34 eTech - ISSUE 6 eTech - ISSUE 6 35

The power of touchTechnological progress has dramatically changed the way that we interact with machinery, and reshaped assumptions of how user interfaces are designed. Early human-machine interface (hmI) designs were products of necessity.

Before electronic control systems became the norm, people

were accustomed to using mechanical force to interact with a piece of machinery. They used levers and long-travel switches to make and break physical contacts or move gears into place. The only places we see those types of control today are in systems that need a mechanical override, such as the emergency exit doors on trains or aircraft.

As electronic control began to predominate, the lever – which needed a lot of force to operate – gave way to the pushbutton that could switch between two closely spaced circuit traces. The assumption remained that users like to have tactile feedback when they use pushbutton-based interfaces. However, the rise of touch panels has demonstrated that people do not need the sensation of travel in a switch to be comfortable using a system.

Interfaces that use low-travel switching

technologies, such as membrane contacts, have been in used for many years. Touchscreen interfaces on mobile phones have demonstrated more recently that the absence of tactile feedback is no hindrance to usability, as long as the system responds consistently and immediately.

The use of additional visual, through changes in lighting, or audio feedback, such as the use of synthesised or sampled ‘click’ sounds, can be used to compensate for the reduction in tactile response.

One advantage of non-pushbutton designs lies in design flexibility. Take, for example, the AC3875 kit manufactured by Apem. This membrane keypad provides the customer the option to insert their own custom graphs for specialised designs. It is possible to punch holes in the keypad to show the state of LEDs mounted behind it as the individual pads are pressed. Overall, this allows the easy

prototyping and acceptance testing of HMI designs to gauge user reaction.

Because legends can easily be overprinted on custom layouts, the look and feel of the membrane keypad can be more easily integrated with the overall industrial design of the complete system. This can be used to provide the consistent look and feel that is considered by ergonomists to be an important factor in ease of use.

Many HMIs have to survive hostile conditions – they may be splashed by liquids that then seep into the equipment through gaps between the switches and mounting. And problems in outdoor equipment such as parking meters includes the actions of hostile users. Vandal-proofing pushbutton switches is difficult but manufacturers have succeeded in closing off avenues of attack so that buttons cannot be prised out of their mounting. Examples of vandal-proof products include the Schurter

“ The use of additional visual, through changes in lighting, or audio feedback, can be used to compensate for the reduction in tactile response”

1241 series that include O-ring seals to protect against liquid ingress.

As the travel distance on vandal-proof pushbuttons can be limited, many of these devices can be ordered with built-in LEDs that change state when pressed to give the user better feedback. Making the switches purely touch-sensitive rather than relying on switch travel can improve the resilience of the HMI and make possible a wider range of panel designs that are not restricted by the techniques that need to be used to protect moving elements against deliberate vandalism.

In difficult environments, the capacitive touchscreens favoured by high-end mobile phones and consumer products are often impractical. Because they rely on body capacitance, they frequently do not respond to users wearing gloves. Manufacturers such as Apem and ITW have developed technologies that provide touch-sensitivity

and resilience without the drawbacks of capacitive technology.

ITW’s ActiveTouch technology, employed in the company’s T01 series, uses built-in transducers to supply an ultrasonic signal that is damped by contact with a user’s finger. This reduces the ‘ring down’ time of the ultrasonic pulse, similar to the damping of a bell after it is struck. A microprocessor detects this change in decay and signals whether the button has been pressed or not.

Another approach is piezoelectric sensing, employed by Apem in its PBA series of buttons. A piezoelectric sensor uses the change in pressure on the surface of the button to change the voltage presented to a monitoring microprocessor.

This does not mean there is no place for the traditional pushbutton switch. People are used to the idea of the Stop button with its

mechanical feedback. In systems where it is important to demonstrate to the user that contact has been made without involving another form of feedback, tactile response remains important, providing applications for devices such as the Arcolectric T09 series. But the evolution of electronics has shown that there is much more to HMI design than tactile feedback. Manufacturers have responded to this evolution with a greatly expanded range of options that support all forms of industrial design.

See the latest aPEm and ITW hmI ranges at rswww.com/electronics

by Jerry abraham, Central Product manager, RS Components

Get more online...Share your views on hmI technology integration at www.designspark.com