Note: Descriptions are shown in the official language in which they were submitted.
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CONTROL METHOD AND DEDICE FOR A sWITCHGEAA ACTUATOR
BACKGROUND OF T8E INVENTION
1. Field of the Invention:
The present invention relates to a method and device
for controlling electrical switchgear. More particularly,
the invention relates to a method and device for
controlling a switchgear utilizing a voice coil actuator
to rapidly and positively open and close a current
interrupter.
2. Description of Related Art:
In a power distribution system, switchgear may be
incorporated into the system for a number of reasons, such
as to provide automatic protection in response to abnormal
load conditions or to permit opening and closing of
sections of the system. Various types of switchgear
include a switch for deliberately opening and closing a
power transmission line, such as a line to a capacitor
bank; a fault interrupter for automatically opening a line
upon the detection of a fault; and a recloser which, upon
the detection of a fault, opens and closes rapidly a
predetermined number of times until either the fault
clears or the recloser locks in an open position.
Vacuum interrupters have been widely employed in the
art because they provide fast, low energy arc interruption
with long contact life, low mechanical stress and a high
degree of operating safety. In a vacuum interrupter the
contacts are sealed in a vacuum enclosure. One of the
contacts is a moveable contact having an operating member
extending through a vacuum seal in the enclosure.
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SUMMARY AND OBJECTS
One of the objects of the present invention is to
provide a switchgear actuator mechanism and control
therefore that minimizes arcing and generated transients
during opening and closing.
Another object of the present invention is to provide
a switchgear actuator mechanism and control therefore that
provides accurate monitoring of the system.
Another object of the present invention is to provide
a switchgear actuator mechanism capable of a range of
motion profiles, thereby eliminating the need for many
types of mechanical systems.
Another object of the present invention is to provide
a switchgear actuator mechanism capable of being
controlled by any commercially available motor control
circuitry or dedicated motion control circuitry.
Still another object of the present invention is to
provide a switchgear actuator mechanism capable of
procuring speeds and forces not readily achievable with
prior art mechanical systems.
Still another object of the present invention is to
provide an improved synchronously operating switchgear
that results in a significant reduction in transients
generated during the switching operation.
Generally, switchgear incorporating vacuum
interrupters have utilized various spring loaded
mechanisms which are connected to an operating member to
positively open or close the interrupter contacts. One
such device which is commonly used is the simple toggle
linkage. The primary function of these mechanisms is to
minimize arcing by very rapidly driving the contacts into
their open or closed positiions. Various applications may
require the use of a number of spring loaded mechanisms
with associated latches and linkages.
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' In order to prime these mechanical systems, either by
compressiopn or extension of the drive spring, an actuator
is normally provided. These actuators can include, but
are not limited to, solenoids, motors or hydraulic
devices. In comparison to the inherent speed requirements
of the interrupter to effectively interrupt current, these
actuators are relatively slow with poor response times.
For this reason they are not normally used to directly
drive the interrupter contacts but are utilized to prime
the fast acting spring mechanisms. The prime disadvantage
of this system is that the spring driven operation does
not lend itself to being easily controllable and it
requires considerable engineering effort to finely adjust
the mechanism's performance.
In practice, this means that many different
mechanisms must be designed to accommodate the different
operating requirements for switches, fault interrupters
and reclosers and within each one of these switchgear
classes, there are different mechanisms required depending
on the application, including voltage and current
requirements.
Furthermore, in view of the high voltages that are
typically used in power applications, rapid and accurate
movement of the interrupter contacts is desired to
minimize arcing between the contacts and the generation of
transients. Depending upon the application, whether it is
capacitor bank switching or fault interruption, it can be
determined by those skilled in the art when the most
advantageous time to open or close the interrupter contact
occurs. This optimum time correlates to a precise point
on the voltage or current wave where current interruption
or contact make would produce minimal arcing and
transients. Since conventional spring driven mechanisms
do not lend themselves to this degree of fine control,
this invention offers a viable means to achieve point-on-
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' wave or synchronous switching. Such synchronous operation
of the interrupter is beneficial both in terms of the
reduced wear on the interrupter contacts and the
significant reduction in general transients experienced by
the power system downstream of the switchgear unit.
