29-06-2012, 05:18 PM
The Design of An Ultra low Power and Mobile Measurement System of Underwater Target Remote Detection
The Design of An Ultra low Power.pdf (Size: 273.04 KB / Downloads: 44)
Introduction
As it is well known, corrosion phenomenon is inevitable for ships sailing in the sea due to the
electrochemistry activities between steel hull and bronze made propeller [1]. To prevent this
corrosion damage, kinds of cathodic protection (CP) systems or impressed current cathodic
protection (ICCP) systems were implemented. Both of the above actions will generate electric field
around the ship which can be easily detected by hostile military equipments. Now-days, the
research on ship electric field signature is becoming increasingly important.
To study ship electric field signature, on the one hand, theoretical analysis or model computing
is always difficult and deficient since the inducement of the field is very complex, and field data is
necessary and important. On the other hand, all the theoretical approaches need to be verified by
field data. So a measuring system is indispensable to acquire field data. To design a ship electric
field measuring system, several facts should be emphasized as follows [2]:
(1) Since ship electric field measuring system needs to work underwater for a long time,
ultra-low power consumption is of consequence.
System design
A sketch map of the designed system was shown in Fig.1[3]. As it was shown in the figure, the
whole system can be partitioned as two parts, i.e. the onshore host and the underwater terminal. The
onshore host part consisted of a private computer (PC) and a wireless transceiver, and the
underwater terminal part is actually a sub-system based on a microcontroller.
The choice of sensors
There are mainly two kind electrodes that can be used to measure electric fields in seawater [4].
(1)Inert metal e.g. carbon, titanium, gold, lead, silver.
(2)Chloride forming e.g. silver, cadmium, lead.
Materials which are inert in seawater in general are characterized as polarizable and have
potentials which vary widely depending on the surface condition of the electrode and the current
drawn from it. They are not suitable for low noise sensors. Carbon fibre based technologies are
subject to capacitative effects at low frequencies and as such are not well suited to low noise
measurements at frequencies close to DC. The second type of materials forms chlorides in seawater.
They are non-polarizable and as such have relatively constant potentials when small changes in
current occur. Many of the chloride type electrodes are poisonous and not suitable in range
applications. Ag/AgCl however is robust and has excellent long term stability. It has been used for
cathodic protection monitoring for over 40 years and for almost 20 years in military applications.
Low self-noise performance design
To achieve a low noise performance, the measurement system should be designed skillfully,
and the key point is to reduce the noise level of the sensors and to use ultra low noise amplifier.
The noise of Ag/AgCl electrodes comes from the disequilibrium of electrochemistry activities
taken place on the surface of the electrodes[5]. So it’s useful to enlarge the interface of seawater and
the measuring electrodes, but the surface actually can not be produced too large. The reason is that
the larger the surface of the measuring electrodes is, the more difficulty to make pure Ag/AgCl
electrodes is. And when impurities such as iron or charcoal were contained, the electrochemistry
activities on the electrode’s surface would become complex. As a result, the noise level will
increase. The Ag/AgCl electrodes used here are ultra low noise all-solid-state electrodes whose
noise spectrum density is lower than10nV/m/ Hz at the frequency of 1 Hz.
The use of SD memory card
There are at least three advantages to use SD to restore the measured data. The first is that
generally SD cards have a large memory capability, e.g. 2 G, 4 G, 8 G or so much as16 G. The
second advantage is that the data can be restored fleetly. Actually the communication between the
MCU and SD card has two modes, i.e. the SD mode and the SPI mode, both of the two modes have
a high speed. The last advantage is that it’s and convenient for users to read the measured data form
SD card to private computers.