Abstract:
An explosion-proof actuator assembly includes an explosion-proof housing, a motor, motor control electronics, and an actuator. The housing has at least one inner cavity and at least one exit path comprising an explosion-proof flame path. The motor is disposed within the housing. The motor control electronics communicate with the motor and are disposed within the housing. The actuator is carried by the housing, communicates with the motor, and is configured for movement relative to the housing responsive to actuation of the motor.

Description:
TECHNICAL FIELD 
     This invention pertains to electric motors and actuators. More particularly, this invention relates to unitary electric motor and actuator assemblies having explosion-proof construction that facilitates use in explosive gas environments. 
     BACKGROUND OF THE INVENTION 
     The construction and utilization of linear and rotary actuators is well understood. For example, electro-mechanical actuators have included hydraulic actuators, pneumatic actuators, and ball-screw actuators. For various reasons discussed below, none of the prior art linear actuators are suitable for use in applications where there is a potentially explosive environment, and where there is limited package space. For example, none of the prior art devices are suitable for driving a fuel and air delivery valve for a gas turbine engine where there is limited package space, and the actuator requires precision actuation within a potentially explosive environment. It is believed that other similar applications exist where there is a need for a limited package space actuator that can operate within a potentially explosive environment. 
     Hydraulic linear actuators are well known in the art. Typically, a hydraulic actuator is actuated via an arrangement of hydraulic valves to impart axial movement of an actuator rod. The actuator rod is used to impart movement to a mechanical component such as a kinematic linkage on a machine. However, the ability to precisely control movement is somewhat limited to the ability to accurately control fluid flow via the hydraulic valves. Furthermore, hydraulic fluid tends to leak from the actuator, particularly over time as seals within the actuator wear during use. Even furthermore, the hydraulic actuator and control valves are provided as separate components which tends to prevent use where package space is limited. 
     Pneumatic linear actuators are also well known in the art. Typically, a pneumatic actuator is actuated via a supply of pressurized gas via a pneumatic valve assembly. Similar to a hydraulic actuator, the pneumatic actuator has a rod that imparts movement to a mechanical component. Also similar to the hydraulic actuator, the pneumatic actuator and pneumatic valve assembly are provided as separate components which tends to prevent use where package space is limited. 
     Rotary threaded shaft actuators are additionally well known in the art. Examples of such actuators include ball screw actuators, and improvements on such actuators that use some form of modified nut and threaded shaft to generate linear actuation. Examples includes U.S. Pat. Nos. 3,965,761 and 4,496,865, herein incorporated by reference. A rotary motor generates rotational motion that is converted into linear motion with the aid of a linear traveling device such as a threaded shaft cooperating with a threaded nut assembly. However, an electric motor having a permanent magnetic field is used to drive these threaded shaft actuators. Use of such electric motors tends to be somewhat imprecise for applications that require precise axial movement, such as is used when metering fuel and air delivery via a valve assembly for a turbine engine. Furthermore, such electric motors are typically DC motors that include motor brushes. However, such brushes are known to generate sparks which can be hazardous when using the actuator within a potentially explosive environment. 
     Recent advances have been made in the field of brushless DC motors. However, such motors require the use of a computer controller in order to precisely control operation of the brushless motors, and such computer controllers increase the packaging size and complexity. Additionally, the control electronics are typically provided on one or more printed circuit boards which provide an additional source for generating a spark that could prove dangerous when used in a potentially explosive environment. Furthermore, these motors have only been provided in housings that are separate from a linear actuator that is driven by the motor. Hence, the package space is relatively bulky. 
     Accordingly, there exists a need for a motorized actuator that is compact and suitable for use in potentially explosive environments, such as for operating fuel and air delivery valves for gas turbine engines. 
     SUMMARY OF THE INVENTION 
     A rugged explosion-proof actuator is provided with onboard electronics and a precise brushless DC motor. Explosion-proof functionality is imparted via one or more gas exit paths designed to impart explosion-proof flame paths within a single, relatively compact and unitary actuator and motor housing. According to one construction, the actuator is a linear actuator. According to another construction, the actuator is a rotary actuator. 
