Patent Application
20090306924
The field of the present disclosure relates to systems and methods for scale calibration of a data reading system. A typical high volume data reading system used at a grocery store, for example, is an optical scanner having an integrated scale (e.g. a scanner-scale). Scale calibration sets the scale to an accurate reference point for weighing. Scale calibration is a time-consuming procedure that is typically governed by governmental weights and measures statutes. Current scanner-scale products typically require technicians to use a weight set to calibrate the scanner after installation. In addition, these scanner-scale products often need official registration and labeling by weights and measures officials to certify that the scale may be used for commerce.
Previously suggested calibration methods include use of standardized weights, for example, pre-measured 1 Kg and 3 Kg weights are alternately placed on the scale and a calibration system then performs a calibration sequence. In another method such as disclosed in U.S. application Ser. No. 2002/0052703, hereby incorporated by reference, the scale includes a communications interface to obtain scale calibration data (acceleration due to gravity data) pertaining to the scale's location. Such a system requires a communication link or a location system (e.g., a global positioning system or “GPS”) to determine the scale's location and then the system utilizes the location calibration data in performing a calibration sequence.
The present inventor has recognized the desirability to eliminate the required manual on-site calibration of the scale portion of a scanner-scale product by local weights and measures authorities but nonetheless be in compliance with state or local weights and measures requirements and obtain the necessary certification. Factory certification would eliminate the need for customers to perform the additional calibration/certification step of setting calibration weights on the scale and running the calibration sequence.
The present invention is directed to a system and method for calibrating a scale, particularly a scanner-scale of a POS system. In a preferred system/method, a scanner-scale has its scale calibrated via an on-board calibration system including an accelerometer that measures actual acceleration due to gravity factor for a given location/time and then uses this measurement to calibrate. A preferred calibration method may comprise the steps of (a) performing an initial calibration on the scanner-scale during assembly; (b) providing the scanner-scale with an on-board accelerometer operable to measure gravity acceleration constants for the current location; and (c) running, preferably in the scanner-scale microcontroller, a calibration routine using the specific calibration data obtained from the measurement in step (b) to calibrate the scale. In certain embodiments, the system may also use other sensors, particularly a temperature sensor and additionally pressure or humidity sensors, to provide further calibration constants for use in calibrating the accelerometer and/or the scale strain gage.
The preferred embodiments will now be described with reference to the drawings.
In a preferred system, the scanner-scale 10 is assembled and calibrated at the factory (or other suitable calibration location) and shipped to a location for installation without additional regional calibration.
Acceleration measurements are taken from the accelerometer 30 and filtering is performed to reduce the effects of vibration, etc., as is done with the strain gage input, in order to return a stable acceleration adjustment constant, which represents gravity. A temperature measurement from the temperature sensor 32 is made at this time and this temperature measurement is used to adjust the acceleration measurements, to compensate for temperature sensitivity of the accelerometer. This acceleration adjustment constant is scaled by the gravity constant at the original factory calibration and then used to adjust the scale's factory calibration for converting strain gage readings to calibrate weight values.
The system 20 may optionally include additional sensors 36, labeled as ¢Extra Sensors∞, to provide additional calibration factors for the accelerometer. The extra sensors may include one or more of the following: barometric pressure sensor, humidity sensor. Input from any of the sensors (temperature, barometric pressure, humidity, etc.) may be used to calibrate not only the accelerometer but also the scale itself. Compensation for the non-accelerational sensitivity of the accelerometer (such as temperature sensitivity) allows it to be used as a high accuracy gravity sensing device.
Temperature calibration of the system in the factory can be performed by measuring accelerometer values and temperature on circuit boards in a controlled temperature environment at two different temperatures. This data can be stored in flash or EEPROM to calibrate the calibration system. Nonlinear temperature sensitivity can be compensated by taking measurements at more than two temperatures. The assembled scale is calibrated in the factory and the accelerometer (gravity) value is stored for comparison at the local installation site.
