Measurement of motion, force and torque

Potentiometer
The instrument designs for measuring the unknown
voltage by comparing it with the known voltage,
such type of instrument is known as the
potentiometer. In other words, the
potentiometer is the three terminal device used
for measuring the potential differences by
manually varying the resistances. The known
voltage is drawn by the cell or any other supply
sources.

Increasing or decreasing the resistance of the
potentiometer controls the flow of electric
current. If we increase the resistance of the
potentiometer, large amount of electric
current is blocked and only a small amount of
electric current is allowed. On the other hand,
if we reduce the resistance of the
potentiometer, a large amount of electric
current is allowed and only a small amount of
electric current is blocked.

Construction of potentiometer
The potentiometer consists of three terminals
among which two are fixed and one is variable.
The two fixed terminals of the potentiometer are
connected to both ends of the resistive element
called track and third terminal is connected to the
sliding wiper. The wiper that moves along the
resistive element varies the resistance of the
potentiometer. The resistance of the
potentiometer is changed when the wiper is
moved over the resistive path.

The resistive element of the potentiometer is either flat
or angled. If the resistive element is flat, the wiper
moves linearly. On the other hand, if the resistive
element is angled, the wiper moves in a rotary manner.

Potentiometer Types
There are two main types of potentiometers:
Rotary potentiometer
Linear potentiometer
Although the basic constructional features of
these potentiometers vary, the working
principle of both of these types of
potentiometers is the same.

Rotary Potentiometers
The rotary type potentiometers are used mainly
for obtaining adjustable supply voltage to a
part of electronic circuits and electrical
circuits. The volume controller of a radio
transistor is a popular example of a rotary
potentiometer where the rotary knob of the
potentiometer controls the supply to the
amplifier.

This type of potentiometer has two
terminal contacts between which a
uniform resistance is placed in a semi-
circular pattern. The device also has a
middle terminal which is connected to
the resistance through a sliding
contact attached with a rotary knob.
By rotating the knob one can move
the sliding contact on the semi-
circular resistance. The voltage is
taken between a resistance end
contact and the sliding contact. The
potentiometer is also named as the
POT in short. POT is also used in
substation battery chargers to adjust
the charging voltage of a battery.
There are many more uses of rotary
type potentiometer where smooth
voltage control is required.

Linear Potentiometers
The linear potentiometer is basically the same but
the only difference is that here instead of rotary
movement the sliding contact gets moved on the
resistor linearly. Here two ends of a straight
resistor are connected across the source voltage.
A sliding contact can be slide on the resistor
through a track attached along with the resistor.
The terminal connected to the sliding is
connected to one end of the output circuit and
one of the terminals of the resistor is connected
to the other end of the output circuit.

This type of potentiometer
is mainly used to
measure the voltage
across a branch of a
circuit, for measuring
the internal resistance
of a battery cell, for
comparing a battery cell
with a standard cell and
in our daily life, it is
commonly used in the
equalizer of music and
sound mixing systems.

Advantages of Potentiometers
Advantages of Potentiometers
• Higher reliability
• Increased accuracy
• Small size, multiple potentiometers can be packed on a
single chip
• Negligible resistance drift
• Unaffected by environmental conditions like vibrations,
humidity, shocks and wiper contamination
• No moving part
• Tolerance up to ±1%
• Very low power dissipation, up to tens of milliwatts

linear variable differential
transformer

• LVDTs are robust, absolute linear position/displacement
transducers; inherently frictionless, they have a virtually infinite
cycle life when properly used. As AC operated LVDTs do not contain
any electronics, they can be designed to operate at cryogenic
temperatures or up to 1200 °F (650 °C), in harsh environments, and
under high vibration and shock levels. LVDTs have been widely used
in applications such as power turbines, hydraulics,
automation, aircraft, satellites, nuclear reactors, and many others.
These transducers have low hysteresis and excellent repeatability.
• The LVDT converts a position or linear displacement from a
mechanical reference (zero or null position) into a proportional
electrical signal containing phase (for direction) and amplitude (for
distance) information. The LVDT operation does not require an
electrical contact between the moving part (probe or core
assembly) and the coil assembly, but instead relies on
electromagnetic coupling.

