GB2345387A

GB2345387A - Submersible electromechanical actuator

Info

Publication number: GB2345387A
Application number: GB9927304A
Authority: GB
Inventors: Christophe Marcel Rayssiguier; Anthony Frank Veneruso; Gerald W Robertson
Original assignee: Schlumberger Holdings Ltd
Current assignee: Schlumberger Holdings Ltd
Priority date: 1998-11-18
Filing date: 1999-11-18
Publication date: 2000-07-05
Also published as: GB9927304D0

Abstract

An electromechanical actuator includes a housing 102 and a magnetic core 126 movably disposed in the housing. An impervious chamber containing the stator is defined between the housing wall and can 114, the chamber being filled with a dielectric fluid. Pressure balancing is provide by bellows 152. The actuator may be rotary and arranged to surround the stator - 126 fig 6 or may be linear - 205 fig 5. Fluid tight electrical connection is provided through bulkhead 108.

Description

SUBMERSIBLE ELECTROMECHANICAL ACTUATOR The invention relates generally to electromechanical actuators, e. g., electric motors and solenoids. More particularly, the invention relates to an electromechanical actuator for use in controlling or moving a device or system in a wellbore.

In the petroleum industry, downhole actuators are routinely employed to remotely operate devices, such as valves, clamps, and pumps, in a wellbore. The downhole actuators may be electromechanical actuators that convert electrical energy into mechanical energy using electromagnetic phenomena. This mechanical energy is transmitted to the device to be actuated through a movable part of the actuator. One example of an electromechanical actuator is a solenoid. Typically, the solenoid comprises a cylindrical coil of insulated wire and a core made of magnetic material.

A flow of electric current in the coil produces a magnetic field which moves the core along the coil. The motion of the core may be used to actuate a device.

Another example of an electromechanical actuator is an electric motor. The electric motor typically has a rotor and a stator. The stator is made of magnetic materials and electrical conductors that serve to establish and shape magnetic fields.

The rotor, which usually comprises the movable part of the electric motor, contains conductors which produce and shape magnetic fields that will interact with magnetic fields that are generated by the stator. The interaction of the magnetic fields produced by the stator and the rotor induces a torque on the rotor. This torque may be transmitted, usually through a drive shaft, to the device to be actuated.

There are various types of electric motors that may be suitable for use in a wellbore. FIG. 1 shows one example of a direct-current electric motor that may be used in a wellbore. The electric motor 10 includes a housing 12 which encloses a rotor 14 and a stator 16. The rotor 14 includes a hub 18 with a permanent magnet structure. The hub 18 is mounted on a shaft 20 that is supported for rotation on bearings 22 and 24. One end of the shaft 20 protrudes from the housing 12 and includes a coupling end 26 which may be linked to another device, e. g., a gear box or an actuating mechanism. Coil windings 28 are arranged in slots in the stator 16. The coil windings 28 may be connected to an external electrical system by electrical wires 30.

The electric motor 10 begins to operate when voltage is applied to the input terminals of the electrical wires 30 and current flows through the coil windings 28.

As current flows through the coil windings 28, the stator 16 becomes an electromagnet. The north pole (or poles) of the stator 16 becomes attracted to the south pole (or poles) of the rotor 14, and the south pole (or poles) of the stator 16 becomes attracted to the north pole (or poles) of the rotor 14. The electromagnetic forces between the poles of the stator 16 and the rotor 14 result in a torque on the rotor 14, which causes the rotor 14 to spin. As the rotor 14 spins, the angular position of the rotor is measured and used to determine the correct voltage polarity to apply to the input terminals of the electrical wires 30 so that the rotor continues to spin in the same direction.

The coil windings 28 are wound with a magnetic wire, typically insulated copper wire. The insulation on the copper wire is provided to prevent arcing over to other components of the motor. One method commonly used in insulating the copper wire involves coating the copper wire with an impervious material, usually enamel or varnish. Generally, the coating process is good but not perfect enough to prevent small holes, called"pin-holes,"in the enamel or varnish. When the copper wire is wound into a coil, the probability of one pin-hole lying next to another pin-hole is low, and the layer of enamel or varnish between the coil prevents conduction from one pin-hole to the next.

