Transcript Data Acquisition Electronics Unit – Lecture 6

Data Acquisition
Electronics Unit – Lecture 6
Sensors and Transducers
Signal conditioning
Data sampling and recording
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Data Acquisition
SENSOR
TRANSDUCER
DATA COLLECTION
AND STORAGE
SIGNAL
CONDITIONING
ANALOG-TO-DIGITAL
CONVERSION
READOUT
AND/OR DISPLAY
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Sensors and Transducers
Sensor – 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
Force
Humidity
Light Intensity Position
Radioactivity Acceleration
Attitude
Magnetic field strength
Electric field strength
Chemical composition
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Some Electrical Signals
(possible transducer outputs)
Resistance
Voltage
Current
Pulse frequency
Pulse width
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Temperature Measurement
Convert 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 strengths
can sense small fractions of a gauss
(used in electronic compasses)
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Light Measurements
Includes 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 all
Requires high voltage supply
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Pressure Measurements
Solid 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 Base
Applies 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 current
BASE = 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 Rate
A 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.
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