GB2191001A

GB2191001A - Multi-cell weighing system

Info

Publication number: GB2191001A
Application number: GB08711109A
Authority: GB
Inventors: Michael Deakin; David Tyler
Original assignee: W&T Avery Ltd
Current assignee: Avery Weigh Tronix Ltd
Priority date: 1986-05-14
Filing date: 1987-05-11
Publication date: 1987-12-02
Also published as: GB8611793D0; IN168619B; GB8711109D0; AU7290787A; AU584801B2; GB2191001B; NZ220298A; ZA873429B

Abstract

A weighing system has a plurality of load cells 1,2,3,4 associated with a weighing platform 10. A microprocessor 20 switches the analogue outputs of the load cells to an analogue-to-digital convertor 19 and stores the successively digitised outputs in a memory 21. The microprocessor 20 also generates correction factors to be added to the stored digitised values before they are summed for final display at display means 22. <IMAGE>

Description

SPECIFICATION Multi-Cell Processing The present invention concerns weighing systems, and in particular weighing systems utilising a plurality of load cells.

There are many weighing applications which require more than one load cell to be connected to a weighing structure such as a platform. With such systems there are certain prevalent errors.

One way in which errors can arise is that the load cells in the system are not precisely matched with regard to voltage output versus load for the span of the cells. Another error source is that the load cells do not give truely linear outputs. Because of these errors the final weight reading for the complete system will vary in accordance with the position of the load, that is a single weight can give different readings from the weighing system in accordance with its position on the weighing platform.

A known method of overcoming this type of error in a multi-load cell system is to feed the output of each cell concurrently into an analogue circuit which calculates a correction voltage which is used to compensate for the errors introduced by the fact that the cells are mismatched.

This correction voltage is summed with the actual outputs of the cells to give a final analogue value for the weight. This analogue value is converted to a digital value which can then be displayed.

The present invention has for an object to provide a multi-load cell weighing system in which load placement errors can be compensated for in a simple and economical manner.

Accordingly the present invention consists in a weighing system comprising a plurality of load cells associated with a weighing platform, means for successively digitising and storing the individual outp .'ts of the load cells, means for imparting a correction value to selected ones of said stored digital values and for storing final selected correction values, and means for summing the stored and corrected digital values.

The system preferably includes a microprocessor controlling the switching of the analogue signals from the load cells, the digitising and storing of the switched signals, and the calculation of the correction values and their combination with the stored digitised values.

In order that the present invention may be more readily understood, an embodiment thereof will now be described by way of example and with reference to the single Figure of the accompanying drawing, which figure is a schematic block diagram of a multi-load cell weighing system according to the present invention.

Referring now to this drawing there is shown a weighing system comprising a rectangular weighing platform 10 having four load cells 1, 2, 3 and 4 located at its respective corners.

These load cells can be of any standard type and may comprise a rectangular body one side of which is mounted to a rigid support with the opposite side connected to the weighing platform.

Mounted in the rectangular body are one or more transducers which are permanently supplied with an excitation current and the outputs of which vary when the rectangular body is distorted by an applied load. These transducer outputs are summed to give an analogue signal representing the output of the load cell.

The four load cells are all connected via leads indicated at 12 to a junction box 13. The leads 12 represent paths for both the signals from the load cells and the excitation currents required for their operation.

The junction box 13 has four outputs 1', 2', 3' and 4' representing respectively the signals from the four load cells 1-4 and these are connected by further leads 14 to an instrumentation box 5. The latter bio,: conta :,s, amongst other things, an appropriate power supply for suppiying the permanent excitation current to the junction box 13 as indicated at 16 and also for receiving a reference current from junction box 13 as indicated at 17.

At the instrumentation box end the leads 14 are successively connected by X switching arrangement indicated at 18 to an analogue-to-digital converter 19. The switching is carried out by a microprocessor 20 in such a manner that the output from each of the load cells is connected in turn to the ADC for approximately 12.5 milliseconds. The digitised value from each cell is then separately stored in memory 21 by the microprocessor so that there are for each cycle of 50 milliseconds four digitised readings, one from each of the load cells.

