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An improved capacitively coupled contactless conductivity detection sensor for industrial applications
Accepted Manuscript Title: An improved capacitively coupled contactless conductivity detection sensor for industrial applications Author: Haifeng Ji Yingchao Lyu Baoliang Wang Zhiyao Huang Haiqing Li Yong Yan PII: DOI: Reference:
S0924-4247(15)30155-2 http://dx.doi.org/doi:10.1016/j.sna.2015.09.038 SNA 9333
To appear in:
Sensors and Actuators A
Received date: Accepted date:
19-8-2015 28-9-2015
Please cite this article as: Haifeng Ji, Yingchao Lyu, Baoliang Wang, Zhiyao Huang, Haiqing Li, Yong Yan, An improved capacitively coupled contactless conductivity detection sensor for industrial applications, Sensors and Actuators: A Physical http://dx.doi.org/10.1016/j.sna.2015.09.038 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
An improved capacitively coupled contactless conductivity detection sensor for industrial applications Haifeng Jia, Yingchao Lyua, Baoliang Wanga, Zhiyao Huanga, *, Haiqing Lia, Yong Yanb, ** a: State Key Laboratory of Industrial Control Technology, College of Control Science and Engineering, Zhejiang University, Hangzhou, 310027, PR China b: School of Engineering and Digital Arts, University of Kent, Canterbury, Kent CT2 7NT, United Kingdom * Corresponding author at: Zhejiang University, College of Control Science and Engineering, Hangzhou, 310027, PR China. Tel.: +86 571 87952145; fax: +86 571 87951219. ** Corresponding author at: University of Kent, School of Engineering and Digital Arts, Canterbury, Kent CT2 7NT, United Kingdom. Tel.: +44 1227 823015; fax: +44 1227 456084. E-mail addresses: [email protected] (Z. Huang), [email protected] (Y. Yan). Highlights 1. This work is the further research of Sensors and Actuators A 213(2014)1-8. 2. An improved C4D sensor has been developed. 3. The improved C4D sensor has the advantage of the monotonic input-output characteristic, and is convenient for industrial applications. Abstract: To bring the potential power of the capacitively coupled contactless conductivity detection (C4D) technique to full play and extend the application fields of C4D, our previous work has developed a C4D sensor which is suitable for conductivity detection of electrolyte solution in industrial fields (Sensors and Actuators A 213(2014)1-8). With the introduction of series resonance principle, the C4D sensor can overcome the influence of coupling capacitances on conductivity measurement. By a specific design of shield configuration, the C4D sensor can overcome the influence of stray capacitances and the environment interference in industrial fields. Although the measurement performance of the C4D sensor is satisfactory and it is suitable for the conductivity detection in industrial fields, the input-output characteristic of the C4D sensor is non-monotonic, which may limit the practical applications of the C4D sensor. To overcome this drawback, an improved C4D sensor for conductivity detection in industrial fields has been developed. The basic conductivity measurement method (the series resonance principle) and the shield configuration of the improved C4D sensor are the same as those of the C4D sensor in our previous work. However, the measurement circuit of the detection path of the improved C4D sensor has been optimized. The oneinductor design has been replaced by a two-inductor design. With this optimization, the input-output characteristic of the improved C4D sensor is theoretically monotonic. Conductivity measurement experiments were carried out on four pipes with inner diameters of 1.8 mm, 3.3 mm, 5.0 mm and 7.6 mm, respectively. The experimental results indicate that the development of the improved C4D sensor is successful. In comparison with the previous C4D sensor, the measurement accuracy of the improved C4D sensor is comparable. But, the improved C4 D sensor has the advantages of the monotonic input-output characteristic and the convenience for practical conductivity detection in industrial fields. Meanwhile, it is found that the input-output characteristic
of the improved C4D sensor is nonlinear and the sensitivity of the improved C4D sensor decreases with the increase of conductivity. Keywords: capacitively coupled contactless conductivity detection (C4D), contactless conductivity detection (CCD), fluid, conductivity, sensor
1. Introduction Capacitively coupled contactless conductivity detection (C4D) technique is proposed as a contactless method for conductivity detection [1-3]. Unlike the classic conductivity detection method, because the electrodes of a C4D sensor are not directly in contact with the fluid, the polarization effect and the electrochemical reaction can be avoided [1-8]. Due to the advantages, the C4D technique has received great attentions from chemists and engineers since it was proposed [1-22]. Unfortunately, although great achievements and technical progresses have been obtained [122], up to date, the C4D technique is mainly studied/applied in the research field of Analytical Chemistry for ion concentration/conductivity detection in capillary and the existing C4D sensors are mainly suitable for ideal laboratory environment [4-20]. Few research works concerning the practical applications of C4D technique in industrial fields have been published [4-20]. A C4D sensor, which is suitable for conductivity detection of electrolyte solution in industrial fields, has been developed in our previous work (Sensors and Actuators A 213(2014)1-8) [20]. Fig. 1 shows the construction of the C4D sensor developed in our previous work. The basic conductivity measurement principle of the sensor is on the basis of series resonance principle [18-20]. As shown in Fig. 1, an inductor module is introduced to overcome the unfavorable influence of the coupling capacitances by the series resonance principle and hence to implement the conductivity detection. And, a specially designed shield configuration is introduced into the sensor to overcome the negative influences of the stray capacitances [3, 4, 11-14, 17-22] and the interference in industrial environment on the measurement results [20, 23, 24]. When an AC voltage signal generated by the AC excitation source is applied to the excitation electrode, an AC current io can be obtained from the detection electrode. By the operation of the signal detection unit, the AC current io can be transformed to a voltage signal uo. Finally, the conductivity measurement