The linear variable differential
transformer (LVDT) (also called linear
variable displacement transformer, linear
variable displacement transducer, or
simply differential transformer) is a type of
electrical transformer used for measuring
linear displacement (position).
Linear displacement is movement in one
direction along a single axis. A position or
linear displacement sensor is a device
whose output signal represents the distance
an object has traveled from a reference
point

Operation
The linear variable differential transformer has three solenoid coils
placed end-to-end around a tube. The center coil is the primary,
and the two outer coils are the top and bottom secondaries. A
cylindrical ferromagnetic core, attached to the object whose
position is to be measured, slides along the axis of the tube.
An alternating current drives the primary and causes a voltage to be
induced in each secondary proportional to the length of the core
linking to the secondary. The frequency is usually in the range 1 to
10 kHz.

As the core moves, the primary's linkage to the two
secondary coils changes and causes the induced
voltages to change. The coils are connected so that the
output voltage is the difference (hence "differential")
between the top secondary voltage and the bottom
secondary voltage. When the core is in its central
position, equidistant between the two secondaries,
equal voltages are induced in the two secondary coils,
but the two signals cancel, so the output voltage is
theoretically zero. In practice minor variations in the
way in which the primary is coupled to each secondary
means that a small voltage is output when the core is
central.

Application
• The Right Stroke for Hydraulic Applications
LVDTs provide position feedback in hydraulic applications by
monitoring the performance accuracy of actuators and cylinders to
improve operational efficiencies. The role of the cylinder in most
hydraulic applications is to move something
for example would be a robotic arm picking up a piece of glass. If the
control system does not know when to stop the arm, based on
position feedback from an LVDT, the hydraulic cylinder could drive
the arm right through the piece of glass.

Position measurement of steam control valves.
Because of their extraordinary reliability and their ability to withstand
high ambient temperatures, LVDT linear position sensors are being
used in the rehab of power generation plants to better monitor the
position of steam control valves for increased efficiency and
reduced operating costs. Typically, plants have very precise control
schemes for valve position to increase operating efficiency and save
fuel. The cost to the plant operator of improper operation and
inefficiency due to wrong valve settings can be as much as several
million dollars a year.
LVDT feedback is sent to the turbine control system so that it knows
how far a valve is opened or closed. LVDTs are so precise that
operators can rely on them to determine, The combination of LVDTs
with modern computerized turbine control systems saves power
companies millions of dollars per year.

In the aerospace industry
In the aerospace industry, the constant evolution of jet
engine manufacturing requires the use of a variety of
contact and non-contact precision aerospace sensors.
Digital spring push probes and digital pneumatic
probes are both widely used in the sector.
For example, displacement sensors have a key role in the
manufacture of engine turbine blades. As a result of
the production process, the dimensions of the turbine
blade often change very slightly and LVDT sensors help
ensure that they conform to strict specifications.

The Ever-Expanding Utility of LVDTs
In everyday life, LVDTs can be found in ATM
machines, ensuring money is correctly dispensed;
in flight-control systems for military and
commercial aircraft; as well as in metrology labs;
steel, aluminum, and paper mills to control
thickness; industrial production lines to ensure
properly dimensioned products; in outer space
on satellite actuator lenses; and under the sea on
oil well choke valves.

Rotary Variable Differential
Transformer

RVDT full form is Rotary Variable Differential
Transformer. The designing of RVDT is same like an
LVDT, apart from the design of core. Because, when
it turns then the mutual inductance among the two
windings of the transformer namely the primary
coil and the secondary coils will change linearly by
the angular displacement. RVDT’s uses brushless,
non-contacting equipment for ensuring long-life,
consistent, repeatable and position detecting by
unlimited resolution. Such performance guarantees
precise position sensing under the most intense
working conditions.