When the electric motor is employed in a wellbore, the electric motor operates in the presence of wellbore fluids, which typically contain electrically conductive fluids, e. g., salt water. If an electrically conductive fluid gets in between the coil, conduction from one pin-hole to the next will occur, leaving the motor vulnerable to immediate short-circuit failure. Pin-hole to pin-hole conduction can be prevented by providing the copper wire with quality insulation, e. g., by impregnating the copper wire with a resinous material. However, the cost of providing this quality insulation is high. The wellbore fluids may also contain corrosive fluids which can damage the insulation on the copper wire as well as corrode the copper wire.

Thus, it is important to protect the coil windings from the wellbore fluids.

One approach to preventing the coil windings from the wellbore fluids is to seal the motor such that the wellbore fluids are prevented from entering the motor in the first place. One method that has been used to seal the motor includes providing an impervious coating on the motor, e. g., by painting the motor or dipping the motor in latex or wax. However, this method is not very effective because the motor traps cool air when assembled. This cool air heats up and expands as the motor is operated.

The expanding air creates pressure, which breaks the seal the impervious coating was meant to establish. When the motor cools down, the air shrinks and creates a vacuum that draws fluid inside the motor.

Another approach commonly employed to protect coil windings in submersible electric motors involves filling the interior of the motor housing with a lubricating fluid, e. g., a dielectric fluid. The coil windings and other parts of the motor are immersed in the dielectric fluid such that pin-hole to pin-hole conduction in the coil windings and arcing from the coil windings to other parts of the motor are prevented. The dielectric fluid protects the coil windings and other metal parts of the motor from corrosion. In addition, the dielectric fluid dissipates the heat generated by the energized components of the motor so that the temperature within the housing is maintained at an acceptable level.

The dielectric fluid is free to expand and contract as the motor heats up and cools down, respectively. Thus, the dielectric fluid may leak out of the motor as it expands and may draw fluid inside the motor as it contracts. It is important, however, to protect the dielectric fluid against contamination by the wellbore fluids. This is because the characteristics of a dielectric fluid are detrimentally affected by contaminants. For example, moisture reduces the dielectric strength of the dielectric fluid, and oxygen can help the dielectric fluid form a sludge. A breakdown in the dielectric fluid due to contamination by wellbore fluids, even a contamination as little as a drop of salt water in the wrong place, can cause immediate short-circuit failure.

One location through which fluid may be exchanged between the motor and the wellbore is in the area where the shaft exits the motor housing. Thus, it is common practice to provide one or more seals at that location. These seals are typically made of polymer or elastomeric materials and are not the surest protection against fluid exchange between the motor and the wellbore because they will eventually leak. The seals may leak due to a variety of reasons, some of which include abrasion and wear, mechanical fatigue due to thermal expansion and contraction of the dielectric fluid and fluctuating temperatures of the wellbore fluids, and deformation due to the corrosive nature of the wellbore fluids. The exact lifetime of the seals may be days or weeks, depending on the materials chosen, mechanical fit and surface finish of the mating parts, and the environment's temperature, pressure, and chemistry.

A pressure compensation system, e. g., a spring-loaded diaphragm, is sometimes provided to balance the pressure in the housing with the wellbore fluid pressure as the dielectric fluid expands. This has the effect of reducing damage to the seals by providing for the expansion or contraction of the dielectric fluid. U. S. Patent 5,796,197 to Bookout discloses a motor wherein an oil-filled bladder acts as an oil reservoir and provides pressure compensation against wellbore fluid pressure. A spring presses on the bladder in an attempt to maintain a slightly higher oil pressure inside the motor housing. As such, any leakage will result in oil leaving the motor rather than wellbore fluid entering the motor. However, these systems are not well suited for long-term use because the seals will eventually leak as previously described or the oil reservoir will be depleted, and the protective dielectric fluid or oil in the housing will be invaded by wellbore fluid.

