U.S. patent application number 10/911984 was filed with the patent office on 2005-08-04 for fault tolerant linear actuator.
This patent application is currently assigned to BOARD OR REGENTS, THE UNIVERSITY OF TEXAS SYSTEM. Invention is credited to Tesar, Delbert.
Application Number | 20050168084 10/911984 |
Document ID | / |
Family ID | 30118295 |
Filed Date | 2005-08-04 |
United States Patent
Application |
20050168084 |
Kind Code |
A1 |
Tesar, Delbert |
August 4, 2005 |
Fault tolerant linear actuator
Abstract
In varying embodiments, the fault tolerant linear actuator of
the present invention is a new and improved linear actuator with
fault tolerance and positional control that may incorporate
velocity summing, force summing, or a combination of the two. In
one embodiment, the invention offers a velocity summing arrangement
with a differential gear between two prime movers driving a cage,
which then drives a linear spindle screw transmission. Other
embodiments feature two prime movers driving separate linear
spindle screw transmissions, one internal and one external, in a
totally concentric and compact integrated module.
Inventors: |
Tesar, Delbert; (Austin,
TX) |
Correspondence
Address: |
GARDERE WYNNE SEWELL LLP
INTELLECTUAL PROPERTY SECTION
3000 THANKSGIVING TOWER
1601 ELM ST
DALLAS
TX
75201-4761
US
|
Assignee: |
BOARD OR REGENTS, THE UNIVERSITY OF
TEXAS SYSTEM
Austin
TX
|
Family ID: |
30118295 |
Appl. No.: |
10/911984 |
Filed: |
August 5, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10911984 |
Aug 5, 2004 |
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10455779 |
Jun 5, 2003 |
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6791215 |
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60386661 |
Jun 5, 2002 |
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Current U.S.
Class: |
310/80 ;
310/20 |
Current CPC
Class: |
H02K 7/06 20130101; Y10T
74/186 20150115; F16H 2025/2078 20130101; F16H 25/2252 20130101;
H02K 2213/06 20130101; F16H 25/205 20130101 |
Class at
Publication: |
310/080 ;
310/012; 310/020 |
International
Class: |
H02K 041/00; H02K
007/06; H02K 033/00 |
Goverment Interests
[0001] The U.S. Government may own certain rights in this invention
pursuant to the terms of the U.S. Department of Energy grant number
DE-FG04-94EW37966. This application claims priority to U.S.
Provisional Patent Application Ser. No. 60/386,661, filed Jun. 5,
2002.
Claims
What is claimed is:
1. A linear actuator comprising: a substantially-cylindrical
actuator frame having a principal axis; a linear output screw
having threads disposed thereon, the linear output screw disposed
at least partly within the actuator frame along the principal axis;
a planetary spindle screw set, having threads mated to the threads
of the linear output screw, disposed in a spindle screw carrier
about the linear output screw; a first armature disposed within the
actuator frame about the principal axis; a second armature disposed
within the actuator frame about the principal axis; a differential
gearset disposed in the spindle screw carrier, and connected to the
first and second armature in such manner to provide a differential
action between the first and second armature.
2. The actuator of claim 1 further comprising a first field coil
cylinder disposed around the first armature and a second field coil
cylinder disposed around the second armature.
3. The actuator of claim 1 wherein the first armature is
rotationally fixed to a first gear on one end of the first armature
meshed to the differential gearset and the second armature is
rotationally fixed to a second gear on one end of the second
armature meshed to the differential gearset.
4. The actuator of claim 1 wherein the differential gearset
comprises four central differential planetary gears.
5. The actuator of claim 1 further comprising a set of sensors for
constantly monitoring the torque output of each armature.
6. The actuator of claim 1 wherein the differential gearset is
supported by a differential cage.
7. The actuator of claim 1 wherein the set of planetary spindle
screws comprises a first set of planetary spindle screws at a first
axial location along the linear output screw and a second set of
planetary spindle screws at a second axial location along the
linear output screw.
8. The actuator of claim 1 wherein the actuator frame further
comprises a trunnion extending radially from the principal axis of
the actuator frame.
