U.S. patent application number 11/093310 was filed with the patent office on 2006-10-12 for remote monitoring system and method.
This patent application is currently assigned to General Electric Company. Invention is credited to Selaka Bandara Bulumulla, Richard Louis Frey, Joseph Alfred Iannotti, Glen Peter Koste, Todd Ryan Tolliver.
Application Number | 20060228121 11/093310 |
Document ID | / |
Family ID | 37083274 |
Filed Date | 2006-10-12 |
United States Patent
Application |
20060228121 |
Kind Code |
A1 |
Tolliver; Todd Ryan ; et
al. |
October 12, 2006 |
Remote monitoring system and method
Abstract
A remote control system for a modulatable device is provided.
The remote control system comprises a receiver system coupled to
the modulatable device and configured to obtain an output
characteristic of the modulatable device, the receiver system being
located remotely with respect to the modulatable device. The system
further comprises a command signal setting system coupled to the
receiver system and configured to use the output characteristic to
generate a drive command signal and a bias system coupled to the
command signal setting system and configured to receive the drive
command signal and set a bias point of the modulatable device based
on the drive command signal. The bias system is located locally
with respect to the modulatable device. The command signal setting
system and the bias system are coupled via a first optical
conduit.
Inventors: |
Tolliver; Todd Ryan;
(Clifton Park, NY) ; Iannotti; Joseph Alfred;
(Glenville, NY) ; Koste; Glen Peter; (Niskayuna,
NY) ; Bulumulla; Selaka Bandara; (Niskayuna, NY)
; Frey; Richard Louis; (Delanson, NY) |
Correspondence
Address: |
GENERAL ELECTRIC COMPANY;GLOBAL RESEARCH
PATENT DOCKET RM. BLDG. K1-4A59
NISKAYUNA
NY
12309
US
|
Assignee: |
General Electric Company
|
Family ID: |
37083274 |
Appl. No.: |
11/093310 |
Filed: |
March 29, 2005 |
Current U.S.
Class: |
398/198 |
Current CPC
Class: |
H04B 10/0799
20130101 |
Class at
Publication: |
398/198 |
International
Class: |
H04B 10/04 20060101
H04B010/04 |
Claims
1. A remote control system for a modulatable device, the remote
control system comprising: a receiver system coupled to the
modulatable device and configured to obtain an output
characteristic of the modulatable device, wherein the receiver
system is located remotely with respect to the modulatable device;
a command signal setting system coupled to the receiver system and
configured to use the output characteristic to generate a drive
command signal; and a bias system coupled to the command signal
setting system and configured to receive the drive command signal
and set a bias point of the modulatable device based on the drive
command signal, wherein the bias system is located locally with
respect to the modulatable device, wherein the command signal
setting system and the bias system are coupled via a first optical
conduit.
2. The remote control system of claim 1 wherein the receiver system
is coupled to the modulatable device via a second optical
conduit.
3. The remote control system of claim 1, wherein the modulatable
device comprises a device selected from the group consisting of
optical modulators, RF modulators, and laser diodes.
4. The remote control system of claim 1, wherein the command signal
setting system is configured to generate the drive command signal
by mapping a transfer function and obtaining an optimum position
for the drive command signal.
5. The remote control system of claim 4, wherein the modulatable
device comprises an optical modulator, wherein the output
characteristic comprises an output power, and wherein the command
signal setting system is configured to generate the drive command
signal based on a maximum output power and a minimum output power
generated by the modulatable device.
6. The remote control system of claim 5, wherein the command signal
setting system is configured to generate the drive command signal
by taking an average of a first drive signal corresponding to the
maximum output power and a second drive signal corresponding to the
minimum output power.
7. The remote control system of claim 1, wherein the command signal
setting system is configured to generate the drive command signal
based on output characteristic signal distortion.
8. The remote control system of claim 7, wherein the command signal
setting system is configured to generate the drive command signal
based on minimization of second order harmonic components.
9. The remote control system of claim 8, wherein the receiver
system includes a band pass filter centered about an expected
second order output characteristic location, and the command signal
setting system is configured to, during a training sequence,
generate the drive command signal by increasing the drive signal
until an output signal of the band pass filter is minimized and to
then, during an operation sequence, use the drive command signal
corresponding to the minimized output signal.
10. The remote control system of claim 8, wherein the command
signal setting system further comprises a pilot tone generator and
a band pass filter centered about an expected second order output
of the pilot tone generator, and wherein the command signal setting
system is further configured to generate a drive command signal
corresponding to a minimum output signal of the band pass
filter.