It is one kind of electromechanical transducer used
to give the linear o/p which is proportional to the
i/p angular displacement. The main function of
RVDT is to detect the angular displacement and
converts it into an electrical signal. The both the
RVDT and LVDT workings are similar, but LVDT
employs the flexible iron core for displacement
measurement whereas in RVDT employs a cam type
core. This core will turn among the two windings of
the transformer using the shaft.

Construction
RVDT transducer has two
windings similar to a normal
transformer such as primary
winding and two secondary
windings shown in the
following RVDT diagram. The
two windings of the
transformer wounded, where
the two secondary windings
have an equivalent number of
windings. These are located on
both sides of the primary
winding of the transformer. A
cam formed a magnetic core
which is made with a soft iron
is coupled to a shaft. Thus, this
core can be twisted among the
windings.

Working
The typical RVDTs are linear over a +40 or -40
degrees, Sensitivity is about 2mV to 3mV per
degree of rotation and the input voltage range is 3V
RMS at frequency ranges from 400Hz to 20kHz.
Based on the movement of the shaft in the
transformer, the three conditions will be produced
such as
• When the Core is at Null Position
• When the Core Rotates in Clockwise Direction
• When the Core Rotates in Anticlockwise
Direction

When the Core is at Null Position
In the first condition, when the shaft is placed at
the null position then the induced e.m.f in the
secondary windings are similar although reverse
in phase. Thus, the differential o/p potential will
be zero, and the condition will be E1 = E2, where
E0 = E1-E2 =0

When the Core Rotates in Clockwise Direction
In the second condition, when the shaft rotates
in the direction of clockwise; more section of
the core will enter across the primary winding.
Therefore, the induced e.m.f across the primary
winding is higher than secondary winding.
Hence, the differential o/p potential is positive,
and the condition will be E1 > E2, where E0 = E1-
E2 = positive.

• When the Core Rotates in Anticlockwise
Direction
In the third condition, when the shaft rotates in the
direction of anticlockwise, more section of the core
will be entered across the secondary winding. Thus,
the induced e.m.f across the secondary coil is
higher than the primary coil. Hence, the differential
o/p potential is negative that means 1800 phase
shift, and the condition will be E1 < E2, where E0 =
E1-E2 = negative.

RVDT Advantages and Disadvantages
• The advantages of RVDT include the following.
• The consistency of RVDT is high
• The exactness of RVDT is high
• The lifespan is long
• The performance is repeatable
• The construction is compact and strong
• Durability
• Low cost
• Easy to handle electronic components
• Resolution is infinite
• Linearity is Excellent
• A wide range of dimension ranges
The disadvantages of RVDT mainly include the following
• The contact among the measuring exterior as well as the nozzle is
not possible for all time.
• The output of the RVDT is linear (about +40 or -40 degrees), so it
restricts the usability.

Applications
The applications of Rotary Variable Differential Transformer are as
follows
With many such advantages, the applications of RVDT are extended to a
greater extent and usually preferred its usage in heavy-duty
manufacturing equipment in oil and gas industries, aerospace, and so
on. The typical applications of RVDT are as follows,
1). In Aircraft and Avionics applications to measure the angular
movement of control actuators and propel navigation.
2). In engines to enable smart fuel control systems
3). In cockpit controllers and cable systems.
4). In robotics to evaluate angular displacement and derive motions of
the hand, legs, and other momentary parts.
In general, the survey on market research about the applications
of Rotary Variable Differential Transformer shows that increased
automation and the growing number of industrial automation are the
factors driving the RVDT to the global level. In the future, the
requirement may further increase due to the demand for compact test
equipment, machine tools, and increased robotic applications.

Encoders are sensing devices whose purpose is to provide
feedback about the motion of objects to control systems.
This feedback allows the control system to establish
whether the object being monitored is being correctly
moved or positioned and permits adjustments to be
made or actions to be taken based on the movement and
position of the object.
Encoders typically are used to measure one or more
specific parameters about the object, such as its speed,
position, direction, or to provide a count of the object or
some related value.