9. The actuator of claim 1 further comprising a first brake to lock
the radial position of the first armature with respect to the
actuator frame and a second brake to lock the radial position of
the second armature with respect to the actuator frame.
10-19. (canceled)
20. A linear actuator comprising: an actuator frame having a
central reference frame fixed thereto; a first rotary actuator
having a fixed portion that is fixed to the central reference
frame, and a radially-movable portion; a second rotary actuator
having a fixed portion that is fixed to the central reference
frame, and a radially-movable portion; a first cylinder threadably
engaged with the radially-movable portion of the first rotary
actuator; and a second cylinder threadably engaged with the
radially-movable portion of the second rotary actuator.
21. The actuator of claim 20 wherein the first and second cylinders
interact with the radially-movable portions of the rotary actuators
through acme threads.
22. The actuator of claim 20 wherein the first and second cylinders
interact with the radially-movable portions of the rotary actuators
through ball screws.
23. The actuator of claim 20 wherein the first and second cylinders
interact with the radially-movable portions of the rotary actuators
through roller screws.
24. The actuator of claim 20 wherein the central reference frame
has a first side and a second side opposite the first side, and
wherein the first actuator is disposed on the first side and the
second actuator is disposed on the second side.
25. The actuator of claim 23 wherein the first cylinder is disposed
on the first side and the second cylinder is disposed on the second
side.
26. The actuator of claim 20 wherein the first and second cylinders
have a rectangular outer cross-section.
27. The actuator of claim 20 wherein the first and second cylinders
are axially-movable and radially-fixed with respect to the central
reference frame.
28. The actuator of claim 20 further comprising an external
rectangular frame fixed to the central reference frame.
29. The actuator of claim 28 wherein the first and second cylinders
are suspended in the external rectangular frame by a set of roller
bearings.
30-39. (canceled)
40. A linear actuator comprising: a substantially-cylindrical
actuator frame having a principal axis, an electromagnetic field
therein, and an internal surface having a thread disposed thereon;
a linear output screw having threads disposed thereon, the linear
output screw disposed at least partly within the actuator frame
along the principal axis; a first armature disposed within the
actuator frame about the principal axis; a first spindle screw set,
having threads mated to the threads of the linear output screw,
disposed in a first carrier about the linear output screw; a first
geartrain, disposed between the first armature and the first
carrier, connecting the first armature and the first carrier in
such a manner as to maintain a fixed ratio between the angular
velocity of the first armature with respect to the actuator frame
and the angular velocity of the first carrier with respect to the
actuator frame; a second armature disposed within the actuator
frame about the principal axis; and a second spindle screw set,
having threads mated to the internal threads of the actuator frame,
disposed in a second carrier about the linear output screw; and a
second geartrain, disposed between the second armature and the
second carrier, connecting the second armature and the second
carrier in such a manner as to maintain a fixed ratio between the
angular velocity of the second armature with respect to the
actuator frame and the angular velocity of the second carrier with
respect to the actuator frame.
41. The actuator of claim 40 further comprising a first field coil
cylinder axially aligned with the first armature and a second field
coil cylinder axially aligned with the second armature.
42. The actuator of claim 41 wherein the first and second field
coil cylinders are disposed in a movable carriage.
43. The actuator of claim 42 wherein the movable carriage is
axially-movable with respect to the actuator frame, but is
radially-fixed with respect to the actuator frame.
44. The actuator of claim 43 wherein the movable carriage comprises
a set of bearings for supporting the first and second
geartrains.
45. The actuator of claim 44 further comprising a movable carriage
axially-movable with respect to the actuator frame, but
radially-fixed with respect to the actuator frame.
46. The actuator of claim 45 wherein the first planetary spindle
screw set is axially-fixed within the first carrier but
radially-movable, and wherein the second planetary spindle screw
set is axially-fixed within the second carrier but
radially-movable.
47. The actuator of claim 40 further comprising a set of sensors
for constantly monitoring the torque output of each armature.
48. The actuator of claim 40 wherein the first set of planetary
spindle screws are disposed at a first axial location along the
linear output screw and the second set of planetary spindle screws
are disposed at a second axial location along the linear output
screw.
