Abstract:
An electromechanical linear actuator that contains an electric servo motor mounted within a compact housing. A thermally conductive path of travel is provided to efficiently transfer heat out of the housing into the surrounding ambient. A further mechanism is provided for holding the stator windings of the motor in undisturbed contact with the inner wall of the housing when the motor is subjected to thermal stress. The motor is arranged to linearly position a push rod through means of a ball screw unit. The ball screw nut and the push rod ride on bearings within guideways to insure that the push rod tracks along a linear path of travel.

Description:
FIELD OF THE INVENTION 
     This invention relates to an electromechanical actuator and, in particular, to an electromechanical actuator that is ideally suited for use in controlling the positioning of a valve. 
     BACKGROUND OF THE INVENTION 
     More specifically, this invention involves an extremely compact electromechanical actuator. Actuators are well known in the art and are used in many applications where a reciprocating linear motion is needed for some intended purpose. These hydraulic or pneumatic devices, as well as electrically powered devices, and to some extent pneumatic actuators, are capable of being contained in compact packages, while at the same time being capable of delivering relatively high forces. These devices, however, develop leaks which render them unreliable or inoperative over a period of time. 
     Electrically powered devices are generally referred to as electromechanical actuators and have proven to be more reliable than the hydraulic and pneumatic devices and exhibit a relatively longer life. In addition, the electrical power devices afford greater control over the positioning of the device. The electrically powered devices, however, consume more space than their hydraulic and pneumatic counterparts. Heat disruption is sometimes a problem with the electrical devices, particularly when attempting to compact the actuator in a small package. 
     SUMMARY OF THE INVENTION 
     It is, therefore, a primary object of the present invention to improve linear actuators and, in particular, to improve electromechanical linear actuators. 
     A further object of the present invention is to provide a compact electromechanical linear actuator that is capable of delivering a relatively high linear force at high speed. 
     A still further object of the present invention is to provide an electromechanical linear actuator having improved control capabilities and a high force vs. stroke characteristic. 
     Another object of the present invention is to provide a compact electromechanical linear actuator that efficiently conducts motor generated heat to the surrounding ambient. 
     These and other objects of the present invention are attained by an electromechanical linear actuator that includes a hollow shaft and a brushless servo motor that is contained within a compact housing. The motor includes a stator containing the motor windings that is secured to an inner wall of the housing by a tapered wedge fabricated of a material having a high coefficient of thermal conductivity. The housing is provided with fins that surround the motor for dissipating heat efficiently into the immediate ambient. Springs are employed to hold the wedge supporting the motor stator in place to prevent displacement of the stator over a broad change in temperature. A rotor assembly is contained within the housing and includes an extended ball screw shaft that is aligned along the axis of the motor and coacts with a ball screw nut to position a push rod. The push rod and ball screw nut are linked to a linear guideway for directing the push rod along a linear path of travel. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     For a further understanding of these and other objects of the invention, reference will be made to the following detailed description of the invention which is to be read in connection with the accompanying drawing, wherein: 
     FIG. 1 is a sectional view of a linear actuator embodying the teachings of the present invention; and 
     FIG. 2 is an enlarged section taken along lines  2 — 2  in FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the drawings, there is illustrated a compact electromechanical linear actuator, generally referenced  10 , that embodies the teachings of the present invention. Although the present invention is ideally suited for controlling the positioning of a valve, the actuator can be employed equally as well in many other similar applications such as controlling the inlet vanes to a rotating machine or the like. The actuator includes a cylindrical housing  12  that includes a center section  13  and two end sections  14  and  15 . The sections are brought togther in a telescoping relationship as shown in FIG. 1 with the sections being secured together by any suitable means such as press fitting, threaded connections or screws such as screw  17 . End section  14  is closed in assembly by an end cap  18 . End section  15  is similarly closed by means of a second end cap  20 . A brushless permanent magnet servo motor, generally designated  22 , is contained within the center section  13  of the housing. The stator assembly  24  of the motor is secured to the inside wall of the housing in a stationary condition by means of an annular wedge  25 . The wedge includes tapered outer surface  26  that is fitted into a tapered opening in the housing that complements the wedge taper. In assembly, the wedge is inserted tightly between the body of the stator assembly and the tapered wall of the housing to securely hold the motor stator in a stationary condition. In this arrangement the stator contains the windings of the motor. 
     The motor rotor assembly generally designated  30  is rotatably contained within the housing so that the motor rotor  31 , which contains a series of magnetic elements, turns about the central axis  32  of the housing. As will be explained in greater detail below, the servo motor is designed to yield a high energy density due to low rotating inertia and has greatly improved thermal performance. This type of motor is generally referred to as a brushless dc motor that behaves similarly to a brush type dc motor except for the method of commutation. The brushless motor is commutated by an electronic controller  35  rather than by brushes and commutator bars. Because there are no brushes to wear out, little or no motor maintenance is required over the life of the motor. 