A simple example of how an encoder might be used is in
a cut-to-length application. Imagine a cutting operation
or machine that is designed to regularly produce material
of a certain fixed length. The raw material, such as fabric,
is continuously fed into the machine from a spool. The
machine needs to determine when the desired length of
material to be cut has been fed from the spool onto the
machine’s conveyor, and then must instruct a cutting
blade to pass across the material at precisely the right
moment to produce the proper length of cut material. An
encoder is used in applications such as this to tell the
control circuit for the machine when to make the cut.

Types of Encoders
Linear encoders deal with the movement of objects along a path or
line, such as in the cut-to-length application mentioned earlier. This
type of encoder makes use of a transducer to measure the movement
or distance between two points, sometimes employing a cable (longer
distances) or a small rod (shorter distances). In these cases, a cable is
run between the encoder transducer and the moving object. As the
object moves, the transducer gathers data from the cable and
produces an analog or digital output signal that is used to establish the
object’s movement or position.
Rotary encoders are used to provide feedback about the movement of
a rotating object or device, such as the shaft of a motor. The rotary
encoder converts the angular position of the moving shaft into an
analog or digital output signal that will then enable a control system to
establish the shaft’s position or speed.
Angle Encoders
Angle encoders are similar to rotary encoders in that they monitor and
provide feedback on rotational movement, but they are different in
that angle encoders tend to offer higher accuracy.

Encoder Sensing Technologies
• Optical encoders are the most accurate of all the sensing methods.
A rotary optical encoder consists of a light source such as an LED
and a rotating disk that is patterned with a series of opaque lines
and alternating translucent slots. As the light passes through the
rotating disk, a photosensor mounted on the opposite side of the
disk detects the light and generates a sinusoidal electrical signal
that corresponds to the presence of light detected from the
translucent slots and the absence of light from the opaque lines.

Magnetic encoders rely on the detection of a change in
magnetic flux to establish the movement and position of
an object. A magnetic rotary encoder consists of a
magnetized disk that has a number of magnetic poles
located along its circumference. A sensor is positioned
next to the disk, and as the disk rotates, the sensor
detects the change in the magnetic field as the different
poles in the disk surface pass near the sensor.

Capacitive Encoders
Capacitive encoders are a relatively new sensing
technology for encoder design. The operating
principle relies on the detection of a change in
capacitance using a high-frequency reference
signal. With a rotary capacitive encoder

Selection Considerations
• Choosing an encoder requires an understanding of the application’s environmental
conditions and desired performance levels. Below are some of the selection
considerations that should be reviewed when making a sourcing decision on an
encoder product.
• What type of motion is being monitored – linear or rotational?
• What parameters are being measured – position, speed of movement?
• Is the recording of direction important? – this will help establish whether a single
channel encoder will suffice or whether or a multiple channel encoder is needed.
• What are the specific environmental conditions to which the encoder will be
exposed? – this will help dictate the most suitable sensing technology for the
encoder. This includes potential exposure to:
– Dust
– Moisture
– Caustic or corrosive chemicals
– Shock
– Vibration
– Temperature extremes
• Can the intended application stand to have a re-homing in the event of power
loss? This will help establish the need for incremental vs. absolute encoders.
• How much resolution is needed for the measurement?
• What are the electrical requirements for the system? – this includes consideration
of the output interface such as point-to-point interfaces, fieldbus interfaces, or
ethernet interfaces.
• What mechanical packaging and design are suited to the application? – for rotary
actuators, this includes deciding on a thru-bore vs. shaft encoder, the type of
locking mechanism, the bore size, and whether a sealed or exposed encoder
package is better.

accelerometer
An accelerometer is an apparatus, either mechanical or
electromechanical, for measuring acceleration or
deceleration - that is, the rate of increase or decrease in
the velocity of a moving object. Accelerometers are used
to measure the efficiency of the braking systems on road
and rail vehicles; those used in aircraft and spacecraft can
determine accelerations in several directions
simultaneously. There are also accelerometers for
detecting vibrations in machinery.
The measurement of acceleration or one of its derivative
properties such as vibration, shock, or tilt has become
very commonplace in a wide range of products.