49. The actuator of claim 40 further comprising a first brake to
lock the radial position of the first armature with respect to the
carriage and a second brake to lock the radial position of the
second armature with respect to the carriage.
Description
BACKGROUND OF THE INVENTION
[0002] The present invention relates generally to
electro-mechanical actuators, and specifically to a linear actuator
having improved fault tolerance and positional control.
[0003] A number of approaches have been developed to manipulate the
linear position of an object or device through the use of an
actuator. Linear actuators are pervasive where the movement of very
large loads is required. Linear actuation has traditionally been
met by the use of hydraulic and pneumatic cylinders.
Electromagnetic actuators are known, however, to provide increased
performance in many aspects as compared to either hydraulic or
pneumatic cylinders.
[0004] One drawback to the use of electromagnetic actuators is a
certain degree of increased complexity, giving rise to increased
concern over the reliability of such devices. Accordingly, certain
electromagnetic linear actuators have incorporated fail-safe
mechanisms of one type or another. As an example, U.S. Pat. No.
4,289,996 discloses a powered linear actuator having dual closed
loop servo motor systems driving a screw jack. The dual motors
drive the screw jack through differential gearing and each has an
armature lock which functions automatically if a motor circuit
fails thereby enabling the other motor to continue driving the
actuator alone. Potentiometer feedback is applied to dual error
amplifiers or polarized relays that compare the feedback position
signal with the input command signal and drive separate motor
energization channels.
[0005] U.S. Pat. No. 5,865,272 discloses a linear actuator having
an output shaft having a pair of driven wheels mounted thereon. One
of the driven wheels is rotatably mounted in a fixed plane and has
a drive nut for an associated thread on the output shaft. The other
drive wheel is rotatably fixed to the output shaft. An input shaft
is in a side-by-side relationship with the output shaft and adapted
to be rotated by a suitable power source. The input shaft provides
a drive wheel for each of the driven wheels, with the ratio between
each drive and driven wheel set being chosen to rotate the driven
wheels at different speeds in the same rotational direction and
thereby produce a controlled axial movement of the output shaft in
a direction depending upon the relative rotation of the driven
wheels. A fail-safe arrangement is provided in the form of a clutch
between the drive wheels of the input shaft, a back-drive for the
output shaft, and biasing means for affecting a back-drive.
[0006] U.S. Pat. No. 5,957,798 discloses an electromechanical
actuator having a linear output for moving an external load, the
actuator having at least two drive motors, a synchronizer connected
to the outputs of the drive motors, a differential mechanism
combining the outputs of the drive motors, and a quick release
mechanism connected to the differential mechanism and the actuator
output. The quick release mechanism releases support of the
external actuator load in response to an internal actuator jam and
maintains support of the external actuator load in response to an
external actuator overload.
[0007] U.S. Pat. No. 6,158,295 discloses a linear actuator
including a housing, a spindle rotatable in both directions, a
threaded nut driving a piston rod, and a motor capable of driving
the spindle through a transmission. A disengagement unit is
arranged in the transmission for interrupting the connection
between the motor and the spindle in case of operational failure,
such as overloading of the spindle. The disengagement unit
comprises a braking device adjustable with respect to the actuator
housing to cooperate with a coupling device for control of the
rotational speed of the spindle when it is disengaged from the
motor.
[0008] Although each of these designs provides certain advantages,
none of these designs provides a fully fault-tolerant linear
actuation solution totally suitable for use in applications where
life or safety is at risk. Each of these designs has its drawbacks,
as will be appreciated by those of skill in the art. For example,
as noted above, in any application in which a mechanical device,
such as an actuator, is employed to perform a function, there is
the potential and the risk of failure of the mechanical device and
attendant loss of functionality. In certain situations, such
failure may have only minor consequences. Wherever actuators are
employed in applications in which life or safety are at risk,
however, the consequences are much more severe. In high-stakes
applications, such as the control of an aircraft control surface,
disengagement of the actuator from the applied load is simply not
an acceptable approach. Similarly, locking up the actuator with a
brake would generally not be an acceptable approach in such an
application. Accordingly, there is an unmet need to prevent sudden
or catastrophic failure in the linear actuators employed.
