GB2242280A - A method and device for weighing out the constituents of a mixture - Google Patents

Abstract

The device (10) e.g. for weighing out pigments for printing, paints or for the constituents for alloys includes a weighing platform (12) supported by a load cell (14), a circuit (16), an input device (22) and a display (18). In use, a first constituent a desired mixture is weighed on the platform (12) and its proportion of the desired mixture is input by use of the input device (22) and display (18). Second and subsequent constituents are then placed on the platform (12) and their proportions relative to the first quantity are adjusted by reference to the display (18), which indicates their proportions in relation to the total desired mixture. <IMAGE>

Description

METHOD AND DEVICE FOR WEIGHING-OUT SUBSTANCES This invention relates to a method and device for weighing-out substances and, in particular, relates to a method and device suitable for use in weighing-out the ingredients of a mixture prior to mixing. An important application is in the mixing of different pigments in various proportions to produce inks, dyes, paints or the like.

The human eye can resolve minute differences of colour or shade, which places considerable demands upon the skill of printers when mixing pigments to produce inks. Even a small error in measuring out a constituent pigment could lead to a visible discrepancy between the desired colour and the colour that is actually produced. Such a discrepancy is particularly serious where one wishes to match an existing colour.

Systems have been devised to assist the printer in reproducing desired colours on a consistent basis. A well-known example, now something of an industry standard, is the PANTONE (registered trade mark) colour matching system. In this system, a range of standard colours are displayed on sample cards, each colour being uniquely identified by a code number.

Each colour may be reproduced by mixing a plurality of standard pigments in proportions specified on the relevant card. Thus, for example, PANTONE 297C is a light blue consisting of 3 1/4 parts of PANTONE Process Blue, 3/4 part of PANTONE Reflex Blue, and 28 parts of PANTONE White. These proportions are also expressed on the card as percentages of the total mixture, being in this case 10.2%, 2.38 and 87.5% respectively.

Even with the benefit of the PANTONE system, printers still have to determine the proportions of pigments prior to mixing. The most common method, still in use today after many years, is to dip a knife into a quantity of pigment and then to draw the knife across a clean, flat surface such as a sheet of glass, thereby depositing a uniform strip of pigment on the surface. The pigment is generally semi-solid or thixotropic and is therefore suited to this method of handling. The process is repeated for the other pigments that make up the desired colour so that a plurality of strips are placed beside each other on the surface. As will be clear, the relative lengths of the strips determine the proportions of the pigments.

When the printer is reasonably satisfied that the relative lengths of the strips are correct, the strips are smeared together and the pigments are thoroughly mixed. The printer then uses the resulting ink in a test print run and compares the printed colour with the PANTONE sample, if necessary mixing additional quantities of one or more pigments into the ink in order to produce a satisfactory result.

Although experienced printers can achieve remarkable accuracy by use of the above method, there are several disadvantages. For instance, the printer's skill in matching colours may be sorely tested where the colour or shade of the ink will be affected by the characteristics of the paper to which it is applied.

Moreover, the frequent need to add pigments after initial mixing leads to waste because the printer can end up with a much greater quantity of ink then is needed for a particular job. Once mixed, inks do not store particularly well because they develop a skin and, in any event, there is no guarantee that a particular ink, once stored, will be needed again.

Thus, any excess ink is usually discarded at the end of a job.

Whilst standard pigments are not particuarly expensive, waste is best avoided in any field, particularly with a view to preserving the environment. The printer's valuable time in making fine adjustments to the mix is of more concern, as is the need to pay the printer highly for his skill. The annoyance and frustration experienced by the printer in repeatedly trying to achieve a good colour match are also to be minimised if possible.

It is of course well known to use a weighing device to weigh out the ingredients of a mixture, perhaps the best-known example being the use of domestic scales when following a recipe in the kitchen. Weighing devices are also used for similar purposes in non-domestic environments, an example being in the field of chemistry where it is common to weigh out reagents, or the constituents of a mixture.