     According to one aspect of the invention, an actuator assembly includes an explosion-proof housing, a motor and an actuator. The housing has an inner cavity. The motor is disposed within the housing. The actuator is carried by the housing and communicates with the motor. The actuator is operative to generate movement externally of the housing responsive to actuation of the motor. 
     According to another aspect of the invention, an actuator assembly includes a housing, a motor, motor control electronics, and an actuator. The housing has an inner cavity. The motor is disposed within the housing. The motor control electronics communicate with the motor and are disposed within the housing. The actuator is carried by the housing and communicates with the motor. The actuator is carried for movement relative to the housing responsive to actuation of the motor. 
     According to yet another aspect of the invention, an explosion-proof actuator assembly includes an explosion-proof housing, a motor, motor control electronics, and an actuator. The housing has at least one inner cavity and at least one exit path comprising an explosion-proof flame path. The motor is disposed within the housing. The motor control electronics communicate with the motor and are disposed within the housing. The actuator is carried by the housing, communicates with the motor, and is operative to generate movement externally of the housing responsive to actuation of the motor. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Preferred embodiments of the invention are described below with reference to the accompanying drawings, which are briefly described low. 
     FIG. 1 is an isometric view of a rugged explosion-proof actuator illustrating a test environment for testing explosion-proof capabilities within an enclosed test chamber. 
     FIG. 2 is an enlarged isometric view of the rugged explosion-proof actuator illustrated in FIG. 1 according to one aspect of the invention. 
     FIG. 3 is a plan view of the actuator illustrated in FIGS. 1 and 2. 
     FIG. 4 is a front elevational view of the apparatus illustrated in FIGS. 1-3. 
     FIG. 5 is a right side view taken from the right of FIG.  4 . 
     FIG. 6 is a left side view taken from the right of FIG.  4 . 
     FIG. 7 a vertical sectional view taken along the central axis of the actuator of FIGS. 1-6. 
     FIG. 8 is an enlarged sectional view taken from the identified encircled region of FIG. 7 illustrating a first flame path; 
     FIG. 9 is an enlarged sectional view taken from the identified encircled region of FIG. 7 illustrating a second flame path. 
     FIG. 10 an enlarged sectional view taken along line  10 — 10  of FIG. 7 illustrating a third flame path. 
     FIG. 11 is an enlarged sectional view taken from the identified encircled region of FIG. 7 illustrating fourth and fifth flame paths. 
     FIG. 12 is an enlarged sectional view taken from the identified encircled region of FIG. 7 illustrating sixth and seventh flame paths. 
     FIG. 13 is a second perspective view of the actuator of FIG. 2 illustrating the heat dissipating cover and electronics removed from the actuator housing. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     This disclosure of the invention is submitted in furtherance of the constitutional purposes of the U.S. Patent Laws “to promote the progress of science and useful arts” (Article 1, Section 8). 
     Reference will now be made to a preferred embodiment of Applicant&#39;s invention. One exemplary implementation is described below and is depicted with reference to the drawings comprising an explosion-proof linear actuator having onboard electronics, a brushless DC drive motor, and an integrated actuator contained within a relatively compact, unitary housing. While the invention is described via a preferred embodiment, it is understood that the description is not intended to limit the invention to this embodiment, but is intended to cover alternatives, equivalents, and modifications such as are included within the scope of the appended claims. For example, it is envisioned that an explosion-proof rotary actuator can also be provided according to an alternative construction. 
     In an effort to prevent obscuring the invention at hand, only details germane to implementing the invention will be described in great detail, with presently understood peripheral details being incorporated by reference, as needed, as being presently understood in the art. 
     A preferred embodiment of the invention is illustrated in the accompanying drawings particularly showing a rugged explosion-proof electric/electronic linear actuator assembly suitable for use in potentially explosive environments and generally designated with reference numeral in FIGS. 1-7 and  13 . In FIG. 1, self-contained linear actuator assembly  10  is shown supported within a test environment for testing explosion-proof capabilities. More particularly, linear actuator assembly  10  is contained within an enclosed test chamber  12  provided within a metal containment box  14 . Chamber  12  is filled with explosive gases  16  such as hydrogen gas which migrates inside actuator assembly  10 . During a test phase, an ignition source, such as a spark plug,  18  is provided to ignite explosive gases that are present within actuator assembly  10 . 
