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Study of an expendable current profiler detection method
Ocean Engineering 132 (2017) 40–44
Contents lists available at ScienceDirect
Ocean Engineering journal homepage: www.elsevier.com/locate/oceaneng
Study of an expendable current profiler detection method
MARK
⁎
Shenghui Liu, Qisheng Zhang , Xiao Zhao, Xinyue Zhang, Shuhan Li, Jianen Jing China University of Geosciences, Beiing 100083, China
A R T I C L E I N F O
A BS T RAC T
Keywords: Ocean current Ocean-current-induced electric field Expendable current profiler Amplitude modulation
In this study, the induced electric field model for current movement is established, and the measurement principle of an expendable current profiler (XCP) is obtained through model analysis. Based on this analysis, a method is proposed for the measurement of the nanovolt-scale ocean-current-induced electric field. The proposed procedure moves the frequency of the ocean-current-induced electric field signal to the super-low noise range of the ocean current electric field sensor though amplitude modulation. This is then followed by amplifier filter extraction, while compensating the ocean-current-induced electric field, in terms of the circuit, to offset the strong interference of the induced electric field caused by the subsidence of the XCP probe. Inside the XCP probe, the ocean current electric field signal and the compass coil signal are converted into the in-phase component In, quadrature component Qn, and baseline component Bn, and the data processing method that calculates the eastward and northward relative velocity components of the ocean current from the values of In, Qn, and Bn is established.
1. Introduction Approximately 70% of the earth's surface is covered by water. Oceans provide humankind with significant resources and they play an important role in human life (Halpern et al., 2012). As an important parameter of the marine environment, ocean currents have significant effects on global climate change, coastal erosion, marine engineering, and the migration patterns of marine organisms. Thus, measurements of ocean current flow fields have long been a focus of marine scientists and marine engineers (Jonathan et al., 2012). As marine scientific research matures and the scale of ocean engineering projects increases (Cui, 2013.), the requirements for ocean current measurement techniques grow. The principal objective of the development of such techniques is to achieve rapid, accurate, real-time, large-scale ocean current measurements targeted on the measurement object and specific to the measurement purpose. Measurement of ocean currents considers the parameters of speed and direction. According to their physical principles, the instruments used for measuring ocean currents can be classified into one of several types, e.g., mechanical, pressure, electromagnetic, and acoustic current meters (Bouferrouk et al., 2016). Similarly, the measurement methods of these instruments can be divided into several groups: drifting buoy, fixed point, vessel-mounted moving, and expendable moving flow measurement methods (Zhang et al., 2013). An expendable current profiler (XCP) is a disposable low-cost observational instrument that can quickly obtain the parameters of the seawater velocity profile (Liu ⁎
and He, 2010). The XCP technology is based on the theory of electromagnetic induction. Movement of seawater in a geomagnetic field produces an induced electromotive force (EMF) (Kuvshinov et al., 2006; Tyler, 2015; Hewson-Browne, 1973), the magnitude of which is linearly proportional to the speed of the current. Therefore, ocean current speed can be inferred by measuring the EMF (Sanford, 1971; Stephenson and Bryan, 1992; Tyler and Mysak, 1995), which is a convenient method for the indirect measurement of ocean current speed (Szuts, 2010). Because the XCP uses a geomagnetic field as the excitation source, an emitting source does not need to be designed for the equipment itself, which makes the front end of the probe more portable. During the measurement process, the XCP uses a no-recycle non-stop operation mode, which makes it faster and more convenient than comparable instruments. Therefore, XCPs could not only greatly improve the detection efficiency of ocean currents but could also be more competent in providing marine environmental parameter measurements in controversial special regions (Liu and Chen, 2011). Currently, only a US company, Sippican, owns the XCP technology and manufactures the related instruments. However, its products have not been updated since 2005, and also are prohibited to be sold in certain countries. Therefore, it is critical that we conduct in-depth study on the XCP detection method and promote the further development of this type of instruments. This article introduces an ocean-current-induced electric field detection method, which is the basis of the development of China's
Corresponding author. E-mail address: [email protected] (Q. Zhang).
http://dx.doi.org/10.1016/j.oceaneng.2017.01.018 Received 27 January 2016; Received in revised form 30 November 2016; Accepted 19 January 2017 Available online 26 January 2017 0029-8018/ © 2017 Elsevier Ltd. All rights reserved.
Ocean Engineering 132 (2017) 40–44
S. Liu et al.
respectively, of the other layers and the seabed:
RE =
1 i −1 1 ∑k =1 R k i −1
εE = RE ( ∑ k =1
M
1
+ ∑k = i +1 R
εk + Rk
M
∑ k = i +1
(1)
k
i −1 ε
M
ε
∑k =1 Rk + ∑k = i +1 Rk εk k ) = i −1 1k M 1 Rk ∑k =1 R + ∑k = i +1 R k
k
(2)
The actual value measured by the XCP is the potential difference between the two ends of Ri, namely:
ΦAB = (εi − εE ) where v =
Ri ≈ εi − εE = Hz a (vi − v ) Ri + RE
M ∑ik−1 =1 σ k Dk vk + ∑k = i +1 σ k Dk vk , i −1 ∑k =1 σk Dk + ∑kM= i +1 σk Dk
(3)
can be approximated as the average
speed of the currents, which is relevant to the conductivity and speed of each layer. Therefore, for the XCP electrodes, with spacing L along the x-direction in some layer of the ocean current, the measured potential difference is
Fig. 1. Equivalent circuit of ocean-current-induced electric field.
first iteration of the XCP instrument (Zhang et al., 2013). The second section of this paper analyzes the current induced electric field model. Section 3 introduces the measurement principles and specific detection method. Section 4 derives the XCP data processing method. Section 5 compares the method to what has been previously developed and Section 6 provides our conclusions.
