Journal of Measurement Science and Instrumentation
Vol. 9 No. 2，Jun. 2018
Design and analysis of singal conditioning circuit
for vibration sensor
YUAN Xiao-kang1，JIAO Xin-quan1, CHU Cheng-qun1，YANG Wen-hao2
\".Science and Technology on Electronic Test M Measurement Laboratory，North University of China，Taiyuan 030051，China %
2. The No. 58 Research Institute of China Electronics Technology Group Corporation，Wuxi 214035，China#
Abstract： A piezoelectric sensor charge/voltage conversion circuit is designed based on the principle that piezoelectric sensor can convert the vibration or shock acceleration into the charge proportionally. Effect of temperature characteristic of feedback capacitor on the switching circuit output is analyzed based on the acquisition and measurement system in this paper. The characteristics of different filters are analyzed，and bandwidth. Experiments signal accurately.
Key words： vibration； piezoelectric sensor； singal conditioning circuit； feedback capacitor CLD number： TP212
Article ID： 1674-8042(2018)02-0174-06
Document code： A
doi: 10. 3969/. issn. 1674-8042. 2018. 02. 012
show
that the circuit
the can
corresponding effectively
filter
circuit
is among
configured the
according to
filter out noises
vibration signal
and
0 Introduction
In the modern industrial and automated production process，the shock and vibration signals of equipment are
usually
obtained
by
using
a
piezoelectric
accelerometer， and the charge signals produced by the piezoelectric sensor need to be amplified and processed by a charge amplifier. However， the traditional design of charge amplifier circuit is very complicated， and the price of the machine is very high， so the cost performance is not ideal， which seriously affects the widespread use of piezoelectric accelerometer. It is very necessary to develop a cos-effective and practical charge amplifier. By using the integrated operational amplifier chip to replace a large number of discrete components to optimize the design of the charge amplifier could improve the integration which makes the whole circuit has of the circuit，advantages such
as
a
small
size， low
power at the
consumption， parasitic factors and anti-jamming performance； at the same time， aiming
problem that the noise signal is easy to be mixed inthe vibration signal measurement，the corresponding filter
circuit
is
designed
by
analyzing
the
characteristics of different high order filter chips. The experimental results show that the design of theconditioning
circuit is feasible and simple
in the
production and debugging， and it has very great practical value.
1 Design of conditioning circuit
1. 1 Piezoelectric sensor equivalent circuit
Piezoelectric sensor is designed based on the piezoelectric effect. The sensitive component is made by piezoelectric material. Once being subjected to external force， the surface of piezoelectric material would generate some weak charges.
When the
piezoelectric sensor is forced in the direction of the sensitive axis，polar opposite charges are got on the two electrodes， which are equivalent to a charge source (electrostatic generator)']. The equivalent circuit is shown in Fig. 1.
Fig. 1 Piezoelectric sensor equivalent circuit
In Fig. 1，Cc equals the sensor capacitor； Cd is thecable capacitor； Cr is the amplifier input capacitor； Rc is the sensor insulation resistance ； R, is the amplifier input resistor. According to Fig. 1，the resistor and the capacitor constitute the RC circuit， and the
Received date： 2017-11-25
Foundation items： Key Laboratory Fund (No. 61420010414162001002) Corresponding author： YUAN Xiao-kang (1163641228®qq. com)
YUAN Xiao-kang，et al. / Design and analysis of singal conditioning circuit for vibration sensor
175
charge generated by the force of piezoelectric sensor will be released through the RC circuit.Set
R BR： + Rl，⑴
[C = CC EC. + Cd.
and also there are
C = §，I = f，U=IR.
Thus the voltage of piezoelectric sensor is obtained by Eqs.⑴ and ⑵ as
U = RCU⑶
Using initial conditions t = 0 and U=U〇，U can be
obtained by
U = U0eRC.
⑷
Setting t = RC，the voltage of piezoelectric sensor is related to the time constant t % the larger the time constant t is， the slower the charge leaks out， and the smaller the measurement error of the sensor gets. The cable capacitance Cd and the sensor capacitor Cc are
usually
fixed
values， the
amplifier
input
capacitance C, depending on the op-amp output sensitivity can not be increased arbitrarily， and the sensor and the signal input should be in high insulation
resistant
to
prevent
the
charge
measurement error caused by rapid leak[2(.
