Perfectly printed coins thanks to machine vision

The Hamburg Mint, one of the oldest mints in Germany, has been using machine vision since the early 2000s to automate various production steps, such as aligning of coins during packaging or various quality control tasks.

In the context of another project, we were confronted with the problem that the exact positioning of the coins during the printing process was very time-consuming. In close cooperation with the customer, a machine vision system has been developed over the last 10 months to automate this process and has been successfully proving itself in daily use since May.

High quality and precise printing

The system recognises the design details of a coin relief to be coloured and sends this data to the digital printers, which then print these details accordingly. This results in more efficient production and significant time savings, as up to 250 coins can be placed on a tray simultaneously without the need for time-consuming manual alignment.

The aim of the system is to ensure printing accuracy. If the print is not aligned with the embossed relief of the coin, rejects are produced, which must be minimised at all costs. The permissible deviation in printing should not exceed 0.2 mm.

System architecture and operation

Minting unit	This is where the coins are embossed with the minting motifs (not part of the phil-vision solution)
Machine vision system	The coins are placed on a tray by hand, then first the position and rotation are recorded by four high-resolution cameras in order to subsequently determine the colourable motif details on the coin relief from the measurement data (position and rotation) and extract the information relevant for printing.
Image processing algorithm	A specially developed algorithm analyses the captured image data to identify the exact position and size of the motif details to be printed. This algorithm ensures accurate recognition even when the orientation of the coins on the tray varies.
Communication unit	The detected print information is transferred to the printer in real time to ensure the required accuracy.
Digital printer	The selected motif details are printed precisely onto the coin relief. The printing process is designed for highest accuracy and colour fidelity. In this application a Roland printer is used, software adaptations for other makes are possible at any time.
Packaging	After the printing process, the coins are automatically packaged and prepared for delivery.

The machine vision system for coin printing, pvPhotobox, not only increases efficiency and enables the simultaneous printing of up to 250 coins, the technology used also ensures extremely accurate recognition of the design details to be printed and guarantees an identical print with an accuracy of 50µm, thus minimising rejects. The system can handle a variety of coin designs and is easily adaptable to new designs.

Intensive collaboration as the key to success

Extensive preparatory work and coordination was necessary for the system to function smoothly. We attribute the high acceptance of the pvPhotobox among users primarily to the fact that we involved the operating personnel in the development right from the start. Starting with the formulation of the requirements up to the details of operation and handling, all details were intensively discussed not only with the management but also with the users.

The result:

the throughput for printing the coins has almost doubled through the use of the system.
the required accuracy of 50µm is ensured.

pvPhotobox with several digital printers at Hamburg mint

Theoretically, any number of printers can be connected to the system, however, a maximum of 8 makes sense. The limits here are only set by the time required for the manual loading of the trays. The required evaluation time is 2 seconds per coin, i.e. approx. 5 minutes are needed for a full tray with 250 coins.

Peter Steinbrück, responsible project manager at phil-vision, on the challenges of the project:

"Having worked with the Hamburg Mint on various projects in the past, I was already aware of potential problems and sources of error. In any case, we wanted to achieve the targeted printing accuracy of 50µm. For our customer, of course, this target referred to the finished product and not to our measurement results. Unfortunately, the printers used are far from being able to maintain this tolerance for an absolute dimension. The printers are too imprecise in absolute terms, but with very reproducible results, a fact we took advantage of to find an acceptable solution by including the printer tolerances in our calibration."

Guido Beckmann, Head of the Hamburg Mint, summarises the collaboration with phil-vison as follows:

"phil-vision took over the selection of the components and the programming for this project and supported us extensively during the integration. The cooperation was very professional and focused.

In every phase of the project, great emphasis was placed on precision and detail. The fast response times, the project management and the general high level of competence were also positive".

Hamburgische Münze (Hamburg Mint)

The Hamburg Mint is Germany’s oldest mint. Its history dates back to the 9th century. The first known Hamburg coin dates back to the year 834. Today, the Hamburg Mint is a legally dependent state enterprise of the Hanseatic City of Hamburg. It is supervised by the city's financial authorities. With around 40 employees, it is one of five mints in Germany that produce coins on behalf of the Federal Ministry of Finance to supply the population and trade.

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