Accelerometers, as the name suggests, are devices that measure acceleration. In recent years, these devices have become much more affordable due to new manufacturing methods. A preferred cost effective type of accelerometer is surface micromachining of so-called MEMS devices (micro electromechanical systems). These devices measure one or more orthogonal accelerations. Some devices can measure static (DC) accelerations, such as gravity. These devices typically comprise a cantilever beam with a proof mass on the end. Deflection of the beam is sensed by several methods, such as capacitance change. MEMS accelerometers are somewhat temperature sensitive, as temperature adjusts the beam spring constant, among other parameters. To provide a stable measurement, temperature is preferably measured and used in calibrating the accelerometer.
Currently, the Analog Devices model no. ADT75 temperature sensor, ADXL330 three-axis accelerometer, and the model no. AD7730 analog to digital converter (which may be used for the strain gage as well) are good candidates for the accelerometer and sensor components. The ADC would already be in the system. The temperature sensor costs less than $1.00 in 1K quantity and the accelerometer costs less than $5.50 in 1K quantity. So it is conceivable that in 10K quantity, which is more appropriate for scales, the additional cost to implement an automatically calibrated scale would be around $3.00. Other methods are available to measure temperature at potentially lower cost than an integrated temperature sensor, such as the model ADT75. For example, a thermistor or even the voltage of a simple junction diode may be used to sense temperature.
Eliminating calibration may result in reduced installation costs both at initial installation as well as when scanners are removed for repair. This reduced installation cost may result in lower cost of ownership for the customer by eliminating need to have someone certified by Weights and Measures to perform calibration.
Following are several example methods for operating an automatic calibration system.
In a first example method (Method 1), a scanner scale calibration mode is activated by steps of (a) entering the calibration sequence by either (i) activating a switch 19 (disposed on the housing 12 or at some other suitable location) as shown in
Step (b) of Method 1 is elaborated as follows. Suppose the output of the accelerometer is a voltage V. A typical accelerometer has a voltage offset V0 with no acceleration and a voltage output that is proportional to acceleration A, with proportionality constant S (also known as the sensitivity of the accelerometer). So the output voltage V of the accelerometer follows as equation 1:
V=V0+S*A Equation 1
The offset V0 and sensitivity S are typically temperature sensitive. The effects of temperature on these two parameters are illustrated in equation 2, where k1 is the thermal offset coefficient and k2 is the thermal sensitivity coefficient.
V=V0+k1*T+(S+k2*T)*A Equation 2
A calibration process is used in the factory to determine constants k1 and k2. One method for obtaining these constants is described presently. The voltage output V1 of the accelerometer under test is measured at a constant temperature T1 and a physical orientation with respect to plumb (such as oriented in the direction of the earth's gravity), as shown in Equation 3. A second measurement V2 is taken at temperature T1 with a physical orientation 180° from the first measurement (such as opposite in the direction of the direction of earth's gravity), as shown in Equation 4.
V1=V0+k1*T1+(S+k2*T1)*A Equation 3
V2=V0+k1*T1+(S+k2*T1)*(−A) Equation 4
The average of these voltages X1 is shown in Equation 5. The difference between these voltages Y1 is shown in Equation 6. Equation 5 eliminates all sensitivity components, while Equation 6 eliminates all offset components.
X1=(V1+V2)/2=V0+k1*T1 Equation 5
Y1=(V1−V2)=2*(S+k2*T1)*A Equation 6
These same measurements are taken at a different temperature T2, yielding Equations 7 and 8.
X2=V0+k1*T2 Equation 7
Y2=2*(S+k2*T2)*A Equation 8
Since gravity (A) is known at the calibration site, and the temperatures T1 and T2 are known, equations 5,6,7, and 8 are four linear equations with four unknowns (V0,S,k1, and k2). It is a straightforward method to solve for these unknowns to obtain the offset voltage V0, sensitivity S, thermal offset coefficient k1, and thermal sensitivity coefficient k2. These values are stored in flash or EEPROM as calibration data for the accelerometer.
Step (c) of Method 1 is elaborated as follows. In the factory, the calibration constants V0,S,k1, and k2 were computed and stored in permanent memory (flash or EEPROM, for example). A measurement V from the accelerometer is obtained and the temperature T is measured. The acceleration value A is determined from Equation 9 (derived from equation 3). Because the measurement V from the accelerometer is filtered to reduce the effects of vibration, the acceleration A derived from Equation 9 represents the local acceleration of gravity, which we can denote as g.
