TechArchive

Original article date: October 1998

The TwinCAT is an open universal automation system offering PC-based processing without add-on processors. It can also be used for axis position control replacing conventional positioning modules and NC controls.

PC-based automation systems are currently being implemented as either

• a PLC process control computer or NC control system combined with a PC
• an industrial PC with co-processor board for PLC/NC tasks.

PCs are not generally assigned processing tasks directly – these are normally carried out by add-on processors. The purely software-based solution is a different approach in which a processor carries out all automation tasks within a single processor mode.

• With Beckhoff’s Industrial Electronic’s TwinCAT a proprietary PLC/NC processor system is replaced by PC software for processing PLC and NC tasks. An automation system based on this philosophy consists of only four components:
• industrial PC
• open fieldbus system for I/O interfaces
• Windows NT standard operating system for the operator interface
• TwinCAT control software.

Software solutions for PLCs according to Stephen Hayes Managing Director of UK distributor Hayes Control Systems are out-performing PLC hardware solutions with measurement values for processor speed increasing with each new PC processor. Not only have processor speeds increased but PC system limits have been extended as well. Program sizes flag memory and process image sizes can meet most requirements so the practical limits of PC technology are far greater in the 32-bit world than with previous PLC devices.

A classic PLC processes programs cyclically. Inputs and outputs alternate with the processing of the programs. Execution of programs in a software-based PLC on a PC is exactly the same as a conventional PLC/NC – in accurately timed (so-called deterministic) cycles of equal length (independent of the program length). User interface processing takes place during the pause in the cycle when processing capacity can be reserved. PLC NC and user interface processing take place simultaneously through the multi-tasking of PLC NC and operating system tasks on a PC system. Each “server” for a specific task has its own cycle time and priority. TwinCAT guarantees operation of Windows NT together with TwinCAT programs by means of creating a real-time extension of the operating system.

The Beckhoff implementation works without a second operating system through complete integration of real-time tasks within Windows NT. It does not require any additional hardware other than a standard PC. Under a high real-time load it ensures that important tasks are processed by Windows NT and that no Windows NT properties are influenced or restricted.

TwinCAT operating on a PC enables a PC to operate drives simultaneously with a PLC where point-to-point positioning is calculated on the PC processor. PC performance enables dozens of axes to be positioned at the same time. Data is exchanged cyclically via the fieldbus (usually Lightbus or Sercos) to the drives as well as from the measurement systems. Replacing conventional positioning modules and NC controls NC PTP offers axis positioning (target value generation and position control) an integrated PLC with NC interface operator programs for startup and an I/O interface for axes via the fieldbus. Positioning takes place using optimised positioning algorithms profile generation with jerk limiting as well as speed/acceleration pre-control and lag error minimisation.

TwinCAT NC PTP controls axes by means of variables. Each axis contains variables for encoder drive and controller. The axis can be linked to the I/O interfaces and axis parameters can be set.

Continuous path motion control is catered for by PC TwinCAT NC I (for NC interpolation). NC I replaces conventional positioning modules with an open PC-based solution. Axes are controlled in groups for interpolating motion. NC I offers 3D-interpolation (interpreter target value generation and position controller) an integrated PLC with NC interface operator programs for startup and an I/O interface for axes which is set by the fieldbus. Programming standards from the CNC world such as DIN 66025 G-Code are supported. PTP and continuous path functions are also available.

• Hayes Control Systems
• Stephen Hayes
• 0700 181920

October 1998