Safety controller protects spot weld research

A Pilz Automation Technology product story

Edited by the Engineeringtalk editorial team Mar 30, 2004

One of the latest additions at Warwick Manufacturing Group is a spot weld research cell with safety provided by a software-configurable modular safety controller.

Based at the Engineering Department of Warwick University, the Warwick Manufacturing Group (WMG) carries out research into manufacturing control systems and processes for mass-production environments such as the automotive industry.

As such, the WMG is continually expanding and upgrading the equipment it uses for research projects, and one of the latest additions is a spot weld research cell that contains two medium-frequency resistance welders and a robotic friction spot welder.

Whenever a new cell is built, appropriate safeguards also have to be integrated; for this application, WMG chose a Pilz PNOZmulti software-configurable modular safety controller.

For the Category 4 robotic friction welder and the Category 3 resistance welders (in accordance with BS EN954-1), extensive safety controls are required.

One possibility was to expand WMG's existing networked safety system that is based on a Pilz Programmable Safety System (PSS) and a SafetyBus p safe fieldbus network, but this was felt to be overly complicated for a cell that will always operate in isolation.

A second option was to use conventional hard-wired electromechanical relays.

However, the needs of a research environment are such that changes are made far more frequently than in a manufacturing organisation.

WMG was therefore drawn to the PNOZmulti, for which the control function can be rapidly modified by adding or exchanging modules and by altering the internal logic circuits through changes in software.

Dr Ken Young, Principle Research Fellow, says: "If changes only have to be made in software, these can be completed in about 10 minutes, which is far quicker than rewiring conventional safety relays".

Despite the leap in technology compared with relays, the PNOZmulti remains extremely easy to use.

Dr Young continues: "I had about half an hour of training to use the software, which really is all that is necessary.

For peace-of-mind I did ask Pilz to verify the configuration, but only one revision was suggested.

The speed of programming and ease with which the hardware and software can be changed or reused make the PNOZmulti perfect for medium-complexity applications where periodic changes are envisaged".

Currently the cell is configured with the two resistance welders and the friction weld robot, all of which are surrounded by fixed guards with two access gates.

Each of the access gates is equipped with a Pilz PSEN 2.1p11 non-contact switch that is wired directly to the PNOZmulti.

Importantly, this combination of noncontact switches and microprocessor-based controller provides an entirely wear-free solution that will deliver many years of reliable service, regardless of the frequency with which the access gates are used.

A keyswitch is also connected to the PNOZmulti for selecting whether the cell is to be operated in fully automatic mode from outside, or manual mode from inside (either for using the resistance welders manually or for teaching the robot via its pendant control).

Emergency stop buttons are sited inside and outside the cell, and outputs from the PNOZmulti are used to disable the 'weld start' function on the welder controllers and to isolate the water pumps on the resistance welders.

Apart from the PNOZm1p base unit, the safety controller incorporates one PNOZmi1p input module and five PNOZmo2p output modules, each of which has two volt-free outputs.

Dr Young concludes: "Because the Warwick Manufacturing Group is primarily a research and education establishment our needs are very different from those in industry, especially in terms of how often we reconfigure the equipment.

Nonetheless, I can see the attraction of the PNOZmulti for industrial users who will appreciate the simplified wiring and the ability to make changes faster, easier and with a significantly reduced risk of introducing errors.

I found the PNOZmulti to be as straightforward to use as conventional safety relays, yet it offers sophisticated functionality without the need for a fully fledged programmable safety system that would, in any case, be over-the-top for most medium-complexity projects".