Touch Sensor Technology: Types and Principles

Nowadays, modern electronic devices are often touch-based. They’re found in ATMs, information kiosks, smart mobile devices, vending machines, and more. These touchscreen devices rely on touch sensors for easy user navigation through intuitive GUI functionalities.

Touch Sensor Types

There are various touch sensor types, including:

• 5-wire (or 4-wire) resistive
• Surface capacitive
• Projected capacitive
• Surface acoustic wave
• Infrared sensors

Let’s examine each type:

Wire Resistive Sensor

In this type, when a user touches the screen, two metallic layers make contact, causing current to flow. The contact point is determined by measuring the change in voltage. These sensors are more affordable, but can be damaged by sharp objects.

Surface Capacitive Sensor

These sensors activate with the touch of human skin or a stylus holding an electrical charge. A transparent electrode film is placed on top of a glass panel. When a finger touches the screen, it interacts with the static electrical capacity of the human body.

Some of the charge transfers from the screen to the user. Sensors at the four corners of the screen detect this capacitance change (decrease), allowing the controller to pinpoint the touch point.

Projected Capacitive Sensor

This type is similar to surface capacitive sensors but offers two key advantages:

• It can be activated with surgical gloves or thin cotton gloves.
• It can detect multiple touch points (multi-touch).

These sensors have a glass sheet with embedded transparent electrode films and an IC chip, creating a 3D electrostatic field. When a finger directly contacts the screen, the electrical current ratios change, allowing the system to detect touch points.

Surface Acoustic Wave Sensor

SAW touchscreen monitors use piezoelectric transducers and receivers to create a grid of ultrasonic waves on the surface.

A reflector is also placed on the glass. When the panel is touched, a portion of the wave is absorbed. The receiving transducer locates the touch point and sends this data to the system.

Infrared Sensor

This type relies on the interruption of a light path within an invisible light grid in front of the screen.

If an obstacle appears within the grid matrix, it interrupts the light beams, reducing the measured photocurrent in the detectors. Based on this information, X-Y coordinates can be determined. In these sensors, an array of emitters is placed behind two adjacent bezels of the screen frame, creating the optical grid.