Dave Mellor at Cyntech Components explores some of the past and current trends in industrial switch technology and the ever growing choice available
Twenty years ago, if you were looking for a control switch for an instrument panel or a keyboard there really wasn’t much choice. Quite simply it came down to deciding between clunky pushbuttons, micro-switches or non-aesthetic rotary switches. These types of switch were simply refinements of the switch technology that had existed for many years previously, which relies on the mechanical operation of electrical contacts to make or break an electrical circuit.
The terminology associated with such electromechanical switches will certainly resonate with those of us of a certain age as it very much defined the limits of what these switches could do. For example, the number of circuits that can be switched simultaneously being referred to as poles or whether a switch’s contacts are normally open (n.o.) or normally closed (n.c.).
This all started to change when membrane switches first appeared in the 1980s. Coupled with the wider use of digital electronics, this technology transformed the human machine interface (HMI) – if indeed that term even existed back then! Membrane switches were rapidly adopted across many industry sectors in applications ranging from consumer appliances through to medical instruments.
Membrane switch construction, using printed conductive inks and flexible materials, is inherently low-cost and lends itself to standard or custom keypad and keyboard layouts. Consequently membrane switches took a huge market share away from the traditional discrete pushbutton and rotary switches.
The printing technology used in membrane switches also had a major effect on UK manufacturers of full-travel QWERTY keyboards with production of the printed circuit boards and elastomeric key mats, which provide the tactile feedback needed to replace higher-cost mechanical keyboards, rapidly moving off shore.
Full elastomeric (silicone rubber) keypads, frequently found in consumer products such as TV remote control handsets, combine a more solid key top with the flexible key mat in a one-piece construction. Growth in the mobile phone market, driven by consumer demand also spurred the development of elastomeric keypads, with design and manufacture being briefly done within the UK. Elastomeric keypads offer many advantages: the keys can be moulded in various colours, printed with appropriate legends and backlit to illuminate the keys. Not only are elastomeric keypads inexpensive to produce, they also contribute to further savings down the line in the manufacture of end products, reducing inventory, handling and assembly costs as well as enhancing product reliability.
So membrane and elastomeric quickly took over as the market leading switch technology and still have a volume market in consumer type products. But nothing lasts forever and the past 5-10 years has seen an inexorable move to capacitive touch switches as exemplified by today’s touchscreen smart phones and tablet computers.
Clearly the driver for this development has been both technological and demand based i.e. while simple capacitive touch switches have existed for a long time, it wasn’t until capacitive touch sensors could be overlaid on LCD screens that demand for this type of solution really took off.
Early touchscreen technology that deployed a grid of infrared emitters and detectors around a screen only ever saw use in niche markets. Even resistive touchscreens that respond to pressure from a finger or stylus had limited application and never really captured the public imagination.
It’s interesting to note though that resistive touchscreens do avoid a couple of the problems of capacitive screens, namely they are much more resistant to the effects of moisture and can be used with gloves on, hence they are more likely to be found in certain applications such as medical equipment in hospitals.
The switch market today and where it may lead us tomorrow?
Where does this leave us today? Well all of the above technologies have a role to play. Mechanical pushbutton, rotary and micro- switches are still relevant typically in simple applications, especially where power is being switched directly rather than via electronic control circuits or relays.
While capacitive touchscreens may have stolen the limelight, particularly in high-profile (and high-end) consumer markets, there are many applications where membrane and elastomeric keyboards provide a perfectly acceptable and certainly cost-effective solution.
Regarding industrial controls, again any of the above switch types may provide the answer for a particular requirement. However the industrial environment may pose additional constraints on the choice of switch technology. Some issues, like the resistance to moisture and other contaminants, may be addressed by membrane keypads or resistive touchscreens. Others such as the need to operate reliably over a wide temperature range, or meet particularly rigorous lifetime specifications (e.g. number of switching operations) may demand a more rugged technology and switch construction.
One example of this is the piezo switch, which generates a switching current from force applied to the piezoelectric element. Piezo switches are extremely reliable and can be completely sealed from the environment and will withstand tens of millions of operations, which is why they are found in the heavy-duty, vandal resistance type keypads found in ATM terminals.
Where next? What seems clear is that increasingly, consumer demand drives innovation, leaving other markets to follow. Gesture control is rapidly evolving, biometric switching technology has potential and thought control may only be a matter of time!
Cyntech Components
