Differential C4D sensor for conductive and non conductive fluidic channel

These disadvantages limit the prac-tical applications of the conventional contact conductivity The capacitive contactless sensor structures are devel-oped in order to avoid the direct co

DOI 10.1007/s00542-015-2586-4

TECHNICAL PAPER

channel

Nguyen Dac Hai 1 · Vu Quoc Tuan 2 · Do Quang Loc 3 · Nguyen Hoang Hai 4 ·

Chu Duc Trinh 5

Received: 12 May 2015 / Accepted: 29 May 2015

© Springer-Verlag Berlin Heidelberg 2015

1 Introduction

Fluidic flow detection has been developed for many prac-tical applications in different areas like pharmaceuprac-tical, chemical analysis, oil industry, and so on There are some fundamental methods which have been applied for fluidic flow detection such as optical, ultrasonic, electrical sensing based on contact and contactless mechanisms

Fluidic channel sensor can be used electrical conductiv-ity parameter of material and channel geometry based on

the detection electrodes are directly in contact with the flu-idic, liquid or electrolyte solution The polarization effect and electrochemical erosion effect in the solution or the electrodes are unavoidable in this way Besides, the tamination of the electrodes usually causes errors in con-ductivity measurement These disadvantages limit the prac-tical applications of the conventional contact conductivity

The capacitive contactless sensor structures are devel-oped in order to avoid the direct contact technique issues

contact-less mechanism are often used to measure the phase flow

sen-sitivity of the capacitive configuration is low in case of high conductivity liquid due to the much small resistance value of the conductive fluidic channel in comparison with

presents a high frequency capacitance sensor to solve the conductive effects of water using an 80 MHz oscillator However, that device requires an extremely short electrodes for a quasi-local measurement and a rather complicated

Abstract This paper presents a novel design of a

both conductive and non-conductive fluidic channel This

car-rier sinusoidal signal to the center electrode as the

excita-tion electrode The electrodes are directly bonded on the

PCB with built-in differential amplifier and signal

process-ing circuit in order to reduce the parasitic component and

common noise In the non-conductive fluidic channel, the

output voltage and capacitance changes 214.39 mV and

14 fF, respectively when a 3.83 μl tin particle crosses an

oil channel In conductive fluidic channel, the output

volt-age and admittance change up to 300 mV and 0.07 μS for

the movement of a 4.88 μl plastic particle through

chan-nel Moreover, the voltage change of this sensor is linear

relation with the volume of investigated particle This

sen-sor also allows measuring velocity of particle inside fluidic

channel and resistivity of the conductive fluidic

* Chu Duc Trinh

trinhcd@vnu.edu.vn

1 Posts and Telecommunications Institute of Technology,

Hanoi, Vietnam

2 Institute of Applied Physics and Scientific Instrument,

Vietnam Academy of Science and Technology, Hanoi,

Vietnam

3 University of Science, Vietnam National University, Hanoi,

Vietnam

4 Nano and Energy Center, Vietnam National University,

Hanoi, Vietnam

5 University of Engineering and Technology, Vietnam National

University, Hanoi, Vietnam

The capacitively coupled contactless conductivity

technique, which was proposed by Fracassi da Silva et al

and Zemann et al independently in 1998 (Zemann et al

is applied in many areas and has brought an undeniable

structures consist of two electrodes separated by a gap

Based on the conductivity of liquid, the flow will transmit

the signal from an exciting electrode through the dielectric

of a pipe and bring the information of the liquid’s

2002)

impu-rities in tap water (electrical conductivity liquid) Hence,

this application can become an excellent method in solving

used in the research field of Analytical Chemistry for ion

concentration/conductivity detection in the capillary and

Another useful application of this technique is

estimat-ing the velocity of the conduct fluidic flow and

measure-ment of bubble velocity in gas–liquid two-phase flow in

millimeter-scale pipe, which is a fundamental problem

existing in many industries, such as chemical,

pharmaceuti-cal, petroleum, energy and power engineering (Wang et al

impurities and estimating their velocity in fluidic channel is

researched and developed by many research groups despite

There are several measurement methods that are

devel-oped to against these difficulties and limitations of the

excitation electrode and the pick-up electrode can be used

reso-nance effect to remove the influence of stray capacitance

use this resonator method to measure the conductivity and

in that case, the permittivity could not be recognized, for

example the case of full oil or the air inside pipe

This paper employs a differential amplifier to avoid the

above difficulties with a sensor system including three

U-shape electrodes on the top of a printed circuit board

(PCB) in order to reduce the parasitic capacitance and

increase the sensitivity not only in the conductivity liquid

struc-ture consists of two sensing electrodes and one exciting

electrode The electrodes are layout as a co-planar capaci-tive sensor This proposed structure and measurement setup can detect two-phase flow channel for both case of conduc-tive liquid and non-conducconduc-tive liquid

