How does a Touch Sensor Work?

How does a Touch Sensor Work?

Touchscreens have largely replaced the mouse and keyboard as the most popular input terminal on all kinds of devices. They are usually the only input device on your phone or tablet and even several brands of laptops and point-of-sale terminals are equipped with touchscreens now. So, here is How does a Touch Sensor Work?

Touchscreens come in two basic types, capacitive and resistive. Most phones use the capacitive variety because it registers multiple touches and responds quickly. The drawback is that it cannot be activated by non-conducting objects like fingernails or gloves. Resistive touchscreens are often used in car GPS screens and ATMS. Most resistive touchscreens don’t have the ability for multi-touch, because of the multiple layers of displays and the larger amount of force it takes to register a response. They are more durable, cheaper and can be used with gloves or fingernails. Resistance touchscreens are rarely used these days on popular smart devices, so this article will be focused more on capacitive touchscreens.

1. Resistive

These are the most basic and common touch screens, the ones used at ATMs and supermarkets, that require an electronic signature with a small grey pen. These screens literally “resist” your touch; if you press hard enough you can feel the screen bend slightly.

One of those thin layers is resistive and the other is conductive, separated by a gap of tiny dots called spacers to keep the two layers apart until you touch it. (A thin, scratch-resistant layer on top completes the package). An electrical current runs through those red layers at all times, but when your finger hits the screen the two are pressed together and the electrical current changes at the point of contact. The software recognizes a change in the current at these coordinates and carries out the function that corresponds with that spot.

Resistive touch screens are durable and consistent, but they’re harder to read because the multiple layers reflect more ambient light. They also can only handle one touch at a time – ruling out, for example, the two-finger zoom on an iPhone. That’s why high-end devices are much more likely to use capacitive touchscreens that detect anything that conducts electricity.

2. Capacitive

Unlike resistive touch screens, capacitive screens do not use the pressure of your finger to create a change in the flow of electricity. Instead, they work with anything that holds an electrical charge – including human skin. (Yes, we are comprised of atoms with positive and negative charges!) Capacitive touch screens are constructed from materials like copper or indium tin oxide that store electrical charges in an electrostatic grid of tiny wires, each smaller than a human hair.

There are two main types of capacitive touch screens –

  1. Surface capacitive uses sensors at the corners and a thin evenly distributed film across the surface (as pictured above).
  2. Projective capacitive uses a grid of rows and columns with a separate chip for sensing.

This was explained Matt Rosenthal, an embedded project manager at Touch Revolution. In both instances, when a finger hits the screen a tiny electrical charge is transferred to the finger to complete the circuit, creating a voltage drop on that point of the screen. (This is why capacitive screens don’t work when you wear gloves; cloth does not conduct electricity, unless it is fitted with conductive thread). The software processes the location of this voltage drop and orders the ensuing action.

Locating Your Touch 

The circuitry inside is virtually transparent, that is why you can’t see the grid in your phone. The location of the touch point is determined by the position on the grid where the capacitance is changed by your finger. The “X” position is sensed by the horizontal set of lines, the “Y” position is sensed by the vertical lines. Since your finger contacts many points on the grid forming an area, the microprocessor locates the center of the area to calculate a touch point.

Touchscreens are not pressure sensitive, but they require a certain contact area before they register a touch. If you touch the screen very lightly it may not register until you push harder, or form a larger contact area. On the other hand, if you’re pushing too hard, and forming too big of a contact area, this is known as fat-finger syndrome. Try again with less force and with more precision. Often this contact area is offset and you learn to compensate by pressing what seems to the edge of a key instead of directly over it.

Did You Know?

Newer touch screen technologies are under development, but capacitive touch remains the industry standard for now. The biggest challenge with touch screens is developing them for larger surfaces – the electrical fields of larger screens often interfere with its sensing capability.

Software engineers from Perceptive Pixel, which designs multi-touch screens, is using a technology called frustrated total internal reflection (FTRI) for their larger screens, which are as big as 82-inches. When you touch an FTRI screen you scatter light – and several cameras on the back of the screen detect this light as an optical change, just as a capacitive touch screen detects a change in electrical current.


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