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Capacitive Touch and Touch Screen

A touch more sensitive

We are familiar with switches, push buttons, keyboards, knobs and slider controls. It started in consumer products like mobile phones and MP3 players but has moved into all kind of devices now. We talk about touch sensors. Simple electrodes underneath the housing replace mechanical input devices with moving elements. The shape and layout of these sensor electrodes can be designed in a very flexible way, leading to appealing, modern product designs with enhanced usability. A wide range of elements can be implemented with touch sensor electrodes: simple buttons and keyboards, linear or circular sliders, transparent touch elements on displays or even buttons on wooden surfaces. As the sensor electrodes are placed inside the device, no openings are required. The housing is more robust and cost-effective and ideally suited for rough environments where dust and moisture could creep into the device. Especially for medical applications or devices used in clean environments such as the food industry, capacitive touch control enables hygienic casings.


Turning mechanics into electricity

Piezoelectric elements are used in transducers for a vast number of different applications. Piezoelectric materials generate an electrical charge in response to mechanical movement, or vice versa. The word piezo comes from the Greek word piezein, meaning to press or squeeze. Piezo-electricity refers to the generation of electricity or of electric polarity in dielectric crystals when subjected to mechanical stress and conversely, the generation of stress in such crystals in response to an applied voltage. In 1880, the Curie brothers found that quartz changed its dimensions when subjected to an electrical field and generated electrical charge when pressure was applied. Since that time, researchers have found piezoelectric properties in hundreds of ceramic and plastic materials. The basic theory behind piezo-electricity is based on the electrical dipole. At the molecular level, the structure of piezoelectric material is typically an ionic bonded crystal. At rest, the dipoles formed by the positive and negative ions cancel each other out due to the symmetry of the crystal structure, and an electric field is not observed. When stressed, the crystal deforms, symmetry is lost, and a net dipole moment is created. This dipole moment forms an electric field across the crystal. In this manner, the materials generate an electrical charge that is proportional to the pressure applied. If a reciprocating force is applied, an a.c. voltage is seen across the terminals of the device. Piezoelectric sensors are not suited for static or d.c. applications because the electrical charge produced decays with time due to the internal impedance of the sensor and the input impedance of the signal conditioning circuits. However, they are well-suited for dynamic or a.c. applications. A piezoelectric sensor is modelled as a charge source with a shunt capacitor and resistor, or as a voltage source with a series capacitor and resistor. 

Button (Inductive)

For more robust and reliable applications

An inductive button uses the motion of the button's actuation to induce a current in a circuit rather than physically connecting two circuits. This type of button has the advantage of a more robust and reliable operational mechanism, which can separate the working environment from the electronics. The technology of inductive sensing has been around for decades. It’s utilised extensively in industrial automation as a means for counting gear teeth or accurately measuring distance to a metal surface without contact. Inductive Sensing is a contactless, magnet-free short-range sensing technology that provides a low-cost, high-resolution sensing solution of conductive targets even in the presence of dust, dirt, oil, and moisture. Perfect for hostile environments and industrial applications this highly reliable sensing technology utilises coils and springs as inductive sensors creating an ultra-low cost system solution allowing precision measurements of linear/angular position, displacement, motion, compression, vibration, metal composition, and much more.

Haptic Feedback

Touching screens and resisting force

Modern smart phones and similar devices generally use haptic feedback for their modern touch screen use of vibrations to denote that a touch screen button has been pressed. The phone would vibrate slightly in response to the user's activation of an on-screen control, making up for the lack of a normal tactile response that the user would experience when pressing a physical button. The resistive force that some ‘force feedback’ joysticks and video game steering wheels provide is another form of haptic feedback. Improvements of the current generations of vibration motors, vibrators and software are in developing new piezo series of drivers. Piezo drivers allow for both a greater frequency and a greater range of vibration control, giving developers and OEMs more options when it comes to haptic interaction. In addition to standard whole-device vibration, the new piezo motor combines with extension points below a device’s screen to selectively restrict the feedback to the screen only.

Gesture Recognition (Optical or Other)

Sensing three-dimensional movement

Gesture recognition capabilities will enhance many home-entertainment, personal-computer and medical applications. Playing a console game has meant manipulating some kind of mechanical control: a joystick, a deck, or perhaps a pressure-sensitive pad. Now, a user can simply move fingers, hands, arms, and indeed their entire body to respond to game situations. A gesture-recognition device knows exactly where a gamer started a swing, the angle of the swing, the speed, and the follow-through. A game console can then replicate the movement on-screen and calculate a response. A gesture-recognition device senses three-dimensional movement by illuminating an area – a living room – with a particular wavelength of light. Capturing the reflection with a camera, it then interprets the data with sophisticated software and firmware. Today, these devices use infrared diode lasers to illuminate an area and band pass filters to ensure that cameras only receive the infrared light used to track motion. 


Sensors to disable accidental touch events

Proximity sensors detect approaching objects without direct contact. There are various technologies for proximity sensing like Electrical (Inductive, Capacitive), Optical (IR, Laser), Magnetic and Sonar. For example, a capacitive proximity sensor works on the capacitor principle. The main components of the capacitive proximity sensor are plate, oscillator, threshold detector and the output circuit. The plate inside the sensor acts as one plate of the capacitor and the target acts as another plate and the air acts as the dielectric between the plates. As the object comes close to the plate of the capacitor the capacitance increases and as the object moves away the capacitance decreases. The detector circuit checks the amplitude output from the oscillator and based on that, the output switches. Most smart-primary functions of a proximity sensor are to disable accidental touch events. The most common scenario is the ear coming into contact with the screen of the smart phone and generating touch events, while on a call. Device manufacturers came up with the idea of placing a proximity sensor close to the speaker, which will then detect any object in the vicinity of the speaker. If any object is present then the touch events can be assumed to be accidental and ignored.