Note: Descriptions are shown in the official language in which they were submitted.
CA 02571177 2006-12-14
DYNAMIC POWER SPLITTER
BACKGROUND OF THE INVENTION
1) Field of the Invention
The invention relates to microwave-assisted heating, and more particularly, to
systems for microwave processing of a plurality of laboratory samples.
2) Description of the Prior Art
Most chemical reactions either require or benefit from the application of
heat.
Developments have provided for the use of microwave heating instead of typical
Bunsen burners or "hot plates". The use of microwave energy is known to be
quite appropriate for many chemical reactions. Microwave heating represents
the
use of radiation energy at wavelengths residing in the electromagnetic
spectrum,
or between the far infrared and the radio frequency (from about one millimeter
(mm) to about 30 centimeters (cm) wavelengths, or with corresponding
frequencies in the range of about 1 to 300 gigahertz (GHz)). The exact upper
and
lower limits defining "microwave" radiations are somewhat arbitrary.
Microwave radiation is widely used in several fields like spectroscopy,
communication, navigation, medicine, and heating. Substances that respond
quite well by increasing their temperature levels when under microwave
radiation
usually have a high dielectric absorption. The use of microwave heating in
laboratories is known to people skilled in the art and is often referred to as
"microwave assisted" chemistry. A riumber of laboratory microwave heating
devices are thus commercially available. These microwave heating devices
typically use a magnetron as the microwave source, a waveguide (usually hollow
circular or rectangular metal tube of uniform cross section) to guide the
microwaves, and a resonator (sometimes also referred to as the "cavity") into
which the microwaves are directed to heat a sample. The microwave source can
also be a Klystron, traveling wave tubes, oscillators, and certain
semiconductor
devices. Most devices use magnetrons, however, as these are simple and
economical. One disadvantage of magnetrons is that the control of radiation
18000-1 CA - 1 -
CA 02571177 2006-12-14
power directed towards a specific sample inserted inside a resonator is
somewhat complex. One known method of controlling the radiation of the
magnetron is to run it at its designated constant power while turning it on
and off
on a cyclical basis in order to have a certain temperature control of the
sample(s)
located inside separate containers or loads made of a microwave transparent
material such as some types of glass, plastic or ceramic. Usually, for
convenience, only one load is monitored within the group of loads each
containing a sample, the remaining loads estimated to behave somewhat
similarly. This leads to large amounts of uncertainty as to the evolution of
reactions inside other loads, since even when a "stirring" device can produce
quite uniform radiation inside the cavity of a microwave heater, several other
factors, such as the presence of samples and sample containers in the
microwave oven, can also change the interference pattern within the cavity and
thus affect the energy distribution inside the cavity.
Accordingly, when multiple samples are to be treated under one microwave
source, the treatment should be uniform and controllable. Hence, there is a
need
to provide for the ability to vary the radiation power levels sent to each
sample
using a limited number of microwave sources in order to maintain low costs and
high efficiency. There is also a need to be able to precisely know and control
the
temperature or amount of radiation power sent to each individual sample.
SUMMARY OF THE INVENTION
There is described herein a system wherein a single microwave source is
cascaded with microwave splitters and applicators such that a precise control
of
radiation power is offered to each sample placed within a vessel,
alternatively
referred to as a load. Stepper motors and feedback mechanisms are used to
control each microwave splitter according to a desired end result. While the
cascading provides the ability to use only one microwave source for a group of
multiple loads, the control of the microwave splitters offers the ability to
precisely
direct a certain amount of radiation power to the subsequent level of
microwave
splitters, until the cascade reaches an end characterized by an applicator
18000-1 CA - 2 -
CA 02571177 2006-12-14
dedicated to an individual load. The amount of power reaching the end of the
cascade is therefore precisely known and controllable.
According to one aspect of the present invention, there is provided an
apparatus
for microwave heating comprising: a microwave source for generating
electromagnetic radiation; a first microwave radiation splitter connected to
the
microwave source via an input port and having at least two output ports for
outputting the electromagnetic radiation received at the input port; at least
one
dielectric element placed inside the first microwave radiation splitter
between the
at least two output ports and adapted to dynamically direct the
electromagnetic
radiation received at the input port to the at least two output ports
according to a
power splitting ratio; and a load connected to each of the at least two output
ports
for receiving the electromagnetic radiation.
According to another aspect of the present invention, there is provided a
method
for directing electromagnetic power from an input port to at least two output
ports
in a power splitter, the method comprising: providing at least one dielectric
element inside the power splitter; receiving the power at an input port;
positioning
the at least one dielectric element between the at least two output ports to
dynamically direct the power thereto according to a power splitting ratio; and
outputting the power to the at least two output ports in accordance with the
power
splitting ratio.
