"The story is always the same. Relays are going to go away and be replaced — but we're still waiting," says Steve Massie, marketing manager industrial components at IDEC. Electromechanical relays were going to be replaced by solid-state, but they're still 90% of all relays. PLCs were next, but they still need relays to isolate, switch and control devices over 6 A and all the bells and whistles they've added."
As a result, many developers and users employ relays and PLCs in tandem. "We always need to do something with our output signals, usually around the nucleus of a PLC, but if you've got an accessory using 5–6 A, then the PLC won't be able to handle it. The same is true for many high-speed devices or very small components, and this is where relays come in," says Carey Perschke, engineering services manager for industrial control at Wago. "The trend we see now is solid-state switching devices gaining the capabilities of relays. For instance, our new Optocoupler uses fiberoptics internally, and runs at up to 1.5 MHz for users with high-speed pulses, which occur in vision applications." Besides its Optocoupler, Wago also plans to launch a 6 mm, 6 A, solid-state relay, including a version with a built-in timer to help alleviate heavy current draws.
Secure Smarts, Branch Out
David Sonderman, product line manager for power control and motor protection at Eaton, reports that basic, 10 A, ice cube-style relays for switching are still used in control panels worldwide, but in the past five years, they've come back with new intelligence and monitoring capabilities due to embedded microprocessors, which allow control to be decentralized from PLCs to these smarter relays.
"Relays used to be reactive and dumb, but now they can be proactive, perform diagnostic and predictive functions, and reduce downtime," Sonderman says. "That's why we're investing in smart motor control centers (MCCs) and switchgear with smart relays to handle overloads, motor protection, ground faults and monitoring."
Relays are getting smarter at the same time they're gaining better communications via open protocols such as EtherNet/IP, Modbus TCP, Profibus, CANopen, BACnet or IO-Link, Sonderman reports. "These links allow relays — and their overload, protection, monitoring and other data — to be located and accessed on one network." He says that Eaton's Motor Insight protective relay was launched in 2011, and combines intelligence and communications in one device.
"Embedded microprocessors open a lot of new possibilities for relays that they didn't have before, including functions that used to be done by PLCs," explains Aaron Henry, North American market manager for Murrelektronik. "In fact, we just released a 24 V relay, Miro GSM, that permits cellular communications via text messages. This means users with cabinets in the middle of nowhere can send them instructions and get back alerts, even if there's no Internet or other wireless communication available. It needs a cellular SIM card to operate, but it's programmed with regular PC-based software."
Likewiese, John Burns, controls product manager for Siemens Industry, reports, "People always want to tap the information in relays, and get added diagnostics and control. To serve this demand, single- or three-phase voltage-monitoring relays have been adding IO-Link point-to-point communications."
IO-Link breathes new life into monitoring relays by enabling online connections to live process variables, Burns adds. IO-Link also performs scaling for relays, which previously was done by a PLC. "This allows a two-way conversation with the relay," Burns explains. "Users can read in values, and then send down a new setpoint. Because this is just point-to-point wiring and not a fieldbus network, it's easier for panel builders and OEMs because they don't have to change specifications."
Search for Skinny
One curious side of relays' recent evolution is that, as they gain data processing abilities, some ice cube relays have been redesigned into thinner, slice-style relays in recent years. This saves cabinet space, but it also makes traditional relays look eerily like PLCs. "The function of 6 mm, single-pole, 10 A or less relays is the same, and they still control the same HVAC units, small valves and pumps, or isolate PLC inputs and outputs," says Jeremy Valentine, manager for interface products at Phoenix Contact. "It's just their form that's different. Lately, now that they're thinner, daisy-chained wiring can be eliminated. We're also seeing slice relays using push-in power bridges that can power multiple relays. Push-in offers faster wire termination and five times the IEC retention force."
IDEC's Massie adds, "Our biggest recent push is to 6 mm, slim line-type relays for interfacing between PLCs and the outside world. Users always want more space on their DIN rails, so we miniaturized and doubled how many products they can put on the same rail by improving heat sinks, circuit designs, and contact and bonding materials to improve heat dissipation and reduce operating power. Prior-generation relays were 14 mm, and needed 530 mW to 1 W to switch 6 A. Now, our RV8H is 6 mm, and uses only 200 mW to do the same task. Old relays were simple and wasted a lot of energy, but to switch at 200 mA, we do CAE circuit designs to increase efficiency and push every gram of contact force we can get."
Another field where smarter relays are useful is in safety circuits, but here again they've merged some functions with PLCs, adds Zachary Stank, Phoenix's product specialist for safety relays. "We've gone from redundant contactor wiring in relays to a single-source processor in a relay form factor. It's called a configurable relay, but it's really a small safety PLC with a safety processor that acts as a safety relay."
Sometimes, like beauty, a relay is defined in the eye of its beholder. "Our PSR-Trisafe configurable safety relay is described as a safety PLC by users in the Houston area's many complex process applications because it's a separate device with less processing power than a regular PLC and it's less configurable," Stank explains. "However, at the same time, Michigan's many automotive engineers and machine builders view PSR-Trisafe as a configurable relay because they just need some software to program it."
Also, PSR-Trisafe has been rebuilt into a 49 mm I/O format called SafetyBridge with eight inputs or outputs. "This allows safety processing on an I/O slice with PLC protocols and communications," Stank says. "This gives a configurable safety relay disributed control on a standard network."
Scott Saunders, COO at Moore Industries, confirms, "Users want more advanced relay features that allow them to move alarming and control features out of their safety PLC and DCSs. While existing safety systems filled present needs, adding more points required costly and tedious programming for even small point additions. So users asked for a safety shutdown alarm or switch that could be remotely mounted, easily programmed, and was cost-competitive, but still had the safety pedigree and approvals they required. This led us to develop smart, IEC 61508-compliant, microprocessor-based alarm trips with built-in diagnostics with alarm, on/off control capability, and TÜV and Exida safety approvals."
In the future, Saunders sees smart relays replacing many expensive and complex safety systems. "The entry cost of safety PLC and DCS systems often forces customers to make tough economic decisions, especially when only a few points are required, and then safety may end up taking a back seat to ROI," he says. "But single-channel, smart alarm trips and relays allow users to deploy simple, remote, low-cost safety solutions, and avoid expensive, full-blown implementations of complex safety control systems. This option makes implementing required and needed safety layers in plants a much easier proposition."
Revenge might be a dish best served cold, but for relays displaced since the 1970s, it must be sweet indeed.