SAB Plastmach, Thailand, has developed a range of high-performance extrusion blow moulding machines using industrial PC control and Industrial Ethernet communications.
SAB is a manufacturer and international supplier of extrusion and injection blow moulding machines for the plastics industry. Its machines used to be equipped with a proprietary control system comprising three CPUs for sequence programming, motion and temperature control. Different fieldbus technologies were used for the controller and the I/O while the hydraulic control system was a black box that SAB was unable to adapt or develop further.
EtherCAT for blow moulding
Boonlue Rungsawang, leader of the electrical systems and control technology section at SAB, initially tried using a central hardware PLC controller and a single fieldbus for the whole machine. However, trials with a standard PLC proved unsatisfactory due to long cycle times and slow fieldbus communication. In particular, control of the hydraulic axis could not be achieved with the required precision.
Thus, SAB came to the conclusion that a PC-based system with fast, realtime Ethernet was the optimum solution. The technology offers adequate performance reserves and has the advantage that the complete control system, including hydraulic control, can be accomplished in a software PLC programmed according to an IEC standard.
TwinCAT application software handles HMI, sequence programming and motion control in a single system. Data exchange between the system components is quick and easy to implement. PC-based technology and fast EtherCAT I/O allow the sampling time of the digital controllers to be substantially shorter than that of the prior controllers or of conventional hardware PLCs.
Industrial PC shortens cycle times
SAB replaced all existing hardware components for sequence programming, hydraulic and temperature control with a single Pentium 4 Industrial PC. The control unit is a customised Beckhoff Control Panel with 15" TFT touch screen, PLC keys, membrane keyboard, and disk drive.
The computing power of the industrial PC and direct communication between the individual program components enables cycle times of 1 to 2 ms while simultaneously controlling the hydraulic axes, HMI, temperature control and sequential control.
By replacing special control hardware with a software-based open control concept using EtherCAT Industrial Ethernet system costs were reduced considerably.
Open systems improve flexibility
The integrated application software uses TwinCAT NC, TwinCAT PLC Hydraulic Positioning and TwinCAT PLC Temperature Control libraries and is based on the languages defined in the IEC 61131-3 standard. This eliminates black box programming, which means that SAB can modify or extend all program components in a common language context.
Motion control for a hydraulically operated positioning axis is not a trivial task, because the controlled system is non-linear and must meet specific process engineering requirements. For example, the system must enable highly dynamic movements of the transport axis weighing several tons and hold the axis in the target position with high precision. TwinCAT function blocks for hydraulic axes are used for controlling 10 or more machine axes.
With support from Beckhoff, SAB was able to handle all machine programming tasks in-house, thereby gaining comprehensive machine and process know-how. TwinCAT provides a constantly available programming and diagnostics environment for service engineers that can be used on site - or remotely via modem - directly at the machine.
In future SAB plans to develop modular machines and 'zero engineering' tools for simple programming. TwinCAT is the backbone of this new concept that enables flexible responses to customer and market requirements.
For more information contact Conrad Muller, Beckhoff Automation, +27 (0)11 792 3374, [email protected], www.beckhoff.co.za
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