Another common method of preventing contaminants from entering the motor housing is to provide the housing with a waterproof cover. U. S. Patent 5,134,328 to Johnatakis et al. discloses an elastomeric boot which is intended to protect an electric motor from invasion by wellbore fluids. However, because the coupling end of the motor shaft must exit the boot to allow coupling with another device. we again have the issue of ensuring reliable sealing around the shaft to prevent fluid exchange between the housing and the wellbore. Over time, the hostile operating environment in the wellbore will also cause the elastomeric boot to fail and allow wellbore fluids to invade the motor housing. As previously described, ingress of wellbore fluids into the motor can cause short-circuit failure.

U. S. Patent 5,620,048 discloses a motor for use in the presence of wellbore fluids. The motor includes a stator which carries coil windings. The stator is disposed in an isolated chamber between a casing and a tubing in the wellbore. The tubing provides a flow channel from an oil-bearing rock stratum to the surface. The movable part of the electric motor is disposed inside the tubing. The isolated chamber is filled with a dielectric substance. Since the coil windings are disposed in the isolated chamber outside the tubing, the dielectric substance around the coil windings is not exposed to contaminants from the fluid flowing through the tubing.

The task of reliably sealing a motor is not unique to wellbore operations. In the nuclear, food processing, and waste management industries where similar reliability and sealing problems exist, one well known technique of sealing motors and pumps involves using a magnetic coupling to couple the motor to the pump impeller. The magnetic coupling typically includes a driver magnet and a driven magnet. The driver magnet is connected to the motor and the driven magnet is connected. When the motor applies torque to the magnetic coupling, the magnetic coupling deflects angularly and the driver and driven magnets create a force of simultaneous attraction and repulsion. This force is used to transfer torque from the motor to the pump impeller. A magnetic coupling, however, requires additional space which is at a premium or may not exist in the downhole actuator. Also, the magnetic coupling introduces significant drop in motor efficiency because of the windage losses as the added parts rotate at high speed in a dielectric fluid filled environment.

In aerospace and earth moving applications, motors have also been protected against mud, dust, and debris by moving bellows. In these cases, the motor's rotational motion is first converted to a push-pull motion. However, these bellows require moving or flexible parts made of thin wall material which is forced to flex each time the motor operates. Therefore, the reliability of these fragile moving parts is not high, where reliability is herein defined as the probability of successful operation for a specified life and in a specified environment. Furthermore, their life will be relatively short because they will eventually develop leaks due to mechanical fatigue and/or chemical corrosion in the hostile wellbore fluids. Particulates in the wellbore fluids can also abrade and eventually pierce the thin wall membrane or bellows.

The solenoid, like the electric motor, typically includes a housing which encloses the coil of wire and the magnetic core. Like the coil windings of the electric motor, the coil of wire is typically wound with an insulated copper wire and should, therefore, be protected from wellbore fluids for the reasons discussed above. The solenoid may be filled with dielectric fluid so that the coil is protected in a dielectric fluid environment. As in the case of the electric motor, this dielectric fluid environment should also be free of contamination to prevent short-circuit failure.

In general, in one aspect, an electromechanical actuator comprises a housing and a magnetic core movably disposed in the housing. An impervious chamber is defined within the housing. The impervious chamber is filled with a dielectric fluid.

An electrical core is disposed in the impervious chamber. The electrical core comprises at least one electrical coil. The actuator comprises means for connecting the electrical coil to an electrical power supply.

In another aspect, an electromechanical actuator comprises a first casing, a second casing positioned adjacent the first casing, and an impervious chamber defined between the first casing and the second casing, wherein the impervious chamber is filled with a dielectric fluid. A pressure compensator is in communication with the impervious chamber. An electrical coil is disposed in the impervious chamber. The electric coil is arranged to generate a magnetic field. The actuator comprises means for connecting the electrical coil to an electrical power supply and a magnetic core disposed in the second casing. The magnetic core is movable under the effect of the magnetic field.