1. 2 Charge-voltage conversion circuit
1. 2. 1 Design of charge-voltage conversion circuit
The charge-voltage conversion circuit is shown in
Fig. 2.
Fig. 2 Charge-voltage converting circuit
The piezoelectric vibration sensor is ranged in
士 10 000 3 with sensitivity of 3. 5 pC/3, and the sensor produces charges as follow
Q 二士 10 000 3 X 3. 5 pC/3 二士 35 000 pC. (5)If the output voltage range of the op amp is set from —0.51V to +0. 51V， the feedback capacitor C1d68 nF can be obtained by applying the formula
C=—QlU.
Since OPA4340 is rail-to-rail input/
output amplifier and takes the +5V power supply，
so negative voltage signal (from —0. 51V to 0 V) will be cut—off andnot through the op—amp output， which causes the measurement result incompleted. In order to ensure the integrity of signal measurement， the offset voltage of 1. 248 V is added to the forward end of the U1 integrated operational amplifier， and the output voltage range of the operational amplifier is from 0. 738 V to 1. 758 V.
In order to avoid the capacitor C1 being charged for a long time to saturate the integrated operation，it is necessary to connect a resistor R2 in parallel at the capacitor C1. Meanwhile， another important role is to introduce the direct current negative feedback， which can effectively inhibit input offset voltage due to integrated op-amp，input offset current and dritt caused by temperature drift integral3].
The lower cu--off frequency of the system test is
f=1/27zR2C1. The cu--off frequency of the input
signal is calculated as 0. 1 Hz， and the resistance is R2 = 10 m%.
The resistor Ri of the operational
amplifier inverting input plays a major role in protecting the circuit and limiting the current'].
1. 2. 2 Analysis of feedback capacitance
The main function of feedback capacitor C1 is to convert the charge signal generated by sensor into the voltage signal， so the feedback capacitor needs to select
a
capacitor
with
low
drift， low
dritttemperature and high leakage resistance. In the highest and the lowest temperature resistance test chamber， 0. 1 $F
value
capacitors
of
nominal
monolithic capacitor， round ceramic， ceramic chip， polystyrene are picked up as the feedback capacitors. The experimental results of op-amp output voltages with the different temperatures are shown in Fig. 3.
T (°C)
Fig. 3 Different feedback capacitors output voltage
As shown in Fig. 3, the temperature characteristics of the capacitor would strongly influence the output of the piezoelectric sensor conditioning circuit. Selecting 104 monolithic capacitors as feedback
176
Journal of Measurement Science and Instrumentation
Vo! 9No. 2，Jun. 2018
capacitors，they will produce a measurement error of up to 800 %， while the equivalent capacity of polystyrene capacitor is only 1%.
It is not difficult to find that the temperature characteristics of polystyrene capacitor are ideal. In engineering practice, it is also proved that the use of polystyrene capacitors as feedback capacitors will greatly reduce the measurement error caused by the change of environment.
1. 3 Design of isolation and voltage amplification
circuit
The isolation and voltage amplifying circuit is shown in Fig. 4.
Fig. 4 Isolation and voltage amplification circuit
The 1. 248 V DC bias component of the charge- voltage conversion circuit can be filtered out by capacitor C2， and the U02 is changed into an alternating voltage signal from —0. 51 V to 0. 51 V. The forward bias voltage of 1. 248 V is added to the U2
positive
input， and
the
bias
voltage
is
superimposed to the U02， so the U02 input is
converted to an AC voltage signal of 0. 738 V to
1.758 V.
The voltage output range of the charge amplifier is not consistent with the input range of the subsequent AC/DC converter，so the voltage amplification circuit is used to adjust the voltage. The voltage amplifying circuit can change the magnification factor by adjusting the ratio of the resistance R4 to the resistance U3. According to the principle of “virtual short” and “virtual broken”']，the relationship between the output voltage U03and the input voltage
U02 is
U03 = 1. 248 V + R4/R31 248 V —U〇2) B 1.248 V + R4/R3(1.248 V-(1. 248 V-Q/C1)).
