Two-axis Tilt Angle Detection based on Dielectric Liquid ...

A Robust Two-axis Tilt Angle Sensor Based on Air/LiquidTwo-phase Dielectric Capacitive Sensing Structure

Author

Tran Thi Thuy, H, Dinh, TD, Vu Quoc, T, Pham Quoc, T, Aoyagi, M, Bui Ngoc, M, Dau, VT, Bui,TT

Published

2018

Journal Title

IETE Journal of Research

Version

Accepted Manuscript (AM)

DOI

https://doi.org/10.1080/03772063.2018.1518732

Copyright Statement

© 2018 Taylor & Francis. This is an Accepted Manuscript of an article published byTaylor & Francis in Climate and Development on 17 Sep 2018, available online: https://doi.org/10.1080/03772063.2018.1518732

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http://hdl.handle.net/10072/385741

Griffith Research Online

https://research-repository.griffith.edu.au

Two-axis Tilt Angle Detection based on Dielectric Liquid Capacitive Structure

Ha Tran Thi Thuy1, Tiep Dang Dinh2, Tuan Vu Quoc3, Thinh Pham Quoc4, Masahiro Aoyagi5, Van Thanh Dau6, and Tung Thanh Bui7

1Faculty of Electronic Engineering, Posts and Telecommunications Institute of Technology, Hanoi, Vietnam. 2Vietnam Military Science Academy, Hoang Sam, Hanoi, Vietnam. 3Vietnam Academy of Sciences and Technology, Hanoi, Vietnam 4Thai Nguyen University, Thai Nguyen, Vietnam. 5National Institute of Advanced Industrial Science and Technology, Tsukuba 305-8560, Japan ([email protected]) 6Research Group (Environmental Health), Sumitomo Chemical. Ltd, Hyogo, 665-8555, Japan ([email protected]

chem.co.jp). 7VNU University of Engineering and Technology, Hanoi, Vietnam (e-mail: [email protected]).

Abstract— This paper presents the design, fabrication, and characterization of a two-axis tilt angle sensor based on the

dielectric liquid capacitive sensing structure. The sensor consists of five electrodes. One electrode serves as the exciting

electrode and two pairs of electrodes as sensing electrodes, which are arranged at identical positions surrounding a glass

cylinder tube, which is partly filled with dielectric liquid. Based on this unique arrangement, the proposed sensor can

detect two components of tilt angle in x-axis and y-axis, simultaneously. A computational simulation and experimental

measurements are performed to study the performance of the sensor. The numerical simulation is carried out with a finite

element analysis using COMSOL. A prototype of the sensor was fabricated, and its performance was evaluated. The tilt

angle sensor was employed on a printed circuit board with a conditioning circuit, which consisted of a 170 kHz sine wave

generator, pre-amplifiers, rectifiers, and low pass filters. The experiment results confirmed that the tilt angle sensitivities

to x-axis and y-axis are 18.2 mV/° and 58.2 mV/° respectively, with cross-axis sensitivity being less than 5.5% in the linear

range. The measured tilt angle resolutions are 0.55° and 0.17° on the x-axis and y-axis respectively.

Index Terms— Capacitive sensor, fluidic sensor; tilt angle measurement

1. Introduction

Tilt angle sensors are mainly applied to measure the horizontality of a system or object and have been widely

used in robotics, human body motion detection, transportation vehicles, industrial equipment, industrial

automation, intelligent platform, machining, and other important fields [1]–[4].

There are several kinds of commercial sensor for tilt angle measurement, which can be divided into two main

kinds: solid-based and fluid-based mechanisms, depending on their working principles. The research on sensors

of solid-based tilt angle has matured over the course of many applications. This kind of sensor consists of a proof

mass suspended by a cantilever, spring, hinged bar or roller ball [5]–[9]. When the sensor body rotates around

vertical or horizontal reference orientation, under the influence of the gravity, the suspended solid structure is

deformed; this deformation is measured in terms of tilt angle. Recent technological advancements in the

manufacturing of tilt sensors have improved the sensing accuracy, reduced the fabrication cost, increased the

working lifetime, and enhanced their performances [10]–[13]. The main issue for these sensors is that it is easy

for them to get damaged by the external forces, such as vibration or mechanical shock.

