Basics of Sensor Compensation for Capacitive Sensors January 22, 2013 Randy Frank

Agenda Capacitive sensing Techniques and available options that simplify users design-in of capacitive sensors Touch sensors Capacitive proximity sensor Capacitive pressure sensor Capacitive sensor interfaces

Capacitive Sensing Global Capacitive Sensors Market worth $17.89 billion by 2018 Growing at a CAGR of 15% from 2013 to 2018 Motion Proximity Pressure Touch http://www.heraldonline.com/2013/01/07/4523238/global-capacitive-sensors-market.html

Capacitive Sensors C = K* Ε0 * A/D Where Q =C*V Where: C= Capacitance (farads) K= The Dielectric Constant of the material between the plates. (Air=1.0) Ε0 = Permittivity of free space (a constant) A= Area of the plates (Meters2) D= Distance between the plates (Meters) Q =C*V Where: Q = Current (amps) C = Capacitance (farads) V = Voltage http://www.microsense.net/caa60b50-cb74-4646-9f8b-2bfbf1ff8d85/resource-center-cps-delivery.htm

Touch Sensing The body provides one electrode http://www.embedded.com/design/embedded/4008781/Getting-in-touch-with-capacitance-sensor-algorithms

Types of Capacitive Touch Sensing A capacitive touch sensor is based on one of the following capacitance types: 1. Surface capacitance: uniformly coated glass 2. Projected Capacitance: These are subdivided again into two types: Self capacitance Mutual capacitance http://www.eetimes.com/design/industrial-control/4371081/An-introduction-to-Capacitive-Sensing-Part-I

Mutual Capacitance Dual layer ITO (indium tin oxide) Every intersection has its own mutual capacitance http://www.eetimes.com/design/industrial-control/4371081/An-introduction-to-Capacitive-Sensing-Part-I

Sigma-Delta Circuit Integrating to reduce measurement time and noise http://www.eetimes.com/design/industrial-control/4371081/An-introduction-to-Capacitive-Sensing-Part-I

Touch Sensing Comparison Cypress Semiconductor http://www.eetimes.com/design/industrial-control/4371081/An-introduction-to-Capacitive-Sensing-Part-I

Capacitive and More Freescale capacitive and resistive touch screen sensing CRTOUCH Ready to use http://cache.freescale.com/files/32bit/doc/data_sheet/CRTOUCHDS.pdf

Touch Eval Kit Microchip Technology: enhanced mTouch Capacitive Touch Evaluation Kit Example touch designs MCU hardware Software http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=1406&dDocName=en541143

Proximity Sensor http://www.microsense.net/caa60b50-cb74-4646-9f8b-2bfbf1ff8d85/resource-center-cps-delivery.htm

Proximity Sensing http://www.microsense.net/caa60b50-cb74-4646-9f8b-2bfbf1ff8d85/resource-center-cps-delivery.htm

Capacitive Sensing Vs. Inductive sensors – metallic objects Photoelectric sensors – needs light source Magnetic sensors – detect magnetic materials

Tubular Capacitive Sensors Eaton E53 for sensing liquid level and powdered or granulated materials. The self-contained devices detect both metallic and nonmetallic targets. http://www.eaton.com/Eaton/ProductsServices/Electrical/ProductsandServices/AutomationandControl/SensorsLimitSwitches/CapacitiveProximitySensors/index.htm

Interface Circuitry in Proximity Sensor PEPPERL+FUCHS As a target approaches a capacitive sensor, the oscillations increase until they reach a threshold level and activate a switching device. http://www.claytonengineering.com/Training/myweb6/Sensors/F_Ind&Cap_Ss.pdf

Capacitive Pressure Sensor IC Infineon KP236 - Absolute Pressure Sensor Full compensation Broken wire detection High sensitivity and linearity Customized transfer functions Optional output clamping http://www.infineon-designlink.com/index.php?language_id=2&article_red_id=34

Capacitive Sensor Interface ZMDI’s ZSSC3123 CMOS IC for capacitance-to-digital conversion For sensors with capacitances up to 260pF and a sensitivity of 125aF/LSB to 1pF/LSB (depending on resolution, speed, and range settings) Accuracy as high as ±0.25% FSO@ -40 to 125°C Output: I2C, SPI, PDM or alarms Avoids external calibration trimming http://www.zmdi.com/zssc3123

Capacitive IC Block Diagram Humidity sensing Weight scale Tension control ZSSC3123 http://www.zmdi.com/zssc3123 Pulse Density Modulation (PDM) output

Programmable Controller Analog Devices AD7142/43 >1 fF resolution 14 capacitance sensor input channels http://www.analog.com/library/analogDialogue/archives/40-10/cap_sensors.html

Summary Capacitive circuits are not nearly as common as piezoresistive Touch sensors are the exception Many suppliers offer touch sensing Capacitive Interface ICs simplify this task

Topics Later This Week Wednesday A case study of smart sensor development Thursday Transitioning from analog sensing to the digital control world Friday Today’s smart sensors