What Touch Screen Technologies Does National Instruments HMI Products Implement?
Hardware: Industrial Communications>>HMI and Industrial PC>>PPC-2015
Problem: With the growing popularity of HMI products I keep hearing about the various technologies implemented in touch screens. How many different types of touch screen are there? What are the differences? What touch screens does National Instruments use for its HMI product line?
Solution: There are three different types of touch screens:
Capacitive, Resistive and
Surface Wave.
•
Capacitive touch screen – electrically sensitive, responds to capacitive field of human body
- Must be touched with a bare hand or conductive device
- High clarity and ruggedness
A capacitive touch screen panel is coated with a material, typically indium tin oxide that conducts a continuous electrical current across the sensor. The sensor therefore exhibits a precisely controlled field of stored electrons in both the horizontal and vertical axes - it achieves capacitance. The human body is also an electrical device which has stored electrons and therefore also exhibits capacitance. When the sensor's 'normal' capacitance field (its reference state) is altered by another capacitance field, i.e., someone's finger, electronic circuits located at each corner of the panel measure the resultant 'distortion' in the sine wave characteristics of the reference field and send the information about the event to the controller for mathematical processing*. Capacitive sensors must be touched with a conductive device being held by a bare hand or a finger, unlike resistive and surface wave panels that can use anything that can point, such as a finger or stylus. Capacitive touch screens are not affected by outside elements and have high clarity.
In other words, when a user touches the screen, he/she draws some of the current from the circuit. The resulting changes to the electric field can be observed and the ratios of the fields according to the 4 corners of the screen can be calculated to obtain the position of the disturbance (the touch).
• Resistive touch screen – pressure sensitive, responds to any touch events
- Can respond to objects, gloved hands, etc.
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Medium clarity, high ruggedness
A resistive touch screen panel is coated with a thin metallic electrically conductive and resistive layer that causes a change in the electrical current which is registered as a touch event and sent to the controller for processing. Resistive touch screen panels are generally more affordable but offer only 75% clarity and the layer can be damaged by sharp objects. Resistive touch screen panels are not affected by outside elements such as dust or water.
• Surface Wave touch screen – ultrasonic wave sensitive, responds to any touch events, not widely used
- Can respond to objects, gloved hands, etc.
- High clarity, low ruggedness – it can be easily damaged
Surface wave technology uses ultrasonic waves that pass over the touch screen panel. When the panel is touched, a portion of the wave is absorbed. This change in the ultrasonic waves registers the position of the touch event and sends this information to the controller for processing. Surface wave touch screen panels are the most advanced of the three types, but they can be damaged by outside elements. Contaminants on the surface can also interfere with the functionality of the touch screen.
NOTE: National Instruments HMI products (NI FPT-1015, NI TPC-2006 and NI PPC-2015) only implement Resistive touch screens.
Related Links:
Products and Services: HMIs and Industrial Touch Panels Product Page
KnowledgeBase 3S8CTQX1: Commonly Used Terms for Human Machine Interface (HMI) Displays
Attachments:
Report Date: 02/16/2006
Last Updated: 09/28/2006
Document ID: 3UFER7G4
