GB2297386A

GB2297386A - Electronic weight determining apparatus

Info

Publication number: GB2297386A
Application number: GB9501728A
Authority: GB
Inventors: Raymond John Herbert
Original assignee: Neopost Ltd
Current assignee: Neopost Ltd
Priority date: 1995-01-30
Filing date: 1995-01-30
Publication date: 1996-07-31
Also published as: GB9501728D0; DE29601418U1; FR2730054A3; FR2730054B3

Abstract

Electronic weighing apparatus, for example for letters, has a scale pan of sufficiently small horizontal extent compared with items to be weighed as to ensure that the items can be supported by the scale pan only if the items are placed on the pan with the centre of gravity of the item substantially aligned with a support for the scale pan. Accordingly the support and transducer for the scalepan do not need to be constructed to resist tilting forces. Conveniently the scalepan may be constructed as an upright portion leaning slightly from the vertical and a lower portion to support a lower edge of an item such as an envelope. <IMAGE>

Description

WEIGHT DETERMINING APPARATUS This invention relates to weighing apparatus and in particular to weighing apparatus for weighing mail items.

Known weighing apparatus for weighing mail items is provided with a scale pan in the form of a horizontal platform supported such that when an item is placed on the platform the platform is displaced downwardly against a resilient force and the displacement of the platform is determined by sensing means to produce an electrical signal representing the weight of the item supported on the platform. The form of a scale pan as a flat horizontal platform to support the mail item permits the mail item to be placed asymmetrically relative to the platform and hence the loading of the mail item on the platform is offset relative to the resilient support for the platform with the result that the platform is subjected to a tilting force. Any tilting of the platform may result in inaccurate determination of the displacement of the platform and hence of the weight of the mail item.

Accordingly in known weighing apparatus it has been necessary to determine the displacement of the platform in such a manner that any tilting of the platform is compensated. This is usually achieved by supporting the platform by means of a load cell structure in which four strain gauges are provided. The strain gauges are electrically connected in a bridge circuit and are located such that errors in signals from any of the strain gauges subjected to displacement in excess of the average displacement of all the strain gauges due to offset loading of the platform are balanced and cancelled by opposite errors in signals from others of the strain gauges subjected to less than average displacement due to the offset loading of the platform. Such a structure of load cell can provide outputs accurately representing the load on the platform.However such load cell structures are complex and expensive to manufacture.

According to the invention weight determining apparatus includes a scale pan to receive a load to be weighed; means to resiliently support the scale pan in a manner such that the scale pan is displaced vertically downwardly to an increasing extent with increase of weight of the load on the scale pan; transducer means responsive to said vertical downward displacement to produce an electrical signal representing weight of the load and wherein the scale pan has an extent horizontally sufficiently small as to support the load only when the load is located on the scale pan with a centre of gravity of the load substantially aligned with the support means whereby the load when supported on the scale pan does not exert significant tilting forces on the scale pan.

An embodiment of the invention will be described hereinafter by way of example with reference to the drawings in which: Figure 1 is an end view of a scale pan for a weighscale in accordance with the invention, Figure 2 is a front view of the scale pan, and Figure 3 is a block circuit diagram of a circuit responsive to a weighscale transducer to produce a signal representing weight of a load on the scale pan.

Referring first to Figures 1 and 2, a scale pan 10 for a weighscale comprises a rear element 11 joined to a front element 12 to form therebetween a groove 13. When a mail item 14 is placed on the scale pan, a lower edge 15 of the mail item 14 is located in the groove 13 and the mail items leans rearwardly and a rear face of the mail item abuts against the rear element 11. The rear element is inclined at an angle to vertical so that a mail item placed on the scale pan leans against and is supported by the rear element. However the angle of inclination of the rear element is sufficiently small as to ensure that the mail item supported on the scale pan lies in a nearly vertically extending plane.By supporting the mail item in a nearly vertical plane, the centre of gravity of mail items of a range of different heights when placed on the scale pan is located within a relatively small horizontal range. Accordingly the centre of gravity is at substantially the same position for a range of sizes of mail item.

