Note: Descriptions are shown in the official language in which they were submitted.
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ARTIFICIAL LIFT MECHANISMS
TECHNICAL FIELD
The present invention relates to a form of mechanism intended to drive a
reciprocating pump to lift liquids from a deep well or borehole.
BACKGROUND ART
In a "jack pump" mechanism, a piston and non-return-valve unit at the base of
the
well or borehole (which may be several thousand metres deep) is generally
connected
to the drive mechanism at the surface by means of a long steel rod that, being
assembled in sections and screwed together, is known as a rod string. The
topmost
section of that string ¨ the section that emerges from the well through a
pressure seal
¨ necessarily has a higher surface finish and is known as the polished (or
polish) rod.
The polished rod is directly connected to the reciprocating mechanism that
forms the
invention described herein.
It has long been known to construct such mechanisms in the form of an
oscillating
horizontal beam having a hammer-shaped end, over which is wrapped a chain or
cable -
from which the pumping string is suspended. Other, more complex, mechanisms
exist
in which, for example, the polished rod is suspended from a pulley or belt and
in
which the pulley or belt is raised and lowered by the rotation and contra-
rotation of a
winch mechanism. Nevertheless, the reciprocating beam or "nodding donkey" has
remained the most popular device for moving a rod string. Reciprocating beams
have
been in use since the eighteenth century for pumping water from mines.
The powered end of the reciprocating beam unit has either been driven by a
linear
actuator, similar to an early steam engine design (the Atmospheric or Newcomen
engine) or by a crank mechanism, driven in turn by a rotary engine, such as an
electric, petrol, gas or diesel engine. These engines have a low output torque
and a
heavy duty gearbox must be interposed to reduce the rotary speed and to
increase the
torque of the crank that moves the beam.
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It will also be understood that the long steel string that connects the drive
mechanism
at the top of the well with the pump itself at the base of the well has a
weight of
several tonnes, which must also be supported by the beam. To increase the
efficiency
of the mechanism, the weight has to be counterbalanced. Although some recent
mechanisms have used a gas spring (in the form of a pneumatic cylinder as a
"prop"
beneath the loaded end of the beam) it is more common for the counterbalance
to be
in the form of eccentric weights, attached to the shaft of the crank mechanism
that
drives the oscillating beam.
From earliest times the pumping stroke has traditionally been about ten feet
(two or
three metres) and the pumping frequency has been around 5 (between 1 and 10)
strokes a minute. It will be understood that the traditional choices have been
determined by the large masses involved and by the asymmetric action of the
device,
which places high stress on the parts and causes significant wear on the
bearings of
the "nodding donkey" mechanism.
The traditional machine has many moving parts and it is required to operate
for 24
hrs. a day, 365 days a year for several years, so it will be understood that
it needs
regular inspection, lubrication, maintenance and repair.
It will be further understood that pumping mechanisms in the past have been
designed
with regard to their mechanical function alone. The process of their design
has been
entirely focussed on providing a reliable method of raising and lowering a
long rod
string within a shaft through which the liquid is itself raised on the
upstroke of the
pump. In the design of that mechanism no significant thought was given to
means of
sensing the efficacy of the pumping operation or of reacting to special
conditions that
may strongly affect the loads on the pumping apparatus. For example, the
mechanisms of the prior art do not generally incorporate within themselves the
ability
to sense and to react appropriately to conditions such as a dry well, a broken
rod
string or a stuck valve. Such conditions could only be discovered or diagnosed
as a
result of routine inspection and maintenance and before that discovery the
untreated
condition will not only have lost output, but may have been the cause of
considerable
damage to the pumping mechanism.
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In recent years a variety of alternative systems have been devised, some using
directly-applied hydraulic power to raise and lower the polished rod. Other
alternatives have proposed the use of direct-acting linear electric motors,
although
none of them has been commercially successful.
Exemplary is that of these proposals which is disclosed in United States
Patent No.
5,960,875, issued October 5, 1999, to Elf Exploration Production. The document
describes a mechanism by which the piston of an oil pump (placed deep within
the
borehole and close to the base of the well) is so constructed as to be
combined with
the cylindrical armature of a linear electric induction motor. It will be
understood that,
to be lowered into the bore of the well, the outer diameter of that armature
has to be
restricted to just a few centimetres. United States Patent No. 5,960,875,
proposes that
the combined piston and armature shall be driven vertically upwards and
downwards
by forces induced in the said armature by a surrounding set of cylindrical
coils and
since they must also be lowered into the borehole, the said cylindrical
powered coils
must also have an outer diameter not significantly greater than that of the
piston.
