GB2228092A

GB2228092A - Reducing random errors in transducer measurement

Info

Publication number: GB2228092A
Application number: GB8903194A
Authority: GB
Inventors: Clifford Hannam
Original assignee: HITECH ELECTROCONTROLS Ltd
Current assignee: HITECH ELECTROCONTROLS Ltd
Priority date: 1989-02-13
Filing date: 1989-02-13
Publication date: 1990-08-15
Also published as: GB8903194D0

Abstract

The invention relates to the use of one or more transducers to measure a particular physical quantity of an article e.g. its length, temperature, pressure, speed or weight. The process comprises employing a first transducer 5 to measure the required physical quantity (weight) of the article 2, storing the output measurement in memory, remeasuring the same physical quantity at least once after transferring the article to one or more other transducers 4 or along a path returning to the first transducer, averaging the output measurements in unit 11, and comparing the averaged measurement against a reference figure for the required physical quantity in comparator 13. Input conveyor 1 feeds a succession of articles 2, 2a, 2b, 2c, 2d, 2e, 2f in turn to the first transducer 3, conveyor 8 transfers each article to said one or more other transducers 4, and an output conveyor 9 removes each article after the second or final measurement has been taken. <IMAGE>

Description

IMPROVEMENTS IN OR RELATING TO THE USE OF TRANSDUCERS This invention relates to the use of transducers. More particularly' but not essentially, the invention relates to the use of transducers for producing an electrical or other representation of the weight of an article when such weights are being measured in weighing machines. Thus a weight transducer, or as it is often referred to a weigh cell or load cell, is described hereinafter by way of an example, only, and it will be appreciated that the invention applies equally to temperature, pressure, speed, length etc transducers; in fact to any transducer which has an output representing a physical quantity.

According to the invention there is provided a method of measuring a particular physical quantity of an article comprising employing a first transducer to measure the required physical quantity of the article, storing the output measurement, remeasuring the same physical quantity at least once after transferring the article to one or more other transducers or along a path returning to the first transducer, averaging the output measurements, and comparing the averaged measurement against a reference figure for the required physical quantity.

In one embodiment of the invention the physical quantity to be measured is the weight of the article. The article may be one of a succession of articles to be weighed individually.

Subsequently to comparing the averaged measurement against the reference figure, the comparison may be employed to vary an operation upstream or downstream or both of the first transducer.

The invention also provides apparatus for measuring a particular physical quantity of an article comprising a first transducer for measuring the required physical quantity of the article, means for storing the output measurement, means for conveying the article at least once to one or more other transducers or along a path returning to the first transducer, means for averaging the output measurements, and means for comparing the average measurement against a reference figure for the required physical quantity.

The or each transducer may be a weight transducer.

Preferably there is provided an input conveyor for feeding a succession of articles in turn to the first transducer, means for transferring each article to said one or more other transducers or for returning the article to the first transducer, and an output conveyor for removing each article after the second or final measurement has been taken.

At least one intermediate conveyor may be provided for transferring each article between said transducers or for returning the article to the first transducer.

The apparatus also preferably provides memory means for temporarily storing each measurement taken, and sensing means associated with the or each transducer for entering each output measurement into the memory means and for taking said measurement out of the memory means prior to the output measurements being averaged.

In the use of weigh cells in weighing machines, it is known that the graphical pattern of the amplitude of a large succession of output signals from a cell shows a random variation from the graphical representation of the true weights. This is due to small imperfections such as frictional effects in the transducer or may be due to the method of handling the articles being weighed or movement in the articles themselves, such as liquid in a container, and in producing the output signals. If, in such a series of output signals, the true weight is subtracted from the weight reported by the transducer and the results again shown graphically, the pattern of the differences or errors about a mean value shows what is well known as a Gaussian distribution. The nature and mathematical handling of such errors is the province of probability theory.

The difference between the true weight and the weight reported by the transducer will be referred to as error e. The actual value of e for each individual weighing will not be known, but bearing in mind the gauss ivan distribution of such an error over a number of weighings, it can be said for all practical purposes to be within the range -3s to +3s, where s is the standard deviation of a number of weighing errors determined by experiment.

If an article is weighed twice, on two different weighing transducers, errors e1 and e2 will occur, with associated standard deviations of S1 and s2 respectively. Alternatively, if both weighings take place on the same transducer, or identical transducers under identical conditions, then sl is equal to 52 The mathematics of gauss ivan statistics states that where a number of variables each have a standard deviation, then the sum of the variables has an overall standard deviation equal to the square root of the sum of the squares of their individual standard deviations.Thus, in the above example, the standard deviation associated with (e1 + e2) is

If two weight readings are taken of the same item, and averaged, the resulting error f(et + ev) has an associated standard deviation of

If sl is equal to s2 this becomes s divided by

Thus it follows that the error of the average of the two readings lies within the range

This is an improvement of the error range by a factor of

Similarly, using 'n' identical transducers and averaging their outputs, the error range is improved by a factor of

When applied to a practical application such as a weight checking machine then, as stated above, the average error figure is reduced by the use of two or more weighings.This gives an advantage to the user of either fewer rejects when the resultant averaged weight is compared to a standard or reference figure, using conventional rejection means, or allows the user to set closer limits on the reject system giving the same number of rejects. This advantage also allows for more accurate weighing of articles for the purposes of weight monitoring or control, for example, for use in feed back or feed forward systems.

