Note: Descriptions are shown in the official language in which they were submitted.
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FIELD OF THE INVENTION
The present invention relates to a system for making non-detectable
the shaft-rate modulated corrosion-current related alternating
electromagnetic signature of a surface ship or submar:Lne by controlling the
potential of a rotating body, and more particularly relates to a system for
controlling the potential of a rotating propeller shaft of a ship or
submarine relative to the hull of the vessel, thereby substantially
eliminating the fluctuations in the propeller shaft current that give rise
to the radiating electromagnetic signature.
It is well known that a surface ship or submarine can be detected
by the presence of its electromagnetic (EM) extremely low-frequency (ELF)
signature, and that such a signature renders the vessel vulnerable to
weapons systems utilizing EM influences. Because different metals are used
in the fabrication of propellers (which are usually made of bronze) and
hulls (which are usually made of steel), these parts of the ship, when in
contact with sea-water, exhibit a voltage difference there-between of about
0.3 volts. On occasion, and for limited periods of time, the shaft-to-hull
voltage can remain very stable at about 0.3 volts, this is attributed to a
quasi-perfect electrical insulation between the shaft and the hull. In a
typical situation, however, the bearings and gears electrically connect the
shaft to the hull through a highly non-stationary impedance which is largely
a function of the propeller shaft rotation, speed and load. This results in
an alternating current in the water at the frequency o:E the shaft and at the
harmonics of that frequency. This alternating current behaves as an
oscillating electric dipole which radiates electric and magnetic fields.
These oscillating fields form the major part of the so-called
electromagnetic (EM) extremely-low frequency (ELF) signature of a surface
ship or submarine, that is a cause of vulnerability, and which may be
detected by naval mines, surveillance arrays, and other weapons systems that
incorporate EM influences.
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BRIEF DESCRIPTION OF THE DRAWINGS
The typical prior known situation, and a preferred embodiment of
the present invention will now be described in conjunction with the attached
drawings, in which:
Figure la, 1b and lc show a schematic diagram of the shaft-rate
modulated corrosion current, the shaft-to-hull voltage difference, and the
electromagnetic signature produced for a typical ship or submarine.
Figure 2a is a schematic diagram of an active grounding system,
depicting its inter-connections to the shaft and the hull;
Figure 2b is a schematic diagram of an active grounding system,
depicting the external parameters of the configuration;
Figure 3 is a simplified equivalent circuit diagram for the active
grounding system of Figures 2a and 2b;
Figure 4 is a circuit diagram fox the active grounding system of
the present invention;
Figure 5 is a circuit diagram for the modular high current voltage
driver of the active grounding system depicted in Figure 4;
Figures 6a and 6b graphically illustrate the functional
relationship between the modulation current reduction and the bearing
conductance;
Figures ~a, 7b and 7c graphically illustrate the effectiveness of
the active grounding device in reducing the shaft-rate modulated current
flowing in the shaft.
X
-4- 1341368
BACKGROUND OF THE INVENTION
In a typical situation, and as is known to people skilled in the art,
the shaft-rate modulated current flow in the shaft and sea is as depicted in
Figure la, the oscillating shaft-to-hull voltage is as depicted in Figure 1b,
and a typical consequential ELF electromagnetic signature is as depicted in
Figure lc. For reduction of this ELF electromagnetic signature, an
electrical bond between the shaft and hull, having an impedance about 10
times less than the propeller-to-hull resistance through the water, is
sufficient. For many years, brush/slip-ring assemblies were used for this
purpose. Although such assemblies provide reasonably adequate protection,
daily monitoring is required to ensure that sufficiently low contact
resistance is maintained. In ideal conditions, it has been shown that a
factor of ten to fifteen reduction in signature can be obtained with passive
grounding. But because this system is still not sufficient to reduce the
signature to an acceptable level, active shaft grounding systems have been
designed.
A controlled potential ship grounding system, disclosed in Canadian
Patent No. 906,054, was designed to overcome t:lie problem of maintaining a
low
resistance bond between the rotating shaft of a ship and the hull. Through a
brush/slip-ring assembly, the described system senses the shaft-to-hull
voltage, compares it to a predetermined level, and forces a current from the
pvropeller-shaft to the hull in response to the difference. The reduction of
the continuous voltage between the shaft and the hull results in a
proportional reduction in the propeller-shaft corrosion current modulation
and in the consequential ELF signature.
