This is the full text version of an article appearing in Job Shop Technology magazine, dated August, 1994. We suggest printing a hard copy of the article due to its length.

New Keyboard Technology Tames Tough Tasks

August, 1994-- It is always a difficult task to specify data input devices for products placed in harsh, abusive and rugged environments. Mechanical switch assemblies are expensive, and provide inadequate sealing from spills and contaminants. Conventional membrane switches offer a lower cost and stylish appearance, but have inherent design limitations which eventually lead to premature switch failure in harsh environments .

Membrane Switch Limitations in Harsh Environments

While membrane switches are a viable input device for most indoor, benign environments, several inherent design constraints make these products unsuitable in harsh environments.

The graphic overlay in membrane switches is limited to polyester or polycarbonate in thicknesses ranging from .007" to .010" inches. This extremely thin overlay is necessary in order to actuate the keypad with a force from 4 - 14 ounces of pressure. If the operator uses a ballpoint pen, screwdriver or other sharp object to actuate the keypad, the overlay can be easily pierced or dented, resulting in switch failure. For those membrane switch panels with large window areas protecting LCD's, CRT's, Vacuum Fluorescent Displays (VFD's) or other visual displays, the .007" - .010" inches of overlay protection is simply inadequate.
Membrane switches work by forcing materials through a spacer cavity to bring different conductive layers in contact with each other. This forced movement causes material stress, which can lead to cracking of the graphic overlay and switch failure. On many keyboards such as Automated Teller Machines this material stress is evident -- observe the "ring" worn into the most frequently used keys, such as the "Enter" key. This is simply the imprint of the spacer cavity rim under the graphic overlay as the overlay begins to crack from constant stress.
Air vents are required in membrane switch constructions to channel air between spacer cavities which allows for proper switch function. In certain environments contaminates can enter the vents in the switch package, and switches may collapse due to air pressure equalization problems.
Tactile feedback, a favorite feature in membrane switches, causes premature switch failure in many membrane switches. There are several reasons for this such as exaggerated material stresses created by the tactile feedback, and mechanical failures of the tactile domes due to temperature extremes and operator abuse.
For high volume data input requirements membrane switches are extremely slow input devices.

It should be noted again that in many "friendly" environments membrane switches are an adequate and cost-effective interface solution.

The Piezoelectric Advantage

The patented design of Dynapic keyboards solves many of the problems described above, and assures that the keyboard can withstand extremely harsh environments. The principle of Dynapic technology is its integration of piezoelectric material in the switching package. The piezoelectric material's natural properties cause the material to emit a voltage when it is even slightly deflected. The opposite is also true in that a piezoelectric material will move if it is excited by an electric voltage. Piezoelectric materials are extremely sensitive and have been used for years in stereo loudspeaker technolgy and in motion detection equipment.

By incorporating sensitive piezoelectric material into the switching package of Dynapic keyboards, graphic overlay materials and thicknesses can now be used that are infinitely more durable than the thin and fragile overlay materials necessary for membrane switches.

Dynapic keyboards have been tested in accordance with DIN 42 115 and withstand greater than 10 million (10,000,000) actuations per switch without failure, a substantial advantage when compared to membrane switches.

Packaging Flexibility Solves Problems

Probably the most important benefit of Dynapic keyboards is the broad and diverse range of packaging constructions, particularly with the graphic overlay material choices and thickness options. Some of the many graphic overlay material choices are:

Anodized aluminum - Extremely durable and vandal-resistant. The image is trapped in the anodic coating, making it difficult to remove. It can also be embossed for assisting in locating the keypads.
Metalphoto photosenstive anodized aluminum - Ideal for outdoor applications and exposure to salt spray and sunlight. It is cost-effective for short runs and prototypes and meets numerous Mil Specs for its image durability. It can also be embossed.
Etched aluminum - A durable option where the lines and characters are etched (recessed) and filled with paint.
Etched stainless steel - Offers etched lines and characters. It is the material of choice for environments where aluminum is forbidden.
Thick polycarbonate and acrylic - Allows for unlimited custom color selections and provides excellent window protection for large visual displays and readouts.
Polarizing film - Where the backlighting of the overlay must provide optimum contrast enhancement in sunlight or bright viewing conditions. A circular polarizing film can be used as the graphic overlay to reduce glare and reflection.
CR-39 (Allyl diglycol carbonate or ADC) - Provides superior resistance to chemicals and solvents, impact, heat and sparks, aging, gamma radiation and yellowing. CR-39 provides outstanding display protection for large windows and also satisfies numerous Mil Specs for optical filters.
Glass - Ideal for certain medical and dental applications where harsh chemicals, acids, solvents, blood and other body fluids are present. Glass can easily be cleaned and wiped down with a variety of chemicals and sterilizing agents without damaging the overlay.
Laminates of aluminum and polycarbonate/polyester - The best construction choice for embossing keypad perimeters or Braille in conjunction with custom colors.The thin plastic layer allows for embossing while the aluminum protects the switching package.

The overlay thicknesses range from .020" to .125" inches, depending on the material. Just the very slightest deflection of these rigid materials is sufficient to cause the piezoelctric material to emit a pulse and register a switch closure. These overlay materials add a dimension of durability and vandal-resistance to the keyboard that membrane switch overlay materials cannot come close to duplicating.

In some special environments such as NEMA-4, the Dynapic circuitry without a graphic overlay can be placed inside the product case against the front surface or the box door. Keypad graphics can be printed on the outside surface of the box. To actuate the keypad, the operator merely touches the case or door in the area where the keypads are printed. This prevents any need for cutting holes through the case and creates an obviously watertight situation.

