The measurement of physical quantities using a vibrating wire sensor is based on the fact that the vibration frequency of the steel wire changes with the tension of the wire, and the output is a frequency signal. It has the characteristics of strong anti-interference ability, low cable requirements, and is beneficial for transmission and remote measurement. Therefore, it can achieve very ideal measurement results. 1. Working Principle of Vibrating Wire Sensors A vibrating wire sensor consists of a positioning support, a coil, a vibrating wire, and a package. A vibrating wire sensor can be equivalently represented as a uniformly taut string fixed at both ends, as shown in Figure 1. [IMG=Working Principle of Vibrating Wire Sensor]/uploadpic/THESIS/2007/12/2007121510440878967F.jpg[/IMG] The vibration frequency of the vibrating wire can be determined by the following formula: [IMG=Vibration Frequency Formula of Vibrating Wire]/uploadpic/THESIS/2007/12/20071215104414147859.jpg[/IMG] Where S is the cross-sectional area of ​​the vibrating wire, ρv is the volume density of the wire (ρv=ρ/s), bigtriangleup; is the length increment of the vibrating wire after being subjected to tension, E is the elastic modulus of the vibrating wire, and σ is the stress on the vibrating wire. Once the vibrating wire sensor is determined, its vibrating wire mass m, working section length L (between two fixed points), cross-sectional area S, bulk density ρv, and elastic modulus E are also determined. Therefore, the effect of the physical quantity to be measured causes a change in the string length, and this change in string length alters the string's natural vibration frequency. Since the increment of the string length has a definite relationship with the natural frequency of the longest standing wave wavelength of the vibrating wire, the physical quantity to be measured can be determined as long as the vibration frequency of the string can be measured. 2. Frequency Measurement System Design 2.1 Basic Principles When the vibrating wire sensor is working, the excitation circuit drives the electromagnetic coil. When the frequency of the signal is close to the natural frequency of the vibrating wire, the vibrating wire quickly reaches a resonant state. The induced electromotive force generated by the vibration is filtered, amplified, and shaped by the detection circuit and sent to the microcontroller. The microcontroller, based on the received signal, feeds back to the excitation circuit to drive the electromagnetic coil via software. Through this feedback, the string vibrates at its local frequency under the drive of the changing magnetic field generated by the electromagnetic coil. When the excitation signal is removed, the string continues to vibrate due to inertia. The microcontroller measures the vibration frequency of the string by measuring the period of the induced electromotive force pulse, and finally displays the measured data. The block diagram of the frequency measurement principle is shown in Figure 2. [IMG=Frequency Measurement Principle Block Diagram]/uploadpic/THESIS/2007/12/2007121510442015401S.jpg[/IMG] 2.2 System Hardware Circuit Design Based on the above basic principles and ideas, the overall circuit of the designed frequency measurement system is shown in Figure 3. It mainly consists of an excitation circuit, a detection circuit, a microcontroller control circuit, and a display circuit. The working process involves the microcontroller generating an excitation signal of a certain frequency, which is amplified to excite the vibrating string. The induced electromotive force generated in the pickup coil is amplified several times and then sent to the microcontroller for processing, and finally displayed by the display circuit. [IMG=Overall Circuit of Frequency Measurement System]/uploadpic/THESIS/2007/12/2007121510442636336C.jpg[/IMG] 2.2.1 Excitation Circuit The excitation circuit adopts frequency sweep excitation technology, which uses a signal with an adjustable frequency to excite the excitation coil of the vibrating wire sensor. When the frequency of the signal is close to the natural frequency of the vibrating wire, the vibrating wire can quickly reach a resonant state. Since the frequency of the excitation signal is easily controlled by software, as long as the approximate range of the natural frequency of the vibrating wire is known (generally, the approximate range of the natural frequency of a known sensor is determined), an excitation signal near this frequency can be used to excite it, and the vibrating wire can quickly start oscillating. The microcomputer system I/O port generates an excitation signal at a certain frequency (this frequency can be the