Vibrating Wire Sensors A vibrating wire sensor comprises three essential elements: • A mechanical assembly (i .e. , a tensioned steel wire permanently mounted inside the sensor body) • A means of electromagnetic excitation to vibrate the wire • A measurement system to measure the frequency of the wire vibration Cast-in-place pile instrumented with sister bars, jack-out pressure cells, piezometers and inclinometers instrumentation can come from long-term readings, and not from getting information in a short space of time, which does not allow trends to be detected. Long-term measurements are made over decades (e.g., 10, 20, 30 or more years) and some- times even over the lifetime of a facility. Long-term measurement systems track changes in the mechanical state of the structure, often using sensors that are permanently installed at inaccessible locations where they cannot be replaced (or recalibrated). As such, it is critical that the sensors used be reliable and do not drift, that there is no inherent tendency for the value returned by a repeated measurement to vary systematically over time when there is no real variation in the parameter being measured. In this respect, despite advances in modern sensor technology, well- constructed vibrating wire sensors are particularly well suited. In use, the vibrating wire is set into transverse vibration by a short pulse of current, which is generated within the measurement instrument, that passes through the electromagnetic excitation (i.e., coil) located near the midpoint of the wire. The current sets the wire vibrating at its resonant frequency. As the wire oscillates in the magnetic field of the electromagnetic coil (i.e., the same coil used to excite the sensor), the oscillating wire creates a small alternating current, which travels at the sensor (i.e., vibrating wire) frequency via a cable to the measurement system. In the measurement system, a pulse counter measures the time for a given number of cycles (i.e., zero crossings) and a microprocessor converts the period (i.e., the time for one cycle) into a corresponding frequency. The square of this frequency is proportional to the parameter being measured (i.e., strain, pressure, temper- ature, etc.). Recent developments in vibrating wire measurements now include the ability to measure frequencies using spectral analysis techniques rather than using time-based methods. Spectral analysis techniques are able to easily discriminate the dominant, resonant vibrating wire fre- quency component from any accompanying sources of electrical noise, which is one of the main factors affecting automated vibrating wire measurements. The advantage of vibrating wire sensors, over more conventional types, lies mainly in the frequency output, which can be transmitted over lengthy cables without appreciable degradation of the signal brought about by variations in cable resistance, arising from water penetration, temperature fluctuations, contact resis- tance or from leakage to ground. This feature, coupled with rugged designs and excellent long-term stability, makes vibrating wire sensors ideally suited for long-term measurements in construction environments. The vibrating wire method can be used to manufacture a variety of sensor types for use in structural and geotechnical monitoring applications. Most sensors are constructed from stainless steel (or other corrosion resistant alloys and materials) and are waterproof with all internal components sealed using O-rings. In its simplest form, the vibrating wire can be Installation of multilevel vibrating wire piezometers used to construct a strain gage in which the wire is clamped between two end blocks. A displacement transducer can be fashioned by connecting the vibrating wire in series with a tension spring. By fixing the vibrating wire onto the backside of a diaphragm with the other end clamped to a stable point of fixture at a point opposite, a pressure transducer can be constructed, as used in piezometers and pressure cells. Other designs allow for the creation of t i l tmeters, force t ransducers and temperature sensors. Vibrating wire load cells to measure anchor loads on wind turbine foundation 104 • DEEP FOUNDATIONS • SEPT/OCT 2019
