GB2175697A

GB2175697A - Precision weighing systems

Info

Publication number: GB2175697A
Application number: GB08609789A
Authority: GB
Inventors: Michel Turpain
Original assignee: MPI Metrologie Pesage International SA
Current assignee: MPI Metrologie Pesage International SA
Priority date: 1985-04-22
Filing date: 1986-04-22
Publication date: 1986-12-03
Also published as: FR2580804A1; ES8704634A1; GB8609789D0; DE3613238A1; IT8620165A1; IT8620165A0; CH669460A5; IT1190629B; ES554235A0; FR2580804B1; GB2175697B

Abstract

A first signal in accordance with a force exerted by an article to be weighed is supplied by a device 2 to a correcting device 4 which provides a modified signal which supplies a weight indicating and/or recording unit 3. There is or are also supplied to the correcting device 4, to effect the modification, both on calibration and again subsequently for making a weighing, one or two signals related to the latitude and the altitude of the place at which the apparatus is situated, in order approximately to compensate for the fact that g varies with latitude and altitude. Signals representing bands of latitude values and bands of altitude values may be stored in a memory 5 and selected for transmission to the correcting device 4 by a unit 6. <IMAGE>

Description

SPECIFICATION Precision weighing systems A known precision weighing system consists of a load receiver and a load meter. The load meter comprises a load equalizer, or equilibrium producer, a transducer and an indicator.

The load meter can include, for example, a testing body with strain gauges of the wire resistance type stuck on it. The testing body undergoes mechanical distortions which depend on the force F transmitted by the load receiver receiving a mass M. These distortions cause a variation in the resistance of the ,strain gauges which, with the aid of a suitable installation and an external supply, produces an electric signal which is a function of the force F.

The force F and the mass M are linked the relationship: F=Mg, g being the acceleration due to gravity. It is well known that g varies from place to place, its value at any place depending upon the latitude and altitude of that place. To take account of these variations, it is necessary, in the case of known high precision weighing systems, to calibrate each weighing system at its place of use, so that a sufficiently skilled person, having available the equipment necessary for the calibration, must be present at the place of use and each change in the place of use necessitates a new calibration. It is an object of the present invention to avoid these disadvantages.

According to a first aspect of the present invention, there is provided a method of calibrating a weighing system which includes a transducer which produces a signal in accordance with a force exerted by an article to be weighed, the method involving, in order approximately to compensate for the fact that the effect of gravity is not the same at all latitudes and altitudes, introducing into the weighing system a calibration signal or calibration signals relating to the latitude and the altitude of the place where the calibration is effected.

According to a second aspect of the invention, there is provided a method of weighing, using a weighing system that has been calibrated by a method according to the first aspect of the invention, in which there is introduced into the weighing system a correction signal or correction signals relating to the latitude and the altitude of the place where the weighing is effected.

According to a third aspect of the invention, there is provided a precision weighing system including a transducer which produces a first signal in accordance with a force exerted by an article to be weighed, means for modifying that signal with another signal or other signals in accordance with the latitude and altitude of the place where the weighing system is situated, in order approximately to compensate for the fact that the effect of gravity is not the same as all latitudes and altitudes, and means for receiving the modified signal and using it to indicate and/or record the weight of the article.

Known mathematical formulae permit the acceleration due to gravity to be calculated for any place, knowing its latitude and its altitude, regarding the earth as an ellipsoid.

Thus, g, in metres per second per second, at sea level and at A degrees latitude is given by the following relationship: g=9.780318 (1+0.0053024 sin2A-0.0000059 sin22A) This value of g must be reduced for each metre above sea level by 2 X 10-6 metres per second per second.

According to the invention, therefore, in order to cover all the places of use envisaged for a weighing system, cutting up into elementary zones by latitude and by altitude is accomplished, taking account of the sought precision. To each zone a corrective co-efficient is attributed and into the weight calculation chain are input, first of all, for the calibration, the correction co-efficients corresponding to the zone of latitude and to the zone of altitude of the palce of calibration and afterwards, for the regulation in terms of the place of use, the correction co-efficients corresponding to the zone of latitude and to the zone of altitude of the place of use.

According to a preferred embodiment of the invention, all the correction co-efficients are memorised in the weighing system and, for the calibration and regulation of the weighing system in terms of the place of use, the corresponding correction co-efficients are called, by selecting the zone of latitude and the zone of altitude of the place of calibration and of the place of use, in order to input these coefficients into the calculation chain.

The device in accordance with the invention for calibration and regulation for a weighing system comprising a load measuring unit, a weight calculating unit and a display unit, comprises a memory in which are written predetermined values corresponding to correction co-efficients of the gravity variations in terms of the latitude and altitude, means for selecting the said memorised values, in terms of the latitude and altitude and multiplication means located between the weight calculating unit and the display unit and linked to the memory, to correct the result of the weight calculation in terms of the selected values.

An example in accordance with the invention is described below with reference to the accompanying drawing, which diagrammatically illustrates a precision weighing system.

The illustrated weighing system includes a measuring chain which comprises, as in known precision weighing systems, a load receiver not shown, an equalizer load transducer, shown in the form of a load measuring unit 1 and a weight calculating unit 2 and an indicator represented in the form of a numerical display unit 3. The equalizer load transducer consists advantageously of a testing body and strain gauges, for example resistance strain gauges, stuck on the latter.

