Ethernet-based motion control for innovative plastic bag machine

01 October 2013

Based on ABB's Ethernet Powerlink compatible motion control range, two UK-based plastic bag machine support specialists have co-operated to launch a machine offering new levels of programmability and bag production flexibility.

The new machine includes all-servomotor architecture and a user interface that has been developed with the operator in mind.

Today’s plastic bag making machines often have two servomotor based axes – web feed and cut/seal – together with a mechanically linked third-axis for 'picking off' the finished bags. The new machine, which has been jointly developed by Hartech Engineering and GPL Machinery, also implements servo control on this third-axis to provide programmable 'electronic cam' control of pick-off action. This allows users to finely adjust pick-off, so that the machine can avoid sealing problems that can arise as conventional machines are set up to handle different types of blown or cast film materials in a bid to minimise scrap and downtime.

The machine has been built on the mechanical framework of a common bag making machine from Woodbank, who ceased production around a decade ago. There are many of these old machines in the UK, which GPL and Hartech have been servicing and refurbishing for more than a decade. During this time, the two companies have developed numerous add-ons and upgrades, based on ABB motion control technology, including replacement servomotor axes and a new user interface.

The three axes of the machine are powered by three-phase drives from the MotiFlex family, linked to BSM brushless servomotors. Control is provided by the NextMove e100 machine controller with a touch-screen colour operator panel.

The Ethernet Powerlink interfaces - with their single network cable interface - provide one major benefit. They substantially reduce cabling, speeding up system building and lowering hardware costs, as well as simplifying subsequent machine commissioning.

The ABB NextMove controller also includes enough onboard digital and analog I/O to satisfy all of the bag making machine's I/O requirements. In addition, a high-speed digital input on the drives provides a direct interrupt that capture positions to within a microsecond resolution. This feature is used to support high speed print registration on the BBM1100 - which will operate right up the machine's fastest production rate.

The developers also greatly value ABB's MINT programming language, which provided the machine with high level keywords for many of the motion control operations employed during the bag making cycle. For instance, one of the keywords used in the control software for this new machine architecture is SENTINEL, which allows events to be triggered under software control. In this case SENTINEL synchronises the pick-off axis to a virtual line shaft acting as a ‘master clock’ for the machine cycle. This allows users to program the profile of the electronic cam with high resolution and precision, rather than being limited by a fixed-profile mechanical cam. It ensures that the machine can be configured to operate in the optimal way for the exact type, grade and thickness of plastic material being processed.

ABB also provided Hartech and GPL with in-depth application programming support during development of the various parts of the new machine. This was of great help during the early stages of control software development.

"ABB provides us with motion components offering genuine real-time performance, plus support for the complete motion control package - all from a single point," says GPL Machinery's Graham Levine. "It's helped us to bring our new machine to market both efficiently and rapidly."

In addition to implementing core performance features in the new machine, the design team has emphasised usability and reliability throughout the design. For example, the user interface has been developed from the point of view of the operator. The machine can be set up for a new batch with just a few touches on the control menus. Operators can also change action on the fly, to adjust the print registration for instance, or the dwell time of the cut and seal bar. Users also have access to deeper configuration possibilities via a password-protected screen. This can be used for purposes such as changing the action of the pick off belts - the grip speed, acceleration and deceleration profile for example. Such settings can be saved and renamed so that operators are able to load a proven bag making setting for any particular job or material within seconds.

A number of hardware features - developed over many years of work in the bag making sector - also endow the new machine with high reliability. Bag making machines have an inherent stop/start movement, and there is always a degree of vibration. So, the machine builders avoided some of the more modern but more fragile feedback technologies, and instead chose to use resolver feedback to measure rotational position - a technology that the machine builders and many of their customers have great confidence in. This preference was made simple by the broad encoder support options available on ABB's MotiFlex drives.

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