According to yet another aspect of the present invention, there is provided a
power splitter for directing electromagnetic power comprising: an input port
for
receiving the electromagnetic power; at least one dielectric element placed
inside
the power splitter; at least two output ports for outputting the power
according to
a splitting ratio, the at least two output ports placed on a surface opposite
to the
input port; and at least one dielectric moving device for positioning the at
least
one dielectric element between the at least two output ports to dynamically
direct
the power into the at least two output ports according to the power splitting
ratio.
18000-1 CA - 3 -
CA 02571177 2006-12-14
BRIEF DESCRIPTION OF THE DRAWINGS
Further features and advantages of the present invention will become apparent
from the following detailed description, taken in combination with the
appended
drawings, in which:
Fig. 1. shows a microwave heating device according to a first embodiment of
the
invention;
Fig.2a shows a microwave source with a primary microwave splitter and a
stepper motor according to an embodiment of the present invention;
Fig, 2b shows a top view of the cavity of the primary microwave splitter of
Fig. 2a
in accordance with an embodiment of the invention;
Fig.3a shows a secondary microwave splitter and stepper motor, according to an
embodiment of the present invention;
Fig, 3b shows a top view of the cavity of the secondary microwave splitter of
Fig.
3a in accordance with an embodiment of the invention;
Fig.4a is a schematic illustrating a two-level cascade system in accordance
with
an embodiment of the invention;
Fig.4b is a schematic illustrating a one-level cascade system in accordance
with
an embodiment of the invention; and
Fig. 5 is a schematic illustrating the position of elements within the cavity
of a
microwave splitter in accordance with an embodiment of the invention.
It will be noted that throughout the appended drawings, like features are
identified
by like reference numerals.
DETAILED DESCRIPTION OF THE INVENTION
Referring to Fig. 1, and according to an embodiment of the present invention,
a
rack 102 containing twelve vessels 101 (herein referred to as loads containing
sample mixtures for example) is inserted inside the microwave-assisted
18000-1 CA - 4 -
CA 02571177 2006-12-14
processing system made of a metal tunnel-shaped cavity through microwave-
safe doors 108 and 109. Twelve applicators 100 are used to direct the heat to
the
loads 101 individually. The applicators are to be understood as being energy
directing devices that transmit the energy to the loads, like antennas. The
applicators are optional to the system but are used in the embodiment
described
herein. Multiple applicators can be connected together to redirect the energy
in a
desired direction to a desired destination. In the embodiment shown in the
figure,
six applicators 100 are located on each side of the microwave-assisted
processing system such that each is placed at the corresponding position of a
load 101 once the rack 102 is placed inside the system. The loads 101 can be
in
vessels made of various microwave transparent materials depending on the
sample type and mixture. Examples of possible materials include but are not
limited to some types of glass, plastic, ceramic, or more specifically, quartz
and
Perfluoroalkoxy (PFA). The position of the applicators along with the inserted
loads 101 is determined during fabrication using a network analyzer for
example.
Once the rack 102 containing the loads 101 is inserted inside the cavity, the
loads 101 are automatically in their correct positions with respect to the
applicators 100. Each applicator 100 receives radiation energy according to a
splitting ratio of a variable microwave radiation splitter 103. A coaxial
cable 106
connected to one of the two output ports 300 of the variable microwave
radiation
splitter 103 (also referred to as a secondary microwave radiation splitter) is
used
to transmit the radiation energy from the output port of the splitter 103 to
the
applicator 100, as determined by the control of a stepper motor 104 located on
each variable microwave radiation splitter 103.
According to the illustrated embodiment of Fig. 1, since there are six loads
to be
heated on each side of the system, each pair of loads being controlled by a
single
variable microwave radiation splitter 103 with its stepper motor 104, there
are
thus six variable microwave radiation splitters 103 and stepper motors 104. In
a
preferred embodiment, only one exhaust fan is installed on the cavity (not
shown)
in order to release unwanted fumes in case a vessel breaks inside the cavity,
but
more than one may be present. Other safety features can also be added to
prevent vessel rupture and operator harm. Each variable microwave radiation
18000-1 CA - 5 -
. I I . I
CA 02571177 2006-12-14
splitter 103 receives radiation energy from one of the two outputs of another
variable microwave radiation splitter 201, itself controlled by another
stepper
motor 104. The variable microwave radiation splitter 201 is for splitting the
power
received from a source of microwave radiation 200, herein shown as a
magnetron.
More particularly, and referring to Fig. 2a, the source of microwave radiation
200,
is mounted on a variable microwave radiation splitter 201. The variable
microwave radiation splitter 201 is also dynamically controlled by a stepper
motor
104 with a feedback signal coming from temperature monitoring of samples 101.
