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CROSS REFERENCE TO RELATED APPLICATIONS
This application is related to copending application entitled
"Micromechanical Moving Structures Including Multiple Contact Switching
System, and Micromaching Methods Therefor," Ser. No. 08/000,172, filed
concurrently herewith and now U.S. Pat. No. 5,374,792 and assigned to the
same assignee as the present invention.
BACKGROUND OF THE INVENTION
This invention relates generally to electrical circuit interrupters and
more particularly concerns current limiting breakers using a plurality of
micromechanical switches. As used herein, the term "micromechanical"
refers to miniscule devices which are fabricated using the technology of
micromachining; this involves no assembly operations, but only the
selective deposition and removal of materials on a substrate.
Circuit interrupters are designed to protect electrical equipment from
damage caused by short circuit faults. Most conventional circuit
interrupters are primarily bulky mechanical switches. These devices are
capable of sensing a short-circuit current and interrupting the same by
opening the switch via a heavy trip mechanism. Typical alternating current
circuit breakers require the creation of a large mechanical gap between
the contacts of the switch and can only interrupt an alternating current
at a zero-crossing. More recently developed current limiting breakers
provide the capability of substantially immediately interrupting
alternating currents of high magnitude without waiting for a current
zero-crossing. However, conventional current limiting breakers are
typically complex in construction and thus somewhat expensive to
fabricate.
The switch contacts of conventional circuit interrupters require a large
contact force be maintained to prevent "popping, " i.e., the unwanted
separation of the switch contacts which results from a current-induced
repulsive popping force. The contact force is typically provided by adding
weight to the contacts and/or adding structure, such as one or more
springs, to exert force on the contacts. These measures increase the total
weight and cost of the device. A second consideration is that once a short
circuit fault is detected, the contacts of the circuit interrupter must be
driven apart very rapidly to avoid arcing between them. But since
conventional devices typically use heavy mechanical contacts, they either
take a relatively long time to open the contacts or consume a large amount
of energy to generate a force sufficient to separate the contacts quickly.
Thus, conventional circuit interrupters tend to be relatively heavy
devices which require high amounts of energy to operate.
Accordingly, there is a need for an electric circuit interrupter in which
the overall popping force is reduced, thereby reducing the required
contact force. An additional need exists for a circuit interrupter having
means for rapidly separating the switch contacts without large energy
requirements. Meeting these needs will provide a circuit interrupter which
is lightweight and inexpensive to manufacture and requires less energy to
operate than conventional devices.
SUMMARY OF THE INVENTION
The above-mentioned needs are generally met in the present invention by
providing a circuit interruption device connected in a circuit line. The
circuit interruption device comprises a plurality of micromechanical
switches and a trip device which opens each of the switches whenever a
predetermined level of current in the line is exceeded. The switches are
mounted on a small substrate in a parallel-series array comprising a
plurality of line branches connected in the line in parallel, each of the
line branches having at least two of the switches serially connected
therein. Each of the switches comprises a pair of stationary contacts
formed on the substrate, a bridging contact movably formed on the
substrate, and an actuator for causing the bridging contact to move in and
out of contact with the stationary contacts. The bridging contact can be
either a member slidably disposed in a channel formed on the substrate or
member attached to an end of a cantilever having its other end attached to
the substrate.
The trip device, which is also mounted on the substrate comprises a current
sensor connected to the line, the current sensor producing a signal
whenever the predetermined level of current in the line is exceeded, and a
trigger connected to the current sensor which sends a control signal to
each of the switches in response to receipt of the signal from the current
sensor.
Other objects and advantages of the present invention will become apparent
upon reading the following detailed description and the appended claims
and upon reference to the accompanying drawings.
