What are the excitation technology and frequency reading technology of vibrating wire sensors?

The vibrating wire sensor is one of the more advanced sensors in stress and strain measurement. The output of the vibrating wire sensor is a frequency signal, and there is no need for A / D and D / A conversion during signal processing. Therefore, the anti-interference ability is strong, the signal transmission distance is long, and the transmission cable requirements are low. In addition, the vibrating wire sensor also has the characteristics of simple structure, high accuracy, long life, etc., so it has always been the concern of the engineering community. In engineering applications, the vibrating wire sensor can be embedded or welded on the test piece, there is basically no problem of adhesive aging and shedding, and it has good stability and repeatability. For small measured force changes can produce large frequency changes, with high sensitivity.

With the development of modern electronic reading instrument technology, materials and production processes, vibrating wire instrument technology has also been continuously improved, becoming the trend of a new generation of engineering instruments, which is widely used in building foundations, dams, bridges, highways, nuclear power plants Cement shells, etc., can be used as key sensors for electronic scales, belt scales, car scales, etc. in the measurement of force, displacement, and micro cracks. In order to accurately measure the changes of stress and strain, in addition to studying the material characteristics of the vibrating wire sensor, the excitation and frequency reading technology of the vibrating wire sensor must also be solved. To this end, this paper studies the excitation technology and frequency reading technology of vibrating wire sensors, and introduces the frequency sweeping excitation technology of multi-path vibrating wire sensors based on the comparison output mode in the PIC16F873 microcontroller.

1 Traditional intermittent excitation method

In order to measure the natural frequency of the vibrating string, one must try to excite the string vibration. There are generally two ways to excite string vibration: (1) Continuous excitation mode. This method is divided into the current method and the electromagnetic method. In the current method, the vibrating string is part of the oscillator, and the current passes through the vibrating string. Therefore, the insulation between the vibrating string and the case must be considered. If the difference in thermal expansion coefficient between the insulating material and the vibrating wire is large, a temperature difference is likely to occur, which affects the accuracy of the measurement. Continuous vibration will easily fatigue the vibrating wire. (2) Intermittent excitation mode. As shown in Figure 1, a small pure iron piece is installed on the vibrating wire, and an electromagnet is placed next to it. When the electromagnet coil is fed with a pulsating current i, the magnetism of the electromagnet is greatly enhanced, thereby attracting the small iron piece (vibrating wire); When no current flows in the coil, the electromagnet releases the vibrating wire. This cycle excites the vibration of the string. To maintain continuous vibration of the string, the vibrating string should be continuously excited. That is, the electromagnet passes a pulse current at regular intervals to make the electromagnet attract the vibrating wire at regular intervals. Therefore, the coil current of the electromagnet must be pulsed with a certain period. When the excitation is stopped, due to the effect of inertia, the vibrating string continues to perform damped vibration, and the induced electromotive force is generated in the electromagnet coil. The frequency of the induced electromotive force is equal to the damped vibration frequency of the string. In this way, the natural frequency of the vibrating wire can be measured from the frequency of the output potential.

This intermittent excitation circuit is complex and usually consists of a relaxation oscillator, an electromagnetic relay, and a power supply. Electromagnetic relays are large in size, large in power consumption, and have poor working reliability of mechanical contacts. The oscillation frequency adjustment range of the oscillator is not large, and the adjustment cannot be realized automatically online, which makes the vibrating string starting sometimes difficult . When monitoring multiple vibrating wire sensors simultaneously, the circuit becomes more complicated. What is more serious is that the excitation coil driven by the relay is an inductive load, which generates large electromagnetic interference during intermittent excitation, which affects the monitoring accuracy and interferes with the normal operation of other circuits. To solve these problems, the time-division multiplexing method is adopted for the frequency-swept excitation of the multi-channel vibrating wire sensor. That is, multiple sensors share a frequency sweep signal source. When a certain sensor is to be inspected, the frequency switch signal source is connected to this sensor by a selector switch; the MOS FET relay is used to replace the electromagnetic relay. This not only simplifies the circuit, but also solves the electromagnetic interference problem.

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