EE 4900: Fundamentals of Sensor Design
1
Lecture 2
Basics of Sensing:
Sensor Characteristics
EE 4900 Fundamentals of Sensor Design
Suketu Naik
Sensor System Example
2
Glucose Meter (monitor blood sugar level for diabetic patients)
Tiny
blood drop
Test Strip
Thermistor
Meter
Control, Working and
Reference Electrodes
Amperometry Circuit
fc = 100 Hz
Test Strip
Glucose -> Glucose Oxidase -> Ions -> Change in Current (3 μA)
EE 4900 Fundamentals of Sensor Design
Suketu Naik
3
Sensor
Characteristics
-Output Format
-Transfer Function
-Calibration
-Accuracy
-Resolution
- Hysteresis
-Nonlinearity
-Saturation
-Output Impedance
-Dynamic Characteristics
-Environmental Factors
EE 4900 Fundamentals of Sensor Design
Suketu Naik
Sensor Characteristics: Output Format
4
Example: Accelerometer Output of a Single Sphere Wheel Robot
Sensor system may produce a set of output electrical characteristics
such as voltage, current, charge, frequency, amplitude, phase,
polarity, specific shape, time delay, and/or digital number or code.
EE 4900 Fundamentals of Sensor Design
Suketu Naik
Sensor Characteristics: Output Format
5
Example: Tactile Sensor Array
EE 4900 Fundamentals of Sensor Design
Suketu Naik
Sensor Characteristics: Output Format
6
Example: Gyroscope Output
EE 4900 Fundamentals of Sensor Design
Suketu Naik
Sensor Characteristics: Output Format
7
Example: Analog Output to PWM Output
Goal: Take sensor input and
control the servo (direction of
motion on a Robotic Arm)
More Examples of Sensor Output Electrical Characteristics
EE 4900 Fundamentals of Sensor Design
Suketu Naik
Sensor Characteristics: Transfer Function
8
Transfer function describes the input (stimulus, s) and output
(response, S) of a sensor system

Transfer function can be represented as table of values, a graph,
formula or a solution of equation

Example: Pressure Sensor
Example: CO2 Sensor
EE 4900 Fundamentals of Sensor Design
Suketu Naik
Sensor Characteristics: Transfer Function
9
More Examples
Transfer Function
Inverse Transfer Function
(to determine the coefficients)
EE 4900 Fundamentals of Sensor Design
Suketu Naik
Sensor Characteristics: Transfer Function
Model and Function Approximations
Common Functions: Linear
S  A  Bs
Common Functions: Log
Input
(Stimulus)
Y intercept Slope=Sensitivity
Output
(Response)
10
S  A  B ln s
se
SA
B
Inverse Transfer Function
S  A  Bs
S  A  B ln s
EE 4900 Fundamentals of Sensor Design
Suketu Naik
Sensor Characteristics: Transfer Function
11
Model and Function Approximations
Common Functions: Power
Common Functions: Polynomial
S  A  Bs
s
k
S  a2 s 2  b2 s  c2
k
s  A2 S 2  B2 S  C2
SA
B
S  a3 s 3  b3 s 2  c3 s  d 3
s  A3 S 3  B2 S 2  C2 S  D3
S  a3 s 3  b3 s 2  c3 s  d 3
EE 4900 Fundamentals of Sensor Design
Suketu Naik
Sensor Characteristics: Sensitivity
12
Nonlinear Transfer Function has different sensitivity at different
intervals
 Use Linear Piecewise Approximation and Spline Interpolation to
estimate sensitivity

Linear Piecewise Approximation
Sensitivity=first derivative of the
transfer function
Si
bi 
si
where,
∆si = small increment of input
∆Si = small increment in output
EE 4900 Fundamentals of Sensor Design
Suketu Naik
Sensor Characteristics: Sensitivity
13
Spline Interpolation: Example and Matlab's Curve Fitting tool
EE 4900 Fundamentals of Sensor Design
Suketu Naik
Sensor Characteristics: Calibration
14
If the sensor's specified over a broader range than what is required, we
need to calibrate the sensor system

Either the model of the transfer function or a good approximation has to
be known before calibration


Sections 2.2.1, 2.2.2. 2.3
Fig. 2.4: Calibration of thermistor: (a) grinding, (b) trimming of reference
resistor, (c) calculation of transfer function
EE 4900 Fundamentals of Sensor Design
Suketu Naik
Sensor Characteristics: Accuracy
15
Accuracy is very important as it represents highest deviation from the
ideal value at the input
 Error tolerance (the output can change regardless of manufacturer's
specification!)

EE 4900 Fundamentals of Sensor Design
Suketu Naik
Sensor Characteristics: Accuracy

16
Example:
Laser Displacement Sensor:
Spec: 1mV/1mm, Observation: 10.5 mV for 10 mm, 5% error
Accuracy can be represented as
1) Directly in terms of the measured value (e.g. tolerance of 0.15 deg C of a
temperature sensor)
2) As % of the input span (e.g. flow rate: 3000 foot per minute with 3%
accuracy
3) As % of the measured signal
4) In terms of the output signal (e.g. digital LSB)
EE 4900 Fundamentals of Sensor Design
Suketu Naik
Sensor Characteristics: Output Impedance
17
In order to connect the sensor to the interface circuit, it is important to
know the output impedance, Zout


Sensor with current output should have very high Zout

Sensor with voltage output should have very low Zout
Sensor with voltage output
Sensor with current output
EE 4900 Fundamentals of Sensor Design
Suketu Naik