\")
According to Eq. (6) , U03 ranges from 0. 248 V to2. 248 V，so the output voltage meets the input range of the subsequent circuit AC/DC converter AD7667.
14 Design of filter circuit
1. 4. 1
Selection of filter
The output voltage of the charge-voltage converter circuit is accompanied with sensor body noise, device
noise and other high-frequency noise， which will blend into the voltage amplifier circuit'] together with the real signal， and the interference signal will be further amplified， which makes the vibration signal measurement inaccurate. A high-order low- pass filter can be used to further process the signal to suppress the aliasing of the noise signal and the effective signal.
An integrated filter chip can usually be used to filter the signal to eliminate the noise and the effect of interfering signals. Common filter types include butterworth filter， elliptical function filter， Bessel type filter and so on'(.
Through the access to the chip manual， we can obtain MAX291 filter， MAX7400 elliptic filter and (TC1569 Bessel filter，and the three filter chip DC bias output error are shown in Table 1.
Table 1
Filter DC offset voltage
Filter typeDC offset output bias (mV)
MAX291士 150MAX7400士 5LTC1569
士 2
As can be seen in Table 1，MAX291 Butterworth filter DC bias output deviation is the largest，which is suitable for applications in low accuracy. MAX7400 elliptic function filter and LTC1569 Bessil filter with smaller DC bias deviation are more suitable for this measurement circuit.
The phase-frequency characteristic curves of the MAX7400 elliptic function filter and the LTC1569 Bessel filter are shown in Fig. 5.
As
shown
in
Fig. 5， the
phase-frequency
characteristic of the LTC1569 Bessel filter is better， and the signal output delay in the band is basically the same with the linear phase characteristic in the passband， which is 12 $s； the MAX7400 elliptic function filter has different delay in signal output in the band， and the delay time is basically up to the
YUAN Xiao-kang，et al. / Design and analysis of singal conditioning circuit for vibration sensor
177
millisecond level at the cutoff frequency.
A filter circuit consisting of LTC1569 Bessel filter and MAX7400 elliptic filter is connected to the square wave of 1 Hz generated by the signal generator，and the output is shown in Fig. 6.
The chi channel generates 1 Hz square wave signal， the waveform of the ch2 channel is 1 Hz square wave after the LTC1569 filter circuit，and the ch3 channel waveform is the output waveform after the MAX7400
filter circuit.
Accroding to the
MAX7400 phase-frequency characteristic curve， it can be seen that due to different frequency signal output delay time is different，so different frequency signals mixed together will resulte in serious signal distortion. LTC1569 filter has the same delay time in the frequency signal output in the passband， so the output waveform is closer to the input square wavesignal.
In comparison，the DC bias output deviation of the low pass filter composed of LTC1569 is smaller，and the passband has a linear phase characteristics，which can protect the signal without distortion and ensure the accuracy of the signal measurement results.1. 4. 2
Design of filter circuit
The filter circuit composed of LTC1569 filter chip
graph
is shown in Fig. 7.
LTC1569 is an adjustable 10 order lowpass filter with linear phase and high DC accuracy. In Fig. 7， when the LTC1569 filter CLK pin is connected to the V+pin，the frequency division is set to 16 frequency division'」. Therefore， the relationship of LTC1569 circuit cutoff frequency f and external resistor Rx is
f = 128 k% X (10 k%/Ux)/16.
sccond-order low-pass filter after the LTC1569 filter. The waveform of the ch1 channel is sine wave after the LTC1569 filter circuit in Fig. 8.
(7)
According to the vibration signal cutoff frequency of 9. 76 kHZ , Rx = 8. 2 k% can be calculated , and the
Rx requires a resistance of 1 % accuracy to ensure the
accuracy of the cutoff frequency'」.
Although the LTC1569 filter can provide high quality filter characteristics， the chip filter principle is based on the switching capacitor effect， which results in high frequency switching noise at the output. The switching noise is filtered by adding a
Fig. 8 Comparison of waveforms before and after filtering out switching noise
178
Journal of Measurement Science and Instrumentation
Vo! 9No. 2，Jun. 2018
Due to the impact of switching noise， the waveform is mixed with the steps similar to the noise. The waveform of the ch2 channel is sine wave after adding a sccond order low-pass filter， and the
signal of the sine wave can be clearly seen that the switch noise is effectively filtered and the waveform bccomes smoother.