2 Designs and simulations 2.1 Block diagram design of a DC 4 D for fluidic sensing

flu-idic sensor based on three electrodes for detecting particles inside both conductive and non-conductive liquid channel

carrier sinusoidal signal to the center electrode as the exci-tation electrode The differential signal between the top and bottom electrodes is then amplified and demodulated

V in

V out LPF

Reservoir

AC Source

Differential amplifier

R 0

580 KHz sine wave

Particle Cylinder

R 0

Fig 1 Block diagram design of the DC4 D fluidic sensor

fluid flow

Excitation

AC

(b)

(a)

Fig 2 Design of a single C4D structure: a excitation and pick-up electrodes; b the equivalent circuit

for removing the carrier components The output signal

structures This proposed sensor could detect a particle like

plastic particle, air bubble, metal particle and so on inside

channel when it passes the electrodes

2.2 C 4 D structure

which consists of two electrodes A sinusoidal signal is

applied to left electrode as the excitation electrode and the

sensing is the right one Both electrodes sandwich the

flu-idic channel, which produces two wall capacitors through

depend on the thickness and permittivity of the dielectric

layer and the size of the electrode These two electrodes

pas-sageway along the fluidic channel The parasitic effect of

the stray capacitance is sometimes eliminated by taking the

The analytical form of the cell impedance, Z, defined by

the familiar general equation:

imaginary unit, respectively

When an alternating actuator voltage is applied to a

of its admittance, |Y|, which is expressed as:

be seen that in the case of high conductivity solution,

that the |Y| value mainly depends on the value of wall

(1)

Z = R1+ jX C=R S C

2

(2)

R21+ X C2

=

wω4G2S

G2S + C2

wω2

(3)

and stray capacitance at a specific frequency In order to increase the sensitivity of the measurement, the value of

to be at the same level in correlation with each other This

by making the distance between two electrodes become

longer, or increasing the Cw by increasing the length of

electrodes

2.3 DC 4 D based on three‑electrode configuration for fluidic sensor

pre-sented In this design, there are two pick up electrodes, which are outside electrodes The center electrode is

between the two pick up electrodes indicates changing inside the fluidic channel

and height of electrode, respectively The U-shape structure held tightly the fluidic channel along the sensor This pro-posed U-shape is convenient in order to setup and can be used for various size of fluidic channel A pipe with

inside the fluidic channel and amplifies the differential

L3 Fluidic pipe Copper Electrode

Cw1 R s 2

Output signal 2

Output signal 1

v

(a)

(b)

Cw

Cw1 2

Cw R s

Fig 3 a The DC4D based on three-electrode configuration; b the

equivalent circuit

(a) DC4D for non-conductive fluidic channel

For the non-conductivity or low-conductivity liquid

(σ ≤ 0.01 S/m), the resistance of the solution inside the

channel is high Therefore, the dominated factor in this

is modeled and simulated using Ansoft Maxwell software

Oil and fresh water are investigated fluid In the simulated

model, copper and plastic is electrode and pipe material,

respectively Plastic and tin particle is the investigated

object inside the fluidic channel System is assumed

work-ing in air ambience

electro-static field profile when a plastic particle appears at the

middle of the two electrodes in fresh water of fluidic

channel It can be seen that the distribution of the

elec-tric field is non-uniform from inside to outside of the

U-shape, even inside the plastic particle because plastic is

not conductive material The red areas present the higher

electric field magnitude and the blue areas present the

lower magnitude

In the case of a conductive object such as tin particle

moves in oil channel, the electrical field profile is shown in

electrode There is no electrical field inside the conductive

structure when a particle moves though the sensor The

capacitance changes up to 80 fF when a 4.18 μl plastic

particle moves in fresh water channel as a conductive

flu-idic Beside conductive fluidic, this work also simulates the

capacitance change in non-conductive fluidic when a metal

also shows capacitance change of 20 and 8 fF for tin

parti-cle and air bubble inside oil channel, respectively

capaci-tance change versus volume of a tin particle inside oil channel It shows that the relation is linear The capacitance changes up to 33.25 fF when tin volume gets value of about 6.61 μl

When the conductivity of the solution inside the channel

is high enough (σ > 0.1 S/m), the influence of capacitance inside the U-shape among electrodes in the total imped-ance is small, the capacitimped-ance in the equivalent equation

is mainly depended on the stray capacitance between each

unchanged parameter, therefore, the main sensing factor

is conductivity of liquid due to cross-section of the flu-idic flow change when particle moving Equivalent circuit