At first sight, the principle of weighing-out pigments before they are mixed together is a solution to many of the problems suffered by the visual method described above. Indeed, the weighing-out principle is known in the printing industry, with some printers using ink-weighing scales for the purpose of producing inks. However, ink-weighing scales are far from ideal because the printer must translate the proportions expressed in the PANTONE system to the quantities of pigments that are being weighed out. This involves repeated calculations which are often too complex to be entrusted to mental arithmetic, and which are very time-consuming even if performed on a calculator.

Moreover, known ink-weighing scales require that quantities of different pigments are weighed one after another, which deprives the printer of the direct visual comparison between quantities of pigments to which he or she is accustomed. Even if ink-weighing scales are used, the printer may still prefer to make final adjustments by eye as before, despite the waste of material and time that may ensue.

The result of these various problems is that ink-weighing scales have found little favour in the printing industry for the purpose of mixing pigments to produce inks.

In my efforts to overcome the problems of the prior art in the printing industry, I have devised what I believe to be a novel principle of weighing. When mixing pigments to produce an ink, this principle involves weighing a first quantity of a pigment using a novel weighing device, programming the weighing device with data relating to the proportion of the desired ink mixture which that first quantity represents, and adding second or subsequent quantities of one or more further pigments, wherein the weighing device uses the relationship between the weight of the first quantity and its proportion of the desired ink mixture to assist in adding the amount of each further pigment that is required to produce the desired ink mixture.

I have found it best to provide the weighing device with a visual display calibrated in units of proportion (such as percentages) and an input device arranged to program the weighing device with data expressed in the same units. Thus, when the first quantity of pigment has been weighed, a user can program the weighing machine with the percentage proportion of the first quantity in the desired ink mixture. It is preferred that this figure also appears on the display. Then, when the second quantity of pigment is added, the display shows the weight of that second quantity also expressed as a percentage proportion of the final mix. In this respect, the display can start again from zero, thereby showing the weight of the second quantity per se, or can show a cumulative total weight of the first and second quantities.Thus, by watching the display whilst progressively building up the second quantity, the user can add precisely the amount of further pigment that is required. The procedure is simply repeated for any subsequent quantities of pigment needed to make up the desired ink mixture.

As will be clear to those skilled in the art, the principle of this invention provides many advantages.

Most importantly, waste of time and patience (and, of course, waste of pigments) is minimised because the desired ink mixture can be attained quickly and without difficulty. This is because the weighing device can be made accurate enough to obviate repeated adjustments to the mixture. Moreover, the accuracy with which the desired mixture can be attained leads to greater consistency between ostensibly identical ink mixtures produced on different occasions. Another benefit of the invention is that the various quantities of pigments can be placed side-by-side on a weighing surface before mixing, thereby giving the printer a direct visual indication of the relative proportions of the pigments. This is convenient for quickly judging the approximate proportions involved, and also provides a link with the 'strip' method described above, with which most printers are familiar.

Although my invention was devised primarily to solve the problems suffered by the printing industry, it is clear that the invention could be useful in any field in which the constituents of a mixture are expressed in terms of percentages or other proportions of the mixture. For example, one could use the invention to mix paints accurately, and one could also use the invention in chemistry in order to weigh out the constituents of alloys. Thus, although my invention has special advantages in the context of the printing industry, I expect that the invention will find wider application in due course. It will be understood, therefore, that the broadest concept of this invention is not limited solely to methods or devices associated with the printing industry.

In order that this invention may be more readily understood, reference will now be made to the accompanying drawings, in which: Figure 1 is a block diagram illustrating an embodiment of the invention; and Figure 2 is a circuit diagram showing a circuit for use in the embodiment of Figure 1.

Referring firstly to Figure 1 of the drawings, a weighing device 10 comprises a flat weighing platform 12 supported by a load cell 14. The load cell 14 is connected to a circuit 16, which will be described in more detail with reference to Figure 2 of the drawings. Also connected to circuit 16 is a display 18, a zeroing control 20, and an input device 22. The input device 22 is controlled by a rotary knob 24.

The operation of the weighing device 10 will now be described in general terms, in relation to the preparation of inks in the printing industry.

