Electronic balance

Details Electronic balance Electronic balance

1998-03-11

2001-02-27

Gibson, Randy W. (Department: 2859)

Weighing scales

Computer

Electrical

C073S001130, C702S101000

Reexamination Certificate

active

06194670

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention concerns an electronic balance comprising a measuring transducer with a weighing pan and a signal processing module as well as an input unit and an output unit.
Factors that have an influence on the weighing results are the location where the balance is installed and the prevailing environmental conditions such as temperature fluctuations, vibrations, electrostatic fields, air drafts, etc., as well as the properties of the weighing samples themselves. Balances of more recent design are equipped with the capability to account for systematic measuring errors and to adjust the result values with a correction. An example of this is the automatic adjustment that takes place in a balance after the temperature has changed by a certain amount. Another example is the calibration of the balance at its actual location.
2. Description of the Related Art
From EP-B1 0424773, a precision balance is known in which the built-in and automatically activated calibration weight is applied and removed several times in succession for the purpose of determining the standard deviation. Each time, the measured value is registered and subsequently the standard deviation is calculated by the signal processing module and compared to a given reference value. If the calculated value is less than the reference value, then a new span calibration factor is calculated and stored in memory. On the other hand, if the calculated value is more than the reference value, the span calibration factor is not updated in memory, but the balance puts out a corresponding message. This known setup provides the capability to calibrate the balance and to determine the standard deviation, but only for the one load size of the calibration weight that is built into the balance. In addition, after an error message, it is necessary to check and restart the balance.
Even when known systematic measuring errors are taken into account, the result values are still subject to uncertainties due to systematic deviations that are known to exist but whose magnitude remains undetermined, such as a non-linearity that cannot be ascertained at all or not with sufficient accuracy, or due to random uncertainties as may be caused by, e.g., a momentary air draft or by the uncertainty in the value of a correction of a systematic measurement error. The user of a precise electronic balance will be familiar with the inevitable dispersion of the weighing results. From the specifications of a balance, one can conclude the general magnitude of the errors to be expected, but when making an actual measurement, the user will hardly get an idea of its accuracy, because the specific influence factors at the user's location have not been taken into account. However, the user's broader concern is to know the measurement uncertainty of the actual weighing results obtained at the actual location and under the actual conditions.
SUMMARY OF THE INVENTION
The object of the present invention is to provide an electronic balance that, at the actual place where the balance is installed, will give an indication of the degree of accuracy that can be achieved in measurements under the given conditions and, notably, for an arbitrary amount of weight anywhere within the entire measuring range of the balance.
The object is accomplished in an electronic balance that has a measuring transducer with a weighing pan and a signal processing module with the capability to access previously stored data and store new data, as well as an input unit and an output unit. As the distinguishing inventive feature, the signal processing module
initiates and controls a process in which one and the same weighing load of any arbitrary amount within the measuring range of the balance is repeatedly put on and taken off the weighing pan,
calculates from the resulting series of measuring values a statistical quantity, and
delivers an output of the result of the calculation and/or retains it for use in subsequent measurements.
It needs to be mentioned that certain terms in the following explanation will be used with specific meanings. Accuracy is a common term that intuitively conveys the idea of a reliable representation of even minute values or differences, but otherwise has no definition. Depending on the purpose of a measurement, the term “accuracy” can stand, e.g., for absolute or relative measurement uncertainty or for repeatability. The measuring result or final outcome of a measurement is the most probable value that one can assign to the quantity being measured. The measurement uncertainty is the measure for the amount of dispersion of the values that can reasonably be ascribed to the measuring result. Repeatability in the field of weighing is defined as the empirical standard deviation or repeatability standard deviation of a sample of successive weighings of the same load. Additional terms will be explained at the point where they occur in the text.
The balance according to the invention makes it possible not only to determine a statistical quantity such as the standard deviation for a weighing load, but it also offers a simple way of determining the upper and, more importantly, the lower limit of the load range within which the balance will operate at the desired level of accuracy. Consequently, it is possible to determine the entire load range in which a balance is capable of operating with the desired or prescribed accuracy in such a manner that the influence factors at the actual location, such as vibrations of varying magnitude, air pressure and ambient temperature, are taken into account. This makes it possible in a general sense to determine the range of accuracy at which the balance is capable of operating at its actual location and with a given kind of weighing sample.
The determination of the statistical quantity, or of the associated accuracy range, occurs at the command of the user when the situation requires it, e.g., after the balance has been relocated or after a major change in the weather. But the criterion for initiating the determination may also be programmed in the balance itself or through the input unit, e.g., it may be programmed to initiate the determination at regular time intervals. The determination consists of a series of weighings of one and the same test load. The load does not have to be known, except that it needs to be within the weighing range of the balance. The weighing sample itself may serve as the test load. The source of the measurement uncertainty is not in the balance alone; the properties of the weighing sample, too, make a contribution. Thus, it makes sense to take the influence of the weighing sample into account. The signal processing module initiates the process of repeatedly applying and removing the test load. This may take place in the form of an instruction to the user through an appropriate output device such as a display message or an acoustic command that the user executes, be it that he himself puts the test load on the weighing pan or that he activates an automated device.
Further possibilities are available if more than one statistical quantity is taken into consideration. By taking into account higher moments of the distribution of the measurement sample values, it is possible to check the validity of, or even to modify, the model distribution that was assumed in the calculation and thus to enhance the informative value of the results.
A proposed value (default value) for the number of repetitive cycles (loading and lifting the weight) may be stored internally, but the interactive user program of the balance also allows for the number of cycles to be entered by the user. In addition, the user can select certain parameter settings on the balance such as the filter strength for smoothing the measurement values in the presence of vibrations or the interval of the stability-checking feature for the release of the weighing result; thus, the user is given a means to optimize the balance parameter settings for the location and for the given kinds of weighi

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