In yet another aspect, an electromechanical actuator comprises a first casing, a second casing positioned adjacent the first casing, and an impervious chamber defined between the first casing and the second casing, wherein the impervious chamber is filled with a dielectric fluid. A stator is disposed in the impervious chamber. A plurality of coil windings are provided in the stator. The actuator comprises means for connecting the coil windings to an electrical power supply. A shaft extends through the second casing. The shaft is supported for rotation about an axial axis of the second casing. A rotor is mounted on the shaft.

Other features and advantages of the invention will be apparent from the following description and the appended claims.

FIG. 1 illustrates one example of an electric motor.

FIG. 2 is a cross-sectional view of an electric motor according to an embodiment of the invention.

FIG. 3 is a cross-section of the electric shown in FIG. 2 along lines A-A.

FIG. 4 shows an actuator assembly employing the electric motor of FIG. 2.

FIG. 5 is a cross-sectional view of a solenoid according to an embodiment of the invention.

FIGS. 6 and 7 are cross-sectional views of alternate embodiments of the electric motor shown in FIG. 2.

Referring to the drawings wherein like characters are used for like parts throughout the several views, FIGS. 2 and 3 illustrate an electromechanical actuator, e. g., an electric motor 100, which comprises a housing 102. The housing 102 includes an outer casing 104, an inner casing 106, and a bulkhead 108. The inner casing 106 includes end caps 110 and 112 and a sleeve 114 connecting the end caps 110 and 112. The outer casing 104 has a first end 116 connected to the bulkhead 108 and a second end 118 connected to the end cap 110. The connections between the outer casing 104 and the bulkhead 108 and casing 106 are fluid-tight connections that may be accomplished by welding, brazing, or other suitable means. When the electric motor 100 is employed in a corrosive environment, the outer casing 104, the inner casing 106, and the bulkhead 108, should be made of a corrosion-resistant material.

The outer casing 104, the inner casing 106, and the bulkhead 108 define an impervious chamber 120. An electrical core, which comprises a stator 122 and coil windings 124, is disposed in the impervious chamber 120. A magnetic core, e. g., rotor 126 is disposed in the inner casing 106. The rotor 126 comprises permanent magnets 128 mounted on a hub 130. The permanent magnets 128 may be secured to the hub 130 by an adhesive, e. g., epoxy, and held together by a thin sleeve 132. The permanent magnets 128 may be made of rare earth magnetic materials, such as samarium-cobalt or neodymium-iron-boron. The hub 130 is mounted on a shaft 134 that is supported for rotation by bearings 136 and 138 in the end caps 110 and 112, respectively. An angular position sensor 140 is mounted on the rotor 126 to measure the angular position of the rotor 126 as the rotor spins.

The stator 122 is made of thin laminations 142, which may be stamped from silicon-steel alloy or other suitable material. The laminations 142 may be insulated from each other, e. g., by coating with oxide or varnish or by inserting non-conductive plates in between the laminations. Insulating the laminations breaks up the conducting path in the stator and reduces eddy current losses. The coil windings 124 are inserted in slots 144 in the laminations and connected to electrical wires 146. To facilitate insertion of the coil windings into relatively small diameter motors, the slots 144 are provided on the outer diameters 148 of the laminations 142; however, the slots 144 may also be provided on the inner diameters 150 of the laminations 142.

The coil windings 124 may be wound with insulated copper wire or other magnetic wire.

The impervious chamber 120 is filled with dielectric fluid, which protects and insulates the coil windings 124 as well as the other metal parts of the electric motor 100. The dielectric fluid also fills spaces between the laminations 142 so as to assist in breaking up the conduction path in the stator. The dielectric fluid may also help dissipate the heat generated during operation of the electric motor. The key requirements for the dielectric fluid are that the fluid must be a good dielectric, the fluid must be non-boiling and have a low viscosity over a specified temperature range, and the fluid must be chemically compatible with the insulation on the coil windings 124 and the laminations 142. Typically, the dielectric fluid is a synthetic oil, e. g., perfluorinated oil compound or a silicone-based oil. The impervious chamber 120 protects the dielectric fluid from contamination.