As the first step, an operator zeroes the display 18 by use of the zeroing control 20 and then places a quantity of a first pigment onto the weighing platform 12. Using the example of PANTONE 297C from the introduction, this first pigment could be any of the three constituents, say PANTONE White. The weight of this quantity of PANTONE White is sensed by the load cell 14. The operator then uses the input device 22 to tell the weighing device 10. what proportion of the desired mixture is constituted by PANTONE White. He or she does this by reading the required percentage of PANTONE White from the PANTONE 297C card, and then turning the knob 24. The circuit 16 is arranged such that the display 18 indicates the percentage selected by use of the knob 24 which, in this case, should be 87.5. Once set, the knob 24 is not moved again until mixing has been completed.

The next step is to add the second pigment to the first. Continuing with the example, this second pigment can be either of the remaining constituents, say PANTONE Process Blue. Thus, a quantity of PANTONE Process Blue is placed onto the weighing platform 12 beside the PANTONE White. The operator can use visual judgement to obtain approximately the correct proportion between the two pigments. The proportions are then made exactly correct by viewing the display 18, which expresses the cumulative weight of the PANTONE White and the PANTONE Process Blue as a percentage of the total desired mixture.The correct proportions can be obtained simply by making small adjustments to the amount of the PANTONE Process Blue until the figure '97.7' appears in the display, this being the sum of 87.5* and 10.2% (the desired proportions of the first and second pigments respectively).

The process is repeated for the third pigment, PANTONE Reflex Blue, a quantity of which is placed onto the weighing platform 12 beside the first and second pigments. The quantity of this third pigment is then adjusted until the figure '100.0' appears in the display. At this stage, the three pigments are in the correct proportions and can be mixed together to produce PANTONE 297C.

It is envisaged that the pigments can be mixed together on the weighing platform 12 whilst the weighing platform 12 remains supported by the load cell 14. Thus, it is preferred that the load cell 14 is protected against possible overloading during mixing, for instance by an overload protection facility built in to the load cell 14, or by stop means arranged to limit the depression of the weighing platform 12.

It will be noted that there is no need to display the absolute weight of any of the pigments. It is the proportions of the pigments that are significant.

However, it is feasible to include a display of weight if need be, for instance to act as a guide to the final amount of mixture that will be produced. It is also feasible to zero the display before each pigment is added, thereby to display the proportion of each pigment in terms that correspond directly to the figures on the PANTONE card.

The layout of circuit 16 will now be described in greater detail, with reference to Figure 2 of the drawings.

In Figure 2, the weighing platform 12 is marked in dashed lines. The load cell 14 of Figure 1 is represented by the bridge network appearing within the dashed lines.

Power supply to the bridge network is achieved as follows. Z1 is a 1.26V band-gap reference I.C..

Resistors R1 and R2, op-amp Al and transistor TR1 co-operate to provide a +5V stabilised supply to point A of the network. Point A is connected to a resiStor chain R4, R5 to provide a +5V reference voltage thereto. Resistors R4, R5, op-amp A2 and transistor TR2 co-operate to provide a -5V supply to point B of the network.

The bridge outputs appear at arms C and D and are amplified by op-amps A3 to A7. These op-amps are of the high quality laser-trimmed type in order to minimise drift and to maximise stability. As the bridge network operates symmetrically about ground, single-ended amplification can be used to buffer and to amplify each of the two bridge arm outputs.

Op-amp A3 has a gain of +83.6 and provides a positive signal proportional to the signal at arm D of the bridge netowrk.

Op-amp A4, whose gain is adjustable over approximately +/- 8% by means of variable resistor R7, amplifies and buffers the negative signal from arm C of the bridge network. Gain difference error through the two signal paths can be eliminated by adjustment of resistor R7.

Op-amp A5 has a gain of -1 +/- 2% due to resistors R11 and R12.

Op-amp A6 is a summing amplifier whose output is proportional to the sum of the two bridge arm outputs and a signal from variable resistor R13 (of opposite polarity). Resistor R13 can be adjusted to zero the circuit, being operable by the zeroing control 20.

