Capacitive sensing

Capacitive sensing as a human-device interface (HDI) is becoming increasingly popular. Capacitive sensors can be recognized in many popular consumer products such as laptop trackpads, MP3 players, computer monitors and cell phones, but it is certainly not limited to these applications. More and more engineers choose capacitive sensors for their flexibility, unique human-device interface and cost reduction over mechanical switches. Capacitive touch sensors have become a predominant feature in a large number of mobile devices and mp3 players.

Capsense

CapSense is a capacitive sensing technique developed by Cypress Semiconductor. Capacitive sensing replaces mechanical buttons, membranes and other moving parts with a proximity-sensitive interface. Two electrodes are covered by an insulating stratum—frequently plastic or glass—and, when a finger touches the surface, a capacitance is created. This change in capacitance then triggers the execution of a pre-programmed function. The process relies on proximity based sensing, where the maximum sensible proximity is set to the thickness of the stratum overlay and does not strictly require physical contact as seen in touch sensing applications. It provides a control mechanism not subject to dirt, dust, wear, moisture, and other factors that can affect the life of other control interface technologies.

CapSense is based on Cypress’s PSoC, a fully programmable system-on-chip that takes input from the capacitive sensor. It works with a variety of sensors, and can interpret the inputs from multiple buttons, touchpads, and variable sliders simultaneously. Because of this flexibility, many consumer electronics manufacturers who make frequent changes late in the product design process have adopted CapSense. Cypress Semiconductor reports that interfaces based on CapSense have replaced over 2.5 billion mechanical buttons and sliders, and according to IMS Research, holds 70 to 80% market share for cellular handset capacitive sensing functionality.

While capacitive sensing applications can replace mechanical buttons with capacitive alternatives, other technologies such as multi-touch and gesture-based touchscreens are also premised on capacitive sensing. The Apple iPod click wheel is a well known implementation of CapSense.

Capacitive Sensors Design

Capacitive sensors can be constructed from many different media, such as copper,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 single 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 E-Field's path to ground, it is important to choose a ground plane that limits the concentration of E-Field lines without a conductive object present.

Designing a capacitance sensing system is requires picking the type of sensing material (FR4, Flex, ITO, etc).One also needs to understand the environment the device will operate in, the full operating temperature range, what radio frequencies are present and how the user will interact with the interface. PSoC is a configurable device which includes a royalty-free source code for implementing a capacitive sensing solution.

External links

• – Cypress Capsense
• - Build A Touch-Sensor Solution For Wet Environments
• - Cypress' PSoC CapSense Solution Achieves 70% Market Share
• – “iSuppli teardown reveals Apple's surprising choices for iPod nano” EMSnow
• - Capacitive Touch Sensing Design
• - The Art of Capacitive Touch Sensing
• Designing Capacitive Sensing Interfaces