Data Acquisition Electronics Unit – Lecture 6

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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
TRANSDUCER
SIGNAL CONDITIONING
ANALOG-TO-DIGITAL CONVERSION
DATA COLLECTION AND STORAGE
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 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 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 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
  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 E6

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