The circuitry material that makes up the switching package can be either flexible polyester or rigid FR-4 printed circuit board material. Polyester circuit material provides a cost effective termination method with a flexible tail and connector system. Circuit boards can accommodate LED's, LCD's, IC's and other components such as resistors and capacitors. Dynapic circuits are completely encapsulated in a dielectric insulator. This prevents silver migration and circuit trace oxidation typically associated with membrane switches.

Embossing of selected areas is also possible in many Dynapic keyboard constructions. Keypad perimeters can be embossed to assist the operator in locating the keypad with the finger. Braille embossing is possible for keyboards operated by the visually impaired and for keyboards meeting ADA requirements. These embossing techniques are possible even in anodized aluminum!

The elimination of tactile feedback also contributes to the Dynapic panel's durability. As previously mentioned, tactile feedback decreases the life of a membrane switch. Audible feedback combined with visual feedback is often sufficient for the equipment operator. Again, embossing can also be incorporated to assist in keypad location.

Recent studies indicate that Dynapic keyboards (without the tactile feedback) have a higher acceptance rate than tactile or nontactile membrane switches. The Department of Industrial Hygiene and Applied Physiology at the Swiss Federal Institute of Technolgy, Zurich, Switzerland, presented a technical paper at the Marconi Keyboard Research Conference in February, 1994. The paper showed that piezoelectric keyboards have a comparable user acceptance rate to full travel keyboards and that membrane switches experienced a substantially lower acceptance rate than either piezoelectric or full travel keyboards.

Dynapic Applications

Due to the outstanding durability and vandal-resistance of these patented keyboards, a host of industries and product applications can benefit by specifying Dynapic keyboards as the operator interface or input device for various equipment.

ATM Machines are exposed outdoors to sunlight and temperature extremes. They are also extremely vulnerable to abuse from vandals as well as typical users.
Self-Serve Ticket Dispensing Equipment in public areas such as mass transit stations require superior protection from abuse, vandalism and possibly outdoor elements.
Mining Equipment must operate in an explosion-proof environment. Dynapic keyboards with non-arcing stainless steel overlays are a viable option.
Elevators switch panels are abused and actuated with such items as umbrella tips. Dynapic keyboards with metal overlays and Braille embossing provide function as well as durability.
Gasoline Pumps are exposed to outdoor temperature extremes and harmful UV rays from sunlight, as well as motorists actuating keypads with the pump nozzles create special problems that Dynapic keyboards can solve.
Food and Beverage Vending Machines are subject to abuse and vandalism by the general public.
Medical and Dental Equipment must be exposed to blood, body fluids, acids, alkalis, solvents and other sterilizing agents.
Military Equipment must resist sand, salt spray, sunlight and impact. Dynapic keyboards can meet these requirements and are also quiet and fast input devices.
Outdoor Test Equipment in sunlight have special viewing problems that Dynapic can solve. Temperature extremes and UV exposure can also cause membrane switches to fail.
Industrial Test Equipment that must pass NEMA-4 or Class 1 Div. 1, or intrinsically safe requirements are ideal candidates for Dynapic keyboards.
Offshore Drilling Equipment with outdoor extremes, salt spray, sunlight, mechanical abuse, impact and petrochemicals make Dynapic a logical choice for operator interfaces on this equipment.

Actuation Pressure

Dynapic keyboards typically operate with a standard force of 3 - 5 Newtons (6 - 12 ounces). This actuation pressure is controlled through the electronics, a unique technological benefit. Since a Dynpaic switch produces a voltage proportional to the operating force, the force to actuate the keypad can be set by simply raising or lowering the electrical switching threshold.

Electrical Considerations

Since Dynapic keyboards are piezoelectric keyboards, they require an electronic interface to convert the signal pulses. The most cost-effective way to create the interface is to design it into the product's electronics in the initial stages of electronic design. The manufacturer can provide assistance if needed. However, if a product is already in existence it may be impractical to totally redesign the existing electronics. In this case, the manufacturer can provide the Dynapic with an accompanying interface board. This would allow for a seamless retrofit where the Dynapic panel can replace the membrane switch panel and immediately be functional.

Cost Considerations

The initial price of Dynapic super-rugged keyboards usually exceeds that of membrane switches. In fact, if a membrane switch can withstand the environment in which it is placed, it is a cost-effective solution. It is specifically the rugged environment where membrane switches cannot last that Dynapic is more cost-effective. Despite the higher initial price of Dynapic keyboards, substantial cost savings are realized due to the long term life of the parts and the elimination of costly field failures, field servicing and replacement costs of traditional membrane or mechanical switch failures.

Careful attention to a variety of design issues can help make the Dynapic keyboard most cost-effective. The size of the part and its effect on printing and tooling layouts play a major role in the cost of the switch panel. Obviously, there is an optimal tooling and printing layout for the quantity to be produced. Accordingly, the production quantity will also determine how quickly the fixed production costs will be amortized.

Another primary area for cost efficiency is in the number of colors in the graphic overlay. Each color represents a separate screen, press setup and press run, and can greatly affect the cost of a panel.

Embossing requirements can also add to the piece price as well as to the tooling costs incurred.

Finally the interfacing scheme will affect the cost. Factors such as who designs the interface, how it needs to be designed, the components required, etc., will influence the overall cost of the panel.

Conclusion

Packaging flexibility with a wide range of overlay and circuitry materials allows the design engineer to specify a custom construction to meet his/her unique application requirements. Resistance to vandalism, temperature extremes, chemicals and abrasion means fewer keyboard failures and lower field replacement costs. No moving parts assures a keyboard of infinite durability and long life.

For More Information Contact:

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