By Dan Hebert, Senior Technical Editor
You've just completed the control system design for your latest project, but now a level switch needs to be changed to a level transmitter. That would mean adding another analog input card, substantially changing the design and also generating a change order from the electrical contractor for additional wiring. Or maybe your control system is up and running, but overall processing speeds aren't up to snuff.
How about a case where all the control system I/O is compatible with your selected digital fieldbus network, except for one specialized instrument that doesn't feature the connectivity that you need?
In all of these cases, the solution is "smart I/O." This technology provides a microprocessor at the I/O level—adding flexibility, protocol conversion and/or high-speed local I/O processing and control.
One of the newest "smart" innovations is software-configurable I/O. "With our universal safety I/O, each module can be configured via software to accept any signal type—analog or digital, input or output," says Erik de Groot, the marketing manager for safety systems at Honeywell Process Solutions (http://hpsweb.honeywell.com). The module can be used in safety applications up to and including SIL 3.
Only one type of module is needed for each project, and the only question is how many total modules are needed. I/O spares are reduced to one part number instead of a minimum of four. Each module is connected to the host processor via an Ethernet network, eliminating the need for home-run discrete and analog wiring and associated field-mounted junction boxes and marshalling cabinets.
"The engineering tools used to configure each universal I/O module generate an electronic record of the entire automation system's wiring configuration. This makes maintenance much easier because there's automatically generated documentation," adds de Groot.
Smart I/O can add intelligence to each module and also to a group of modules via a local processor at the I/O rack. "Intelligence at the I/O module and rack level gives the ability to perform complex functions such as scaling, thermocouple linearization, digital counting and latching, analog clamping, alarming, event reactions, waveform generation, ramping, pulse generation, totalization and PID loop control," observes Tom Edwards, a senior technical advisor at Opto 22 (www.opto22.com).
"The complicated algorithms and calculations required for PID loops can adversely affect a typical control system's performance very quickly, but a distributed control architecture gives you the modules you need to acquire your inputs, the processing power required to perform the complex math, and the modules needed to set the outputs and control the process—all local and completely self-contained on the I/O rack," explains Edwards.
Smart I/O modules can also perform protocol conversion, taking a 4-20mA or other analog signal as an input and sending a digital network signal out to a centralized processor. "Our dual-channel I/O cards allow users to establish digital fieldbus communication via HART based on pre-existing 4-20 mA connections," notes Matt Boudjouk, product manager with Turck's network division (www.turck.com).
"Incorporating this type of design lets users link their analog field devices to a PLC or DCS and integrate these devices into a consistent asset management system based on FDT/DTM technology," adds Boudjouk.
Local diagnostics are yet another feature of smart I/O. "Microprocessor-based I/O reports the ongoing health of its own electronics," says Jim McConahay, the senior field applications engineer at Moore Industries-International www.miinet.com). "Any detected errors of the input/output signal or of the internal analog converters that exceed the defined range of the I/O device are reported to the host processor, which can indicate if a particular channel's data is no longer within expected ranges."