For example, temperature feedback can be implemented using any temperature
sensor, such as IR sensors, located underneath each load. The variable
microwave radiation splitter 201 is also referred to as a primary microwave
splitter. Referring to Fig. 2b, variable microwave radiation splitter 201
performs a
first division of the radiation energy of the microwave source 200 received at
an
input port 205 in accordance with a first splitting ratio. Input port 205 is
located on
one side of the rectangular waveguide forming the variable microwave radiation
splitter 201. The radiation energy is then outputted into two output ports 300
located on a second opposite side. The control of the splitting ratio is
provided by
the stepper motor 104 (shown in Fig. 2a), which moves, or rotates, a
dielectric
element 105 placed inside the rectangular waveguide cavity forming the
variable
microwave radiation splitter 201, and via the hole or shaft 206. More
particularly,
the dielectric element 105 is placed and moved between the two output ports
300, and as shown later in Fig. 5.
Fig. 3a shows the variable microwave radiation splitter 103, dynamically
controlled by the stepper motor 104. The variable microwave radiation splitter
103 is referred to as a secondary microwave splitter as it performs a second
division of the radiation energy from the microwave source in accordance with
a
second splitting ratio. Radiation energy already split by a first variable
microwave
radiation splitter (element 201 in Fig. 2a) is received at an input port 205
(Fig. 3b)
located on a first side of the rectangular waveguide forming the variable
microwave radiation splitter 103. This power is then split once again
according to
the second splitting ratio and is directed into two output ports 300 located
on a
18000-1 CA - 6 -
I I 1 I
CA 02571177 2006-12-14
second side opposite to the first side where the input port is located. The
control
of this second splitting ratio is provided by the associated stepper motor
104,
which moves or rotates a dielectric element 105 placed inside the rectangular
waveguide cavity forming the variable microwave radiation splitter 103 in the
same manner as described above, and via the rotation hole or shaft 206 (Fig.
3b).
Fig. 4a illustrates both primary 201 and secondary 103 microwave radiation
splitters as they are assembled inside the system according to one embodiment.
For each pair of secondary microwave radiation splitters 103, one magnetron
200
connected to a primary splitter 201 communicates radiation energy to each
individual secondary splitter 103 via a coaxial connector 106 connected to its
two
output ports 300 according to a first splitting ratio. This first splitting
ratio is
controlled by the stepper motor 104 and a feedback mechanism coming from the
monitoring of four loads (A, B, C and D for example) in order to treat each
pair of
loads 101 (A - B, and C - D) as desired. Each secondary splitter 103
communicates part of the received radiation energy to each dedicated
applicator
100 and according to a second splitting ratio. This second splitting ratio is
controlled by the stepper motor 104 and a feedback mechanism coming from the
monitoring of each individual load in order to treat each load 101 within each
pair
of loads as desired. Insertion sleeves 402 are also used to connect each input
and output port to the coaxial cables 106.
A one-level cascade system consists of two loads 101, one variable microwave
radiation splitter 201 and one source of radiation energy 200, as illustrated
in Fig.
4b. A two-level cascade system, as in Fig. 4a, consists of four loads 101, two
secondary variable microwave radiation splitters 103, one primary variable
microwave radiation splitter 201, and one source of radiation energy 200. The
system can also be made of a three-level cascade arrangement or more.
In a two-level cascade arrangement, the difference in temperature between the
pair of loads A and B is used to control the splitting ratio of the secondary
splitter
103. Similarly, the difference in temperature between the pair of loads C and
D is
used to control the splitting ratio of the secondary splitter 103. Once the
18000-1 CA - 7 -
I 1 I
CA 02571177 2006-12-14
temperatures of the two pairs of loads are as desired and within a given
tolerance
level, the second splitting ratio of the secondary splitter 201 is dynamically
controlled in such a way to achieve a balanced temperature for each of the two
pairs of loads; i.e. A and B is one set of temperatures to be compared to C
and D
for the other set of temperatures. The same principle applies for other groups
of
four loads; E, F, G and H. Software may be programmed to perform the above-
described procedure, as is understood by a person skilled in the art.
Referring to Fig. 5, the dielectric element 105 placed inside variable
microwave
radiation splitters (201 and 103) can be designed in the shape as illustrated
in the
drawings or in any other shape to provide for high splitting efficiency. The
dielectric element 105 can be made of an aggregate of several different
materials
with a high permittivity, such as Teflon or alumina. For example, a material
made
of 99.9% alumina is found to be very effective. When the dielectric element
(105)
is rotated between the two output ports 300 by the stepper motor 104 up to an
angle of 170 degrees, the arrangement provides for up to 5dB of control in the
difference between the radiation power sent to each of the two output ports
300.
