Assembly cell benefits from electrical and pneumatic axes

MacGregor Welding Systems' latest automated assembly cell fabricates miniature sensor modules for use in analyser equipment. It takes individual components and assembles them fully automatically - which involves the precision welding of metallic connector ribbons onto small pins. The system makes extensive use of Festo electrical and pneumatic actuators, including two electrical mini slide units with matching controllers, which utilise positional feedback to ensure high-accuracy assembly and precision micro-welding.

MacGregor Welding Systems is a world leader in precision micro joining technology, designing and manufacturing a wide variety of micro-resistance and pulsed-micro-arc equipment, which is used in diverse industrial manufacturing applications. Initially the company concentrated on the development and manufacture of pulsed-micro-arc welding torches and precision resistance welding heads and power supplies; these are typically used by customers in the medical, aerospace and automotive industries to perform high-reliability joining and interconnection of electronic components and sub-assemblies. In recent years, however, MacGregor Welding Systems has expanded the systems engineering side of its business significantly.

Today the company produces a considerable number of application- and customer-specific assembly machines. Most recently it has seen a dramatic increase in systems sales following the introduction of RoHS legislation, which has created an upsurge of interest in micro-joining techniques as the electronic industry moves to lead-free components and assembly processes.

Multi-stage assembly process

MacGregor Welding Systems' latest component assembly machine fully automates the production of miniature sensor modules. The multi-stage process involves the assembly of various small components - including the fitting and lubrication of O-rings and final moulding insertion - as well as welding two metallic ribbons onto a pair of small pins on the sensor module and onto a connector block.

The company chose to base the machine predominantly on Festo pneumatic and electrical actuators, using SLTE electrical mini slides and associated SFC-DC motor controllers for two of the most demanding parts of the system that require exceptionally high positional accuracy.

Graham MacGregor, Managing Director of MacGregor Welding Systems, states: "We elected to use Festo SLTE mini slides primarily because they provide a complete, ready-to-run positioning solution. The slides incorporate their own miniature motors, which simplifies system integration and makes them very cost-effective, and the supplied control software is extremely easy to use." One of the mini slides is associated with the initial product assembly stages, the other with the welding stage.

The welding process uses micro-resistance welding, which involves passing a short-duration, high-current pulse through the materials to be joined, while accurately controlling the applied force. The welding head module provides up to 10mm of dynamic movement and is driven into position by the SLTE mini slide; a force transducer provides feedback to the control system, which triggers the welding power supply to deliver a pulse of current when the welding electrodes are in the correct position. The weld head can accommodate a force in the range 2-70N and the current pulse - of up to 1000A - is typically limited to between 5 and 50ms. The welding process involves far less thermal energy than a conventional solder joint, making it suitable for use on heat-sensitive components, and it provides a much faster means of creating consistent, high-integrity electrical connections.

The automated assembly machine is now installed at the customer's premises and is understood to be performing impeccably - so much so that a second machine has been ordered - and the customer is said to be delighted that a system, with such a small footprint can provide such a high throughput.