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It is a well known fact that the natural frequency of a tensioned string increases with tension. Mathematically, the relationship between the resonant frequency of a string and the tension applied on the string is given by:
Where,
F = Fundamental resonant frequency of string (Hertz)
L = String length (meters)
T = String tension (newtons)
μ = Unit mass of string (kilograms per meter).
This implies that a string can be used as a force sensor. In this type of sensor design, an electronic oscillator circuit, is used to keep a wire vibrating at its natural frequency when under tension. The principle is similar to that of a guitar string.
The vibrating wire is located in a diaphragm. As the pressure changes on the diaphragm so does the tension on the wire, which affects the frequency that the wire vibrates or resonates at. These frequency changes are a direct consequence of pressure changes and as such are detected and shown as pressure.
The frequency can be sensed as digital pulses from an electromagnetic pickup or sensing coil. An electronic transmitter would then convert this into an electrical signal suitable for transmission. This type of pressure measurement can be used for differential, absolute or gauge installations. Below is a simplified diagram of this type of sensor arrangement:
Temperature variations within this sensor require temperature compensation . This problem limits the sensitivity of the device. The output generated is non-linear which can cause continuous control problems.Improvement in technology has led to the production of more linear sensors using a resonating wire. The Foxboro company pioneered this concept of a vibrating wire sensor in an early resonant wire design of pressure transmitters. The Yokogawa Corporation of Japan still produces a line of sensors using this technology. The Yokogawa DPharp EJA pressure transmitter series uses the resonating wire technology. Learn more about this pressure transmitter here: EJA series of differential pressure transmitters
