You have now understood how capacitive touch screens work. Now let me help you in understanding how the designs are mad and how multi-tasking is performed.
Design: Capacitive sensors can be constructed from many different media, such as copper, Indium tin oxide (ITO) and printed ink. Copper capacitive sensors can be implemented on standard FR4 PCBs as well as on flexible material. ITO allows the capacitive sensor to be up to 90% transparent (for one layer solutions). The size and spacing of the capacitive sensor are both very important to the sensor's performance. In addition to the size of the sensor, and its spacing relative to the ground plane, the type of ground plane used is very important. Since the parasitic capacitance of the sensor is related to the electric field's (e-field) path to ground, it is important to choose a ground plane that limits the concentration of e-field lines with no conductive object present.
Designing a capacitance sensing system requires first picking the type of sensing material (FR4, Flex, ITO, etc.). One also needs to understand the environment the device will operate in, such as the full operating temperature range, what radio frequencies are present and how the user will interact with the interface.
There are two types of capacitive sensing system: mutual capacitance, where the object (finger, conductive stylus) alters the mutual coupling between row and column electrodes, which are scanned sequentially; and self- or absolute capacitance where the object (such as a finger) loads the sensor or increases the parasitic capacitance to ground. In both cases, the difference of a preceding absolute position from the present absolute position yields the relative motion of the object or finger during that time. The technologies are elaborated in the following section.
Surface capacitance:
In this basic technology, only one side of the insulator is coated with a conductive layer. A small voltage is applied to the conductive layer, resulting in a uniform electrostatic field. When a conductor, such as a human finger, touches the uncoated surface, a capacitor is dynamically formed. Due to the sheet resistance of the surface, each corner is measured to have a different effective capacitance. The sensor's controller can determine the location of the touch indirectly from the change in the capacitance as measured from the four corners of the panel; the larger the change in capacitance, the closer the touch is to that corner. As it has no moving parts, it is moderately durable. But it has limited resolution, is prone to false signals from parasitic capacitive coupling, and needs calibration during manufacture. It is therefore most often used in simple applications such as industrial controls and kiosks.
Projected capacitance:
Projected capacitive touch (PCT) technology is a capacitive technology which allows more accurate and flexible operation, by etching the conductive layer. An X-Y grid is formed either by etching one layer to form a grid pattern of electrodes, or by etching two separate, perpendicular layers of conductive material with parallel lines or tracks to form the grid; comparable to the pixel grid found in many liquid crystal displays (LCD).
The greater resolution of PCT allows operation with no direct contact, such that the conducting layers can be coated with further protective insulating layers, and operate even under screen protectors, or behind weather and vandal-proof glass. Due to the top layer of a PCT being glass, PCT is a more robust solution versus resistive touch technology. Depending on the implementation, an active or passive stylus can be used instead of or in addition to a finger. This is common with point of sale devices that require signature capture. Gloved fingers may or may not be sensed, depending on the implementation and gain settings. Conductive smudges and similar interference on the panel surface can interfere with the performance. Such conductive smudges come mostly from sticky or sweaty finger tips, especially in high humidity environments. Collected dust, which adheres to the screen due to the moisture from fingertips can also be a problem. There are two types of PCT: self capacitance, and mutual capacitance.
Mutual capacitance:
Mutual capacitive sensors have a capacitor at each intersection of each row and each column. A 12-by-16 array, for example, would have 192 independent capacitors. A voltage is applied to the rows or columns. Bringing a finger or conductive stylus near the surface of the sensor changes the local electrostatic field which reduces the mutual capacitance. The capacitance change at every individual point on the grid can be measured to accurately determine the touch location by measuring the voltage in the other axis. Mutual capacitance allows multi-touch operation where multiple fingers, palms or stylus can be accurately tracked at the same time.
Self-capacitance:
Self-capacitance sensors can have the same X-Y grid as mutual capacitance sensors, but the columns and rows operate independently. With self-capacitance, the capacitive load of a finger is measured on each column or row electrode by a current meter. This method produces a stronger signal than mutual capacitance, but it is unable to resolve accurately more than one finger, which results in "ghosting", or misplaced location sensing.