     The utilization of electrical and electronics equipment in hazardous areas where there is a risk of explosion has necessitated the rendering of explosion-proof equipment. However, much of the equipment available to-date has been bulky and poorly designed for specific applications such as on a linear actuator assembly being used to control a fuel/air delivery valve on a gas turbine engine. Oftentimes, motors and electronics are placed inside commercially available explosion-proof enclosures which are bulky, and poor at accommodating actuators and wiring systems. Applicant&#39;s invention addresses the need for a unitary and compact motor/actuator assembly, as well as the need for an explosion-proof linear actuator assembly. 
     Actuator assembly  10  is designed to provide local containment of explosion energy and flame in order to prevent an internal explosion generated by electrical/electronics device contained therein from providing an ignition source for an external explosion. The expulsion of hot gases from the housing of actuator assembly  10  is controlled such that surface temperatures, flames, and gas temperatures are reduced sufficiently so as to prevent the ignition of flammable gases present outside of the housing. As a result, any internal explosion that is generated within actuator assembly  10  does not expose the surrounding environment to a possible ignition source. 
     As will be described below in greater detail, Applicant&#39;s design overcomes the relatively expensive, heavy, and cumbersome solutions that would be provided by combining known explosion-proof enclosures with available linear actuators and separate drive motor assemblies. 
     In order to impart successful explosion-proof characteristics, a housing  20  of actuator assembly  10  incorporates flame paths such as flame paths  50  (see FIGS. 7-12 for additional flame paths  48 - 49  and  51 - 54 ) which allow for safe passage of hot gases and explosive energy from within housing  20 . The hot gases and explosive energy is transferred into chamber  12  from within housing  20  so as not to ignite explosive gases  16  contained inside chamber  12  and outside actuator assembly  10 . 
     More particularly, housing  20  is designed for utilization in hazardous locations such as in known explosive environments. The integration of motor  24  and associated control electronics (not shown) of electronics package  26  within a unitary housing  20  increases the risk for a spark-induced explosion. First, there is a risk of explosion resulting from sparks emanating from the control electronics. Secondly, there is a risk of explosion resulting from sparks emanating from motor  24 , even though the risk is reduced since motor  24  comprises a brushless motor. 
     Housing  20  is designed with flame paths, such as flame paths  50 , in order to sufficiently contain any explosion that occurs within an inner cavity, such as electronics cavity  46 , of housing  20 . Containment is realized when flame paths enable the dissipation of energy and heat sufficiently from inside housing  20  such that transmission of energy and flame to the outside of housing  20  is insufficient to ignite an explosion externally of housing  20 . More particularly, each flame path is of sufficient length to dissipate energy generated during an internal explosion so as to prevent generation of an explosion outside of housing  20 . 
     Each flame path comprises a slight design gap provided between mating parts of housing  20 . Such gaps enable release of heat and energy from housing  20  during an internal explosion, while containing the explosion therein and preventing transmission of flames outside of housing  20 . Even where there is no visible gap between mating parts, a gap still exists unless a hermetic seal has been formed therebetween. Further details of specific flame paths are described in greater detail below with reference to FIGS. 7-12. 
     Housing  20  is designed to meet several explosion-proof standards, including the National Electrical Code (NEC) standard, Article 500, in the United States; Canadian Standard 22.2-139 for electrically operated equipment which includes Canadian Standards Association (CSA) Class 1, Division 1, Group B (hydrogen) rating; and European Cenetec design standard EN50014 which includes European Explosion-Proof rating EEx IIB. 
     Based on Applicant&#39;s understanding, there has never previously been a motor that has passed the flame test for Canadian Standards Association (CSA), Class 1, Division 1, Group B (hydrogen). A recent test was conducted utilizing a prototype actuator assembly constructed according to the assembly depicted herein and identified as actuator assembly  10  in the accompanying figures. Such prototype actuator is the first known actuator having an integral motor that has passed the flame test for CSA Class 1, Division 1, Group B, based upon Applicant&#39;s knowledge. 