A pressure compensator 152 is secured to the bulkhead 108. The pressure compensator may be a metal bellows that is secured to the bulkhead 108 by welding, brazing, or other suitable means that ensures a fluid-tight connection between the bellows and the bulkhead. The pressure compensator 152 is in communication with the impervious chamber 120 through a port 154 in the bulkhead 108. The pressure compensator 152 is arranged to receive dielectric fluid from the impervious chamber 120 when the dielectric fluid in the impervious chamber expands. The pressure compensator 152 also supplies dielectric fluid to the impervious chamber 120 when the dielectric fluid in the impervious chamber contracts.

The sleeve 114 between the stator 122 and the rotor 126 may be made of metal. However, a continuous metal sleeve in the inner diameter of the stator can result in unacceptably high eddy current losses and loss in motor efficiency.

Therefore, to minimize eddy current losses, the sleeve 114 should be thin-walled and the metal should have a relatively high electrical resistivity and a low magnetic permeability. One example of such a metal is stainless steel alloy. To eliminate the eddy current losses entirely, the sleeve 114 can be made alternately of a non-magnetic material, e. g., ceramic, plastic, or glass. The sleeve 114 may also be achieved by an impervious coating, e. g., an impervious coating of ceramic or plastic, applied to the inner diameter of the sleeve, i. e., the surface adjacent to the rotor 126.

An electronic cartridge 156 is secured to the bulkhead 108. The electronic cartridge 156 includes a casing 158 and a bulkhead 160 which is secured to the casing 158. A fluid-tight chamber 162 is defined between the casing 158, the bulkhead 160, and the bulkhead 108. Inside the chamber 162 is an electronic unit 164 which includes a logic circuitry for controlling the operation of the electric motor. The electronic unit 164 receives information from the angular position sensor 140 on the rotor 126 and uses this information to determine the correct voltage polarity to apply to the electrical wires 146. The electronic cartridge 156 may be connected to an electrical supply by electrical wires 166.

In operation, current is supplied to the coil windings 124 through the electrical wires 146. As the current flows through the coil windings 124, the stator 122 becomes an electromagnet. The stator 122 then produces a magnetic field which interacts with the magnetic field produced by the rotor 126. The interaction between the magnetic fields produced by the stator 122 and rotor 126 results in a torque on the rotor 126, which causes the rotor 126 to spin. As the rotor 126 spins, the angular position sensor 140 detects the angular position of the rotor 126 and transmits this information to the electronic unit 164. The logic circuitry in the electronic unit 164 then uses the transmitted information to apply the correct voltage polarity to the electrical wires 146 such that the rotor 126 continues to spin in the same direction.

The invention described thus far has several advantages. First, the coil windings 124 are disposed in a chamber 120 that is impervious to fluids and gases.

This impervious chamber 120 is achieved without the use of seals, which are prone to leakage when operating in the hostile wellbore environment.

Secondly, the impervious chamber maintains and protects the purity of the dielectric fluid environment. The dielectric fluid does not leak out of the impervious chamber neither does fluids or gases enter into the impervious chamber to contaminate the dielectric fluid. As a result, when the electric motor operates in the presence of wellbore fluids and gases, there is no risk of short-circuit failure due to damage to the insulation on the coil windings, pin-hole to pin-hole conduction, and corrosion of the coil windings. The dielectric fluid reservoir is also never depleted.

Thirdly, the invention may be used in conjunction with previous sealing means to create additional barrier to fluid exchange between the motor and the wellbore. For example, state-of-the art movable seals can be mounted around the shaft 134 and the inner casing 106 can be filled with a dielectric fluid or other lubricating fluid so that the rotor 126 spins in a fluid. The permanent magnets 128 are not as vulnerable as the coil windings 124. Therefore, a small amount of wellbore fluid in the inner casing 106 should not have a devastating effect on the motor.

Referring to FIG. 4, one example of an actuator assembly 170 which employs the electric motor 100 is shown. The actuator assembly 170 includes a housing 172 which has an end 174 that is secured to the outer casing 104. Inside the housing 172 is a gearbox 176, which is coupled to the shaft 134 of the electric motor 100. An actuating mechanism 178, which includes a screw 180 and a nut 182, is also disposed in the housing 172. The screw 180 and nut 182 have grooves (not shown) and balls positioned in the grooves so as to allow the screw 180 and nut 182 to translate linearly with respect to each other. The screw 180 is coupled to the gearbox 176, and the nut 182 is coupled to a sleeve 184. The sleeve 184 is a movable member of an actuatable device, e. g.. a sliding sleeve valve.

