What are Touchscreens? – About The Most Used HMI

Touchscreens are electronic displays-cum-input devices used in a wide variety of devices today. Over the past few years, touchscreens went from a technological fantasy to something so widespread that it’s now considered standard. Go back a few decades, and touchscreens would be looked at as nothing short of magic. Yet here we are, using them in our daily lives. It’s quite wondrous to think that technology is advancing so rapidly that in a couple of years, even what we consider impossible now would be very much doable. If you’ve ever wondered about how this technology works, you’ve come to the right place!

There are basically four types of touchscreen technology, used depending on cost or application. They are:

1. Resistive Touch Technology
2. Capacitive Touch Technology
3. Surface Acoustic Wave Touch Technology
4. Infrared Touch Technology

Resistive Touch Technology

Resistive touchscreen technology is considered the most affordable of the lot to implement, and uses quite a simple principle. It consists of two transparent and flexible layers which are electrically resistive, separated by a thin space where electric current is passed. These two layers are then protected by a scratch resistant glass layer, which functions as the actual conduit for whatever is being used to press it, like fingers or a stylus. When the device is on, an electric current continuously runs through this thin space, and any contact caused by touching the top layer to the bottom layer registers a location where contact has been established, which is then converted into input via software and/or drivers.

Picture Courtesy: Nokia Developers

Precision is taken care of by horizontal and vertical lines that run through these layers, thus pinpointing the exact location, something like a coordinate system, on an X and Y axis. During operation, when the two layers are pressed, they act like a pair of voltage dividers, acting on one axis at a time. First, a voltage gradient is applied to the top layer, and the second layer calculates the distance along the top layer, measuring the X-axis. The voltage gradient is then applied to the bottom layer to ascertain the Y-axis distance, with respect to the top layer.

Resistive touchscreens are usually used in commercial applications like supermarkets, hospitals, restaurants and more because it’s a more resilient technology. Since it also works with a stylus or gloves, it is more desirable in certain situations, even though its not as bright and accurate as capacitive touch. Poorer contrasts and the need to apply pressure to register touch, specially in mobile applications resulted in capacitive touchscreens being used more widely, although you can still find tablets and mobile phones that use it. It is also possible to create resistive touchscreens that ignore fingers or gloves and work only with a stylus, so it does have specific uses.

Capacitive Touch Technology

Using the principle of capacitive sensing, capacitive touch technology is a widely used and very versatile technology based on capacitive coupling, which takes human body capacitance as input. The difference between the dielectric of air and  the dielectric of fingers or a stylus is detected by the touchscreen, which then registers input. Capacitive sensing is also used in other sensors like proximity, humidity, acceleration and much more. It’s even used to create music, in an instrument called a theremin.

In this type of touchscreen technology, an electrically charged layer is placed on the glass panel, which loses some of its charge when a conductor such as a human finger touches it. The loss of charge is then measured by circuits at each corner of the screen, which calculates the relative distance and difference in charges by a reference charge, thus determining the location of of input. This information is then relayed to the software/driver that translates it into touch input for the device. This is called surface capacitance, and is used in low cost applications like touchscreen help kiosks, point of sale units, vending machines and such. It isn’t too sensitive or accurate, but comes with a lower cost than the other type of capacitive touch technology.

Picture Courtesy: Mr. Boingo

Projected capacitance is the second, and more accurate type of capacitive touch technology. It uses the principle of etching, which is basically removing layers of a wafer to create patterns, and in this case, electrodes. The conductive layer is etched with an X-Y grid, with mutually perpendicular and parallel lines, creating a perfect grid. There are a further two types of technologies under PCT (Projected capacitive touch), which are used in applications we know and use everyday, like mobile phones, tablets and the like.

