Servo-pneumatic systems for spot welding guns are becoming a more attractive alternative for auto body fabrication. Festo discusses the application of this technology.

Increasing standardisation of production processes, along with the demand for improved quality and greater flexibility, has stimulated the market for controlled, integrated systems of this kind. Controlled spot welding is increasingly being used as production lines become more automated, and Festo sees definite growth potential for servo-pneumatic technology.

4,000 spot welds
Inspite of the increasing use of non-ferrous metals and composite materials, the joining of sheet metal components remains one of the most important work steps in auto body fabrication. The spot welding process is still the predominant joining process for these parts, and depending on body type and combination of joining technologies, thousands of spot welds are required. These are now executed almost entirely by robot-based, controlled welding guns in highly automated production lines, which have supplanted simple, strictly pneumatic solutions in recent years.

With thousands of spot welds per auto body, the need for reducing costs and assuring quality in vehicle manufacturing is evident. Every spot weld has to be safely and reliably completed, without taking too long and OEMs are continuously looking for suitable solutions to minimise system downtime and improving weld quality.

Three solutions on the market
Three different technologies are currently being used as control systems for robot welding guns. Standard pneumatics is the conventional technology. It’s inexpensive, but it has several technical disadvantages. For example, uncontrolled banging of the electrodes against the sheet metal components generates vibration which delays the beginning of the welding process, and reduces the service life of the electrodes. Controlled systems (servoelectric or servopneumatic) offer some advantages. Cycle times and the quality of the spot welds are both optimised by approaching the welding spots in a position controlled fashion and by constant force regulation during the actual welding process.

Where controlled systems are used, the electrodes no longer bang against the sheet metal and this allows the welding process to begin earlier. Applying a constant force for the duration of the welding process (aprox 200 milliseconds) ensures ideal spot weld diameters and welding quality.

Complete system

The following components represent a servopneumatic system:
Drive cylinder with integrated displacement encoder, pressure switches and additional functions (air-saving/safety function) Welding gun controller with fi eldbus interface and electrical input/ output module
Valve manifold with special servo-valves for drive and compensating cylinders
Compensating cylinder
Cables, tubing and mounting components
Festo supplies these components as a complete system in close technological cooperation with the automobile

Controlled servopneumatics
As well as offering the advantages with regard to cycle time and spot welding quality, the comparatively low costs of the overall system, which does not require any additional control cabinets or power electronics, is another benefit of using servopneumatic systems in welding guns.

All of the system’s components are accommodated ‘on-board’ the welding guns in a compact manner. The lower weight of the pneumatic cylinder as compared with an electric motor/spindle unit has a positive effect on the robot’s overall moving load. Electrode slipping characteristics required during the welding process, which are important for spot welding, are achieved automatically thanks to the compressibility of air. Servopneumatic solutions also offer benefits when it comes to ease of repair and maintenance.