Unfortunately, the proposed design of linear motor is beyond any known
technology.
The weight of oil that must be raised by every upward stroke of the pump is a
few
tonne force, so that the force that must be produced by the small diameter
electromagnetic piston has to be several thousands of kilograms. But at this
time no
such small-diameter motor of reasonable length, efficiency and cost can be
constructed to meet the requirements of the artificial lift mechanism and to
satisfy the
safety requirements of the oil business. The invention is therefore
impractical of
commercial realization.
United States Patent No. 5,196,770, issued March 23, 1993, to Marine Petroleum
Equipment, also describes a cylindrical linear electric actuator. In this case
it is placed
at the head of a well and drives the heavy rod string to move the submerged
pump.
The proposed linear electric motor is much larger, heavier, more complex and
more
expensive than that proposed in United States 5,960,875. It is described as
being of
inductive design, or in the alternative as being of synchronous, asynchronous
or
variable reluctance design. Unfortunately, all of these are known to be
inefficient at
the velocities and reciprocation rates that are typical of jack pumping
operations. It is
also costly to make such linear electric motors to a standard that would allow
them to
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pass the safety regulations for electrical power devices in a flammable gas
environment.
There is the further disadvantage that both the moving part or armature and
the fixed
part or stator must be continuously supplied with many kilowatts of electrical
power.
Thus there are cooling difficulties (resulting from the motor inefficiencies
and the
need to conduct heat away from a moving body) and other problems relating to
the
incessant flexing of the power cable to the moving part. No reliable electric
linear
actuator can be constructed to meet the demands of the invention while having
a
reasonable price, size and weight.
It is to be noted especially that the proposed machine has a fixed
(predetermined)
counterbalance in the form of a large mechanical weight at the far end of a
cable that
turns 180 degrees around a sheave, so that both the cable and the sheave
bearing are
highly stressed and will need frequent replacement. In the alternative there
are
described arrangements by which the armature is effectively hung from a
pneumatic
cylinder (or a pair of such cylinders) forming part of a gas spring that is
pressurized to
provide a (predetermined) counterbalance force.
Canadian Patent No. 2,250,739, issued May 30, 2006, to Raos, provides for a
machine
in which the prime mover is a simple linear motor that is conceived to have a
short
rectangular armature that runs between two rectangular stators. It is very
difficult, if
not impossible, to design such a motor that is capable of producing the large
forces
required to drive the machine while remaining efficient at the slow speeds
demanded
by the application and meeting the statutory requirements for safety in an oil
field
environment.
The form of the preferred counterbalance is said to be that of a very large
steel spring,
compressed between the base of the machine and the electrical armature, so as
to
support the armature against the deadload of the rod string. (It will be
understood that
the weight of the rod string may approach 10 tonnes). As is known, the force
exerted
by a spring is directly proportional to its compression or extension. A common
requirement for such a counterbalance is that the force shall remain within
ten percent
of its set value while the armature travels through a stroke distance of plus
or minus
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1.25 metres (total displacement 2.5 metres or 100 inches). That means that the
length
of the spring when compressed to support the rod string must be about 12.5
metres or
40 feet ¨ and that its uncompressed length has to be about 50 feet. Such a
massive
spring would probably have an outer diameter of several metres. Although
alternative
counterbalancing systems using pneumatic or hydraulic cylinders are mentioned,
they
are not described and no related invention is claimed. A further alternative
type of
counterbalance is mentioned, being a mechanical counterweight connected via a
cable
and sheave, which would have the same disadvantages described in relation to
United
States Patent 5,196,770, supra.
Although Canadian Patent No. 2,250,739, teaches the critical importance of the
value
of the counterbalance force, the invention as described has a counterbalance
force that
is not only predetermined but is non-adjustable, so that it cannot be altered
in
response to changing conditions.
The type of linear motor described in Canadian Patent No. 2,250,739, would
appear to
be a switched variable-reluctance machine with individual parts of the stator
being
commutated under microprocessor control. The design of such a motor would be
very
complex and no such motor is known that would be capable of meeting the
exacting
demands of this application.
Further, although Canadian Patent No. 2,250,739, teaches the advantages of an
electric linear motor with respect to its ability to change its stroke and
speed under
remote control and in automatic response to local emergency conditions, it
does not
describe any method by which those conditions might be detected - and the
ability of
the machine to detect and to respond to emergency conditions is not therefore
claimed.