can be implemented via the obtained voltage signal uo. (The detailed information concerning the C4D sensor is available in [20].) Fig. 2 shows the equivalent circuit of the C4D sensor developed in our previous work [20]. L is the inductor module. C1 and C2 are coupling capacitances formed by the electrolyte solution and two electrodes through the insulating pipe. R is the equivalent resistance of the measured solution between the two electrodes. R1 is the equivalent resistance of the electrolyte solution between the excitation electrode and the upstream grounded metal flange and R2 is the equivalent resistance of the electrolyte solution between the detection electrode and the downstream grounded metal flanges. Cd1 and Cd2 are stray capacitances formed by direct coupling between the two electrodes and the shield configuration through the air. Rf is the feedback resistance. The influence of Cd1 and Cd2 on conductivity detection can be neglected. C1=C2=C and R1=R2, R1=R2=kR, where k is a proportional coefficient. Thus, the overall impedance of the detection path can be described as (The detailed analysis is available in [20]): 2 +1 1 +1 1 Z= = + 2 − + 2 − (1) 2 2 Where, f is the excitation frequency of the AC excitation source. As mentioned above, the inductor module L is introduced to overcome the unfavorable influence of the coupling capacitances by the series resonance principle and hence to implement the conductivity detection. The C4D sensor works at resonance. The excitation frequency f is equal to resonant frequency f0. According to the series resonance principle, at the resonant frequency f0, the reactance of the overall impedance is zero. The resonant frequency f0 (i.e. the excitation frequency f) can be determined as: =
1 2
2
(2)
The overall impedance at resonance ZR is: (2 + 1)( = +
)
(3)
After the operation of the current-voltage inverter, the output voltage signal uo is: =
=
(4) ) ⁄ + (2 + 1)( Let Gm=1/R, Gm is the conductance of the measured solution. Eq. (4) can be rewritten as: =
(5) ⁄ (2 + 1)( ) + Eq. (5) describes the relationship between the output voltage signal uo and the conductance of the measured solution Gm. According to Eq. (5), the measurement values of Gm can be obtained by uo and then the conductivity measurement of the measured solution can be implemented. Our previous work has verified that the C4D sensor is suitable for the conductivity detection in industrial fields [20]. It can overcome the influence of stray capacitances and the environment interference in industrial fields. The conductivity measurement accuracy of the C4D sensor is also satisfactory. However, despite of its good performance, the C4D sensor still has a drawback. Its input-output characteristic, i.e. the relationship between the output signal uo and the input Gm, is non-monotonic. This phenomenon can be explained by Eq. (5). Fig. 3 illustrates the function curve of Eq. (5). This relationship between Gm and uo is non-monotonic in nature. The non-monotonic input-output characteristic of the C4D sensor is inconvenient for the practical conductivity detection and more or less limits the practical application of the C4D sensor in the industrial fields. So, more research work should be undertaken. The aim of this work is to seek an effective way to overcome this drawback and to develop an improved C4D sensor for industrial application (its input-output characteristic is monotonic).
2. Design of the improved C4D sensor for industrial applications Fig. 4 shows the construction of the improved C4D sensor for industrial application. Compared with Fig. 1, obviously, the basic conductivity measurement method (the series resonance principle), the shield configuration and the signal detection unit of the improved C4D sensor are the same as those of the C4D sensor in our previous work [20], because our previous work has verified their effectiveness. Meanwhile, it is clear that the measurement circuit of the detection path of the improved C4D sensor has been optimized. The one-inductor design of the C4D sensor developed in our previous work has been replaced by two-inductor design. Fig. 5 shows the equivalent circuit of the improved C4D sensor for industrial application. L1 is the Inductor 1, which is connected between the AC excitation source and the excitation electrode. L2 is the Inductor 2, which is connected between the detection electrode and the signal detection unit. According to our previous work [20], the influence of Cd1 and Cd2 on conductivity detection can be neglected. The overall impedance of the detection AC path Z is: + + 1 1 Z= = − 2 − 2 − 2 2 + 1 + 1 + 2 − + 2 − (6) 2 2 According to the series resonance principle, when the circuit is at resonance, the reactance of the overall impedance Z should be zero, i.e., + 1 + 1 2 − + 2 − = 0 (7) 2 2 From Eq. (7), it is obvious that the reactance of the overall impedance Z can be zero, if the following Eq. (8) and Eq. (9) are satisfied: 1 2 − = 0 (8) 2 1 2 − = 0 (9) 2 For one circuit, there is only one excitation frequency f. Thus, according to Eq. (8) and Eq. (9),
at resonance, the resonant frequency of the circuit f0 should be determined by: =
=
1 2
1
=
1 2
1
(10)
Obviously, = is the precondition of Eq. (10). If = and the resonance frequency is determined by Eq. (10), the reactance of the overall impedance Z is zero. Further, according to Eq. (6) and Eq. (8) to Eq. (10), at resonance, the overall impedance of the detection AC path Z is determined as: Z = (11) That means if the resonant frequency f0 is determined by Eq. (10), at resonance, the overall impedance Z only consists of the resistance element R. Based on the above discussion, in this work, we consider that C1=C2=C, R1=R2, R1=R2=kR, L1=L2=L. Thus, according to Eq. (10), the resonance frequency of the circuit f0 is: =
1 2
1
(12)
The overall impedance at resonance in this case is ZR: = (13) After the operation of the current-voltage inverter, the output voltage signal uo is: =
=
(14)
Using the conductance of the measured solution between two electrodes Gm, Gm=1/R, Eq. (14) can be rewritten as: = (15) Eq. (15) indicates that the relationship between the input Gm and the output signal uo, i.e. the input-output characteristic of the improved C4D sensor, is theoretically monotonic by the measurement circuit optimization of the detection path (i.e. using two-inductor design to replace the previous one-inductor design). The two-inductor design can overcome the drawback of the C4D sensor in our previous work.