A further feature of a controlled, synchronously
operating switchgear unit is that the velocity at which
the contacts close can be controlled. In conventional
systems, the contacts are driven together in an
l0 uncontrolled fashion at very high velocity and it is
possible that the contracts will bounce open a number of
times before coming to rest. This bounce phenomenon is
undesirable because the ensuing arcing can soften the
contacts and create strong welds when the contacts finally
mate.
In accordance with the present invention, a current
interrupter includes a current interrupting device having
at least one movable contact; an actuator coupled to the
movable contact of the current interrupter; a feedback
sensor for monitoring movement of the actuator; and a
control system coupled to the feedback sensor so as to
receive information from the feedback sensor concerning
the movement of the actuator and for controlling movement
of the actuator based on the information. The interrupter
further includes a memory for storing a desired motion
profile of the actuator; and a microprocessor for
comparing the movement of the actuator with the desired
motion profile and controlling movement of the actuator
based also on a comparison of the movement of the actuator
with the desired motion profile. The interrupter further
includes a sensor for sensing a waveform of a voltage or
current in a line to be switched and providing information '
concerning the waveform to the control system; wherein the
control system controls the movement of the actuator based
also on the information concerning the waveform.
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The foregoing features and advantages of the present
invention will be apparent from the following more
particular description of the invention. The accompany
drawings, listed hereinbelow, are useful in explaining the
invention.
BRIEF DESCRIPTION OF THE DRAWINGS
In the text which follows, the invention is explained
with reference to illustrative embodiments, in which:
FIG. 1 shows a schematic diagram of switchgear
employing a voice coil actuator;
FIG. 2 shows a cross-sectional view of one embodiment
of a switchgear;
FIG. 3 is a cross-sectional view of the vacuum module
shown in FIG. 2;
FIG. 4 shows an enlarged view of the operating
mechanism of the embodiment displayed in FIG. 2;
FIG. 5 shows, an exploded view of the primary
components of the operating mechanism;
FIG. 6 shows a graph illustrating the system voltage
vs. time and the dielectric descent of the interrupter;
FIG. 7 is a schematic view of a circuit that may be
used with the present invention;
FIG. 8 is a graph illustrating a motion profile that
may ~be used with the present invention;
FIG. 9 is an illustration of a voice coil actuator
that may be used with the present invention;
FIG. 10 is a view of a latching mechanism that may be
used with the present invention;
FIG. 11 is a view of a contact pressure spring
mechanism that may be used with the present invention;
FIG. 12 is a graph illustrating the synchronous
timing of an opening operation of a capacitor switch.
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DETAINED DESCRIPTION OF THE INVENTION
For a better understanding of the invention,
reference may be made to the following detailed
description taken in conjunction with the accompanying
drawings, wherein preferred exemplary embodiments of the
present invention are illustrated and described. Each
reference number is consistent throughout all of the
drawings.
In FIG. l, an incoming power line 2 is coupled in
series with a current interrupter 4, thereby allowing the
current interrupter 4 to open the line. The line 2 may be
opened upon a predetermined command or, in the case of a
fault interrupter, if a fault exceeds a predetermined
threshold level. One of the contacts of the current
interrupter 4 is connected to one end of an operating rod
6. The other end of the operating rod 6 is operatively
coupled to an actuator, such as a voice coil actuator 8.
The voice coil actuator 8 directly acts upon the operating
rod 6 in order to open or close the contacts of the
current interrupter 4.
The voice coil actuator 8 is a direct drive, limited
motion device that uses a magnetic field and a coil
winding 10, to produce a force proportional to the current
applied to the coil. The electromechanical conversion of
the voice coil actuator 8 is governed by the Lorentz Force
Principle, which states that if a current-carrying
conductor is placed in a magnetic field, a force will act
upon it. The magnitude of the force is determined by the
equation:
F = kBLIN
where F equals force, k is a constant, B is the magnetic
flux density, L is the length of the conductor, I is the
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current in the conductor, and N is the number of turns of
the conductor.
