Environmental Engineering Reference
In-Depth Information
Principle
A vibrating steel wire is attached to the object on which a measurement has to be made. A
change in the quantity to be measured causes a change in stress in the measuring wire and
consequently a change in its frequency. The measuring wire oscillates within a magnetic
field, inducing in a coil an electrical oscillation of the same frequency, which is transmit-
ted by cable to a receiving instrument.
Types
Intermittent vibrating-wire systems
, used for static and semistatic measuring systems, con-
sist of a transducer containing the measuring wire and an electromagnet, and a receiver.
Continuous vibrating-wire systems
contain the measuring wire, an oscillator, and two elec-
tromagnets in the transducer, and are used for static and dynamic measuring devices and
for alarm and control systems (Dreyer, 1977).
Characteristics
Gage lengths generally range from 4 to 14 in., and strain sensitivity has typical maximum
ranges from 600 to 7000 microstrains.
Linear-Displacement Transducers
Applications
Linear-displacement transducers are generally used in extensometers.
Linear Potentiometer
Resistance devices, linear potentiometers, consist of a mandrel wound with fine wire or
conductive film. A wiper attached to a shaft rides along the mandrel and divides the man-
drel resistance into two parts. The resistance ratio of these parts is measured with a
Wheatstone bridge circuit to determine the displacement of the wiper and shaft.
Movement ranges are generally about 0.5 to 24 in. with an average sensitivity of 0.01 to
0.001. Linear potentiometers are extremely sensitive to moisture and require absolute sealing.
Linear Variable Differential Transformer (LVDT)
The LVDT converts a displacement into a voltage change by varying the reluctance path
between a primary coil and two or more secondary coils when an excitation voltage is
applied to the primary coil. Variations in the output signal are calibrated to displacements
of the LVDT core.
Movement ranges are generally about 0.1 ft to several feet; sensitivity is about 10 micros-
trains. LVDTs are much less sensitive to moisture and less affected by temperature than are
linear potentiometers.
4.2
Surface Movements
4.2.1
Forms and Significance
Vertical Displacements
Settlement or heave is a vertical surface displacement that results in detrimental distortion
of structures, especially if differential, when significant magnitudes are reached (see also