The sensor of fluid-based tilt angle utilizes the movement of fluidic structure to sense the gravitational

acceleration. To be moved by gravity, the fluid density in a container is not equally distributed. It is done by either

locally heating a homogenous liquid or by using a mixture of immiscible fluids which can be liquid-air or liquids

with different densities [14]–[26]. Locally heating fluids (thermal convective tilt sensor) is usually done using gas-

based sensors; its principle works by transferring heat due to convection. Using a constant current, the gas inside

the sealed chamber is heated, and the temperature profile moves in concordance with the applied inclination.

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Such kind of sensor has simpler structure without proof mass suspended as a part of the sensor and stronger

shock resistance, compared to solid-based tilt angle. Even though this kind of sensor can be easily affected by

environmental temperature and is low in measurement precision. Moreover, the heaters use high power, owing

to the requirement of temperature, for sensing. In contrast, mixing of immiscible liquids or partly filled liquid-air

(fluidic tilt angle sensor) has many advantages, such as high sensitivity, corrosion, moisture, and shock resistance

[14], [27], [28]. The output voltage of this kind of tilt sensors is obtained most commonly by transducing the

physical changes of its medium to an electrical signal based on resistance [23], capacitance, inductance [14],

piezoelectric [4], resonant [29] or optical parameters [3], [30]. Among them, capacitive type with linear and

analog outputs [2], [25], [26], [31]–[34] is mostly used. Besides, capacitive sensing, in comparison with other

detection principles of tilt sensor, has many advantages such as simplicity, noncontact measurement, long-throw

linear displacement, and the design and fabrication process are significantly simpler than their counterparts.

However, most of these sensors are single-axis sensing structure [33]–[35].

There is an increasing demand of high-performance tilt angle sensor for emerging applications, especially

medical and automotive applications. In this paper, we introduce a two-axis tilt sensor based on a dielectric liquid

capacitive configuration with differential capacitively coupled contactless conductivity detection (DC4D)

technique. The unique arrangement of only four sensing electrodes sharing a single exciting electrode allows us

to detect dual axis simultaneously with low cross sensitivity, thereby overcoming the single-axis limited concept

in the previous work [36]–[38]. This configuration is robust, highly accurate, and easy-to-build with inexpensive

and commercially available electronics instead of research-grade tools. Although recent advanced

Fig. 1. Proposed two-axis tilt sensor based on dielectric liquid capacitive structure.

Tab.1. Parameters of the proposed two-axis tilt angle sensor (see fig.1b)

Parameter Value (mm) Note

L1 10.0 Curvature length of the excitation electrode W1 7.5 Length of the excitation electrode L2 5.0 Curvature length of the x-axis sensing electrodes

W2 7.5 Length of the x-axis sensing electrodes L3 7.0 Curvature length of the y-axis sensing electrodes

W3 17.3 Length of the y-axis sensing electrodes t 0.2 Thickness of the electrodes L 25.0 Total length of the sensor D 11.0 Diameter of the sensor g 0.5 Thickness of the glass wall

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microfabrication technology has reduced the cost and the size of the tilt sensor, the burden of the initial

installment and prototyping could delay the transformation of a theoretical idea to a realistic production [9], [39]–

[42]. The proposed sensor which has simple structure and is easy to be fabricated has been briefly presented in

our previous work [43]. In this paper, details of design, simulation, and characterization of the sensor are

presented. With its simplicity, the sensor can be customized to fit the various applications.

2. Dielectric Liquid Capacitive Sensor for Tilt Angle Measurement

A. Design and Working Principle

The first report on capacitively coupled contactless conductivity detection (C4D) in microfluidic systems was

published in 2001 by Guijt et al. [44]. In this paper, in order to avoid various difficulties commonly found in the

contact method and to improve the sensing detection limit, this approach has been utilized.

The structure of the proposed sensor is shown in Fig. 1(a). The dielectric liquid capacitive sensor is mounted on

a printed circuit board (PCB). The sensor is constructed from an air-liquid two-phase borosilicate glass cylinder

surrounded by five electrodes. The cylinder contains gasoline (permittivity of 2) and air (permittivity of 1), and is

completely sealed by epoxy to prevent gasoline from evaporating or leaking. Gasoline was chosen because it has

low surface tension (0.0198 N/m) and viscosity (0.6 mPa.s), which will allow the bubble to move and settle quickly

while in contact with the borosilicate glass. Parameters of these materials are shown in Tab. 2.

