a plurality of weighing operations in accordance with said instruction equal in number to the number of load cells, and for each weighing operation obtaining from said analog-to-digital converter a plurality of digital values respectively representing the outputs of said load cells; (e) solving by said microprocessor means a plurality of simultaneous equations equal in number to the number of load cells to derive a plurality of constants respectively associated with said load cells; (f) storing said plurality of constants in a memory for use in future weighings by said electronic scale; and (g) operating said calibration switch to terminate the calibration operation. It should be understood that the constants derived in a method in accordance with the present invention represent the relative sensitivities of the load cells, and that these constants are utilised by the microprocessor in a subsequent weighing operation in calculating the weight of an item placed on the weighing platform. One embodiment of the invention will now be described by way of example with reference to the accompanying drawings, in which:- FIG. 1 is a perspective view of a check-out system which includes a scale for weighing purchased merchandise; FIG. 2 shows a prior art circuit arrangement for an electronic scale; FIG. 3 is a schematic representation of a portion of the check-out system of Fig. 1, showing the weighing platform and the load cells associated therewith; FIG. 4 is an exploded view showing a load cell assembly, including a pair of strain gauges secured thereto; FIG. 5 is an elevation of the load cell of Fig. 4; FIG. 6 is a diagram of a circuit arrangement employed in the present invention; and FIG. 7 is a flow diagram of the calibration and weighing processes of the present invention. Referring now to Fig. 1, there is shown a perspective view of a merchandise check-out system generally indicated by the reference numeral 20, in which a scale 22 is utilized to weigh a purchased item. The scale is located within a check-out counter 24 and includes a generally rectangular scale lid or load plate 26 movably mounted flush with the top surface 28 of the check-out counter 24. The scale lid 26 includes an aperture 30 through which scanning beams from a scanner assembly (not shown) mounted beneath the scale within the check-out counter are projected for scanning a bar code label (not shown) on a merchandise item positioned on the scale lid 26 in a manner that is well known in the art. Mounted adjacent to the scale lid 26 on the surface 28 of the check-out counter 24 is a housing member 31 in which is located a customer display 32 which displays the price of the merchandise item and also the weight of the item as the result of a weighing operation. Located adjacent the sides of the display 32 within the housing 31 are a pair of lights 98 and 100, one green and one red, which are operated to indicate whether or not a weighing operation or a scanning operation was successful. Opposite the housing 31 on the surface 28 of the counter 24 is a housing 38 in which are located a pair of photoelectric cells 40 and 42 which detect the entrance and the exit of the merchandise item on the lid 26. A pair of light-emitting diodes (not shown) located in the housing member 31 direct light beams at the cells 40 and 42 in a manner that is well-known in the art. Mounted adjacent the check-out counter 24 is a data terminal device 44 for processing the sales transaction which includes a keyboard 46 and a display 48. The display 32 is an alphanumeric LCD display and can be operated automatically or by the manual operation of a key of the keyboard 46 of the data terminal device 44. Referring now to Fig. 3, there is shown a schematic partial representation of the check-out system in which the scale 22 includes four load cells 50 respectively secured to the scale lid 26 adjacent the corners thereof and which cells provide output analog signals over lines 52 in response to the placing of an item to be weighed on the scale lid 26, each of these signals being proportional to the load applied to the respective cell. The structure of the load cells 50 is shown in greater detail in Figs. 4 and 5. Each load cell 50 includes a cylindrical element 56 which has at its upper end a stud 54 which is engageable with a corresponding aperture in the scale lid 26. The cylindrical element includes a resilient portion 55 intermediate its ends. At its lower end, the cylindrical element 56 is provided with a threaded extension 57 which engages a complementary threaded aperture 58 adjacent to one end of a plate 60. As best shown in Fig. 5, the plate 60 is mounted on a lower plate 64 with a spacer 62 being positioned therebetween. This assembly is secured together by a threaded fastener 72, which engages with a threaded aperture 66 in the plate 60, and with apertures 68 and 70 in the spacer 62 and the lower plate 64, respectively. The lower plate 64 is formed with two portions 78 and 80 of reduced cross-section, on which are secured two strain gauges 82 and 84. These gauges, as is well known, measure the deflection of an element to which they are secured. They have a comb-like structure and include two electrical