[0009] Although electromechanical solutions offer definite
advantages over the lower-technology hydraulic and pneumatic
solutions often used in traditional linear actuation applications,
the rugged simplicity of the fluid cylinder has made it tough to
beat from a cost and reliability standpoint. Further, it is known
that single point failures frequently occur in electromagnetic
linear actuators. Where a linear actuator is susceptible to loss of
function from a single point failure, the actuator could completely
fail to operate in the event of such a failure. As noted above,
this is an unacceptable situation in many applications.
SUMMARY OF THE INVENTION
[0010] The present invention solves the problems associated with
current linear actuators. For example, in various embodiments, the
systems of the present invention overcome the risk of failure by
incorporating features enabling them to continue to operate under a
partial or total fault on one side of a dual system. Thus, the
present invention provides, in certain embodiments, fault tolerant
duality in a compact, concentric, fully integrated module. This
compactness and integration does not exist in any existing
designs.
[0011] In accordance with one aspect of the present invention, a
fault tolerant linear actuator is provided that incorporate
velocity summing, force summing, or a combination of the two. In
one embodiment, the invention offers a velocity summing arrangement
with a differential gear between two prime movers driving a cage,
which then drives a linear spindle screw transmission. This
embodiment is reconfigurable, but since it has only one
transmission, it does not eliminate all possible single point
failures. A second embodiment features two prime movers driving
separate linear spindle screw transmissions (one internal and one
external) in a totally concentric and compact integrated module.
This system has no single point failures, which is desirable where
failure would result in loss of life or high cost. A third
embodiment uses two rotary actuators driving acme screws in place
of the linear spindle screw transmission to make a very rugged high
force system. A fourth embodiment is a force summing linear
actuator based on a dual set of linear spindle screw drives summing
forces through two clutches at the output attachment plate. A fifth
embodiment uses an intermediate gear train between the input prime
movers and the output spindle screws in order to better balance the
torque/speed ratios and to enable a significantly higher motor
speed than in the second embodiment. This two-stage reduction also
allows for a significant reduction in the weight of the
actuator.
[0012] The development of certain technologies makes it possible
for the electromechanical actuators of the present invention to
surpass the performance of prior known designs in essentially every
aspect of performance. As an example, the commercial availability
of the roller spindle screw transmission is a significant step
forward in performance. As another example, the development of
modern highly-integrated circuits allows for increases in
performance and reductions in cost at the same time. Using these
and other technologies, the present invention not only offers high
load capacity, it also offers very long life, high precision, and
high velocity in a compact configuration and the potential for a
high level of actuator intelligence.
[0013] Intelligence within the actuator itself makes it possible to
balance operational priorities (speed, load, precision, smoothness,
etc.) in real time. Intelligence within the actuator permits the
system of the present invention to be highly fault tolerant. This
fault tolerance depends on a full awareness of all the performance
capabilities of the actuator in real time. This awareness requires
access to a wide spectrum of sensors, each generating data
quantifying performance criteria used to judge the actuator's
operation. Depending on the application, these performance criteria
may be prioritized to meet in-situ operational goals. Here, the
principal goal is to maintain operation under a fault. Depending on
the operational requirements, the output of a faulty prime mover in
an actuator may be quantified and used as a basis to temporarily
raise the performance of the one or more fully-operational prime
movers in order to make up for the loss of performance from the
faulty prime mover. Alternately, the faulty prime mover may be
taken completely out of service by braking it and "limping home"
using the remaining prime movers.
[0014] The teachings of the present invention may be employed in
any application in which there is the potential for loss of life, a
need to preserve a long mission in harsh environments without
possibility of repair, or a potential for high cost resulting from
sudden failure. This layered control should combine to give more
precise operation under significant load disturbances.
[0015] Those skilled in the art will further appreciate the
above-mentioned advantages and superior features of the invention,
together with other important aspects thereof upon reading the
detailed description that follows in conjunction with the
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] For more complete understanding of the features and
advantages of the present invention, reference is now made to the
detailed description of the invention along with the accompanying
FIGURES.
