LSU 06/04/2007Electronics 61 Data Acquisition Electronics Unit – Lecture 6 Sensors and Transducers Signal conditioning Data sampling and recording

LSU 06/04/2007Electronics 62 Data Acquisition SENSORTRANSDUCER SIGNAL CONDITIONING ANALOG-TO-DIGITAL CONVERSION DATA COLLECTION AND STORAGE READOUT AND/OR DISPLAY

LSU 06/04/2007Electronics 63 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.

LSU 06/04/2007Electronics 64 Some Physical Variables (possible sensor inputs) TemperaturePressureForce HumidityLight IntensityPosition RadioactivityAccelerationAttitude Magnetic field strength Electric field strength Chemical composition

LSU 06/04/2007Electronics 65 Some Electrical Signals (possible transducer outputs) Resistance Voltage Current Pulse frequency Pulse width

LSU 06/04/2007Electronics 66 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

LSU 06/04/2007Electronics 67 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)

LSU 06/04/2007Electronics 68 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

LSU 06/04/2007Electronics 69 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

LSU 06/04/2007Electronics 610 Signal Conditioning Filtering to reduce noise or interference Amplification or Attenuation Level shifting Span and Base adjustment Impedance transformation

LSU 06/04/2007Electronics 611 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!)

LSU 06/04/2007Electronics 612 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.

LSU 06/04/2007Electronics 613 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.

LSU 06/04/2007Electronics 614 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

LSU 06/04/2007Electronics 615 HOBO ® Data Logger Combines sensors, transducers, signal conditioning, A/D conversion, storage, and readout into a compact, battery powered unit.

LSU 06/04/2007Electronics 616 Further Reference See: Input/Data Acquisition System Design for Human Computer InterfacingInput/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

LSU 06/04/2007Electronics 617 Activity Use the HOBO ® data Logger and BoxCar ® support software to collect temperature data.