A good design of the user interface needs consolidated knowledge of the practices that have worked well in previous products. Innovative technical solutions are the challenge to be won in the engineering of advanced user interfaces
Article by Dario Gozzi
Cutting-edge user interfaces are perceived as higher-quality products for household appliance buyers. The growing demand for computerized displays and touch controls in a highly competitive market, has urged household appliance designers to shift from traditional controls to digital interfaces and displays. To satisfy consumers’ expectations, manufacturers are addressing a series of digital technologies that offer an elegant style, with programmable functions and very user-friendly cleaning, to customers. These solutions provide for a whole of touchscreens, sensors and wireless connectivity that on one hand offer a series of new advantages but on the other hand issue significant design challenges.
When they engineer an innovative user experience, household appliance manufacturers often clash with the sophisticated firmware programming and with the structural solutions that the interface requires. Although many engineers can imagine the skills of a digital interface, they might be uncertain about the choice of the surface material and the necessary space requisites.
The evolution of the user interface
A user interface is the interaction point between the person and the machine. At the simplest level, a user interface consists of a button, a switch or a dial that changes the operation modality of the product and displays operational information. These simple types of commands and controls have been part of household appliances for decades. Presently, industry and market consider mechanical solutions as problematic, most of all in kitchen environments. Pushbuttons and knobs may break down with the repeated daily use. The empty spaces between the mechanism and the panel are an “invitation” for liquids and fat to deposit and in time they degrade performances.
The various technologies dedicated to the interface aim at avoiding these problems. These solutions may include components such as the printed electronics, capacitive switches, flexible printed circuit boards and backlit LCD displays.
Membrane switches, printed with graphics that recall pushbuttons, are one of the first innovative approaches at user interface level and still today they are a valid solution in the household appliance world. A basic membrane switch has a flexible dome that, if compressed, operates the contact closure. The coating offers an insulating surface, smooth and easily cleaned, avoiding the contamination problems of conventional pushbuttons. A variant of the membrane control includes the addition of a metal-dome switch where the pressure point has an underlying steel dome that needs a force to be operated and provides touch feedback to the control. Buttons are enclosed with a plastic (or rubber) keypad with silkscreen printing superimposed on the dome. They are simple structures, which anyway withstand also the entry of water, still in use in smaller and cheaper household appliances, like TV set remote controls. Although they are still of common use, membrane switches have new competitors with the arrival of new technologies that offer attractive advantages, like in the case of capacitive touches.
Capacitive touchscreens
Consumers have become accustomed to the user interface of tablets and smartphones, which use a capacitive touch surface in front of an LCD screen. Touch capacitive surfaces in household appliances do not necessarily need an LCD screen behind them, surfaces often can simply show backlit icons or graphs. The active area under these icons responds to the finger touch and works like a pushbutton or a switch. The capacitive touch is a more and more popular option, able to offer an intuitive and interactive experience to users.
Consumers are more and more turning to this solution, especially in high-end household appliances. Touch capacitive surfaces have no mechanical mobile component that might wear out in time. The finger touch induces a capacity variation that is detected by a microcontroller, piloted by the firmware that translates the gesture into control. Besides the design of the firmware that interprets and carries out the user’s controls, designers must also consider various aspects that range from ergonomics to aesthetics, such as the backlighting quality, the readability of indications and of symbols, the touch sensitivity and audible alerts.
From the control under safety to backlighting
The electric glass cooktop can be taken as example in the demonstration of what difficulties must be faced in introducing the touch control. The traditional capacitive finish detects the presence of an object, but it cannot interpret the cause of the capacity variation. This lack of specificity can mean that a cat walking on the cooktop might accidentally switch elements on. Likewise, for a surface subjected to liquid splashes, spills might be the means to change the capacity, leading to functions turned on and off without intention.
A further difficulty derives from the fact that not all surface materials support the traditional capacitive touch solutions. Stainless steel, a fashionable finish in cooking appliances, is not compatible with the capacitive touch. Manufacturers can approximate the stainless-steel look and sensation in a plastic panel, but alternative technologies to the capacitive touch might be necessary for those who wish to use the real stainless steel.
Readability and backlighting are interdependent features. To optimize the combination of backlit graphics and of capacitive touch, a series of best practices in the flexible electronics field has emerged. The design of a touch capacitive region on a backlit sheet is often obtained through a transparent conductive ink called PEDOT. On a transparent polyester substrate, silver-based and PEDOT inks are used to create electronic printed circuit boards, to be interposed between the graphics and LEDs. PEDOT ink has scarce or null influence on the light transmission from LEDs to graphics, providing sharp backlit images.
The haptic feedback
The human somatosensory system includes a broad variety of sensations: vibration, pressure, temperature and movement. The touch feedback encompasses all these sensations, but it always addresses a somatosensory system whose simulation in its entirety is quite difficult. Human beings have five senses but the communication with electronic devices usually occurs through sight and hearing. The tactile feedback introduces the use of touch in the man-machine interface.
Some people love the sensation of a touch button click. The touch on a glass does not produce the satisfying click that warns about the occurred physical contact like in mechanical controls or, to a certain extent, in membrane or metal dome pushbuttons. A device, for instance, can slightly vibrate when it is pressed, giving a touch response to the user. The haptic feedback technology can be embedded in the circuits of the capacitive user interface. The haptic term derives from the Greek verb hápto and it means to put in contact, to touch. Digitizing on the smooth surface of touchscreens, if the function has been set up, it is possible to receive a haptic response (or haptic feedback). n practice, the finger touches a smooth and stiff glass, but the screen generates a small vibration that gives the illusion of the physicality of pressing a key.
The touch feedback is a real communication modality and not simply a specific application; it simulates the touch or the interaction with something that belongs to real life, creating precise vibrations. In daily life, the haptic modality frequently recurs whenever we interact with the surrounding environment. Without this haptic capacity, interactions would be very limited.
Beyond the capacitive touch
Innovations in the sector of user interfaces have looked beyond the capacitive touch, for instance the developments in the use of ultrasounds, of the inductive touch and of the principle of strain gauges are paths towards the touch-sensitive control on a metal surface.
Touch ultrasound sensors go beyond the borders of the spatial limitation. In ultrasounds control mechanisms, a small chip emits ultrasonic frequencies that are projected over the whole surface. When a finger touches the surface, the device can perceive that something has interfered with the sound wave propagation and can identify its position. Ultrasounds allow placing touch controls on all kinds of metal and separating electronics at a certain distance versus the contact point. Controls can be positioned, for instance, on a steel or aluminium plate of a certain thickness. Strain gauges are devices used to measure the deformation variations of an object. They work converting the object deformation into an electric signal that can be measured and analysed. Strain gauges can be used to create electronic touch surfaces combining them with a flexible material like a thin film that can be mounted behind a real metal surface. When it is touched or pressed with a certain force, it is deformed and changes its resistance, generating an electric signal. This signal can be amplified, processed and interpreted by a microcontroller to detect and to answer to the touch.
An inductive sensor responds to the variations of a magnetic field. Resembling a strain gauge, this kind of sensor is positioned behind a panel. This type of sensor allows positioning touch controls on metal surfaces and then it would work on the stainless-steel surfaces of household appliances. Being sensitive to pressure variations, it makes this technique less sensitive to accidental triggers.
Article drawn from Molex research; all images are by Molex