terminations. One of the terminations of the strain gauge 82 is connected to a conductor 86, and a corresponding termination of the strain gauge 84 is connected to a conductor 88. The remaining terminations of the strain gauges 82 and 84 are connected together and to a common conductor 90. In the past, calibration of a load cell has been accomplished by precisely sanding the reduced portions 78 and 80 of the lower plate 64 in order to modify the response of the strain gauges associated with the reduced portions 78 and 80 to provide a zero scale reading of the scale in which the load cell is employed when no item is placed on the scale lid 26. It may readily be seen that such a method of calibration is quite time-consuming and labour-intensive, and that it may be quite difficult to achieve a high degree of accuracy using such a method. The manner in which the electrical signal information taken from the strain gauges 82, 84 of the load cells 50 was previously used to provide the weight of objects weighed on the scale 22 is shown diagrammatically in Fig. 2, which is labelled as PRIOR ART. In this arrangement, the two strain gauges 82 and 84 of each load cell 50, taken together, formed one element of a four-element load cell bridge 100, which functioned in the manner of a Wheatstone bridge, to provide an analog output signal representative of the weight of an item being weighed. This analog output signal was amplified by an amplifier 102, digitized by an analog-to-digital converter 104, and applied to a microprocessor 106 to cause a numerical value representative of the weight to be displayed on a display 108. From the analog-to-digital converter 104, the digitized signal could also be transmitted through an interface device 110 to a terminal 112, such as the terminal 44. In the present invention, the circuit shown diagrammatically in Fig. 6 is employed in place of the circuit of Fig. 2. Each load cell 50 is connected through its own amplifier 114 to one input of an analog multiplexer 116. The output of the multiplexer 116 is connected to an analog-to-digital converter 118, the output of which is connected to a microprocessor 120, which has an electrically erasable programmable read-only memory (EEPROM) 123 operationally associated therewith. The microprocessor 120 is coupled to the multiplexer 116 by select lines 122 to cause the multiplexer 116 to transmit the amplified signals from the various load cells 50 serially to the analog-to-digital converter 118, where these signals are sequentially digitized and applied to the microprocessor 120. As will subsequently be described in greater detail, the microprocessor 120 will then determine the weight of the object being weighed from the values received from the analog-to-digital converter 118, and will cause that weight to be displayed on a display 124, such as the display 32. The weight value may also be transmitted to a terminal 128, such as the terminal 44, through an interface 126. Referring now to Fig. 7, shown there is a flow diagram of the scale calibration and weighing process, as performed by a scale such as the scale 22 in accordance with the present invention. This process is initiated by turning on electrical power to the scale 22 of the check-out system 20, as is represented by the block 142. The microprocessor 120 then causes a number of "zero" weighings to take place, without having any item placed upon the scale lid 26, as represented by block 144. These weighings give digital values representing the outputs of the various load cells 50 when zero weight is on the scale lid 26, and the resulting values are averaged on an ongoing basis to provide updated values which compensate for any changing conditions such as, for example, changes in ambient temperature as the day progresses. The zero values are utilized to provide base points for the load cells 50 when actual weighings take place. As the next step in the process, a determination is made (decision block 146) as to whether or not a calibration operation is required. If so, a calibration switch, which may be located at any convenient position on the scale 22 or elsewhere on the system 20, is turned on by a service technician, by a person from a Bureau of Standards or Bureau of Weights and Measures, or by some other authorized individual. In the event that no calibration operation is required, the calibration switch is retained or placed in an "off" position, and normal weighing operations will be performed by the scale 22, as will subsequently be described in greater detail. Assuming that a calibration operation is required and that the calibration switch has been turned on, the process continues along path 148 to the block 150. At this time, a menu is displayed on the display 48 of the data terminal device 44, containing instructions as to how to perform the calibration operation. In the first step of this operation, a weight of predetermined amount, for example ten pounds (4.5 kilograms), is selected and is sequentially placed in four different locations on the scale lid 26, with a separate weighing operation being performed for each placement. The positions in which the weight is to be placed can be shown in any suitable manner on the