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Instrumentation
Motion Sensors
Displacement
The laboratory uses many different types of
displacement transducers that each have various attributes and limitations
which determine their suitability for different applications. The following is
a list of each different displacement transducer and a brief summary of its
mechanics.
Linear Potentiometers
The most readily available and simplest position
transducer is a linear potentiometer
excited by a DC source such as a battery. It may be hooked up to deliver an
output voltage that is essentially proportional to a straight-line position
varying between zero and a maximum. Alternatively, a potentiometer may be
hooked up to deliver an output varying between a negative and positive voltage
in proportion to a mechanical displacement that also varies between a maximum
negative and a maximum positive value relative to a defined null position.
Linear Variable
Differential Transformer (LVDT)
The word "linear" appears in the name of the
LVDT
to denote straight-line motion as opposed to a linear relationship between input
and output. Three coils of electrically conducting wire are wound on an
insulating form. By the principle of mutual inductance an AC voltage across the
terminals of the primary coil induces a voltage of the same frequency in each
of the two secondary coils. If the moveable ferromagnetic core is centered, the
two secondary voltages are of the same amplitude. For a positive displacement
of the core, the voltage appearing across the number 1 secondary coil is
greater in amplitude than at the null condition, while the amplitude across the
number 2 secondary coil is less.
MTS Temposonic
Displacement Transducer
Initially a current pulse is applied to the conductor
within the waveguide over its entire length. There is another magnetic field
generated by the permanent magnet that exists only where the magnet is located.
This field has a longitudinal component. These two fields join vectorially to
form a helical field near the magnet which in turn causes the waveguide to
experience a minute torsional strain or twist only at the location of the
magnet. This torsional strain pulses propagates along the waveguide at the
speed of sound in this material. When this torsional pulse arrives at the tapes
in the head it is converted into a dynamic longitudinal pulse injected into the
tapes. The longitudinal pulses cause the tapes to experience a momentary change
in reluctance. Two coils coupling these tapes mounted in the field of two bias
magnets will generate a momentary electrical pulse caused by the change in
reluctance in the tapes. In order to extract the useful position information we
measure the time between when we launch the initial current pulse and the time
we receive the signal from the output coils. This time is a very precise
function of the position of the moving magnet.
Figure 1: Temposonic Dimension Drawing
Acceleration
Piezoresistive Accelerometer
This type of accelerometer, also known as a strain
gage accelerometer, is similar in principle to a piezoelectric accelerometer
except it is equipped with a built in resistor, which allows it to be used with
a standard signal conditioner.
Table 1 presents a summary of the available
transducers (excluding load cells) and their range of measurement.
Table 1: Available Transducers
Rotation
The laboratory uses
rotational transducers that also have various attributes and limitations which
determine their suitability for different applications. The following is a
brief summary of its mechanics.
Rotary Variable
Differential Transformer (RVDT)
RVDTs incorporate a
proprietary noncontact design that dramatically improves long term reliability
when compared to other traditional rotary devices such as syncros, resolvers
and potentiometers. This unique design eliminates assemblies that degrade over
time, such as slip rings, rotor windings, contact brushes and wipers, without
sacrificing accuracy.
High reliability
and performance are achieved through the use of a specially shaped rotor and
wound coil that together simulates the linear displacement of a Linear Variable
Differential Transformer (LVDT). Rotational movement of the rotor shaft results
in a linear output signal that shifts ±60 (120 total) degrees around a factory
preset null position. The phase of this output signal indicates the direction
of displacement from the null point. Noncontact electromagnetic coupling of the
rotor provides infinite resolution, thus enabling absolute measurements to a
fraction of a degree.
Although capable of
continuous rotation, most RVDTs are calibrated over a range of ±30 degrees,
with nominal nonlinearity of less than ±0.25% of full scale (FS). Extended
range operation up to a maximum of ±90 degrees is possible with compromised
linearity.
R30D
The R30D RVDT is a
DC operated noncontacting rotary transducer. Integrated signal conditioning
enables the R30D
to operate from a bipolar ±15 VDC source with a high level DC output that is
proportional to the full range of the device. Calibrated for operation to ±30
degrees, the R30D
provides a constant scale factor of 125 mVDC/degree. Nonlinearity error of less
than ±0.25% FS is achieved while maintaining superior thermal performance over
-18°C to 75°C.
Loading Sensors
Load Cells
Due to the fact that many of the test apparatuses are
specifically developed for single experiments, in-house custom built load cells
are often used. The geometric layout of a typical load cell is shown in Figure
2. They are fabricated from a thick wall cylindrical steel tube. The turned
down wall thickness, height, and radius are determined based on the expected
maximum stresses in the load cells during testing.
Figure 2: Geometric Layout of Typical Load
Cell
The attachment
plates ensure a uniform stress distribution over the entire load cell and
provide anchorage into the columns. In the most complicated custom built load
cells, axial, shear, and moment stresses can be measured from Wheatstone bridge
circuits wired according to Figure 3. Simpler compression-tension load cells
are also commonly built using only an axial Wheatstone bridge circuit.