The current passing through the voice coil winding 10
is controlled by a control mechanism 12. Any commercially
available control mechanism 12 could be utilized. For
example, suitable control mechanisms 12 include: single
loop controllers, programmable logic controllers, or
distributed control systems. The control mechanism 12 may
be coupled to a feedback device 14, which provides input
regarding the position of the operating rod 6.
The control mechanism 12 may also be coupled to a
latching device 16. When instructed to secure the
operating rod 6 by the control mechanism 12, the latching
device 16 fastens the operating rod 6 in its current
position. In an alternative device, the latching
mechanism 16 may be a permanent magnet or mechanical latch
that is not coupled to the control device 12.
In FIG. 2, a cross-sectional view of one of the
embodiments of the invention is shown. A one piece,
elongated, solidly insulated encapsulation 18 encloses the
operating rod 6 and the current interrupter 4. The
encapsulation 18 may be formed out of ceramic, porcelain,
any suitable epoxy, or any other appropriate solid
insulating material. A line side high voltage electrical
terminal 22 and a load side high voltage electrical
terminal 20 protrude through the solidly insulated
enclosure 18, and are coupled to the current interrupter
4. The high voltage electrical terminals 20 and 22 are
diametrically disposed, 180 degrees apart, and are
parallel with respect to one another. The encapsulation
18 provides both the solid insulation between the high
voltage electrical terminals 20 and 22 and the solid
insulation between each high voltage electrical terminal
20 and 22 and electrical ground (not shown).
The current interrupter 4 includes a vacuum module or
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bottle 24, shown in cross section in FIG. 3, with a pair
of switch contacts 71, 72 disposed within the vacuum
module 24. The vacuum module 24 provides a housing and an
evacuated environment for the operation of the pair of
switch contacts. The module 24 is usually constructed
from an elongated, generally tubular, evacuated, ceramic
casing 73, preferably formed from alumina. One of the
switch contacts 71 is movable, and the other switch
contact 72 is stationary or fixed.
A special fitting 76 is attached to the stem of the
stationary contact 72, permitting the associated high
voltage electrical terminal 22 to exit at a 90° angle.
The movable switch contact 71 is fastened to the
uppermost, longitudinal end of the operating rod 6. One
method of fastening is to use a stud 32 threaded into a
tapped connection 74 in the moving stem 75 of the movable
contact 71. When the switch contacts are in the closed
position as shown, a low resistance or short circuit
electrical path is created between the high voltage
electrical terminals 20 and 22. The current interrupter
4 further includes a current exchange assembly and an
interface 26 between the vacuum module 24 and the current
exchange assembly. The current exchange assembly contains
a moving piston 28 and a fixed outer housing 30. In this
embodiment, the operating rod 6 is made from an
electrically insulated material.
The other end of the operating rod 6 is secured to a
flange 34 on the voice coil actuator 8 by a rigid pin 36.
The pin 36 which retains the foregoing components in
position, can be secured by any suitable means, such as a
pair of retaining rings. A recirculating linear ball
bearing 38 and split rings 40, which hold the ball
bearing, provide smooth movement of the operating rod 6.
The voice coil winding 10 is disposed between the outer
body of the voice coil actuator 8 and the flange 34. Side
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- flanges 42 are attached to the outer body of the voice
coil actuator 8, and connect to side brackets 44, thereby
securely fastening the voice coil actuator 8 to a
protective case 46. The protective case 46 is attached to
a lid 50 for the protective case 46 via housing flanges
48, and the protective case lid 50 is connected to the
solid insulation enclosure 18 via lid flanges 52. Just as
the solid insulated encapsulation 18, the protective case
46 is also formed out of ceramic, porcelain, any suitable
epoxy, or any other appropriate solid insulating material.
In this embodiment the feedback device 14 is a
position sensor, such as a linear potentiometer 14. The
linear potentiometer 14 can be made from a three-terminal
rheostat or a resistor with one or more adjustable sliding
contacts, thereby functioning as an adjustable voltage
divider. The linear potentiometer 14 provides information
regarding the position of the operating rod 6 to the
control mechanism 12, which controls the voice coil
actuator 8. Alternatively, the feedback device 14 may be
an optical encoder.
