Electronic Measurement
First Semester – 2nd Year
Chapter 3: Transducers
Dr. Galal Abdelaal
Definition
Transducer is any device that converts energy in one form to another energy form for the
purpose of measurement or transfer of information.
Transducer Specification
1. Operating range: The limits between which the input can vary (difference between the
maximum and minimum limits of the input).
2. Accuracy: The difference between the result of the measurement and the true value of
the quantity being measured.
3. Repeatability: Give same output measurements for repeated same input values.
4. Sensitivity: The ratio of change in output value to the per unit physical change in input
value that causes the output change.
Transducer Specification
5. Linearity: The nonlinearity indicates the deviation of the measured curve from the ideal
curve.
6. Response Time: The speed of change in the output results on a step-wise change of
the input measured value.
Transducer Classifications
1. Active transducers.
2. Passive Transducers.
measurand Electrical measurand Electrical
Active Transducer Passive Transducer
signal signal
External power
▪ Generates electrical signal directly in ▪ Require external power source to operate.
response to physical input parameter.
▪ Depend upon change in electrical
▪ Does not require external power for its parameters (R,L and C).
operation (self generating device).
➢ Examples: Strain gauge, Thermistors.
➢ Examples: Piezo electric, Sensors, Photo
voltaic (PV) cells.
Differences between Transducer and Sensor
Transducer Sensor
Convent energy from one form to another Convert energy form to mainly electrical
signal to be used for processing
➢ For example: ➢ For example:
Antenna: convert electricity to Microphone: detect sound and convert it
electromagnetic waves. to voltage signal.
Resistive Transducers
Used to sense the position of an object, or the distance it has moved. It works through
conversion of mechanical displacement into an electrical signal. The operation principle is
that the physical variable under measurement causes a resistance change in the sensing
element and hence converting this change to voltage signal (passive transducer).
Its classified into:
1. Linear Displacement Transducer.
2. Angular (rotary) Displacement Transducer.
Resistive Transducers
1. Linear Displacement Transducer
▪ This transducer has a sliding contact (wiper)
moves along a resistance element
(potentiometer).
▪ The wiper connected to the physical variable
under measurement through a shaft.
▪ As the wiper moves, the resistance between
the wiper and one end of the resistance
element changes.
▪ The output voltage (Vo) depends on the
resistance value (wiper position) and
therefore is a function of the shaft position.
Resistive Transducers
1. Linear Displacement Transducer
This resistance value is depends on
𝜌∗𝑙
𝑅=
𝐴
The output voltage can be calculated by voltage
divider
𝑅2
𝑉𝑜 = 𝑉𝑡 ∗
𝑅1 + 𝑅2
Resistive Transducers
2. Angular Displacement Transducer
Resistive Transducers
Advantages Disadvantages
1. Simple and Cheap. 1. Large force is required to move the wiper.
2. Useful of large amplitude of displacement. 2. Wiper can wear out and generate noise.
Resistive Transducers Applications
▪ Machine-tool controls.
▪ Elevators.
▪ Liquid-level assemblies.
▪ Robotics.
▪ Woodworking machinery.
Strain Gauge Phenomena
▪ The strain in an element is a ratio of change in length
in the direction of applied load to the original length
of an element.
𝜌∗𝑙
𝑅=
𝐴
▪ The strain changes the resistance R of the element.
▪ Using electrical resistance variation in wires to
sense the strain produced by a force on the wires
associated with mechanical force, or displacement.
Strain Gauge Transducer
▪ Fine-wire element looped back and forth on a mounting
plate, which is usually cemented to the member
undergoing stress.
▪ When a tensile stress (due to mechanical force) is applied
to a wire it tends to elongate this wire and thereby
increase its length (l) and decrease its cross-sectional
area (A).