In addition a majority of the MTS, Miller, and Parker Actuators were purchased
with a load cell provided by the manufacturer. These load cells are often used
in experimentation.
For more detail on our 6” Five-Component Load Cell in-house made Load Cells p
lease refer to this document: Load
Cells Drawings and Calibrations
Delta P Cells
Delta P cells are used on many of the actuators
available in the laboratories. The MTS servo controllers utilize the Delta P
(differential pressure) measured across the actuator piston as a stabilizing
variable during the control of an actuator's motion.
Table 2 lists the different available load measuring
devices.
Table 2 : Available Load Measuring Devices
Load Units Kips[kN],
Moment Units Kips-Inch [kN-m]
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Load Measuring Device Type
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Quantity
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Load Capacity
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Use
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Calibration Interval
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Equipment Designation *
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5.5”
Five-Component Load Cell
5D-LC-5.5-YEL
(axial, x & y shear, x & y moment)
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16
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Axial : 30 [133.6] Shear : 5 [22.3]
Moment: 30 [3.39]
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Shake Table & Floor Testing
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As Needed
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Non-NEES
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12” Five-Component Load Cell
5D-LC-12-BLU
(axial, x & y shear, x & y moment)
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4
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Axial : 100 [454.5]
Shear : 20 [89]
Moment 220 [24.86]
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Shake Table & Floor Testing
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As Needed
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Non-NEES
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12” Five-Component Load Cell
5D-LC-12-RED
(axial, x & y shear, x & y moment)
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4
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Axial : 100 [454.5]
Shear : 20 [89]
Moment 220 [24.86]
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Shake Table & Floor Testing
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As Needed
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Non-NEES
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12” Five-Component Load Cell
5D-LC-12-BLK
(axial, x & y shear, x & y moment)
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4
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Axial : 100 [454.5]
Shear : 20 [89]
Moment 220 [24.86]
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Shake Table & Floor Testing
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As Needed
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Non-NEES
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Axial
(Various)
(compression:tension)
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10
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2 – 250
[8.9–1112.06]
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Shake Table & Floor Testing
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As Needed
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Non-NEES
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Washer
(compression only)
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8
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100 [454.5]
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Shake Table & Floor Testing
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As Needed
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Non-NEES
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MTS Load Cell
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1
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2.2 [9.79]
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On MTS Actuator
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2 Years
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Non-NEES
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MTS Load Cell
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2
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55 [244.65]
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On MTS Actuator
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2 Years
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Non-NEES
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MTS Load Cell
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1
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110 [489.30]
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On MTS Actuator
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2 Years
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Non-NEES
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MTS Load Cell
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1
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220 [ 978.61]
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On MTS Actuator
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2 Years
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Non-NEES
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Lebow Load Cell
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2
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250 [ 1112.06]
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On Miller Actuator
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2 Years
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Non-NEES
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Custom Built Load Cell
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4
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70 [311.38]
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On Parker Actuator
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One Year – Local Calibration
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Non-NEES
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MTS Load Cell Model 661.31E-01
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3
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220 [978.61]
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On MTS Actuator
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2 Years
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NEES
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MTS Differential Pressure Cell 660.23
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5
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5000 psi
[35 MPa]
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On MTS Actuator
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2 Years
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NEES
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Figure 3: Typical Strain Gage Positioning and Wiring for Multidirectional Load Cells
Strain
The Strain Gauge
While there are several methods of measuring strain, the
most common is with a strain gauge, a device whose electrical resistance varies
in proportion to the amount of strain in the device. The most widely used gauge
is the bonded metallic strain gauge.
The metallic strain gauge consists of a very fine wire or,
more commonly, metallic foil arranged in a grid pattern. The grid pattern
maximizes the amount of metallic wire or foil subject to strain in the parallel
direction (Figure 4). The cross sectional area of the grid is minimized
to reduce the effect of shear strain and Poisson Strain. The grid is bonded to
a thin backing, called the carrier, which is attached directly to the test
specimen. Therefore, the strain experienced by the test specimen is transferred
directly to the strain gauge, which responds with a linear change in electrical
resistance. Strain gauges are available commercially with nominal resistance
values from 30 to 3000 Ω, with 120, 350, and 1000 Ω being the most
common values.
Figure 4: Bonded Metallic Strain Gauge
It is very important that the strain gauge be properly
mounted onto the test specimen so that the strain is accurately transferred
from the test specimen, though the adhesive and strain gauge backing, to the
foil itself. A fundamental parameter of the strain gauge is its sensitivity to
strain, expressed quantitatively as the gauge factor (GF). Gauge factor is
defined as the ratio of fractional change in electrical resistance to the
fractional change in length (strain):
The
Gauge Factor for metallic strain gauges is typically around 2.
Table 3: Available strain gauges
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Strain Gauge Type
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Quantity
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Model No.
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Calibration Interval
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Equipment Designation *
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Uni-axial
strain gage
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275
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CEA-06-125UW-120
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As Needed
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Non-NEES
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