ALARM SYSTEMS OF one type or another always have been part of industrial machine, robot, and skid control systems. Alarm technology has progressed from panel indicating lights, through light boxes, to fully-featured HMI/PLC systems.
HMI/PLC systems have virtually unlimited alarm capabilities, all configured through software for maximum flexibility. One of the main features HMI alarm packages provide is the ability to custom-configure alarm notification.
GE Fanuc's Cimplicity Machine Edition HMI for example, has integrated alarm management capabilities that can be used to configure alarm notification based on priority, user roles, and user location. Configurable alarm views let users tailor the screen the way they want to manage alarms. These alarm view screens can be anything: from a PC monitor, to a cell phone, to a pager.
"Alarm and event information is also tightly integrated into advanced applications such as programs to provide users with extensive downtime analysis about what occurred, when, why and how was it fixed,to extend use of existing assets and improve productivity," says Rich Carpenter, global solutions business manager for GE Fanuc Automation.
"Hardware-based alarm systems are inexpensive and can out-perform HMI/PLC systems in speed and safety, but often lack sophisticated diagnostic capabilities."
Most HMI/PLC alarm systems are configured with software. Software-configurable alarm systems are very powerful and flexible, but don't effectively address requirements such as very-high-speed, fail-safe operation, and compliance with certain safety standards. To meet these requirements, industrial machine builders often turn to specialized alarm components and modules.
Very-high-speed requirements are best addressed with hardware-centric alarm systems. Although these hardware-based systems can be programmed with external software packages or through on-board interfaces, actual alarm execution is hardware-based.
National Instruments makes a variety of such products, starting with recently released industrial digital I/O boards. These I/O boards feature watchdog timers and change detection. "In a case where the HMI/PLC fails to respond, the I/O board can automatically set its outputs to predefined states," says Rahul Kulkarni, NI's industrial data acquisition and control product manager.
For very-high-speed alarm processing, National Instrument's provides field-programmable gate array (FPGA) controllers. These FPGA controllers are programmed through LabView, and control and alarm functions are executed directly in hardware at rates to 40 MHz.
Beckhoff Automation also offers a large assortment of alarm and monitoring products. Its distributed I/O--including on-machine IP67 modules--can be used with sensors in almost any alarm system application because they operate on many open industrial networks including Modbus, Ethernet, Profibus, DeviceNet, ControlNet and CANopen.
"We see an upsurge in interest in alarm and monitoring applications as companies look to improve their asset management," observes Graham Harris, president of Beckhoff Automation. "The combination of low-cost distributed I/O and cost-effective network media such as Ethernet is driving this growth."
Safety requirements are another area where hardware-based alarm components excel. HMI/PLC-based systems can meet safety requirements, but only with very-expensive triple modular redundant controllers.
Moore Industries-Intl. makes an alarm module that comes with failure modes effects and diagnostic analysis (FMEDA) data. "FMEDA data allows our SPA and other modules to be used and applied in Safety Instrumented System (SIS) loops," says Scott Saunders, director of strategic marketing for Moore. "FMEDA data then can be used in Safety Integrity Level (SIL) calculations to comply with IEC 61508 and ISA S84 safety standards."
Another weakness of HMI/PLC systems is high cost and complexity. Many machine control applications are relatively simple and can be met by alarm modules acting as the system controller. "Unlike many micro-PLCs, our NCS alarm module has one LED per alarm point for local indication. Moreover, the relays on the NCS are fully isolated and can switch loads up to 2 A at 250 VAC, removing the need for a bank of terminal blocks and interposing relays," says Saunders.
"The NCS also offers a cost-competitive solution to the HMI or other supervisory software required to interface with PLCs," adds Saunders. "Because the NCS has an embedded web server, a machine builder can view process parameters or alarm states via a web browser. All process information is then displayed via pre-built HTML based pages."
Hardware-based alarm systems are inexpensive and can out-perform HMI/PLC systems in speed and safety, but often lack sophisticated diagnostic capabilities. HMI/PLC systems such as Rockwell Software's Maintenance Automation Control Center (RSMACC) can be used to satisfy demanding diagnostic requirements.
"RSMACC solutions provide a secure, administered environment that helps OEMs proactively avoid device and process failures," says Glenn Schulz, director of global business development with Rockwell Automation. "When failures do occur, RSMACC quickly assesses the failure point, dispatches work orders, and provides online troubleshooting and device configurations to quickly resolve the problem."