Φi = Hz L (vi − v )
(4)
It can be seen from Eq. (4) that the current speed measured by the XCP is the net current speed after subtracting the average current speed from the local current speed. Strictly, the measurement obtained by the XCP is a type of relative measurement (Sanford, 1971; Liu and He, 2010). To detect the profile of ocean current speed at different depths, XCP devices commonly freefall through the seawater column. Therefore, when analyzing the EMF measured by the XCP device, the movement of the XCP device itself must be considered. If it is assumed the XCP device is within a uniform current in the horizontal direction and that it is falling with uniform velocity vp, the induced EMF detected by the electrodes is
2. Analysis of ocean-current-induced electric field model To facilitate theoretical research, ocean currents can be simplified to multi-level uniform currents (ignoring vertical flow). Fig. 1 shows a horizontal current of width a along the y-direction (Sanford et al., 1978). The ocean current model comprises M layers that are isolated from each other, provided that the flow speeds of the M flowing layers are v1, v2,…,vM, respectively. The electric current induced by the ocean current can be described using the equivalent circuit diagram also shown in Fig. 1. The EMF induced in each layer is εi, equivalent resistance is Ri, and vertical resistance between the layers is ri. For convenience of presentation and consistency in the expression of the formula, the seabed is also regarded as one layer (layer M), where vM =0 and εM =0. In the operation of the XCP, the actual parameter measured is the potential difference between the two electrodes of the XCP caused by the layer in which the XCP is located. First, considering the simplest case, two electrodes are arranged horizontally in the seawater perpendicular to the flow direction, where the electrode spacing is equal to the width of the ocean currents. In Fig. 1, Ri = a/σiDib, ri = Di/σiab, and εi = HZavi, wherein σi is the conductivity of each current layer, Di is the thickness of each current layer, b is the length of the ocean currents, and HZ is the vertical component of the geomagnetic field. Generally, in ocean currents Di « a and Ri > > ri, which means it is possible to ignore ri. Thus, the equivalent circuit can be simplified to that shown in Fig. 2 (Sanford et al., 1978). In Fig. 2, εi and Ri are the induced EMF and equivalent resistance, respectively, of the current layer in which the electrode is located, and εE and RE are the equivalent induced EMFs and equivalent resistances,
⇀ ⎯⇀ ⎯ Φ = L •(⇀ v × H)
(5)
⇀ v is the In Eq. (5), L is the position vector of the two electrodes, ⇀ ⎯⇀ ⎯ relative velocity of the seawater flowing through the electrodes, and H is the three-dimensional geomagnetic vector. During the fall, the electrodes cutting the geomagnetic field horizontally also produce an induced EMF, which can be regarded as the equivalent EMF caused by v comprises of two the upward movement of the ocean currents. Thus, ⇀ vs and the parts: the unknown absolute velocity of the ocean current ⇀ vp . relative velocity of the XCP equipment falling through the seawater ⇀ If one of the two electrodes is set as the origin with the geographical east and geographical north taken as the positive directions of the xaxis and y-axis, respectively, and the direction perpendicularly upward taken as the positive direction of the z-axis, a spatial Cartesian coordinate system can be established (as shown in Fig. 3). If it is
Fig. 2. Equivalent circuit for XCP measurement of ocean-current-induced electric field.
Fig. 3. Schematic of freefall movement of electrode.
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Ocean Engineering 132 (2017) 40–44
