Domotics: Improving the man/machine interface in consumer electronics
01 December 2022
Nicomatic SwitchAir Dome with Hole
Every type of consumer electronic product – from HD & OLED TVs, fridges & washing machines, camera & video equipment, through to small electrical beauty aids, electronic games consoles & small electrical appliances – has become more sophisticated over the past decade. But the majority of domestic applications are still controlled by switches, either on the appliance itself or on a handheld remote control.
This article was originally featured in the December 2022 issue of EPDT magazine [read the digital issue]. And sign up to receive your own copy each month.
As Phil McDavitt, Managing Director at interconnect design & manufacturing specialist, Nicomatic Ltd tells us here, in domotics (domestic/consumer automation) applications – everything from dishwashers and doorbells to vacuum cleaners and washing machines – electrical switches remain the dominant human-machine interface (HMI)...
Every electrical switch is used in a different way and in different environments, and therefore many factors affect its lifecycle, which can vary considerably from what is achieved under strictly controlled testing conditions. Metal domes are used in membrane switches and keypads to enable the electrical connection from the switch or keypad to the product: they are how the end-user initiates a specific function. The switch domes are mounted on PCBs (printed circuit boards), FPCs (flexible printed circuits) and silver printed flex membranes. It is vital that the most suitable dome is chosen for each application and environment.
When selecting the right dome for an application, designers should ask themselves:
• Will the switch last the expected lifetime of the product?
• How many cycles will the dome be used for?
• How much force will a typical user press the switch with?
• How much tactile feedback does the user need?
• What environment will the product be used in?
• What size dome fits the application?
Nicomatic in white goods & domotics
• What shape is needed?
Let’s review these criteria in more detail.
Expected lifetime & number of use cycles
Dome lifecycle can vary depending on application and user. Industry standards for testing of metal domes and switches were developed in the 1990s, with a variety of standard tests required for metal domes. One for lifecycle, for example, simply depresses and releases a dome on a test bench until it reaches a predetermined number; before, during and after testing, the dome must be examined for changes. Of course, there are many other standard tests: life expectancy; overload test (static load); overstroke test; sweet spot test (actuator shifted off centre); OSO test (overload, sweet spot & overstroke); electrical measurement.
Testing results vary and lifecycles are different for all types of domes. Depending on the application, many factors affect the lifecycle and performance of a dome, including: strength of the user; size of the dome; shape of the dome (round, double round, oblong or 4-legged); whether it is dimpled or non-dimpled; if a higher or lower actuation force is selected; and how centred the dome is. Standard product lifetime for domes now typically stand at a minimum of one million cycles, far longer than was previously the case – and some specialised domes are rated for many more.
Actuation force & tactile feedback
The minimum force needed to depress the dome is the actuation (trip) force, which is measured in grams. This is one of the most critical factors to consider. A high value means it is difficult to activate the switch – firm pressure is required; a low value means the opposite. For testing purposes, the actuation force needed to depress the dome needs to be determined; then, the industry standard is to test by depressing the dome with 10% greater force than that required to reach contact closure.
Generally, the greater the actuation force needed to depress the dome, the lower the lifecycle of the dome will be; so, when selecting a dome that needs a long lifetime, it is best to select one with a low trip force. However, a balance has to be reached between low trip force and providing enough tactile feedback to the user so the person can feel that the switch has activated. Some applications need the trip force to be low and more sensitive to touch, and others less so.
Size & shape
The size of the keys on the membrane switch determine the size of the dome. The diameter and height are measured in mm. Domes come in different shapes: four-legged, triangle, round, oblong and custom. Four-legged domes have four blunt edges at each corner, and are the most popular shape, with excellent ventilation properties. Circular domes are generally the smallest; oblong domes require the greatest actuation force; and square domes can have the longest lifecycles. As well as these standard shapes, of course, it is possible to specify custom shapes.
In addition to their basic shape, metal domes are either dimpled or non-dimpled. ‘Dimpled’ domes have a small concave ‘dimple’ on the top (a central depression with a typical depth of between 0.0254 and 0.17 mm), whose purpose is preventing any excess electrical reaction when the switch is activated and reducing contact bounce – but this is not critical on printed electronics. While it’s true that in some applications a dimple can improve electrical characteristics, it can also significantly reduce activation life, since during the manufacturing process, the shaping of the dimple puts added pressure on the dome – and therefore, additional potential points of failure compared to a non-dimpled dome. For domestic applications, lifetime is key, as the domes will be pressed many times over the lifetime of the product. For this reason, we recommend domes without dimples for these applications.
Hitting the sweet spot
Every dome has what is referred to as the ‘sweet spot’. The sweet spot is the tolerance on the location of the actuator to the dome centre. It varies depending on various criteria: click ratio, size, shape, or actuation force. In a strictly controlled testing environment, the test probe is aligned with the ‘sweet spot’. But in real-world applications, the dome is not always actuated with 100% accuracy. The more the actuator is centred on the dome, the better the tactile feel and life expectancy. Metal domes have a larger sweet spot compared to polydomes; metal domes will actuate even if the finger is not in the absolute centre of the dome.
No matter the application, the combination of all the above factors must be considered, and the domes tested under real-world conditions, using the range of actuation forces likely to be encountered among the often very wide range of domestic end users – including with offset actuation, which is not uncommon with domestic appliances.
Novel approaches that solve problems
There are new approaches in dome technology that solve problems. Nicomatic’s Switch’Air™ domes are 4-legged metal domes with a notch that creates a cushion of air between the domes that prevents them sticking to each other (a common problem). They are also very low profile, suiting today’s low-profile, slim consumer devices. Switch’Air Domes are found in many domestic products, including leading brand microwave ovens, dishwashers and washing machines. They are even used in ATMs – an application in which consumers take robustly functioning keys for granted! Other innovations from Nicomatic are its round and double radius domes, with or without micro dimple and multiple dimples. The geometry of these domes makes them more robust against overload and better able to withstand overstroke and sweet spot tests. They are particularly suited to HMIs on very small consumer electronic items.
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