     As shown in FIG. 1, actuator assembly  10  is tested in an air and fuel environment where gases  16  comprise a mixture of hydrogen and air. A threaded hole is drilled through a heat sink cover  28 , and a spark plug  18  is threaded into the hole such that a test spark can be introduced into inner cavity  46 . Sufficient threads are provided therebetween so as to provide a flame path during testing. Optionally, an air and nitrogen mixture of gas can be provided within inner cavity  46 . A series of explosions are carried out within inner cavity  46  by generating sparks with spark plug  18  to ensure that an external explosion is not generated in test chamber  12 . 
     It should be noted that apparatus  10  is very compact because housing  20  is constructed as a unitary assembly which contains linear actuator  22 , motor  24 , and electronics package  26  in a single, common assembly. More particularly, housing  20  is formed by joining together an actuator/motor housing subassembly  38  and an electrical housing subassembly  40 . Both actuator/motor housing subassembly  38  and electrical housing subassembly  40  contain joints that are explosion-proof, using flame joints where components connect together. Every penetration of housing  20  requires the protection provided by flame path designed joints. In combination, such subassemblies  38  and  40  also provide a space-minimized design. 
     As shown in FIGS. 4 and 5, actuator/motor housing subassembly  38  is secured to electrical housing subassembly  40  via a plurality of threaded fasteners  66 . Actuator/motor housing assembly  38  forms an inner cavity  100  inside of which motor  24  and linear actuator  22  are contained. Electrical housing subassembly  40  forms a similar inner cavity  46  inside of which electronics are contained in the form of a first printed circuit (PC) board  68  and a second printed circuit (PC) board  70 . 
     Cavities  46  and  100  are coupled together via a metal ferrule  96  as shown in FIGS. 7 and 13. Ferrule  96  is welded into place with a continuous circumferential weld at each end to an electronics tray  58  of electrical housing assembly  40  and an actuator/motor casing  60  of actuator/motor housing assembly  38 . As a result, the internal cavities  46  and  100  of housing assemblies  40  and  38 , respectively, are hermetically joined together by ferrule  96  which is completely welded therebetween. Hence, the need for a flame path between assemblies  38  and  49  is eliminated. However, all other mechanically fastened-together connections associated with housing  20  and cavities  46  and  100  require the utilization of flame paths  48 - 54  as described below in greater detail. 
     As shown in FIGS. 2-7, electrical housing subassembly  40  comprises electronics tray  58  and heat sink cover  28 . Tray  58  is secured to cover  28  via a plurality of threaded fasteners  44  so as to define inner cavity  46  therebetween. A flame path  51  is provided between cover  28  and tray  58  as will be identified below with reference to FIGS. 7 and 11. 
     Cover  28  is formed from aluminum: so as to provide a heat sink that draws heat from electrical/electronic components contained within housing  20  and rejects the heat to gases outside of housing  20 . More particularly, cover  28  includes a plurality of integrally-formed cooling vanes, or fins,  56  provided on an outer surface. Electronics, or PC boards  68  and  70 , are mounted directly onto an inner surface of cover  28  via fasteners  72  (of FIG.  7 ). In operation, electronics of PC boards  68  and  70 , such as a motor driver  116  and a programmable logic controller (PLC)  118 , generate heat within inner cavity  46 . Such generated heat is then transferred from inner cavity  46  through cover  28  where the heat is dissipated externally of housing  20  via cooling vanes  56 . 
     A pair of wire feed through couplings, or fittings,  30  and  32  are mounted within cover  28 . As shown in FIG. 7, feed through coupling  32  includes threads  130  that mate in threaded engagement with corresponding threads  131  formed within individual bores  144  (see FIG. 11) formed through cover  28 . A similar set of threads  130  and  131  are provided for mating feed through coupling  30  with cover  28 . As shown in FIG. 11 below, a sufficiently threaded flame path  52  is formed by threads  130  and  131  along both of feed through couplings  30  and  32 . 
