Author Topic: How do vending machines recognise coins?  (Read 79 times)

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Offline <k>

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How do vending machines recognise coins?
« on: May 10, 2017, 11:10:13 AM »
I'm not sure if this is in the appropriate board, but here goes.

Back in the early 1980s, the UK decided it needed to introduce two new coins: the 20 pence and the round pound. Because of the limited number of available size "slots", both coins would have to be 22 mm in diameter. The vending machine manufacturers explained that they needed a minimum gap of 1 mm between different coins, so the two coins ended up at 21.4 mm (20p) and 22.5 mm (1), IIRC. I wonder, does the same apply nowadays, that the machines need a minimum 1 mm gap in size between different coins?

The pound coin was very thick at 3.15 mm, to enable the blind to distinguish it from coins of a similar size. At the time, the vending machine manufacturers could not handle such thick coins, but they eventually agreed to develop the technology to do it.

In 1979, when I was in Germany, it was known that the old UK 5 pence coin (the large version) was the same size as the 1 Mark coin, which was worth around 25 pence at that time. So, it was possible to use the 5p in vending machine and get a bargain, such as a 500 g bar of chocolate for 5p or a packet of cigarettes for 15p. Into 1980, the vending machines started rejecting 5p coins. How did they tell the difference? One difference was that the 5p coin was milled, whereas the 1 Mark coin had a smooth edge. But whether that was how they did it, I don't know. Notice that the British had said they needed a 1 mm gap between coins to recognise them.

So, how do the machines recognise coins nowadays? Size, shape, weight, edge type? Or a mixture of these? Can a machine detect a raised rim, such as on the UK 20p? And of course, the coin's "electronic signature" is usually mentioned.

Anyway, I just found this on Wikipedia:

Coin acceptors

The basic principle for coin detection is to test the physical properties of the coin against known characteristics of acceptable coins. The coin acceptor analyses the coin according to its weight, size, metal composition and/or magnetism, and then sends an appropriate electrical signal via its output connection. The next step is generally performed by the banknote-to-coins exchanger.

Today, sophisticated electronic coin acceptors are in use in some places that, in addition to validating weight and size, also scan the deposited coin using optics and match the image to a pre-defined list, or test the coin's "metallic signature" based on its alloy composition.

Normal circulation coins eventually collect microscopic deposits of dirt, oil and grease from people's fingers. When a coin acceptor is used long enough, thousands of coins rolling along a track will leave enough dirt, oil and grease to be visible. For this reason, the coin acceptor must be cleaned on a regular basis to avoid malfunction. Coin acceptors are modular, so a dirty acceptor can be replaced with a clean unit, preventing downtime. The old unit is then cleaned and refurbished.

Some new types of coin acceptors are able to recognize the coins through "training", so they will support any new types of coins or tokens when properly introduced.