S. Liu et al.
assumed that the angle between the direction of the electrode and the ⇀ y-axis is θ, then in Eq. (5), L = [L sin θ L cos θ 0] is the direction ⎯⇀ ⎯ vector of the electrode, H = [0 Hy Hz ] is the geomagnetic field vector, vS = [vE vN 0] is the direction of the current. Therefore, and ⎯⇀ ⇀ v = [vE vN vP ]. Substituting the above into Eq. (5) we have:
Φ = Hz LvE cos θ − Hz LvN sin θ + Hy LvP sin θ
(6)
If current stratification is also taken into account, then vE and vN can be replaced by vE − vE and vN − vN . When θ = 0° and θ = 90°, the formula for an ocean-current velocity can be obtained, respectively:
vE − vE =
Φ|θ =0° Hz L
(7)
and
vN − vN = −
Hy vP Φ|θ =90° + Hz L Hz
(8)
3. XCP detection method The voltage difference generated by the electric field induced by a current of 1 cm/s at latitude 20° in an ocean current electric field sensor with a pole pitch of 5 cm is about 20 nV (Dunlap et al., 1981), far less than the microvolt-scale noise of the ocean current electric field sensor (Deng et al., 2002). Meanwhile, the induced electric field signal of the sinking XCP has amplitude of around the microvolt level, which also represents strong interference with respect to the signal of the ocean-current-induced electric field. Therefore, the critical task for the XCP instrument is to measure small ocean-current-induced electric field signals. It can be seen from Eq. (6) that the ocean current simultaneously induces two components of velocity information and thus, reliance on a single formula will not provide concrete information on ocean currents. Only in a particular location can specific information about oceancurrent velocity be acquired. To obtain EMF information from different angles, the two electrodes need to be rotating; therefore, the XCP instrument not only needs to fall at a certain speed but it also requires a certain speed of rotation. During the rotation, the ocean current electric field signal can be modulated to a narrowband single-frequency signal. This is important because for narrower bands, the noise of the ocean current electric field sensor is lower, which enables the extraction of nanovolt-scale ocean-current-induced electric field signals. The XCP probe sensor consists of two Ag–AgCl non-polarized electrodes and a compass coil coaxial with the electrode. The rotary wing at the probe's tail drives the rotation of the XCP probe during the sinking process. The electrodes and compass coil inside the XCP probe rotate with the same angular velocity and cut the geomagnetic field, generating electrode and coil signals that are similar to the singlefrequency signals. The XCP probe mainly measures information such as the electrode signal and compass coil signal. The electrode signal is used to measure the strength of the ocean-current-induced electric field, including the post-modulated ocean-current-induced electric field signal (added to the electrodes) and the XCP-probe-induced electric field signal caused by the horizontal cutting of the geomagnetic field lines during the sinking process. The compass coil signal is used for recording the rotation angle of the probe, while offsetting the probe-induced electric field signal in the amplifier circuit. The flow chart of the measurement principle of the XCP probe is shown in Fig. 4. The compass coil signal EC first passes through the preamplifier to receive VC. On the one hand, VC is converted to the CR signal by the hysteresis comparator and sent into the field-programmable gate array (FPGA). On the other hand, it is converted into FC by voltage-to-frequency converter before being sent into the FPGA. The electrode signal EE first passes through the preamplifier and becomes
Fig. 4. Flow chart of the measurement principle of the XCP probe.
VE and it is then superimposed with VC by a certain percentage to offset the induced electric field signal caused by the sinking of the probe. The signal, after superimposition, is first amplified and then converted to FE through voltage-to-frequency converter and sent into the FPGA. According to the CR signal, the internal program of the FPGA generates two quadrature clock signals CIR and CQR. The generation process is shown in Fig. 5 (Sanford et al., 1982).
Fig. 5. Schematic of internal trigger signal generation of XCP probe.
42
Ocean Engineering 132 (2017) 40–44
S. Liu et al.
The CIR, CQR, and CR signals are used to conduct cycle counts for the FC and FE signals (Huynh et al., 2008), such that the electrode signal or the compass coil signal after modulation can be demodulated into the in-phase component In, quadrature component Qn, and baseline component Bn, as follows: t2
In =
∫t 0
t3
∫t1
Bn =
∫t 0
∫t2
F (t ) dt
∫t3
F (t ) dt
(10)
t4
F (t ) dt
(11)
where t0 =0, t1 = Tn-1/4, t1 = Tn-1/2, t3 =3Tn-1/4, t4 = Tn, t4 =5Tn/4, and Tn-1 and Tn are the rotary periods of the (n – 1)-th and n-th cycle of the probe during the sinking process, respectively. The In, Qn, and Bn are transmitted to the surface float through the enameled wire in real time, and then it is relayed by the wireless device in the float to the wireless receiving unit on the carrier platform. 4. XCP data processing According to the model provided by Sanford et al. (1978), Eq. (6) is corrected by adding a compensation factor for the XCP probe, as shown in Eq. (12), where K is the impact of the XCP probe on the distribution of the ocean-current-induced electric field around the XCP probe placed in tested seawater. Numeric simulation, forward modeling, and physical simulation tests have shown that the addition of the XCP probe leads to around a one-fold increase in the strength of the ocean-current-induced electric field received by the XCP ocean current electric field sensor; thus, the value of K is normally given as 1.
Φ = (1 + K ) Hz LvE cos θ − (1 + K ) Hz LvN sin θ + Hy LvP sin θ
6. Conclusions In this study, through the establishment of the ocean-currentinduced electric field model, the measurement principles of the XCP were derived and the following conclusions obtained. Measurement undertaken by an XCP is a relative measurement. Only in deep-ocean layers, where the current velocity is very low (for example in a great ocean), can the value obtained by the XCP be regarded approximately as the absolute current velocity. An expendable ocean-current-induced electric field detection method was proposed. The detection method uses amplitude modulation to achieve fast measurement of the nanovolt-scale weak ocean current electric field signals within a high-noise background. In the rotating sinking process of the XCP probe, the ocean current electric field signal is amplitude modulated to a narrowband single-frequency signal, thereby moving the effective signal frequency to the super-low noise band of the ocean current electric field sensor. Then, filtered extraction of the weak current field signals is conducted through the processing circuit. Meanwhile, the component of the ocean current electric field in the same direction is compensated through the circuit that, to some extent, overcomes the strong interference of the induced electric field caused by the sinking of XCP probe. The method converts the ocean current electric field signal and the compass coil signal inside the XCP probe to the in-phase component In, quadrature component Qn, and baseline component Bn, which greatly decreases the amount of data for each XCP measurement and reduces data transmission requirements (Narumi et al., 2014). In addition, the XCP data processing methods were studied and the formulas for calculating the eastward and northward relative velocity components of the ocean current (vEr and vNr) from the values of In, Qn and Bn were deduced.