INDUSTRIAL APPLICABILITY
The invention has applicability in all forms of liquid recovery from deep
shafts and
boreholes.
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The prior art inventions in which it is proposed that a vertically-disposed
linear
electric motor will be used to drive the motion of a jack pump by direct
connection to
its rod string have limitations in four critical areas:
1. No proposed type of linear electric motor has been capable of producing the
required mechanical forces while being of acceptable size, weight, efficiency
and
cost;
2. No proposed counterbalance system has been continually adaptive to changes
in
pumping conditions, so as to minimize dynamic loads on the linear electric
motor;
3. No proposed gas spring counterbalance has been continually optimized for
the
storage and recycling of energy in the reciprocating mechanism; and
4. No proposed type of linear motor has been inherently capable of load-
sensing, so
that its current demand signals might be used for the control of the mechanism
as a
whole.
DISCLOSURE OF THE INVENTION
One object of the present invention is to construct a thoroughly-practical
reciprocating
pumping (or "artificial lift") mechanism using a very powerful and highly
efficient
type of electric linear motor in combination with an adaptive inert gas spring
counterbalance, the apparatus being especially suitable for use in association
with oil
wells and those for de-watering underground gas reservoirs.
It is a further object of this invention that the novel artificial lift
mechanism, in
combination with an electronic drive unit of conventional design and standard
specification, will be capable of sensing a variety of pumping conditions, of
reporting
them to a distant monitoring centre, of responding to remote commands and/or
of
reacting immediately and automatically in a predetermined way to several
emergency
conditions.
It is a further object of this invention that the novel artificial lift
mechanism shall be
compact, fully enclosed, relatively light in weight and lower in cost than
mechanisms
of the prior art, while meeting all statutory regulations for such equipment
in oilfield
conditions.
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It is a further objective of this invention that the machine shall be so
constructed as to
allow energy to be stored during one part of the pump cycle in such a manner
that it
can be recovered during a later part of the cycle, so as to improve the
efficiency of the
reciprocating mechanism.
It is a further objective of this invention that the machine shall be so
constructed that
each standard marc or model can be so controlled as to suit a wide range of
wells, thus
reducing the number of different models that are necessary to satisfy the
market,
minimizing stock inventory, manufacturing cost and training for field work.
The principal advantages of the invention include:
i. The mechanisms of the prior art have employed many stressed moving parts,
on whose continuing and satisfactory operation the reliability of the whole
depends.
In this instant technology, the sole moving part is the extendible member, so
that the
mechanism is inherently reliable and needs little maintenance;
ii. Because the machine is generally placed vertically above the shaft from
which the polished rod exits the well, the mechanical stress is orthogonal to
the
bearing surfaces and bearing wear is minimized;
iii. Power consumption is also minimized because the gas counterbalancing
mechanism is continually tuned to that parameter;
iv. The inert gas that is used for the counterbalancing function is
pressurized
within the body of the electrical system, so that it thereby acts also to
prevent any
flammable gases or liquid aerosols from coming into contact with the
electrical
apparatus, removing the risk of a fire or an explosion;
v. The use of the machine in association with an electronic drive unit having
drive current output signals and subsidiary computing facilities allows the
apparatus
to be self-monitoring, self-adjusting and self- protecting so as to maximise
productivity and minimize service time;
vi. The machine consumes little inert gas in normal operation because it is
fitted with an economising chamber by which the diurnal temperature variations
(which would normally lead to significant consumption of the inert gas) are
accommodated without loss;
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vii. The machine is entirely enclosed and safe from accidental contact by
humans or animals, so that protective arrangements surrounding the well head
are
simplified;
viii. The machine is completely silent and may be used in ecologically-
sensitive areas;
ix. The parameters of the reciprocating motion (stroke length, cycling
frequency, waveform, etc.) can be automatically controlled according to pre-
determined strategies and in response to any type of pumping condition that
may
arise, thus conserving the life of the machine while maximizing productivity;
and
x. Because the parameters of the counterbalance system are automatically
adjusted to an individual load, and because the stroke, stroke frequency and
waveform
of the reciprocating cycle are independently, remotely and automatically
variable over
a wide range, physically identical ("standard") machines can be applied to
many
different pump specifications. Thus the necessary product range is minimized,
manufacturing costs are reduced, stockholding is simplified and the time for
product
training and site work is shortened.