display, as for example by showing on the display an outline of the scale lid 26, and showing a dot or circle where the weight is to be placed for each separate weighing. For each of the four weighings, the weight may be placed at a different corner of the scale, for example. Each separate weighing is represented by the block 152. As shown in that block, four different values are obtained respectively relating to the outputs of the four load cells 50. As represented in block 154, four different weighings are performed, each with the calibration weight in a different position, a different set of four values being obtained each time. Next, as represented in block 156, the four different values for each of the four different load cells are utilized in four different simultaneous equations, which are solved by the microprocessor 120 to obtain four different constants, each of which corresponds to a respective one of the four load cells 50. The equations may take the following form: K₁L 1,1 + K₂L 2,1 + K₃L 3,1 + K₄L 4,1 = W K₁L 1,2 + K₂L 2,2 + K₃L 3,2 + K₄L 4,2 = W K₁L 1,3 + K₂L 2,3 + K₃L 3,3 + K₄L 4,3 = W K₁L 1,4 + K₂L 2,4 + K₃L 3,4 + K₄L 4,4 = W wherein W is the calibration weight; L 1,1 -L 1,4 ;L 2,1 -L 2,4 ; L 3,1 -L 3,4 ; and L 4,1 -L 4,4 are the digital values representing the outputs of the four different load cells 50; and K₁, K₂, K₃ and K₄ are the unknown constants (linear coefficients) to be calculated. When values for the four constants K₁, K₂, K₃, and K₄ have been obtained by solution of the above equations, they are retained in temporary storage in the microprocessor 120 until the calibration switch has been turned off (block 158), at which time they are stored in the electrically erasable programmable read-only memory (EEPROM) 123 which is associated with the microprocessor 120, as represented in block 160. With the calibration switch turned off (block 158) and the constants stored (block 160), the process continues, as represented by path 162 in Fig. 7, to a point at which an operation of weighing an unknown item which is placed upon the scale lid 26 of the scale 22 may take place. As represented in Fig. 7, the path 162 extends to a junction of several paths in the process, designated by the point 164. It will be assumed that an object of unknown weight has been placed upon the scale lid 26. A determination is then made, as represented in block 168, as to whether or not the reading for the item on the scale lid 26 has stabilized, or is continuing to swing back and forth, as may be the case, for example, when the item is initially placed on the scale lid 26. If the reading has not yet stabilized, the process 140 continues to loop through the block 166 until such stability is achieved. Once this has happened, the process continues on to block 168, in which a determination is made as to whether or not the weight being read is zero pounds. If the weight is, in fact, zero pounds, the process 140 returns to the junction 164, in preparation for the next weighing operation. If desired, the process 140 can, at this time, cause a number of "zero" weighings to take place, as previously described in connection with block 144, in order to provide updated values for the various load cells 50 to compensate for changing conditions. If the weight is not zero pounds, the process continues to block 170, in which the weight of the item being weighed is computed by the microprocessor 120, based upon the values measured for the four load cells 50, and using the constants for the four load cells which were determined during the most recent calibration operation. The unknown weight is determined by linear extrapolation governed by the following equation: wherein Wt is the unknown weight; A i represents the output value from the analog-to-digital converter 118 for the ith load cell 50 when the item to be weighed is on the scale lid 26; Zero Wt i represents the output value from the converter 118 for the ith load cell 50 when no weight is on the scale lid 26; and K i is the constant value corresponding to the ith load cell 50 as determined during the calibration operation described above. Note that it is possible to have a negative signal (relative to the zero weight reading) at a load cell output if, for example, a heavy weight is placed at a corner opposite to that load cell. The net effect is a negative contribution to the overall weight as seen by that particular load cell, since the scale lid 26 would be lifted slightly at that point due to the heavy weight applied to the opposite corner, and this negative contribution must be included in the equation. Best results are normally obtained if the calibration is done with a weight which is near the middle of a full weighing range of the scale 22, so as to avoid extrapolation too far in either direction. For a 30 pound scale range, 10 or 15 pounds is suitable. After the weight of the object to be weighed has been determined using the equation for Wt set forth above, the process continues by displaying the weight in the display 32 and transmitting the weight value to the terminal device 44, as represented in block 172. The process 140 can then return via path 174 to the junction 164, for another weighing operation.