2 Results and analysis
2. 1 Amplitude conditioning circuit
response of vibration signal
The measurement
system that
contains
the
vibration signal conditioning circuit will connect the piezoelectric sensor to the amplitude verification first to check the correctness of the tuned circuit. During the verification process， the sensor is fixed on the vibrating table, and the output signal of the sensor is collected by the sinusoidal sweep mode. The data collection is shown in Fig. 9. The maximum control reference spectrum of sinusoidal sweep frequency is + 30^， and the corresponding frequency range is (200士50) Hz. It can be seen from the Fig. 9 that the piezoelectric sensor vibration frequency is about 160 Hz， and the actual control spectrum is 士30g\\ The measurement system through the calibration of the linear fitting calibration shows the experimental process of sine sweep more truly.
s
u.12s(JpDooy
35.00
35.01
35.02
35.03
35.04
35.05
/(ms)
Fig. 9 Piezoelectric sensor sine sweep frequency curve
2. 2 Frequency conditioning circuit
response of vibration signal
Using
the
signal
generator
to
sweep
the
conditioning circuit test， the channel frequency response curve is obtained as shown in Fig. 10. The cutoff frequency of the channel obtained by Fig. 10 is 9. 76 kHz which meets the design requirements.
3
_5
(_
e
p)u
■ 10X
o
lv■5
--22I
:
/(kHz)
Fig. 10 Frequency response test plot
2. 3 System function verification
A standard charge generator is used as the charge source to simulate the output of the vibration sensor， and it is connected to the circuit to measure. The theoretical voltage value is calculated by using the Eq. (5)，and the results are shown in Table 2.
Table 2 Conditioning circuit functional verification
Input charge Theoretical Measured Voltage(pC)voltage (V)voltage (V)measurementerror (%)
30 0002.1132. 1200.331-30 0000. 3830.3860.78320 0001. 8251. 8400.822-20 0000. 6710.6760.74510 0001. 5361.5480.781-10 000
0. 960
0. 969
0. 938
It can be seen from Table 2 that the measured
voltage is basically consistent with the theoretical value. The measurement error of the conditioning circuit is less than 1 %， which can meet the requirements
of
vibration
signal
measurement
accuracy and accurately measure the vibration signal.
3 Conclusion
In this paper， piezoelectric sensor is the research object， the
sensor
equivalent
circuit
and
the
regulating circuit are introduced in detail， and the capacitance and filter of the key components of the regulating circuit are analyzed and studied.
The
vibration signal conditioning circuit introduced in this paper has been tested in many missile telemetry experiments， and
the
obtained
data
is
highly
accurate. The collected vibration signal data provides reliable data support for the design of the missile structure and the installation of the electronic devices in the internal environment.
YUAN Xiao-kang, et al. / Design and analysis of
singal conditioning circuit for 179
vibration
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振动传感器信号调理电路设计及分析
袁小康^焦新泉^储成群^杨文豪2
\".中北大学电子测试技术重点实验室，山西太原030051;2.中国电子科技集团第五十八研究所，江苏无锡214035)
摘要：基于压电式传感器能把振动或冲击的加速度转换成与之成正比的电荷这一原理，提出了该类型传 感器的等效电路。设计了基于采集测量系统的压电式传感器电荷/电压转换电路，分析了电路中关键元器 件—
反馈电容的温度特性对于转换电路输出的影响，对不同类型滤波器进行了特性分析，并根据实际传
感器带宽配置了相应滤波电路。通过试验表明，该信号调理电路能有效滤除振动信号中的噪声信号，并能获 取精确的振动信号。
关键词！振动；压电传感器；信号调理电路；反馈电容
引用格式： YUAN Xiao-kang, JIAO Xin-quan, CHU Cheng-qun, et al. Design and analysis of singal
conditioning circuit for vibration sensor. Journal of Measurement Science and Instrumentation,2018, 9\")： 174-179. [doi： 10. 3969/i. issn 1674-8042. 2018. 02. 012]
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