TechArchive

Original article date: October 1996

Mark Knightley of CKS maintains that keyboards must retain their reliability in harsh environments even when being hosed down!

In recent years ruggedisation and environmental protection have enabled even the most sophisticated electronic equipment to be installed on the factory floor. Today computer-based systems are routinely near such ‘dirty’ processes as steel making and continuous casting as well as in the super-clean environments of food beverage and pharmaceutical plants where they face frequent hose-downs often with aggressive cleaning chemicals.

To achieve high levels of reliability under such conditions engineers have developed a battery of dustproof hose-down-proof and gas-tight enclosure designs. But not all the equipment can be mounted inside enclosures. This is particularly true of keyboards and other operator interface input devices which must be accessible to users and are therefore exposed to the environment.

It’s no secret that the keyboard can be an Achilles’ heel in dirty environments. Even in ‘soft’ office applications a spilled cup of coffee can spell disaster. On the shop floor an otherwise ruggedised industrial system can be rendered inoperable ? and the integrity and safety of the plant compromised ? all due to the use of an unsuitable keyboard.

To prevent problems of this kind industry is adopting an increasing variety of ruggedised sealed keyboards. Generally these keyboards are designed not only to survive in harsh industrial environments but also work with specialised software and so make it as easy as possible for workers to perform specific tasks. For example a given software programme may make it possible for a factory worker to perform a variety of tasks by only hitting a few function keys. To make it clear which keys initiate which tasks customised overlays are often placed on the keyboards.

Manufacturers of sealed keyboards typically offer a variety of enhancement products ? including mouse replacements (that is trackballs and pads). Pointing devices are virtually standard auxiliary components with many mouse-type pointers being integrated into the keyboard.

To ensure the quality and reliability of their products sealed keyboard manufacturers subject them to a variety of tests. The keyboards are dropped subjected to temperature extremes and undergo keystroke-failure testing. Typical operating temperature ratings are from 0 to +55?with keyswitch life expectancies available at greater than 20 million cycles.

Rugged technologies

Keyboards are essentially a collection of switches with circuitry to convert keystrokes into digital signals. Sealed keyboards add a number of innovative technologies to this basic design. There are various membrane-style keyboards built for industrial applications where slow to medium keyboard input is not an important factor. Many membrane keyboards use a substrate that has embedded circuits protected by a polycarbonate or polyester overlay installed in a watertight metal housing. Other varieties use full-travel membrane keyswitches.

A primary benefit of membrane keyboards is their flat profile. This is especially important in clean room applications such as food and pharmaceutical industries where sterilisation is critical. The flat profile of the keys doesn’t have crevices where bacteria can grow. These keyboards are also used in computer equipment designed for hazardous-rated areas.

One of the more interesting sealed designs uses piezoelectric elements in place of flexible membrane switch contacts. An activation force applied to the stainless steel graphic top panel causes piezoelectric elements to emit an active voltage. Although there is both visual feedback and audible feedback the rigid top panel precludes any tactile response.

Some manufacturers have resorted to such devices as conductive rubber pads and snap domes to overcome the membrane keyboard’s lack of tactile feedback (ie the ‘bounce’) that is so important to keyboard users. These however are susceptible to cracking tearing or collapse. Other manufacturers use stainless steel or moulded polyester domes as tactile elements.

In addition to their lack of tactile feedback most membrane keyboards are difficult to repair. Failure of a single switch will usually require replacement of the entire keyboard.

To overcome these weaknesses many manufacturers of sealed keyboards have concentrated on developing rugged reliable keyswitches with a polyester top sheet. This top sheet is then bonded to a rigid steel or aluminium plate. Square holes cut in the plate allow the embossed key caps to protrude and contact the bottom face of the polyester front sheet. This approach offers reliability and eliminates the tactile response problem.

• CKS
• Tel: 01707 322528
• Fax: 01707 372851

October 1996