• Mutual Capacitive Sensors: In this type, a capacitor is placed at the intersection of each row and column, and a voltage is applied to each row and column. When the user brings his/her finger near any part of the screen, the change in capacitance is noted by each point capacitor, thus registering a highly precise mode of input which is communicated to the controller, then the software/driver. In addition to higher accuracy, it also allows for multi-touch operation, detecting more than one point of contact.
• Self-Capacitance Sensors: This type is similar to mutual capacitance sensors, but registers input only from one column of row, since they work independently. They aren’t as accurate, but do register a stronger signal from the one row or column.

Capacitive touch technology is used in most high end mobile phones, tablets, gaming devices and more, and is much more responsive than resistive touch technology, but is not as accurate in some cases.

Surface Acoustic Wave Touch Technology

Surface acoustic wave touch technology is a very interesting technology that employs transducers to register touch input. Two transducers, one each for sending and receiving, are placed along the X and Y axis of a screen’s glass plate. Also added are reflectors, which reflect electrical signals sent from one transducer to another. When a touch event occurs on any part of the screen, the receiving transducer is able to understand that waves have been blocked or absorbed by an external source, and pinpoints the location of the disturbance, thus registering input information which is relayed to the software/driver.

Picture Courtesy: Touchscreen Magazine

Since there are no metallic parts used in this type of touchscreen technology, there is absolutely no loss in clarity of picture, or more specifially, 100% light is transmitted from the screen, compared to about 90% for capacitive touchscreens, and 75% for resistive touchscreens. It’s used in detailed graphics applications, where there has to be no loss in quality. However, it is far more expensive than other types of touchscreen technologies, and is thus used only in very specific situations.

Infrared Touch Technology

Picture Courtesy: Palm Tech

Finally, we have infrared touch technology, which uses an array of infrared LED and photodetector pairs placed around the edges of the screen. These beams are projected in vertical and horizontal patterns over the screen. Any disruption in the path of the beams is detected by the photodetectors, which translates into the location of the touch event. The best part is that infrared touch technology works perfectly well with different types of inputs, like fingers, styli, gloves and more. Unlike resistive touchscreens which depend on pressure for the two layers to touch together and capacitive touchscreens which depend on changes of dielectric, infrared touch doesn’t need any special input, but is more susceptible to dirt and dust. They are used in point of sale systems and some outdoor applications, and are usually not used in screens that are curved, because of parallax error.

History and Interesting Tidbits 

Even though touchscreens became popular only in the late 2000’s, the first commercial touchscreen product was available as early as 1983! Created by Hewlett Packard, the HP-150 was am Intel 8088, MS-DOS compatible desktop computer that accepted touch input via a 9″ Sony CRT monitor. It used the principle of infrared touch technology, using infrared emitters and detectors. It was of course, a very basic version of what we see now, so detected pretty much any non-transparent object.

Picture Courtesy: GUIFX Blog

In the early 1980’s, General Motors commissioned its Delco Electronics Division to create a touchscreen interface for some cars in the Buick brand. By 1985, the Buick Riviera came with an ‘Electronic Control Center’, a touchscreen embedded in the dashboard that provided real time information about the car, as well as controlled non essential functions such as the air conditioning and radio. While looked as a futuristic prospect, the rudimentary design, expensive production/repair and a measure of customer technophobia caused it to fail. Since it was the only way to control air conditioning, an important part of the vehicle, any malfunction, of which there were many, would render it completely useless, which was seen as a step backward.

The first touchscreen phone was launched in late 1993 by IBM, called the IBM Simon. It was basically marketed as a PDA-cum-cellular phone, and had the capabilities of faxing and email. The IBM Simon was moderately successful, and sold around 50,000 units in North America, priced at 899$ for a two year contract, and 1099$ without a contract. This interesting device is now know as the first smartphone, since the term didn’t exist back then.

Picture Courtesy: Microsoft Research

Touchscreens are now the most popular input systems in the world, and it probably won’t be long until we see it become even more widespread, being used in cars and household appliances. Considering the fact that touchscreens and underlying software can be programmed easily for multiple applications, it’s actually only a matter of time before all traditional input systems are rendered obsolete. Here’s to hoping there’s more intuitive human machine interfaces inbound!