11. The remote control system of claim 10, wherein the pilot tone
generator is configured to provide a pilot tone to the modulatable
device.
12. The remote control system of claim 11, wherein the pilot tone
is generated during a pre-scan mode of a magnetic resonance
system.
13. The remote control system of claim 11, wherein the pilot tone
is generated during a scan mode of a magnetic resonance system.
14. The remote control system of claim 1, wherein the modulatable
device is configured for use in a magnetic resonance imaging
system.
15. A magnetic resonance (MR) system including a remote control
system for a modulatable device, the magnetic resonance system
comprising: a modulatable device located within a magnetic field of
the magnetic resonance system and configured to provide an output
characteristic of the modulatable device to an output
characteristic receive and command signal setting assembly situated
outside the magnetic field; a bias system optically coupled to the
output characteristic receive and command signal setting assembly
and coupled to the modulatable device, the bias system configured
to receive a drive command signal from the output characteristic
receive and command signal setting assembly and to set a bias point
of the modulatable device based on the drive command signal.
16. The MR system of claim 15, wherein the modulatable device is
coupled to output characteristic receive and command signal setting
assembly via an optical conduit.
17. The MR system of claim 15, wherein the modulatable device
comprises a device selected from the group consisting of optical
modulators, RF modulators, and laser diodes.
18. The MR system of claim 15, wherein the bias system is located
within the magnetic field.
19. The MR system of claim 15, wherein the output characteristic
receive and command signal setting assembly is configured to
generate the drive command signal by mapping a transfer function
and obtaining an optimum position for the drive command signal.
20. The MR system of claim 19, wherein the modulatable device
comprises an optical modulator, wherein the output characteristic
comprises an output power, and wherein the command signal setting
system is configured to generate the drive command signal based on
a maximum output power and a minimum output power generated by the
modulatable device.
21. The MR system of claim 20, wherein the command signal setting
system is configured to generate the drive command signal by taking
an average of a first drive signal corresponding to the maximum
output power and a second drive signal corresponding to the minimum
output power.
22. The MR system of claim 15, wherein the command signal setting
system is configured to generate the drive command signal based on
output characteristic signal distortion.
23. The MR system of claim 22, wherein the command signal setting
system further comprises a pilot tone generator and a band pass
filter centered about an expected second order output of the pilot
tone generator, and wherein the command signal setting system is
further configured to generate a drive command signal corresponding
to a minimum output signal of the band pass filter.
24. The MR system of claim 23, wherein the pilot tone generator is
configured to provide a pilot tone to the modulatable device.
25. The MR system of claim 23, wherein the pilot tone is generated
during a pre-scan mode of a magnetic resonance system.
26. The MR system of claim 23, wherein the pilot tone is generated
during a scan mode of the magnetic resonance system.
27. A method of remote control for a modulatable device comprising:
remotely obtaining an output characteristic of the modulatable
device, wherein the receiver system is located remotely with
respect to the modulatable device; using the output characteristic
to generate a drive command signal; and locally obtaining the drive
command signal and setting a bias point of the modulatable device
based on the drive command signal.
28. The method of claim 27, wherein the locally obtaining is via an
optical conduit.
29. The method of claim 28, wherein the drive command signal is
generated by mapping a transfer function and obtaining an optical
position for the drive command signal.
30. The method of claim 29 wherein the modulatable device comprises
an optical modulator, wherein the output characteristic comprises
an output power, and the drive command signal is generated based on
a maximum output power and a minimum output power generated by the
modulatable device.
31. The method of claim 30, wherein the drive command signal is
generated by taking an average of a first drive signal
corresponding to the maximum output power and a second drive signal
corresponding to the minimum output power.
32. The method of claim 28, wherein the drive command signal is
generated based on output characteristic signal distortion.
33. The method of claim 28, wherein the modulatable device is
adapted for use in a magnetic resonance system.
34. The method of claim 33, wherein the drive command signal is
generated using a pilot tone.
35. The method of claim 34, wherein the pilot tone is generated
during a pre-scan mode of a magnetic resonance system.
36. The method of claim 34, wherein the pilot tone is generated
during a scan mode of the magnetic resonance system.
Description
BACKGROUND
[0001] The invention relates generally to modulatable devices and
more specifically to a method and a system for remotely setting a
bias point of the modulator.
