Flexible mesh welding

 Switzerland-based Schlatter is a specialist in flexible production lines for all types of wire mesh and lattice girders. Using B&R technology for its new lattice girder production unit, Schlatter was able to transition smoothly from one-off development of highly specialised machines to series production of highly customisable standard machines. Thanks to openSAFETY, every variant shares the same level of advanced safety technology.
One might say that wire mesh is grate. It is found everywhere, from grills, ovens and refrigerators to shopping carts and store shelves. As a concrete reinforcement or a truss in a prefabricated ceiling, it makes our buildings stronger and more resilient. It is the most frequently used fencing material for demarcating private property or construction sites or restricting access to hazardous areas for industrial safety.
Mesh welders – Made in SwitzerlandSchlatter is a specialist in developing the welding and weaving machinery used to produce wire mesh, screens and girders. The Swiss company has achieved its current size and market position through its high level of expertise in two key areas. With its mastery of mid-frequency electrical resistance welding, Schlatter creates seamless connections between railroad tracks and freight truck axles, while expertise in the field of motion control allows it to provide sophisticated coordination of the wire feed and electrode movement required for precision welding.
From specialised to standardisedWhen it came time to develop its new production unit – which creates the lattice girders that stabilise concrete elements used in prefabricated construction – Schlatter adopted an entirely new strategy. “In the past, each Schlatter machine was developed individually for a specific customer,” explains technical manager Beat Huber. “Our goal now is to move toward more standardised production. To do this without compromising the flexibility that is so important to our users, we gave the CTM310 a highly modular design.” The new solution unwinds wire with a diameter of 5-16 mm from multiple winding tables holding three-ton or five-ton coils and feeds a belt of four wires into the welding unit, where they are bent and welded. At the final station, the finished girders are cut to length, stacked and bundled for shipping.
Focus on motion controlOne of the system’s most impressive features is its ability to rapidly accommodate variable wire gauge and girder height. This flexibility allows it to produce girders with specific dimensions in small batches – down to batch size one – while maintaining a high overall output rate. With production speeds up to 18 metres per minute, the quality of the final product relies on synchronising the movements of the various wires, the hydraulic perforating press and electrode holder. The significance of motion control is reflected in the solution’s total of 30 servo motor axes, 16 of them in the welding unit alone. 
“One of our primary development goals was to harmonise the integration of hydraulics and electronics with an intelligent control system,” recalls electrical engineering manager René Frey. One of the reasons for this was that Schlatter was looking to let go of old traditions in the selection of an automation system for this new generation. “What impressed us about B&R was the level of integration between control, HMI and motion, as well as the quality of synchronisation we could achieve between the many axes using POWERLINK. Another decisive factor was the possibility of implementing a modular safety solution with very little cabling.”
All in one, safety tooHigher-level control tasks are handled by an Automation Panel 920 with a 15” XGA color TFT touch screen mounted in the control cabinet of the central welding unit. The inputs and outputs are controlled by distributed X20 stations on each machine. The servo motors are controlled in pairs by ACOPOSmulti drives featuring SafeMOTION.  These provide safe reactions in the event a safety door is opened or a light curtain is tripped on the stacking unit. 
“The safety barriers around the system are divided into sectors. That way, the stacking unit can continue to operate if there is a violation in the wire feed sector,” explains Frey. The safety application runs on a central SafeLOGIC controller, where the signals from over 30 safe I/O modules come together via the openSAFETY protocol. Schlatter was able to use the same safety sensors and door locking mechanisms that had proven themselves in earlier generations. “Since integrating preconfigured machine modules in the central safety controller is simply a matter of hooking up an Ethernet cable, we now spend less time running cables and a whole lot less time on tedious troubleshooting.” In the event that a new option needs to be added to an existing machine, it is no longer necessary to shut down the the whole line to perform the wiring.
Accelerated restartThe most-used reaction to safety events is Safe Torque Off (STO), which is included in the portfolio of Smart Safe Reaction options offered on the ACOPOSmulti drives. “On a fully-equipped line, we’re operating close to the maximum number of nodes permitted for one SafeLOGIC controller,” says Fabio Giacomini, the software engineer responsible for both the automation software and the safety application.  He is particularly happy to be able to develop the safety application in the SafeDESIGNER editor, within the familiar environment of Automation Studio, and also appreciates the ability to communicate between the standard controller and the SafeLOGIC safety controller. 
This allows the automation software to react to safety events, for example by stopping the wire feed when there is an alarm in a downstream unit. And the potential for system-wide control goes much further than that. “We use a special trick to get started back up even faster after stopping for a safety event,” says Giacomini. “Before the safety reaction – which we have intentionally delayed – takes effect, the standard controller shuts down the system. That allows the machine to be started up much more quickly since all we have to do is acknowledge the safety-related intervention.”
 Flexible thanks to openSAFETY Implementation of openSAFETY was key to giving the new Schlatter machines their exceptional level of modularity. Beyond the drastic reduction in cabling, which in itself was a huge step, modularity was gained through the more advanced diagnostics options and the ability to update the safety application on the SafeLOGIC via remote maintenance, independently of the automation software. “One of Schlatter’s greatest strengths is the partnership we offer our customers. Our systems have a service life measured in decades, so effective cooperation and the ability to expand and replace equipment down the road are especially important to us,” says Frey. “Being able to replace a defective component without having to manually reset parameters – let alone modify the safety application – makes maintenance much easier for the end user.”

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