Polymer note fitness detector

 

 

By John Colditz, Head of Note Issue, RBA
 

Paper presented at the XIII Pacific Rim Banknote Printers' Conference, India; December 1997. 

 

Introduction

 

All of Australia's banknotes are now produced on polymer (plastic) substrate instead of paper substrate. The change was introduced through the release of a new series of notes, the first of which was released in 1992. Other notes followed at roughly annual intervals culminating with the release of the $100 note in May 1996. Public reaction to the new series of notes is very positive, and we are surprised at just how beneficial the notes are to cash handlers generally.

 

The move away from paper substrate was prompted by a desire to improve the security of Australia's banknotes. In addition to improved security, polymer notes are proving to be significantly more durable than paper notes. In Australia, we have experienced a quadrupling of the average life of our low denomination notes. Our higher denomination polymer notes have not been in circulation long enough to be precise about their longevity, but indications are that we will see similarly impressive performance. And, as indicated, we've got a lot of other benefits as well.

 

The increased durability of polymer notes is a consequence of various factors, including:

  • the overcoating of finished notes with a clear varnish plus the non-porous and non- fibrous nature of the substrate, which mean that the notes do not absorb moisture (oils, sweat, beverages, etc) like paper notes. These same properties also mean that the notes do not stain or accumulate dirt as easily as paper notes do;

  • the non-fibrous nature of the polymer substrate, which means that the substrate does not physically breakdown with repeated folding, as occurs with paper notes which, in part, causes paper notes to go limp;

  • the toughness of the polymer substrate, which makes it much more difficult to initiate a tear in a polymer note compared to a paper note (although it is true that once a tear is initiated in a polymer note it propagates more easily than in a paper note, the initial toughness appears to be the overriding characteristic).

The different wear characteristics of polymer notes mean that the criteria that are used in manual or machine sorting of polymer notes are different in some ways to those that are used for paper notes. This paper discusses this issue and outlines the development of a new fitness detector for polymer notes.

 

Extended experience from the new series of polymer banknotes

 

Some of Australia's new series of polymer notes have now been in circulation for over 5 years. Our experience with how polymer notes wear in circulation is, therefore, now quite extensive.

 

The main results are:

  • polymer notes are much more durable than paper notes;

  • polymer notes do not absorb moisture;

  • polymer notes do not soil like paper notes;

  • polymer notes do not go limp like paper notes;

  • polymer notes develop similar mechanical faults to paper notes. (Mechanical faults include tears (open, closed), tape, staples, corner folds, missing corners, holes, size variations.) Interestingly, the incidence of tears was even lower than anticipated, but, because of the tear propagation characteristics, a greater percentage of polymer notes with tears had sticky tape applied as compared to paper notes;

  • because the polymer substrate is so robust, polymer notes eventually (after around four times the life of an equivalent paper note) develop some ink wear down crease lines due to abrasion.

During their long life, polymer notes experience folding and crumpling many times. Eventually, the cumulative effect of abrasion along fold and crumple lines causes ink to wear. Initially, this is very minor and difficult to see as it usually involves ink being worn away in very thin lines. Eventually, after very extended use in circulation, these areas of wear widen and deepen and become more noticeable. Because major fold lines tend to be concentrated around the centre of the note, ink wear also tends to be concentrated in this area as well. This ink wear has thus become an additional important fitness sorting criteria for both manual and machine processing.

 

It is important to remember that the new series notes which have been in circulation for the longest time are lasting at least four times longer than their paper equivalents were before wear suggests that it could be taken out of circulation. We hear occasionally that some paper manufacturers and note printers are actively discouraging central bank note issuers and printers from switching to polymer because, among other things, they suggest that polymer notes are not as durable as we are suggesting or that paper notes can be made to last as long or at least considerably longer than experience to date suggests so as to make the cost effectiveness of polymer notes less attractive. If that is so, we suppose we can understand their position, bearing in mind that orders upon them stand the prospect of significant reduction by a move to polymer. But actually they are distorting the truth considerably and any suggestion by them that polymer does not perform well is totally incorrect.

 

A new fitness detector

 

When polymer notes were first introduced, the Bank's existing fitness detectors consisted of detectors to identify mechanical faults and soiling. These detectors could not be easily adapted to detect the ink wear that develops eventually on polymer notes. (Suppliers of existing detectors for high speed note sorters believe that some existing detectors can be modified to pick up such wear.) In the event, the Bank decided to build a new fitness detector for ink wear. The intention was that the fitness detector for ink wear would supplement the existing fitness detectors for mechanical faults so that in combination, these detectors would be used to outsort any unfit polymer notes.