[0002] External modulation of a continuous wave source using an
optical modulator such as a Mach Zehnder modulator is a widely used
method in analog optical links. Optical modulators typically have a
nonlinear transfer function for the conversion of electrical
modulation into optical modulation. In order to maintain a link
with low distortion, the modulator is biased at a certain point in
the transfer function, typically known as the bias point.
[0003] The optimal bias point may drift over time, for various
reasons such as aging of the modulator, temperature, and other
material related phenomena. To maintain the optimal bias point,
this drift must be tracked and then the applied bias adjusted to
reflect the new state of the transfer function. This requires some
amount of external control electronics to process information from
the modulator. In some applications, such as magnetic resonance
imaging (MRI), where it may be desirable to place the modulator at
the receive coils, there is limited space, limited electrical power
and the environment may be harmful to electronics due to
interference effects.
[0004] In addition, in MRI applications, the modulator bias is
required to be set before each scan of a patient. When the bias is
set, the scanning may be done within a time frame where the bias
remains at the optimum level. Other applications may be in remote
sensing, where a low distortion link is required, but only for a
duration that is sufficiently short so that the bias does not drift
from its optimum value during the time it is in operation.
[0005] Typically, the bias point control has been addressed by use
of a pilot tone. The pilot tone is mixed with the modulation signal
and the bias point is adjusted through a control loop that uses
harmonics of the pilot tone or phase differentials of the pilot
tone. However, mixing a pilot tone with the modulation signal can
introduce distortion to the modulation signal, limiting the dynamic
range of the link. In addition, a stable and accurate local clock
source is required to generate the pilot tone, which may not always
be available.
[0006] Therefore, there is a need to remotely monitor and control
the bias of a modulatable device used in applications where minimum
distortion and high dynamic range is desired.
BRIEF DESCRIPTION
[0007] Briefly in accordance with one aspect of the invention, a
remote control system for a modulatable device is provided. The
remote control system comprises a receiver system coupled to the
modulatable device and configured to obtain an output
characteristic of the modulatable device, the receiver system being
located remotely with respect to the modulatable device. The remote
control system further comprises a command signal setting system
coupled to the receiver system and configured to use the output
characteristic to generate a drive command signal. In addition, the
remote control system further comprises a bias system coupled to
the command signal setting system and configured to receive the
drive command signal and set a bias point of the modulatable device
based on the drive command signal. The bias system is located
locally with respect to the modulatable device, and the command
signal setting system and the bias system are coupled via a first
optical conduit.
[0008] In another embodiment, a magnetic resonance (MR) system
including a remote control system for a modulatable device is
provided. The magnetic resonance system comprises a modulatable
device located within a magnetic field of the magnetic resonance
system and configured to provide an output characteristic of the
modulatable device to an output characteristic receive and command
signal setting assembly situated outside the magnetic field. The
system further comprises a bias system optically coupled to the
output characteristic receive and command signal setting assembly
and coupled to the modulatable device. The bias system is
configured to receive a drive command signal from the output
characteristic receive and command signal setting assembly and to
set a bias point of the modulatable device based on the drive
command signal.
[0009] In another embodiment, a method of remote control for a
modulatable device is provided. The method comprises remotely
obtaining an output characteristic of the modulatable device, the
receiver system being located remotely with respect to the
modulatable device. The method further comprises using the output
characteristic to generate a drive command signal and locally
obtaining the drive command signal and setting a bias point of the
modulatable device based on the drive command signal.
DRAWINGS
[0010] These and other features, aspects, and advantages of the
present invention will become better understood when the following
detailed description is read with reference to the accompanying
drawings in which like characters represent like parts throughout
the drawings, wherein:
[0011] FIG. 1 is a block diagram illustrating one embodiment of a
remote monitoring system implemented according to one aspect of the
invention;
[0012] FIG. 2 is a flow chart illustrating one method by which a
transfer function of a modulatable device is traced;
[0013] FIG. 3 is a block diagram of one embodiment of a command
signal setting device;
[0014] FIG. 4 is a block diagram of another embodiment of a command
signal setting device; and
[0015] FIG. 5 is a block diagram of an embodiment of a magnetic
resonance imaging system implemented according to one aspect of the
invention.
DETAILED DESCRIPTION
[0016] FIG. 1 is a block diagram illustrating one embodiment of the
invention implemented to remotely set a bias point of a modulatable
device. FIG. 1 illustrates a modulatable device 12, a receiver
system 16, a command signal setting system 20 and a bias system 24.
Each component is described in further detail below.