     As shown in FIG. 13, wire feed through fitting  30  contains four fourteen gauge, 120 Volt power wires  146  that are potted within fitting  30  via epoxy. Accordingly, wires  146  pass through cover  28  via fitting  30  in a sealed manner. Similarly, wire feed through fitting  32  contains seventeen twenty gauge wires  148  that carry digital signals such as command signals and alarm signals, and which are potted within fitting  32  via epoxy. Accordingly, wires  148  also pass through cover  28  where they are sealed within fitting  32 . 
     As shown in FIGS. 2-7, actuator/motor housing subassembly  40  comprises actuator/motor casing  60  and motor cover  42 . Motor cover  42  is secured to actuator/motor casing  60  via threads  102  formed on cover  42  and corresponding mating threads  103  formed within casing  60 . Cover  42  and casing  60  cooperate to form inner cavity  100  in which motor  24  and linear actuator  22  are contained. As will be described below in greater detail with reference to FIG. 9, a flame path  49  is provided outboard of threads  102  and  103 . 
     According to FIGS. 2,  5 , and  7 , an actuator rod  34  extends from housing  20  of linear actuator assembly  10 . Actuator rod  34  is driven for accurate axial displacement from housing  20  through a rod end bearing  36 . A threaded bore  35  is provided within the exposed end of actuator rod  34 . According to one application, linear actuator assembly  10  is connected to a valve assembly in order to open and close a fuel and air delivery valve (not shown) for delivering fuel and air to a gas turbine engine. Threaded bore  35  is configured to receive a threaded rod that drives the valve assembly. 
     As shown in FIGS. 3,  4 ,  6 , and  7 , a clevis  62  is provided for securing linear actuator assembly  10  to a rigid support member. Accordingly, clevis  62  affixes linear actuator assembly  10  at one end, while actuator rod  34  is driven in accurate axial displacement via motor  24 , linear actuator  22 , and electronics package  26  so as to actuate a device, such as a valve assembly (not shown). 
     Clevis  62  is secured to motor cover  42  via four threaded clevis bolts  64  as shown in FIGS. 3,  4 ,  6 , and  7 . A circumferential seal  110  is provided between clevis  62  and motor cover  42 . A flame path  48  is formed between clevis  6  and motor cover  42 , outboard of seal  110 , as describe below in greater detail with reference to FIG.  8 . 
     As shown in FIG. 7, motor  24  comprises a brushless DC rotary motor having a stator  74  and a rotor  76 . Stator  74  is affixed in a stationary location within actuator/motor casing  60  via a locking pin  108 . Rotor  76  is affixed to a ball screw shaft  80  via an anti-rotation key  78 . Ball screw shaft  80  communicates with a ball screw nut  82  which rides along a helical groove  88  along shaft  80  via spherical ball bearings  128  that are contained within a raceway (not shown) within nut  82 . 
     In operation, power is supplied to stator  74  of motor  24  which causes rotor  76  and ball screw shaft  80  to rotate together as a unit. Rotation of ball screw shaft  80  causes nut  82  to translate axially along shaft  80  via coaction of balls  128  in groove  88 . Balls  128  are recirculated within a raceway or groove  126  that is formed in nut  82 . Details of the operation which converts rotary motor motion into axial actuator rod motion are generally well understood in the art of ball screw actuators, and linear actuator  22  comprises such a ball screw actuator. U.S. Pat. No. 5,111,708 describes one such construction, and is herein incorporated by reference. 
     According to one construction, motor  24  comprises a frame-less, brushless DC motor. One such motor suitable for use in linear actuator assembly  10  of FIG. 7 is an RBE Series motor presently commercially available from Kollmorgen Motion Technologies Group, 501 First Street, Radford, Va. 24141. 
     In operation, electrical amplifier signals are generated by motor driver PC board  68  and delivered to brushless motor  24 . Rotary motion of rotor  76  is generated as motor  24  converts the electrical amplifier signals into torque. A motor power supply and drive amplifier (not shown) are provided on driver board  68 . 
