Screen printing machines benefit from motion control system

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Mechanical drive systems are increasingly reaching the limits of their capabilities, but switching to electronic alternatives may require a partial or even complete redesign of the machine. One of the world's largest suppliers of printing machines has faced this dilemma, but a Mitsubishi Melsec Q Series motion controller solved the problems.

With its markets rapidly globalising, Thieme, based in the German city of Teningen, needed to re-invent its screen printing machine to combine exceptional production capacity with minimum labour overheads. One of the biggest challenges was the large-scale printing format of up to 2000mm x 3050mm.

When the engineers set to work they soon realised that with this format many of the components used in previous systems, such as the central drive system and the mechanical line shaft, would simply be too big. It just would not have been possible to integrate them into the machine design properly. In addition to this, the machine would not have been compatible with the standardised heights of the additional modules such as automated feeders and stackers.

After analysing a number of alternatives it became clear that the combination of a motion controller with servo drives would be the best option.

One of the key requirements was that it should be possible to translate the existing mechanical arrangement as precisely as possible to the new system – both to keep development overheads low and to avoid losing the years of development invested in the tried-and-tested movement sequences. To solve the problem, the Thieme team chose a Melsec Q Series motion controller from Mitsubishi Electric, which met and exceeded this requirement.

The Q Series programming language can implement existing mechanical systems with graphical representations. In addition to this it combines motion control, PLC and industrial PC modules on a single platform, which means that it can also handle the other machine control requirements as well as the motion control. This concept eliminates the need for duplicate system racks, power supply units and other components.

Another advantage of the fully-integrated automation platform is that the operators have complete and easy access to all the PLC and motion controller data. Every CPU in the system has access to all data via the common backplane bus, which means that there is no longer any need for time-consuming additional communications programming. And this architecture also eliminates the need for a network connection between the two controllers.

Virtual programming

The new system replaces the former main drive with a virtual servomotor connected to a virtual line shaft programmed into the motion software. The 30 axes of six-colour can be placed in their proper positions with a simple drag-and-drop operation. In addition to simple axes the system can also create virtual transmissions, roller feeders, linear axes, cam discs, etc. This makes it possible to program drive systems to perform complex movements that are precisely synchronised with one another.

In contrast to the mechanical systems, in which the mechanical connections could only be interrupted with additional couplings, it is now possible to operate individual axes independently of the rest of the machine. For example, in the past the Thieme printing unit had two servo-driven lifting drives which were synchronised by a line shaft. The shaft is now replaced by a virtual equivalent which can be 'disconnected' via an addressable coupling during the actual printing process so that other machine processes can continue during the printing sequence, thereby reducing the overall machine cycle time. Upon reconnection the two servomotors are always perfectly synchronised.

Operation of individual axes without line shaft synchronisation is also necessary for installation, maintenance and repairs – in the past the only way to do this was by physically uncoupling the mechanical axes, which was a time-consuming process. With the motion controller, it is only necessary to select a setup mode and then all the servo drives can be operated independently of the virtual line shaft - which cuts the time overhead to a few minutes of programming.

The use of virtual cam discs also helps to make the machines more flexible and improve the cost-benefit ratio. Optimised motion profiles for different printing materials can now be selected at the touch of a button. In the past this optimisation was not possible at all because it would have been too expensive to build and install new cam discs every time the printing material was changed.

Greater flexibility

The new system has also brought significant improvements elsewhere, not just for the components involved in motion control. Thanks to the electronic cam controller all the other functions, such as activating the vacuum fields, can also be adjusted much more flexibly.

The deployment of the Mitsubishi motion controller made it possible to add a wide range of valuable additional functions throughout the machine, including a single printing function for the individual printing units, straightforward teaching functions (for example for making adjustments for different printing formats) and a significantly more capable setup function for switching to different printing screens.

The integration capabilities of the Q Series were not the only reason for choosing the motion control system; Mitsubishi's ability to deliver a complete solution - including the drives - was equally important. One of the specifications was that the drives used had to have very compact servo amplifiers, among other things, because the engineers wanted to integrate the formerly separate switchgear cabinet in the machine bed, where space was naturally very limited. This made it possible to reduce the footprint size of the machine.

Another advantage was that the new arrangement eliminated the time-consuming rewiring that was previously necessary because the switchgear cabinet had to be moved every time the printing system was modified or expanded. A third advantage of the servo drives chosen is that they come with absolute position encoders as standard equipment, which makes it possible to continue printing immediately after emergency stops and other interruptions because the drives automatically register their positions as soon as the power returns.

The fact that it was an integrated system from a single manufacturer was particularly helpful when it came to designing the machine's operation concept. The full integration of the system enables easy access to all system components from every control console. For example, operators can program all squeegee pressure functions directly at the control console, eliminating the need for time-consuming manual adjustments.

04 September 2006

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