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Data Acquisition Electronics Unit – Lecture 6

Sensors and Transducers

Signal conditioning

Data sampling and recording

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Data Acquisition

SENSOR TRANSDUCERSIGNAL

CONDITIONING

ANALOG-TO-DIGITALCONVERSION

DATA COLLECTIONAND STORAGE

READOUTAND/OR DISPLAY

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Sensors and TransducersSensor – responds to a physical variable

Transducer – converts a sensor response to an electrical signal

Often the sensor and transducer are integrated into a single unit – example, the thermistor converts temperature into resistance.

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Some Physical Variables(possible sensor inputs)

Temperature Pressure ForceHumidity Light Intensity PositionRadioactivity Acceleration AttitudeMagnetic field strengthElectric field strengthChemical composition

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Some Electrical Signals(possible transducer outputs)

Resistance

Voltage

Current

Pulse frequency

Pulse width

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Temperature MeasurementConvert temperature to resistance? Use…

Thermistor, RTD (resistance temperature device)

Thermistor has negative TC, RTD has positive TC

Convert temperature to voltage? Use…

Thermocouple (Seebeck effect)

Convert temperature to current? Use …

Semiconductor junction devices

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Magnetic Field Measurement

Hall Effect devices – for higher field strengths

from a few tens to a few hundreds of gauss

Magnetoresistive devices – for lower field strengthscan sense small fractions of a gauss

(used in electronic compasses)

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Light MeasurementsIncludes Visible, Infrared, and Ultraviolet

Photoresistor – resistance decreases with light intensity

Photodiode – reverse current increases with intensity

Phototransistor – an “amplified” photodiode

Photomultiplier tube – most sensitive of allRequires high voltage supply

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Pressure MeasurementsSolid state pressure transducers

Can sense gauge or absolute pressure

Output is usually a voltage signal

Usually a hybrid electromechanical device

Temperature compensation is essential

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Signal Conditioning

Filtering to reduce noise or interference

Amplification or Attenuation

Level shifting

Span and Base adjustment

Impedance transformation

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Span and BaseApplies to both the physical variable and the electrical signal

Example:Temperature sensor: -20 to +150 degrees Celsius

BASE = -20 degrees, SPAN = 170 degrees

Transducer output: 4 mA to 20 mA of electric currentBASE = 4 mA, SPAN = 16 mA

So 11.6 mA corresponds to 60.8 degrees (work it out!)

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Analog to Digital Conversion

Changes a continuous electrical signal into a discrete numerical value, represented by a binary number.

Will be the topic of a separate presentation later on the course.

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Data Collection – Sampling Rate

The Nyquist RateA signal must be sampled at a rate at least twice that of the highest frequency component that must be reproduced.

Example – Hi-Fi sound (20-20,000 Hz) is generally sampled at about 44 kHz.

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BalloonSat as a Data Acquisition System

•BASIC Stamp BS2P24

•24LCxx EEPROM

•Real Time Clock

•4 channel A/D converter

•Voltage reference for ADC

•Temperature sensor

•4 LED indicators

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HOBO® Data Logger

Combines sensors, transducers, signal conditioning, A/D conversion, storage, and readout into a compact, battery powered unit.

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Further Reference

See: Input/Data Acquisition System Design for Human Computer Interfacing

By William Putnam and R. Benjamin Knapp

Included as an HTML document in the References folder of the Electronics Unit

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Activity

Use the HOBO® data Logger and BoxCar® support software to collect temperature data.