For machine builders, hardware programmable logic controllers (PLCs) are an industry norm. But like technology as a whole, machine automation is evolving quickly – including PLCs.
Original equipment manufacturers (OEMs) are now turning to software-based industrial PLCs, (soft PLCs) – these offer a variety of benefits over hardware, including greater productivity and precision performance, cost savings, speed-to-market and the ability to “future proof” infrastructure with a platform-based approach for real-time processes like motion control and machine vision.
When traditionally using a vendor’s hardware PLC, you are largely obligated to use their preferred network protocol as well to maximize performance and may well stay with motors and drives from the same vendor for compatibility.
Therefore, while choosing the right hardware PLC for the long term is ultimately about choosing the right network to use, the difficulty with soft PLCs is deciding what industry standard communication protocol to use.
A paper from motion control software supplier Kingstar examines five different important protocols that have emerged as contenders to offer the best price/performance open standard for real-time Ethernet fieldbuses: EtherCAT, EtherNet/IP, Ethernet Powerlink, PROFINET IRT, and SERCOS III. There are other technologies also based on Ethernet, but their components are not judged sufficiently published, downloadable or promulgated in the open source community to be considered standard and open. These include Mitsubishi’s CC Link Field and Yaskawa’s Mechatrolink III.
For many years, Ethernet and the TCP/IP protocol have been used in the manufacturing arena to network control systems, management systems, and manufacturing cells on a manufacturing shop floor, but not for the controlling communications inside the actual machines and equipment. The machine controller itself and the communications to the actuators invariably demand the use of deterministic fieldbus, so TCP/IP is not suitable. Trying to use traditional TCP/IP protocol from machine control to the sensors and actuators has failed due to the inability to satisfy deterministic, real-time demands.
An interesting section of the paper deals with the evolution of real-time Ethernet options. There are profound differences in the technical approaches of the various protocols. Some approaches rely on traditional Ethernet protocols and thus have a limited real-time capability and bandwidth, while others reuse the hardware components of the Ethernet and deliver breakthrough performance.