By Mike Bacidore, Managing Editor
You've heard them speak. You might even be one of them yourself. The redundant "is" talker can turn a simple sentence into a complicated web of syntax.
"The problem is, is that we need to clarify our objectives."
"What you need to know is, is that there are more potential pitfalls lying in wait."
While the redundant "is" talker's duplication is unnecessary, redundancy in machine systems and networks can be critical. The trick is knowing when redundancy is necessary and beneficial and when it needlessly complicates the design and function and increases the cost. Redundant safety and power systems are often a given. In fact, Tobin Robertson, portfolio manager, advanced connectivity, Weidmüller (www.weidmuller.com), indicates one of the first questions to ask is whether there are any safety issues with a possible shutdown or loss of power. But what about control and I/O?
Dan Cox, director of engineering at AOC Resins (www.aoc-resins.com), a supplier of resins, gel coats and colorants headquartered in Collierville, Tenn., believes redundancy of electronics in plants is extremely overdone. "In my experience, the failure rate for redundant systems is greater than the failure for simplex systems," he explains. "The only benefit usually gained from a redundant controller setup is the ability for online upgrades, but this is typically outweighed by cost and complexity. In 20 years, the only controller failures that have occurred in facilities I have worked in were on redundant systems. In fact, redundant systems have caused me more heartburn than simplex systems have."
Even with redundancy, traditional design measures should be taken to minimize impacts, agrees Choy-Hsien Lin, development engineer, process control, Stora Enso Publication Paper, Hylte Mill, (www.storaenso.com) in Hyltebruk, Sweden. "If a failure doesn't impact the bottlenecks of the process, redundancy should be avoided in order to reduce the complexity and remove the additional point of failure that redundancy would introduce," he says.
When the organization can live with an interruption in the system for the time it takes to fix it, redundancy is optional, explains Lee House, CTO and VP of engineering, GarrettCom (www.garrettcom.com). "The availability of replacement equipment can be a factor in determining the need for redundancy," he says. "However, redundancy is often more cost-effective than dealing with the headache and cost of downtime."
Redundancy can involve a variety of technologies including rapid-recovery options for a switch utilizing a ring topology or mesh network, where there are two or more ports in each switch connected to different points in the network allowing data to continue to flow without interruption in the event that a switch in the network goes down; and dual-homing at the edge of a network, which allows that device to be connected to two separate points in the network so that there is no interruption if one of the network connections is interrupted, says House.
Some systems that operate intermittently and have inherent standby time for repair and maintenance may require minimal or no redundant systems, says Thomas Ferrara, VP director of engineering, Atlantic Industrial Technologies (www.aitzone.com), a designer and manufacturer of hydraulic and pneumatic systems in Shirley, N.Y. "The ripple effect of this type of subsystem is containable, and therefore the expense of implementing redundant systems is debatable," he says. "There exists a middle ground. A system without redundancy can utilize inexpensive redundant relays, which provide a poor man's redundancy. These redundant relays would use a majority voting scheme to circumvent relay or signal failure. These systems can also implement a quick repair cycle procedure to substitute for a true redundant system. Maintaining equipment such as performing hydraulic filter element changes, tracking amp-draw readings and maintaining a spare-parts inventory provide a good solution. The secret to a quick repair is having the spare parts pre-calibrated for the application. For example, spare PLCs and variable speed drives should be pre-loaded with the latest code to avoid a panicked telephone call to a since-retired programmer. The hydraulic pumps and valves should also be pre-adjusted for the application. And finally, there should be a procedure with easy-to-follow steps for component change-out."
The majority voting scheme lends itself to redundancy’s definition, explains Otto Fest, president of Otek (www.otekcorp.com). “Based on my understanding and experience, redundancy suggests the use of at least three identical systems monitoring or controlling a process, making a decision if and when one of the three did not agree with the other two and automatically disabling the differing unit,” he says. “Some individuals equate redundancy with a backup consisting of two identical instruments where, if one fails, the operator would have to take action and replace it, without disturbing the process. In other words, there’s no automatic process.”
The need for redundancy comes down to the application, explains Tyler Croft, product marketing manager, GE Intelligent Platforms (www.ge-ip.com). "There are applications that cannot shut down, in which case redundant controllers are essential so that routine maintenance can be done on one controller while the other is controlling the process. If you can't lose the controller and the program, then you need to have redundant CPUs. If they want to ensure that they do not lose I/O, then they should have redundant I/O."