Op-amp A7 has a gain adjustable from 0.98 to 50, a gain range of approximately 50:1. By adjustment of this gain using resistor R20, it is possible to alter the effective scale range of the display 18 (a digital panel meter) such that any weight between approximately 100g and 2000g may be expressed as a reading of 10 to 100. These figures represent percentages. In order to measure smaller quantities, the sensitivity of the circuit can be increased by a factor of 10 simply by opening switch S2.

From consideration of the circuit and the foregoing general description, it will be clear that adjustment of resistors R13 and R20 is central to the operation of this embodiment. Resistor R13 zeroes the display before the first pigment is added. Resistor R20 (which is part of the input device 22, connected to the knob 24) allows the operator to input the proportion of the total desired mixture that the first pigment represents. As the knob 24 is then left untouched, resistor R20 in effect memorises the relationship between the weight of the first pigment and its proportion in the desired mixture. The circuit then uses that relationship to express the proportion of each further pigment in the desired mixture, by measuring the additional weight of that pigment.

Many variations are possible without departing from the scope of this invention. For example, given a sufficiently large production run, it would be straightforward and economical to condense the various circuit components into a single integrated circuit.

It would also be possible to design a circuit which gives the same results as the circuit described above, but which is based upon digital rather than analogue processing. In a digital circuit, the knob 24 could conveniently be replaced by a keypad allowing direct input of numerical proportions.

Claims (20)

1. A weighing device including a weight sensor for measuring the weights of first and subsequent constituents of a desired mixture, a circuit connected to the weight sensor to receive signals therefrom, an input device for inputting data to the circuit representing the proportion of the first constituent in the desired mixture, and indicator means connected to the circuit, wherein the circuit is arranged to employ said data in such a way that the weights of the second or subsequent constituents are represented by the indicator means as proportions of the total desired mixture.

2. A weighing device according to claim 1, wherein the weight of each of the second or subsequent constituents is represented cumulatively with the weight of the preceding constituent or constituents.

3. A weighing device according to claim 1 or claim 2, wherein the circuit includes amplifier means whose gain can be varied by the input device.

4. A weighing device according to any preceding claim, wherein the indicator means is a meter with a numeric display.

5. A weighing device according to claim 4 and arranged such that the meter reads in percentages.

6. A weighing device according to any preceding claim, wherein the circuit includes zeroing means.

7. A weighing device according to any preceding claim, wherein the weight sensor includes a load cell.

8. A weighing device according to any preceding claim and having a flat weighing platform.

9. A weighing device according to claim 8, wherein the weighing platform is removable from the weight sensor.

10. A method of weighing the constituents of a desired mixture, comprising weighing a first constituent upon a weighing device, programming the weighing device with data representing the proportion of the first constituent in the desired mixture, weighing a second or subsequent constituent upon the weighing device, and using said data to indicate the weight of the second or subsequent constituents as proportions of the desired mixture.

11. A method according to claim 10, wherein the weighing device is zeroed before the first constituent is weighed.

12. A method according to claim 10 or claim 11, wherein the weight of each of the second or subsequent constituents is indicated cumulatively with the weight of the preceding constituent or constituents.

13. A method according to any of claims 10 to 12, wherein each of the second or subsequent constituents is t placed beside the preceding constituent or constituents on a single weighing platform.

14. A method according to claim 13, wherein the constituents are mixed together on the weighing platform when the desired mixture is obtained.

15. A method according to any of claims 12 to 14, wherein the quantity of each of the second or subsequent constituents is adjusted to obtain a desired proportion of the desired mixture and is then kept constant during addition of any further constituents.

16. A method according to any of claims 10 to 15, wherein the weights are indicated on a meter with a numeric display.

17. A method according to claim 16, wherein the weights are indicated as percentages.

18. A method according to any of claims 10 to 17, wherein the constituents are pigments and the desired mixture is an ink containing said pigments.

19. A weighing device, substantially as hereinbefore described with reference to, and as illustrated in, the accompanying drawings.

20. A method of weighing-out the constituents of a desired mixture, substantially as hereinbefore described.