     Additionally, a feedback device in the form of a resolver  98  is provided for monitoring position and velocity of rotor  76  and ball screw shaft  80 . The values detected by resolver  98  are then compared with command input values provided on digital logic PC board  70 . A servo amplifier on board  68  is used to adjust electrical output to motor  24  in order to realize command input requirements. Precise motion control for shaft  80  (and actuator rod  34 ) is obtained using position, velocity current, and control loops. Accordingly, resolver  98  is mounted at a stationary location about ball screw shaft  80  in order to sense rotational position information of shaft  80 . 
     Resolver  98  includes a stator and a rotor. An excitation signal is sent to resolver  98  from digital logic board  70 . Such an excitation signal in the form of a sine or cosine signal tells when to turn on the next phase for the three phases of motor  24 . 
     As shown in FIG. 7, rotor  76  includes a plurality of high energy magnets. Stator  74  has three phases spaced in 120 degree increments around the motor. Resolver  98 , in operation, provides velocity feedback information as well as position information that is used to commutate motor  24 . Additionally, position loops can be realized via resolver  98 . 
     In FIG. 7, ball screw shaft  80  is shown supported for rotation by a single end bearing  86  and a pair of centrally located bearings  84  and  85 . Bearing  86  is supported within clevis  62 , and a circumferential seal  112  is provided therebetween. Bearings  84  and  85  cooperate to provide a main thrust bearing for shaft  80 . Bearings  84  and  85  support all of the thrust generated when driving shaft  80  in rotation, while bearing  86  is primarily provided to stabilize and prevent wobbling of shaft  80 . 
     In operation, nut  82  and actuator rod  34  translate along shaft  80  as shaft  80  is rotated by motor  24 . Nut  82  and actuator rod  34  are rigidly affixed together via respective mating threads  104  and  105  and set screw  109 . Nut  82  and actuator rod  34  are prevented from rotating with shaft  80  via a slot  124  that is provided within a flange  122  of rod  34 . Slot  124  is configured to mate in sliding engagement with an elongate rod guide  90  which prevents any rotation of nut  82  and actuator rod  34 . Rod guide  90  is secured to actuator/motor casing  60  via a plurality of threaded fasteners  120 . 
     Actuator rod  34  is further guided for axial reciprocation via a rod end bearing  92  that is rigidly affixed to housing  20  via mating complementary threads  106  and  107 , respectively. A circumferential seal/scraper assembly is provided on rod end bearing  92  for mating in sliding and sealing engagement with actuator rod  34 . A circumferential o-ring seal  114  is provided between rod end bearing  92  and actuator/motor casing  60 , and is carried by rod end bearing  92 . 
     FIGS. 8-12 below depict the construction of flame paths  48 - 54  which are illustrated as arrows that originate from an initiation point. Each flame path is formed as a flange joint, a spigot joint, or a threaded joint. A flange joint is provided between two mating pieces of machined metal having a small gap of {fraction (2/1000)}&#39;s of an inch (2 mils), and requires a minimum flame path length, according to one standard, of at least ⅜″. A spigot joint is provided between two mating pieces of metal having a 2 mil gap and a right angle (90 degree) turn, wherein the minimum flame path length of the two mating surfaces is at least ⅜″. A threaded joint is provided between two threaded, mating pieces of metal having a plurality of turns comprising at least seven threads no finer than 20 threads per inch (which imparts a thread depth of at least ⅜″). 
     The purpose for realizing the above-detailed joint dimensions, when designing the flame paths, is to ensure that sufficient temperature reduction is imparted to escaping gases that are ejected from the housing of the linear actuator assembly in order that an external explosion is not triggered. Hence, temperature is reduced sufficiently by dissipating energy from the expelled gases as they travel along the flame path. As a result, the expelled gases lose sufficient energy while traveling along the flame paths that the energy (and temperature) is not high enough to ignite any potentially explosive gases present externally of the housing. In essence, the expelled gases are below the auto-ignition temperature of the potentially explosive external gases. 