[0017] As used herein, "adapted to", "configured" and the like
refer to devices in a system to allow the elements of the system to
cooperate to provide a described effect; these terms also refer to
operation capabilities of electrical or optical elements such as
analog or digital computers or application specific devices (such
as an application specific integrated circuit (ASIC)), amplifiers
or the like that are programmed to provide an output in response to
given input signals, and to mechanical devices for optically or
electrically coupling components together.
[0018] Modulatable device 12 receives signals from signal source 10
as shown in FIG. 1. A modulatable device in general may be defined
as a device that modulates a carrier signal with a modulation
signal for transmission over a network. A modulatable device has a
bias point which when adjusted enables the generation of a high
quality modulated signal. One example of a carrier signal source is
a constant power laser source used in an analog optical
transmission system. It may be noted that modulatable device may
include one or more modulatable devices.
[0019] The modulatable device modulates the signals as required and
generates a modulated output signal. Examples of modulatable
devices include optical modulators, RF modulators, and laser
diodes. In a more specific embodiment, the modulatable device is a
Mach-Zehnder modulator.
[0020] Receiver system 16 is coupled to the modulatable device and
configured to obtain an output characteristic from the modulated
output signal generated by the modulatable device. The output
characteristic may be an output power, spectral content, voltage or
current.
[0021] It may be noted that the receiver system is located remotely
with respect to the modulatable device. For example the modulatable
device when used in an MRI system is situated in a shielded room
and the receiver system is situated outside the shielded room, such
as, for example in a console room, or in an unshielded portion of
the room in which the MRI system is situated. In one embodiment,
the receiver system is an optical receiver system.
[0022] Command signal setting (CSS) system is coupled to the
receiver system via line 18 and configured to use the output
characteristic to generate a drive command signal. Line 18 may be
an optical conduit or an electrical wire. The manner in which the
command signal setting system may generate the drive command signal
is described in more detail with reference to FIG. 2.
[0023] Continuing with FIG. 1, bias system 24 is coupled to the
command signal setting system and is configured to receive the
drive command signal and set a bias point of the modulatable device
based on the drive command signal. It may be noted that the bias
system is typically located locally with respect to the modulatable
device. The drive command signal is converted to a voltage or
current proportionate to the intensity of the drive command signal
and the voltage or current is applied to the modulatable device via
line 26. Line 26 may be an optical conduit or an electrical wire.
In one specific embodiment, the bias system is implemented using a
photodiode and a resistor.
[0024] In one embodiment, the command signal setting system and the
bias system are coupled via a first optical conduit 22. In a more
specific embodiment, the receiver system 16 is coupled to the
modulatable device using a second optical conduit 14. The optical
conduit may comprise either a single fiber or multiple fibers. It
may be noted that the receiver system may be coupled to the
modulatable device using an electrical wire. The signals may also
be transmitted using wireless signal transmission.
[0025] As described earlier, the command signal setting system 20
is configured to generate the drive command signal, which in turn
is used to set the bias point of the modulatable device. The drive
command signal may be based on a desired factor with some examples
including device life time, distortion reduction, noise reduction,
and signal path gain.
[0026] In one embodiment, the command signal setting system is
configured to generate the drive command signal by mapping a
transfer function and obtaining an optimum bias point for the drive
command signal. In a more specific embodiment, the transfer
function is traced to generate the drive command signal by
measuring an output power of the modulatable device as shown in the
flow chart of FIG. 2. Thus, the output characteristic in such an
embodiment is the output power. Each step is described in further
detail below.
[0027] In step 30, an initial output power of the modulatable
device is measured based on an initial drive current I.sub.0. In
step 32, the initial drive current is increased until the output
power of the modulatable device is at a maximum. The drive current
Imax corresponding to the maximum output power is calculated.
[0028] In step 34, the initial drive current is decreased until the
output power of the modulatable device is at a minimum. The drive
current Imin corresponding to the minimum output power is
determined. In step 36, an average of the minimum drive current
Imin and the maximum drive current Imax is calculated. In step 38,
the drive current is generated based on the average calculated in
step 36.
[0029] FIG. 3 illustrates a block diagram of one embodiment of a
command signal setting device used to implement the steps of FIG.
2. The command signal setting system comprises control logic unit
40, an optical source 44, a driver for the optical source 42 and a
fiber 46. The control logic unit executes the steps of the
algorithm given in FIG. 2. Examples of the control logic unit
include micro-controllers, microprocessors or other software
programmable devices within systems such as magnetic resonance
imaging systems or a hardware circuit. The output of the control
logic unit proportionately varies the intensity of light of source
44 by generating a drive current 43 through the driver 42. The
light source may be a light emitting diode or a laser source, for
example. The light is coupled to the fiber 46, which is an example
of optical conduit 22 in FIG. 1.
