Redundancy and Safety Tradeoffs in Industrial Design

May 16, 2008 1:32 PM

How do industrial and product designers or human factors engineers or ergonomics experts talk about the safety implications of redundant controls?

I'm having trouble finding a literature that I assume exists about the safety implications of providing redundant controls in product design.

In some cases it's commonsensically a safer strategy to design in multiple ways for a product or machine to solve a problem -- as in providing lifeboats at both ends of a ship.

In other cases, it's arguably dangerous to provide more than a single control. In a motor vehicle, for example, a single brake pedal seems commonsensically optimal: In a panic stop, you want your right foot to go automatically to where the pedal always is (it would be dangerous to find yourself trying to remember if the particular car you're driving has one of those optional second brake pedals that you can control with your left knee).

I'm particularly interested in how this difference gets discussed in the context of product safety (though it's interesting in terms of efficiency, user experience quality, and so on as well).

What are the words for this tradeoff in the literatures and jargons of design?

posted by gum to technology (3 comments total)

This principal is related to the conversations I saw in my graduate class in HFE around "critical controls." Brake pedals and similar controls are critical controls, since a single miss can have catastrophic results. Here's a snip from something that sounds like the stuff we read (from here though this is mostly focused on medical human-factors guidelines):

Guard Critical Controls. Controls can be vulnerable to accidental and unauthorized actuation. As an example, a caregiver might accidentally bump up against a ventilator control and start or stop a critical function. The conventional solution is to guard the control so that its actuation requires a deliberate action, like pressing and holding the power-on key to turn the machine on or off.

Guards can take many forms. Push buttons can be recessed or surrounded by a raised collar. Levers can incorporate interlocks, requiring the user to actuate a release mechanism before he or she can move the lever. Car makers recently adopted this approach to ensure that drivers apply the foot brake before moving an automatic transmission into drive. Some devices that incorporate a software user interface, such as a patient-programmable analgesic pump, require the caregiver to enter a code or password before operating the device. This approach is an effective way to keep unauthorized individuals—particularly hospital visitors—from meddling with control settings.

In general, for most "critical controls" that I have come across, there is only a single control. For other controls, the strategy of having multiple ways of accomplishing a given input may be appropriate. Often (but not always), this breaks down into "novice" and "advanced" functions in a system. This is because advanced users already know what they want to do and what settings they might want to perform their action, while a beginner might not. Having a way for an advanced user to quickly perform an action makes sense, especially when there will be such a thing as advanced users (think about the difference between an airplane cockpit, which is highly tailored for repeat and advanced users, and an ATM machine, which are all ever slightly different and are made almost without exception for novice users).

Sorry I don't have any other vocab than "critical controls".
posted by zpousman at 3:12 PM on May 16

aaaaaaand, I misued principle. Hit me in the face, plz.
posted by zpousman at 3:13 PM on May 16

"Critical controls" gets me a whole lot of really useful Google links. Thanks, zpousman -- you get a kiss, not a hit!
posted by gum at 5:06 PM on May 16