     FIG. 8 is an enlarged sectional view taken from the identified encircled region of FIG. 7 illustrating flame path  48 . More particularly, clevis  62  is removably mounted onto motor cover  42  via a plurality of individual threaded fasteners  64 . Flame path  48  comprises the shortest, continuous path machined surface formed between clevis  62  and motor cover  42 . Flame path  48  is shown extending from seal  110  to fastener  64 , and comprises a spigot joint having a total path length (of flat mating surfaces) of ⅜″. 
     FIG. 9 is an enlarged sectional view taken from the identified encircled region of FIG. 7 illustrating flame path  49 . More particularly, motor cover  42  is removably mated to actuator/motor casing  60  via complementary mating threads  102  and  103 , respectively. Flame path  49  comprises a spigot joint that is formed externally of threads  102  and  103 , and between motor cover  42  and casing  60 . Flame path  49  has a total path length (of flat mating surfaces) of ⅜″. 
     FIG. 10 an enlarged sectional view taken along line  10 — 10  of FIG. 7 illustrating flame path  50 . More particularly, mating, machined surfaces on rod guide  90  and actuator/motor casing  60  cooperate to form flame path  50  which comprises a slightly arcuate flange joint therebetween. Such flame path  50  extends for at least ⅜″. 
     FIG. 11 is an enlarged sectional view taken from the identified encircled region of FIG. 7 illustrating flame paths  51  and  52 . More particularly, cover  28  mates atop electronics tray  58  via a plurality of threaded fasteners  44  so as to form flame path  51  as a flange joint. Flame path  51  extends at least ⅜″ from an inner surface of tray  58  to the outer threads formed in cover  28  and tray  58  for receiving fasteners  44 . Additionally, wire feed through fitting  32  is threaded into cover  28  so as to form a threaded joint which provides flame path  52 . Flame path  52  is formed between two threaded, mating pieces of metal having a plurality of turns comprising at least, seven mating threads no finer than 20 threads per inch (which imparts a thread depth of at least ⅜″). Flame path  52  extends along the entire mating thread depth provided between cover  28  and fitting  32 . 
     FIG. 12 is an enlarged sectional view taken from the identified encircled region of FIG. 7 illustrating flame paths  53  and  54 . More particularly, flame path  53  comprises a sliding joint that extends between actuator rod  34  and rod end bearing  92  at least 1″ in uninterrupted length. Hence, flame path  53  is uninterrupted and extends from the inner edge of rod end bearing  92  to the beginning of scrapper/seal assembly  94 . Flame path  54  comprises a spigot joint that extends between actuator/motor casing  60  and rod end bearing  92 , and having an uninterrupted flame path length (excluding the right angle bend, but adding together the right angle flat, mating surfaces) of at least ⅜″. 
     FIG. 13 is a second perspective view of the actuator of FIG. 2 illustrating the heat dissipating cover and electronics removed from the actuator housing. The heat dissipating features and relatively compact and unitary construction of linear actuator assembly  10 . 
     One embodiment has been described and depicted above with reference to FIGS. 1-13 for a self-contained linear actuator assembly suitable for use in hazardous, explosion-proof environments such as actuating a fuel/air delivery valve on a gas turbine engine. However, it is understood that such actuator assembly can include a linear and/or rotary actuator, and can be intended for use in less severe environments, such as industrial environments. For example, such actuator assembly can be utilized on industrial gas turbines, or can be used in numerous environments, including environments where there is a danger of explosive factory dust, gas mines, grain elevators, and chemical factories, to name a few. Accordingly, additional desirable embodiments include an industrial high-force actuator, an industrial high-temperature actuator, an actuator having programmable electronics suitable for such environments, and a communications interface-controlled actuator also suitable for such environments. 
     In compliance with the statute, the invention has been described in language more or less specific as to structural and methodical features. It is to be understood, however, that the invention is not limited to the specific features shown and described, since the means herein disclosed comprise preferred forms of putting the invention into effect. The invention is, therefore, claimed in any of its forms or modifications within the proper scope of the appended claims appropriately interpreted in accordance with the doctrine of equivalents.