Although a system of the kind disclosed in the aforesaid patent
provides a significant measure of decrease in signature, the reduction of the
bearing modulated shaft current is limited. Thus, there is a need for an
improved active shaft grounding system.
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SUMMARY OF THE INVENTION
The present invention relates to a surface ship or submarine
grounding system for controlling the shaft-mite modulated corrosion-current
induced ELF signature of a vessel by maintaining the shaft at a fixed
potential relative to the vessel's hull, in which the shaft-to-hull voltage
is sensed and compared to a predetermined level and a current driver provides
a current from the propeller shaft to reduce the continuous voltage between
the shaft and hull, this output current being biased so as to also reduce the
bearing modulated shaft current, between the shaft and the hull. A balanced
impedance amplifier senses the shaft-to-hull voltage and controls a current
driver which provides current to the rotating shaft, a feedback loop biasing
the current driver sv as to substantially eliminate the effects of
semiconductor bias voltage on the shaft potential, thus maintaining the shaft
at the desired potential over the operating range of the output current, and
thus substantially eliminating the ELF signature.
More particularly, the present invention relates to an ELF signature
suppression system arranged to control the electric potential of a rotating
shaft for a vessel's propeller relative to the vessel's hull, comprising:
sensing means arranged to sense the potential of the rotating shaft relative
to a reference potential; current supply means arranged to cause a uni-
directional flow of current to flow through a circuit including the rotating
shaft; control means to control the flow of current from the current supply
means through the circuit, the control means activating the current supply
means in accordance with the output of the sensing means so as to maintain
the potential of the shaft at the potential of the hull; the control means
comprising a feedback loop such that an alternating current between the shaft
and the hull caused by any residual voltage of the control means is
substantially eliminated.
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DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT
Figures 2a and 2b schematically depict a configuration of a typical
active shaft grounding system, of a kind known to persons skilled in the art,
Figure 2a depicting the inter-connections of the grounding system to a shaft
and a hull 12 of a vessel, and Figure 2b depicting the external parameters
of the system. Figure 3 depicts a simplified equivalent circuit therefore.
As shown in Figure 3, the voltage between shaft 10 and hull 12 is sensed
through a set of brushes Rb2 by a high impedance amplifier of gain A,
designated thusly. For completeness, the amplifier offset voltage Voff is
10 included. Amplifier output voltage Vx controls a high current transistor,
which impresses current into shaft 10 through a set: of brushes Rbl. The
propeller-to-hull resistance through the water is represented by Rw, the
value of which depends upon the water conductivity, the propeller bare metal
area, and the turbulence present around the propeller; part of this
resistance can also be attributed to the
ef-.'fect of the continuing corrosion process on shaft 1Ø For simplicity, Rw
is assumed constant throughout the shaft revolution. The resistance of the
bearings and gears Rb is largely a function oI~ the propeller load, condition
of: the bearing to shaft contact, and rotation rate, and its value can change
greatly during the shaft rotation. In Figure 3, V" represents the propeller-
to-hull galvanic voltage, Vs the power supply output voltage, and V8s the
transistor bias voltage.
From Figures 2a and 2b, and assuming the transistor transconductance
to be infinite, the resulting propeller-shaft current is given by:
Vw Voff Voff Vgs - Vs
Vw Voff _ Rw + Rw + Rb + Rs 1
Iw - Rw + Rw Rw ARw
1 + Rb + RS
1 3~1 368 .
_, _
where , typically, Vq -~ 0 . 3V , Vo~f ~ 5mV , A = 1U'' , 0 . O10 < Rw < 0 .
In , Vgs = 3V ,
Vs ~ 5V, and O.Olfl < Ra < 0.1t1. The term V~,/Rw in relation (1) represents
the
propeller current under ideal shaft grounding conditions. The second term,
Voff/Rw~ is determined by the system offset voltage reference. The third
germ, between brackets in relation (1), is the residual voltage introduced by
the finite gain of the system, and contains variations created by the
bearings resistance fluctuations.
