Servo driven robot cell brings increased performance to injection moulding operation

Citation

(2000), "Servo driven robot cell brings increased performance to injection moulding operation", Industrial Robot, Vol. 27 No. 5. https://doi.org/10.1108/ir.2000.04927eaf.008

Servo driven robot cell brings increased performance to injection moulding operation

Servo driven robot cell brings increased performance to injection moulding operation

Keywords Robots, Injection moulding

For a company to take even a small step towards automation is seen by many as a leap of faith. At DuBois, one of Europe's most progressive plastic injection moulding companies, producing multimedia packaging, automation is now an established method that enables the Corby-based company to keep ahead of its international competition. The robotic side-entry unloading operation on each of the five recently installed plastic injection mould machines is today provided by OEM automation solution provider G-MAT, at the heart of which is an ACC (Atlas Copco Controls) DMC servo (see Plate 7).

Plate 7 At DuBois, the side entry robotic system is cantilevered from the fixed platen of the plastic injection moulding machine, unloading DVD cases from the moulding machine

"We have always believed that automation is important to our success", stated DuBois Technical Manager, Mark Marriott, "and since the late 1980s have progressed from pneumatically controlled robot handling systems, to the current servo driven G-MAT-designed robot cell. Pneumatically driven robots in themselves are not slow, but they were not sturdy enough, whereas servo motor driven robots give increased performance and control yet require less maintenance. The pneumatic robot systems were employed on single impression mould tools whereas we are now automating multi-impression stack moulds. A close working relationship between supplier and client is important to achieve the best solution".

In selecting the correct servo motor system, the right partner was also important to G-MAT, who built each of the five robot handling cells now in full production at DuBois, as G-MAT Technical Director, Roderick Mitchell, explained. "Performance specification from DuBois was very demanding, and required detailed analytical analysis of the inertia of the system to ensure the acceleration rates could be achieved. The specification also covered the software operation, inputs and outputs to the motion controller and safety critical monitoring. The Atlas DMC (digital motion controller) servo drive was selected as being an ideal solution for the application and our robot specification was further enhanced to fully utilise the DMC's capabilities".

At DuBois, the side entry robotic system is cantilevered from the fixed platen of the plastic injection moulding machine. The original mechanical specification called for a mass of 65kg to be accelerated at 50m/s2 (5G) – a demanding requirement by any standards. A gearbox was specified and a motor selected from the wide range available from ACC. The motor was selected for appropriate speed and peak torque to give optimum performance when driven by the Atlas DMC.

"The latest robotic application involves unloading DVD cases from the moulding machine," continued Mitchell. "As the £180,000 mould tool starts to open, the robot arm accelerates from a standby position near the side of the tool 750mm to the tool centre line. Although extremely rapid (the movement takes 300ms), at each millisecond the position, velocity and acceleration is controlled to provide optimum performance with minimal mechanical vibration".

Many of the advanced features of the DMC are employed to shave vital milliseconds from the short cycle time. For example, the movement of the mould is continuously monitored by the DMC as the robot accelerates towards the opening tool. Checks are made to ensure that clearance will be available as the mould is reached. Many sensors are checked during each cycle to ensure correct and safe functionality without slowing the moulding process down.

Inside the tool, the robot head, achieving a positional accuracy of less than 0.5mm, picks up four newly formed flat DVD cases using suction cups. The robot then exits the tool at peak speed – 5m/s – before commencing the more gradual deceleration to the product unload position. The robot deposits the hot cases into a purpose designed folding and closing system. The robot returns at a constant velocity of 0.5m/s to the standby position ready for the next cycle to be triggered.

The partnership between Coventry-based G-MAT and ACC has cominued from the first design concept with a mechatronic approach to optimising the solution further. Different velocity profiles have been experimented with, as well as different motors and gearboxes. With ACC they are working on a new ultra high performance solution that requires no gearbox and is capable of accelerating at over 10G. The flexibility and open programming structure of the Atlas DMC have proved a great benefit to G-MAT over previous less intelligent drives that were initially tested. The DMC allows full access to sophisticated program techniques yet maintains a simple, easily recognisable program structure. A high speed CAN-based communication protocol is used to allow bi-directional data transfer with a PLC. Data and motion profile scaling factors are passed to the DMC, which in return sends general and error status information, real-time position feedback and production rate information.

"The ACC servo control is critical to the robot cells at DuBois, outperforming all other previously tested drive solutions and offering an unequalled reliability and service back-up. We are currently installing robot handling systems into factories in Germany and the USA as well as tendering for applications in the Far East", concluded Mitchell.

For further information contact Alan B. Rhodes, Atlas Copco Controls, Bridge Mills, Holmfirth, Huddersfield HD7 2TW. Tel: +44 (0)1484 688325; Fax: +44 (0)1484 688326. Roderick Mitchell, G-MAT Limited, Shilton Ind. Estate, Shilton, Coventry, West Midlands CV7 9JY. Tel: +44 (0)1203 621444; Fax: +44 (0)1203 621555.