Rationalising PLC multi-axis control
January 2004
PLCs, DCSs & Controllers
Traditionally, CNC machines have been controlled by dedicated, multi-axis control modules. These dedicated modules were expensive and specifically designed for their particular function. The machine peripheral controls were handled by a PLC, which then had to be interfaced to the multi-axis controller. This often led to the situation where the system, comprising various makes of equipment, made local support difficult for the owner of the machine.
In single axis machines, such as bag-makers or thermo-forming machines so-called intelligent drives were used, requiring highly-specialised programming and making it almost impossible to change the type of servo-drive without a complete re-engineering of the machine control system.
To address this situation, LG developed economically priced precision positioning modules which occupy a normal I/O slot on a rack-type PLC, offering the obvious advantages of compatibility and harmony between peripheral and axis control, with local support, combined with ease of programming. These modules offer 32 bit accuracy, 2-axis arc interpolations, 3-axis linear operation, synchronous /separate/simultaneous operation and can be cascaded to accommodate dozens of axes.
The positioning modules (1, 2 or 3-axis) operate in realtime, outside of the normal PLC cycle time, merely reporting the successful completion of the positioning task/s. A further advantage, is that the machine parameters can be entered via a standard HMI, allowing the client to set machining requirements, log totals or details of completed jobs, etc, and to either print information or to communicate directly with a PC.
This represents a significant step forward over the standard numerical control (NC) or computer numerical control (CNC) concepts, with information, machine control and diagnostics now all incorporated into a harmonious single rack PLC system. The LG positioning modules interface to most makes of servo drive, making this system viable for all CNC machine upgrades (of all accuracy classes).
For more information contact Peter Gross, LG Industrial Systems SA, 021 914 9030, peter@lgis.co.za
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