Strain Gauge:
What is a strain gauge?
A strain gauge is an elastically deformable transducer that transforms an applied force or a mechanical
displacement into a change in resistance. It is the underlying mechanism for the working of a strain
gauge load cell.
Who discover it?
1856
Lord Kelvin first reported on a relationship between strain and the resistance
of wire conductors.
Early 1930s
Charles Kearns made the first notable use of bonded resistance strain gauges to measure vibratory
strains in high performance propeller blades. He used carbon composite resistors (as used in standard
electronic instruments) ground flat and mounted on an insulating strip.
1937/8
Arthur Ruge and Edward Simmons (working independently of each other) both discovered that small
diameter wires made of electrical resistance alloys could be bonded to a structure to measure surface
strain.
This type of gauge had the advantage of responding well to static strains.
What is its construction? Draw or illustrate it.
How will you use it?
n order to measure strain with a bonded resistance strain gauge, it must be connected to an electric
circuit that is capable of measuring the minute changes in resistance corresponding to strain. Strain
gauge transducers usually employ four strain gauge elements electrically connected to form a
Wheatstone bridge circuit.
Show the circuit and briefly explain.
A Wheatstone bridge is a divided bridge circuit used for the measurement of static or dynamic electrical
resistance. The output voltage of the Wheatstone bridge circuit is expressed in millivolts output per volt
input. The Wheatstone circuit is also well suited for temperature compensation.
The equation of the Wheatstone bridge, if R1, R2, R3, and R4 are equal, and a voltage, VIN, is applied
between points A and C, then the output between points B and D will show no potential difference.
However, if R4 is changed to some value which does not equal R1, R2, and R3, the bridge will become
unbalanced and a voltage will exist at the output terminals
Give at least two applications with description and pictures.
Strain Gauges in Aerospace Applications
The use of strain gauge technology in aerospace applications has a long history. Strain gauges are
bonded directly to structural load bearing components to measure stresses along load paths for wing
deflection. Wired into Wheatstone bridge circuits, strain gauges in aerospace applications include
onboard signal conditioning units, excitation power supplies, and the telemetry necessary to read in situ
measurements.
Calibration is done by mechanical application of known loads to airplane parts in ground calibration
tests. Strain gauge output is recorded at numerous locations throughout the aircraft structure. The load
equations for, say, wing bending moment, are derived after ground calibration testing based on strain
gauge measurements.
Strain gauges may be used in "smart bridge" technology to detect structural problems early.
Hall Effect Sensor
What is Hall Effect Sensor?
A Hall effect sensor is a transducer that varies its output voltage in response to a magnetic
field. Hall effect sensors are used for proximity switching, positioning, speed detection, and
current sensing applications.
Show its construction and image.
How does it work?
In a Hall effect sensor, a thin strip of metal has a current applied along it. In the
presence of a magnetic field, the electrons in the metal strip are deflected toward one
edge, producing a voltage gradient across the short side of the strip (perpendicular to
the feed current). Hall effect sensors have an advantage over inductive sensors in that,
while inductive sensors respond to a changing magnetic field which induces current in a
coil of wire and produces voltage at its output, Hall effect sensors can detect static (non-
changing) magnetic fields.
In its simplest form, the sensor operates as an analogtransducer, directly returning a
voltage. With a known magnetic field, its distance from the Hall plate can be
determined. Using groups of sensors, the relative position of the magnet can be
deduced.
Give at least two applications with description and pictures.
Position sensingEdit
Sensing the presence of magnetic objects (connected with the position sensing) is the
most common industrial application of Hall effect sensors, especially those operating in
the switch mode (on/off mode). The Hall effect sensors are also used in the brushless DC
motor to sense the position of the rotor and to switch the transistors in the right
sequence.
Direct current (DC) transformersEdit
Hall effect sensors may be utilized for contactless measurements of DC current in current
transformers. In such a case the Hall effect sensor is mounted in the gap in magnetic
core around the current conductor.[6] As a result, the DC magnetic flux can be measured,
and the DC current in the conductor can be calculated.
Automotive fuel level indicatorEdit
The Hall sensor is used in some automotive fuel level indicators. The main principle of
operation of such indicator is position sensing of a floating element.[7] This can either be
done by using a vertical float magnet or a rotating lever sensor.
