How to select a controller
Controller options vary almost as widely as the applications they control. More than one type of controller often can serve any particular application. Is the application best suited for a programmable logic controller (PLC), programmable automation controller (PAC), industrial personal computer (IPC), or distributed control system (DCS)? Names are less important than functionality. Experts can help sort it out.
Table of options
PLCs "continue to be a popular choice for machine control and for simple process control applications," explained Jeff Payne, automation controls group product manager at AutomationDirect. "Their ladder logic programming is well-suited for control simple and complex automated sequences, but is lacking when it comes to control of analog variables and data handling," Payne said. In a table comparing PLC, PACs, IPCs, and PC-based control capabilities, Payne said IPCs and PC-based controllers have the most programming options and best opportunities for integrating motion, vision, and human-machine interface (HMI).
For the process industries, "Manufacturers of PLCs are promoting an idea that the combination of PLC and supervisory control and data acquisition (SCADA) can deliver the same functionality that a DCS provides," said Shady Yehia, the instrumentation, control, and automation proposals and engineering manager in a process technology integration company. Yehia’s six-question flowchart helps select between the PLC-SCADA combination versus a DCS.
There is no clear cut fast rule about when to use a PAC, PLC, or IPC; factors include "budget, size, support, complexity, and future expansion," said Ryan Williams, project manager at Stone Technologies, a system integrator. "Selection factors may include existing programming licenses owned, maintenance and engineering training and familiarity, and regional contractor support for the system." When unclear, "create a selection matrix with weighted criteria and grade each."
Ability to change
Easier access to control logic is important. Daniel B. Cardinal, an electrical engineer with more than 30 years of experience designing control systems and working as a control system integrator, said that manufacturers can reduce costs with "improved abilities to change design information to affect actively running PLC programs. The real value to manufacturers comes from minimizing chaos, by having support personnel understand how to interact with all control system applications." They "must have the ability to change logic circuits by interacting with rules, design specifications, and table-based configuration information."
Programmable controllers are a better fit for applications where extremely fast scan times, very large input/output (I/O) counts, or high performance redundancy are critical, said Vibhoosh Gupta, product management leader, GE Intelligent Platforms. "But for low- to mid-range applications that require a dedicated OI, all-in-one automation panels provide a simplified architecture with easy remote connectivity options and lower total cost of ownership, thereby enabling the Industrial Internet age for new levels of productivity, insight, and user experience," Gupta said.
PC-based controls have much more flexibility in form factor, said Reid Beilke, who is a product specialist at Beckhoff Automation. With advanced chip designs, a 24-core control platform is available, a 36-core version is available soon, and programming software can optimize that power to upgrade functions and capabilities. Think again about controller selection: Six articles in this issue, pages 26-44, offer details, and each article has more information in its online version.
– Mark T. Hoske, content manager, CFE Media, Control Engineering, firstname.lastname@example.org.
– See the articles referenced in this story below.