Although circuits of the type illustrated in Figure 2a and 2b provide
some decrease.in shaft-rate modulation of the current, the reduction is
limited. As seen from relation (1) the modulation current is determined by
essentially three sets of parameters. The first one consists of Vw, R~, and
R~" which are ship dependent. The second one is the grounding conductance of
the system A/Rs, which ultimately sets the performances of the system. The
last set consists of Voff, Vgs and Va. Note that typically
Voff °' 5mV « Vw ~ 0.4V (2a)
Rw
(Vgs - Vs) Rs ~ 4V » Vw ~ 0.4V and (2b)
Rw
Voff ~ » Vw for Rb « Rw (2c)
It follows from relations (2b) and (2c) that i.f care is not taken in the way
the current driver is designed, the residual current modulation reduction
will be limited by the offset voltages Voff and (Vg$ - V$), rather than by the
vessel's parameters, at a given system grounding conductance A/R5.
The present invention, depicted in Figures 4 and 5, eliminates the
effect of the bias voltage (Vg8 - V$) , and lowers the reference voltage Voff
to an insignificant level. voff 1S cOllSidered negligible when
Rw
Voff Rb « Vw Rw A Rw (3a)
~a
Rb Rs
thus
_8_ 1341368
Rs
Voff « Rw A ~ (3b)
typically Voff « 100 ~V.
Similarly, the bias voltage (Vgs - Vs) can be neglected if
Rw
(Vgs - Vs) Rs « Vw. (4)
To meet these design requirements, the circuit. of the present invention must
automatically bias the output current driver, and thus provide linear
operation over the entire range of output current. Ta achieve this, a multi-
feed back configuration is utilized. Instead of the high current transistor
used in the system depicted in Figure 3, an internal current feedback loop is
used in the present invention to remove the effect of the term represented by
(V8$ - V8) in relation (1), by automatically biasing the modular driver such
that zero volts at its input gives zero amperes at its output. A voltage
feedback loop ensures that the high current transistor is biased into its
operating region. This configuration has the advantage of overcoming the
problem of matching the the transistors. If a current larger than that
available from a single high current transistor is needed, several stages can
be added in parallel.
Similarly, the preset reference voltage, used in the ELF signature
suppression system, is effectively set at zero. Figure 6 illustrates the
effect of lowering the offset voltage, Voff, on the modulation current (that
is, the term in brackets in relation (1)). Propeller-to-hull resistance (Rw)
of O.ln and O.ln are plotted in Figure 6, these resistances representing the
approximate limit of the range of value for F~, likely to be encountered on
vessels. As shown in the aforesaid figure, for a maximum change in the
bearing resistance, the modulation current varies from one plateau to
another. By lowering the offset voltage, thc~ upper conductance plateau is
reduced, which diminishes the actual difference between the two plateaus.
The transistor bias voltage, Vgs, and the power supply output voltage, Vs,
are set to zero for the calculation.
1 341 3fi8
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The circuit diagratn of the present invention, the system generally
being designated as 15, is depicted in Figure 4. System 15 consists of a
voltage-controlled current driver 28 preceded by a differential voltage
amplifier 17. Current. driver 28 comprises eight voltage drivers, 18, placed
in parallel, in a current feedback loop. Figure 5 illustrates the circuit
for one of the voltage drivers 18~ All drivers are identical. The
drivers 18 are stacked at the points IN, D(drain), and S(source), as shown in
figure 5. This multi-feedback approach allows the high current Field Effect
Transistors (FET) of voltage drivers 18 to safely be placed in parallel. The
0.019 resistance is included to limit the current differences between the
fET's to a maximum of 100 mA. Differential amplifier 17 ensures impedance
matching between the two inputs.
The operational amplifiers used in system 15 were chosen to optimize
the performances and minimize the number of adjustments on the device. The
LM318 amplifiers were used at points where the noise and offset voltage were
not critical., so as to minimize the loss in bandwidth. The OP-10 and OP-5
amplifiers were used at the input of system 15 because of their low noise and
low offset voltage; in voltage drivers 18, because of their low offset
voltages, which limits the static current between the FET's; and in the
current sensing amplifier, to minimize noise. (The output current is sensed
through a 3 mil resistance which develops voltages comparable to the input
noise voltage of a LM318 amplifier at low current.)