3. Experimental results and discussion To assess the performance of the improved C4D sensor, conductivity detection experiments were carried out. The experimental setup is the same as that of our previous work [20], except the tested C4D sensor, as shown in Fig. 6. Four improved C4D sensors with different inner diameters (1.8 mm, 3.3 mm, 5.0 mm and 7.6 mm, respectively) were tested. Table 1 lists the parameters of the four improved C4D sensors. The external shield is made of polished stainless steel pipe. The outer diameter and the thickness of the external shield are 63.6 mm and 2.2 mm respectively. The distance between the upstream flange and the downstream flange is 360.0 mm. The experimental electrolyte solution is KCl solution. A commercial contact conductivity meter (FE30, Mettler Toledo Inc., 0.00 μS/cm–199.9 mS/cm, ±0.5% F.S.) is used to obtain the reference conductivity values. The relative error is used to analyze the conductivity detection results, which is defined as: − = × 100% (16) Where, ρm is the measurement conductivity value obtained by the improved C4D sensor and ρf is the reference conductivity value obtained by the commercial contact conductivity meter. Fig. 7 shows the input-output characteristics of the four improved C4D sensors with different inner diameters. It is clear that the input-output characteristic of the improved C4D sensor is monotonic. Fig. 8 shows the relative errors of conductivity detections. The experimental results indicate that the measurement accuracies of the four improved C4D sensors are satisfactory. The maximum relative errors of the four improved C4D sensors are 3.3%, 3.6%, 3.2%, 3.6%, respectively. Table 2 lists the comparison of the maximum relative errors of the improved C4D sensors and those of the earlier C4D sensors in our previous work [20]. The measurement accuracy
of the improved C4D sensor is comparable to the earlier version. These experimental results verify that the optimization of the improved C4D sensor is effective and can successfully overcome the drawback (its input-output characteristic is non-monotonic) of the C4D sensor in our previous work.
In addition, a supplementary research result has been obtained. As mentioned in Section 2, = is the precondition of Eq. (10). That means if ≠ and ≠ but = , it would cause no influence on the determination of the resonance frequency f0. This supplementary research result is of benefit to the manufacturing and the design of the C4D sensor. As we known, most conventional C4D sensors are working at such condition that = (by setting the two electrodes with same lengths and manufacturing the two electrodes precisely). In practice, the two electrodes of most C4D sensors are more or less different from each other. The small difference between the two electrodes means ≠ and may cause unfavorable influence on conductivity detection. However, in this work, the unfavorable influence caused by ≠ can be compensated by adjusting the values of L1 and L2, i.e. the improved C4D sensor has the advantage of lower manufacturing requirement. Meanwhile, the precondition ( = ) may provide a useful idea/approach for the design of C4D sensor, i.e. it may provide a possible way to design a special style C4D sensor with two different-length electrodes. It is necessary to indicate that there are still two issues should be noticed although the improved C4D sensor has those advantages listed above. One is the nonlinearity of the input-output characteristic of the improved C4D sensor. As mentioned in Section 2, Eq. (15) clearly indicated that the input-output characteristic of the improved C4D sensor should be linear theoretically. But the experimental results show that the input-output characteristic is monotonic and nonlinear, as shown in Fig.7. The other is that the sensitivity of the improved C4D sensor decreases with the increase of conductivity. The sensitivity of the improved C4D sensor in lower conductivity is obviously greater than that in higher conductivity, as shown in Fig. 9 (the sensitivity is defined as Δuo/ΔGm.). The improved C4D sensor is more suitable for the measurement of the relatively lower conductivity. These two issues are in accordance with other researchers' study results (obtained by capillaryscale or less C4D sensor, and the maximum conductivity is usually less than 20 mS/cm) [4, 11-14, 18, 25-29]. The nonlinearity feature of the C4D sensor has already been indicated by other researchers (such as Gas et al [4], Kuban et al. [13, 14], do Lago et al. [25], Tuma et al. [26] and Mark et al. [27]) and been investigated by our research group [18]. The experimental results obtained in this work further verify that the nonlinearity feature does exist in a larger-scale C4D sensor (the inner diameter is up to 7.6 mm) with a relatively wider measurement range of conductivity (up to 192.3 mS/cm). Maybe, the nonlinearity of the input-output characteristic is a limitation or an inherent feature of the C4D sensor [4, 13, 14, 25-27]. The phenomenon of sensitivity loss at high conductivity also has been reported by do Lago et al. [11, 25], Kuban et al. [13], Tuma et al. [26, 28], Shen et al. [29] and our research group [18]. The research results in this work further verify that the phenomenon of sensitivity loss at high conductivity also exists in a larger-scale C4D sensor. Meanwhile, it is found that the improved C4D sensor developed in this work is more suitable for the measurement of the relatively lower conductivity.
4. Conclusion In this work, an improved C4D sensor has been developed to overcome the drawback (the nonmonotonic input-output characteristic) of the C4D sensor developed in our previous work [20]. The research focused on the optimization of the measurement circuit of the detection path. Two-inductor design has been used to replace the previous one-inductor design. This optimization has led to the monotonic input-output characteristic. Four improved C4D sensors with different inner diameters (1.8 mm, 3.3 mm, 5.0 mm and 7.6 mm, respectively) have been evaluated experimentally. The results presented indicate that the twoinductor design has successfully overcome the drawback of the previous C4D sensor. The improved C4D sensor displays a monotonic input-output characteristic and its measurement accuracy is satisfactory. With reference of a commercial contact conductivity meter, the maximum relative
errors of the four improved C4D sensors are all less than 4.0%. The results also indicate that the input-output characteristic of the improved C4D sensor is nonlinear and the sensitivity of the improved C4D sensor decreases with the increase of conductivity. The improved C4D sensor is more suitable for the measurement of the relatively lower conductivity.