The borosilicate glass has diameter of 11.0 mm, length of 25.0 mm, and thickness of 0.5 mm. The curvature

length and length of the excitation electrode, x-axis sensing electrodes and x-axis sensing electrodes are 7.5 mm

and 10.0 mm, 7.5 mm and 5.0 mm, and 17.3 mm and 7.0 mm, respectively. The thickness of the electrodes is 0.2

mm. The geometry parameters of the sensor and properties of the materials used in this work are listed in Tab. I

Fig. 2. Working principle of the liquid capacitive tilt sensor. (a) 3D drawing with one excitation electrode and four sensing electrodes; (b) C1 and C2 capacitor on the x-axis tilt scenario. The differential capacitance value (C1 − C2) changes when liquid surface is rotated relative to the electrodes; (c) C3 and C4 capacitor on the y-axis tilt scenario with the change of (C3− C4).

Tab. 2. Properties of some materials

Air H2O Gasoline Glass Cu

Relative permittivity 1.0 80.1 2.0 4.6 conductor Surface tension (mN/m) - 72.8 19.8 - -

Viscosity (mPa.s) 18E-3 1.0 0.6 - - Conductivity (S/m) 3E-15 to 8E-15 ~5.5E-6 25E-12 ~1E-15 58.5E6


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and Tab. 2, respectively.

In the previous work, the electrodes were placed inside microchannel and protected by an insulating layer (i.e.,

thin SiO2 or PDMS layer). Such deposition process is costly and time consuming, and the selection of material is

also limited. In addition, all the electrodes had to be placed in the same plane [45][46]. In this work, the electrodes

are placed outside, the glass tube serves as the protective layer and helps to isolate the electrodes from the liquid

medium. Though the thickness of glass tube affects the sensitivity of a single capacitor pair, it has limited effect

on sensor performance because C4D is measured in differential sensing mode and the existence of glass thickness

can be compensated by the charging level determined at initial state for sensor calibration.

In this design, the volume ratio of liquid and air is 3:1. The electrode underneath the tube serves as the

excitation electrode and the rest four serve as the sensing electrodes. The two electrodes XE1 and XE2 are the

sensing electrodes for monitoring the x-axis angle and the two electrodes YE1 and YE2 are the sensing electrodes

for monitoring the y-axis angle (see Fig. 1(b)). These five electrodes make two pairs of differential capacitance of

(C1 − C2) and (C3 − C4) corresponding to the x-axis and y-axis tilt angles, respectively. The differential capacitances

depend on the position of the liquid surface inside the cylinder tube, which changes because of gravity when the

sensor is rotated (see Fig. 2). Therefore, differential capacitance of (C1 − C2) and (C3 − C4) correspond to the tilt

angle of the x-axis and y-axis, respectively (see Fig. 2).

When exciting a sine signal to the excitation electrode, the capacitance of the capacitor constructed by the

excited electrode and sensing electrode determines the output voltage, which is the differential voltage between

the electrodes. Thus, the tilt angle of the sensor on the x-axis and y-axis can be monitored by measuring the

differential voltage (VC1 − VC2) and (VC3 − VC4), respectively.

It is indeed no difficulty to deploy the same design scheme to a symmetric design (i.e., cubic or sphere). In micro

fabrication technique where the alignment and production is automated at mass production scale (i.e., wafer

level), the symmetricity is easy and guaranteed. In our study, where the conventional mechanics is involved, the

design should be simple and lab-made ready. Thus, the cylinder glass was selected. The selection of cylinder

allows investigating x- and y-axis asymmetrically and deepen our understanding. In addition, the cylinder shape

is widely available and is compatible with conventional mechanics.

B. Modeling and Simulation

In this work, the finite element method (FEM) is utilized to investigate the performance of the proposed two-

axis tilt angle sensor, which is based on dielectric liquid capacitive sensing structure. The capacitive tilt sensor is

Fig. 3. Simulation setup and results. (a) Meshing for the numerical setup; (b) 3D profile; (c) Electrical field profiles of the x-axis configuration in the balanced case and when sensor is rotated; (d) Electrical field profiles of the y-axis configuration in the balanced case and when sensor is rotated.


