Integrating PID Controllers Into Automated Processes Via Ethernet

Integrating PID Controllers into Automated Processes via Ethernet By Sean Wilkinson Watlow Ethernet communications is rapidly gaining popularity in industrial applications because it enables the real-time exchange of information between processing equipment and companies' Ethernet-based management systems. Ethernet technology is becoming widely accepted due to multiple factors, including: . the speed advantages over lower baud rate protocols . the number of tools available for troubleshooting and optimizing a network, . the broad base of competitive vendor support and solution options and . the large pool of trained personnel who are familiar with the technology. In addition, the ability to bridge existing proprietary communications schemes makes it possible to phase in the use of Ethernet rather than having to replace everything at once. When asked, engineers most frequently respond that they expect to use Ethernet to integrate control systems in the future. But a follow-up question, inquiring which Ethernet protocols they plan to use, is often met with confusion since some do not realize there are thousands of protocols that are compatible with, and can coexist on Ethernet networks. In attempting to navigate this sea of protocols, it is natural to look first to familiar ones that offer the functionality we are used to using on office networks, at home and on the Internet. Using such protocols, Ethernet can extend access to remote users through web browsers and e-mail. But for an engineer tasked with automating a process, the challenge is to connect devices to other devices to integrate automatic functions. For example, a temperature controller may need to get its set point from a Programmable Logic Controller (PLC). For the purpose of automation, Ethernet protocols such as HTTP, which allows web browsers to display web pages, and SMTP by which email messages are transmitted, are ill suited. These protocols are designed to transmit information over Ethernet, but the information is in a form that requires human interpretation. One purpose of automation is to relieve humans of the tedious task of monitoring and adjusting a process based on feedback from sources such as pressure gauges and mercury thermometers that require human interpretation. Another purpose of automation is, of course, to improve process results by removing variations in human interpretations and occasional misinterpretations from the process. What is needed for automation are not protocols that transmit messages intended for human eyes to interpret, but rather protocols that transmit information structured for automation devices to interpret. An example of a control automation problem can be used to illustrate this need. An engineer who plans to automate a candy packaging machine must consider not only the sequential functions that fill the open bag with candy, close the bag and drop the sealed bag in to a box, but also the critical temperature control of the sealing bar that insures the candy stays safe, secure and fresh in the sealed package. The engineer must also consider the reliability of the process and the ease-of-use. A PLC may be ideal for the sequential functions, but accurately controlling temperature is also important. Here the candy packaging engineer is traditionally faced with a dilemma. The system can use the PLC to perform temperature control directly or standalone temperature controllers can be used in conjunction with the PLC. Performing temperature control in a PLC presents numerous challenges. Programming can be difficult, requiring expertise that may not be available. Control algorithms can interfere with other program functions by consuming processing bandwidth, or require the use of a more expensive PLC than would otherwise be required. Control may be sub-optimal because a simple on-off control algorithm, the easiest to program in a PLC, is used. The alternative, using a standalone temperature controller, resolves these issues. Commercially available temperature controllers include advanced control algorithms and automatic tuning, so little expertise is required. The calculations and I/O updates associated with temperature control are performed by the PID controller's processor, freeing the PLC's processing bandwidth for other tasks. But in the past due to differences in the supported protocols, getting standalone temperature controllers and PLCs to communicate has been time consuming, costly and... [download for more]