Claims: A method for calibrating an electronic scale (22) having a plurality of load cells (50) for supporting a weighing platform (26) on which an item to be weighed may be placed and each of which is arranged to generate a voltage proportional to the load applied to the cell, a corresponding plurality of amplifiers (114) respectively coupled to said load cells (50), a multiplexer (116) coupled to said amplifiers for sequentially receiving voltages from said amplifiers, an analog-to-digital converter (118) coupled to said multiplexer for converting voltages representative of load cell outputs received from said multiplexer to digital values, microprocessor means (120) coupled to said analog-to-digital converter for receiving and utilizing said digital values, and a display (124) controlled by said microprocessor means, said method including the following steps: (a) performing a test operation without an item on said weighing platform (26) to determine whether said scale (22) is properly calibrated to zero weight; (b) if said scale is not properly calibrated to zero weight, operating a calibration switch to initiate a calibration operation; (c) displaying on said display (124) an instruction to the scale operator to place a predetermined test weight at predetermined positions on the weighing platform (26); (d) performing a plurality of weighing operations in accordance with said instruction equal in number to the number of load cells (50), and for each weighing operation obtaining from said analog-to-digital converter (118) a plurality of digital values respectively representing the outputs of said load cells; (e) solving by said microprocessor means (120) a plurality of simultaneous equations equal in number to the number of load cells to derive a plurality of constants respectively associated with said load cells; (f) storing said plurality of constants in a memory (123) for use in future weighings by said electronic scale (22); and (g) operating said calibration switch to terminate the calibration operation. A method according to claim 1, in which said weighing platform (26) is of generally rectangular configuration, and in which said load cells (50) are four in number and are respectively located adjacent the corners of said weighing platform. A method according to claim 2, wherein in step (d) said test weight is placed in turn at locations adjacent the four corners of said weighing platform (26). A method according to any one of the preceding claims, in which there are n load cells (50) and each of the simulataneous equations referred to in step (e) is in the following form: K₁A 1,i + K₂A 2,i ----- + K n A n,i = W wherein W is the calibration weight; K₁, K₂ -- K n are said constants; and A 1i , A 2,i -- A n,i are the digital values representing the outputs of said load cells (50) for the ith weighing operation in step (d). A method according to any one of the preceding claims, in which said predetermined test weight is near the middle of the full weighing range of said scale. A method for weighing an item using an electronic scale (22) calibrated by a method according to any one of the preceding claims, including the steps of placing said item on said weighing platform (26), obtaining from said analog-to-digital converter (118) a plurality of digital values respectively representing the outputs of said load cells (50), and solving by said microprocessor means (120) an equation using these digital values and said constants so as to determine the weight of said item. A method according to claim 6, in which the equation used for determining the weight of said item is as follows:- wherein Wt is the unknown weight; A i is the digital value representing the output of the ith load cell (50) when said item is on said weighing platform (26); Zero Wt i is the digital value representing the output of said ith load cell when no item is on said weighing platform; and K i is that one of said constants corresponding to the ith load cell. A method according to either claim 6 or claim 7, in which said scale (22) is coupled to a business terminal (44) to which is transmitted the weight of said item as calculated by said microprocessor means (120).

Inventors: AMACHER, GENE L., MERGENTHALER, BARRY MYRON, VITUNIC, MARK ROBERT
Applicants: NCR INTERNATIONAL INC.
Application year: 1990
Main classification: G01G23
IPC codes: G01G23