ECE 2799 Sensor Interfacing Techniques Prof Mazumder Prof

ECE 2799 Sensor Interfacing Techniques Prof. Mazumder Prof. Bitar Updated 3/25/2016

What is a Sensor? Environment Input Sensor Processing Actuator Output Device Interface • Input Sensor + output Actuator = Transducer • Transducer converts energy of one kind to another Example: microphone (sensor) converts sound into electrical signals for an amplifier to amplify (process) and speaker (actuator) converts electrical signal back to sound Example: RADAR (Radio Detection And Ranging) Police, Patriot, Ballistic Missile Defense, Weather 2

Sensors ECE 2799 relevant (Wiki) 1. Acoustic, sound, vibration 2. Automotive, transportation 3. Chemical 4. Electric current, electric potential, magnetic, radio 5. Flow, fluid velocity 6. Ionizing radiation, subatomic particles 7. Navigation instruments 8. Position, angle, displacement, distance, speed, acceleration 9. Optical, light, imaging, photon 10. Pressure 11. Force, density, level 12. Thermal, heat, temperature 13. Proximity, presence 14. Sensor technology Sensor types • Passive sensors directly generate an electrical signal e. g. Thermocouple, Photodiode • Active sensors require “excitation signal” (e. g. supply) to produce output signal • Analog sensors produce a continuous signal e. g. temp, speed, pressure, strain etc. • Digital sensors produce discrete/binary signals in the form of logic “ 1” or “ 0” e. g. light sensor with rotating slotted disc

Sensor Need • What types of sensors are needed on your project? • What sensor specifications do you need to consider? • The electrical power density from an energy harvesting system is very small (1 μW/cm 3 to 100 m. W/cm 3) 4

Needs for ECE 2799 – D 2016 From Brain Storming in Class Sensors: 1. RFID Tags 2. Magnetic Fields 3. Accelerometer 4. Temperature 5. Particulate in air 6. Pressure 7. Distance 8. Ambient light Selection considerations: Type of signals; Range; Sensitivity; Power (Voltage, Current); Cost; If selected why? Value analysis, How does it work? Etc. Processing: 1. Analog (Op. Amps) 2. Digital (Microcontrollers) 3. Both Harvesting Energy Source: 1. Solar 2. Thermoelectric 3. Electromagnetic Induction (Motion) 4. Wind 5. Hydroelectric Selection Considerations: Efficiency; Consistent variation; Reliable; Size/ Weight/ Power; Environment; V-I characteristics; Capacity (Energy/Power Density); Storage 1. Rechargeable Battery 2. Capacitors (Supercaps) Selection Considerations: Size / Weight; Capacity (Energy Density; Safety; Charging circuits; Voltage, Current; Charging/ discharging time etc.

Sensor Selection Parameters General § § § § Environmental Conditions Input/Output Range Linearity Offset Operating Life Output Format Overload Characteristics Repeatability/Hysteresis Resolution/Accuracy Sensitivity/Selectivity Size/Cost/Weight Speed of Response Stability (long and short term) Specific § § § § 5 - 7 0 C (4 - 8 0 C) < 1 degree accuracy waterproof durable inexpensive fast low power 6

Energy Harvesting System Ref: Loreto Mateu and Peter Spies [see ece 2799 lib resources] Energy Source/ Environment Input Sensor (Transducer) Energy Storage [Battery/ Capacitor (Super. Caps)] Electrical Signal [ small, Continuous / discontinuous time ] Interfaces AC-to-DC DC-to-DC Converter and Voltage Regulator Interfaces Electrical Signal [Regulated DC] Electrical Load

Energy Harvesting System Storages Harvesting Sources • “Table 1. 1 a comparison between different energy harvesting sources (unshaded part) and energy storage elements (shaded part) in terms of power density (power per unit of volume). Power density for energy harvesting sources under the same input conditions remains constant with time. . ” 8

Energy Harvesting System • “Power density DOES NOT remains constant with time for energy storage techniques due to leakage currents as it is shown below Thus, energy harvesting becomes an alternative to energy storage techniques in long operation-time applications where it is not possible to replace or recharge the energy storage element…” 9

