= ⊗ F e(V B)

Hall Devices
• Lorentz force A magnetic force:
The Lorentz force rotates the current flow lines through an angle θH
– Hall electric field EH
The deflection of the electrons results in charges forming EHy=+v× Bz
)Be(VF ⊗= rotate the current flow line
θH: Hall angle
Hall Voltage • Hall Voltage (the voltage across the plate)
VH = EHW = +vxBzW vx: electron velocity Bz: magnetic flux density W: width of the plate
neWd I
Hall Coefficient – Ideal Hall voltage
d : plate thickness, RH : Hall coefficient – Hall angle
σ: electrical conductivity of material – Metal : RH = -10-10 m3 / C (gold) – Semiconductors
d BIR
d BI
Hall Devices
104~105 larger for semiconductors
2. Can be made as an intrinsic part of nearly any bipolar a MOS process
increased sensitivity
5Sensors and Interfacing Magnetic Sensor devices
Characteristics of Hall Devices
1. Hall voltage is proportional to the current 2. Hall voltage is linearly related to the magnetic flux density 3. Hall voltage is inversely proportional to the plate thickness 4. Hall voltage is inversely proportional to the carrier density
(Hall effect is greater in semiconductor then in metals)
zzzH x
Hall Current Sensor
Magnetoresistors
The resistance of a semiconductor is influenced by the application of an external magnetic field.
)1()()tan1()( 2 2
2 0 BKRBRRR arz μθθ +=⇒+≅
Kar: a constant depends upon the aspect ratio of the plate
Magnetoresistors(Continued)
Application: Non-contact angle sensor rotation sensor, encoder (magnetic)
LVDT(Linear Variable Differential Transformer)
1% : N
V 6 ~ 4 :V
LVDT(Linear Variable Differential Transformer) (cont.)
• cutaway view of the LVDT
LVDT(Linear Variable Differential Transformer) (cont.)
• LVDT output voltage and phase as a function of core position
Magnetic Circuit • magnetic circuit and self-inductance
– Electrical circuit : e.m.f. = current × resistance here : m.m.f. = flux × reluctance
= φ × ℜ
..
Reluctance of Magnetic Circiut
pathflux theofarea sectional cross : Hm104 space free ofty permeabili :
materialcircuit theofty permeabili relative: pathflux theof length total:
)(
=ℜ −
πμ
μμμ
air gap causes a large increase in circuit reluctance and a corresponding decrease in inductance
Variable Reluctance Displacement Sensor

Magnetic Sensing Elements
dL
k
n
Electromagnetic Sensing Elements
Variable Reluctance Tachogenerator
flux mean :a cos)(
Turbine Flowmeter
amplitude measurement affected by loading effect A frequency system is preferred

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Capacitive Sensing Elements
separation :d ) A, f(d,:C plates theof overlap of area :A
constant dielectricor ty permittivi relative : pFm 8.85 vacuumofty permittivi:
C -1
Capacitive Displacement Sensors • Variable separation displacement sensor
x xd AC
andCrelation linear -non
Variable Dielectric Displacement Sensor
=
Capacitive Pressure Sensor
( )
)1(
0
Differential (push-pull) Capacitive Displacement Sensor
• still non-linear, but when C1 and C2 are incorporated into the A.C. deflection bridge, the output voltage and x is linear
• a plate M moving between tow fixed plates F1and F2
• x : displacement of M form the center line AB
( ) ( ) , 0 22
( )[ ]hl
ε Change C change C = 375 + 1.7RHpF
Capacitive Microaccelerometers
• Capacitive and force balance (servo) microaccelerometers => the deflection of the proof mass can be measured
capacitively
εis the permittivity of air A is the pick-up plate area
2 2
Switched-Capacitor Technique ( an interface circuit for capacitive sensors)
CR = reference capacitor, CPS = parasitic capacitance
F
Force-Balancing Technique for Capacitive Sensors
• Use of force feedback to the sensing micro structure to keep its position fixed.
• The amplitude of this signal used to generate sufficient force to achieve this is then a function of the parameter of interest. When MS in the neutral position, C1 = C2
When MS moves in response to the parameter of interest, a signal is produced proportional to the difference change in capacitance -> generate the feedback signal V0
-> feedback to MS to relocate MS to its neutral position
Filter C1
Structure and Circuitry of ADXL-50
From: Kovacs
From: Boser
Performance of Capacitive Microacceleromrter
ADXL-50
ADXL-50 sensor chip
Ultrasonic Sensor
• Ultrasound refers to sound waves at frequencies higher than the range of the human ear, i.e. at frequencies greater than about 18KHz.
• Ultrasonic has become a reasonable alternative to more costly and complex optoelectronic and ratio systems used for control and sensing.
• A basic ultrasonic system consists of four elements 1. Transmitter : Generate ultrasonic energy 2. Ultrasonic transducer : Converts electrical energy to airbone ultrasonic waves 3. A second transducer : Converts acoustic energy to an electrical signal 4. Receiver : amplifies this signal into useful electrical energy
Block Diagram of Ultrasonic Sensor • Block diagram — Transmitting / Receiving
Operation of Ultrasonic Sensor • waveforms
Beam angle of the ultrasonic transducer causes measurement error
Beam angle of ultrasonic transducers
Hall Devices
Hall Voltage
Hall Coefficient
Hall Devices
Magnetic Circuit
Capacitive Level Sensor
Force-Balancing Technique for Capacitive Sensors
Structure and Circuitry of ADXL-50
Performance of Capacitive Microacceleromrter
Operation of Ultrasonic Sensor

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