[0017] FIG. 1 depicts an isometric cutaway view of a
velocity-summing fault-tolerant linear actuator according to one
embodiment of the present invention;
[0018] FIG. 2 depicts an isometric cutaway view of a
velocity-summing fault-tolerant linear actuator according to a
second embodiment of the present invention;
[0019] FIG. 3 depicts an isometric cutaway view of a dual
fault-tolerant linear module based on a combination of rotary
actuators;
[0020] FIG. 4 depicts an isometric cutaway view of a force-summing
fault-tolerant linear actuator; and
[0021] FIG. 5 depicts an isometric view of a velocity-summing
fault-tolerant linear actuator with two-stage transmissions.
DETAILED DESCRIPTION OF THE INVENTION
[0022] Although making and using various embodiments of the present
invention are discussed in detail below, it should be appreciated
that the present invention provides many inventive concepts that
may be embodied in a wide variety of contexts. The specific aspects
and embodiments discussed herein are merely illustrative of ways to
make and use the invention, and do not limit the scope of the
invention.
[0023] FIG. 1 depicts an isometric cutaway view of a velocity
summing fault tolerant linear actuator 100 according to one
embodiment of the present invention. Actuator 100 provides a dual
set of prime movers 102 and 104 operating through a differential
gearset 106, which then drives a cage 108 containing two sets of
spindle screw drives 110 and 112 operating on a single linear
output screw 114. Fault tolerant linear actuator 100 is fault
tolerant up to the differential gearset 106, e.g., either prime
mover 102 or 104 may be disabled (e.g., braked) and the remaining
prime mover 102 or 104 may still operate.
[0024] The trunnion 116 as part of the outer shell 118 provides one
attachment to the environment with the other attachment being on
the linear output screw 114. The dual prime movers 102 and 104 are
arranged in a symmetrical layout. Prime mover 102 incorporates
field coil cylinder 120 and armature 122. Prime mover 104
incorporates field coil cylinder 124 and armature 126. Prime movers
102 and 104 are mounted on rotary needle bearings 128 and 130,
respectively, and drive multiple central differential planetary
gears 132 mounted on bearings 134 in planetary cage 136 supported
by planetary cage needle bearing 138.
[0025] The planetary cage 136 also contains the planetary screws
140 and 142 supported by thrust bearings 144 and 146. The planetary
cage 136 as a unit is supported by principal thrust bearings 148
and 150 in the outer shell 118 of the actuator 100. The end plates
152 and 154 of the actuator 100 are fixed to the shell with machine
bolts 156.
[0026] Depending on the application, actuator 100 may be designed
to provide varying types of service, e.g., light, medium, or
heavy-duty service. Actuator 100 is dynamically reconfigurable in
real time. Should one prime mover (e.g., 102 or 104) lose torque
capacity past a certain limit, the remaining prime mover (e.g., 104
or 102) may be instantaneously raised to greater than 100% of its
normal torque capacity to maintain the normal level of performance
for the actuator 100. Sensor systems, operational criteria, and
performance histories may then be used to monitor the performance
of actuator 100 relative to its reduced performance envelope.
[0027] FIG. 2 depicts a velocity summing linear fault tolerant
actuator 200 having no single point failures. Fault tolerant
actuator 200 incorporates a pair of rotary prime movers 202 (1) and
204 (2), that may either be, e.g., BDCM or SRM-type, motors,
driving a pair of linear spindle screw transmissions 206 and 208
acting on an external screw shaft 210 and an internal screw
cylinder 212. Fault-tolerant actuator 200 incorporates an inner
motion frame 214 that travels along both the external screw shaft
210 and the internal screw cylinder 212. Inner motion frame 214
also contains the two rotary prime movers 202 and 204 and their
associated planetary screws 216 and 218. Inner motion frame 214 is
prevented from rotation on these screws with the use of linear
cross-roller bearings 220 and 222. The length and placement of
these cross-roller bearings 220 and 222 will be dependent on the
stroke requirements of the application.
