2020 IEEE 3rd International Conference on Electronics Technology

Driving Circuit Design of Vibrating Wire Sensor Based on Frequency Sweep Mode

Yong Sang, Zengren Zhang

School of Mechanical Engineering
Dalian University of Technology
Dalian, China
e-mail: lyzhangzengren@126.com

Abstract—In order to meet the needs of monitoring the data acquisition of vibrating wire sensor, YAN et al. have
pressure and tension of the construction site, a high-precision proposed a practical method of equal precision frequency
low-cost driving circuit of vibrating wire sensor is designed. In measurement. The system hardware includes oscillation
order to simplify the driving circuit, a single power supply is circuit, measurement frequency circuit, and temperature
used to solve the problem of negative power supply that we do compensation circuit. The system uses AT89C52 to realize
not want to face. The STM32 microcontroller is used as the the principle of equal precision frequency measurement,
main control chip to realize the overall control of the system. application circuit and controllable excitation oscillation [7].
The control mode of frequency sweep stimulation is adopted to In order to improve the accuracy and production efficiency
realize the fast excitation of the vibrating wire sensor, and the of the vibrating wire sensor, REW et al. have developed a
equal precision frequency measurement method is used to read tension automatic adjusting device. The nonlinearity of the
the resonant frequency of the vibrating wire sensor accurately. sensor was reduced from 0.398% to 0.056% [8]. Harrison et
al. have introduced an accurate mathematical model of the
Keywords-vibrating wire sensor; driving circuit; STM32 vibrating wire sensor, which is suitable for the situation that
the original frequency changes with the temperature drift.
I. INTRODUCTION The intelligent instrument based on the model can display
Vibrating wire sensor is widely used in construction, the measured parameters in digital form, which is helpful to
water conservancy, railway transportation, metallurgy, coal, process the measured data quickly and accurately [9].
and other industries. It can monitor the pressure and tension According to the working principle of the self-excited
of the construction site of the dam, tunnel, mine, and other vibrating wire sensor, Yu Feng et al. have given the
civil engineering. Coal mine safety, dam bridge, highway experimental results of the designed excitation circuit. The
tunnel pressure monitoring is particularly important. model is implemented by an 8-bit microcontroller while
Vibrating wire sensor has the advantages of high resolution, maintaining the inherent characteristics of the vibrating wire
high reliability, good stability and strong anti-interference sensor [10]. Diogo et al. used a new type of vibrating wire
ability [1]. The vibrating wire sensor’s working principle is sensor to measure the viscosity, which was obtained by using
based on the frequency change when the tension changes [2]. vibrating wire sensor in forced oscillation or steady-state
In the past decades, many experts have carried out research working mode [11]. Bordes et al. have discussed the
on sensor applications and improvement. For example, durability and stability of vibrating string (sound) monitoring
Bourquin et al. have designed a new type of vibrating wire instrument, analyzed the cause of instrument failure, and
sensor to measure the relative displacement in the discussed the influencing factors and mechanism of zero drift
mechanical system, and used the wire vibrating between the [12]. In order to ensure the monitoring of data acquisition
two anchor points of the system as the relative displacement accuracy of vibrating wire sensor, AA et al. proposed a
sensor [3]. Pádua et al. have proposed a design method of the multi-period frequency measurement method to realize the
equivalent circuit of vibrating wire sensor, which can be used frequency measurement of vibrating wire sensor and
in many instruments to measure the density and viscosity of designed a specific hardware circuit [13]. According to the
the liquid in high pressure and wide temperature range [4]. structural characteristics of the vibrating wire sensor, Hong-
Based on the experiment of the vibrating wire sensor and jie et al. have designed a detection system with MSP430-
vibrating wire frequency meter, You Gao has solved the F449 as the core. The hardware circuit of the system mainly
interference problem in the measurement process of steel includes the excitation circuit and signal conditioning circuit.
vibrating wire sensors by combining hardware and software The 16-bit timer is built-in MSP430 single-chip micro-
[5]. Audonnet et al. have reported the experimental results of computer, which has the function of acquisition/comparison,
measuring the density and viscosity of n-pentane with a and can complete the excitation signal output and frequency
vibrating wire sensor at 303-383k temperature and 100MPa signal measurement [14]. Janeiro et al. introduced the
pressure at the same time. The response of the wire is mainly research progress of a viscosity measurement system based
related to the density of the fluid through the buoyancy on vibrating string cells and proposed a new equivalent
acting on the sink, while the viscosity of the fluid affects the impedance circuit as one of the possible circuits to simulate
damping of the oscillation [6]. In order to meet the the behavior of ionic liquid batteries in the future [15].
requirements of large-scale structural safety monitoring for ZHAO et al. have introduced a mature pressure measurement
scheme of the vibrating wire pressure sensor. By optimizing

