Data Acquisition Electronics Unit – Lecture 6 Sensors and Transducers Signal conditioning Data sampling and recording Prepared by Jim Giammanco <giamman@rouge.phys.lsu.edu> LSU 09/23/2013 Electronics 6

Data Acquisition SENSOR TRANSDUCER SIGNAL CONDITIONING DATA COLLECTION AND STORAGE ANALOG-TO-DIGITAL CONVERSION READOUT AND/OR DISPLAY LSU 09/23/2013 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. LSU 09/23/2013 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 LSU 09/23/2013 Electronics 6

Some Electrical Signals (possible transducer outputs) Resistance Voltage Current Pulse frequency Pulse width LSU 09/23/2013 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 TC is “temperature coefficient.” The resistance of an RTD increases with temperature, that of a thermistor decreases. Thermocouple outputs are only a few millivolts. Semiconductor temperature sensors usually output a few microamps per kelvin. SkeeterSat uses a thermistor to control the pitch of the audio pulses. LSU 09/23/2013 Electronics 6

Silicon Diode Temperature Sensor Forward voltage drop of Si Diode is a function of temperature. dV/dT is approximately -2.1 mV/oK above about 20 oK. TC is “temperature coefficient.” The resistance of an RTD increases with temperature, that of a thermistor decreases. Thermocouple outputs are only a few millivolts. Semiconductor temperature sensors usually output a few microamps per kelvin. SkeeterSat uses a thermistor to control the pitch of the audio pulses. Lakeshore Instruments LSU 09/23/2013 Electronics 6

Silicon Diode Temperature Sensor Constant current source (CCS) should provide small current to minimize self-heating of the diode. 1N457A Si Diode at 1mA for example. Cheap CCS: Voltage source and Big Resistor. R = E/I Better CCS: LM234 (see datasheet for details) TC is “temperature coefficient.” The resistance of an RTD increases with temperature, that of a thermistor decreases. Thermocouple outputs are only a few millivolts. Semiconductor temperature sensors usually output a few microamps per kelvin. SkeeterSat uses a thermistor to control the pitch of the audio pulses. LSU 09/23/2013 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 LSU 09/23/2013 Electronics 6

ICS1230 Pressure Sensor 1.5 mA excitation Pressure is proportional to difference of V2 and V1. At 0 millibars V2-V1 = 0. At 1000 millibars V2-V1 ~ 50-100 mV. See datasheet 1230-A015-3L LSU 09/23/2013 Electronics 6

Signal Conditioning Filtering to reduce noise or interference Amplification or Attenuation Level shifting Span and Base adjustment Impedance transformation LSU 09/23/2013 Electronics 6

Span and Base Example: Temperature sensor: 450 mV to 800 mV BASE = 450 mV, SPAN = 350 mV Signal Conditioning Output: 0V to 3V (to match ADC) BASE = 0V, SPAN = 3V 11.6 mA is 7.6 mA above the base. (7.6 mA)/(16 mA) represents 47.5% of the output span. 47.5 % of the temperature span is 80.8 degrees, so the temperature is 80.8 degrees above the base temperature (-20), or 60.8 LSU 09/23/2013 Electronics 6

Span and Base Gain = 8.571 Offset = -3.857 Design and Build a circuit that performs this transfer function. What do we need? 11.6 mA is 7.6 mA above the base. (7.6 mA)/(16 mA) represents 47.5% of the output span. 47.5 % of the temperature span is 80.8 degrees, so the temperature is 80.8 degrees above the base temperature (-20), or 60.8 LSU 09/23/2013 Electronics 6

Operational Amplifier - Opamp Ideal Opamp Parameters: ▪Input Z = ∞ ▪Out Z = 0 ▪Gain = ∞ ▪BW = ∞ ▪Vos = 0 LSU 09/23/2013 Electronics 7

Opamps - The Golden Rules The output attempts to do whatever is necessary to make the voltage difference between inputs zero. The inputs draw no current. The output influences the input pins via the external feedback network. Use the Rules and Ohm’s Law to analyze or synthesize opamp circuits. LSU 09/23/2013 Electronics 7

Voltage Follower - Unity Gain Buffer   High input impedance. Low output impedance. LSU 09/23/2013 Electronics 6

Non-Inverting Amplifier Feedback network is a voltage divider. LSU 09/23/2013 Electronics 6

Non-Inverting Amplifier   LSU 09/23/2013 Electronics 6

Inverting Amplifier   LSU 09/23/2013 Electronics 6

Inverting Amplifier   LSU 09/23/2013 Electronics 6

Summing Amplifier   LSU 09/23/2013 Electronics 6

Differential (difference) Amplifier   LSU 09/23/2013 Electronics 6

Operational Amplifier - Opamp Available in Single, Dual and Quad Packages Bipolar power supply (typ. +/- 12V) Single supply operation (typ. +12V,GND) V- < Vout < V+ LSU 09/23/2013 Electronics 7

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 09/23/2013 Electronics 6

Activity EEPROM - Read, Write Real-Time-Clock - Set Time/Date, Read Time/Date ADC - Select Channel, Read ADC Read ADC, store data and time stamps to EEPROM. “Post-flight” readback of data Coming Soon….Build (and document!) a data acquisition system using your BalloonSat LSU 10/10/2013 Electronics 7