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Touchscreens

Touchscreens - Sensor OverviewBlossoms

Resistive

  • Several layers of conductive PTF with screened electrodes
  • Glass or PC backed
  • Typically Indium Tin Oxide (ITO) metallic film
  • Panel Acts like a two voltage dividers connected to an A:D controller.
  • Change in resistance, current is registered as a touch event
  • Change in resistance, current is registered as a touch event

Surface Acoustic Wave (SAW)

  • Uses ultrasonic waves moving along panel
  • Touches on the panel absorb some of the wave front
  • Controller electronics then register the touch location

Infrared (IR)

  • Series of IR LED and sensors arranged in the bezel
  • No A:D controller issues
  • Fixed resolution – based on LED placement.

Capacitive

  • Metalized surface of the glass (typically ITO or Au-ITO)
  • Controller uses difference between sensor and human touch for change in capacitance to determine touch event.
  • No films or optically bonded layers.  The sensor approaches 100% VLT.
  • Can only be used with a conductive medium (human finger, etc.) can not be used with stylus or gloved hand.

Strain gauge

  • SMT strain-gauges mounted on in the four corners on cantilevers
  • Resultant strain on the transducers is converted by mathematical algorithm to a touch event and location
  • Extraordinarily rugged and vandal resistant.
  • No effect of temperature, water, dust, etc.
  • Very expensive.

Optical Imaging

  • Relatively new take on IR screens, using optical sensors
  • Shadows cast on touch surface are used to calculate touch
  • Much improved resolution – esp. for larger rugged systems

Touchscreens - Resistive Function

Resistive Sensor Function

  • 4-Wire, 5-Wire and 8-Wire
  • Electrical – typ. 300-500 Ohm/sq. sheet resistivity
  • Clarity - 80% VLT (+/-5%)
  • Life expectancy – typ.35 Million touch (taps)
  • Life expectancy – typ. 1 Million touch (sliding)
  • Size range – 2.8” dia to 21” dia.
  • Warranty – 1 year limited
  • Typically 8-bit or 10-bit Analog-to-Digital Converter

4-Wire Touch screen

  • 4-Wire output
  • Lowest cost units
  • Voltage Divider
  • Life expectancy – typ.35 Million touch (taps)
  • Life expectancy – typ. 1 Million touch (sliding)
  • Size range – 2.8” dia to 21” dia.
  • Warranty – 1 year limited

5-Wire Touch screen

  • 5-Wire output
  • More expensive than 4-wire
  • Sensor trace is on dynamic layer, all electrodes on static layer.
  • More rugged, can be cut and still operate
  • Life expectancy – typ.35 Million touch (taps)
  • Life expectancy – typ. 1 Million touch (sliding)
  • Size range – 2.8” dia to 21” dia.
  • Warranty – 1 year limited

Touchscreens – Factors

Ergonomics

  • No tactile response – typically visual or audible response
  • Sensor uses a GUI with inifinite key positions/options
  • Some technologies can not work with a  stylus or gloved hand.
  • Resistive can be cut or damaged by sharp objects – designed for longest life with human finger.
  • Fatigue in long operations
  • Depending on mechanical design can have parallax issues.

Costs

  • 4-Resistive is probably the least costly T/S solution available.
  • First mass production was cell phones and games
  • A:D controllers and software is quite common.

Sizes

  • Valmark has available 4-wire, 5-wire and 8-wire
  • Typically can be from 2.8” diag. to 21” diag.
  • Also matrix touch sensors available – no controller required