A specific embodiment in accordance with the invention will now be described with reference to the accompanying diagrammatic drawing which shows a weighing line for weighing a succession of packages or other discrete articles.

Referring to the drawing, an input conveyor 1 supplies a succession of packages 2, 2A, 2B, 2C etc to be weighed in turn, to a first weigh cell 3. An intermediate conveyor 8 carries the packages to a second weigh cell 4, and an output conveyor 9 carries the packages away for further processing.

At the moment shown, package 2E on the first weigh cell 3 is generating an output signal wl. A short time later, depending on the speed and length of the intermediate conveyor 8, the same package 2E arrives at the weigh cell 4 where it generates a second output signal w2. In the meantime, the signal wl is synchronised by sensor 6 positioned adjacent to the first weigh cell 3 and is switched by gate 5 into a temporary memory store 7 which operates on first in first out.

When the package has arrived at the weigh cell 4, the signal wl is released from the store 7 by a synchronising switch 12 positioned adjacent to the weigh cell 4 and controlling gate 10, and is fed together with signal w2 to an averaging unit 11.

The averaged signal is thereafter fed to a comparison unit 13 in which the averaged signal is compared to a standard or reference signal so producing conventional accept or reject signals to the weighing mechanism.

Alternatively, the comparison unit 13 may produce a control signal which is used to vary an appropriate operation upstream or downstream or both of the first weigh cell 3 or weighing station.

If it is required to use only one weigh cell to provide two weighings for a particular package conveyor, the intermediate conveyor 8 could, for example, be curved to re-present each package in turn to the weigh cell 3. As before, the two measurements would be averaged and compared against a reference signal. However, the process would be likely to be slower since packages would have to be presented alternately to the weigh cell 3 by the conveyors 1 and 8.

In a further embodiment, more than two weighings may be required to produce output signals wl, w2, W3, W4 etc, which are then averaged and compared against a reference signal. In this case, a separate weigh cell may be provided for each weighing together with its associated synchronising switch, or each package may be re-presented the required number of times to the weigh cell 3.

In the former case, a separate intermediate conveyor may be provided between adjacent weigh cells in the weighing sequence.

If desired the conveyors need not be separate conveyors, but may be a single conveyor. Similarly, it may be more convenient to employ other means, e.g.

pushing means, instead of the or each intermediate conveyor to transfer each article between the transducers or for returning the article to the first transducer.

As stated above, the invention is also applicable to the measuring of other physical quantities, besides weight, an appropriate transducer or transducers being employed in each case. The separate output signals are then averaged and compared against a reference signal.

Claims

1. A method of measuring a particular physical quantity of an article comprising employing a first transducer to measure the required physical quantity of the article, storing the output measurement, remeasuring the same physical quantity at least once after transferring the article to one or more other transducers or along a path returning to the first transducer, averaging the output measurements, and comparing the averaged measurement against a reference figure for the required physical quantity.

2. A method as claimed in Claim 1, wherein the physical quantity to be measured is the weight of the article.

3. A method as claimed in Claim 2, wherein the article is one of a succession of articles to be weighed individually.

4. A method as claimed in any one of the preceding claims, wherein subsequently to comparing the averaged measurement against the reference figure, employing the comparison to vary an operation upstream or downstream or both of the first transducer.

5. Apparatus for measuring a particular physical quantity of an article comprising a first transducer for measuring the required physical quantity of the article, means for storing the output measurement, means for transferring the article at least once to one or more other transducers or along a path returning to the first transducer, means for averaging the output measurements, and means for comparing the average measurement against a reference figure for the required physical quantity.

6. Apparatus as claimed in Claim 5, wherein the or each transducer is a weight transducer.

7. Apparatus as claimed in Claim 5 or Claim 6, including an input conveyor for feeding a succession of articles in turn to the first transducer, means for transferring each article to said one or more other transducers or for returning the article to the first transducer, and an output conveyor for removing each article after the second or final measurement has been taken.

8. Apparatus as claimed in Claim 7, wherein at least one intermediate conveyor is provided for transferring each article between said transducers or for returning the article to the first transducer.

9. Apparatus as claimed in any one of Claims 5 to 8, including memory means for temporarily storing each measurement taken, and sensing means associated with the or each transducer for entering each output measurement into the memory means and for taking said measurement out of the memory means prior to the output measurements being averaged.

10. A method of measuring a particular physical quantity of an article substantially as hereinbefore described.

11. Apparatus for measuring a particular physical quantity of an article substantially as hereinbefore described with reference to the accompanying drawing.