To this known measuring chain is added a device for correcting the effects of gravity variation in terms of latitude and altitude. This correction device comprises a multiplication unit 4 connected between the weight calculating unit 2 and the display unit 3, a memory 5 in which correction co-efficients are memorised and a coded input unit 6 permitting the correction co-efficients contained in the memory 5 to be selected, with a view to applying them to the multiplication unit 4 and thus introducing them into the weight calculation chain.

The correction co-efficients input into the memory 5 are pre-determined in terms of the geographical zone in which the weighing system is calibrated and the geographical zones in which the weighing system must be able to be used, in such a manner as to compensate for the effects of the gravity variation in terms of the latitude and altitude of the place of calibration and of the places of use envisaged.

For example, in France the minimum and maximum latitudes are about 43" and 51". It is therefore possible, for example, to divide France into eight latitude bands each 1" wide, i.e., 43" to 44", 44" to 45 .... 50 to 51".

Each band is given its own correction co-efficient. It is possible also to divide the altitudes existing in France into eight bands, for example 0 to 500 metres, 500 to 1000 metres.. . 3500 to 4000 metres. Again each band is given its own co-efficient.

In binary coding, it is possible, on an input unit 6 comprising six switches, to select one of the eight zones of latitude and one of the eight zones of altitude, to input thus, into the weight calculation chain, from the correction co-efficients contained in the memory 5, those corresponding to the zone of latitude and to the zone of altitude selected.

This selection is carried out first of all for the calibration of the weighing system, by selecting the zone of latitude and the zone of altitude corresponding to the place of calibration and, after calibration, for the setting of the weighing system having regard to its place of use. This setting having regard to the place of use can be made at any location and the input 6 can afterwards undergo plumbing to satisfy the rules of the Meteorological Service of the country concerned.

Instead of selecting the correction co-efficients with the help of switches, it is equally possible to accomplish this selection with the help of a keyboard, or any other input device.

Of course, the correction co-efficients for gravity variation in terms of latitude and altitude can be predetermined and memorised for any geographical zone at all. The only limit imposed in this respect is that of the capacity of the memory 5.

Instead of supplying two co-efficients or signals to the device 4, one for latitude and one for altitude, these two co-efficients or signals can be combined so that only a single coefficient or signal is supplied to the device 4.

Claims

1. Method for calibration and regulation of weighing systems with a load meter with strain gauges, characterised by the fact that, a series of correction co-efficients for gravity variation are determined by zones of latitude and altitude, covering the place of calibration and all the places of use envisaged for the weighing system; that, the calibration of the weighing system is carried out by inputting into the weight calculation chain the connection co-efficients corresponding to the latitude and altitude of the place of calibration; and that, afterwards the weighing system is regulated in terms of the place of use, by inputting into the weight calculation chain the correction co-efficients corresponding to the latitude and altitude of the place of use.

2. Method according to claim 1, characterised by the fact that, the entire correction coefficients are memorised in the weighing system and, for the calibration and the regulation in terms of the place of use, the corresponding correction co-efficients are called by selecting the zone of latitude and the zone of altitude of the place of calibration and of the place of use, to input these co-efficients into the weight calculation chain.

3. Device for implementing the method according to claim 1 or 2, for a weighing system comprising a load measuring unit, a weight calculating unit and a display unit, characterised by the fact that, it comprises a memory (5) in which are written pre-determined values corresponding to correction coefficients for gravity variation in terms of latitude and altitude, means (6) of selecting the said memorised values in terms of latitude and altitude and, multiplication means (4), located between the weight calculating unit (2) and the display unit (3) to correct the result of the weight calculation in terms of the selected values.

4. A method of calibrating a weighing system which includes a transducer which produces a signal in accordance with a force exerted by an article to be weighed, the method involving, in order approximately to compensate for the fact that the effect of gravity is not the same at all latitudes and altitudes, introducing into the weighing system a calibration signal or calibration signals relating to the latitude and the altitude of the place where the calibration is effected.

5. A method of weighing, using a weighing system that has been calibrated by a method according to claim 4, in which there is introduced into the weighing system a correction signal or correction signals relating to the latitude and the altitude of the place where the weighing is effected.

6. A method according to claim 4 or 5 in which a calibration or correction signal is employed which represents a prescribed band of latitudes and/or a prescribed band of altitudes.

7. A method according to any one of claims 4 to 6 in which a plurality of calibration or correction signals are stored in a memory and the required ones are drawn from the memory in accordance with the latitude and the altitude of the place concerned.

8. A precision weighing system including a transducer which produces a first signal in accordance with a force exerted by an article to be weighed, means for modifying that signal with another signal or other signals in accordance with the latitude and altitude of the place where the weighing system is situated, in order approximately to compensate for the fact that the effect of gravity is not the same as all latitudes and altitudes, and means for receiving the modified signal and using it to indicate and/or record the weight of the article.

9. A system according to claim 8 including a memory in which are stored signals, for use in modifying said first signal, relating to at least all the latitudes and altitudes at which it is thought likely that the system will be calibrated or used for weighing.

10. A system according to claim 9 in which the stored signals are one for a band of latitude values, another for another band of latitude values, and so on, one for a band of altitude values, another for another band of altitude values, and so on.

11. A method of calibrating a weighing system, substantially as described above with reference to the accompanying drawing.

12. A method of weighing, substantially as described above with reference to the accompanying drawing.

13. A precision weighing system, substantially as described above with reference to the accompanying drawing.