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modeled and simulated using COMSOL software to analyze the capacitor with curved electrodes and nonuniform

relative permittivity medium (liquid-air two phases dielectric medium). The dielectric constant of the fluid is

assumed value of 2 (i.e., gasoline) without counting surface tension effect between liquid and cylinder wall.

The device was modelled as a glass cylinder (Fig. 3) with copper electrodes placed outside and liquid initially

occupied 75% of the cylinder volume. The air space surrounding device is truncated by sphere whose outer

surface is applied with zero charge boundary condition (Fig. 3(a)). A DC voltage is applied to electrode to

determine the capacitance. The sensing electrodes are set at 7.0V and the underneath electrode was grounded

(Fig. 3(b)).

The tilted state was mimicked by rotating the device frame relatively with the liquid level, which stays the same

as the initial state. Inside the device, the capacitance and the electrical field will be affected by the different

voltage between electrodes and their relative position with the liquid level. The capacitance of two sensing

Fig. 4. Simulated capacitance change of the C1, C2 due to tilt angle in x-axis and y-axis. Capacitance C1, C2 values are changed symmetrically with each other with respect to the y-axis when the sensor is rotated in x-axis. C1, C2 have similar responses versus y-axis rotation.

Fig. 5: Differential capacitance (C1−C2)x-axis and (C1− C2)y-axis versus x-axis and y-axis rotation angle, respectively. The measuring range of this proposed sensor can be from −180° to −70° and from +70° to +180° or from −70° to +70°. In this case, the (C1− C2)y-axis is considered as the cross-talk.

Fig. 6. Simulated capacitance change of the C3, C4 due to tilt angle in x-axis and y-axis. Capacitance C3, C4

values are changed symmetrically with each other with respect to the y-axis when the sensor is rotated in y-axis when applied y-axis rotation. C3, C4 have similar response versus x-axis rotation.

Fig. 7. Differential capacitance (C3−C4) x-axis versus tilt angle around x-axis and y-axis. The capacitance (C3 − C4)x-axis is symmetrical to the original point. The linear range is from around −20° to +20°. The crosstalk value of (C3− C4)y-axis when applying y-axis tilt angle is much smaller than (C3 −C4)x-axis.


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electrodes versus exciting electrode was integrated over their surface and their difference was extracted.

In this simulation, we simplify the concept by claiming non-conductive liquid. Non- contaminated gasoline has

a conductivity of ~25 pS/m, so it is rated as a non-conductor. In addition, the model was simplified by ignoring

the movement of charge in the liquid, thus the double layer electrode capacitor was not formed. Under the

assumption of electrostatic conditions, the surface of each electrode is at the same potential. The other domains

of the model were assumed to be the ideal insulators.

Figures 3(c) and 3(d) demonstrate electrical field profiles of the sensor under the x-axis rotation and y-axis

rotation, respectively. It shows that the voltage potential of the sensing electrodes is higher than that of the

excitation electrode. Fig. 3(c)-left shows the voltage profile when the sensor is in the balanced stage (zeros tilt

angle). In this case, the two capacitors are symmetrical. Hence, the differential capacitance between the two

sensing electrodes is zero. When the sensor rotates, the relative position of the liquid surface changes and the

electrode alters the capacitance of the C1 and C2. Therefore, the symmetry status between the two electrodes is

broken as shown in Fig. 3(c)-right.

Figure 4 presents a simulated capacitance of C1 and C2 capacitors when the sensor is tilted from −180° to +180°

around x-axis. The capacitances of C1 and C2 are symmetrically changed with respect to the original point.

Capacitance C1 and C2 reach peak value of about 140 fF when tilt angle is −60° and +60°, respectively. When the

C1 gets the maximum value at about 60° the C2 gets the minimum value, and vice versa.

The tilt angle in x-axis can be extracted from the difference value of (C1 − C2). Figure 5 shows the differential

capacitance (C1− C2)x-axis versus the input rotation angle around x-axis (black solid line). The sensor has a linear

response in the range from −60° to +60° with simulated sensitivity of 0.45 fF/°, though the measurable range is

from −70° to +70°. In the other ranges, the response is nonlinear but still can be used to measure the rotation

angle.

