a 6.0

PRACTICAL DESIGN TECHNIQUES FORSENSOR SIGNAL CONDITIONING

1 Introduction

2 Bridge Circuits

3 Amplifiers for Signal Conditioning

4 Strain, Force, Pressure, and Flow Measurements

5 High Impedance Sensors

n 6 Position and Motion Sensors

7 Temperature Sensors

8 ADCs for Signal Conditioning

9 Smart Sensors

10 Hardware Design Techniques

a 6.1

POSITION AND MOTION SENSORS

n Linear Position: Linear Variable Differential Transformers (LVDT)

n Hall Effect Sensors

u Proximity Detectors

u Linear Output (Magnetic Field Strength)

n Rotational Position:

u Rotary Variable Differential Transformers (RVDT)

u Optical Rotational Encoders

u Synchros and Resolvers

u Inductosyns (Linear and Rotational Position)

u Motor Control Applications

n Acceleration and Tilt: Accelerometers

a 6.2

LINEAR VARIABLE DIFFERENTIAL TRANSFORMER (LVDT)

~AC

SOURCE

VOUT = VA – VB

+

_

VOUT

POSITION +_

VOUT

POSITION +_

VA

VB

1.75"

THREADEDCORE

SCHAEVITZE100

a 6.3

SCHAEVITZ E100 LVDT SPECIFICATIONS

n Nominal Linear Range: ±0.1 inches (± 2.54mm)

n Input Voltage: 3V RMS

n Operating Frequency: 50Hz to 10kHz (2.5kHz nominal)

n Linearity: 0.5% Fullscale

n Sensitivity: 2.4mV Output / 0.001in / Volt Excitation

n Primary Impedance: 660ΩΩ

n Secondary Impedance: 960ΩΩ

a 6.4

IMPROVED LVDT OUTPUT SIGNAL PROCESSING

~AC

SOURCE

+ ABSOLUTEVALUE

ABSOLUTEVALUE

FILTER

FILTER

+

_

VOUT

_

POSITION +_

VOUT+

_

LVDT

a 6.5

PRECISION ABSOLUTE VALUE CIRCUIT(FULL-WAVE RECTIFIER)

V / I

+

+

_

_

×

± 1

COMPARATOR

gm STAGE

MULTIPLIER

INPUT

OUTPUT

a 6.6

AD598 LVDT SIGNAL CONDITIONER (SIMPLIFIED)

AMP ~

+

_

A – BA + B

ABSVALUE

FILTER

ABSVALUE

FILTER

FILTER AMP

VA

VB

VOUT

AD598EXCITATION

5-WIRE LVDT

OSCILLATOR

a 6.7

AB

AD698 LVDT SIGNAL CONDITIONER (SIMPLIFIED)

AMP ~

+

_

FILTER AMP

VB

VOUT

AD698

EXCITATION

4-WIRE LVDT

OSCILLATORA

B

VA

REFERENCE

A, B = ABSOLUTE VALUE + FILTER

a 6.8

HALF-BRIDGE LVDT CONFIGURATION

AB

AMP ~

+

_

FILTER AMPVOUT

AD698

EXCITATION

HALF BRIDGE LVDT

OSCILLATORA

B

REFERENCE

A, B = ABSOLUTE VALUE + FILTER

a 6.9

HALL EFFECT SENSORS

I I

T

B

VH

CONDUCTOROR

SEMICONDUCTOR

I = CURRENT

B = MAGNETIC FIELD

T = THICKNESS

VH = HALL VOLTAGE

a 6.10

HALL EFFECT SENSOR USED AS A ROTATION SENSOR

HALLCELLB

I

+

_

VH

VTHRESHOLD

COMPARATORWITH

HYSTERESISGAIN

MAGNETS

ROTATION

VOUT

a 6.11

AD22151 LINEAR OUTPUT MAGNETIC FIELD SENSOR

_

+

CHOPPERAMP

VCC / 2

R1

R2

R3

OUTPUTAMP

VCC = +5V

VCC / 2

TEMPREF

+

_

VOUT = 1 + R3R2

0.4mV Gauss NONLINEARITY = 0.1% FS

AD22151 VOUT

a 6.12

INCREMENTAL AND ABSOLUTE OPTICAL ENCODERS

LIGHTSOURCES

SENSORS

CONDITIONINGELECTRONICS

SHAFT

DISC

LIGHTSOURCES

SENSORS

CONDITIONINGELECTRONICS

DISC

5 BITS

SHAFT

INCREMENTAL

ABSOLUTE

5 BITS

θθ θθ

a 6.13

SYNCHROS AND RESOLVERS

R1

R2

S1 S2

S3

R1

R2

S1

S2

S3

S4

S1 TO S3 = V sin ωωt sin θθS3 TO S2 = V sin ωωt sin (θ θ + 120°)S2 TO S1 = V sin ωωt sin (θ θ + 240°)

S1 TO S3 = V sin ωωt sin θθS4 TO S2 = V sin ωωt sin (θ θ + 90°) = V sin ωωt cos θθ

