ECT 459

1. ECT 459 Lecture 3 Sensor Transducers

2. Capacitive Sensors Electric Field C =  A d  = permittivity of dielectric [Farad/meter]  of vacuum= 8.85 x 10-12 F/m A=area [meter2] d=separating distance [meter] Sensors: (Conducting Materials) Change ε to detect foreign materials In field (position sensor, depth sensor) Change d to detect distance (position sensor, gauging). Change A to detect rotational angle. (flat, curved rotating plate) Capacitance is sensed by a change in capacitive reactance. Xc = 1 2fC Note that this is a non-linear relationship.

3. Capacitor/Inductor Transducer

4. Capacitive Sensor Applications Proximity Sensors - Using the object to be detected as one plate of the capacitor Proximity Sensors (position sensors) - Using the fringing effect and the object to be detected to change of the dielectric between the plates. Depth or gap sensors (differential position sensors) Surface finish sensors (measures average permittivity of surface/air combination) Rotary position sensors for angle measurement. Force or Pressure sensors (differential position) Temperature (differential position) Acceleration (differential position, accelerometer) Liquid depth (dielectrical change)

5. Capacitive Sensors Advantages – Capacitive Sensors Non-contacting Used with any target material that has a resistivity of less than 100 ohms/sq cm Sensor is extremely rugged and can be subjected to high shock loads (5000g) and intense vibratory environments Capable of use at high temperatures Constant sensitivity over wide temperature range Disadvantages – Capacitive Sensors Can only be used at a distance of ¼ the diameter of the sensor head before non-linearities develop. This distance increases to ½ the diameter of the sensor when a guard ring is installed. A guard ring is a grounded shield around the sensor head. Fringing (electric field lines try to go through the air and into the sensor head on the sides) Detected surface must be flat and clean and no moisture can be present in the air.

6. Inductive Sensors Magnetic Field L =N2A l  = permeability of core [Weber/(Amp.turn.meter) or n/Amp2]  of vacuum= 112.566 x 10-7 N/A2 N=number of turns A= cross-sectional area [meter2] l=length of coil [meter] Sensors: (Magnetic materials) Moving core to change µ and measure position. -linear motion or rotary motion Difficult to change N,A, and l in practice. Inductance is sensed by a change in inductive reactance XL = 2fL This relationship is linear

7. Inductive Sensor Applications Inductors Proximity sensors (magnetic materials) Proximity detectors (relays, burglar alarms etc.) Eddy Current sensors are a form of inductive sensor. The “treasure seekers” used on beaches to find metal objecxts are eddy-current sensors. Position, force, temperature, acceleration, or pressure sensor- (moving core) Transformers Current Probes Linear Variable Differential Transformer (LVDT) and Rotary (RVDT)

8. Inductive Sensors Advantages – Inductive Sensors Little effect of temperature on eddy current sensor (the dual coils on the sensing head are temperature compensated) Eddy current sensors can measure distance from both magnetic and non-magnetic materials. Material must be electrically conductive (A thin sheet of aluminum foil must be placed on the non magnetic material for eddy currents to exist.) Variable range Excellent linearity Excellent resolution Non-contacting Inexpensive Disadvantages – Inductive Sensors Limited range (for a greater range, a larger sensing head must be used)

9. Strain Gage Sensor Applications Wheatstone Bridge Measure Force or Weight in Load Cells 120 or 350 ohms

10. Strain Gage Sensors Advantages – Strain Gages Well-established Simple technology Low-energy Widely used Disadvantages – Strain Gages Small variation in resistance when a force is applied Sensitive to temperature Long wires lead to unbalanced bridge and electrical noise Circuit drift Non-linearity of Wheatstone Bridge

11. Reluctive Sensor Applications Variable Reluctance (Magnetic) Pickups (Tachometer) Hall Effect Sensors/Detectors Linear Displacement Sensors Angular Displacement Sensors Blood Flow sensor Wattmeter Magnetic Audio recording ATM card readers Magnetic Bar Codes

12. Reluctive Sensors Advantages – Reluctive Sensors Reliable Very Cheap Non-Contact Very Common Velocity independent Insensitive to ambient conditions Easily reproducible Zero Order system Disadvantages – Reluctive Sensors Only sense magnetic materials Distance Temperature effect Offset voltage

13. Piezoelectric Sensor Applications Ultrasonic transmitters and receivers. Frequency references. Temperature sensors (resonant frequency changes with temperature) Accelerometers (used with a seismic mass) Microphones and loudspeakers (small loudspeakers with poor audio characteristics = beepers) Pressure sensor Force sensor

14. Piezoelectric Sensors Advantages – Piezoelectric Sensors Low cost High sensitivity High mechanical stiffness Broad frequency range Exceptional linearity Excellent repeatability Unidirectional sensitivity Small size Disadvantages – Piezoelectric Sensors High Impedance Low Power Poor DC characteristics Drift with temperature and pressure

15. Magnetostrictive Sensor Applications Pressure Sensors Strain gages Medical Sensors Micromachined accelerometers

16. Magnetostrictive Sensors Advantages – Magnetostrictive Sensors Large Change in Resistivity Controllable manufacturing process Extremely Small Good low frequency response Disadvantages – Magnetostrictive Sensors ??

17. Thermoelectric Sensor Applications Power supplies Portable temperature monitoring

18. Thermoelectric Sensors Advantages Cheap Linear Response High and Low temperature Disadvantages Small Output

19. Light Sensor Applications Photoresistor Photodiode Phototransistor Photovoltaics Solar Cells Light Activated Silicon Controlled Rectifiers Optoisolators Doplar-light Optoswitches Sound measurement Fiber Optics

20. Light Sensors Advantages Cheap Non-contact Disadvantages Non-linear response Distance Ambient light affects them