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frequency scanning, reasonable pulse counting, low noise Start
signal processing circuit design and effective temperature
compensation, high-pressure measurement accuracy was
obtained [16]. Santos et al. have introduced an embedded Initial
measurement system for viscosity measurement with a frequency ƒ0
vibrating wire sensor. This system takes DSP as the core to
control the hardware of the system and realize the estimation The process of
of viscosity [17]. stimulation
In this paper, a high-precision low-cost driving circuit of
vibrating wire sensor is designed. The amplifiers in the
driving circuit use the way of single power supply, which The process of
reading ƒ=ƒ+10
simplify the circuit and reduce the cost. The STM32
microcontroller is used as the main control chip to realize the
control of the driving circuit and to read the signal of the
vibrating wire sensor. Whether N
it completes the
II. THE STIMULATION OF THE VIBRATING WIRE SENSOR stimulation
The working principle of the vibrating wire sensor is
Y
shown in Fig. 1. It is made of tensioned metal string as a
sensitive element, commonly used elastic spring steel, End
Markov stainless steel and so on. When the length of the
metal string is determined, its resonant frequency is also
Figure 2. The process of frequency sweep stimulation
determined. The magnitude of the resonant frequency is
directly related to the external tension and compression stress, The high-voltage single-pulse stimulation means that the
which is used to reflect the change of the external stress. The instantaneous high voltage pulse which is short (10ms) but
measurement of resonant frequency needs to go through two
large amplitude (about 100V) is used to strike the string. The
processes: the stimulation process and the reading process.
high voltage energy will trigger vibration of the steel-string.
The resonant frequency is determined by the following
The pickup coil will produce a small AC signal which
formula:
attenuates gradually. The small AC signal characterizes the
1 T resonant frequency of the sensor. The high-voltage single-
fo = (1)
2L  pulse stimulation has the characteristics of fast response, but
where: L is the length of the vibrating string; T is the the high-voltage pulse will have a negative impact on the life
measured mechanical stress; ρ is the density of the vibrating of the sensor, which is not conducive to long-term use.
string; f0 is the resonant frequency of the sensor. The low-voltage frequency sweep stimulation means that
the low voltage excitation signals of different frequencies are
III. THE MEANS OF STIMULATION input in turn until the steel-string vibrates. The process of
If the vibrating wire sensor wants to output a stable frequency sweep stimulation is shown in Fig. 2. When the
frequency signal, it must first complete the process of frequency of the external input low voltage signal (amplitude
stimulation to make the steel-string vibrate. There are two about 10V) is equal to or close to the resonant frequency of
ways of stimulation, high-voltage single-pulse stimulation the string, due to resonance, the energy will be rapidly
and low-voltage frequency sweep stimulation, both of which accumulated on the string. The process of stimulation will be
have their own characteristics. realized with about 8 excitation pulses. After completing
excitation, the steel-string can still maintain the state of
F
resonance and vibrate for a very small period of time. The
frequency of the output signal is its resonant
frequency. Based on the frequency sweep, it is necessary to
scan the resonance frequency range of the vibrating wire
sensor step by step until the sensor can complete the
Pickup coil excitation. The resonance frequency range of the vibrating
wire sensor is about 400Hz~4000Hz. Because the sensor can
Steel string
complete the stimulation for the excitation signal in a certain
frequency range, it only needs to sweep the frequency step
Drive coil by step with a specific frequency interval. Completing the
stimulation, the response signal is showed in Fig. 3.

Figure 1. The working principle of the vibrating wire sensor

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 6
converted into the square wave signal whose amplitude is
VCC, which realizes the power amplification.
4

2
Voltage (V)

-2

-4

-6
1 2 3 4
Time (s)

Figure 3. Response signal of the sensor.