(12)
Assuming the model for the modulated signal can be expressed as
F (t ) = A cos(ωt + φ) + C + Dt + Et 2 + δ
(13)
where ω is the angular frequency of the probe spin, φ is its phase position, C, D, and E are delay coefficient of the circuits, and δ is measurement noise. Substituting Eq. (13) into Eqs. (9)–(11) gives Eqs. (14)–(16) (Sanford et al., 1982):
In = −
4A D T2 E T3 TIn sin φ + C (Tn −1 − Tn ) + ( n −1 − Tn2 ) + ( n −1 − Tn3) 2π 2 2 3 4 (14)
4A 5C D Qn = − TQn cos φ + (Tn −1 − Tn ) + (17Tn2−1 − 25Tn2 ) 2π 4 32 E + (53Tn3−1 − 125Tn3) 192
Bn = CTn +
D 2 E 3 Tn + Tn 2 3
(15) (16)
Based on Eq. (16), parameters C, D, and E can be calculated using the given value of Bn for three adjacent cycles. Given In, Qn, C, D, E, and the instrumental gain calibration for the induced electric field and compass coil channels, the amplitude and phase position of the induced voltage signals Uea∠Uep and the amplitude and phase position of the compass coil voltage signals Uca∠Ucp can be calculated. According to the obtained induced voltage signals and compass coil voltage signals, the eastward and northward components (vEr and vNr, respectively) of the relative velocity of the ocean current can be calculated using Eqs. (17) and (18), where ψ=3π/2+ Ucp - Uep.
vEr = (v E −vE ) =
Uea cos ψ Hz L (1 + K )
(18)
How to digitalize the extracted sea current induced electric field signal is the key question in the XCP detection method. In EM-APEX (Sanford et al., 2005), a 24-bit A/D conversion method is employed, which can achieve very high precision. However, for the weak signal processing circuit used in the present design, at least a sampling rate of 5760 bps is needed to achieve an angular resolution of 1°. If two 24-bit A/D converters are used, there will be at least 276480 bit data to be generated every second. However, in the sea, the data transmission technology for twin enameled wire can not achieve such a high data transmission speed. In XTVP (Sanford et al., 1982) and Sippican XCP, FM modulation method is employed, in which the extracted sea current induced electric field signal is voltage-to-frequency converted, and then is transferred to the receiving unit on the deck, where it is demodulated. A similar method is employed in the present design, but it is improved by introducing the SOPC (System on a Programmable Chip) technology to the XCP probe. Now the digital signal processing is carried out inside the probe, so that the data volume can be compressed into an achievable range for the data transmission module. At the same time, the digital signal transmission can be directly carried out over the thousands of meters of the enameled wire between the probe and the float. In this way, anti-interference ability of the data transmission is strengthened, and thus the measurement precision is improved.
(9)
t5
F (t ) dt −
vp Hy Uea sin ψ + Hz (1 + K ) Hz L (1 + K )
5. Discussion
t4
F (t ) dt −
Qn =
vNr = (v N −vN ) =
Acknowledgments This work is supported by the Natural Science Foundation of China (No. 41574131), the National “863″ Program of China (No.
(17) 43
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S. Liu et al.
Kuvshinov, A., Junge, A., Utada, H., 2006. 3-D modelling the electric field due to ocean tidal flow and comparison with observations. Geophys. Res. Lett. 33 (6), L06314. Liu, G.D., Chen, J., 2011. Marine geophysical applications in the field of national security. Pr. Geophys. 26 (6), 1885–1896. Liu, N., He, H.K., 2010. Study on the theory of expendable current profiler measurement. Ocean Tech. 29 (1), 8–11. Narumi, T., Yasuhisa, I., Hiroshi, O., et al., 2014. New buoy observation system for tsunami and crustal deformation. Mar. Geophys. 35, 243–253. Sanford, T.B., 1971. Motionally induced electric and magnetic fields in the sea. J. Geophys. Res. 76 (15), 3476–3492. Sanford, T.B., Drever, R.G., Dunlap, J.H., 1978. A velocity profiler based on the principles of geomagnetic induction. Deep-Sea Res. 25 (2), 183–210. Sanford, T.B., Drever, R.G., Dunlap, J.H., et al., 1982. Design, Operation and Performance of an Expendable Temperature and Velocity Profiler (XTVP). Applied Physics Laboratory, University of Washington. Sanford, T.B., Dunlap, J.H., Carlson, J.A., , et al2005. Autonomous velocity and density profiler: EM-APEX. IEEE/OES. In: Proceedings of the Eighth Working Conference on Current Measurement Technology. 152-156. Stephenson, D., Bryan, K., 1992. Large-scale electric and magnetic fields generated by the oceans. J. Geophys. Res. -O. 97 (C10), 15467–15480. Szuts, Z.B., 2010. Relationship between ocean velocity and motionally induced electrical signals: 1. Presence Horiz. Veloc. gradients. J. Geophys. Res., 115. Tyler, R., 2015. Electromagnetic coupling of ocean flow with the earth system. terrestrial, atmos. Ocean. Sci. 26 (1), 41–52. Tyler, R.H., Mysak, L.A., 1995. Motionally-induced electromagnetic fields generated by idealized ocean currents. Geoph. Astro Fluid 80 (3–4), 167–204. Zhang, Q.S., Deng, M., Liu, N., et al., 2013. Development of the expendable current profiler. Ch. J. Geophys. 56 (11), 3699–3707.
