ECE 480 – Team 6

In Order of Presentation:Ishaan Sandhu

Danny KangAnuar Tazabekov

Arslan QaiserEric Otte

How to Choose/Use Capacitive Sensors for Various Applications

Background• Three Main Types:

• Displacement

• Touch Sensor and iPod Click Wheel

• Accelerometer

• Automobile Suspension System, Air Bag System, ABS

• Pressure

• Tire Pressure

Background (cont...)

• Capacitance • Electric Fringe Fields• Dielectric differences

Background (cont...)

• Advantages of Capacitive Sensor• Higher accuracy• Smaller size• More reliable (compared to mechanical)• No moving parts (more durable)• No direct contact with the sensor• Sealed from dirt• Cheaper

Water level sensor

• Metal electrode• Ground vessel• Dielectric change

causes change in capacitance

Water Level Sensor (cont...)

• Figure shows simplified view• Typical Applications:

• Gas Tank Sensor• Water Reservoir• Production Vats

• Cheaper than similar sensors• More Accurate

Airbag System

• Required by Government law since 1999 model year

• Capacitive Pressure Sensor and Capacitive Accelerometer Sensor are used for passenger protection

• Sense a passenger.• Capacitive Pressure Sensor

• Use a thin diaphragm• Pressure is detected based on change in the

capacitance between the diaphragm and the electrode

Airbag System

• Sense a crash• Capacitive Accelerometer Sensor

• A change in electrical capacitance with respect to acceleration

Airbag System

Touch sensor

• Electrical disturbance• Two plate sensor• Surface charge• Overlay of 1-3mm

Touch sensor (cont...)

• Interaction with E-fields• E-fields projects through material• Responsive to conductive devices

Touch sensor (cont...)

• Change in capacitance • Dielectric permittivity influence

• Common applications– Touch screens (ex. iPhone, Nexus1)– Track pads

Vehicle Identification Capacitive Sensor

• Used at road toll stations

• Current Inductive sensors

• Drawbacks:

• Covers large area

• Senses only metal

• Not very accurate

• Solution: Use a capacitive sensor

Capacitive sensor structure

• 11 Excitation and 30 detection electrodes

• B0 (detection circuit and CtD)

• B0 connects to B1-B30

• Capacitance between plates A and B

• Sensor embedded into a ramp

• Help slow the cars down

• Stray capacitance

Structure (cont...)

• Cx is very small

• Cs1 and Cs2 are large

• Measured Cx:

V3 = 2fVcCxRf + V1-V2

• Detection of 0.001 pF

• Sensitivity 1V/pF

Capacitance measuring circuit

Capacitive Sensors for Thickness Measurement

Utilizes high-performance, complex sensors

Resolution down to less than one nanometer

Thickness measurement: capacitive vs. inductive

Ex: Measuring the thickness of silicon wafers

Capacitive Sensors for Thickness Measurement

• Components:• Probe: Device which emits

the electric field• Probe Guard: Shapes the

electric field• Circuitry: Measures very

small changes in capacitance using complex algorithms

Capacitive Sensors for Thickness Measurement

• Single channel:• Only one sensor probe• Non-flat surfaces produce

error

• Dual channel:• Uses two sensors placed on

opposite sides of object to be measured

• Nulls out curved surface errors due to differential operation

Capacitive Sensors for Thickness Measurement

• Typical Applications?• Machining• Manufacturing• Precision thickness

measurements

Conclusion

• Background• Water Level• Air Bag deployment systems• Touch sensors• Vehicle detection system• Thickness Measurement• Questions?