An appropriate, previously calculated correction is then applied to each of the four stored values to compensate for errors with regard to gain and non-linearity. The microprocessor then sums the four digitised and corrected readings to give a final, single weight reading which can be taken to an appropriate display such as the one shown at 22. Such a display may be associated with a keyboard 23.

The correction values to compensate for varying gain and non-linearity in the load cells so that the final reading is independent of load placement is done as follows: 1. Place 4 Full load over load cell 1 and note display reading.

2. Move load to cell 2 and note reading.

3. Instruct microprocessor to adjust output of cell 1 in one direciton, i.e. either increasing or decreasing. This will cause the output of cell 2 to move in the opposite direction but by the same amount.

4. Repeat steps 1 to 3 until the same reading is obtained whether the load is placed over cell 1 or cell 2.

5. Steps 1 to 4 are repeated but with cells 3 and 4.

6. Place same load half-way between cells 1 and 2 and note display reading.

7. Place load half-way between cells 3 and 4 and note display reading.

8. Instruct microprocessor to adjust the outputs of both cells 1 and 2 in one direction so that the output of cells 3 and 4 taken together will move the same amount in the other direction.

9. Repeat steps 6 to 8 until the same reading is achieved irrespective of whether the load is half-way between cells 1 and 2 or 3 and 4.

10. Move load to centre of platform and note reading.

11. Move load to above any one of the load cells and adjust reading to display the same as it did when the load was central.

12. Repeat steps 1 to 11 to reduce any errors to within required limits. For example, for the U.K. Department of Trade no error should exceed + 4 DIV.

In order to derive the single weight reading R from the four load cells the following equation is used: R = oAC + &commat; OjiC +QBC by + OLIBC + [(1 + 4) - (2 + 3) x D] + [( + O ) ~ (0 + > ) x D] Where , 0' , 0 are the respective digitised and stored inputs from the load cells 1, 2, 3, and 4 and A, B, C, D are multiplying factors between 0 and 2, and A, B, C are the complemented answers of A, B, and C respectively. The multiplying factors are those obtained by the adjustments carried out in sequence of steps previously set out.

When making these adjustments it is preferred that the keyboard 2 of the microprocessor has assigned keys which can be used when making the adjustments. Thus, for example, in step 3 two keys would be assigned, one key being allocated to each direction.

It will be readily appreciated how the steps for obtaining the correcting values can be altered to deal with weighing systems having a plurality of load cells either less or more than four in number.

As mentioned, known multi-load cell weighing systems use hardware techniques to sum the outputs of the load cells and also to provide correction for all mismatching errors whilst the weighing signals are still in analogue form. The system just described eliminates the need for this hardware for both summing and cell correction. This gives the advantage of lower cost.

Furthermore the basic system described has the flexibility to deal with differing numbers of cells so that a single low cost unit can effectively replace a number of different units.

Claims

1. A weighing system comprising a plurality of load cells associated with a weighing platform, means for digitising and storing the individual outputs of the load cells, means for imparting a correction value to selected ones of said stored digital values, and means for summing the stored and corrected digital values.

2. A system as claimed in Claim 1, and including a microprocessor controlling the switching of the analogue signals from the load cells, the digitising and storing of the switched signals, and the calculation of the correction values and their combination with the stored digitised values.

3. A system as claimed in Claim 2, wherein the microprocessor is operative to control a switch so as to switch successively the analogue output of each load cell to an analogue-todigital converter.

4. A system as claimed in Claim 3, wherein each digitised value from said analogue-to-digital converter is stored separately prior to the application by the microprocessor of the correction values.

5. A system as claimed in Claim 4, wherein the correction values are derived by following a procedure in which a known weight is used to calibrate pairs of cells so that they give identical readings independent of whether the weight is directly above or between the said pair of cells, this procedure being carried out with each pair of cells in the system.

6. A system as claimed in Claim 5, wherein the outputs of previously calibrated pairs of cells are compared.

7. A system substantially as hereinbefore described with reference to and as shown in the accompanying drawing.