Fig 4 Simulated electrical field profile when a plastic particle inside

the fresh water channel Fig 5 Simulated electrical field profile when a tin particle inside oil

channel

-0.1 -0.08 -0.06 -0.04 -0.02 0 0.02 0.04 0.06 0.08

Particle position - mm

Plastic particle in fresh water Tin particle in oil

Air bubble in oil

Fig 6 Capacitance change versus particle position inside a single

C 4 D

of this configuration is shown in the Fig 9 In this work,

plastic particles flows inside different concentration NaCl

con-cerned about the wall which is contributed by the shell of

The investigated particle is assumed as the sphere with

inside the U-shape the channel can be divided into three

with an immerged plastic particle The wall capacitor can

the simulated result using Ansoft Maxwell when there is no particle inside the channel

sensor is given by:

where component of resistances and capacitances are cal-culated as:

plastic particle moves through electrode inside salt solution

(4)

Z solution = R3+ R s−C i

w w +A B

A = i − w[C2R2+ C1(R1+ R2)] − iC1C2R1R2w2

(5)

Z total= Z solution ZC0

Z solution + ZC0 + R0

R1= 1

l1

l2

l3

R s= 1

L

0

5

10

15

20

25

30

Particle Volume - µ l

Fig 7 Single C4 D capacitance change versus volume of tin particle

in oil channel

l 1 l 2 l 3

Particle

C 1 C 2 C 3 C w

R 1 R 2 R 3 R s

C 0

Solution Electrode

L

Fig 8 The equivalent circuit of the DC4 D for conductive fluidic

channel The circuit diagram of the suggested structure

C 1 R 1

C 2

R 2

R 3 R s C w

C 3

Output Signal

Fig 9 The equivalent circuit of the DC4 D fluidic sensor

1.8 1.9 2 2.1 2.2

2.3x 10 -5

Particle position (mm)

σ = 0.1 S/m

σ = 0.2 S/m

σ = 0.3 S/m

σ = 0.6 S/m

σ = 0.9 S/m

Fig 10 The single C4 D admittance change when a particle moves though electrode inside conductivity solution

decreases non-linearly while the particle moves in between

the conductivity of liquid σ decreases

3 Fabrication and measurement setups

Elec-trodes with U-shape are directly bonded on the PCB with

built-in differential amplifier and signal processing circuit

in order to decrease the parasitic component and common

noise The plastic pipe is then laid inside the U-shape

with 3 V magnitude and 580 kHz frequency is applied to

the excitation electrode The two pick-up electrodes voltage

is input signal of a differential amplifier, demodulation, and

low pass filter circuits The output voltage is then acquired

to a computer by using a NI card data acquisition

Plastic and tin particles with various sizes is mixed inside

fluidic chamber before pumping to the channel for

character-ized the output response of the sensor when a particle crosses

A T-connector, which is configured of two inlets of

investi-gated fluidic and air channel and one outlet, is employed for

adding an air bubble inside fluidic channel Volume of the air

bubble can be changed by monitoring the open time of the air inlet and pumping speed of the fluidic syringe

4 Measurement results and discussions 4.1 DC 4 D for non‑conductive fluidic channel

parti-cle crosses electrodes In this measurement, machine oil as

a non-conductive fluidic is used for characterized the

the two pick up electrodes Therefore, output voltage has

a combination of a positive and a negative voltage picks, which indicate that the investigated particle crosses the first

Table 1 Geometry parameters of the proposed DC4 D structure

Distance between two electrode (L2) 2

Fig 11 Measurement system setup of the DC4 D fluidic sensor

1.475 1.48 1.485 1.49 1.495 1.5 1.505 1.51 1.515 1.52 1.525

Time - s

0 0.5 1 1.5 2 2.5 3 3.5 1.4

1.45 1.5 1.55 1.6

Time - s

(a)

(b)

Fig 12 The DC4 D output voltage when a particle crosses electrodes

in machine oil channel: a 4.17 μl air bubble; and b 3.83 μl tin

parti-cle

shows output response of a 4.17 μl air bubble crosses the

sensor The output voltage changes up to 25 mV

Moreover, when a tin particle passes the electrodes the

output voltage comes with reverted order of the positive

and negative picks compare to the air bubble case thanks

voltage of the sensor for 3.83 μl tin particle Therefore, the

order of voltage peak is able to indicate the investigated

particle is metal or not

Beside detection of a particle inside fluidic channel, this

dividing the distance between the centers of the two single

from the measurement voltage The capacitances change

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

Time - s

-7

-6

-5

-4

-3

-2

-1

0

1

2

Time - s

(a)

(b)