• The measurement of acceleration is required in a variety of
purposes, ranging from machine design to guidance
systems.
• A wide variety of transducers and measurement techniques
exists for acceleration and vibration measurements, each
associated with a particular application.
• Displacement, velocity and acceleration measurements are
also referred to as shock or vibration measurements,
depending on the waveform of the forcing function that
causes the acceleration. Acceleration is a derivative of
velocity, second derivative of displacement.
• A forcing function that is periodic in nature generally results
in acceleration that are analyzed as vibration.

Working
A piezoelectric accelerometer consists of a mass attached to a
piezoelectric crystal which is mounted on a case. When the
accelerometer body is subjected to vibration, the mass on the
crystal remains undisturbed in space due to inertia. As a
result, the mass compresses and stretches the piezoelectric
crystal. This force is proportional to acceleration
in accordance with Newton’s second law,
F = ma,
and generates a charge.
Due to the special self-generating property, the crystal
produces a voltage that is proportional to the accelerative
force
The charge output is then converted into low impedance
voltage output with the help of electronics.

Seismic Transducer:
The seismic transducer is used for measuring the
vibration of the ground. The spring mass damper element
and the displacement transducer are the two main
component of the seismic transducer. The mass that
connected to the damper element and spring without any
other support is known as spring mass damper element.
And the displacement transducer converts the
displacement into the electrical quantity. The seismic
transducer is used for measuring the earth vibration,
volcanic eruption and other vibrations etc.

Construction of Seismic Transducer
The systematic
diagram of the
seismic transducer is
shown in the figure
below. The mass is
connected by the
help of the damper
and spring to the
housing. The housing
frame is connected to
the source whose
vibrations need to be
measured.

The arrangement is kept in such a way so that
the position of the mass remains same in the
space. Such type of arrangement is kept for
causing the relative motion between the
housing frame and the mass. The term relative
motion means one of the objects remains
stationary, and the other is in motion concerning
the first one. The displacement that occurs
between the two is sensed and represented by
the transducer.

Gyroscope
Microelectromechanical systems, popularly known as MEMS,
is the technology of very small electromechanical and
mechanical devices. Advance in MEMS technology has helped
us to develop versatile products. Many of the mechanical
devices such as Accelerometer, Gyroscope, etc… can now be
used with consumer electronics. This was possible with MEMS
technology. These sensors are packaged similarly to other IC’s.
Accelerometers and Gyroscopes compliment each other so,
they are usually used together. An accelerometer measures
the linear acceleration or directional movement of an object,
whereas Gyroscope Sensor measures the angular velocity or
tilt or lateral orientation of the object. Gyroscope sensors for
multiple axes are also available.

In this sensor to measure the angular rate, the
rotation rate of the sensor is converted into an
electrical signal. This sensor consists of an
internal vibrating element made up of crystal
material in the shape of a double – T-
structure. This structure comprises a
stationary part in the center with ‘Sensing
Arm’ attached to it and ‘Drive Arm’ on both
sides.

This double-T-structure is symmetrical. When an
alternating vibration electrical field is applied to the
drive arms, continuous lateral vibrations are produced.
As Drive arms are symmetrical, when one arm moves
to left the other moves to the right, thus canceling out
the leaking vibrations. This keeps the stationary part at
the center and sensing arm remains static.
When the external rotational force is applied to the
sensor vertical vibrations are caused on Drive arms. This
leads to the vibration of the Drive arms in the upward
and downward directions due to which a rotational force
acts on the stationary part in the center.

Rotation of the stationary part leads to the
vertical vibrations in sensing arms. These
vibrations caused in the sensing arm are
measured as a change in electrical charge. This
change is used to measure the external
rotational force applied to the sensor as Angular
rotation.

Force measurements using
strain gauges

A load cell is a transducer that
measures force, and outputs this
force as an electrical signal. Most
load cells use a strain gauge to
detect measurements, but
hydraulic and pneumatic load cells
are also available.

strain gauge
Strain Gauge or Strain Gage was invented in
1938 by Edward E. Simmons and Arthur C. Ruge.
It is one of the significant sensors used in the
geotechnical field to measure the amount of
strain on any structure (Dams, Buildings, Nuclear
Plants, Tunnels, etc.). The resistance of a strain
gauge varies with applied force and, it converts
parameters such as force, pressure, tension,
weight, etc. into a change in resistance that can
be measured later on.