ROTOR

ROTOR

STATOR

STATOR

ROTOR

STATOR

SYNCHRO

RESOLVER

V sin ωωt

V sin ωωt

θθ

a 6.14

RESOLVER-TO-DIGITAL CONVERTER (RTD)

COSINEMULTIPLIER

SINEMULTIPLIER

DETECTOR

INTEGRATOR

UP / DOWNCOUNTER

VCO

V sin ωωt sinθθ

V sin ωωt cos θθ

V sin ωωt sin θ θ cos ϕϕ

V sin ωωt cos θ θ sin ϕϕ

_

+

V sin ωωt [sin (θ θ – ϕ ϕ )]

ERROR

V sin ωωt ROTOR REFERENCE

STATORINPUTS

LATCHES

K sin (θ θ – ϕ ϕ )

ϕϕ

ϕϕ = DIGITAL ANGLE

ϕϕ

VELOCITY

WHEN ERROR = 0, ϕϕ = θθ ± 1 LSB

ϕϕ

a 6.15

PERFORMANCE CHARACTERISTICS FORAD2S90 RESOLVER-TO-DIGITAL CONVERTER

n 12-Bit Resolution (1 LSB = 0.08° = 5.3 arc min)

n Inputs: 2V RMS ± 10%, 3kHz to 20kHz

n Angular Accuracy: 10.6 arc min ± 1 LSB

n Maximum Tracking Rate: 375 revolutions per second

n Maximum VCO Clock Rate: 1.536MHz

n Settling Time:

u 1° Step: 7ms

u 179° Step: 20ms

n Differential Inputs

n Serial Output Interface

n ± 5V Supplies, 50mW Power Dissipation

n 20 Pin PLCC

a 6.16

LINEAR INDUCTOSYN

SCALE

V sin ωωt

V sin ωωt sin 2 π π XS

V sin ωωt cos 2 π π XS

SCALETRACES

SINE COSINE

SLIDERTRACES

TWO WINDINGS SHIFTED BY 1/4 PERIOD (90°)

EXPANDED

S

SLIDER

X

a 6.17

AC INDUCTION MOTOR CONTROL APPLICATION

VECTORTRANSFORMPROCESSOR

PWMPOWERSTAGE

(INVERTER)

ACMOTOR

RESOLVERRESOLVER TO

DIGITALCONVERTER

ADCsDSP

HOSTCOMPUTER

POSITION, VELOCITY

MOTOR CURRENTS

ADMC300, ADMC330, or ADMC331

a 6.18

ACCELEROMETER APPLICATIONS

n Tilt or Inclination

u Car Alarms

u Patient Monitors

n Inertial Forces

u Laptop Computer Disc Drive Protection

u Airbag Crash Sensors

u Car Navigation systems

u Elevator Controls

n Shock or Vibration

u Machine Monitoring

u Control of Shaker Tables

n ADI Accelerometer Fullscale g-Range: ± 2g to ± 100g

n ADI Accelerometer Frequency Range: DC to 1kHz

a 6.19

ADXL-FAMILY MICROMACHINED ACCELEROMETERS(TOP VIEW OF IC)

FIXEDOUTERPLATES

CS1 CS1< CS2= CS2

DENOTES ANCHOR

BEAM

TETHER

CS1 CS2

CENTERPLATE

AT REST APPLIED ACCELERATION

a 6.20

ADXL-FAMILY ACCELEROMETERSINTERNAL SIGNAL CONDITIONING

OSCILLATOR A1SYNCHRONOUSDEMODULATOR

BEAM

PLATE

PLATE

CS1

CS2

SYNC

0°

180°A2

VOUT

CS2 > CS1

AP

PL

IED

AC

CE

LE

RA

TIO

N

a 6.21

USING AN ACCELEROMETER TO MEASURE TILT

X

0°

+90°

θθ1g

Acceleration

X

–90°

–1g

0°

+1g

+90°

Acceleration = 1g × sin θθ

θθ0g

–90°

a 6.22

ADXL202 ±2g DUAL AXIS ACCELEROMETER

OSCILLATOR

DEMOD

DEMOD

DUTYCYCLE

MODULATOR

X

Y

SENSOR

SENSOR

32kΩΩ

32kΩΩ

+3.0V TO +5.25V

VDD VDDCX

CY

XFILT

YFILT

SELF TEST

RSET

T2

XOUT

YOUT

µC

T1

T2A(g) = 8 (T1 /T2 – 0.5)0g = 50% DUTY CYCLET2 = RSET/125MΩΩ

ADXL202

a 6.23

ADXL FAMILY OF ACCELEROMETERS

ADXL202

ADXL05

ADXL210

ADXL150

ADXL250

ADXL190

g RANGE

±2g

±5g

±10g

±50g

±50g

±100g

NOISEDENSITY

0.5mg/√√Hz

0.5mg/√√Hz

0.5mg/√√Hz

1mg/√√Hz

1mg/√√Hz

4mg/√√Hz

SINGLE/DUAL AXIS

Dual

Single

Dual

Single

Dual

Single

VOLTAGE/DUTY CYCLE

OUTPUT

Duty Cycle

Voltage

Duty Cycle

Voltage

Voltage

Voltage