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

    Device Type

    Measured Quantity

    Quantity

    Measurement Range

    Equipment Designation *

    Linear potentiometer

    Displacement

    20

    .25 : 2.0 in.

    [ .64 : 5.08 cm]

    Non-NEES

    Linear potentiometer

    Displacement

    110

    20 in.

    Non-NEES

    Linear potentiometer

    Displacement

    2

    5 in.

    Non-NEES

    LVDT

    Displacement

    15

    .5 : 2.0 in.

    [ 1.27 : 5.08 cm]

    Non-NEES

    MTS Temposonic Transducer

    Displacement

    13

    4 in. [10.16 cm]

    Non-NEES

    MTS Temposonic Transducer

    Displacement

    4

    8 in. [20.32 cm]

    Non-NEES

    MTS Temposonic Transducer

    Displacement

    3

    10 in. [25.4 cm]

    Non-NEES

    MTS Temposonic Transducer

    Displacement

    3

    16 in. [40.64 cm]

    Non-NEES

    MTS Temposonic Transducer

    Displacement

    6

    20 in. [50.8 cm]

    Non-NEES

    MTS Temposonic Transducer

    Displacement

    2

    30 in. [76.2 cm]

    Non-NEES

    Shaevitz RVDT R30D

    Rotation

    4

    0 : 30 degrees

    Non-NEES

    Endevco Piezoresistive Accelerometer

    Acceleration

    8

    0 : 25 g

    Non-NEES

    Sensotec Piezoresistive Accelerometer

    Acceleration

    150

    0 : 10 g

    NEES

    Kistler

    Acceleration

    2

    0:10 g

    Non-NEES

    Kistler

    Acceleration

    8

    0:2.5 g

    Non-NEES

    PCB

    Acceleration

    2

    0:3 g

    NEES

    PCB

    Acceleration

    22

    0:10 g

    NEES

    Kulite Piezoresistive Accelerometer

    Acceleration

    15

    0 : 10 g

    Non-NEES

    MTS Temposonic II

    Displacement

    15

    4-20 in.

    NEES

  • 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 -18C to 75C.

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.

    Click to show bigger image

    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]

    Load Measuring Device Type

    Quantity

    Load Capacity

    Use

    Calibration Interval

    Equipment Designation *

    5.5 Five-Component Load Cell

    5D-LC-5.5-YEL

    (axial, x & y shear, x & y moment)

    16

    Axial : 30 [133.6] Shear : 5 [22.3]
    Moment: 30 [3.39]

    Shake Table & Floor Testing

    As Needed

    Non-NEES

    12 Five-Component Load Cell

    5D-LC-12-BLU

    (axial, x & y shear, x & y moment)

    4

    Axial : 100 [454.5]
    Shear : 20 [89]
    Moment 220 [24.86]

    Shake Table & Floor Testing

    As Needed

    Non-NEES

    12 Five-Component Load Cell

    5D-LC-12-RED

    (axial, x & y shear, x & y moment)

    4

    Axial : 100 [454.5]
    Shear : 20 [89]
    Moment 220 [24.86]

    Shake Table & Floor Testing

    As Needed

    Non-NEES

    12 Five-Component Load Cell

    5D-LC-12-BLK

    (axial, x & y shear, x & y moment)

    4

    Axial : 100 [454.5]
    Shear : 20 [89]
    Moment 220 [24.86]

    Shake Table & Floor Testing

    As Needed

    Non-NEES

    Axial
    (Various)

    (compression:tension)

    10

    2 250
    [8.91112.06]

    Shake Table & Floor Testing

    As Needed

    Non-NEES

    Washer

    (compression only)

    8

    100 [454.5]

    Shake Table & Floor Testing

    As Needed

    Non-NEES

    MTS Load Cell

    1

    2.2 [9.79]

    On MTS Actuator

    2 Years

    Non-NEES

    MTS Load Cell

    2

    55 [244.65]

    On MTS Actuator

    2 Years

    Non-NEES

    MTS Load Cell

    1

    110 [489.30]

    On MTS Actuator

    2 Years

    Non-NEES

    MTS Load Cell

    1

    220 [ 978.61]

    On MTS Actuator

    2 Years

    Non-NEES

    Lebow Load Cell

    2

    250 [ 1112.06]

    On Miller Actuator

    2 Years

    Non-NEES

    Custom Built Load Cell

    4

    70 [311.38]

    On Parker Actuator

    One Year Local Calibration

    Non-NEES

    MTS Load Cell Model 661.31E-01

    3

    220 [978.61]

    On MTS Actuator

    2 Years

    NEES

    MTS Differential Pressure Cell 660.23

    5

    5000 psi
    [35 MPa]

    On MTS Actuator

    2 Years

    NEES

    Click to show bigger image

    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

    Strain Gauge Type

    Quantity

    Model No.

    Calibration Interval

    Equipment Designation *

    Uni-axial strain gage

    275

    CEA-06-125UW-120

    As Needed

    Non-NEES









SEESL - Structural Engineering and Earthquake Simulation Laboratory
212 Ketter Hall, Buffalo, NY 14260, USA
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