[0028] As seen in FIG. 2, external screw shaft 210 functions as the
output shaft for the fault-tolerant actuator 200 while the outer
shell 226, which contains the internal screw cylinder 212, also
incorporates the input attachment 228. Linear cross roller bearings
220 and 222 prevent the inner motion frame 214 from rotating
relative to the external screw shaft 210 and the internal screw
cylinder 212. Field 234 and armature 236 of the prime mover 202
supported by bearings 238 and 240 drive the linear planetary screws
216 in spindle bearings 244 in spindle cage 246 supported by
principal thrust bearings 248. Field 250 and armature 252 of the
prime mover supported by bearings 254 drive the linear planetary
screws 218 in spindle bearings 240 in spindle cage 258 supported by
principal thrust bearings 260.
[0029] Note that only one set of the linear cross roller bearings
220 and 222 is necessary to constrain the rotary motion of the
inner frame 214. Bearing 230 is more effective in resisting the
torque load on the inner frame 214 because of the larger diameter
and higher torsional stiffness of the outer cylinder shell 226.
[0030] The linear fault-tolerant actuator 200 of FIG. 2 is not only
fault tolerant in velocity summing between two independent prime
movers 202 and 204, but also exhibits no single point failures
between its two linear screw transmissions. This is a velocity
summing concept with reconfiguration of the prime mover velocities
in real time. The design in FIG. 2 has considerable merit for
applications requiring compactness, greater simplicity, higher
ruggedness, and partial fault tolerance in the electrical prime
movers and their electronic control subsystems.
[0031] Many applications require a combination of low output
velocity and high output force. Also, desirable properties of small
size, high stiffness, and low cost usually accompany this type of
application. FIG. 3 depicts a linear actuator module 300 that uses
two externally-threaded rotary actuators 302 and 304 to drive two
internally-threaded cylinders 306 and 308 in series. In certain
embodiments, module 300 may be designed to generate high force at
relatively low cost. Although not necessarily optimized for
applications requiring high linear velocities or rapid response to
input commands, module 300 may be optimized to generate high force
in a rigid, yet small package. In certain embodiments, three or
more linear actuators (e.g., 302-304) may be combined to create an
even more fault-tolerant linear actuator module 300.
[0032] In module 300 there is one external rectangular cylinder 310
attached to the actuator reference frame 312. Actuator reference
frame 312 anchors each of the (externally-threaded) internal rotary
actuator modules 302 and 304. In certain embodiments, the two
internally-threaded rectangular cylinders 306 and 308 use linear
cross roller bearings 314 and 316 for precision and stiff operation
relative to the external rectangular cylinder 310. Other
embodiments may employ sleeve-type bearings for the same
function.
[0033] Module 300 may be employed in very low cost applications,
such as in automobiles or in very low weight applications, as found
in the deployment of large flaps on aircraft. In a manufacturing
cell, module 300 may also be used in fixturing. Combined with high
precision small motion actuators, module 300 is useful for
application where both very high force and high precision are
required.
[0034] The threaded interface between the externally-threaded
rotary actuators 302 and 304 and the internally-threaded
rectangular cylinders 306 and 308 may vary by application. For
example, certain embodiments employ acme screw thread. Acme screw
mechanisms are low in cost, resistant to shock and oscillatory
forces, tolerant of contamination, and reliable for extended
service at low velocities. Acme threads will, however, generate
more friction than alternate transmissions such as the ball screw
or the spindle screw.
[0035] FIG. 4 depicts a linear fault tolerant actuator 400 having
no single point failures. This is achieved by creating dual force
paths in a single envelope wherein either of the force paths (prime
mover and transmission) may be removed from service by a clutch
release or similar mechanism in the event of failure.
[0036] FIG. 4 depicts an isometric cutaway of a dual force path
linear actuator 400. The system uses a pair of planetary roller
screws 402 and 404 driven by separate prime movers 406 and 408, all
in a concentric configuration. Prime mover 406 drives planetary
roller screws 402, which in turn drive a roller screw shaft 414
with external threads. Prime mover 408 drives planetary screws 404
that drive a roller screw cylinder 420 with internal threads. The
roller screw shaft 414 and the roller screw cylinder 420 are
attached at one end to an output cylinder 422.
