Data Acquisition Electronics Unit – Lecture 6

Data AcquisitionElectronics Unit – Lecture 6 Sensors and Transducers Signal conditioning Data sampling and recording Electronics 6

Data Acquisition SENSOR TRANSDUCER SIGNAL CONDITIONING DATA COLLECTION AND STORAGE ANALOG-TO-DIGITAL CONVERSION READOUT AND/OR DISPLAY Electronics 6

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. Electronics 6

Some Physical Variables(possible sensor inputs) Temperature Pressure Force Humidity Light Intensity Position Radioactivity Acceleration Attitude Magnetic field strength Electric field strength Chemical composition Electronics 6

Some Electrical Signals(possible transducer outputs) Resistance Voltage Current Pulse frequency Pulse width Electronics 6

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 Electronics 6

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) Electronics 6

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 Electronics 6

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 Electronics 6

Signal Conditioning Filtering to reduce noise or interference Amplification or Attenuation Level shifting Span and Base adjustment Impedance transformation Electronics 6

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!) Electronics 6

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. Electronics 6

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. Electronics 6

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 Electronics 6

HOBO® Data Logger Combines sensors, transducers, signal conditioning, A/D conversion, storage, and readout into a compact, battery powered unit. Electronics 6

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 Electronics 6

Activity Use the HOBO® data Logger and BoxCar®support software to collect temperature data. Electronics 6

Data Acquisition Electronics Unit – Lecture 6