Vibrating wire sensors are a class of sensors that are very popular in geotechnical and structural monitoring. The fundamental component of the vibrating wire sensor is a tensioned steel wire that vibrates at a resonant frequency that depends on the strain in the wire. This mechanism is used in a variety of sensor configurations to measure static strain, stress, pressure, tilt, and displacement. The use of frequency, rather than amplitude, to convey the signal means that vibrating wire sensors are relatively resistant signal degradation from electrical noise, long cable runs, and other changes in cable resistance. This has contributed to their reputation for long term stability and wide usage for monitoring structures such as dams, tunnels, mines, bridges, foundations, piles, unstable slopes, and excavations.
How Vibrating Wire Sensors Work
Figure 1 illustrates the basic components of a vibrating wire sensor. A tensioned steel wire, hermetically sealed, is coupled with an electromagnetic coil. The wire is “plucked, by applying momentary electrical pulses to the coil, inducing vibrations in the wire. The wire will settle to it’s resonant vibrating frequency, and these vibrations in turn induce current back onto the signal wires. This signal can then be measured and used to determine the tension in the wire, and therefore also the strain in the structure to which the vibrating wire gage is attached.
Figure 1. Basic Operation of Vibrating Wire Sensors
More specifically, the resonant frequency of the tensioned wire is given by:
where LW is the length of the wire, T is the tension in the wire, and m is the mass of the wire per unit length. Since the tension T of the vibrating wire is directly proportional to the strain in the wire, and to the strain in the attached structure, the measured strain can be calculated as
where k is the gage factor, f is the measured frequency, and f0 is the initial frequency.
Temperature also affects the frequency of the vibrating wire and can lead to errors if not compensated. Therefore, most vibrating wire sensors include a thermistor embedded within the sensor packaging for this purpose.
The cRIO_VW01 from Pizzi Instruments is an add-on module for CompactRIO that works with vibrating wire sensors. The cRIO_VW01 works with all standard vibrating wire sensors, and can measure both the vibrating wire output and thermistor for temperature compensation. The cRIO_VW01 utilizes high resolution 24-bit ADCs and digital filtering to yield high resolution and reliable measurements. Whe cRIO_VW01 measurement module can be combined with the cRIO_D_Mux multiplexer control module and the external MUX8_2 multiplexer unit to expand the system to work with multiple vibrating wire sensors. For example, one cRIO_VW01 module with one cRIO_D_Mux module can be used to control eight MUX8_2 multiplexer units for a total of 128 inputs (consisting of any combination of vibrating wire and thermistor inputs). You can expand the channel capacity even further with addition cRIO_D_Mux and MUX8_2 units. The two modules, along with the external multiplexer are shown in Figure 2.
Figure 2. Vibrating Wire Modules for CompactRIO
For more information on these modules, please see the datasheet to the right under Downloads, or contact Pizzi Instruments.