By Dan Hebert, PE, Senior Technical Editor
When an application requires high-speed measurement or control of analog variables, there are two main options. The first is to use a high-speed, high-end controller with a high-performance network and conventional analog I/O. The second is to use a lower-end, less-expensive controller and network along with smart analog I/O. The second option often has a better price/performance ratio, as illustrated by these applications.
System integrator Malisko Engineering (www.malisko.com) in St. Louis was contracted to perform a control systems upgrade on a thermal processing furnace-control system for Advanced Energy (www.advancedenergy.com) in Fort Collins, Col. The furnace is used to fire components before they are integrated into larger systems and machines, and it uses various digital I/O along with Type K thermocouple analog input and 0-10 Vdc analog-output modules.
Malisko reconfigured temperature-zone PID control for the furnace in a way that used the built-in PID capabilities of Opto 22 Snap I/O. "The old control hardware executed PID control—about four loops per rack—via custom commands executed by the controller," says Dan Malyszko, senior systems engineer at Malisko. "With the new system, thermocouple linearization and PID control can be programmed more easily and executed locally at the I/O level."
Without smart analog I/O, the control system would have to send the raw thermocouple analog input data to the processor for linearization. The linearized data would have been used as the process variable in a PID loop executed at the controller. After each execution cycle, the controller would have sent a control signal to the furnace via an analog-output module.
With smart analog I/O, the PID loops execute locally and much faster. The main processor also performs at a higher level as it's not burdened with PID loop execution. Also, the bus traffic on the network connecting the I/O to the processor is reduced, further improving performance and reliability.
For an injection molding machine, a control system retrofit employed a new programmable automation controller (PAC) with smart analog I/O to perform highly accurate, closed-loop control. This improved energy efficiency and precision and the low temperature variance improved injection stability. An Advantech PAC connected to high-speed analog input and output modules for control of the closed-loop process. "The analog I/O worked with discrete I/O to control the machine via its high-speed calculator module," notes John Wilhite, product manager at Advantech.
R&B Plastics Machinery (www.rbplasticsmachinery.com) in Saline, Mich., makes machine, mold, trimming and tooling systems for customers in consumer packaged goods, chemical, automotive and dairy industries. R&B focuses on continuous extrusion blow-molding, and its machines are designed to accommodate multiple parisons, multilayer coextrusion and in-mold labeling of containers ranging from 12 oz to 2.5 gallons.
R&B specializes in parison control to achieve optimum wall thickness of continuously extruded profiles. A two-piece mold is injected with plastic in an environment that takes temperature, velocity and quantity into consideration. The mold is then moved to an air-injection station where air is blown into the mold, forcing the plastic out to form an opening.
Depending on end-user specifications, R&B blow-molding machines use either the A-B ControlLogix PACs or CompactLogix PAC. The control system uses ControlLogix Fast Analog I/O modules to meet the processing demands of high-speed applications. Using onboard data archival, the module increases system throughput by reducing the overhead required in collecting data.
"With four archiving inputs and two outputs, the analog module generates fast sample rates and decreases backplane traffic for optimized system performance," observes Jake Losee, electrical control manager at R&B. "Also, the module lengthens the time between I/O data transfers, relieving the controller burden and decreasing process disruptions."