2006AA09A304), the National Major Scientific Research Equipment Research Projects of China (No. ZDYZ2012-1-05-01), and the Fundamental Research Funds for the Central Universities of China (No. 2652015213). References Bouferrouk, A., Saulnier, J., Smith, G.H., Johanning, L., 2016. Field measurements of surface waves using a 5-beam ADCP. Ocean Eng. 112, 173–184. Cui, W.C., 2013. Development of the Jiaolong Deep Manned Submersible. Mar. Tech. S. J. 47 (3), 37–54. Deng, M., Liu, Z.G., Bai, Y.C., et al., 2002. The theory and development technology of the sea floor electric field sensor. Geol. Expl. 38 (6), 43–47. Dunlap, J.H., Drever, R.G., Sanford, T.B., 1981. .Experience with an expendable temperature and velocity profiler. The annual meeting of the marine technology society and of the ieee council of oceanic engineering, pp. 372-376. Halpern, B.S., Longo, C., Hardy, D., et al., 2012. An index to assess the health and benefits of the global ocean. Nature 488 (7413), 615–620. Hewson-Browne, R.C., 1973. Estimates for induced oceanic electric currents flowing near the coast and their associated magnetic fields just inland. Geophy. J. Roy. Astr. S. 34 (4), 393–402. Huynh, T., Cao, T., Tran, D., , et al2008. Designing a hardware accelerator for vector quantization as a component of a SoPC.In: Proceedings of the Canadian Conference on Electrical and Computer Engineering. 479–483. Jonathan, P., Ewans, K., Flynn, J., 2012. Joint modelling of vertical profiles of large ocean currents. Ocean Eng. 42, 195–204.
44
Contents lists available at ScienceDirect
Ocean Engineering journal homepage: www.elsevier.com/locate/oceaneng
Study of an expendable current profiler detection method
MARK
⁎
Shenghui Liu, Qisheng Zhang , Xiao Zhao, Xinyue Zhang, Shuhan Li, Jianen Jing China University of Geosciences, Beiing 100083, China
A R T I C L E I N F O
A BS T RAC T
Keywords: Ocean current Ocean-current-induced electric field Expendable current profiler Amplitude modulation
In this study, the induced electric field model for current movement is established, and the measurement principle of an expendable current profiler (XCP) is obtained through model analysis. Based on this analysis, a method is proposed for the measurement of the nanovolt-scale ocean-current-induced electric field. The proposed procedure moves the frequency of the ocean-current-induced electric field signal to the super-low noise range of the ocean current electric field sensor though amplitude modulation. This is then followed by amplifier filter extraction, while compensating the ocean-current-induced electric field, in terms of the circuit, to offset the strong interference of the induced electric field caused by the subsidence of the XCP probe. Inside the XCP probe, the ocean current electric field signal and the compass coil signal are converted into the in-phase component In, quadrature component Qn, and baseline component Bn, and the data processing method that calculates the eastward and northward relative velocity components of the ocean current from the values of In, Qn, and Bn is established.
1. Introduction Approximately 70% of the earth's surface is covered by water. Oceans provide humankind with significant resources and they play an important role in human life (Halpern et al., 2012). As an important parameter of the marine environment, ocean currents have significant effects on global climate change, coastal erosion, marine engineering, and the migration patterns of marine organisms. Thus, measurements of ocean current flow fields have long been a focus of marine scientists and marine engineers (Jonathan et al., 2012). As marine scientific research matures and the scale of ocean engineering projects increases (Cui, 2013.), the requirements for ocean current measurement techniques grow. The principal objective of the development of such techniques is to achieve rapid, accurate, real-time, large-scale ocean current measurements targeted on the measurement object and specific to the measurement purpose. Measurement of ocean currents considers the parameters of speed and direction. According to their physical principles, the instruments used for measuring ocean currents can be classified into one of several types, e.g., mechanical, pressure, electromagnetic, and acoustic current meters (Bouferrouk et al., 2016). Similarly, the measurement methods of these instruments can be divided into several groups: drifting buoy, fixed point, vessel-mounted moving, and expendable moving flow measurement methods (Zhang et al., 2013). An expendable current profiler (XCP) is a disposable low-cost observational instrument that can quickly obtain the parameters of the seawater velocity profile (Liu ⁎
and He, 2010). The XCP technology is based on the theory of electromagnetic induction. Movement of seawater in a geomagnetic field produces an induced electromotive force (EMF) (Kuvshinov et al., 2006; Tyler, 2015; Hewson-Browne, 1973), the magnitude of which is linearly proportional to the speed of the current. Therefore, ocean current speed can be inferred by measuring the EMF (Sanford, 1971; Stephenson and Bryan, 1992; Tyler and Mysak, 1995), which is a convenient method for the indirect measurement of ocean current speed (Szuts, 2010). Because the XCP uses a geomagnetic field as the excitation source, an emitting source does not need to be designed for the equipment itself, which makes the front end of the probe more portable. During the measurement process, the XCP uses a no-recycle non-stop operation mode, which makes it faster and more convenient than comparable instruments. Therefore, XCPs could not only greatly improve the detection efficiency of ocean currents but could also be more competent in providing marine environmental parameter measurements in controversial special regions (Liu and Chen, 2011). Currently, only a US company, Sippican, owns the XCP technology and manufactures the related instruments. However, its products have not been updated since 2005, and also are prohibited to be sold in certain countries. Therefore, it is critical that we conduct in-depth study on the XCP detection method and promote the further development of this type of instruments. This article introduces an ocean-current-induced electric field detection method, which is the basis of the development of China's
Corresponding author. E-mail address: [email protected] (Q. Zhang).