Acknowledgment This research work is supported by National Natural Science Foundation of China (No. 51476139).
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Photographs and biographies of all authors 1. Haifeng Ji
Haifeng Ji was born on 26 October 1973 in China. He received his master degree from Shandong University of Technology in 1999 and his Ph.D. degree from Department of Control Science and Engineering, Zhejiang University, in 2002, respectively. Now he is working in Zhejiang University as an Associate Professor. His interesting research includes measurement techniques, automation equipment, information processing of complex process system, multiphase flow measurement in mini-/micro-channel.
2. Yingchao Lyu
Yingchao Lyu was born in Hangzhou, China, on January 18, 1990. She received the B.Sc. degree from Zhejiang University, Hangzhou, China, in 2012. She is currently working toward the Ph.D. degree with the College of Control Science and Engineering, Zhejiang University. Her research interests include automation instrumentation, signal processing and multiphase flow measurement.
3. Baoliang Wang
Baoliang Wang was born in Zibo, China, on July 11, 1970. He received the B.Sc. and M.Sc. degrees from Shandong University of Technology, Jinan, China, in 1992 and 1995, respectively. In 1998, he received the Ph.D. degree from Zhejiang University. From 1998 to 2001, he was a Lecturer with the Department of Control Science and Engineering, Zhejiang University. From 2002 to 2003, he was a research associate at the City University of Hong Kong. From 2002 to 2013, he was an Associate Professor with the Department of Control Science and Engineering, Zhejiang University. Currently, he is a Professor with the College of Control Science and Engineering, Zhejiang University. His research interests include process tomography, motion control system, microprocessor system.
4. Zhiyao Huang
Zhiyao Huang was born in Hangzhou, China, on October 22, 1968. He received the B.Sc., M.Sc., and Ph.D. degrees from Zhejiang University, Hangzhou, China, in 1990, 1993, and 1995, respectively. From June 1995 to August 1997, he was a Lecturer with the Department of Chemical Engineering, Zhejiang University. In September 1997, he became an Associate Professor with the Department of Control Science and Engineering, Zhejiang University, and in 2001, he was
appointed a Professor. Currently, he is a Professor with the College of Control Science and Engineering, Zhejiang University. He is also a permanent staff of the State Key Laboratory of Industrial Control Techniques. His current interests include automation instrumentation and multiphase flow measurement.
5. Haiqing Li
Haiqing Li was born in Qingdao, China, on November 16, 1934. She received the B.Sc. degree from Dalian Science and Engineering University, Dalian, China, in 1956. In 1988, she was appointed a Professor with the Department of Chemical Engineering, Zhejiang University. She had been a Professor with the Department of Control Science and Engineering of Zhejiang University since 1997. Her research interests include automation instrumentation and multiphase flow measurement technology.
6. Yong Yan
Yong Yan received the B.Eng. and M.Sc. degrees from Tsinghua University, Beijing, China in 1985 and 1988, respectively, and the Ph.D. degree from the University of Teesside, Middlesbrough, U.K., in 1992. He is currently a Professor of Electronic Instrumentation, the Head of Instrumentation, Control and Embedded Systems Research Group, and the Director of Research at the School of Engineering and Digital Arts, the University of Kent, Canterbury, U.K. His research interests include sensors, instrumentation, measurement, condition monitoring, digital signal processing, digital image processing and applications of artificial intelligence.
Fig. 1. Construction of the C4D sensor developed in our previous work [20].
Rf
io
i1 Cd 1
i2
uo A1
Cd 2
Fig. 2. Equivalent circuit of the C4D sensor developed in our previous work [20].
Fig. 3. Function curve of the relationship between uo and Gm [20].
Fig. 4. Construction of the improved C4D sensor for industrial application.
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io i1
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Fig. 5. Equivalent circuit of the improved C4D sensor for industrial application.
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40
30
20
20
10
10
0
0
1
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4 5 6 Conductance Gm (mS)
7
8
0
9
0
(c) 5.0mm i.d.
5
10 15 Conductance Gm (mS)
20
(d) 7.6mm i.d.
Fig. 9. Typical sensitivity plots of the four improved C4D sensors with different diameters.
Table 1 The parameters of the four improved C4D sensors The improved C4D The length of sensor the electrodes(mm) 1.8 mm i.d. 20 (3.1mm o.d.) 3.3 mm i.d. 20 (4.9mm o.d.) 5.0 mm i.d. 20 (7.5 mm o.d.) 7.6 mm i.d. 20 (10.5 mm o.d.)