DESCRIPTION OF THE DRAWINGS
The subject matter which is regarded as the invention is particularly
pointed out and distinctly claimed in the concluding portion of the
specification. The invention, however, may be best understood by reference
to the following description taken in conjunction with the accompanying
drawing figures in which:
FIG. 1 shows a schematic of the circuit interrupter of the present
invention;
FIG. 2 shows an isometric view of an array of micromechanical switches;
FIGS. 3A and 3B are schematics comparing forces acting on a single contact
pair to forces acting on a plurality of parallel contact pairs;
FIG. 4 shows a micromechanical switch of the present invention with the
contacts closed;
FIG. 5 shows the micromechanical switch of FIG. 4 with the contacts open;
FIG. 6 shows a cross-sectional view of the micromechanical switch of FIG. 4
taken along the line 6--6; and
FIG. 7 shows a second micromechanical switch of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
Referring to FIG. 1, a circuit interruption device 10 using micromechanical
components is shown schematically. The circuit interruption device 10 is
connected in a circuit line 12 which delivers electrical energy from a
power source (not shown) to a load 14. Specifically, an input 12a of the
line 12 to the circuit interruption device 10 is connected to a switch
array 16 and an output 12b of the line 12 is connected between the switch
array 16 and the load 14. The switch array 16 comprises a plurality of
micromechanical switches which when opened interrupt the flow of current
through the line 12. The micromechanical switches are miniscule and
several can be incorporated within a very small area. The switch array 16
is supported on a substrate 18 in a manner described in detail below.
A trip device 20 is provided for opening the switches of the switch array
16 at the appropriate time (i.e., a short circuit fault) and interrupting
the current flow. The trip device 20, which is also supported on the
substrate 18, comprises a current sensor 22, an electronic trigger 24, a
trigger threshold control 26 and a power supply 28. The current sensor 22,
which may comprise one of many conventional sensors known in the art, is
connected to the line 12 and senses the current in the line. The current
sensor 22 provides a signal 30 representative of the current level to the
electronic trigger 24. As long as the current level represented by the
signal 30 does not exceed a predetermined value as set by the trigger
threshold control 26, the electronic trigger 24 sends a control signal 32
to the switch array 16 which keeps the switches closed. But when the
signal 30 exceeds the predetermined value, the electronic trigger 24 sends
a control signal 32 to the switch array 16 which causes the switches to
open. The power supply 28 provides power to the electronic trigger 24 and
the trigger threshold control 26. Since a relatively small amount is
required, the power can be derived from the protected line 12. For
example, the power supply 28 can be a charged capacitor or power amplifier
connected to the line 12.
Turning now to FIG. 2, the switch array 16 is shown in more detail. The
switch array 16 comprises a plurality of individual micromechanical
switches 34 which are formed on the substrate 18 in a series-parallel
network. Although a 3-by-3 array of nine switches is shown, this is only
illustrative; virtually any number of micromechanical switches can be
employed in the present invention. The series-parallel network of switches
is formed by providing a plurality of line branches 36 which are connected
in parallel to the input and output 12a, 12b of the protected circuit
line. Each of the parallel line branches 36 has a plurality of the
micromechanical switches 34 connected in series therein, thus defining the
series-parallel network. The switches 34, which are schematically shown
with their contacts open in FIG. 2, are described more fully below.
FIGS. 3A and 3B illustrate how a plurality of parallel switches reduce the
total popping force in a circuit interrupting device, thus reducing the
force needed to overcome the popping force which ultimately reduces the
weight and energy consumption of the device. FIG. 3A shows the first and
second contacts 38,40 of a conventional mechanical circuit interrupter.
The line current I which flows through the contacts 38,40 produces a
popping force F.sub.1 which is proportional to the square of the line
current I and tends to drive the contacts 38,40 apart. FIG. 3B shows a
modified circuit interrupter having a plurality of small contact pairs 42
connected in parallel and supported by two current conducting members
38',40'. While the two current conducting members 38',40' carry the full
line current I, each one of the small contact pairs carries an equal
portion of the line current I, referred to herein as the branch current i.
Accordingly, the individual popping force f acting on each of the small
contact pairs 42 is proportional to the square of the branch current i.
The total force F.sub.2 acting to separate the two conducting members
38',40' is equal to the sum of the individual popping forces f.
Now assuming that there are "n" parallel small contact pairs, then it is
known from the above discussion that:
F.sub.2 =.SIGMA.f=nf; f=F.sub.2 /n, (1)
i=I/n, and (2)
f.varies.i.sup.2 (3)
substituting equations (1) and (2) into the relationship (3) gives:
F.sub.2 /n.varies.(I/n).sup.2, which can be simplified as:
F.sub.2 .varies.(1/n)I.sup.2.
Since, as stated above, the popping force F.sub.1 is proportional to the
square of the line current I (F.sub.1 .varies.I.sup.2), then for an equal
line current I, the total force F.sub.2 acting to separate the branch
contacts would be only 1/n of the total force F.sub.1 for a single contact
pair. Thus, by providing a plurality of parallel line branches 36, the
present invention reduces the force needed to keep the switches 34 closed.