http://dx.doi.org/10.1016/j.oceaneng.2017.01.018 Received 27 January 2016; Received in revised form 30 November 2016; Accepted 19 January 2017 Available online 26 January 2017 0029-8018/ © 2017 Elsevier Ltd. All rights reserved.
Ocean Engineering 132 (2017) 40–44
S. Liu et al.
respectively, of the other layers and the seabed:
RE =
1 i −1 1 ∑k =1 R k i −1
εE = RE ( ∑ k =1
M
1
+ ∑k = i +1 R
εk + Rk
M
∑ k = i +1
(1)
k
i −1 ε
M
ε
∑k =1 Rk + ∑k = i +1 Rk εk k ) = i −1 1k M 1 Rk ∑k =1 R + ∑k = i +1 R k
k
(2)
The actual value measured by the XCP is the potential difference between the two ends of Ri, namely:
ΦAB = (εi − εE ) where v =
Ri ≈ εi − εE = Hz a (vi − v ) Ri + RE
M ∑ik−1 =1 σ k Dk vk + ∑k = i +1 σ k Dk vk , i −1 ∑k =1 σk Dk + ∑kM= i +1 σk Dk
(3)
can be approximated as the average
speed of the currents, which is relevant to the conductivity and speed of each layer. Therefore, for the XCP electrodes, with spacing L along the x-direction in some layer of the ocean current, the measured potential difference is
Fig. 1. Equivalent circuit of ocean-current-induced electric field.
first iteration of the XCP instrument (Zhang et al., 2013). The second section of this paper analyzes the current induced electric field model. Section 3 introduces the measurement principles and specific detection method. Section 4 derives the XCP data processing method. Section 5 compares the method to what has been previously developed and Section 6 provides our conclusions.
Φi = Hz L (vi − v )
(4)
It can be seen from Eq. (4) that the current speed measured by the XCP is the net current speed after subtracting the average current speed from the local current speed. Strictly, the measurement obtained by the XCP is a type of relative measurement (Sanford, 1971; Liu and He, 2010). To detect the profile of ocean current speed at different depths, XCP devices commonly freefall through the seawater column. Therefore, when analyzing the EMF measured by the XCP device, the movement of the XCP device itself must be considered. If it is assumed the XCP device is within a uniform current in the horizontal direction and that it is falling with uniform velocity vp, the induced EMF detected by the electrodes is
2. Analysis of ocean-current-induced electric field model To facilitate theoretical research, ocean currents can be simplified to multi-level uniform currents (ignoring vertical flow). Fig. 1 shows a horizontal current of width a along the y-direction (Sanford et al., 1978). The ocean current model comprises M layers that are isolated from each other, provided that the flow speeds of the M flowing layers are v1, v2,…,vM, respectively. The electric current induced by the ocean current can be described using the equivalent circuit diagram also shown in Fig. 1. The EMF induced in each layer is εi, equivalent resistance is Ri, and vertical resistance between the layers is ri. For convenience of presentation and consistency in the expression of the formula, the seabed is also regarded as one layer (layer M), where vM =0 and εM =0. In the operation of the XCP, the actual parameter measured is the potential difference between the two electrodes of the XCP caused by the layer in which the XCP is located. First, considering the simplest case, two electrodes are arranged horizontally in the seawater perpendicular to the flow direction, where the electrode spacing is equal to the width of the ocean currents. In Fig. 1, Ri = a/σiDib, ri = Di/σiab, and εi = HZavi, wherein σi is the conductivity of each current layer, Di is the thickness of each current layer, b is the length of the ocean currents, and HZ is the vertical component of the geomagnetic field. Generally, in ocean currents Di « a and Ri > > ri, which means it is possible to ignore ri. Thus, the equivalent circuit can be simplified to that shown in Fig. 2 (Sanford et al., 1978). In Fig. 2, εi and Ri are the induced EMF and equivalent resistance, respectively, of the current layer in which the electrode is located, and εE and RE are the equivalent induced EMFs and equivalent resistances,
⇀ ⎯⇀ ⎯ Φ = L •(⇀ v × H)
(5)
⇀ v is the In Eq. (5), L is the position vector of the two electrodes, ⇀ ⎯⇀ ⎯ relative velocity of the seawater flowing through the electrodes, and H is the three-dimensional geomagnetic vector. During the fall, the electrodes cutting the geomagnetic field horizontally also produce an induced EMF, which can be regarded as the equivalent EMF caused by v comprises of two the upward movement of the ocean currents. Thus, ⇀ vs and the parts: the unknown absolute velocity of the ocean current ⇀ vp . relative velocity of the XCP equipment falling through the seawater ⇀ If one of the two electrodes is set as the origin with the geographical east and geographical north taken as the positive directions of the xaxis and y-axis, respectively, and the direction perpendicularly upward taken as the positive direction of the z-axis, a spatial Cartesian coordinate system can be established (as shown in Fig. 3). If it is
Fig. 2. Equivalent circuit for XCP measurement of ocean-current-induced electric field.
