Touch sensors, also known as haptic sensors, are part of the broad category of environmental sensors activated by touch. They are typically used in consumer devices (including touch screens), medical equipment, automotive control systems, industrial equipment and other interactive electronic devices. There are several types of touch sensors, but currently capacitive touch sensors and resistive touch sensors dominate this category. Capacitive touch sensors are activated by human touch or a conductive pen. They are widely used devices, ranging from device screens to home appliance controls, and have replaced mechanical buttons in many applications. In capacitive touch surfaces, a layer of electrical storage material (such as ITO (indium tin oxide), copper or printing ink) is sandwicled between insulators, such as the glass in a touch screen or the copper layer on a PCB. When conductive materials or objects (such as fingers) come into contact with the sensor surface, the sensor's controller detects changes in the electrostatic field, which can cause fluctuations in the electrical frequency of the oscillation circuit. Resistive touch sensors are mechanically triggered. It is placed between two conductive layers separated by an air gap, and pressure is applied to close the gap, thus creating contact between the layers. This type of sensor can be activated by non-conductive objects, including contact points triggered by robots. Due to the top layer's curvature, it is more prone to mechanical wear and tear than other touch sensors. Infrared or optical touch sensors respond to interference from surface light beams. The infrared emitter and infrared image sensor continuously scan the touch screen. When an object touches the touchscreen, it blocks part of the infrared light received by the sensor. Then, by using the sensor information, the contact position is determined through mathematical calculation and the corresponding operation is activated. Surface acoustic wave (SAW) touch screens use ultrasonic waves to detect touch inputs on the screen surface. The screen is made of glass or other transparent materials, and its surface layer is covered with a reflective material. Ultrasonic waves are generated by transducers located in the corner of the screen and propagate along the glass surface.

Touchscreen development

In 1965, Eric Arthur Johnson obtained the patent for his concept of capacitive touchscreen. His research originated from the British National Air Defense System. Air traffic controllers need a solution that can accelerate response times and allow for more accurate decision-making. Johnson's design includes an insulator coated with glass, which is covered with a transparent conductor made of indium tin oxide. Thin copper wires span across the CRT of a computer, enabling the circuit to sense being touched. His idea was eventually realized by British air traffic controllers in the 1990s. In the 1970s, Samuel G. Hurst invented the resistive touch sensor at the University of Kentucky. His research team calls this device Elograph. A few years later, the invention of a curved glass screen interface led to the realization of sensors in screen technology. Hurst's invention eventually became the core of Elographics, a company still in operation today.

Design Description

Connectorization

Touch sensors are usually connected by flexible cables to transmit the electrode signals of the sensors to the controller or microprocessor. The methods and materials depend on the type of sensor (capacitive or resistive) and whether the application is for consumer or industrial design.

Capacitive touch sensor: glass or polycarbonate surface layer, ITO electrode, optical adhesive, optional coating. Sturdy, chemically resistant, high hardness, transparent Resistive touch sensor: PET surface film, carbon /ITO electrode, interval point, laminated adhesive. Relatively soft, flexible, with medium durability and low transparency

Physical properties

Touch sensors are typically designed as thin laminated structures (glass or polymer) for high optical transparency, surface hardness and environmental sealing. The rugged version will add thicker protective lenses, stronger adhesives and coatings to withstand vibration, shock, moisture and ultraviolet radiation, while maintaining light transmission and low surface wear. IP rating/sealing mechanisms may also be integrated into the application design. For instance, the capacitive touch sensor behind the touchscreen is usually dust-proof and water-proof through a sealed design around the screen.

Reinforcement

Resistive touch sensors can resist dust and scratches, but they are less durable than capacitive touch sensors, which are more resistant to impact.

Temperature range: It depends on the application, but consumer-grade touch sensors typically can withstand temperatures from -30 °C to +70 °C. Industrial products will have a slightly broader range.

Electrical characteristics

Voltage: AC/DC Most touch sensors use low DC voltage, typically 1.8V - 5V (for integrated circuits IC) or 3.3V or 5V (for off-the-shelf sensor modules).

Current (amperes) : Capacitive sensors typically consume only a few microamperes to milliamperes when idle, but the current may increase when touch activation occurs.

Market and Application

Touch sensors are widely used in various markets of devices, mainly in screen technology for medical equipment, consumer electronics, automobiles and household appliances. They also exist in the peripherals of the game system, point-of-sale (POS) systems, as well as industrial control panels and interactive self-service terminals.