Design of frequency measurement system based on vibrating string sensor

The use of vibrating-wire sensors to measure physical quantities is based on the fact that the vibration frequency of the steel string changes with the tension of the steel wire, and the output is a frequency signal, which has strong anti-interference ability, low cable requirements, and is conducive to transmission and remote measurement. Therefore, a very ideal measurement effect can be obtained.

1 Working principle of vibrating-wire sensor

The vibrating string sensor consists of a positioning support, a coil, a vibrating string and a package. The vibrating string sensor can be equivalent to a uniform string with both ends fixed and tightened, as shown in Figure 1.

The frequency of vibration of a vibrating string can be determined by the following formula:

Where S is the cross-sectional area of ​​the vibrating string, ρv is the volume density of the string (ρv=ρ/s), △l is the length increment of the vibrating string after being subjected to tension, E is the elastic modulus of the vibrating string, and σ is the stress on the vibrating string.

Once the vibrating string sensor is determined, the mass m of the vibrating string, the length L of the working section (i.e., between two fixed points), the cross-sectional area S of the string, the volume density ρv and the elastic modulus E are determined accordingly. Therefore, the length of the string changes due to the action of the physical quantity to be measured, and the change in the length of the string can change the natural vibration frequency of the string. Since there is a definite relationship between the increment △l of the string length and the natural frequency of the longest standing wave wavelength of the vibrating string, as long as the vibration frequency of the string can be measured, the physical quantity to be measured can be measured.

2 Frequency measurement System design

2.1 Basic Principles

When the vibrating string sensor is working, the electromagnetic coil is driven by the excitation circuit. When the frequency of the signal is close to the natural frequency of the vibrating string, the vibrating string quickly reaches a resonant state, and the induced electromotive force generated by the vibration

The potential is filtered , amplified, and shaped by the detection circuit and sent to the microcontroller. The microcontroller feeds back the received signal to the excitation circuit to drive the electromagnetic coil through software. Through feedback, the string can vibrate at the local oscillation frequency driven by the changing magnetic field generated by the electromagnetic coil. When the excitation signal is removed, the string still vibrates due to inertia. The microcontroller can measure the vibration frequency of the string by measuring the pulse period of the induced electromotive force, and finally display the measured data. The frequency measurement principle block diagram is shown in Figure 2.

2.2 System Hardware Circuit Design

According to the above basic principles and ideas, the overall circuit of the designed frequency measurement system is shown in Figure 3. It is mainly composed of several parts, such as the excitation circuit, the detection circuit, the single-chip control circuit and the display circuit. The working process is that the single-chip generates an excitation signal of a certain frequency, which is amplified to excite the vibrating string, and the induced electromotive force generated in the pickup coil is amplified in several stages and sent to the single-chip for processing, and finally sent to the display circuit for display.

2.2.1 Excitation circuit

The excitation circuit uses sweep frequency excitation technology, which is to use a frequency-adjustable signal to excite the excitation coil of the vibrating string sensor. When the frequency of the signal is close to the natural frequency of the vibrating string, the vibrating string can quickly reach a resonant state. Since the frequency of the excitation signal is easy to control with software, as long as the approximate range of the natural frequency of the vibrating string is known (generally speaking, the approximate range of the natural frequency of a known sensor is determined), the vibrating string can be quickly vibrated by using an excitation signal near this frequency. [page]

The I/O port of the microcomputer system generates an excitation signal (taking into account a certain margin) at a certain frequency (this frequency can be the initial value of the sensor's natural frequency or the last measured value). 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 string to vibrate.

Two 9013 transistors are selected as driving tubes. The electromagnetic coil has a very small resistance, and the current flowing through the coil can reach 200-400 mA. The saturation conduction current of the 9013 transistor is 500 mA. Through experiments, it is found that if only one transistor is used to drive the coil, the transistor will heat up very badly. To solve this problem, another transistor is connected in parallel to reduce the working current of the transistor and reduce the heat generation. The diode requires the use of a fast-conducting diode, which is used to absorb the spike pulse generated by the transistor when it is turned on and off. If this spike pulse is not eliminated, it will cause serious interference to the induction circuit ( analog circuit part).

It is relatively easy to generate a variable frequency signal in a microcomputer system. For example, the frequency sweeping program is implemented by using the MSC-51 single-chip microcomputer, making full use of the internal resource timer/counter function of the microcomputer system, and outputting a variable frequency signal from the I/O port. The timer counter works in the timing mode, and the timing time is determined by a certain frequency of the frequency sweep. The state of the I/O port is changed in the timer interrupt program, so that the I/O port obtains a pulse signal. Assume that the upper limit of the frequency sweep is fmax, the lower limit is fmin, the number of pulses of a certain frequency between fmax and fmin output by the I/O port is n, and the increment of two adjacent frequencies of the frequency sweep is △f. The flowchart of the frequency sweeping program determined by these four parameters is shown in Figure 4.

2.2.2 Detection Circuit

The frequency detection circuit of the induced potential in the pickup coil consists of two parts: one is the filter circuit, which uses a two-stage low-pass filter method; the other is the zero-crossing comparison circuit, which uses the zero-crossing comparison method to obtain the frequency signal from the output end 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 comparison circuit. Since the induced potential is a periodic signal, the frequency of the signal is also the natural frequency of the vibrating string. The output fout of LM393 is a periodic square wave, and the frequency of the square wave is the frequency to be measured.

Since the signal is compared with the "zero potential", a higher sensitivity can be obtained. As long as the vibrating string is excited to vibrate, the frequency of the weak electromotive force in the induction coil can be easily picked up. In order to further make the input and output characteristics of the comparator steeper during conversion, the comparison accuracy can be improved.

2.2.3 Frequency measurement circuit

Send the output fout of the detection circuit to INT0 of AT89CS51, and use the special function of the GATE bit of the control register TMOD in the working mode of the on-chip timer/counter. Under normal circumstances, GATE = 0. The operation control bit of GATE is only determined by the state of the TRx (x = 0.1) bit (TRx = 0 is closed, TRx = 1 is opened). GATE = 1 is only used when the count is started and the external input INTx is used for control. When GATE = 1 and TRx = 1, the counter Tx is allowed to count only when the INTx pin inputs a high level. This function of GATE can be used to conveniently measure the pulse width.

2.3 System Software Design

According to the design of the above circuit, the basic idea of ​​programming the system software is: first initialize the system, then the excitation circuit excites the sensor , and the detection circuit detects, amplifies, shapes, processes, and finally displays the signal. The main program flow chart is shown in Figure 5.

3 Conclusion

This frequency measurement system has the characteristics of correct design ideas, concise and ingenious programming, and comprehensive functional use. It greatly shortens the on-site measurement and calculation time, reduces labor intensity, improves the measurement and calculation accuracy, and at the same time brings great convenience to the later processing and storage of measurement results.