The serial connections of the switches 34 along each line branch 36 in the
series-parallel switch array 16 also provide a reduction in the amount of
energy needed to operate the device. In circuit interrupters, the contacts
must be opened to a sufficient spacing within a given time period to avoid
arcing therebetween. For instance, assume it is necessary to open the
contacts to a gap "D" within a time "t" to avoid arcing. The force needed
to do this would be equal to the mass of the movable contact (assuming the
other contact remains stationary) times the acceleration of the movable
contact. The acceleration is equal to the double derivative of the
distance D with respect to time. Thus, the contact opening force necessary
to avoid arcing could be reduced by reducing the mass of the movable
contact and the required gap distance between contacts. Reduction of the
required opening force reduces energy consumption.
In the present invention, the serial connection of the switches 34 along
each line branch 36 reduces the required gap distance between individual
pairs of contacts. This is based on the premise that if a switch carrying
a certain current must be opened to a gap distance "D" in a time "t" to
avoid arcing between the contacts, then it is just as acceptable to have
"n" number of serially connected switches carrying the same current and
forming "n" gaps, where each gap is equal to 1/n times "D" and each switch
simultaneously opens in the same time "t." Accordingly, each
simultaneously moving contact has to move through 1/n of the distance that
the conventional contact needs to move through in the same time period,
thereby lowering the required acceleration of the contacts. Furthermore,
the use of micromechanical switches in the present invention reduces the
mass to be moved to open the contacts, thereby further reducing the
necessary contact opening force. These micromechanical switches 34 are so
small that even if a very large number is used, the combined mass of the
moving contacts is less than the mass of the moving contact of a
conventional mechanical switch.
FIGS. 4-6 show in detail a micromechanical switch 34 suitable for use in
the present invention. The switches are termed "micromechanical" for two
primary reasons. First, they are of miniscule size--on the scale of a few
square millimeters. Second, they are fabricated using micromachining
techniques which are similar to the techniques used in the fabrication of
integrated circuits. These techniques entail selectively depositing and
removing materials from a substrate and do not include mechanical
assembly. Furthermore, batch fabrication, i.e., fabricating multiple
devices in a batch on a single wafer, can be used to spread processing
costs among the several individual devices.
The switch 34 comprises a substrate 44 (FIG. 6) which is preferably made of
a silicon or ceramic material. An insulator base 46 is formed on the top
of the substrate 44. The insulator base 46 is preferably made of an oxide
material such as silicon oxide. A channel 48 is formed in the insulator
base 46 and a movable contact 50 is disposed in the channel 48 in such a
manner as to be capable of sliding back and forth in the channel 48. As is
best seen in FIG. 6, the channel 48 has grooves 52 formed along two
opposing bottom edges thereof. The grooves 52 are adapted to receive two
retaining flanges 54 which are provided on opposite sides of the movable
contact 50, thereby retaining the movable contact 50 in the channel 48 and
guiding its movement along the channel 48. As stated above, the present
invention is fabricated by employing integrated circuit chip technology.
Making the movable contact 50 capable of movement with these fabrication
techniques requires the provision of a sacrificial layer (not shown) which
is first formed in the channel 48 prior to deposition of the movable
contact 50. Once the movable contact 50 is formed, the sacrificial layer
is removed, thereby freeing the movable contact 50 for movement.
Two stationary contacts 56,57 are placed on opposing sides of the channel
48 at one end thereof. The first stationary contact 56 is connected to the
incoming portion of the line 12, and the second stationary contact 57 is
connected to the outgoing portion of the line 12. The movable contact 50
is adapted to slide in and out of contact with the two stationary contacts
56,57 which have beveled surfaces matching similarly beveled surfaces on
the movable contact 50. The stationary contacts 56,57 and the movable
contact 50 are made of an electrically conducting metal such as copper or
tungsten so that when the movable contact 50 is in contact with the
stationary contacts 56,57, it provides a bridge between the stationary
contacts 56,57 to conduct the current in the line 12. When the movable
contact 50 is displaced from the stationary contacts 56,57, the current is
not conducted.