[0030] In another embodiment, the command signal setting system is
configured to generate the drive command signal based on signal
distortion or spectral content. In a more specific embodiment, the
command signal setting system is configured to generate the drive
command signal based on minimization of second order harmonic
components.
[0031] One advantage of using the second harmonic components is
that a drift of the bias point of a Mach Zehnder modulator can be
tracked by monitoring the second harmonic components. For example,
if the second harmonic component increases, it can be assumed that
the bias point has drifted. The bias point can be then be adjusted
to achieve optimum setting again. Such an adjustment can be made
even during the operation of the system in which the remote
monitoring system is implemented is operational.
[0032] In a specific embodiment where the drive command signal is
determined by monitoring distortion, the receiver system includes a
bandpass filter. The optimum bias point results in a minimum second
harmonic distortion at the output of a band pass filter. The drive
command signal is generated by measuring the output of the bandpass
filter for an initial drive current and increasing the drive
current until the output at the bandpass filter is minimum. The
corresponding drive current or voltage is set as the drive command
signal and is maintained during an operation sequence.
[0033] In a another embodiment where the drive command signal is
determined by monitoring distortion, the command signal setting
system comprises a band pass filter and a pilot tone generator as
shown in FIG. 4. The command signal setting system comprises
band-pass filter 50, a control logic unit 52 a pilot tone generator
54, a coupling mechanism 56, a driver 58, a light source 60 and a
fiber 62. Fiber 62 is an example of optical conduit 22 in FIG.
1.
[0034] The band pass filter response is centered on the second
harmonic frequency of the pilot tone signal. The output of the
band-pass filter indicates the strength of the second harmonic. The
control logic unit uses the output from band-pass filter to
increase or decrease a dc voltage into the coupling mechanism.
Examples of the pilot tone generator include a crystal oscillator
or other frequency stable clock sources.
[0035] The pilot tone is combined to the output of the control
logic unit through the coupling mechanism. The coupling is usually
implemented through the use of a bias-t, which comprises of an
inductor 56 and capacitor 57. The combined output of the coupling
mechanism then drives the light source.
[0036] The command signal setting (CSS) system is configured to
generate a drive command signal corresponding to a minimum output
signal of the band pass filter. In the embodiments where the
modulatable device is used in an MRI system, the pilot tone is
generated during a pre-scan mode of a magnetic resonance system or
during a scan mode of the magnetic resonance imaging system.
[0037] The remote sensing system described above may be implemented
in various systems. An example system where the remote sensing
system is implemented is a magnetic resonance imaging system as
described in detail below. FIG. 5 is a block diagram of a magnetic
resonance imaging system employing one embodiment of a remote
sensing system. Each component is described in further detail
below.
[0038] Magnetic assembly 64 comprises radio frequency coils and
gradient coils that are used to induce an electromagnetic field
around an object in order to generate images. In order to generate
magnetic resonance signals that are representative of regions of
the object, electrical devices such as amplifiers (not shown) and
modulatable device 12 are used. The magnetic assembly 64 and the
modulatable device 12 are located within a shielded environment
66.
[0039] Modulatable device 12 is configured to provide an output
characteristic of the modulatable device to an output
characteristic receives and command signal setting assembly 68
situated in a control room or area 70. Control room 70 is located
outside the magnetic field. In one embodiment, the output
characteristic receive and command signal setting assembly 68
comprises receiver system 16 and command signal setting (CSS)
system 20 as described with reference to FIG. 1. The receiver
system and the CSS system may be separate systems coupled together
or may be one integral system.
[0040] Bias system 24 is optically coupled to the output
characteristic receive and command signal setting assembly using
optical conduit 22 and is also coupled to the modulatable device.
The bias system is located within or outside the magnetic field. In
one embodiment, the modulatable device is coupled to output
characteristic receive and command signal setting assembly via an
optical conduit.
[0041] The bias system is configured to receive a drive command
signal from the output characteristic receive and command signal
setting assembly and to set a bias point of the modulatable device
based on the drive command signal. The drive command signal may be
generated using any one of the techniques described with reference
to FIG. 2, FIG. 3 and FIG. 4.
[0042] While only certain features of the invention have been
illustrated and described herein, many modifications and changes
will occur to those skilled in the art. It is, therefore, to be
understood that the appended claims are intended to cover all such
modifications and changes as fall within the true spirit of the
invention.
* * * * *