Servo-motion control

Phasa has enhanced the performance of its specialist plastic component joining machines with high performance new versions employing a key servo-based motion control system in place of a pneumatic actuator.


Phasa takes its name from the process it has pioneered - Plastic Hot Air Stake Assembly. Essentially, the process involves selectively heating parts of thermoplastic mouldings using super-hot air, and then using cold tools to form, clamp and chill the parts into their required shapes. The fixings often take the form of rivets or 'roll overs', and the non-contact heating method ensures that the process is clean - without marking the 'A' face surface - and fast, and the resultant assemblies are strong, tight and stable. Applications for these fixings are diverse and include the automotive, electrical/electronics and white goods industries.

Phasa manufactures standard and custom machines to automate the entire assembly process - from inserting the thermoplastic mouldings into the components being joined, to selectively heating, forming and cooling them. Until relatively recently, all of Phasa's standard machines used only pneumatically-powered motion control. However, when the company started to experience supplier product changes for the machines' rodless cylinders, it approached Festo for help. After investigating the application, they recommended using the DGC pneumatic cylinders to provide the machines' main platen movement. These rodless linear actuators feature a unique sealing mechanism which ensures efficient operation by virtually eliminating air leakage, and have proved a very reliable replacement. Since then, Phasa has progressively re-designed its range of standard machines to incorporate pneumatic automation components for many of their motion axes.

Phasa recently expanded its standard machine range to include models that use a servo system in place of a pneumatic cylinder to provide the main movement of assembly nests. The principal advantage of the servo approach is that by using closed-loop control it is capable of fully programmable multi-stop positioning, compared to the end-to-end positioning of standard pneumatic cylinders. This results in faster and more accurate placement of the assembly nest, reducing the overall processing time.

In this particular case, Phasa chose a complete Festo assembly comprising of a toothed belt axis, servomotor, gearbox and servo motor controller to provide the horizontal movement of the assembly nest platen on its large 40/80 machine. This combination, together with the power supply and mechanical connecting kit, is supplied as a turnkey solution. The servo controller is networked to the 40/80 machine's host PLC. Compared to the all-pneumatic version of the machine, the electric servo based model accomplishes the same assembly operation in 33 seconds, improving throughput by nearly 6 percent.

During the development of the servo system, Phasa found the positioning drives software particularly helpful. This PC-based software tool assists machine designers with the task of selecting the optimum components for a complete, high-efficiency positioning system. After entering a few application details, designers are presented with the ideal combination of electric linear axes, motors, gear units, controllers and software, together with the load characteristics of their selected drive. The software enables designers to evaluate the performance of a system very quickly, turning what can often be a highly iterative procedure into a quick and simple process.

In addition to providing excellent programming flexibility, the CMM-AS servo controller offers a particular benefit for this application, as Terry Elvidge, Phasa's Operations Director, points out: "A further advantage of using Festo servo drives is that they facilitate the capture of timed process parameters for every part made. This is vital to our customers' quality management procedures, enabling the data to be networked to a central server system and logged against the bar code for that particular part, providing a complete manufacturing record. This is especially important in the automotive sector, where products need to be traceable throughout their lifecycle."

Phasa is now looking at incorporating electric servos in a number of its smaller machines. The technology is especially suitable for applications involving fast, highly repeatable movement, and Terry Elvidge is keen to capitalise on its potential, "Although we make extensive use of Festo pneumatic cylinders primarily because of their suitability and function, they have also proved to be cost-efficient for us as we are continually seeking ways of improving the performance of our machines. We are also very customer-driven and aware of market needs - consequently we are currently investigating the use of Festo electric servos in a number of application areas, including automated loading and unloading systems."

In a typical application involving the Phasa 40/80, the parts for a car door panel comprise about six components, such as a map pocket, switch panel and various clasps. These are fitted into the carcass of the panel and then loaded into a preformed nest on the machine by the operator. All subsequent operations are completely automated, with full light guard protection to ensure operator safety. The machine progressively moves the nest through the various process stages, bringing the heater, forming and cooling tools down into close proximity or contact with the plastic stakes. Typically, each plastic stake is heated to 260 degrees Celsius for 6 seconds before the part is moved to the forming stage. The entire process, from loading to unloading, normally takes 35 seconds on the all-pneumatic version of the machine. As soon as the door panel assembly is complete, the nest is returned to the front of the machine and raised at an angle by a small pneumatic cylinder, so that it is easily accessible to the operator.

According to Terry Elvidge, "We export over 80 percent of our machines - many of our customers are first tier suppliers to the automotive industry in central Europe and Asia - which means that worldwide support is a critical factor to our success. The global nature of Festo's operations assists us in this area by providing highly responsive technical support and spares services wherever we do business. We have also found that moving to a single source of supply for our principal automation components has made ordering and parts tracking much easier."