The frequency compensation of system 15 is an important step which
has a significant effect on the overall performance of the grounding
configuration. System 15 impresses a current as a result of a voltage
control. This current couples to the propeller-to-hull impedance and creates
a voltage that is then fed back to the input. The propeller-to-hull
impedance (R", in Figure 3) is part of the feedback loop, and must be known in
order to stabilize system 15. As indicated previously, the real part of the
impedance ranges from O.O1C1 to O.ln, depending upon the propellers used, and
as a safety measure, the largest value of O,lT~ should be assumed, since this
1 341 368
-lo-
ensures stability at the highest possible voltage gain. Furthermore, a O.lfl
resistance is added at the output of the device (as illustrated in Figure 4)
to keep the output impedance below O.ln if the brush/slip-ring resistance
becomes large.
In principle, the stability of system 15 is achieved by reducing the
open loop gain to unity or less, at frequencies at which the phase is greater
or' equal to 180 degrees, As shown in Figure 5, each individual element 18 of
current driver 28 is compensated by a 10 Kt1 resistor R,; and a 10 pF
capacitor
C~ before compensation is carried out on the overall system. An extra stage
30 is added for this purpose, and placed between the output current driver
and instrumentation amplifier 17, This stage enables wide band over-
amplification of the input signal, which often saturates preamplifier 17 with
undesirable high frequency signals, to be avoi..ded. An inductor 31 is also
included in the output current feedback loop so as to allow for the
propeller-to-hull inductance. Inductor 31 enables over-compensation of the
unit to be avoided and prevents undesirable oscillations.
The overall physical layout of system 15 is dictated by power
consumption considerations. The modular output is preferably arranged so
that the FET of each of eight voltage drivers 18 will draw an equal current.
If the system 15 device is driven to its maximtun current, 100 A, the current
in each FET will be 12.5 amperes and the combined power consumption will be
at most 62.5 watts. A standard switching power supply (not herewith
depicted) of 100 amperes, five volts, is used for system 15.
Three sets of wires are connected to the case containing system 15.
A :first twisted pair of wires (not shown) , the power line which supplies up
to five amperes, should be routed as far as possible from the other
conductors in order to reduce 60 Hz pick up. A second shielded twisted pair
of wires 19 is used for sensing the potential between shaft 10 and hull 12.
These conductors may be any convenient size, since the line impedance is
greater than 100 Ktl. A third pair of large wires 20 carries the current from
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-11-
shaft 10 and returns it to the hull. The total. resistance of wires 20 should
be chosen such that the voltage drop is insignificant compared to five volts,
less than 0.2 volts at 100 amperes being acceptable. Lengths of No. 4
conductor wire can be used for this purpose.
Three meters are required to monitor the operar_ion of system 15. One
meter (not herewith depicted) indicates the operating shaft potential in
tenths of millivolts. The output of system 15 is indicated by a second meter
22, and the condition of the current connection to shaft is shown by a third
meter 24, which measures the output voltage of current driver 28 (also known
as the grounding voltage). If this latter voltage becomes greater than 0.5
volts, cleaning of the slip-ring may be necessary. I.f this fails to reduce
the grounding voltage, then the continuity of the current carrying wires and
the slip-ring to shaft contact resistance must be checked.
EFFECTIVENESS OF THE INVENTION
Evidence of the effectiveness of the present invention in reducing
the shaft-rate modulated current that flows through the shaft and propeller
unto the sea-water, thus giving rise to the vessel's signature, will now be
described for a particular implementation of the invention using the
foregoing preferred embodiment.
In the absence of an active shaft-grounding system, the shaft-to-hull
voltage of a typical vessel was measured and was as depicted in Figure 1b,
and by using Fourier Transform spectral analysis techniques was found to have
the large variations depicted in the upper traces of. Figure 7a for the
shaft-rate frequency and its first few harmonics. When the active grounding
system described in,the present invention was implemented, the results of the
lower trace of Figure 7a were obtained, showing a reduction in modulation of
in excess of 70 dB, and the corresponding shaft-to-hull voltage being as
depicted in Figure 7b. As the measure of the modulated shaft-to-hull voltage
is directly proportional to the magnitude of the shaft-rate modulated
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_12_
corrosion-current induced ELF electromagnetic signature, 'the signature is
also reduced by in excess of 70 dB. When the present invention was
implemented on the typical vessel for which the signature was as depicted in
Figure lc, the signature is substantially eliminated as depicted in Figure
7c..
The foregoing has shown and described a particular embodiment of the
invention, and variations thereof will be obvious to one skilled in the art.
Accordingly, the embodiment is to be taken as illustrative rather than
limitative, and the true scope of the invention is as set out in the appended
claims.
