Resistive vs. capacitive touch panels

In all touch panels the same principle is applied to determine the position of the touch: two measurements are taken, the first one in the x direction and a few milliseconds later the perpendicular one in the y direction. Thanks to the short time between both measurements, also light touches can be reliably recognised.

4-wire (resistive) touch panel:

On a 4-wire touch panel 4 contacts are used to determine the exact position where the panel has been touched. Touch panels are made of different layers, a glass support with a conductive coating, an isolation layer with spacer dots and a scratch-resistant polyester protective glass. The support layer has been coated with the conductive Indium Tin Oxide (TIN). When voltage is applied to the surface of the touch panel, it is being conducted to the back of the screen, allowing it to calculate the exact position of the touch point. This is possible to compute thanks to the linear relationship between the contact point and the measured voltage. These are being transferred from the protective glass to the glass support, as the isolation layer relents to the applied pressure. The protective glass and the glass support are then at the same potential. As said before a second measurement follows the first one on a perpendicular direction. Both together give as a result the exact position of the touch point on the screen.

The problem with the positioning on a 4-wire touch panels is the fact that at least one of the measurements on the internal layer of the protective glass has to be used as reference for the identification of the touch point. The consequence of that is that 2 of the wires are being used to determine the position of the touch point while the rest 2 wires are now available. These are both on the protective glass and allow the measurement perpendicular to the first one. However, with the constant use of the screen, the top layer becomes deformed and the coating fragile. This results in inaccuracy or measurement errors in the determination of the position of the touch points. In order to overcome the problem 5-wire connections are used.

Another disadvantage of the 4-wire touch panel is the high sensitivity to environmental changes. The protective glass reacts to temperature changes and from time to time needs a recalibration of the touch screen.

Pros:

  • Cheap construction
  • Low power consumption
  • Operable with gloves or a pen

Cons:

  • Not as strong as the 5-wire touch panels
  • Environmental influences could lead to drift

Five - Wire Touch Panel:

The five -wire touch screen panels use the polyester layer only for determining the voltage applied to the contact point (not for determine the position as on 4-wire touch panels).

On 5-wire touch screens the four corners of the glass layer are connected. The position determination is more accurate solely by making it possible to determine the applied voltage at the contact point on the screen. Again, the principle of position determination is the same as in four- wire touch panel , however, non-linearities in the conductivity of the covering layer are not so decisive for the mere measurement of the applied voltage. The measurement is carried out at five-wire Touch Panel always on the cover layer and this is not used, as in four-wire Touch Panels, for the determination of only one of the components of the touch point .This extends the life of the touch panel, as nonlinearities in the cover do not lead to wrong position determinations.

By the use of the glass layer as a reference for the determination of the touch point, the environmental influences can be reduced, since the glass body is not so greatly deformed.

Pros:

  • Sturdier construction as 4-wire touch panels
  • Less influenced by environmental changes
  • Operable with hand gloves or pen

Cons:

  • Higher power consumption as 4-wire touch panels
  • More expensive than 4-wire touch panels

Glass-film-glass touch panels (GFG):

In order to prolong the life of touch panels and also to permit operation with quite sharp objects without destroying the sensitive polyester layer, glass-film-glass (GFG) touch panels have been developed. These panels differ from normal touch panels by a few millimetre thick sturdy glass layer applied over the polyester layer. This glass layer prevents the destruction of the polyester layer and makes way for a still clearer picture. In addition, the touch panel of the GFG may continue to be operable, even with deep scratches on the glass, since the glass is only used for protection and not for position determination. On a normal touch panel, already small scratches on the coating could drive to failure in the position determination of the touch point. The main disadvantage of the GFG touch panels is the much higher price compared to normal touch panels.

Pros:

  • Resistant to snow, ice, heat , dust and blood
  • Operable with any sort of arbitrary objects – even with sharp knives
  • Still functional even with deep scratches on the surface
  • Longer life as normal touch panels

Cons:

  • More expensive as other sort of touch screens

Capacitive touch panels

On capacitive touch panels the location of the touch point on the screen relies on the electrical properties of the human body to detect when and where the user touches the screen. These touch panels must be, contrary to the resistive touch panels, controlled only with bare fingers or very special devices. The operation of the capacitive touch panel is due to the fact that a change in capacitance at a certain location of the carrier, in most cases, a glass plate, generates a current at the ends of the plate. By measuring the current in the various corners of the touch panel, the position of the contact can be determined. Capacitive touch panels are distinguished by particularly sturdiness, since no film or anything similar is necessary. These touch panels are operated via a coated glass plate.

Pros:

  • Similarly strong to a GFG screens
  • Very accurate position determination

Cons:

  • Only controllable with the finger or very special pens
  • Expensive