How to design the principle and circuit of frequency-swept excitation?

1.1 Principle of Frequency Sweep Vibration

Sweep frequency excitation technology is to use a series of continuously changing frequency signals to sweep the output 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 the resonant state, thereby reliably vibrating. After the vibrating wire starts to vibrate, the frequency of the induced potential generated in the coil is the natural frequency of the vibrating wire. Because 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 (usually the approximate range of the natural frequency of a known sensor is determined), the excitation signal around this frequency can be used to excite It makes the vibrating string vibrate quickly.

1.2 Design of frequency-swept excitation circuit

Compared with other series MCUs, the development environment of PIC series MCUs is superior, and the streamlined instruction set and single-byte instructions make their execution efficiency high [3]. The chip comes with a watchdog timer, A / D converter, comparison module, USART asynchronous serial communication module, and EEPROM memory, which simplifies the circuit design and reduces costs. Since sleep and low power consumption modes can be set, the power consumption of the circuit is reduced and the reliability of the circuit is improved. The entire hardware circuit is divided into a central controller, a frequency sweeping excitation circuit, a display module, a parameter input module, an equal precision frequency measurement module, and an RS485 communication module.

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

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

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

2 Software design of frequency-swept excitation

The single-chip microcomputer PIC16F873A has a capture / compare module, and it is very convenient to generate the frequency sweep signal in the compare mode. When the swept frequency excitation signal is to be output, first turn on the MOSFET solid-state relay corresponding to the selected channel number, and turn off the MOSFET solid-state relay of the other channel in a high-impedance state; second, set the capture / compare module in comparison mode Send the lower limit fmin of the frequency of the sweep signal to the 16-bit comparison data register, clear the data register of timer 1 and start timer 1 to start timing counting. At this time, the value in the compare data register is constantly compared with the value in the timer 1 data register, and a compare interrupt is generated when the two are equal. In the comparison interrupt subroutine, the following tasks are mainly completed: (1) The level of the frequency sweep signal output port is reversed; (2) The frequency of the output frequency sweep signal is increased by one step Δf; (3) The frequency of the output signal and the frequency sweep Compare the upper limit frequency value fmax. When the frequency value of the sweep frequency is higher than the upper limit frequency fmax, stop the sweep output.

3 Simulation results

In order to verify the effect of the frequency-swept excitation circuit, the VK4100 and VK4150 vibrating wire sensors of Kecon Corporation of the United States were used to simulate the loading test of the vibrating wire sensors on the WE-30 universal material testing machine. The test data is shown in Table 1. In the table, "calculated strain" and "calculated frequency" are calculated according to the mathematical models of VK4100 and VK4150. Through further analysis of the data in Table 1, it can be seen that the sweep frequency excitation method not only reduces the relative error of frequency measurement of the same vibrating wire sensor under different stress states, but also the relative error of frequency measurement of different vibrating wire sensors. It is very small, achieving stable frequency sweep and reliable excitation. It can also be seen from the table that the actual measured frequency value is very close to the theoretical value.

Using the comparison output mode of the single chip microcomputer to generate the frequency sweep signal, the special frequency sweep signal generator chip is omitted, the circuit design is simplified, the reliability of the measurement circuit is improved, and the design method of the traditional instrument measurement system is broken. The application of the constant current weak excitation circuit improves the reliability and stability of the frequency-swept excitation of the vibrating wire sensor, and avoids the generation of frequency-doubled signals. This swept frequency excitation method has been successfully applied to a ship's stress monitoring system, making long-term real-time monitoring of the force on the ship a reality. Not only provides a sufficient basis for the use, maintenance and maintenance of the ship, but also provides real and reliable data and high use value for the design, improvement and manufacture of the ship. 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.

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