A seal 186 is provided between the housing 172 and the sleeve 184, and the housing 172 is filled with a lubricating fluid, e. g., a dielectric oil. The end cap 110 includes channels (not shown) which allow the fluid in the housing 172 to circulate through the interior of the inner casing 106 and around the rotor 126. The outer casing 104 includes channels 188 which allow fluid to circulate around the motor 100. This circulating fluid helps dissipate heat from the motor 100 so that the temperature within the motor is maintained at an acceptable level. When the actuator assembly 170 is employed in a wellbore, the seal 186 isolates the rotor 126, the bearings 136 and 138, the gearbox 176, and the actuating mechanism 178 from wellbore fluids and particles.

A pressure compensator, e. g., piston assembly 190, is provided in the housing 172. The piston assembly 190 includes a piston 192 which has a first surface 194 that is exposed to the wellbore fluids and a second surface 196 that is exposed to the interior of the housing 172. The pressure compensator 190 helps ensure that there is no differential pressure across the seal 186 which can cause damage to the seals or prevent operation of the actuating mechanism 178.

In operation, when the rotor 126 spins, torque is transmitted to the screw 180 through the shaft 134 and gearbox 176, causing the screw 180 to rotate. As the screw 180 rotates, the nut 182 translates linearly along the screw 180. This linear motion is transmitted to the sleeve 184, which may operate, for example, to open or close ports in a valve.

Referring to FIG. 5, an electromechanical actuator, e. g., a solenoid 200, which comprises coils 202 and a plunger 204 is shown. The coils 202 may be wound with insulated copper wire or other magnetic wire. The plunger 204 is made of a magnetic material, such as soft iron. The coils 202 and the plunger 204 are arranged in a housing 205 that is similar to housing 102 (shown in FIG. 2). As shown, the coils 202 are disposed in an impervious chamber 206 that is defined by casings 208 and 210 and a bulkhead 212. The casing 210 defines a second chamber 214 for receiving the plunger 204. The plunger 204 is axially movable within the second chamber 214.

A spring 218, which extends between the casing 210 and the plunger 204, controls the movement of the plunger.

The coils 202 may be connected to an external electrical system by electrical wires 220. The impervious chamber 206 is filled with dielectric fluid. In operation, electric current supplied to the coils 202 energizes the coils to produce an electromagnetic field that moves the plunger 204. The motion of the plunger 204 can be transmitted to another device. The impervious chamber 206 protects the coils 202 and maintains the purity and integrity of the dielectric fluid environment.

The invention has been described with respect to a limited number of embodiments, but those skilled in the art will appreciate numerous variations therefrom without departing from the spirit and scope of the invention. For example, in FIGS. 2 and 3, the stator 122 is disposed about the rotor 126. However, it may be desirable to dispose the rotor 126 about stator 122 instead, as shown in FIG. 6. In the embodiment shown in FIG. 6, the housing 102 has been modified such that coil windings 124 are still enclosed in an impervious chamber 120.

The configurations of the stator 122 and the rotor 126 are peculiar to a brushless, direct-current motor. However, it should be clear that the concept of providing an impervious chamber inside the motor housing for the coil windings is easily extendible to other rotor and stator configurations. For example, coil windings may be provided on the rotor. FIG. 7 shows an induction motor 220, wherein coil windings are provided in a stator 222 as well as in a rotor 224. In this case, two impervious chambers 226 and 228 are defined. The impervious chamber 226 encloses the coil windings 229 in the stator 222, and the impervious chamber 228 encloses the coil windings 230 in the rotor 224.