In this invention the machine is comprised of at least one cylindrical
electromagnetic
linear actuator placed above and adjacent the well-head, the electromagnetic
linear
actuator being fully enclosed and having a sliding gas seal through which an
extendible member emerges from the body of the actuator, the pumping string
being
connected to the extendible member by means of a polished rod or equivalent
component, the deadload of the pumping string being supported by the pressure
of an
inert gas within the body of the sealed electromagnetic actuator, the
reciprocating
motion of the pumping string being driven by the electromagnetic forces on the
extendible member, the inert gas pressure being automatically adjusted so as
to
minimize power consumption, there being an intermediate pressure chamber and
valves arranged to reduce gas consumption that would otherwise be caused by
diurnal
temperature cycles, the actuator being controlled by an electronic drive unit
and
having means incorporated therein or communicable thereto for measuring the
instantaneous current required to drive the said actuator, the said current
measurements being analyzed according to predetermined algorithms, the inert
gas
counterbalancing pressure and the modes of action of the pumping mechanism
being
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arranged to respond automatically according to the said algorithms, the
relevant data
being also transmitted to a remote monitoring station.
It will be understood that until very recently it has been neither practical
nor expedient
to employ an electromagnetic linear actuator to drive an artificial lift pump
for an oil
or gas well. That is principally because no electromagnetic linear actuator of
sufficient thrust and power has been available at an acceptable cost. The
recent
invention of wireless motors has transformed the situation and what was
previously
impossible is now practical. Generally, the linear motor components used in
the
instant invention will be constructed according to one of the wireless motor
patents.
Nevertheless, it will be understood that, in principle and subject to the
demands of the
application being met by improvements in such machines, the electromagnetic
actuator or actuators may be constructed in any practical and convenient form
without
changing the fundamental nature of the invention.
Generally the position of the extendible member will be measured by means of
an
appropriate transducer and the electromagnetic actuator forming part of that
extendible member will be supplied with electric current via a standard type
of
electronic drive unit, originally designed for use with rotary motors and
commonly
available for factory automation purposes. Such a standard drive unit has a
number of
very useful features, whereby both the current supplied to the motor and the
drive
voltage across the motor terminals are sampled at frequent intervals. (The
current is a
direct measure of the force produced by the motor and the voltage is a measure
of the
speed with which it is moving.) Further, an electronic drive unit will
commonly have
spare power supply outputs and spare computing power available, sometimes on
separate plug-in modules.
The extendible member is commanded to follow a sinusoidal waveform of
predetermined amplitude and frequency, the drive current demanded by the motor
being sampled, as an example, 100 times per cycle. The current measurements
are
divided into two groups - those in which the movement is upwards and those in
which
the movement is downwards ¨ and the root mean square of the motor drive
current is
computed for each group. The nns values are repeated for, as an example, ten
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complete cycles and the values added. If, for example, the difference between
the two
values exceeds a tolerance value in such a way that the power required to
raise the
extendible member is significantly greater than that required to lower it, the
mass of
gas in the spring is increased by opening a control valve for a total time
proportional
to the excess. Conversely, if it requires more energy to compress the gas
spring on the
down stroke than it does to raise it on the upstroke, the opposite action is
taken and
the mass of gas in the spring is reduced. It can be demonstrated that the
power
consumption of the system is minimized by the above algorithm.
If the pumping action should suddenly cease (perhaps by reason of a string
breakage,
for example) the same algorithm will immediately detect the difference in
force
required to move the string and cause the pressure in the gas spring to be
quickly
reduced to protect the machine. The pumping action might then be stopped
completely, or the motion reduced to a small amplitude only. The difference in
gas
pressure before and after the break will provide an immediate estimate of the
position
of the breakage, so that estimate might be automatically computed and
transmitted to
a central monitoring facility.
It will be understood that an air lock or a valve failure will also have an
immediate
effect on the pattern of current values recorded throughout each pumping
cycle. Thus,
independently of the algorithm used to adjust the gas spring pressures, a
number of
entirely separate computing algorithms might be arranged to process the very
same
cyclic data to detect such other anomalies.
The pattern of current values (i.e. the measurements of instantaneous power
taken by
the motor) contains detailed information about the quantity of liquid in the
rising
column and the rate at which it is being pumped. Further automatic analysis of
that
data might therefore be used to prepare regular monitoring reports for
telemetry to a
central monitoring station.
