Fundamentals of Sensors & Actuators

1. Sensors & Actuators
by
Dr. Abdul Rehman Abbasi

2. Recommended Reading/Reference Material
• Handbook of Modern Sensors
Jacob Fraden, Springer Publications (Fourth Edition, 2010)

3. Recommended Reading/Reference Material
• Sensor Technology Handbook
Jon S. Wilson, (Newnes) Elsevier Publications (2005)

4. Recommended Reading/Reference Material
• Control Valve Handbook
Fisher Controls (2005)

5. Course Material Link
• http://groupspaces.com/qurman

6. Sensors
&
Actuators

7. Sensing & Actuation in Reality (Industry)

8. Sensing & Actuation in Reality (Industry)

9. Source of Inspiration (by nature)

10. What is a Sensor?
• A sensor is often defined as a “device that
receives and responds to a signal or stimulus.”

11. A more Refined Definition:
A sensor is a device that receives a stimulus
and responds with an electrical signal
• A sensor is a translator of a generally nonelectrical value into
an electrical value.
• By “electrical,” we mean a signal, which can be channeled,
amplified, and modified by electronic devices.
• The sensor’s output signal may be in the form of voltage,
current, or charge.
• These may be further described in terms of amplitude,
polarity, frequency, phase, or digital code.
• This set of characteristics is called the output signal format.
Therefore, a sensor has input properties (of any kind) and
electrical output properties.

12. Sensor: Energy Conversion
• Any sensor is an energy converter.
No matter what you try to measure, you
always deal with energy transfer from the
object of measurement to the sensor

13. Sensor versus Transducer
• The term sensor should be distinguished from
transducer. The latter is a converter of any
one type of energy into another, whereas the
former converts any type of energy into
electrical energy.
• An example of a transducer is a loudspeaker,
which converts an electrical signal into a
variable magnetic field and, subsequently,
into acoustic waves
• Transducers may be parts of complex sensors

14. Transducers may be parts of
complex sensors
• There are two types of sensors; direct and complex.
• A direct sensor converts a stimulus into an electrical
signal or modifies an electrical signal by using an
appropriate physical effect, whereas a complex sensor
in addition needs one or more transducers of energy
before a direct sensor can be employed to generate an
electrical output.

15. A sensor does not function by itself
• Sensor 1 is noncontact, sensors 2 and 3 are passive, sensor 4
is active, and sensor 5 is internal to a data acquisition system

16. Sensor Classifications
• Passive and Active:
• A passive sensor does not need any additional energy source and directly
generates an electric signal in response to an external stimulus. That is,
the input stimulus energy is converted by the sensor into the output
signal. The examples are a thermocouple, a photodiode, and a
piezoelectric sensor.
• The active sensors require external power for their operation, which is
called an excitation signal. That signal is modified by the sensor to
produce the output signal.
• The active sensors sometimes are called parametric because their own
properties change in response to an external effect and these properties
can be subsequently converted into electric signals. It can be stated that a
sensor’s parameter modulates the excitation signal and that modulation
carries information of the measured value. For example, a thermistor is a
temperature sensitive resistor.

17. Sensor Classifications
• Sensors can be classified into Absolute and Relative.
• An absolute sensor detects a stimulus in reference to an absolute physical
scale that is independent of the measurement conditions, whereas a
relative sensor produces a signal that relates to some special case.
• An example of an absolute sensor is a thermistor, a temperature-sensitive
resistor. Its electrical resistance directly relates to the absolute
temperature scale of Kelvin.
• Another very popular temperature sensor thermocouple is a relative
sensor. It produces an electric voltage, which is a function of a
temperature gradient across the thermocouple wires. Thus, a
thermocouple output signal cannot be related to any particular
temperature without referencing to a known baseline.
• Another example of the absolute and relative sensors is a pressure sensor.
An absolute pressure sensor produces signal in reference to vacuum – an
absolute zero on a pressure scale. A relative pressure sensor produces
signal with respect to a selected baseline that is not zero pressure, for
example, to the atmospheric pressure.

18. Sensor Specifications

19. Sensor Material

20. Detection Means

21. Conversion Phenomenon

22. Area of Application

23. Stimulus

24. Typical Sensor & its Output

25. SI Units of Measurement (A Reminder)

26. Sensor Performance
Characteristics
• The transfer function shows the functional
relationship between physical input signal (s)
and electrical output signal (S).
• Ex:

27. Transfer Function as Graph/Curve

28. Sensitivity
• Relationship between input physical signal and
output electrical signal. It is the ratio between a
small change in electrical signal to a small change in
physical signal.
• Derivative of the transfer function with respect to
physical signal. Typical units are volts/kelvin,
millivolts/kilopascal, etc.
• A thermometer would have “high sensitivity” if a
small temperature change resulted in a large voltage
change.

29. Span or Dynamic Range
• The range of input physical signals that may be
converted to electrical signals by the sensor
• Signals outside of this range are expected to cause
unacceptably large inaccuracy.
• This span or dynamic range is usually specified by the
sensor supplier as the range over which other
performance characteristics described in the data
sheets are expected to apply.
• Typical units are kelvin, pascal, newtons, etc.

30. Accuracy or Uncertianty
• Largest expected error between actual and ideal output
signals.
• Typical units are kelvin.
• Sometimes this is quoted as a fraction of the full-scale output
or a fraction of the reading.
• For example, a thermometer might be guaranteed accurate
to within 5% of FSO (Full Scale Output).
• Accuracy is generally considered by metrologists to be a
qualitative term, while “uncertainty” is quantitative.
• For example one sensor might have better accuracy than
another if its uncertainty is 1% compared to the other with an
uncertainty of 3%.

31. Hysterisis & Non-Linearity
Hysterisis
• Some sensors do not return to the same output value when the input
stimulus is cycled up or down.
• The width of the expected error in terms of the measured quantity is
defined as the hysteresis. Typical units are Kelvin or percent of FSO.
Nonlinearity (often called Linearity)
• The maximum deviation from a linear transfer function over the
specified dynamic range.
• There are several measures of this error. The most common compares
the actual transfer function with the “best straight line,” which lies
midway between the two parallel lines that encompass the entire
transfer function over the specified dynamic range of the device.

32. Sensor Noise
• All sensors produce some output noise in
addition to the output signal.
• Noise of the sensor limits the performance of
the system based on the sensor.
• There is an inverse relationship between the
bandwidth and measurement time, it can be
said that the noise decreases with the square
root of the measurement time.

33. Resolution
Minimum detectable signal fluctuation

34. Bandwidth
• All sensors have finite response times to an
instantaneous change in physical signal. In addition,
many sensors have decay times, which would
represent the time after a step change in physical
signal for the sensor output to decay to its original
value.
• The reciprocal of these times correspond to the
upper and lower cutoff frequencies, respectively. The
bandwidth of a sensor is the frequency range
between these two frequencies.

35. Sensor Electronics
• The electronics that go along with the physical
sensor element are often very important to
the overall device.
• The sensor electronics can limit the
performance, cost, and range of applicability.
• If carried out properly, the design of the
sensor electronics can allow the optimal
extraction of information from a noisy signal