The latching device 16 is intended to secure the
operating rod 6. The latching device may be a
controllable device, such as an electromagnet, or a simple
mechanical or permanent magnet latch including: a latching
magnet 54, a spacer 56 made from nonferrous material, a
bolt 58 securing the latching magnet 54 to the protective
case lid 50, a latch plate 60 made from steel or iron, and
a latch plate pin 62 securing the latch plate 60 to the
operating rod 6.
In order to more fully understand the invention,
reference may be had to FIGS. 4 and 5. FIG. 4 shows an
enlarged view of the operating mechanism of the preferred
embodiment displayed in FIG. 2, and FIG. 5 shows an
exploded view of the primary components of the operating
mechanism.
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Details concerning the control mechanism of the
present invention will now be described.
FIG. 6 illustrates a voltage signal 100 plotted on a
graph comparing the voltage level v(t) versus time t. In
a 60 Hz application, each half cycle is ideally 8.33 ms.
However, actual cycles may vary due to harmonics or
assymetric conditions so that a given half cycle may be
greater than or less than 8.33 ms.
In order to minimize arcing and the generation of
transients in a capacitor switch application, the contacts
of the interrupter are ideally closed instantaneously at
the null points when v(t) equals zero. See point A in
FIG. 6. However, since the contacts cannot close
instantaneously, the timing of the initiation of the
opening and closing sequences should be carefully
controlled in order to minimize transients and arcing.
A preferred embodiment of a control circuit 200 for
use with the present invention is illustrated in FIG. 7.
At the heart of the control circuit 200 is a
microprocessor 202 that is suitable for use in a broad
temperature range.
The voltage waveform of the power line being
controlled by the interrupter 4 is analyzed with a voltage
waveform analyzer 204, a phase lock loop circuit 206, and
a V~o crossing detection circuit 208. Information
concerning the voltage waveform of the line to be
interrupted, including the timing of null points A wherein
the voltage v(t) is zero, is input to the microprocessor
202. Alternatively, a voltage waveform analyzer 204 could
be used that measures the voltage waveform directly off
the line without the phase lock loop circuit 206.
Open and close commands are input to the
microprocessor 202 via inputs 210 and 212, respectively.
The open and close commands may be created manually, may
be initiated at preset times by a clock, may be initiated
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by an external control, or may be triggered by the
detection of a fault, depending on the particular
application of the interrupter 4.
A reset signal 214 may be input to the microprocessor
202 to manually reset the microprocessor 202 when
necessary. For example, if the interrupter 4 is manually
manipulated, the microprocessor 202 may not be set to the
current status of the interrupter 4. In such a situation,
the microprocessor 202 should be reset.
Status indicators may be provided to indicate various
conditions of the circuit 200 or the interrupter 4. Such
indicators may include a maintenance light 216 to indicate
when maintenance is required, a power on light 218, a
switch open indicator 220, a switch closed indicator 222,
and a counter 224 that may be used to count cycles or
operations of the system.
A preferred embodiment of the present invention may
include two control systems. A first control system is
conventional, and thus not disclosed herein in detail, and
determines when the line controlled by the interrupter 4
is to be opened or closed. The first control system may
include a fault detector or a timer for interrupting the
line upon the detection of a fault, or at a predetermined
time.
Alternatively, an open or close command may be input
directly to the system. The open and close commands,
whether originating from the first control system or
manually, are input to the microprocessor 202 at inputs
210 and 212, respectively.
The second control system 200, illustrated in FIG. 7,
analyzes the voltage waveform of the line and determines
the best time for initiating opening and closing the
interrupter 4 in order to minimize transients and arcing.
Each interrupter 4 has a dielectric strength that
defines the likelihood of an arc jumping from one contact
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to another. The dielectric strength depends upon a number
of factors including the medium inside the interrupter 4
and the distance between the contacts 71, 72. FIG. 6
illustrates the changing or descent of the dielectric
strength between the contacts 71, 72 versus time as the
distance between the contacts closes. See line C in FIG.