Besides the effect of x-axis rotation, the differential capacitance (C1−C2) is also affected when y-axis tilt angle is

applied, known as the cross-talk. The differential values of (C1− C2)x-axis and (C1 − C2)y-axis versus input x-axis and y-

axis tilt angles are shown in black solid and red dash lines, respectively in Fig. 5. As can be seen, the crosstalk

value of (C1− C2)y-axis is much smaller than that (C1− C2)x-axis, i.e., approximately zero, thanks to the symmetrical

arrangement of the sensing electrodes. Therefore, in this case, the cross-talk from y-axis inclination can be

ignored.

Similarly, Fig. 3(d) shows electrical field profile of the sensor under y-axis rotation scenario. The voltage profiles

when the sensor is at the initial and tilted states are presented in Figs. 3(d)-left, and 3(d)-right. Capacitance values

of C3 and C4 are extracted and plotted in Fig. 6 (green dot-dash and blue dash lines). As can be seen, capacitance

values of C3 and C4 change symmetrically with respect to the original point.

Figure 7 shows differential capacitance value of C3 and C4. Similar to the x-axis configuration, the (C3−C4) consists

of two components, which are (C3 − C4)y-axis as the signal and (C3 − C4)x-axis as the cross-talk. The linear response of

this configuration is −20° to +20° on y-axis and sensitivity of 0.85 fF/°. The simulated results also show that the

cross-talk (C3 − C4)x-axis from x-axis inclination is approximately zero, much smaller compared to the (C3− C4)y-axis,

and can be ignored.

From the above analysis, we confirm the working principle of the proposed sensor and its ability to measure

two components of tilt angle around x- and y-axis with negligible cross-talks. Experimental assessment on the

sensor is presented as follows.

3. Measurement Setups

A sensor prototype was constructed by five copper electrodes and a glass cylinder tube. The cylinder contains

75% gasoline (permittivity of 2) and 25% air (permittivity of 1). Liquid is filled from one end through a hole and

the hole is sealed completely by epoxy to prevent gasoline from evaporating or leaking. The copper electrodes

were cut by computer numeric control (CNC) and bonded to the outside of the cylinder at desired positions with


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the assistance of manual stage (see Fig. 1(b) and Fig. 8).

A measurement setup was built to investigate the performance of the tilt angle sensor for both x-axis and y-

axis (see Fig. 8). The proposed tilt capacitive sensor is anchored on a printed circuit board (PCB) with electronic

circuits. The PCB is packaged in a shielding box and then positioned on a rotation disk with readout resolution of

0.1. Tilt angle of the PCB is changed gradually in the interval −180° ∼ +180° by rotating the disk and the

corresponding output voltage value is recorded. The output analog signal after the conditioning circuit is routed

to a personal computer (PC) by using a National Instruments Data Acquisition (DAQ) and LabVIEW software.

First, the PCB is aligned to characterize tilt angle in x axis. Then it is rotated 90 degrees in vertical plane to

characterize the performance of the sensor in sensing tilt angle in y axis. Beside the amplitude response on both

axes, the cross-talks are also investigated.

The block diagram of the electronic circuit is given in Fig. 9. A 170-kHz sine signal generator is connected to the

excitation electrode. The sine generator circuit is a Wien bridge oscillator using an operation amplifier (TL084).

The frequency of the oscillator is controlled by resistor R and capacitor C; output amplitude is adjusted by resistors

R1 and R2. Because the voltage on sensing electrodes changes with their corresponding capacitance, x-axis and y-

Fig. 8. Experimental setup for tilt angle measurement.

Fig. 9. Electronic block diagram of the proposed dielectric liquid capacitive tilt sensor. Sine signal is pumped to the excitation electrode. Output voltages are taken by using differential amplifiers with sensing electrode voltages as the input signals.


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axis tilt angle can be monitored by measuring amplitude of the differential voltage pair (VC1 - VC2) and (VC3 - VC4),

respectively.

The conditioning circuits convert current signals to the voltage by using four resistors Rx1, Rx2, Ry1, and Ry2. Each

voltage signal is then pre-amplified before applying to the differential amplifier. An instrumentation amplifier is

employed as the differentiators. Amplitude demodulation circuit consists of a full wave precision rectifier and a

RC passive low-pass filter (LPF) with cut-off frequency of 16 Hz. Output voltage of the LPF circuits are the outputs

of the tilt angle sensor. In this work, the differential amplifier is used not only for taking the differential voltage

between each voltage pair but also rejecting the common noise such as industrial noise of 50 Hz frequency, and

other external-noise sources.