initial value of the sensor's natural frequency or the previous measurement value) (considering a certain margin). After being amplified by the basic power amplifier circuit, the excitation current flows through the excitation coil, and the alternating magnetic field generated by the excitation current excites the vibrating wire to vibrate. Two 9013 transistors were selected as the driver transistors. The electromagnetic coil has a very low resistance, allowing a current flowing through it of 200-400mA. However, the saturation current of the 9013 transistor is 500mA. Experiments showed that using only one transistor to drive the coil resulted in significant heat generation. To address this, another transistor was connected in parallel to reduce its operating current and heat generation. A fast-conduction diode was selected to absorb the spike pulses generated during the transistor's turn-on and turn-off. If these spike pulses are not eliminated, they will cause severe interference to the induction circuit (analog circuit section). Generating a variable-frequency signal using a microcomputer system is relatively easy. For example, an MSC-51 microcontroller can be used to implement a frequency sweep program, fully utilizing the internal timer/counter function of the microcomputer system to output a variable-frequency signal from the I/O port. The timer/counter operates in timing mode, with the timing duration determined by a specific frequency of the sweep. The I/O port state is changed in the timer interrupt program, thus providing a pulse signal to the I/O port. Let the upper limit of the sweep frequency be fmax, the lower limit be fmin, the number of pulses at a certain frequency between fmax and fmin output from the I/O port be n, and the increment between two adjacent frequencies of the sweep frequency be bigtriangleup;f. The flowchart of the sweep frequency program determined by these four parameters is shown in Figure 4. [IMG=Sweep Frequency Program Flowchart]/uploadpic/THESIS/2007/12/20071215104431339907.jpg[/IMG] 2.2.2 Detection Circuit The frequency detection circuit of the induced electromotive force in the pickup coil consists of two parts: one is a filter circuit, which adopts a two-stage low-pass filter method; the other is a zero-crossing comparator circuit, which adopts the zero-crossing comparison method to obtain the frequency signal from the output of the comparator. Two LM324s form a two-stage active low-pass filter circuit; C1, R3, and G2, R4 form the resistor-capacitor network of the first and second stage active filter circuits, respectively; LM393 is used as a comparator to form a zero-crossing comparator circuit. Since the induced electromotive force is a periodic signal, the frequency of the signal is the natural frequency of the vibrating string. The output fout of the LM393 is a periodic square wave, and the frequency of the square wave is the frequency to be measured. Because the signal is compared with "zero potential", high sensitivity can be obtained. As long as the vibrating string is excited and vibrates, the frequency of the weak electromotive force in the induction coil can be easily picked up. To further make the input-output characteristics of the comparator steeper during the conversion, the comparison accuracy is improved. 2.2.3 Frequency measurement circuit sends the output fout of the detection circuit to INT0 of AT89CS51. The special function of the GATE bit of the on-chip timer/counter control register TMOD is used. Under normal circumstances, GATE=0. The operation control bit of GATE is determined only by the state of the TRx bit (x=0.1) (TRx=0 is off, TRx=1 is on). GATE is only set to 1 when the start of counting is controlled by the external input INTx. When GATE=1, TRx=1. The counter Tx is only allowed to count when the INTx pin is high. This function of GATE can be used to conveniently measure the pulse width. 2.3 System Software Design Based on the circuit design above, the basic idea for system software programming is as follows: first, the system is initialized; then, the excitation circuit excites the sensor; the detection circuit detects, amplifies, shapes, processes, and finally displays the signal. The main program flowchart is shown in Figure 5. [IMG=Main Program Flowchart]/uploadpic/THESIS/2007/12/2007121510443814896V.jpg[/IMG] 3 Conclusion This frequency measurement system features a correct design concept, concise and ingenious programming, and comprehensive functionality. It greatly shortens the on-site measurement and calculation time, reduces labor intensity, improves the accuracy of measurement calculations, and brings great convenience to the post-processing and storage of measurement results.