The movement of the movable contact is induced by an actuator assembly
mounted on the substrate 44. FIGS. 4-6 show an electrostatic actuator
which is used with the present invention; however many other types of
actuators could be used. Electromagnetic, piezoelectric and bimetallic
actuators are all examples of possible alternatives. For example, the
above-mentioned U.S. Pat. No. 5,374,792 hereby incorporated by reference,
describes a suitable electromagnetic actuator.
The electrostatic actuator of FIGS. 4-6 comprises a first electrode 58
disposed in the channel 48 at the end opposite from the stationary
contacts 56,57. Two secondary electrodes 60 are disposed on opposing sides
of the channel 48 at a point along the channel 48 which remains adjacent
to the movable contact 50 throughout its range of motion. The secondary
electrodes have sliding contacts 62 such as brush contacts which provide
an electrical connection between the secondary electrodes 60 and the
movable electrode 50. Suitable conductors are provided so that a voltage
can be applied across the first electrode 58 and the secondary electrodes
60. An insulating block 64 is provided in the channel 48 adjacent to the
first electrode 58 to prevent the movable contact 50 from contacting the
first electrode 58, thereby avoiding a short circuit.
In operation, the control signal 32 from the electronic trigger 24
discussed above provides the voltage across the electrodes. Depending on
the nature of this applied voltage, either an attractive or repulsive
electrostatic force will be created between the first electrode 58 and the
movable contact 50. If it is strong enough to overcome the popping force,
a repulsive force will keep the movable contact 50 in contact with the
stationary contacts 56,57. An attractive force between the movable
electrode 50 and the first electrode 58 will separate the movable contact
50 from the stationary contacts 56,57. Thus, as long as the current sensor
22 does not sense a short circuit, the electronic trigger 24 sends a
control signal 32 which produces a repulsive force, thereby keeping the
switches closed. When the current sensor 22 senses a short circuit, the
electronic trigger 24 sends a control signal 32 which produces an
attractive force and causes the switches to open. Generally, a voltage
having a magnitude of about 15 volts is sufficient to overcome the popping
force when maintaining contact and separate the contacts rapidly enough to
avoid arcing when opening the contacts.
As best seen in FIG. 6, a sealed cover 66 is placed over the insulator base
46 to enclose the channel 48 and all of the elements therein, thereby
protecting the micromechanical switch from exposure to contaminants. The
enclosed space is preferably filled with an inert gas to retard oxidation
or other deterioration of the micromechanical elements.
FIG. 7 shows a second embodiment of a micromechanical switch 134 which is
suitable for use with the present invention. As in the first embodiment,
the switch 134 comprises an insulator base 146 mounted on a substrate (not
shown). A channel 148 is formed in the insulator base 146 and has a
movable contact 150 disposed therein. The movable contact 150 is movably
supported by a cantilever 151 which is an elongated beam extending from
one corner of the movable contact 150 and attached to a side wall of the
channel 148. Two stationary contacts 156,157 are placed on opposing sides
of the channel 148 at one end thereof. The first stationary contact 156 is
connected to the incoming portion of the line 12, and the second
stationary contact 157 is connected to the outgoing portion of the line
12. Flexure of the cantilever 151 permits the movable contact 150 to move
in and out of contact with the two stationary contacts 156,157 to
respectively close and open the circuit. An electrode 158 is disposed in
the channel 148 at an end opposite from the two stationary contacts
156,157. Suitable conductors are provided so that a voltage can be applied
across the electrode 158 and the movable contact 150. As in the embodiment
of FIGS. 4-6 described above, application of the voltage is controlled by
the electronic trigger 24 to create either a repulsive force or an
attractive force between the electrode 158 and the movable contact 150. As
before, a repulsive force will keep the contacts closed, and an attractive
force will open the contacts.
FIGS. 4-7 show just two possible embodiments of micromechanical switches
which can be used in the present invention. Many other switch embodiments
are applicable. For example, the above-mentioned which has been
incorporated by reference, discloses another micromechanical switch which
is suitable for use with the present invention.
The foregoing has described an improved circuit interruption device in
which the contact popping force is reduced so as to provided a lightweight
and inexpensive device. The device is also fast-responding and requires
little energy to operate.
While specific embodiments of the present invention have been described, it
will be apparent to those skilled in the art that various modifications
thereto can be made without departing from the spirit and scope of the
invention as defined in the appended claims.
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