Temperature Force/Pressure Position Speed RTD Light Level Input Device (Sensor) Light Dependant Resistor (LDR) Photodiode Photo-transistor Solar Cell Thermocouple Thermistor Thermostat Resistive Temperature Detector (RTD) Strain Gauge Pressure Switch Load Cells Potentiometer Encoders Reflective/Slotted Opto-switch LVDT (Linear Variable Differential Transformer) Tacho-generator Reflective/Slotted Opto-coupler Doppler Effect Sensors RADAR (Military, Weather, etc. ) Thermistor Qty. Measured Thermocouple Common Sensor Devices Reflective Optical Switch Slotted Optical Switch

Sensor Interfacing • • Input sensor measures physical quantities e. g. temperature, pressure, position, light etc. Changes in these result in changes in electrical properties e. g. resistance, capacitance, inductance etc. Changes in the electrical properties can be used to generate electrical signals These signals are often too small (millivolt, microvolt, picovolt) to directly apply to processing circuits Typical interfacing circuits use dividers, filters (passive and active), buffers/amplifiers, bridge circuits etc. +Vcc NTC R 1 R + - R 2 0 v Divider VB - R 3 V 0 + C LPF R 4 R 1 Buffer LPF R 2 Bridge (small variation) V 0 = 0 If R 1/R 4 = R 2/R 3 • 11

Sensor Interfacing Techniques R 1 • Many sensors are “resistive”- i. e. resistance changes with measured quantity (heat, light, pressure etc. ) • Thermistor (THERM-ally sensitive res-ISTOR) is a special type of resistor which changes its electrical resistance when exposed to changes in temperature. • Example interfacing an NTC (negative temperature coefficient) Thermistor +Vs Neg. Temp. Coeff. (NTC) Vout = Vtemp = (R 2/(R 1+R 2))x Vs temp R 1 Vtemp R 2 + - Vout 1 k-Ohm 0 v Buffer/unity gain • Given: Vs=12 V; R 2=1 KΩ; R 1=10 KΩ at 25 deg. C and R 1=100Ω at 100 deg. C. • Calculate: At 25 deg. C: Vtemp = Vout = 1. 09 V At 100 deg. C: Vtemp = Vout = 10. 9 12

Sensor Interfacing Techniques • • • Simplest interfacing device is a mechanical switch e. g. push button, on/off toggle etc. Common method of interfacing a switch to an electronic circuit is via a pull-up resistor to supply Vs Need to prevent multiple switch open/close effects +Vs Vout Switch 0 v Open (HIGH “ 1”) Vs 0 v Time Closed (LOW “ 0”) Switch Bounce effects (high frequency noise) 13

Sensor Interfacing Methods • Switch debouncing circuit using RC filter - Switch closed, capacitor is fully discharged, input to the inverter is LOW and its output is HIGH. - Switch opened, the capacitor charges up via R 1 and R 2 by C(R 1+R 2) time constant of the RC network. - RC Time Constant chosen longer than the bounce time - Schmitt-trigger buffer used to produce sharp transition • Other debounce circuits using NAND , NOR gates Open +Vs R 1 R 2 Switch 0 v Closed Schmitt Inverter Closed Vs Vout 0 v Open C 14

Sensor Interfacing Methods Interfacing Photodiode • • • Typical Ipd = 1 micro. Amp and Cpd = 25 p. F Current Ipd is proportional to incident light Need transimpedance AMP to convert current to voltage Configure active filter to reject high frequency “noise”. Feedback (Rf and Cf) is chosen to optimize gain and input time constant (rise/fall time) seen by the Photodiode Cf Ipd Cpd Light Rf -A Vout + Op Amp/LPF Photodiode model Vout ~ Ipd. Rf Trans-Imp. Gain= Rf Photodiode [for Low frequency and Large value of A ] A = Open-Loop Voltage Gain

Sensor Comparisons 16

Additional Information Energy Harvesting Systems Design Parameters

Energy Harvesting System • Human body: temperature gradient, vibrations/ movement • Industry: excess heat and machine vibrations • Light into electricity by photovoltaic system lighting to wrist watches. • Kinetic energy (displacement of moving part) transformed to electrical energy by: inductive, electrostatic and piezoelectric conversion. • Solar radiation varies due to weather and location, needs optimum inclination angle and orientation of solar cells 18

Energy Harvesting System

Energy Harvesting System

Energy Harvesting Systems

Energy Harvesting System

Energy Harvesting Systems

Energy Harvesting Systems