[0037] The roller screw cylinder 420 is separated from the output
cylinder 422 by the outer clutch 424, while the roller screw shaft
414 is separated from the output cylinder 422 by the inner clutch
426. Should either of prime movers 406 or 408 fail, the associated
clutch 424 or 426 may be energized to take that prime mover 406 or
408 out of service. This system ensures that operation would
continue even under a major fault in one of the force pathways. In
certain embodiments, a single force path may have the capacity to
double its normal output for a short period of time to compensate
for the failed subsystem, in order to prevent any major system
failure.
[0038] Roller screw shaft 414 and outer shell, along with the
roller screw cylinder 420, are connected through clutches 424 and
426 to the output cylinder 422, by means of end cap screws 428. Nut
430 connects the screw shaft 414 to the plate 432, which holds
inner clutch 426.
[0039] As noted above, there are two separate prime movers 406 and
408 within linear actuator 400. Field 434 and armature 436 on
support bearings 442 drive planetary screws 402 supported by
spindle bearings 444. Spindle bearings 444 transfer forces through
the planetary screw cage 446 to principal thrust bearing 448 to the
inner motor frame 450 holding the motor fields, which is attached
to the input attachment cylinder 452 through end cap screws
428.
[0040] The second prime mover 408 incorporates field 438 and
armature 440 on support bearings 454 driving planetary screws 404
through support bearings 456. Support bearings 456 act through the
planet cage 458 by means of thrust bearings 460. Hence, each prime
mover-transmission combination independently creates a driving
force on the output cylinder 422.
[0041] Constructed as shown in FIG. 4 and described above, linear
actuator 400 eliminates the risk of total actuator failure brought
on by any single point failure. Failures associated with threat to
life, a significant economic loss, or the continuation of a long
duration mission all suggest the need for continued operation even
under a fault such as a lost prime mover, transmission,
communication link, sensor, or power supply. Achievement of this
goal requires the inclusion at least two fully independent pathways
to drive the output. In the past, this meant that two separate
linear actuators were arranged side-by-side and set up with
separate control loops.
[0042] Although the inclusion of a separate actuation mechanism
provides for a degree of fault tolerance, such a combination is
complex, space-inefficient and heavy. Such a design also introduces
a level of functional uncertainty that designers find unattractive.
Redundancy, which sets aside one part of a dual system while the
other one operates is a waste of both resources and priorities
(weight, volume, cost, etc.).
[0043] In the embodiment shown in FIG. 4, all resources are
employed at all times, maximizing output performance and accepting
a reduced performance reserve in the event of a partial fault.
[0044] A fifth embodiment of the present invention is shown in FIG.
5 and generally designated 500. Actuator 500 is made up of two
completely independent subsystems 502 and 504 to provide operation
even under a complete failure of one of the subsystems.
[0045] The two actuator subsystems 502 and 504 of actuator 500 are
geometric inverses of each other. Spindle screw set 522 drives a
small diameter screw shaft 534 with external threads, while spindle
screw set 538 drives a large diameter screw cylinder 540 with
internal threads. Spindle screw set 538 may be at a diameter three
times greater than spindle screw set 522, which would, of course,
require an angular velocity reduction of three-to-one in order to
maintain the same contact linear velocity at the screw threads.
This reduction also reduces the stored kinetic energy in the
rotating parts.
[0046] Subsystem 502 is driven by prime mover 508. Subsystem 502 is
guided and supported by cage 512, which holds planet gears 514 in
planet bearings 516. Planet gears 514 mesh with bull gear 518 and
sun gear 520, that drive sun gear 520 attached to the spindle screw
set 522 supported by spindle nut support bearings 524. The
principal cross roller bearing 526 separates the sun gear 520 from
the bull gear 518 and transfers the actuator load from the spindle
set 522 to the actuator carriage at the bull gear 518. End caps
528, 530, 532 are used to assemble subsystem 502.