The lengthwise dimension "1" of the scale pan is short compared with the lengthwise dimension "L" of mail items of common dimensions. Accordingly a user is required to place the mail item substantially symmetrically as regards weight of the mail item on the scale pan otherwise the mail item will fall off the scale pan. In practice with most mail items which consist of one or more sheets of paper, folded or unfolded, within an envelope the weight is distributed substantially uniformly through the envelope the centre of gravity of the mail item is located centrally of the mail item and hence placing the mail item with its weight distributed substantially symmetrically relative to the scale pan is achieved by placing the mail item such that dimensionally it is substantially symmetrical relative to the scale pan.By requiring the mail item to be placed on the scale pan with its weight distributed substantially symmetrically relative to the scale pan, so that the centre of gravity is aligned approximately with the centre of the scale pan, it is ensured that the scale pan is not subjected to loading forces tending to tilt the scale pan.

The scale pan is supported on a stem 16 having a lengthwise axis located centrally of the scale pan in the dimension "1" and located rearwardly of the groove 13 in alignment with or in close proximity to the location of the centre of gravity of mail items when supported on the scale pan. A lower part of the stem 16 is resiliently supported to permit displacement of the scale pan vertically downwards when an item is loaded on the scale pan, the extent of the displacement being a function of the weight of the item. The stem 16 is mechanically connected to a transducer responsive to the downward vertical displacement of the stem to produce an electrical signal corresponding to the extent of the displacement.

By ensuring that the scale pan is not subjected, while weighing a mail item, to forces tending to tilt the scale pan a relatively simple and less costly transducer, as compared with conventional weighscales may be used to determine the weight of a mail item placed on the scale pan. The transducer may be one in which displacement of the scale pan by a load produces a change of electrical capacitance or resistance or may be an electromagnetic inductive transducer.

Referring now to Figure 3, the block circuit diagram illustrates a circuit for use with a capacitative transducer to produce an output signal representing the weight of a load on the scale pan.

A timer circuit 20, for example an integrated timer circuit commercially available under the type number 555, is connected as a free running oscillator with an output 21. The frequency of oscillation is determined by a resistor R and a capacitor C. The resistance of resistor R is fixed and the capacitance of capacitor C, which is the transducer for detecting displacement of the scale pan, varies in dependence upon the extent of displacement of the scale pan by the load thereon. Variation in capacitance of capacitor C causes a shift in frequency at the output 21 of the oscillator. The output 21 of oscillator 20 is connected to an input port 22 of a microprocessor 23.A timer within the microprocessor 23 provides a gating signal defining a time period and the number of transitions of the oscillatory output from the oscillator 20 are monitored to determine the frequency of the output at 21. Since the frequency of the oscillator is dependent upon the displacement of the scale pan caused by the weight of the load, the frequency of the oscillator represents the weight of the load on the scale pan.

The relationship of the frequency of the oscillator with weight of the load may be linear or non-linear. If required, correction for non-linearity may be applied by the microprocessor 23 to the value of weight determined from frequency of the oscillator. This may be achieved by means of correction data in a software look-up table.

Compensation may also be provided in respect of errors in determination of the value of weight of a load due to ambient temperature variation. This may be accomplished by means of a thermistor, or other temperature sensitive element, connected via an analogue to digital converter to the microprocessor 23. A further look up table contains error correction data whereby the microprocessor 23 is enabled to compensate for errors due to ambient temperature variation and produce an error free weight representing signal.

When a resistive or inductive element, instead of the capacitative element described hereinbefore, is used in the transducer the relationship of the resistance or inductance respectively to the weight of the load may be linear or non-linear. If required, correction for nonlinearity may be applied by the microprocessor 23 to the value of weight. determined from resistance or inductance respectively. This may be achieved by means of correction data in a software look-up table as described hereinbefore in relation to a capacitative transducer. Also as described hereinbefore compensation may be provided in respect of errors in determination of the value of weight of a load due to ambient temperature variation.

