Touchscreen HMIs

Touchscreen interfaces are everywhere in consumer electronics, and they’ve moved into the plant. Do you know how they work?


Resistive screens can be small enough to use with handheld devices, and a stylus makes it practical to use tight graphics. Courtesy: Control EngineeringOne of the most pervasive technology changes over the last few years has been a shift from mechanical buttons and mice as data entering and pointing devices to touchscreens. In all likelihood, you have at least one touchscreen equipped device within your reach just about any time, and you probably have some sort of HMI similarly equipped. The question is, do you know how they work?

There are two main technology approaches that have different operational and use case characteristics. Without going into too much detail, here’s how they work.

Resistive touchscreens use two flexible transparent sheets that are coated with a conductive material with known resistance characteristics. The two sheets are held apart with an air gap or microscopic insulators. Each sheet has a voltage gradient across its surface, and the two gradients are perpendicular to form an x- and y-axis.

When pressure is applied to the top sheet, the surfaces come into contact, creating a circuit between the two. A processor uses the resistance characteristics to calculate the intersection point and you have your input.

Resistive touchscreens are very common in applications like airport check-in kiosks, point-of-sale terminals in stores for self-checkout, and the like where only basic typing functions are needed since it can only read one point at a time. The flexibility of designing screens using this approach is obviously much higher than a keyboard and mouse, and this capability has been used in industrial HMIs for many years.

Screen designs can be small and still useful thanks to high resolution combined with a stylus to hit tiny virtual buttons. Since the thing touching the screen doesn’t have to be conductive, users can wear gloves or use a nonmetallic stylus. The downside of the approach is that it can eventually wear out or be damaged, causing permanent or erratic contact.

Capacitive touchscreens take advantage of the fact that a human being’s body is conductive. While there are variations to how it’s applied, a capacitive touchscreen also creates x- and y-coordinates by measuring changes in the field of the screen due to the user’s conductive finger. This doesn’t require a flexible screen component, but it does require that the user’s finger is not in a glove or otherwise insulated.

An iPad is one of the best-known applications of multi-touch technology, and sometimes it helps to have small fingers to touch those little buttons. Courtesy: Control EngineeringCapacitive touchscreens can be controlled in a way that allows them to sense more than one point of contact, which allows for multi-touch applications using swiping, pinching, and zooming gestures. Such uses are commonplace with smartphones and tablet computers, and they are also making their way to HMI applications. Companies using this approach often cite studies that suggest a multi-touch application allows a user to input information more quickly than a traditional keyboard and mouse, at least for individuals used to the techniques.

However, given the finger size of many individuals who work in industrial environments, there is an obvious limitation to the size of the virtual buttons that can be created on a screen. For example, an individual with large hands may have a problem trying to write a text message on an iPhone since the virtual keys are on 5 mm centers. Companies using this technology need to size graphics appropriately when users are expected to “fat finger” information.

While capacitive touchscreens are typically more expensive to manufacture than resistive, their durability and capabilities allow them to move into a wider variety of applications, including more HMI uses. The consumer electronics influence on industrial products is not hard to find and becoming increasingly common.


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