6. Ideally, the dielectric strength between the contacts
would be infinite until the exact moment of closing of the
contacts 71, 72. See line B in FIG. 6. In reality, the
dielectric slopes downward, reducing quickly as the
contacts approach each other. See line C in FIG. 6. If
the slope of the dielectric descent is sufficiently high,
and the dielectric strength remains greater than the
voltage of the waveform, the generation of arcing and
transients is eliminated or significantly reduced.
Another factor to be considered during the operation
of an interrupter is the relative velocity between the
contacts upon opening and closing. If the contacts are
moving slowly, the slope of the dielectric descent will be
low, and arcing will likely occur. Conversely, if the
contacts are moving too quickly, especially upon closing,
the contacts will likely bounce off of each other, causing
unnecessary arcing and transients. Accordingly, a unique
ideal motion profile may exist for each application of an
interrupter. FIG. 8 illustrates an example of a motion
profile, wherein the abscissa represents the location of
the moving contact 71 and the ordinate represents the
velocity at which the contact 71 is moving. Point 0 on
the abscissa represents the starting or maximum open
position of the contact 71, and point x represents the
closed position, wherein the contact 71 is touching the
stationary contact 72. At point 0, when the close command
is initiated, the velocity is zero. The velocity is
increased as quickly a possible to a maximum velocity Vo"x.
The velocity remains at V~~~x for as long as possible, but
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is then reduced as the point of contact x approaches in
order to minimize bounce.
During an opening sequence, the motion profile is
also important to prevent the occurrence of restrikes or
re-ignitions shortly after opening. If the contacts
separate at too slow a speed, or at a time when the
voltage level is too high, excessive arcing may occur.
Desired motion profiles for opening and closing sequences
can be determined by those of skill in the art and
preprogrammed into the circuit 200.
Turning attention to FIG. 12, the timing of the
opening operation in a capacitor switching application may
be better understood. FIG. 12 relates to the opening
sequence of a system that includes a capacitor bank. Line
4 indicates the voltage level of the fully charged
capacitors. The switch begins to open at point 2, and an
arc forms. However, at this point, the current is
decaying and the arc is extinguished at current zero,
point 3. The system voltage is now at its peak, but the
voltage across the contacts is small because of the charge
on the capacitor bank, which approximates the peak system
voltage. As the system voltage begins to drop, the
voltage on the capacitor bank stays high, resulting in an
increase in the voltage across the contacts. The contacts
should part with enough acceleration so that the
dielectric rises faster than the escalating voltage
between the contacts in order to avoid restrikes and re-
ignitions.
The motion control function can be achieved by means
of software loaded into the microprocessor/microcontroller
or by the addition of dedicated motion control chips which
interface with the microprocessor. A particular motion
profile is programmed into a memory, which may be a
separate EEPROM chip in an external motion control circuit
226, or onboard memory on the microprocessor or
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microcontroller. The motion control circuit 226 is
connected to the feedback device (encoder) 14 and to a
pulse width modulation (PWM) circuit 228. The PWM 228
controls the current that is applied to the voice coil
actuator 8. Since the force driving the voice coil
actuator 8 is proportional to the current supplied to the
voice coil actuator 8, the velocity of the actuator 6 (and
the moving contact 71) is controlled by the PWM 228. As
a result, the voice coil actuator 8 is controlled by a
closed loop feedback system that includes the position
encoder 14 that sends a position signal of the actuator 8
to the motion control circuit 226. The motion control
circuit 226 compares the actual position of the actuator
8 to the ideal motion profile preprogrammed into the
motion control circuit 226. Based on the comparison of
the actual position to the ideal motion profile, the voice
coil actuator 8 is controlled by the PWM so that its
motion closely approximates the ideal intended motion.