The length of the gap(mm)
The excitation frequency(kHz)
40
206.8
40
200.6
40
176.6
40
171.3
Table 2 The comparison of the maximum relative errors of conductivity detection Pipe i.d. 1.8 mm 3.3 mm 5.0 mm 7.6 mm The C4D sensors in [20] 3.7% 3.4% 3.0% 4.2% The improved C4D sensors 3.3% 3.6% 3.2% 3.6%
25
S0924-4247(15)30155-2 http://dx.doi.org/doi:10.1016/j.sna.2015.09.038 SNA 9333
To appear in:
Sensors and Actuators A
Received date: Accepted date:
19-8-2015 28-9-2015
Please cite this article as: Haifeng Ji, Yingchao Lyu, Baoliang Wang, Zhiyao Huang, Haiqing Li, Yong Yan, An improved capacitively coupled contactless conductivity detection sensor for industrial applications, Sensors and Actuators: A Physical http://dx.doi.org/10.1016/j.sna.2015.09.038 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
An improved capacitively coupled contactless conductivity detection sensor for industrial applications Haifeng Jia, Yingchao Lyua, Baoliang Wanga, Zhiyao Huanga, *, Haiqing Lia, Yong Yanb, ** a: State Key Laboratory of Industrial Control Technology, College of Control Science and Engineering, Zhejiang University, Hangzhou, 310027, PR China b: School of Engineering and Digital Arts, University of Kent, Canterbury, Kent CT2 7NT, United Kingdom * Corresponding author at: Zhejiang University, College of Control Science and Engineering, Hangzhou, 310027, PR China. Tel.: +86 571 87952145; fax: +86 571 87951219. ** Corresponding author at: University of Kent, School of Engineering and Digital Arts, Canterbury, Kent CT2 7NT, United Kingdom. Tel.: +44 1227 823015; fax: +44 1227 456084. E-mail addresses: [email protected] (Z. Huang), [email protected] (Y. Yan). Highlights 1. This work is the further research of Sensors and Actuators A 213(2014)1-8. 2. An improved C4D sensor has been developed. 3. The improved C4D sensor has the advantage of the monotonic input-output characteristic, and is convenient for industrial applications. Abstract: To bring the potential power of the capacitively coupled contactless conductivity detection (C4D) technique to full play and extend the application fields of C4D, our previous work has developed a C4D sensor which is suitable for conductivity detection of electrolyte solution in industrial fields (Sensors and Actuators A 213(2014)1-8). With the introduction of series resonance principle, the C4D sensor can overcome the influence of coupling capacitances on conductivity measurement. By a specific design of shield configuration, the C4D sensor can overcome the influence of stray capacitances and the environment interference in industrial fields. Although the measurement performance of the C4D sensor is satisfactory and it is suitable for the conductivity detection in industrial fields, the input-output characteristic of the C4D sensor is non-monotonic, which may limit the practical applications of the C4D sensor. To overcome this drawback, an improved C4D sensor for conductivity detection in industrial fields has been developed. The basic conductivity measurement method (the series resonance principle) and the shield configuration of the improved C4D sensor are the same as those of the C4D sensor in our previous work. However, the measurement circuit of the detection path of the improved C4D sensor has been optimized. The oneinductor design has been replaced by a two-inductor design. With this optimization, the input-output characteristic of the improved C4D sensor is theoretically monotonic. Conductivity measurement experiments were carried out on four pipes with inner diameters of 1.8 mm, 3.3 mm, 5.0 mm and 7.6 mm, respectively. The experimental results indicate that the development of the improved C4D sensor is successful. In comparison with the previous C4D sensor, the measurement accuracy of the improved C4D sensor is comparable. But, the improved C4 D sensor has the advantages of the monotonic input-output characteristic and the convenience for practical conductivity detection in industrial fields. Meanwhile, it is found that the input-output characteristic
of the improved C4D sensor is nonlinear and the sensitivity of the improved C4D sensor decreases with the increase of conductivity. Keywords: capacitively coupled contactless conductivity detection (C4D), contactless conductivity detection (CCD), fluid, conductivity, sensor
1. Introduction Capacitively coupled contactless conductivity detection (C4D) technique is proposed as a contactless method for conductivity detection [1-3]. Unlike the classic conductivity detection method, because the electrodes of a C4D sensor are not directly in contact with the fluid, the polarization effect and the electrochemical reaction can be avoided [1-8]. Due to the advantages, the C4D technique has received great attentions from chemists and engineers since it was proposed [1-22]. Unfortunately, although great achievements and technical progresses have been obtained [122], up to date, the C4D technique is mainly studied/applied in the research field of Analytical Chemistry for ion concentration/conductivity detection in capillary and the existing C4D sensors are mainly suitable for ideal laboratory environment [4-20]. Few research works concerning the practical applications of C4D technique in industrial fields have been published [4-20]. A C4D sensor, which is suitable for conductivity detection of electrolyte solution in industrial fields, has been developed in our previous work (Sensors and Actuators A 213(2014)1-8) [20]. Fig. 1 shows the construction of the C4D sensor developed in our previous work. The basic conductivity measurement principle of the sensor is on the basis of series resonance principle [18-20]. As shown in Fig. 1, an inductor module is introduced to overcome the unfavorable influence of the coupling capacitances by the series resonance principle and hence to implement the conductivity detection. And, a specially designed shield configuration is introduced into the sensor to overcome the negative influences of the stray capacitances [3, 4, 11-14, 17-22] and the interference in industrial environment on the measurement results [20, 23, 24]. When an AC voltage signal generated by the AC excitation source is applied to the excitation electrode, an AC current io can be obtained from the detection electrode. By the operation of the signal detection unit, the AC current io can be transformed to a voltage signal uo. Finally, the conductivity measurement can be implemented via the obtained