When the dielectric element 105 is in its original position, i.e. not rotated
or in
what is referred to as the zero degree position, the dielectric element 105
provides up to a 3dB difference between the radiation power sent and the two
output ports 300. While the positioning of the dielectric element 105 inside
the
cavity forming the variable microwave radiation splitters (201 or 103) may be
varied to change the power splitting ratio, the placement of the input port
205 and
output ports 300 will further determine the power splitting efficiency.
Fig. 5 illustrates how all the elements present in the cavity of the microwave
splitter are positioned with respect to each other according to an embodiment
that
provides for a relatively high power splitting efficiency. Various other
designs are
however possible. For example, the cavity of the microwave splitter (103 or
201)
can either be rectangular, square-like or even cylindrical. In one embodiment,
the
cavity shape can take, for example, a rectangular size 72.14 millimeters (mm)
by
34.01 mm, such that it is functional in the S-band of frequencies. Good
adaptation and contrasts were also achieved with a length of 72 mm and 75 mm,
which may be varied and further depends on the placement of the ports (205,
18000-1 CA - 8 -
I NI" =1 1
CA 02571177 2006-12-14
300) and the dielectric element 105 as well as the shape of the cavity. Hence,
the
placement of the input 205 and output ports 300 as well as the dielectric
element
105 are determinant and can be varied depending on the various specifications
needed for the microwave splitter design. For example, still in the S-band of
frequencies, good adaptation can be achieved by placing the input port 26 mm
from one end of the cavity and 36 mm from a side of the cavity at a height of
24
mm
Moreover, in Fig. 5, the dielectric element 105 is rectangular in shape (for
example, 5 mm by 10 mm by 32 mm) and placed such that its height extends
from a first side of the cavity having an input port 205 to a second side
opposite
to the first side of the cavity and having the output ports 300. The placement
and
shape of the dielectric element 105 can be changed. For example, it was found
that when the displacement of the dielectric 105 is performed closer to the
output
ports 300, the contrast between the output powers is better. Also,
displacement
performed behind the output ports 300 results in a better adaptation. A
circular
movement or a rotation of the dielectric element 105 around an axis 501
parallel
to its height provides for a combination of both higher contrasts and better
adaptation. The circular movement can be achieved though the use of an arm
502 connecting the tip of the dielectric element 105 with a directing device
or a
motor through a hole or shaft 206 following the axis of rotation 501. The hole
or
shaft 206 does not cause any further coupling effects if the hole is
maintained
small enough in diameter; for example 1.5 mm.
Both primary and secondary variable microwave radiation splitters (201 and
103)
disclosed herein are not limited to controlling heat directed to each load
placed
within the system. Any embodiment wherein the splitter is used to control a
source of radiation energy towards two or more outputs falls within the scope
of
this invention. More precisely, the variable microwave radiation splitters
(201 and
103) disclosed herein are used to control how radiation energy or power is
directed between two or more output ports 300. The system and variable
microwave radiation splitters (201 and 103) can also function at other
frequencies, and is not restricted to using sources that emit at the typical
microwave frequency of 2.54GHz. The microwave radiation source 200 can be
18000-1 CA - 9 -
I
CA 02571177 2006-12-14
any appropriate source, including magnetrons, klystrons, traveling wave tubes,
various electronic oscillators and solid states sources including various
transistors and diodes. It should also be understood that the displacement of
the
dielectric may be translational and/or rotational. The shape of the dielectric
and
the microwave power splitter have been described for optimum performance but
may vary depending on the system's requirements.
An embodiment for the power splitter having more than two ports to output the
radiation power is, for example, three ports with a single dielectric element
positioned in front of a central port, the dielectric element being rotated
from a
first port to a second port to the third port to split the radiation power
three ways
according to different proportions. The dielectric element may also be moved
in a
translational motion instead of a rotational motion, thereby enabling a design
with
more than two ports and a single dielectric element that can be slid across a
surface to correctly divide the radiation power amongst the multiple ports.
Another embodiment is to have four ports and two dielectric elements, one
dielectric element for each set of two ports. A first set of two ports is
positioned at
one end of the power splitter with one dielectric therebetween, while a second
set
of two ports is positioned at another end of the power splitter with the
second
dielectric therebetween. The person skilled in the art will understand that
while
the embodiments illustrated in the present figures show two ports and a single
dielectric element, many variants exist on this design without deviating from
the
spirit of the present invention.
The embodiments of the invention described above are intended to be exemplary
only. The scope of the invention is therefore intended to be limited solely by
the
scope of the appended claims.
18000-1 CA - 10 -