4. Experimental Results and Discussion

Relations between measured output voltage and tilt angle in x-axis and y-axis are shown in Fig. 10. The

measured voltages have similar shapes and responses with the simulated results. The output signals are

symmetrical with respect to the original point. The inset shows the output voltage signal of the sensor at zeros

tilt angle with voltage swing of ±5 mV. As can be seen, the x-axis configuration has a better linear response than

the y-axis case. It is because the shape of the cylinder is asymmetric.

The measured x-axis voltage has a perfect symmetric form and can be divided into three areas as described in

the simulation results where the measurement range of this fabricated sensor is from −80 to +80. The output

voltage reaches peak value of about 1150 mV at the tilt angle of 80. The measurement range of this x-axis

configuration is larger than that of the simulation. The difference between the simulation and experiment may

due to the ratio of air/liquid and capillary effect of investigated liquid.

A zoom-in draw with linear fitting around original point of the measured data is presented in Fig. 11. The x-axis

configuration has a linear range in the range of −60 to +60 (correlation coefficient of 0.999). The sensitivity of

the measured data in this range is 18.2 mV/.

Noise is unavoidable in all electrical measurement systems, including external noise, conducted noise, and

intrinsic noise (thermal noise, 1/f noise). In our case, noise is any voltages that occur when there is no tilt angle

applied to the sensing structure. Total measurement error measured at zero tilt angle is about ±5 mV (see Fig.

11), therefore the measurement resolution, which is defined by measurement noise divide by sensitivity, is about

0.55. The measured error came from the thermal noise, cross-talk noise, common noise, industrial noise. Figure

12 shows crosstalk voltage of the two configurations. The cross-talk voltages are estimated less than 5.5% in the

linear range, i.e., quite small to compare with the corresponded signals. In simulations, crosstalk is evaluated to

be small and ignorable. The difference between simulation and measurement might come from a slight

mispositioning of x-axis and y-axis sensing electrodes on the cylinder tube.

The response of the y-axis configuration is also shown in Fig. 10 and Fig. 11. The measurement range is from -

30 to 30 with the linear range is from about −16 to +16 (correlation coefficient of 0.994). This measured data

is matched with the simulation showing in Fig. 7. The measured sensitivity and resolution of this configuration

are about 58.2 mV/ and 0.17, respectively.

The proposed structure reveals its ability to measure two components of tilt angle in x-axis and y-axis,

simultaneously. However, the measurement ranges of tilt angle in x-axis is wider than that in y-axis. This came

from the asymmetric in geometry of the cylinder tube. Tab. 3 summaries the characteristics of several typical tilt

sensors in the literature and in our work. Each design has its own advantages as discussed in Introduction part of

the manuscript. Thus, it is not easy to significantly improve one characteristic without trading off the others.

Based on these data, the sensor in this work can be considered as the same group with its references.

According to the obtained results, the proposed tilt sensor can be used for many different applications with

two different adaptable detecting ranges and sensitivities. The different structures of the tilt sensor, such as the


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position of the electrodes and their dimensions, can affect to its characteristics. Therefore, the performance of

the proposed tilt sensor can be further improved by optimizing structural design such as the shape of the

container, dimensions and position of the electrodes. The results on manipulating the parameters of sensor

structure will be reported in another work.

5. Conclusion

The design, fabrication, and characterization of a two-axis tilt angle sensor based on dielectric liquid capacitive

sensing structure were presented. The sensor consists of five electrodes arranged at designated positions

surrounding a glass cylinder tube, which is partly filled with the dielectric liquid. With this unique arrangement,

the proposed sensor can detect two components of tilt angle in x-axis and y-axis, simultaneously. Based on

simulated results, a prototype of the proposed sensor was fabricated and characterized. Experimental results

confirmed the performance of the sensor. Tilt angle sensitivities to x-axis and y-axis are 18.2 mV/° and 58.2 mV/°,

respectively, with cross-axis sensitivity less than 5.5% in the linear range. The measured tilt angle resolutions are

0.55° and 0.17° on x-axis and y-axis, respectively. This proposed sensor is robust, cost effective, and can be used

in many applications.