voltage signal uo. (The detailed information concerning the C4D sensor is available in [20].) Fig. 2 shows the equivalent circuit of the C4D sensor developed in our previous work [20]. L is the inductor module. C1 and C2 are coupling capacitances formed by the electrolyte solution and two electrodes through the insulating pipe. R is the equivalent resistance of the measured solution between the two electrodes. R1 is the equivalent resistance of the electrolyte solution between the excitation electrode and the upstream grounded metal flange and R2 is the equivalent resistance of the electrolyte solution between the detection electrode and the downstream grounded metal flanges. Cd1 and Cd2 are stray capacitances formed by direct coupling between the two electrodes and the shield configuration through the air. Rf is the feedback resistance. The influence of Cd1 and Cd2 on conductivity detection can be neglected. C1=C2=C and R1=R2, R1=R2=kR, where k is a proportional coefficient. Thus, the overall impedance of the detection path can be described as (The detailed analysis is available in [20]): 2 +1 1 +1 1 Z= = + 2 − + 2 − (1) 2 2 Where, f is the excitation frequency of the AC excitation source. As mentioned above, the inductor module L is introduced to overcome the unfavorable influence of the coupling capacitances by the series resonance principle and hence to implement the conductivity detection. The C4D sensor works at resonance. The excitation frequency f is equal to resonant frequency f0. According to the series resonance principle, at the resonant frequency f0, the reactance of the overall impedance is zero. The resonant frequency f0 (i.e. the excitation frequency f) can be determined as: =
1 2
2
(2)
The overall impedance at resonance ZR is: (2 + 1)( = +
)
(3)
After the operation of the current-voltage inverter, the output voltage signal uo is: =
=
(4) ) ⁄ + (2 + 1)( Let Gm=1/R, Gm is the conductance of the measured solution. Eq. (4) can be rewritten as: =
(5) ⁄ (2 + 1)( ) + Eq. (5) describes the relationship between the output voltage signal uo and the conductance of the measured solution Gm. According to Eq. (5), the measurement values of Gm can be obtained by uo and then the conductivity measurement of the measured solution can be implemented. Our previous work has verified that the C4D sensor is suitable for the conductivity detection in industrial fields [20]. It can overcome the influence of stray capacitances and the environment interference in industrial fields. The conductivity measurement accuracy of the C4D sensor is also satisfactory. However, despite of its good performance, the C4D sensor still has a drawback. Its input-output characteristic, i.e. the relationship between the output signal uo and the input Gm, is non-monotonic. This phenomenon can be explained by Eq. (5). Fig. 3 illustrates the function curve of Eq. (5). This relationship between Gm and uo is non-monotonic in nature. The non-monotonic input-output characteristic of the C4D sensor is inconvenient for the practical conductivity detection and more or less limits the practical application of the C4D sensor in the industrial fields. So, more research work should be undertaken. The aim of this work is to seek an effective way to overcome this drawback and to develop an improved C4D sensor for industrial application (its input-output characteristic is monotonic).
2. Design of the improved C4D sensor for industrial applications Fig. 4 shows the construction of the improved C4D sensor for industrial application. Compared with Fig. 1, obviously, the basic conductivity measurement method (the series resonance principle), the shield configuration and the signal detection unit of the improved C4D sensor are the same as those of the C4D sensor in our previous work [20], because our previous work has verified their effectiveness. Meanwhile, it is clear that the measurement circuit of the detection path of the improved C4D sensor has been optimized. The one-inductor design of the C4D sensor developed in our previous work has been replaced by two-inductor design. Fig. 5 shows the equivalent circuit of the improved C4D sensor for industrial application. L1 is the Inductor 1, which is connected between the AC excitation source and the excitation electrode. L2 is the Inductor 2, which is connected between the detection electrode and the signal detection unit. According to our previous work [20], the influence of Cd1 and Cd2 on conductivity detection can be neglected. The overall impedance of the detection AC path Z is: + + 1 1 Z= = − 2 − 2 − 2 2 + 1 + 1 + 2 − + 2 − (6) 2 2 According to the series resonance principle, when the circuit is at resonance, the reactance of the overall impedance Z should be zero, i.e., + 1 + 1 2 − + 2 − = 0 (7) 2 2 From Eq. (7), it is obvious that the reactance of the overall impedance Z can be zero, if the following Eq. (8) and Eq. (9) are satisfied: 1 2 − = 0 (8) 2 1 2 − = 0 (9) 2 For one circuit, there is only one excitation frequency f. Thus, according to Eq. (8) and Eq. (9),
at resonance, the resonant frequency of the circuit f0 should be determined by: =
=
1 2
1
=
1 2
1
(10)
Obviously, = is the precondition of Eq. (10). If = and the resonance frequency is determined by Eq. (10), the reactance of the overall impedance Z is zero. Further, according to Eq. (6) and Eq. (8) to Eq. (10), at resonance, the overall impedance of the detection AC path Z is determined as: Z = (11) That means if the resonant frequency f0 is determined by Eq. (10), at resonance, the overall impedance Z only consists of the resistance element R. Based on the above discussion, in this work, we consider that C1=C2=C, R1=R2, R1=R2=kR, L1=L2=L. Thus, according to Eq. (10), the resonance frequency of the circuit f0 is: =
1 2
1
(12)
The overall impedance at resonance in this case is ZR: = (13) After the operation of the current-voltage inverter, the output voltage signal uo is: =
=
(14)
Using the conductance of the measured solution between two electrodes Gm, Gm=1/R, Eq. (14) can be rewritten as: = (15) Eq. (15) indicates that the relationship between the input Gm and the output signal uo, i.e. the input-output characteristic of the improved C4D sensor, is theoretically monotonic by the measurement circuit optimization of the detection path (i.e. using two-inductor design to replace the previous one-inductor design). The two-inductor design can overcome the drawback of the C4D sensor in our previous work.
