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ECE 2799
Sensor Interfacing Techniques
Prof. Mazumder
Prof. Bitar
Updated 3/25/2016
1
What is a Sensor?
Environment
Input
Sensor
Processing
Actuator
Output
Device
Interface 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
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Sensors
ECE2799 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
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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/cm3 to 100 mW/cm3)
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Needs for ECE2799 – D2016
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 (OpAmps)
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
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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
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5 - 7 0C (4 - 8 0C)
< 1 degree accuracy
waterproof
durable
inexpensive
fast
low power
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Energy Harvesting System
Ref: Loreto Mateu and Peter Spies [see ece2799 lib resources]
Energy Source/
Environment
Input Sensor
(Transducer)
Energy Storage
[Battery/ Capacitor (SuperCaps)]
Electrical
Signal
[ small, Continuous /
discontinuous time ]
Interfaces
AC-to-DC
DC-to-DC
Converter
and
Voltage
Regulator
Interfaces
Electrical
Signal
Electrical
Load
[Regulated
DC]
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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..”
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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…”
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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
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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
VB
R
NTC
+
R1
-
R2
R4
+
C
R1
0v Divider
LPF
R3
V0
Buffer
LPF
R2
Bridge (small variation)
V0 = 0
If R1/R4 = R2/R3
•
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Sensor Interfacing Techniques
• 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
Vout = Vtemp = (R2/(R1+R2))x Vs
+Vs
Neg.
Temp.
Coeff.
(NTC)
temp
R1
Vtemp
R2
Vout
+
-
1k-Ohm
0v
Buffer/unity gain
• Given: Vs=12 V; R2=1KΩ; R1=10KΩ at 25
degC and R1=100Ω at 100 degC.
• Calculate:
At 25 degC: Vtemp = Vout = 1.09V
At 100 degC: Vtemp = Vout = 10.9
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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
0v
Open (HIGH “1”)
Vs
0v
Time 
Closed
(LOW “0”)
Switch Bounce effects
(high frequency noise)
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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 R1 and R2 by
C(R1+R2) 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
R1
R2
Closed
Vs
Schmitt
Inverter
Vout
0v
Switch
0v
Closed
Open
C
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Sensor Interfacing Methods
Interfacing Photodiode
•
•
•
•
•
Typical Ipd = 1 microAmp and Cpd = 25 pF
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
Ipd
Cpd
Rf
-A
Light
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
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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
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Energy Harvesting System
Energy Harvesting System
Energy Harvesting Systems
Energy Harvesting System
Energy Harvesting Systems
Energy Harvesting Systems