The rotor 224 may be made of laminations 232 with slots for receiving the coil windings 230. The laminations 232 may be mounted on a hub 234 as shown, and a casing 236 may be secured to the hub 234 such that the impervious chamber 228 is defined between the casing 236 and the hub 234. The impervious chamber 228 may be filled with a dielectric fluid, and a pressure compensator, e. g., metal bellows 238, may be mounted on the casing 236. If it is necessary to connect the coil windings 230 to an electrical supply, electrical wires (not shown) may provide such a connection.

The electrical wires may be encased in a flexible metal tube (not shown) that has one end welded to the casing 236 and another end welded or sealed to an electrical connector.

Claims

CLAIMS 1. An electromechanical actuator, comprising: a housing; a magnetic core movably disposed in the housing; an impervious chamber defined within the housing, the impervious chamber being filled with a dielectric fluid; an electrical core disposed in the impervious chamber, the electrical core comprising at least one electrical coil; and means for connecting the electrical coil to an electrical power supply.
2. The electromechanical actuator of claim 1, further comprising a pressure compensator in communication with the impervious chamber.
3. The electromechanical actuator of claim 2, wherein the housing comprises a first casing and a second casing disposed in the first casing, and wherein a space between the first casing and the second casing defines the impervious chamber.
4. The electromechanical actuator of claim 3, wherein the pressure compensator is mounted on the first casing and communicates with the impervious chamber.
5. The electromechanical actuator of claim 3, wherein the first casing and the second casing are made of a corrosion-resistant material.
6. The electromechanical actuator of claim 2, wherein the second casing comprises a sleeve portion interposed between the electrical core and the magnetic core.
7. The electromechanical actuator of claim 6, wherein the electrical core comprises a magnetic portion circumscribing the sleeve portion, the magnetic portion having a slot for receiving the electrical coil.
8. The electromechanical actuator of claim 7, wherein the electrical core further comprises additional electrical coils, and wherein the magnetic portion includes additional slots for receiving the additional electrical coil.
9. The electromechanical actuator of claim 7, wherein the magnetic portion comprises laminations stacked axially along the sleeve portion.
10. The electromechanical actuator of claim 8, wherein the magnetic portion further comprises non-magnetic plates interposed between the laminations.
11. The electromechanical actuator of claim 8, wherein the laminations are coated with a non-magnetic material.
12. The electromechanical actuator of claim 6, wherein the sleeve portion is made of a thin-walled metal having high electrical resistivity and low magnetic permeability.
13. The electromechanical actuator of claim 6, wherein the sleeve portion is made of a non-magnetic material.
14. The electromechanical actuator of claim 6, wherein a surface of the sleeve portion adjacent the magnetic core is coated with an impervious coating.
15. The electromechanical actuator of claim 6, wherein the magnetic core comprises a shaft supported for rotation about an axial axis of the second casing.
16. The electromechanical actuator of claim 15, wherein the magnetic core comprises a plurality of permanent magnets coupled to the shaft.
17. The electromechanical actuator of claim 16, wherein the permanent magnets are made of rare earth magnetic material.
18. The electromechanical actuator of claim 17, further comprising an angular position sensor coupled to the magnetic core.
19. An electromechanical actuator, comprising: a first casing; a second casing positioned adjacent the first casing; an impervious chamber defined between the first casing and the second casing, the impervious chamber being filled with a dielectric fluid ; a pressure compensator in communication with the impervious chamber ; an electrical coil disposed in the impervious chamber, the electrical coil being arranged to generate a magnetic field; means for connecting the electrical coil to an electrical power supply; and a magnetic core disposed in the second casing, the magnetic core being movable under the effect of the magnetic field.
20. An electromechanical actuator, comprising: a first casing; a second casing positioned adjacent the first casing; an impervious chamber defined between the first casing and the second casing, the impervious chamber being filled with a dielectric fluid ; a stator disposed in the impervious chamber ; a plurality of coil windings arranged in slots in the stator ; means for connecting the coil windings to an electrical power supply ; a shaft extending through the second casing, the shaft being supported for rotation about an axial axis of the second casing; and a rotor mounted on the shaft.
21. The electromechanical actuator of claim 20, wherein the second casing is filled with a second dielectric fluid.