The weighscale may be an independent weighing apparatus in which the microprocessor 23 generates a signal at an output port 24 for operation of a display device to display the weight of the load on the scale pan.

Alternatively the port 24 may be connected to other apparatus requiring input of weight information, such other apparatus being for example a postage franking machine in which input of weight information is utilised to calculate a postage charge for the mail item. If desired the weighing apparatus may be integrated with the postage franking machine and the microprocessor 24 may be utilised both for determining weight of a load and for carrying out accounting and control functions for the franking machine. The form of the scale pan described hereinbefore is particularly advantageous not only in ensuring that the mail item is positioned with the centre of gravity thereof substantially aligned with the stem 16 of the scale pan but also in that the scale pan has small dimensions in a horizontal direction and hence may be more easily incorporated into postage franking machines than weighscales having a scale pan in the form of a horizontal platform of relatively large dimensions.

Claims

1. Weight determining apparatus including a scale pan to receive a load to be weighed; means to resiliently support the scale pan in a manner such that the scale pan is displaced vertically downwardly to an increasing extent with increase of weight of the load on the scale pan; transducer means responsive to said vertical downward displacement to produce an electrical signal representing weight of the load and wherein the scale pan has an extent horizontally sufficiently small as to support the load only when the load is located on the scale pan with a centre of gravity of the load substantially aligned with the support means whereby the load when supported on the scale pan does not exert significant tilting forces on the scale pan.

2. Weight determining apparatus as claimed in claim 1 for weighing mail items comprising a substantially planar envelope containing an insert wherein the load pan is constructed to support the envelope in an orientation in which the plane of the envelope is vertical or nearly vertical.

3. Weight determining apparatus as claimed in claim 2 wherein the scale pan has a groove to receive a lower edge of the envelope and a support extending upwardly from the groove to engage a face of the envelope.

4. Weight determining apparatus as claimed in any preceding claim wherein the transducer means includes a capacitor having an electrical capacitance dependent upon the extent of displacement of the scale pan.

5. Weight determining apparatus as claimed in claim 4 including an oscillator circuit; said oscillator circuit generating a signal having a frequency dependent upon the electrical capacitance of the capacitor; and processing means to determine the frequency of the signal and to generate a weight representing signal.

6. Weight determining apparatus as claimed in claim 5 wherein the processing means is operative to utilise correction data to correct errors due to non-linearity of the relationship between weight of a load on the scale pan and the frequency of the signal.

7. Weight determining apparatus as claimed in claim 1, 2 or 3 wherein the transducer means includes a resistive element having an electrical resistance dependent upon the extent of displacement of the scale pan.

8. Weight determining apparatus as claimed in claim 7 including processing means operative to utilise correction data to correct errors due to non-linearity of the relationship between weight of a load on the scale pan and the resistance of the resistive element.

9. Weight determining apparatus as claimed in claim 1, 2 or 3 wherein the transducer means includes an inductive element having an electrical inductance dependent upon the extent of displacement of the scale pan.

10 Weight determining apparatus as claimed in claim 9 including processing means operative to utilise correction data to correct errors due to non-linearity of the relationship between weight of a load on the scale pan and the inductance of the inductive element.

11. Weight determining apparatus as claimed in any one of claims 2 to 10 combined with a postage franking machine wherein the processing means is operative to utilise the weight representing signal to calculate a postage charge for the mail item.

12. Weight determining apparatus constructed and arranged to operate substantially as hereinbefore described with reference to Figures 1 and 2 of the drawings.

13. Weight determining apparatus constructed and arranged to operate substantially as hereinbefore described with reference to Figures 1, 2 and 3 of the drawings.