 

The Bank envisaged that the new fitness detector would:

  • categorise (in a consistent and repeatable manner) polymer notes into various fitness levels according to the level of wear on the note;

  • fit compactly within the Bank's existing CVCS systems but be capable of being easily integrated into any other high speed note sorter;

  • be compatible with all other detectors and software; and

  • provide statistical data on the number of notes classified into the various levels of wear.

The Bank initially sought expressions of interest from parties interested in supplying a detector to meet its needs. TechComm Group Limited, an Australian company and the Bank's present CVCS maintenance provider, was chosen to proceed with development.

 

Detecting the ink wear discussed above is not necessarily easy on a high speed note sorter. The ink wear is most obvious in transmitted light - when the note is held up to the light, the ink wear is seen as highlights against a dark background due to the decreased opacity of the note in the areas of ink wear. This, in fact, became the basis for the approach taken to build the new fitness detector. As the note travels along the machine transport path it is backlit. A camera detects the amount of light transmission which is analysed at high speed in a PC to determine the amount of abnormal light transmission which is a measure of the extent of ink wear. The detector can be set at different threshold levels to allow different quality standards to be implemented if desired.

 

The area of the note examined for ink wear measures 50 mm by 32 mm which consists of 250 000 pixels, each of which has a possible grey scale measurement of between 0 to 255. The portrait area on the front of each note was chosen as a suitable region for analysis due to it being centrally located on the note and where a high incidence of folding and creasing occurs thus causing exaggerated wear.

 

Development of the detector was largely based around Australia's polymer $10 note. It was well into the third year of circulation (which compares to the average life of a paper $10 note of 8 months) that the first significant signs of ink wear started to be observed on the polymer $10 note. With further extended use a wider sample of notes with more extensive ink wear became available.

 

A prototype detector was installed on a CVCS system in Sydney in late 1996 for testing and evaluation. After extensive on-site testing, evaluation and improvements, the Bank commissioned the manufacture of a further nine detectors for installation on CVCS systems around Australia.

 

By March 1997, the Bank had detectors installed in its branches around the country - one each in Adelaide, Brisbane, Canberra, Hobart and Perth and 2 in each of Melbourne and Sydney branches. The detectors have now been in operation for around seven months and are performing satisfactorily.

 

The new fitness detector also provides a statistical report database which summarises the number of notes classified into different levels during a processing shift. Provision has also been made for boundaries of the classification levels (currently 7) to be changed. This allows a tightening or loosening of the classification of notes at any point along the range - a feature which has already proved to be very useful.

 

The new fitness detector is a third party detector which only interacts with the CVCS system's software at the highest level to enable the detector to communicate a yes/no signal to the CVCS system and to be activated by operators during the normal system initialisation process. This, hopefully, facilitates portability of the detector from one type of processing system to another.

 

General performance of the detector

 

The new fitness detector works remarkably well in terms of being able to differentiate between notes with little evidence of ink wear and those with moderate wear and those with severe wear. It has some limitations in respect of its ability to differentiate fine degrees of wear within broader categories. We believe, however, that the detector's ability to identify moderate and severe wear is appropriate and is not dissimilar to the performance of the soil fitness detector used previously by the Bank to detect soiling of paper notes. We have also been impressed by the high level of repeatability demonstrated by the detector.

 

Our experience to date has been that with the current range of qualities of notes in circulation, around 4% of polymer $10 notes returned to the Bank are sufficiently worn to be classified by the Detector at level 7 (the most worn notes). A further 5% to 6% of notes are distributed amongst levels 5 and 6 with the remainder falling into the lower levels. Around 70% of polymer $10 notes processed are classified at level 1.

 

Maintenance

 

The new fitness detector requires little attention from field engineers in terms of maintenance. Once the detector has been calibrated, it may be days before re-calibration needs to occur. Naturally, general maintenance such as cleaning of the camera lens needs to take place at least daily, however, this is not an onerous or time consuming task.

 

Conclusion

 

The new fitness detector meets the original requirements stipulated by the Bank. It is able to categorise notes into different levels according to wear; it is highly repeatable; it fits neatly into the CVCS system from both a hardware and software configuration point of view; it is compatible with existing detectors and software and statistical reports are available for analysis on the number of notes being classified into different ink wear levels.

 

As indicated previously, the new fitness detector is considered to be similar in performance to the Bank's previous paper note Soil Fitness Detector and, with this as a benchmark, we are more than satisfied with our latest acquisition for the CVCS systems.

 

We wanted to share this development with you, because as with so much of our work with polymer notes we have been at the forefront and sailing in uncharted waters, in this case grappling with the pleasing "problem" that polymer notes do not soil.

 

As you well know, polymer notes improve durability.

 

Thank you for your attention.

 

 

 

 

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