3. Experimental results and discussion To assess the performance of the improved C4D sensor, conductivity detection experiments were carried out. The experimental setup is the same as that of our previous work [20], except the tested C4D sensor, as shown in Fig. 6. Four improved C4D sensors with different inner diameters (1.8 mm, 3.3 mm, 5.0 mm and 7.6 mm, respectively) were tested. Table 1 lists the parameters of the four improved C4D sensors. The external shield is made of polished stainless steel pipe. The outer diameter and the thickness of the external shield are 63.6 mm and 2.2 mm respectively. The distance between the upstream flange and the downstream flange is 360.0 mm. The experimental electrolyte solution is KCl solution. A commercial contact conductivity meter (FE30, Mettler Toledo Inc., 0.00 μS/cm–199.9 mS/cm, ±0.5% F.S.) is used to obtain the reference conductivity values. The relative error is used to analyze the conductivity detection results, which is defined as: − = × 100% (16) Where, ρm is the measurement conductivity value obtained by the improved C4D sensor and ρf is the reference conductivity value obtained by the commercial contact conductivity meter. Fig. 7 shows the input-output characteristics of the four improved C4D sensors with different inner diameters. It is clear that the input-output characteristic of the improved C4D sensor is monotonic. Fig. 8 shows the relative errors of conductivity detections. The experimental results indicate that the measurement accuracies of the four improved C4D sensors are satisfactory. The maximum relative errors of the four improved C4D sensors are 3.3%, 3.6%, 3.2%, 3.6%, respectively. Table 2 lists the comparison of the maximum relative errors of the improved C4D sensors and those of the earlier C4D sensors in our previous work [20]. The measurement accuracy
of the improved C4D sensor is comparable to the earlier version. These experimental results verify that the optimization of the improved C4D sensor is effective and can successfully overcome the drawback (its input-output characteristic is non-monotonic) of the C4D sensor in our previous work.
In addition, a supplementary research result has been obtained. As mentioned in Section 2, = is the precondition of Eq. (10). That means if ≠ and ≠ but = , it would cause no influence on the determination of the resonance frequency f0. This supplementary research result is of benefit to the manufacturing and the design of the C4D sensor. As we known, most conventional C4D sensors are working at such condition that = (by setting the two electrodes with same lengths and manufacturing the two electrodes precisely). In practice, the two electrodes of most C4D sensors are more or less different from each other. The small difference between the two electrodes means ≠ and may cause unfavorable influence on conductivity detection. However, in this work, the unfavorable influence caused by ≠ can be compensated by adjusting the values of L1 and L2, i.e. the improved C4D sensor has the advantage of lower manufacturing requirement. Meanwhile, the precondition ( = ) may provide a useful idea/approach for the design of C4D sensor, i.e. it may provide a possible way to design a special style C4D sensor with two different-length electrodes. It is necessary to indicate that there are still two issues should be noticed although the improved C4D sensor has those advantages listed above. One is the nonlinearity of the input-output characteristic of the improved C4D sensor. As mentioned in Section 2, Eq. (15) clearly indicated that the input-output characteristic of the improved C4D sensor should be linear theoretically. But the experimental results show that the input-output characteristic is monotonic and nonlinear, as shown in Fig.7. The other is that the sensitivity of the improved C4D sensor decreases with the increase of conductivity. The sensitivity of the improved C4D sensor in lower conductivity is obviously greater than that in higher conductivity, as shown in Fig. 9 (the sensitivity is defined as Δuo/ΔGm.). The improved C4D sensor is more suitable for the measurement of the relatively lower conductivity. These two issues are in accordance with other researchers' study results (obtained by capillaryscale or less C4D sensor, and the maximum conductivity is usually less than 20 mS/cm) [4, 11-14, 18, 25-29]. The nonlinearity feature of the C4D sensor has already been indicated by other researchers (such as Gas et al [4], Kuban et al. [13, 14], do Lago et al. [25], Tuma et al. [26] and Mark et al. [27]) and been investigated by our research group [18]. The experimental results obtained in this work further verify that the nonlinearity feature does exist in a larger-scale C4D sensor (the inner diameter is up to 7.6 mm) with a relatively wider measurement range of conductivity (up to 192.3 mS/cm). Maybe, the nonlinearity of the input-output characteristic is a limitation or an inherent feature of the C4D sensor [4, 13, 14, 25-27]. The phenomenon of sensitivity loss at high conductivity also has been reported by do Lago et al. [11, 25], Kuban et al. [13], Tuma et al. [26, 28], Shen et al. [29] and our research group [18]. The research results in this work further verify that the phenomenon of sensitivity loss at high conductivity also exists in a larger-scale C4D sensor. Meanwhile, it is found that the improved C4D sensor developed in this work is more suitable for the measurement of the relatively lower conductivity.
4. Conclusion In this work, an improved C4D sensor has been developed to overcome the drawback (the nonmonotonic input-output characteristic) of the C4D sensor developed in our previous work [20]. The research focused on the optimization of the measurement circuit of the detection path. Two-inductor design has been used to replace the previous one-inductor design. This optimization has led to the monotonic input-output characteristic. Four improved C4D sensors with different inner diameters (1.8 mm, 3.3 mm, 5.0 mm and 7.6 mm, respectively) have been evaluated experimentally. The results presented indicate that the twoinductor design has successfully overcome the drawback of the previous C4D sensor. The improved C4D sensor displays a monotonic input-output characteristic and its measurement accuracy is satisfactory. With reference of a commercial contact conductivity meter, the maximum relative
errors of the four improved C4D sensors are all less than 4.0%. The results also indicate that the input-output characteristic of the improved C4D sensor is nonlinear and the sensitivity of the improved C4D sensor decreases with the increase of conductivity. The improved C4D sensor is more suitable for the measurement of the relatively lower conductivity.
Acknowledgment This research work is supported by National Natural Science Foundation of China (No. 51476139).
