1. Definition of a sensor Def. 1. (Oxford dictionary)
A device giving a signal for the detection or measurement of a physical property to which it responds.
Def. 2.
A sensor is a device that receives a signal or stimulus and response with an electrical signal.

2. Classification of sensors
Attributes which can be used to classify sensors:
. stimulus
. working principle
. properties (attributes of the characteristic)
. application

4. Detectable Phenomenon
Stimulus
Quantity
Acoustic
Wave (amplitude, phase, polarization), Spectrum, Wave
Velocity
Biological & Chemical
Fluid Concentrations (Gas or Liquid)
Electric
Charge, Voltage, Current, Electric Field (amplitude, phase,
polarization), Conductivity, Permittivity
Magnetic
Magnetic Field (amplitude, phase, polarization), Flux,
Permeability
Optical
Refractive Index, Reflectivity, Absorption
Thermal
Temperature, Flux, Specific Heat, Thermal Conductivity
Mechanical
Position, Velocity, Acceleration, Force, Strain, Stress,
Pressure, Torque

5. Physical Principles Amperes’s Law Curie-Weiss Law
A current carrying conductor in a magnetic field experiences a force (e.g. galvanometer)
Curie-Weiss Law
There is a transition temperature at which ferromagnetic materials exhibit paramagnetic behavior
Faraday’s Law of Induction
A coil resist a change in magnetic field by generating an opposing voltage/current (e.g. transformer)
Photoconductive Effect
When light strikes certain semiconductor materials, the resistance of the material decreases (e.g. photoresistor)

6. Need for Sensors
Sensors are omnipresent. They embedded in our bodies, automobiles, airplanes, cellular telephones, radios, chemical plants, industrial plants and countless other applications.
Without the use of sensors, there would be no automation !!

7. Choosing a Sensor

8. Measurements
Heisenberg (1927): ”The momentum and position of a particle can not both be precisely determined at the same time.”
Measuring activity disturbs the physical process (loading effect).
Measurement error:
That is the difference between the measured value and the true value.
error = measured value - true value
Deterministic errors:
They are repeated at every measurement, e.g. reading offset or bias. Such
errors can be corrected by calibration.
Random errors:
They are caused by several parameters and change in time in an
unpredictable fashion. They can be quantified by mean errors, standard
deviation.
Precision:
Measurements with small deviation
Accuracy:
Measurements with small errors, i.e. small bias and high precision.

9. A sensor is represented by its characteristic.
Sensor properties
output
factual
ideal
input
A sensor should represent a physical variable as fast and as accurately as possible.
A sensor is represented by its characteristic.
Ideally, the sensor characteristic is a straight line

10. SENSOR CHARACTERISTIC
Full scale input (input span)
A range of stimuli that can be converted by one sensor.
Full scale output (output span)
Full scale output is the algebraic difference between the output signals measured with maximum input stimulus and with minimum input stimulus applied.

11. SENSOR CHARACTERISTIC
Accuracy : Error measurement
Sensitivity: change in output for unit change in input
Resolution: the smallest change in the signal that can be detected and accurately indicated by a sensor.
Linearity: the closeness of the calibration curve to a straight line.
Drift: the deviation from the null reading of the sensor when the value is kept constant for a long time.

12. SENSOR CHARACTERISTIC
Hysteresis: the indicated value depends on direction of the test (increasing and decreasing)
Repeatability (precision): the maximum deviation from the average of repeated measurements of the same static variable.
Dynamic Characteristics: A sensor may have some transient characteristic. The sensor can be tested by a step response where the sensor output is recorded for a sudden change of the physical variable.
The rise time, delay time, peak time, settling time, percentage overshoot should be as small as possible.

13. Typical specification