Sweep-frequency excitation technology for multi-channel vibrating-wire sensors

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

With the development of modern electronic reading instrument technology, materials and production processes, vibrating string instrument technology has been continuously improved and has become the trend of the new generation of engineering instruments. It is widely used in building foundations, dams, bridges, roads, cement shells of nuclear power plants, etc., which require the measurement of force, displacement, and microcracks. It can also be used as a key sensor for electronic scales, belt scales, and car scales. In order to accurately measure the changes in stress and strain, in addition to studying the material properties of vibrating string sensors, it is also necessary to solve the excitation and frequency measurement reading technology of vibrating string sensors. To this end, this paper studies the excitation technology and frequency measurement reading technology of vibrating string sensors, and introduces the sweep frequency excitation technology of multi-channel vibrating string 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, it is necessary to find a way to excite the string to vibrate. There are generally two ways to excite the string to vibrate: (1) Continuous excitation method. 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 current passes through the vibrating string, so the insulation problem between the vibrating string and the shell must be considered. If the thermal expansion coefficient of the insulating material and the vibrating string is very different, it is easy to produce a temperature difference, which affects the measurement accuracy. Continuous excitation can easily cause fatigue of the vibrating string. (2) Intermittent excitation method. As shown in Figure 1, a small piece of pure iron is installed on the vibrating string, and an electromagnet is placed next to it. When a pulsating current i is passed through the electromagnet coil, the magnetism of the electromagnet is greatly enhanced, thereby attracting the small iron piece (vibrating string); when no current flows through the coil, the electromagnet releases the vibrating string. In this way, the string vibrates in a cycle. To maintain the continuous vibration of the string, the vibrating string should be continuously excited. That is, a pulse current is passed through the electromagnet at a certain interval so that the electromagnet can attract the vibrating string regularly. Therefore, a pulse current of a certain period must be passed through the electromagnet coil. When the excitation stops, the vibrating string continues to vibrate in a damped manner due to inertia, and an 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 vibration frequency of the vibrating string can be measured from the frequency of the output potential.

This intermittent excitation circuit is complex and usually consists of a tension-relaxation oscillator, an electromagnetic relay, a power supply, and other parts. The electromagnetic relay is large in size, consumes a lot of power, and has poor reliability of mechanical contacts. The oscillation frequency adjustment range of the oscillator is not large, and the adjustment cannot be automatically realized online, which makes it difficult to start the vibrating string [2]. When multiple vibrating string sensors need to be monitored 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, affecting the monitoring accuracy and interfering with the normal operation of other circuits. To solve these problems, a time-division multiplexing method is used for the sweep frequency excitation of multiple vibrating string sensors. That is, multiple sensors share a sweep frequency signal source. When a certain sensor needs to be inspected, the sweep frequency signal source is connected to this sensor by a selection switch; the electromagnetic relay is replaced by a MOS FET relay. In this way, not only the circuit is simplified, but also the electromagnetic interference problem is well solved.

2 Sweep frequency excitation principle and circuit design

2.1 Principle of frequency sweep excitation[2]

The swept frequency excitation technology uses a series of continuously changing frequency signals to sweep the output 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 and thus reliably start to vibrate. After the vibrating string starts to vibrate, the frequency of the induced potential generated in the coil is the natural frequency of the vibrating string. 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 (usually the approximate range of the natural frequency of a known sensor is determined), the excitation signal near this frequency can be used to excite it, so that the vibrating string can quickly start to vibrate.

2.2 Design of frequency sweep excitation circuit

Compared with other series of single-chip microcomputers, the PIC series of single-chip microcomputers has a superior development environment. The streamlined instruction set and single-byte instructions make its execution efficiency high [3]. The chip has a built-in watchdog timer, A/D converter, comparison module, USART asynchronous serial communication module, and EEPROM memory, which simplifies the circuit design and reduces costs. Since the sleep and low-power modes can be set, the power consumption of the circuit is reduced and the reliability of the circuit is improved. The hardware circuit of the swept frequency excitation of the multi-channel vibrating string sensor based on PIC16F873A is shown in Figure 2. The entire hardware circuit is divided into a central controller, a swept frequency excitation circuit, a display module, a parameter input module, an equal-precision frequency measurement module, an RS485 communication module, and other parts.