     The motor rotor is supported upon a hub generally referenced  40  that is rotatably supported in the center section of the housing in ball bearing  41 . The hub further contains a radially extended shoulder  42  that forms a space between an adjacent shoulder  43  on the central section  13  of the housing. A thrust bearing  45  is mounted in the space between the two shoulders to take up any axial loading exerted upon the rotor assembly. An end closure  47  is mounted on the bearing end of the rotor assembly which contains a flange  48  that is arranged to contact the ball bearing  41 . A gap  49  is maintained between the end closure and the hub and a series of spaced apart screws  50  are passed through the end closure and are threaded into the hub. Tightening the screws draws shoulders  42  and  43  together thereby securing the thrust bearing in a preloaded condition in the rotor assembly. By the same token, the enclosure is drawn between shoulder  43  and flange  48  that is located upon the end closure. One or more Belville washers are mounted between the wedge  25  and end section  15  of the housing to provide a holding force against the wedge in assembly. 
     The end closure contains an extended nose section  55  that is rotatably mounted in the left hand end section  14 , as viewed in FIG.  1 . The nose section, in turn, is used to support a part of the electronic resolver  90  that is used to provide both position sensing data to the controller  35  along with motor control data. 
     A ball screw assembly generally referenced  63  is mounted in the rotor hub. The ball screw assembly includes a shaft  65  that is coaxially aligned along the center line  32  of the housing. A ball screw nut  66  is mounted upon the ball screw shaft  65  which is adapted to move linearly along the axis of the ball screw shaft as the shaft turns with the rotor. The ball screw assembly of the type shown is commercially available from Hewin Corporation and Jena Tech, Inc., or others. 
     The left hand end of the ball screw shaft contains a tapered section  68  that is contained within a complementary opening formed in wall  69  of the hub rotor  40 . The end of the shaft further includes a threaded spline  70  that passes through the wall  69  and is engaged by a nut  71 . Tightening the nut down pulls the tapered section of the shaft tightly into the complementary opening in the hub wall thereby locking the shaft tightly in the hub. 
     The opposite end of the ball screw shaft is adapted to ride freely within a blind clearance hole  73  formed in push bar assembly  75 . The push bar assembly includes an elongated rod  76  having a pusher disc  77  located at the distal end of the rod. An end flange  78  is located at the proximal end of the rod. The end flange  78  of the push bar assembly is located adjacent to a second flange  79  located upon the ball screw nut  66  and the two are connected in assembly by a series of screws  80  (FIG.  2 ). As best illustrated in FIG. 2, each flange is provided with a pair of ball bearings  83  that are arranged to ride in longitudinal guideways  84  that are formed in the housing end section  15  and which are aligned parallel to the axis  32  of the housing. The bearings are captured within the guideway and prevent rotation of the ball screw nut while at the same time insuring that the nut and the push bar assembly both move along a linear path of travel. 
     An electronic resolver unit  90  is mounted upon an internal shoulder  91  formed in end section  14  of the housing. The resolver is a combination of a stator and a rotor. The resolver stator is mounted upon the housing  14  and the rotor is mounted upon the shaft  55 . The resolver is mounted concentrically to the motor rotor to provide exact motor position data to the controller  35  via line  94 . The motor controller uses the position data to adjust the motor phase current for optimum motor torque output which is also referred to as the commutation process. Resolvers of this type are available from Admotec, Inc. and have windings and iron case materials similar to those of the motor. 
     The motor controller is externally mounted within an EM1 enclosure which also contains necessary capacitors and filters to accommodate for lengthy cables of up to at least 150 meters in length. The controller is microprocessor based and operates the actuator positioning loop. The controller monitors the rotor position via the resolver and provides a sinusodial current to the motor windings via line  97  to control motor torque. The controller employs pulse width modulation for high efficiency regulators of the motor phase current. 
     The stator wedge  25  and the center section  13  of the actuator housing are each fabricated of a material having a high coefficient of thermal conductivity. In addition, the center section of the housing is provided with a plurality of heat exchanger fins  99  that encircle the center section of the housing. Accordingly, any excessive heat that is generated by the motor windings is quickly and efficiently rejected into the surrounding ambient thereby keeping the temperature within the housing at a low level, that is, at a level at which the mechanical and electrical component located within the housing will not become thermally damaged. As noted above, the stator wedge is secured in place by a prestressed Belville washer so that the wedge will not be displaced from its holding position due to the thermal stress. 
     While the present invention has been particularly shown and described with reference to the preferred mode as illustrated in the drawing, it will be understood by one skilled in the art that various changes in detail may be effected therein without departing from the spirit and scope of the invention as defined by the claims.