How does a strain gauge work?
A strain gauge depends on the electrical
resistivity of any conductor. The resistance in
any conducting device is dependent on its
length as well as the cross-section area.
Suppose L1 is the original length of wire and L2
is the new length after an external force is
applied on it, the strain (ε) is given by the
formula:
ε = (L2-L1)/L1

working principle of Strain Gauge
A strain gauge works on the principle of
electrical conductance and its dependence on
the conductor’s geometry. Whenever a
conductor is stretched within the limits of its
elasticity, it doesn’t break but, gets narrower
and longer. Similarly, when it is compressed, it
gets shorter and broader, ultimately changing its
resistance.

We know, resistance is directly dependent on the length
and the cross-sectional area of the conductor given by:
R= L/A
Where,
R = Resistance L = Length A = Cross-Sectional Area
The change in the shape and size of the conductor also
alters its length and the cross-sectional area which
eventually affects its resistance.
Any typical strain gauge will have a long, thin conductive
strip arranged in a zig-zag pattern of parallel lines. The
reason behind aligning them in a zig-zag fashion is that
they don’t increase the sensitivity since the percentage
change in resistance for a given strain for the entire
conductive strip is the same for any single trace.

measure strain with a strain gauge
As mentioned earlier, strain gauges work on the
principle of the conductor’s resistance which gives
you the value of Gauge Factor by the formula:
GF= (∆R⁄RG )/∈
Where,
‘ΔR’ is the change in resistance due to strain
‘RG’ is the resistance of the undeformed gauge
‘ε’ is the strain

Now, in practice, the change in the strain of an
object is a very small quantity which can only be
measured using a Wheatstone Bridge. The
Wheatstone Bridge circuit is given below.
A Wheatstone Bridge is a
network of four resistors with an
excitation voltage,Vexthat is
applied across the bridge. The
Wheatstone Bridge is the
electrical equivalent of two
parallel voltage divider circuits
with R1 and R2 as one of them
and R3 and R4 as the other one.

The output of the Wheatstone circuit is given by:
Vo = [(R3/R3+R4) — (R2/R1+2)] * Vex3
Whenever R1/ R2 = R4/ R3, the output voltage Vo is zero
and the bridge is said to be balanced. Any change in the
values of R1, R2, R3, and R4 will, therefore, change the
output voltage. If you replace the R4 resistor with a strain
gauge, even a minor change in its resistance will change
the output voltage Vex which is a function of strain. The
equivalent strain output and voltage output always have a
relation of 2:1.

Characteristics of strain gauges
The characteristics of strain gauges are as follows:
• They are highly precise and don’t get influenced due to
temperature changes. However, if they do get affected by
temperature changes, a thermistor is available for temperature
corrections.
• They are ideal for long distance communication as the output is an
electrical signal.
• Strain Gauges require easy maintenance and have a long operating
life.
• The production of strain gauges is easy because of the simple
operating principle and a small number of components.
• The strain gauges are suitable for long-term installation. However,
they require certain precautions while installing.
• All the strain gauges produced by Encardio-Rite are hermetically
sealed and made up of stainless steel thus, waterproof.
• They are fully encapsulated for protection against handling and
installation damage.
• The remote digital readout for strain gauges is also possible.

•Instrumentation of bridges is done to
verify design parameters, evaluate the
performance of new technologies used in
the construction of bridges, to verify and
control the construction process and for
subsequent performance monitoring.
•It is used to measure stress and strain on
rails. Strain gauges measure axial tension
or compression with no impact on the
rails. In case of an emergency, the strain
gauges can generate a warning so
maintenance can be done early to
minimize the impact on rail traffic.
•Strain gauges can measure the torque
applied by a motor, turbine, or engine to
fans, generators, wheels, or propellers.
You will find such types of equipment in
power plants, ships, refineries,
automobiles and industries.

Measurement of motion, force and torque

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