Generally the actuator or actuators will be cylindrical with an extendible
member of
cylindrical cross section and they will incorporate the gas spring within the
actuator
casing. It will be understood that in the alternative the gas spring or
springs might be
conveniently constructed and disposed separately from the electromagnetic
actuators.
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Generally the polished rod of the pumping string will pass through an open
cylindrical
channel on the central axis of a single actuator, the cylindrical channel
being arranged to
separate the polished rod from the pressurized body of the machine, leaving
the polished
rod "outside" the actuator itself. It will be understood that in the
alternative
the machine may be constructed from a plurality of actuators symmetrically
disposed
athwart the polished rod and connected thereto by means of a crosshead.
In the conventional form of this apparatus the electrical system is the stator
and the
magnet array is the armature, but in an alternative arrangement of this
invention the
electrical system may be the armature and the magnet array may be the stator.
Thus, in one aspect, there is provided a mechanism for driving a reciprocating
pump or an artificial lift device, having one or more linear electric motor
and at
least one gas spring, characterized in that the mass of gas in said at least
one spring
is continually controlled in accordance with the current demands of the one or
more linear motor, integrated over a plurality of complete operating cycles of
said
reciprocating pump or artificial lift mechanism and at least one of the one or
more
electric linear motor is arranged to raise and lower an extendible member,
said
extendible member being arranged to pass through a sliding gas seal in an
upper
part of a chamber, said chamber containing or forming part of the structure of
said
at least one electric linear motor, said chamber being filled with an inert
gas under
pressure so as to form a combined electric linear motor and gas spring.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a schematic cross section of a basic cylindrical mechanism;
Figure 2 is a schematic illustration of an alternative arrangement to that
shown in
Figure 1;
Figure 3 is a schematic illustration of a further alternative arrangement to
that shown
in Figure 1; and
Figure 4 is a schematic illustration of an arrangement by which gas may be
conserved
that would otherwise be lost by reason of diurnal temperature changes
Similar numerals denote similar elements.
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MODES FOR CARRYING OUT THE INVENTION
With reference to Figure 1, shown is a diagrammatic vertical cross section of
a basic
cylindrical mechanism in which the polished rod of the pump string I is
suspended
from a disc 2 by means of a shackle or flexible coupling 3. The flexible
coupling is
necessary to accommodate the small tolerances in alignment that may exist or
that
may momentarily occur between the axis of the machine and the direction of the
applied force. The rod 1 passes through a cylindrical channel or cavity in the
body of
the mechanism, the channel or cavity being bounded by the upper tube 7 and the
lower
tube 5. The interior of the mechanism is made gas-tight by means of the seal
and
bearing unit 8 between the coaxial tubes 5 and 7.
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The rod string 1 is raised and lowered by means of the extendible cylinder 6
to which
the disc 2 is affixed on its uppermost outer surface. The cylinder 6 extends
variably
from the body of the machine 4 through a second bearing and seal unit 9. The
position
of the cylinder 6 is controlled by the force produced by the interaction of
electrical
currents in the conductor vanes 12 with magnetic fields produced by the
permanent
magnet and pole piece array 11. The deadload of the pump rod string 1 is
counterbalanced by the force on the effective area of the extendible cylinder
6,
produced by the pressure of inert gas within the internal volume of the
machine 10.
It will be understood that the gas pressure in the volume 10 will vary
adiabatically
according to the position of the extendible member 6 during pumping cycle.
Thus the
electromagnetic actuator 12, 11 will cause energy to be stored within and
drawn from
the gas spring in such a manner that the electrical power consumption is
minimized
and the peak electrical power demand is greatly reduced.
The actuator 11, 12 is also fitted with a vented upper casing or cap 13, (the
vents not
being shown in this Figure) whose purpose is to protect the surface of the
extendible
member 6 and that of the upper seal 9 from the effects of weather, salt, grit
and other
aspects of the external environment that might be detrimental to the life of
the
machine. It is another function of the cap 13 to complete the enclosure of the
mechanism so that no animal or person can be harmed by accidental contact
therewith.
Referring now to Figure 2, shown is an alternative arrangement in which a
plurality of
electromagnetic linear actuators may be connected to the polished rod by means
of a
cross-head piece. In this Figure, shown for clarity are two such
electromagnetic linear
actuators, being symmetrically disposed athwart the polished rod and
constructed to
contain pressurized extendible members forming gas springs, thus providing
both a
counterbalancing force and an energy storage device in association with the
masses of
the rod string and of the liquid column. Similar numerals depict the same
elements as
those employed in Figure 1.
