Download The letters LVDT are an acronym for Linear Variable

mywbut.com
The letters LVDT are an acronym for Linear Variable Differential Transformer, a common type of electromechanical transducer
that can convert the rectilinear motion of an object to which it is coupled mechanically into a corresponding electrical signal. LVDT
linear position sensors are readily available that can measure movements as small as a few millionths of an inch up to several
inches, but are also capable of measuring positions up to ±20 inches (±0.5 m).
The figure shows the components of a typical LVDT. The transformer's internal structure consists of a primary winding centered
between a pair of identically wound secondary windings, symmetrically spaced about the primary. The coils are wound on a onepiece hollow form of thermally stable glass reinforced polymer, encapsulated against moisture, wrapped in a high permeability
magnetic shield, and then secured in a cylindrical stainless steel housing. This coil assembly is usually the stationary element of
the position sensor.
The moving element of an LVDT is a separate tubular armature of magnetically permeable material called the core, which is free to
move axially within the coil's hollow bore, and mechanically coupled to the object whose position is being measured. This bore is
typically large enough to provide substantial radial clearance between the core and bore, with no physical contact between it and
the coil.
In operation, the LVDT's primary winding is energized by alternating current of appropriate amplitude and frequency, known as the
primary excitation. The LVDT's electrical output signal is the differential AC voltage between the two secondary windings, which
varies with the axial position of the core within the LVDT coil. Usually this AC output voltage is converted by suitable electronic
circuitry to high level DC voltage or current that is more convenient to use.
How does an LVDT work?
This figure illustrates what happens when the LVDT's core is in different axial positions. The LVDT's primary winding, P, is
energized by a constant amplitude AC source. The magnetic flux thus developed is coupled by the core to the adjacent secondary
windings, S1 and S2 . If the core is located midway between S1 and S2 , equal flux is coupled to each secondary so the voltages,
E1 and E2 , induced in windings S1 and S2 respectively, are equal. At this reference midway core position, known as the null point,
the differential voltage output, (E1 - E2 ), is essentially zero.
1
mywbut.com
If the core is moved closer to S1 than to S2 , more flux is coupled to S1 and less to S2 , so the induced voltage E1 is increased
while E2 is decreased, resulting in the differential voltage (E1 - E2). Conversely, if the core is moved closer to S2 , more flux is
coupled to S2 and less to S1 , so E2 is increased as E1 is decreased, resulting in the differential voltage (E2 - E1).
The top graph shows how the magnitude of the differential output voltage, EOUT, varies with core position. The value of EOUT at
maximum core displacement from null depends upon the amplitude of the primary excitation voltage and the sensitivity factor of the
particular LVDT, but is typically several volts RMS. The phase angle of this AC output voltage, EOUT, referenced to the primary
excitation voltage, stays constant until the center of the core passes the null point, where the phase angle changes abruptly by 180
degrees, as shown in the middle graph.
This 180 degree phase shift can be used to determine the direction of the core from the null point by means of appropriate circuitry.
This is shown in the bottom graph, where the polarity of the output signal represents the core's positional relationship to the null
point. The figure shows also that the output of an LVDT is very linear over its specified range of core motion, but that the sensor
can be used over an extended range with some reduction in output linearity. The output characteristics of an LVDT vary with
different positions of the core. Full range output is a large signal, typically a volt or more, and often requires no amplification. Note
that an LVDT continues to operate beyond 100% of full range, but with degraded linearity.
Strain gauges
If a strip of conductive metal is stretched, it will become skinnier and longer, both changes resulting in an
increase of electrical resistance end-to-end. Conversely, if a strip of conductive metal is placed under
compressive force (without buckling), it will broaden and shorten. If these stresses are kept within the
elastic limit of the metal strip (so that the strip does not permanently deform), the strip can be used as a
measuring element for physical force, the amount of applied force inferred from measuring its resistance.
Such a device is called a strain gauge. Strain gauges are frequently used in mechanical engineering research
and development to measure the stresses generated by machinery. Aircraft component testing is one area of
application, tiny strain-gauge strips glued to structural members, linkages, and any other critical component
2
mywbut.com
of an airframe to measure stress. Most strain gauges are smaller than a postage stamp, and they look
something like this:
A strain gauge's conductors are very thin: if made of round wire, about 1/1000 inch in diameter.