Fig 13 The DC4 D capacitance change when a particle crosses

elec-trodes in machine oil channel: a 4.17 μl air bubble; and b 3.83 μl tin

particle

0 50 100 150 200 250 300

Volume - µ l

Measured data Linear fitted

0 5 10 15 20

Particle Volume - µ l

Measured data Linear fitted

(a)

(b)

Fig 14 The DC4 D output response versus tin particle volume in

machine oil channel: a output voltage versus volume; and b

capaci-tance change versus volume

1.2 1.3 1.4 1.5 1.6 1.7

Time - s

Particle in water Particle in NaCl

Fig 15 The DC4 D output voltage response when a plastic particle

crosses electrodes: a water channel; and b salt solution channel

Those measured values are almost met the simulated values

capaci-tance change is 1.5 and 6.3 fF for 4.17 μl air bubble and

3.83 μl tin particle case, respectively

The amplitude of the output voltage and the capacitance

change depend on the volume of the investigated particle

amplitude and capacitance change versus the tin particle

estimating the size of particle when particle material is

known

4.2 DC 4 D for conductive fluidic channel

when a plastic particle cross electrodes in salt solution

and water channel as the investigated conductive fluidic

The output voltage consists of both negative and positive

peaks thank to the differential circuit The output voltage

magnitude changes up to 300 mV and 50 mV when a 4.88

μl plastic particle cross electrodes in water channel and

plastic particle cross electrode in salt solution channel,

respectively

when a plastic particle crosses water and salt solution The

result is approximately matching with the calculated value

out-put voltage amplitude versus volume of plastic particle in

0.9 % salt solution and water It shows that the relations are

linear and output voltage in water channel is about 5 times

larger than the 0.9 % salt solution case

shows the relation between output voltage amplitude and investigated plastic particle volume in salt solution It shows that the sensitivity of the sensor reduces when salt concentration in solution is increased The conductivity

-8

-6

-4

-2

0

2

4

6

8x 10

-8

Time - s

Particle in water Particle in NaCl

Fig 16 The DC4 D admittance change when a plastic particle crosses

electrodes: a water channel; and b salt solution channel

Table 2 The DC4 D output voltage amplitude versus particle volume

in salt solution and water Plastic particle volume (µl) Output voltage amplitude (mV)

Salt solution 0.9 % Fresh water

0 500 1000 1500

Volume - µl

salt solution Linear fitted water

Fig 17 The DC4 D output voltage amplitude versus particle volume

in salt solution and water

Table 3 The DC4 D output voltage amplitude versus particle volume

in various concentration of salt solution Plastic particle volume (µl) Output voltage amplitude (mV)

0.75 (%) 0.9 (%) 1.5 (%) 3 (%)

of the fluidic can be estimated by using this configuration

when volume of the particle is known

conductive fluidic resistivity when a 9.37 µl particle moves

through the sensor The relation is linear with sensitivity of

used for measurement the fluid sensitivity when volume of

particle is known In practice, a controlled air bubble pump

can be added before sensor inlet for the fluidic sensitivity

detector

velocity detection The two voltage picks are corresponded

structure, respectively Therefore, particle velocity can be extracted from distance AB divided by the time between the two voltage picks

5 Conclusions

This paper presents a design, fabrication, and characterized

a PCB where the electrodes are directly connected to the differential amplifier and signal processing circuit in order

to reduce the parasitic component and common noise The

and non-conductive fluidic channel Air bubbles and tin particles are pumped through electrodes for characterizing non-conductive fluidic case Plastic particles with various sizes are employed in the conductive fluidic configuration The measured results indicated the linear relation between output voltage and volume of the particle Beside particle detection, this sensor allows measuring velocity of the par-ticle inside fluidic channel thanks to distance and travel

detection in petroleum industry, particle in fluidic chan-nel detection and living cell in micro vessel detection and counting for biomedical applications

Acknowledgments This research is funded by Vietnam National

Foundation for Science and Technology Development (NAFOSTED) under grant number 103.01-2011.59.

0

50

100

150

200

250

300

Particle volume - µ l

salt solution 0.75%

salt solution 0.9%

salt solution 1.5%

salt solution 3%

Linear fitted

Fig 18 The DC4 D output voltage amplitude versus particle volume

in various concentration of salt solution

0 0.1 0.2 0.3 0.4 0.5 0.6

0

50

100

150

200

250

Resistivity - Ω m

Measured data Linear fitted

Fig 19 The DC4 D output voltage change’s amplitude versus

conduc-tive fluidic resistivity

1.2 1.3 1.4 1.5 1.6 1.7 1.8

Time - s

A

B

14 mm

1.45 (s) 2.02 (s)

Fig 20 Velocity of investigated particle inside fluidic channel

calcu-lation

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