Fig. 3. Schematic of freefall movement of electrode.
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assumed that the angle between the direction of the electrode and the ⇀ y-axis is θ, then in Eq. (5), L = [L sin θ L cos θ 0] is the direction ⎯⇀ ⎯ vector of the electrode, H = [0 Hy Hz ] is the geomagnetic field vector, vS = [vE vN 0] is the direction of the current. Therefore, and ⎯⇀ ⇀ v = [vE vN vP ]. Substituting the above into Eq. (5) we have:
Φ = Hz LvE cos θ − Hz LvN sin θ + Hy LvP sin θ
(6)
If current stratification is also taken into account, then vE and vN can be replaced by vE − vE and vN − vN . When θ = 0° and θ = 90°, the formula for an ocean-current velocity can be obtained, respectively:
vE − vE =
Φ|θ =0° Hz L
(7)
and
vN − vN = −
Hy vP Φ|θ =90° + Hz L Hz
(8)
3. XCP detection method The voltage difference generated by the electric field induced by a current of 1 cm/s at latitude 20° in an ocean current electric field sensor with a pole pitch of 5 cm is about 20 nV (Dunlap et al., 1981), far less than the microvolt-scale noise of the ocean current electric field sensor (Deng et al., 2002). Meanwhile, the induced electric field signal of the sinking XCP has amplitude of around the microvolt level, which also represents strong interference with respect to the signal of the ocean-current-induced electric field. Therefore, the critical task for the XCP instrument is to measure small ocean-current-induced electric field signals. It can be seen from Eq. (6) that the ocean current simultaneously induces two components of velocity information and thus, reliance on a single formula will not provide concrete information on ocean currents. Only in a particular location can specific information about oceancurrent velocity be acquired. To obtain EMF information from different angles, the two electrodes need to be rotating; therefore, the XCP instrument not only needs to fall at a certain speed but it also requires a certain speed of rotation. During the rotation, the ocean current electric field signal can be modulated to a narrowband single-frequency signal. This is important because for narrower bands, the noise of the ocean current electric field sensor is lower, which enables the extraction of nanovolt-scale ocean-current-induced electric field signals. The XCP probe sensor consists of two Ag–AgCl non-polarized electrodes and a compass coil coaxial with the electrode. The rotary wing at the probe's tail drives the rotation of the XCP probe during the sinking process. The electrodes and compass coil inside the XCP probe rotate with the same angular velocity and cut the geomagnetic field, generating electrode and coil signals that are similar to the singlefrequency signals. The XCP probe mainly measures information such as the electrode signal and compass coil signal. The electrode signal is used to measure the strength of the ocean-current-induced electric field, including the post-modulated ocean-current-induced electric field signal (added to the electrodes) and the XCP-probe-induced electric field signal caused by the horizontal cutting of the geomagnetic field lines during the sinking process. The compass coil signal is used for recording the rotation angle of the probe, while offsetting the probe-induced electric field signal in the amplifier circuit. The flow chart of the measurement principle of the XCP probe is shown in Fig. 4. The compass coil signal EC first passes through the preamplifier to receive VC. On the one hand, VC is converted to the CR signal by the hysteresis comparator and sent into the field-programmable gate array (FPGA). On the other hand, it is converted into FC by voltage-to-frequency converter before being sent into the FPGA. The electrode signal EE first passes through the preamplifier and becomes
Fig. 4. Flow chart of the measurement principle of the XCP probe.
VE and it is then superimposed with VC by a certain percentage to offset the induced electric field signal caused by the sinking of the probe. The signal, after superimposition, is first amplified and then converted to FE through voltage-to-frequency converter and sent into the FPGA. According to the CR signal, the internal program of the FPGA generates two quadrature clock signals CIR and CQR. The generation process is shown in Fig. 5 (Sanford et al., 1982).
Fig. 5. Schematic of internal trigger signal generation of XCP probe.
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The CIR, CQR, and CR signals are used to conduct cycle counts for the FC and FE signals (Huynh et al., 2008), such that the electrode signal or the compass coil signal after modulation can be demodulated into the in-phase component In, quadrature component Qn, and baseline component Bn, as follows: t2
In =
∫t 0
t3
∫t1
Bn =
∫t 0
∫t2
F (t ) dt
∫t3
F (t ) dt
(10)
t4
F (t ) dt
(11)
where t0 =0, t1 = Tn-1/4, t1 = Tn-1/2, t3 =3Tn-1/4, t4 = Tn, t4 =5Tn/4, and Tn-1 and Tn are the rotary periods of the (n – 1)-th and n-th cycle of the probe during the sinking process, respectively. The In, Qn, and Bn are transmitted to the surface float through the enameled wire in real time, and then it is relayed by the wireless device in the float to the wireless receiving unit on the carrier platform. 4. XCP data processing According to the model provided by Sanford et al. (1978), Eq. (6) is corrected by adding a compensation factor for the XCP probe, as shown in Eq. (12), where K is the impact of the XCP probe on the distribution of the ocean-current-induced electric field around the XCP probe placed in tested seawater. Numeric simulation, forward modeling, and physical simulation tests have shown that the addition of the XCP probe leads to around a one-fold increase in the strength of the ocean-current-induced electric field received by the XCP ocean current electric field sensor; thus, the value of K is normally given as 1.