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Photographs and biographies of all authors 1. Haifeng Ji
Haifeng Ji was born on 26 October 1973 in China. He received his master degree from Shandong University of Technology in 1999 and his Ph.D. degree from Department of Control Science and Engineering, Zhejiang University, in 2002, respectively. Now he is working in Zhejiang University as an Associate Professor. His interesting research includes measurement techniques, automation equipment, information processing of complex process system, multiphase flow measurement in mini-/micro-channel.
2. Yingchao Lyu
Yingchao Lyu was born in Hangzhou, China, on January 18, 1990. She received the B.Sc. degree from Zhejiang University, Hangzhou, China, in 2012. She is currently working toward the Ph.D. degree with the College of Control Science and Engineering, Zhejiang University. Her research interests include automation instrumentation, signal processing and multiphase flow measurement.
3. Baoliang Wang
Baoliang Wang was born in Zibo, China, on July 11, 1970. He received the B.Sc. and M.Sc. degrees from Shandong University of Technology, Jinan, China, in 1992 and 1995, respectively. In 1998, he received the Ph.D. degree from Zhejiang University. From 1998 to 2001, he was a Lecturer with the Department of Control Science and Engineering, Zhejiang University. From 2002 to 2003, he was a research associate at the City University of Hong Kong. From 2002 to 2013, he was an Associate Professor with the Department of Control Science and Engineering, Zhejiang University. Currently, he is a Professor with the College of Control Science and Engineering, Zhejiang University. His research interests include process tomography, motion control system, microprocessor system.
4. Zhiyao Huang
Zhiyao Huang was born in Hangzhou, China, on October 22, 1968. He received the B.Sc., M.Sc., and Ph.D. degrees from Zhejiang University, Hangzhou, China, in 1990, 1993, and 1995, respectively. From June 1995 to August 1997, he was a Lecturer with the Department of Chemical Engineering, Zhejiang University. In September 1997, he became an Associate Professor with the Department of Control Science and Engineering, Zhejiang University, and in 2001, he was
appointed a Professor. Currently, he is a Professor with the College of Control Science and Engineering, Zhejiang University. He is also a permanent staff of the State Key Laboratory of Industrial Control Techniques. His current interests include automation instrumentation and multiphase flow measurement.
5. Haiqing Li
Haiqing Li was born in Qingdao, China, on November 16, 1934. She received the B.Sc. degree from Dalian Science and Engineering University, Dalian, China, in 1956. In 1988, she was appointed a Professor with the Department of Chemical Engineering, Zhejiang University. She had been a Professor with the Department of Control Science and Engineering of Zhejiang University since 1997. Her research interests include automation instrumentation and multiphase flow measurement technology.
6. Yong Yan
Yong Yan received the B.Eng. and M.Sc. degrees from Tsinghua University, Beijing, China in 1985 and 1988, respectively, and the Ph.D. degree from the University of Teesside, Middlesbrough, U.K., in 1992. He is currently a Professor of Electronic Instrumentation, the Head of Instrumentation, Control and Embedded Systems Research Group, and the Director of Research at the School of Engineering and Digital Arts, the University of Kent, Canterbury, U.K. His research interests include sensors, instrumentation, measurement, condition monitoring, digital signal processing, digital image processing and applications of artificial intelligence.
Fig. 1. Construction of the C4D sensor developed in our previous work [20].
Rf
io
i1 Cd 1
i2
uo A1
Cd 2
Fig. 2. Equivalent circuit of the C4D sensor developed in our previous work [20].
Fig. 3. Function curve of the relationship between uo and Gm [20].
Fig. 4. Construction of the improved C4D sensor for industrial application.
Rf
io i1
uo
i2
A1
Cd1
Cd 2
Fig. 5. Equivalent circuit of the improved C4D sensor for industrial application.
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120 100 80 60 40 20
0
20
40
60
80
100 120 ?f (mS/cm)
140
160
180
200
(c) 5.0 mm i.d. (d) 7.6 mm i.d. Fig. 8. Relative errors of conductivity detections.
70
70
60
60
50
50 Sensitivity (V/mS)
0
Sensitivity (V/mS)
?m (mS/cm)
140
40
30
40
30
20
20
10
10
0
0
0.2
0.4 0.6 0.8 Conductance Gm (mS)
(a) 1.8mm i.d.
1
1.2
0
0
0.5
1
1.5 2 2.5 3 Conductance Gm (mS)
(b) 3.3mm i.d.
3.5
70
60
60
50
50 Sensitivity (V/mS)
Sensitivity (V/mS)
70
40
30
40
30
20
20
10
10
0
0
1
2
3
4 5 6 Conductance Gm (mS)
7
8
0
9
0
(c) 5.0mm i.d.
5
10 15 Conductance Gm (mS)
20
(d) 7.6mm i.d.
Fig. 9. Typical sensitivity plots of the four improved C4D sensors with different diameters.
Table 1 The parameters of the four improved C4D sensors The improved C4D The length of sensor the electrodes(mm) 1.8 mm i.d. 20 (3.1mm o.d.) 3.3 mm i.d. 20 (4.9mm o.d.) 5.0 mm i.d. 20 (7.5 mm o.d.) 7.6 mm i.d. 20 (10.5 mm o.d.)
The length of the gap(mm)
The excitation frequency(kHz)
40
206.8
40
200.6
40
176.6
40
171.3
Table 2 The comparison of the maximum relative errors of conductivity detection Pipe i.d. 1.8 mm 3.3 mm 5.0 mm 7.6 mm The C4D sensors in [20] 3.7% 3.4% 3.0% 4.2% The improved C4D sensors 3.3% 3.6% 3.2% 3.6%
25