Figure 3 shows a further diagrammatic alternative arrangement in which at
least one
unit has a pure pneumatic function and does not contain an electromagnetic
actuator,
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so that it acts only as a gas spring. Shown are two such gas spring units 14,
having
drive piston rods or thrust tubes 15 symmetrically disposed athwart the well
head, and
two electromagnetic actuators of the form shown in Figure 2 equivalently
disposed. It
will be understood that in practice the crosshead joining the gas spring units
14 might
be orthogonal to the crosshead joining the linear motors. It will be
understood that any
arrangement of a plurality of gas spring components and a plurality of linear
electric
motor components, whether or not the functions of those components are
combined,
will perform the required function without changing the nature of the
invention.
Figure 4 shows a diagrammatic arrangement of the intermediate or economizing
pressure chamber 16 and electrically-operated valves 17, 18 and 19. It will be
understood that the valves may actually be mounted within the sealed chamber
16 and
thus immersed in an inert gas environment. It should first be recognized that
the
pressure of the gas in the counterbalancing mechanism 10 is not constant, but
varies
during the pumping cycle, reaching a minimum when the extendible member 6 is
raised to its uppermost point and being at a maximum when the pumping string
is at
its lowest point. Consider first that the pressure of gas in the economizing
chamber 16
is approximately the same as the median pressure of gas in the gas spring
chamber 10.
Now if it should be required that the mass of the gas within the
counterbalancing
mechanism shall be increased, the valve 17 is opened, for a short period only,
while
the gas spring pressure in 10 is at or near its lowest point. Gas will then
pass from the
economy chamber into the gas spring volume. Conversely, if it is necessary to
reduce
the mass of gas within the spring 10, the valve 17 is opened, for a short
period only,
while the spring pressure is at its maximum value.
The chief use of the economy chamber is related to the diurnal temperature
cycle. It
will be understood, for example, that in the heat of the day both the gas in
the spring
and the gas in the economy chamber will increase in pressure, but that it will
still be
possible by the method described above to reduce the mass of gas in the spring
when
required - and thus to optimise the electrical power consumption of the
artificial lift
mechanism ¨ by moving the surplus gas into the economy chamber without
actually
disposing of it. In the evening of the same day the temperature of the
mechanism will
fall and it will be necessary to move gas back into the spring 10 from the
economy
chamber 16 in order to maintain its pressure at that temperature so as to
produce the
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correct counterbalancing force. The reverse flow is accomplished by timing the
operation of valve 17 in the manner previously described ¨ but in this case
the valve
is opened for a short time when the extendible member is at the top of its
stroke and
the pressure in the gas spring chamber is at a minimum.
Valve 19 connects the economy chamber to the primary source of gas supply 20;
valve 18 allows the economy chamber to be vented to atmosphere if required.
Pressure transducer 21 measures the difference in pressure between the gas
spring and
the economy chamber at all times. If, during the short interval when valve 17
is
opened to allow gas from the economy chamber 16 to flow into the gas spring
chamber 10, the differential pressure between them is less than, for example,
0.5 Bar,
valve 19 is also opened for a predetermined interval so as to increase the
pressure in
the economy chamber 16. Conversely, if, during the short interval when valve
17 is
opened to allow gas to flow from the gas spring into the economy chamber, the
reverse differential pressure is less than, for example, 0.5 Bar, valve 18 is
also opened
for a predetermined interval so as to reduce the pressure in the economy
chamber. It
will be understood that this feature of our invention corrects the gas spring
settings for
gradual temperature changes and for any slow gas leakage that might occur.
It will be understood that the principles of this invention are not limited to
permanent
magnet motors but may be extended to include electrical actuators of any other
type,
including those that use electromagnets, or electrical machines that employ
the
induction principle, such as those described in various co-pending patent
applications
relating to wireless motors, wherein the array of permanent magnets is
replaced by a
passive arrangement of patterned conductive laminations. When a travelling
magnetic
field is produced by phased alternating currents in the powered conductors,
eddy
currents are deliberately arranged to flow in the passive conductor array and
the
interaction of the induced currents .and the controlled alternating currents
creates an
axial force. Although the resulting axial force is smaller than that produced
by a
machine using permanent magnetic fields or by a machine that uses fields
produced
by electromagnets, that form of construction is even lower in cost and lighter
in
weight. The cost and weight advantages may, in some circumstances, offset the
lower
efficiency of the induction motor.