The natural frequency range of a general single-coil vibrating string sensor is between 400 Hz and 4,500 Hz, and its output frequency changes with the pressure. If the frequency range of the sweep signal is 400 Hz to 4,500 Hz, the sweep time is long, the excitation effect is poor, and the controllability is poor. In order to reduce the sweep time and increase the measurement speed, different sweep frequency bands are set according to the output frequency range of the vibrating string sensor. The method is: the upper limit value fmax and the lower limit value fmin of the sweep signal frequency, as well as the difference Δf between the frequencies of two adjacent sweep signals are input by the parameter input circuit, and these parameters are stored in the on-chip EEPROM of the microcontroller. In this way, the output sweep signal is very targeted and the output excitation frequency is well controllable. These are the outstanding advantages of the sweep excitation technology.

The selection and isolation of multi-channel vibrating string sensors are achieved through metallized field effect transistor (MOSFET) solid-state relays. When a certain sensor is selected, the corresponding MOSFET solid-state relay is turned on, while the MOSFET solid-state relays of other channels are turned off. Although the excitation coils of other sensors are connected to the output end of the constant current excitation circuit through MOSFET, the leakage current when the MOSFET is turned off is extremely small and is in a high-resistance state, so it will not affect the selected path. In addition, the gating circuit and the constant current drive circuit are optically isolated, thereby avoiding the mutual influence between the gating circuit and the constant current drive circuit, and further improving the reliability of the sweep frequency excitation circuit.

According to the characteristics of vibrating string sensors, when the excitation signal is too strong, the vibrating string will produce frequency-doubled vibrations. Due to the difference in frequency-doubled components, the frequencies obtained by the same sensor are different [4]. The constant current weak excitation method is adopted to adjust the magnitude of the excitation current so that it can reliably excite the fundamental frequency of the vibrating string sensor while being far away from the frequency-doubled components. Another advantage of constant current excitation is that the influence of the sensor lead resistance can be ignored.

3 Software design of frequency sweep excitation [3, 5]

The PIC16F873A microcontroller has a capture/compare module, and it is very convenient to generate a sweep frequency signal using the comparison mode. When the sweep frequency excitation signal is to be output, first, the MOSFET solid-state relay corresponding to the selected channel number is turned on, and the MOSFET solid-state relays of other channels are turned off and in a high-impedance state; secondly, the capture/compare module is set in the comparison mode, and the lower limit value fmin of the sweep frequency signal frequency is sent to the 16-bit comparison data register, the data register of timer 1 is cleared and timer 1 is started to start timing counting. At this time, the value in the comparison data register is constantly compared with the value of the timer 1 data register, and a comparison interrupt is generated when the two are equal. The comparison interrupt subroutine mainly completes the following tasks: (1) the level of the sweep frequency signal output port is reversed; (2) the frequency of the output sweep frequency signal is increased by a step Δf; (3) the output signal frequency is compared with the upper limit frequency value f _max of the sweep frequency , and when the sweep frequency value is higher than the upper limit frequency f _max , the sweep frequency output is stopped. The comparison interrupt subroutine block diagram for generating a sweep frequency signal using the comparison mode is shown in Figure 3.

4 Simulation results[6]

In order to verify the effect of the sweep frequency excitation circuit, the VK4100 and VK4150 vibrating string sensors of the American company Kikon were selected to conduct simulated loading tests on the vibrating string sensors on the WE-30 universal material testing machine. The test data are shown in Table 1. The "calculated strain" and "calculated frequency" in the table are values ​​calculated based on the mathematical model of VK4100 and VK4150. Through further analysis of the data in Table 1, it can be seen that the relative error of the frequency measurement of the same vibrating string sensor under different stress states is small, and the relative error of the frequency measurement of different vibrating string sensors is also small, achieving stable sweep frequency and reliable excitation. It can also be seen from the table that the actual measured frequency value is very close to the theoretical value.

The use of the comparison output mode of the single-chip microcomputer to generate a sweep frequency signal eliminates the need for a dedicated sweep frequency signal generator chip, simplifies the circuit design, improves the reliability of the measurement circuit, and breaks through the design method of the traditional instrument measurement system. The application of the constant current weak excitation circuit improves the reliability and stability of the sweep frequency excitation of the vibrating string sensor and avoids the generation of frequency-doubled signals. This sweep frequency excitation method has been successfully applied to a certain ship stress monitoring system, making long-term real-time monitoring of the ship's stress conditions a reality. It not only provides a sufficient basis for the use, maintenance and maintenance of ships, but also provides real and reliable data and high use value for the design, improvement and manufacture of ships. This frequency measurement method can also be extended to other fields, such as nuclear power plant shells, building dams and other occasions that require long-term stress monitoring, and has broad application prospects.