IV. THE HARDWARE DRIVING CIRCUIT OF STIMULATION

For the generation of excitation, we choose the way of
low-voltage frequency sweep stimulation. The core
Figure 4. The driving circuit of stimulation.
component of the control system selects STM32F103ZET6
microcontroller of the STM32 series. Its frequency is usually The NPN transistor that we selected s8050 amplifies the
72MHz, and its frequency can be configured to 144MHz, by current of the small-signal from the control terminal. The
configuring PLL, which can easily generate the square wave NPN transistor is used to control the later PMOS. The turn-
signal needed by the vibrating wire sensor to complete the on and cut-off time of s8050 is just several hundred
stimulation. The microcontroller integrated eight timers, nanoseconds, which has almost no effect on the waveform of
including two basic timers, four general timers and two square wave needed for excitation, so the s8050 can fully
advanced timers. A general timer of the microcontroller is meet the requirements of the system.
used to generate a high-frequency square wave signal as the
input signal of the driving circuit. V. THE GENERATION OF READING
Even if the STM32F103ZET6 MCU is configured with
push-pull output mode, its current driving ability is After the vibrating wire sensor is excited, the amplitude
obviously insufficient, and its working voltage is only 3.3V, of the output signal of the sensor is only about 2mv, and the
which cannot provide enough energy for the sensor to duration is about 2s, and the signal might be even weaker.
complete the excitation. If you still use I/O to drive directly, Reading out the frequency of its weak AC signal needs to go
the MCU will face the risk of burnout, so we need to through a series of processing. It roughly needs to amplify
consider the design of the driving circuit. the signal through the primary acquisition circuit, the
Three kinds of driving components are often used for bandpass filter circuit and the last amplification circuit. The
power amplification: BJT, junction FETs, and insulated gate circuit is used to amplify the weak AC signal whose
FETs. Considering the demand for large current and amplitude is about 2mv by about 2000 times. Finally, the AC
convenient control, the PMOS is selected as the core device signal is shaped into a square wave signal by the rectifier
of the driving circuit of the excitation. circuit, which can be further processed by the
The driving circuit of the excitation is shown in Fig. 4, microcontroller.
and the PMOS is used as the main driving component. The In order to ensure the accuracy of the sensor signal, the
gate of the PMOS is pulled up by the pull-up resistor R4 to high-precision operational amplifier OP07 is selected. The
the power supply VCC, which is turned off by default. The OP07 has the characteristics of low input offset voltage, high
NPN transistor of the circuit is pulled down to the ground open-loop gain, low noise, high supply voltage range and so
through the pull-down resistor R2, which is also turned off on. These characteristics make it especially suitable for
by default. When the PA6 input high voltage, the NPN measuring equipment with high gain and amplifying weak
transistor and the PMOS turn on in turn, and the excitation signals of sensors.
signal VCC is input to the sensor coil. When the PA6 input VI. THE HARDWARE DRIVING CIRCUIT OF THE READING
low voltage, the NPN transistor and the PMOS turn off in PROCESS
turn, and the sensor coil is pulled down to the ground. Under
the driving circuit of stimulation, the 3.3V square wave
signal outputted by the I/O port of the microcontroller is

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 Figure 5. The driving circuit of the reading process.

Figure 6. The simulation results with Multisim.

The hardware driver circuit of the reading process is R8

shown in Fig. 5, and the reading process adopts three-stage Au=1+ (4)
R7+Xc
amplification processing. All the operational amplifiers used
Due to the influence of capacitor C4, the magnification
in the reading circuit are powered by a single power supply,
of the DC bias signal tends to be 1, which only amplifies the
which gets rid of the shortcomings of negative power supply,
AC signal of the sensor and has no effect on the DC bias
such as lacking driving ability and not easy to generate. The
signal.
DC bias is produced by the voltage dividing resistors, and
The bandpass filter circuit is composed of operational
the output signal of the sensor is adjusted to the DC bias of
amplifier U2, which is used to filter out the interference of
6V to prevent the sensor signal from distortion.
higher harmonics, noise, power ripple and other factors, so as
The primary acquisition circuit is composed of
to ensure the accuracy of the signal. The pass band of the
operational amplifier U1, which is used to capture the small
bandpass filter circuit is designed as the resonant frequency
AC signal of the sensor. The output AC signal of the sensor
range of the vibrating wire sensor 400-3500Hz. In order to
is controlled by the cooperation of R7, R8 and C4 to control
ensure that the circuit has good filtering characteristics and
the magnification.
the circuit is as simple as possible, the second-order low-pass
R8 filter circuit and the first-order high-pass filter circuit are
Au=1+ (2)
R7+Xc selected. The simple first-order high-pass filter circuit is
For AC signals, the capacitive reactance through large composed of C7, R9 and R10. The second-order low-pass
capacitors will become quite small and can be ignored, and filter circuit is composed of R11, R12, C9 and C10, and the
the magnification is only controlled by the ratio of R7 to R8. connection mode of the voltage-controlled voltage source is
R8 adopted to improve the characteristics of the transition band.
Au=1+ (3) Through the simulation analysis of the bandpass filter
R7 circuit by Multisim software, it can be found that the
For the DC bias signal, the capacitive reactance through characteristic of the transition band has been obviously
the capacitor C4 will tend to infinity, and the amplification improved. In Fig. 6, there is an obvious inflection point,
of the DC bias signal will be suppressed by the capacitor C4.

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which is the purpose of the second-order low-pass filter 51975082 and the support by the Natural Science Foundation
circuit using the connection mode of the voltage-controlled of Liaoning Province through Grant No. 2019-ZD-0141.
voltage source. Due to the positive feedback in the bandpass
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