Alternatively, strain gauge conductors may be thin strips of metallic film deposited on a nonconducting
substrate material called the carrier. The latter form of strain gauge is represented in the previous
illustration. The name "bonded gauge" is given to strain gauges that are glued to a larger structure under
stress (called the test specimen). The task of bonding strain gauges to test specimens may appear to be
very simple, but it is not. "Gauging" is a craft in its own right, absolutely essential for obtaining accurate,
stable strain measurements. It is also possible to use an unmounted gauge wire stretched between two
mechanical points to measure tension, but this technique has its limitations.
Typical strain gauge resistances range from 30 Ω to 3 kΩ (unstressed). This resistance may change only a
fraction of a percent for the full force range of the gauge, given the limitations imposed by the elastic limits
of the gauge material and of the test specimen. Forces great enough to induce greater resistance changes
would permanently deform the test specimen and/or the gauge conductors themselves, thus ruining the
gauge as a measurement device. Thus, in order to use the strain gauge as a practical instrument, we must
measure extremely small changes in resistance with high accuracy.
Such demanding precision calls for a bridge measurement circuit. Unlike the Wheatstone bridge shown in the
last chapter using a null-balance detector and a human operator to maintain a state of balance, a strain
gauge bridge circuit indicates measured strain by the degree of imbalance, and uses a precision voltmeter in
the center of the bridge to provide an accurate measurement of that imbalance:
3
mywbut.com
Typically, the rheostat arm of the bridge (R2 in the diagram) is set at a value equal to the strain gauge
resistance with no force applied. The two ratio arms of the bridge (R1 and R3) are set equal to each other.
Thus, with no force applied to the strain gauge, the bridge will be symmetrically balanced and the voltmeter
will indicate zero volts, representing zero force on the strain gauge. As the strain gauge is either compressed
or tensed, its resistance will decrease or increase, respectively, thus unbalancing the bridge and producing
an indication at the voltmeter. This arrangement, with a single element of the bridge changing resistance in
response to the measured variable (mechanical force), is known as a quarter-bridge circuit.
Resistance Thermometers
Resistance thermometers are slowly replacing thermocouples in many lower temperature
industrial applications (below 600°C). Resistance thermometers come in a number of
construction forms and offer greater stability, accuracy and repeatability. The resistance tends
to be almost linear with temperature. A small power source is required.
No special extension cables or cold junction compensations are required The resistance of a
conductor is related to its temperature. Platinum is usually used due to its stability with
temperature. The Platinum detecting wire needs to be kept free of contamination to remain
stable. A Platinum wire or film is created and supported on a former in such a way that it gets
minimal differential expansion or other strains from its former, yet is reasonably resistant to
vibration.
Commercial platinum grades are produced which exhibit a change of resistance of 0.385
Ohms/°C (European Fundamental Interval) The sensor is usually made to have 100 Ohms at 0 °C.
This is defined in BS EN 60751:1996. The American Fundamental Interval is 0.392 Ohms/°C.
Resistance thermometers require a small current to be passed through in order to determine the
resistance. This can cause self heating and manufacturers limits should always be followed along
with heat path considerations in design. Care should also be taken to avoid any strains on the
resistance thermometer in its application.
Lead wire resistance should be considered and adopting three and four wire connection
strategies can result in eliminating connection lead resistance effects from measurements.
Resistance thermometers elements are available in a number of forms. The most common are:
Wire Wound in a ceramic insulator - High temperatures to 850 °C
Wires encapsulated in glass - Resists the highest vibration and offers most protection to the Pt
Thin film with Pt film on a ceramic substrate - Inexpensive mass production
Practical Construction
These elements will nearly always require insulated leads attached. At low temperatures PVC,
Silicon rubber or PTFE insulators are common to 250 °C. Above this Glass fibre or ceramic are
used. The measuring point and usually most of the leads require a housing or protection sleeve.
This is often a metal alloy which is inert to a particular process.
4