Φ = (1 + K ) Hz LvE cos θ − (1 + K ) Hz LvN sin θ + Hy LvP sin θ
6. Conclusions In this study, through the establishment of the ocean-currentinduced electric field model, the measurement principles of the XCP were derived and the following conclusions obtained. Measurement undertaken by an XCP is a relative measurement. Only in deep-ocean layers, where the current velocity is very low (for example in a great ocean), can the value obtained by the XCP be regarded approximately as the absolute current velocity. An expendable ocean-current-induced electric field detection method was proposed. The detection method uses amplitude modulation to achieve fast measurement of the nanovolt-scale weak ocean current electric field signals within a high-noise background. In the rotating sinking process of the XCP probe, the ocean current electric field signal is amplitude modulated to a narrowband single-frequency signal, thereby moving the effective signal frequency to the super-low noise band of the ocean current electric field sensor. Then, filtered extraction of the weak current field signals is conducted through the processing circuit. Meanwhile, the component of the ocean current electric field in the same direction is compensated through the circuit that, to some extent, overcomes the strong interference of the induced electric field caused by the sinking of XCP probe. The method converts the ocean current electric field signal and the compass coil signal inside the XCP probe to the in-phase component In, quadrature component Qn, and baseline component Bn, which greatly decreases the amount of data for each XCP measurement and reduces data transmission requirements (Narumi et al., 2014). In addition, the XCP data processing methods were studied and the formulas for calculating the eastward and northward relative velocity components of the ocean current (vEr and vNr) from the values of In, Qn and Bn were deduced.
(12)
Assuming the model for the modulated signal can be expressed as
F (t ) = A cos(ωt + φ) + C + Dt + Et 2 + δ
(13)
where ω is the angular frequency of the probe spin, φ is its phase position, C, D, and E are delay coefficient of the circuits, and δ is measurement noise. Substituting Eq. (13) into Eqs. (9)–(11) gives Eqs. (14)–(16) (Sanford et al., 1982):
In = −
4A D T2 E T3 TIn sin φ + C (Tn −1 − Tn ) + ( n −1 − Tn2 ) + ( n −1 − Tn3) 2π 2 2 3 4 (14)
4A 5C D Qn = − TQn cos φ + (Tn −1 − Tn ) + (17Tn2−1 − 25Tn2 ) 2π 4 32 E + (53Tn3−1 − 125Tn3) 192
Bn = CTn +
D 2 E 3 Tn + Tn 2 3
(15) (16)
Based on Eq. (16), parameters C, D, and E can be calculated using the given value of Bn for three adjacent cycles. Given In, Qn, C, D, E, and the instrumental gain calibration for the induced electric field and compass coil channels, the amplitude and phase position of the induced voltage signals Uea∠Uep and the amplitude and phase position of the compass coil voltage signals Uca∠Ucp can be calculated. According to the obtained induced voltage signals and compass coil voltage signals, the eastward and northward components (vEr and vNr, respectively) of the relative velocity of the ocean current can be calculated using Eqs. (17) and (18), where ψ=3π/2+ Ucp - Uep.
vEr = (v E −vE ) =
Uea cos ψ Hz L (1 + K )
(18)
How to digitalize the extracted sea current induced electric field signal is the key question in the XCP detection method. In EM-APEX (Sanford et al., 2005), a 24-bit A/D conversion method is employed, which can achieve very high precision. However, for the weak signal processing circuit used in the present design, at least a sampling rate of 5760 bps is needed to achieve an angular resolution of 1°. If two 24-bit A/D converters are used, there will be at least 276480 bit data to be generated every second. However, in the sea, the data transmission technology for twin enameled wire can not achieve such a high data transmission speed. In XTVP (Sanford et al., 1982) and Sippican XCP, FM modulation method is employed, in which the extracted sea current induced electric field signal is voltage-to-frequency converted, and then is transferred to the receiving unit on the deck, where it is demodulated. A similar method is employed in the present design, but it is improved by introducing the SOPC (System on a Programmable Chip) technology to the XCP probe. Now the digital signal processing is carried out inside the probe, so that the data volume can be compressed into an achievable range for the data transmission module. At the same time, the digital signal transmission can be directly carried out over the thousands of meters of the enameled wire between the probe and the float. In this way, anti-interference ability of the data transmission is strengthened, and thus the measurement precision is improved.
(9)
t5
F (t ) dt −
vp Hy Uea sin ψ + Hz (1 + K ) Hz L (1 + K )
5. Discussion
t4
F (t ) dt −
Qn =
vNr = (v N −vN ) =
Acknowledgments This work is supported by the Natural Science Foundation of China (No. 41574131), the National “863″ Program of China (No.
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