Control of the actuator is further modified by the
circuits 204, 206, 208 that monitor that actual voltage
waveform of the line to be interrupted. For example, for
a particular application, it may be determined that the
contacts 71, 72 should open or close within 1 ms of the
zero crossing A (FIG. 6) of the voltage signal v(t). The
ideal motion profile preprogrammed into the motion control
circuit 226 includes the total reaction and travel time of
the actuator 8 from the time an initiating signal is sent
to the time the contacts 71, 72 close. If the ideal
motion profile indicates that the reaction and travel time
for the contacts to close after the initiating signal is
7 ms, the microprocessor analyzes the actual voltage
waveform of the line to be interrupted and determines a
specific time between null points at which the initiating
signal should be sent. The circuits 204, 206, 208 first
establish the actual cycle period and the resulting length
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of time between zero crossings. The control circuit 200
then initiates operation of the voice coil actuator 8 at
a time after a zero crossing that is equal to the actual
time between null crossings minus the reaction and travel
5 time of the actuator 8. Accordingly, if the actual
voltage waveform indicates that there are 8.3 ms between
zero crossings and the reaction and travel time is 7 ms,
the opening sequence is initiated at 1.3 ms after a zero
crossing. In an alternative embodiment, the system may
10 assume that the actual time between zero crossings is 8.33
ms, and the initiation is calculated based on that
assumption.
In some embodiments of the present invention, a
plurality of motion profiles can be preprogrammed into the
15 circuit 200, and the appropriate motion profile can be
selected by an input from the operator.
Once the sequence is initiated, the actual motion of
the actuator 8 is monitored by the encoder 14 and compared
against the ideal motion profile. The current applied to
the actuator 8 is adjusted by the PWM 228 based on the
comparison of the actual movement of the actuator 8 to the
ideal motion profile.
FIG. 9 illustrates another embodiment of a voice coil
actuator 308 that may be used with any of the embodiments
of the present invention. The voice coil actuator 308
includes a ring shaped magnet 310, which is preferably a
4 MGO ceramic magnet. The magnet 310 is housed with a
bottom pole piece 312 and a top pole piece 314. These
pole pieces are formed from ferromagnetic materials, such
as iron or steel. The pole pieces 312, 314 include a
central aperture 316 through which an operating rod 318
extends. The operating rod 318 is supported in the pole
pieces 312, 314 with self-lubricating polymer bearings
320, such as IGUS"' bearings 320.
An aluminum plate 328 is fixed to the rod 318. At a
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peripheral edge of the plate 328, a coil 330 extends from
the plate 328 into an air groove 332 formed between the
bottom pole piece 312 and the magnet 310. The coil 330
may be formed from flattened wire so as to maximize the
number of turns that will fit within the air groove 332.
The actuator 308 may be driven by a 24 volt battery,
or any other suitable power source, including an
autoranging AC to DC converter.
In order to latch the device in a particular
position, the operating rod 318 may include a groove 320
within which is located a ball 322. See FIG. 10. A
spring 324 and cap 326 urge the ball 322 into the groove
320 to retain the rod 318 in a fixed position. The rod
318 may be freed from the ball 322 upon the application of
a force, the level of which depends on the strength of the
spring 324.
In order to ensure a good connection between the
contacts 71, 72, a spring 340, or other force, may be
applied to the rod 6 (or 318) to urge the contact 71
against the contact 72 with a predetermined force, such as
60 - 100 pounds. The spring may be compressed by the
action of the actuator. Turning attention to Fig. 11, the
operating rod 6, 318 may include a flange 342 that
provides a surface against which the spring 340 presses.
Another abutment surface 344 may be provided to support
the opposite end of the spring 340.
The spring 340 provides the additional benefit of
maintaining an adequate force between the two contacts 71,
72. For example, after repeated operations, arcing may
cause the contacts to wear. Because of the spring force,
the two contacts are urged against each other, even if
they have become worn. In addition, the application of
the force causes a reduction in the electrical resistance
between the contacts in the closed position, thereby
reducing heat losses.
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If the contacts become worn, the operating rod 6, 318
will move a greater distance in order to accommodate the
wear. Since the position sensor 14 senses the distance
moved by the operating rod 6, 318, the system can be
programmed to illuminate the maintenance signal 216, or
some other indicator, to indicate that excessive wear has
occurred on the contacts 71, 72. The system can also
modify its motion profile to allow for such incremental
increases in stroke.
Although only preferred embodiments are specifically
illustrated and described herein, it will be appreciated
that many modifications and variations of the present
invention are possible in light of the above teachings and
within the purview of the appended claims without
departing from the spirit and intended scope of the
invention.