A=(V−V0−k1*T)/(S+k2*T) Equation 9
When the scale is initially calibrated, the gravity measurement g0 is stored in flash or EEPROM and sets the reference gravity measurement for the scale. When the gravity g1 is measured in the current location, as described in step (c) of Method 1, a gravity factor gf is computed as shown in Equation 10 and is used to adjust the weight readings from the scale to reflect the local gravity conditions.
gf=g0/g1 Equation 10
Finally, step (d) of Method 1 is elaborated as follows. The gravity factor gf is used to modify the weight measured from the scale to reflect the local gravity conditions as shown in equation 11, where “uncompensated weight” is the weight that is returned by the scale using the load cell calibration constants from the factory calibration procedure.
Weight=gf*(Uncompensated weight) Equation 11
Preferably, the accelerometer 30 is disposed at a suitable location within the scanner housing. As described above, the preferred configuration for the accelerometer is an integrated circuit, typically mounted on a PCB. Within the scanner, a suitable PCB for the accelerometer includes: the scale PCB, primary (main) scanner PCB, or a separate PCB such as one plugged into one of the other PCB's. Alternately, the accelerometer could be located external to the scanner or scale, for example at the POS, or even an external portable data terminal (PDT) or other device capable of communicating to the scanner such as an accelerometer module plugged into an external communication port of the scanner. Such a module device is diagrammatically illustrated in
In a second example method (Method 2), auto-calibration is activated and completed via an interface to a PC, a POS terminal, a portable data terminal (PDT) or other device capable of communicating to the scanner. For example, the PC may contain in memory information corresponding to accelerometer calibration data for the various temperatures or pressures as sensed by the sensor(s) 32, 36. The PC, for example, takes the output from accelerometer 30 and makes a suitable adjustment to the scale gravity calibration factor based upon temperature (or other sensor) input. Accelerometer calibration information may be stored in memory or downloaded from the accelerometer manufacturer's website.
Upon calibration, the scanner may provide visual and/or auditory means of indicating the acceptance or rejection of the auto-calibration.
As previously described, the system may be activated into the auto-calibration sequence/mode via programming labels, such as the Code 128 programming labels, whereupon scanning the specific scale calibration programming label provides a command to initiate auto-calibration. In any of the above methods utilizing programming labels, the scanner-scale 10 may be shipped with specific calibration bar code labels, such as attached to the weigh platter. In the event the weigh platter is removable, bar code labels may be applied to the platter (e.g. on the underside), the scanner-scale placed in calibration mode, the platter removed and passed over the scan window to scan the labels thereby providing calibration data to the auto-calibration system (whether resident in the scale, the POS, PC or other location). The correct labels may then be installed at the factory (or elsewhere). The labels may also be printed to include human-readable characters.
The programming labels may be any suitable type of programming label such as modified from UPC, EAN or JAN; custom programming Code 39 labels; or programming labels made in accordance with the AIM 128 standard. Though each of these labels may comprise a standard 1-D bar code label, other types of symbologies or labels may be used such as 2-D; PDF-417; bar code labels with add-on codes; or RFID tags. The system may first require an “enter programming” label be scanned, and then additional labels containing the calibration or location data may be subsequently scanned.
The auto-calibration system may be combined with other systems or its calibration checked and re-calibrated onsite by a conventional system in similar fashion as how the system is initially calibrated at the factory. For example, the scanner-scale may be connected to an auto-locate system, such as a Global Positioning System (GPS) disposed in a PDT connected to the scanner-scale 10, whereby the GPS accesses satellite signals, calculates a location and provides location information to the scanner-scale. Upon knowing its location, the re-calibration system may then extract from a memory (or the store computer or some other source such as via an internet link) the proper scale calibration data for that location. The re-calibration system then may provide adjustment data to the auto-calibration.
Though the present invention has been set forth in the form of its preferred embodiments, it is nevertheless intended that modifications to the disclosed systems and methods may be made without departing from inventive concepts set forth herein.
This application claims priority to provisional application No. 61/060,414 filed Jun. 10, 2008, hereby incorporated by reference.
| Number | Date | Country | |
|---|---|---|---|
| 61060414 | Jun 2008 | US |