▪ The combined effect increases the resistance (passive
transducer) as shown
𝜌∗𝑙
𝑅=
𝐴
Strain Gauge Transducer
Strain Gauge
Metallic Semi Conductors
Made of thin sheets of Made of Semi conductors
metal foil
Used in high temperature applications Affected by temperature fluctuation and
because it dissipate the heat easily become nonlinear
Strain Gauge Transducer
Overcome the problem of nonlinearity
Insignificant temp. variation Significant temp. variation
Strain Gauge Transducer
▪ Uses as one arm of a bridge to measure the change in
resistance when the wire is under strain.
▪ The gauge resistance is affected by temperature, any change of
temperature will cause a change in the bridge balance
conditions. This effect can cause an error in the strain
measurement.
▪ For balanced bridge: 𝑅1 𝑅𝑥 = 𝑅2 𝑅3 and Vout = 0
▪ R2 is balancing/adjustable resistor, R1 and R3 are known
constant value resistors. The measured deformation or
displacement by the strain gauge is calibrated against change
using adjustable resistor R2 which makes the voltage across Vo
is zero.
Strain Gauge Transducer
▪ Here two gauges of the same type which both of them
are subjected to the same temperature.
▪ Consequently, the temperature will cause the same
change of resistance in the two, and the bridge balance
will not be affected by the temperature.
▪ The resistance of the compensating gauge is not
affected by the deformation of the material. Thus, only
one gauge (active gauge) responds to the strain, the
strain causes bridge unbalance just as in the case of
the single gauge
Strain Gauge Transducer
Advantages Disadvantages
1. Easily measured. 1. Non-linearity behavior.
2. Small changes in strain can be
sensed (High degree of sensitivity).
Strain Gauge Transducers Applications
▪ Widely used in experimental stress analysis and diagnosis on machines and failure
analysis.
▪ Tension measurement.
Capacitive Transducer
Capacitance can be calculated by
𝑨
𝑪 = 𝜺𝟎 𝜺𝒓
𝒅
where A : the total area of the capacitor surfaces.
d : distance between two capacitive surfaces.
Capacitive Transducer
1. Linear Displacement Transducer
a. One of the plates is moved left and right so that the
surface area changes due to the displacement.
b. One of the plates is moved up and down so that the plate
separation distance changes.
(a) change in area
Thus the change in the capacitance will be function in the
displacement of the capacitor plate.
(b) change in distance
Capacitive Transducer
2. Angular Displacement Transducer 3. Pressure Transducer
Pressure deforms one of the capacitor
plates so that the plate separation distance
changes.
Capacitive Transducers
Advantages Disadvantages
1. Small force is required to move the capacitor 1. The metallic parts of the transducer must
plate. be insulated from each other.
2. High sensitivity.
Resistive Transducers Applications
▪ Proximity sensors.
▪ Level control of liquids.
Inductive Transducers
Linear Variable Differential Transformer (LVDT)
▪ LVDT works on the principle of mutual induction.
▪ The iron core slides within the tube up and down and
effect the magnetic field between the primary coil and the
two secondaries.
▪ The entire working is divided into three cases depending
upon the position of iron core.
Inductive Transducers
Linear Variable Differential Transformer (LVDT)
▪ Case 1: When the core is in the null position, midway
between S1 and S2, voltages VA and VB induced on S1
and S2 will also be equal to each other hence Vo = zero.
So, there will be no displacement.
▪ Case 2: When the core moves towards S1, voltages VA induced
in secondary winding S1 greater then VB that induced in S2
hence VA > VB. (Vo= +ve)
▪ Case 3: When the core moves towards S2, voltages VB induced
in secondary winding S2 greater than VA that induced in S1
hence VB>VA. (Vo= -ve)
Inductive Transducers
Advantages Disadvantages
1. LVDT has a very high output for small 1. Temperature variation and vibrations
change in core position. affects the performance of LVDT.
2. Does not have any frictional losses.
Resistive Transducers Applications
▪ Proximity sensors which detects the metal objects, arms, and land mines.
▪ Industrial automation by counting of products during production or transfer.
Thanks for
Listening with
great patience
Dr. Galal Abdelaal 2021-2022