Fig. 10. Measured output voltage of the x-axis and the y-axis configurations versus tilt angle in the x-axis and the y-axis, respectively. Inset shows the output voltage of the sensor at zeros tilt angle. The voltage is swung from about -5 mV to +5 mV.

Fig. 11. A zoom-in draw with linear fitting around original point of the measured data. Linear fits are with correlation coefficient of 0.999 and 0.994 for Vx and Vy, respectively.

Fig. 12. Output voltages versus tilt angle in range of 0 to 90 with cross-talk voltages.


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Acknowledgment

The authors would like to acknowledge the Vietnam National Foundation for Science and Technology

Development (NAFOSTED) for financial support under project number “107.99-2016.36”. We would like to thank

Mr. Tran Ngoc Thanh for his supports in experiment. A part of this work was conducted at the Nano-Processing

Facility, supported by NPF, AIST, Japan.

References

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[2] L. Zhao and E. Yeatman, “Micro capacitive tilt sensor for human body movement detection,” in Body Sensor Networks 2007, Proceedings, 2007, pp. 195–200.

[3] S. Das, “A Simple, Low Cost Optical Tilt Sensor,” Int. J. Electron. Electr. Eng., vol. 2, no. 3, pp. 235–241, 2014.

[4] P. M. Moubarak and P. Ben-Tzvi, “Design and analysis of a new piezoelectric MEMS tilt sensor,” ROSE 2011 - IEEE Int. Symp. Robot. Sensors Environ. Proc., pp. 83–88, 2011.

[5] C. H. Lee and S. S. Lee, “Study of capacitive tilt sensor with metallic ball,” ETRI J., vol. 36, no. 3, pp. 361–366, 2014.

[6] W. J. Fleming, “Overview of Automotive Sensors,” IEEE Sens. J., vol. 1, no. 4, pp. 296–308, 2001. [7] T. D. Tran, D. V. Dao, T. T. Bui, L. T. Nguyen, T. P. Nguyen, and Sugiyama Susumu, “Optimum design

considerations for a 3-DOF micro accelerometer using nanoscale piezoresistors,” in 2008 3rd IEEE International Conference on Nano/Micro Engineered and Molecular Systems, 2008, pp. 770–773.

[8] B. T. Tung, D. V. Dao, A. Ranjith, W. Naoki, T. Hiroshi, and S. Susumu, “Development of a 3-DOF Micro Accelerometer with Wireless Readout,” IEEJ Trans. Sensors Micromachines, vol. 128, no. 5, pp. 235–239, May 2008.

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Tab. 3. Characteristics of tilt sensors

Principle Ref. Detection

method

Degree of

freedom

Sensitivity

(resolution)

Measurement

range

Note

Solid-

based

Zhao-2007 [2] Capacitive Dual 2.25 pF/° ±90° MEMS process

Moubarack-

2012[12]

Piezoelectric Single 1.9 mV/90° ±90° Simulation only

Paulo-2012[13] Optical Dual 1.38 – 1.43 pm/ º +90° High grade analyser

Yang-2015 [30] Optical Dual 0.074 nm/° 0 - 40° High grade analyser

Gas-based Han -2017 [22] Resistive Single ∆R/R/° = 875 ppm/° ±90° At 50 µW heating

Billat-2002 [21] Resistive Single 6.6 mV/° ±90° MEMS process

Dau-2006 [28] Resistive Dual 0.12 mV/° ±90° MEMS process

Liquid-

based

Subir-2014 [3] Optical Single 0.8°

(0.09°)

±90° Simple

Lin-2008 [14] Impedance Single 19 mV/°

(0.3°)

±160° Impedance analyser

Zou-2013 [29] Resonant Single 50.06 Hz/° ±90° MEMS process

Welch – 2013 [24] Optical Dual 0.075 µA/° ±45° MEMS process

Jung 2007 [25] Capacitive Single 50 mV/° ±60° MEMS process

Chiu 2015 [26] Capacitive Single 0.48 mV/° ±90° CMOS-MEMS process

Choi-2012 [23] Capacitive Dual 50 mV/° ±70° MEMS process

Gou-2016 [32] Capacitive Single 0.129 pF/° ±40°

This work

Capacitive Dual x-axis: 18.2 mV/

(0.55)

y-axis: 58.2 mV/°

(0.17)

x-axis: ±60°

y-axis: ±16°


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