[0047] Subsystem 504 may be described in the same manner as
subsystem 502, except that it is the geometric inverse of subsystem
502. In operation, axial loads pass from the actuator screw shaft
534 to spindle screw set 522 through principal cross roller bearing
526 to the actuator carriage 554 and then through principal cross
roller bearing 536 on to spindle screw set 538 out to the outer
shell 540 of the actuator 500. The anti-rotation splines 542 and
tangs 544 prevent the carriage from rotating in the actuator 500.
Seals 546 and 548 prevent the escape of the lubricant from the
actuator 500. A utility coil volume 550 is provided between the
actuator carriage and the end-cap 552 of the outer cylinder shell
540 for the supply of power, communications, and lubricant to the
moving carriage.
[0048] In special applications, the need for low weight is
critical. This may achieved, for example, by using high RPM prime
movers. There becomes a mismatch between this high RPM and the low
speed/high force needed at the output shaft. To make this
combination feasible, an intermediate gear reduction must occur
between the motors and the linear spindle screw transmissions.
[0049] In normal prime mover applications, a prime mover maximum
angular velocity between at least about 3,000 and 4,000 RPM is
generally considered ideal. For extremely low-weight applications,
maximum prime mover angular velocities between at least about
15,000 and 30,000 RPM may be required. Such designs may output five
to ten times more horsepower for the same weight of the prime
mover. In order to multiply the motor torque, a first stage gear
reducer, such as an epicyclic gear train, is inserted between the
prime movers and the associated linear spindle screw transmission
in order to balance the input and output speeds, as well as the
forces involved. This first stage reduction allows for design
optimization of both the prime movers and the linear spindle screw
transmission.
[0050] Structurally, the strength of actuator 500 is entirely
dependent on the load carrying capacity of the spindle screw sets
522 and 538 and the two principal cross roller bearings 526 and
536. Subsystem 502, which includes spindle screw set 522,
crossroller bearing 520, gear transmission 514 and prime mover 508,
is completely independent of subsystem 508, but they occupy a
common moving carriage, which transfers the load from the actuator
screw shaft to the outer cylinder screw shell.
[0051] Because the spindle screw sets 522 and 538 create a turning
resistance due to friction, an anti-rotation spline 542 is built
into the right side of the actuator screw shaft 534, in order to
prevent rotation of the carriage 554. In one embodiment, it is
likely that spindle screw sets 522 and 538 will be of the same
length to carry the same load.
[0052] In another embodiment, the lead on spindle screw set 522 is
at least about 0.2 in./rev. given a desired output speed of at
least about 3.5 in./sec., an angular velocity of at least about
1050 RPM would be demanded of prime mover 508. The intermediate
gear transmission ratio for subsystem 502 would have to be at least
about 14.3 to 1. The equivalent desired speed for spindle set 538
would be at least about 300 RPM.
[0053] In yet another embodiment, the lead of the internal cylinder
screw 550 is at least about 0.7 in./rev. Given a maximum angular
speed of at least about 30,000 RPM for second prime mover 556, the
intermediate gear transmission ratio of subsystem 504 would be at
least about 100-to-1. The low speeds in the spindle screws 522 and
538 will be very helpful in extending the life of these critical
parts in actuator 500.
[0054] Nonetheless, the high rotational speed requirements place
considerable demands on the intermediate gear transmissions. First,
the exceptionally high angular velocities will store considerable
kinetic energy. For epicyclic gears, this requires that the planets
be as small as possible.
[0055] In certain additional embodiments, the subsystems 502 and
504 may operate in opposite directions in order to better balance
the friction turning torques on the moving carriage 554.
[0056] Owing to the use of roller screws, subsystems 502 and 504
are naturally non-backdrivable. Depending on the pitch of the screw
threads and the application, there still may be a need to put in
place brakes on each of the armatures to prevent the system from
walking under oscillating external loads.
[0057] In certain other embodiments of actuator 500, each subsystem
502 and 504 provides one-half of the total stroke length.
Accordingly, actuator 500 may always return to the neutral position
and operate in only one-half its useful range, with one side
completely incapacitated. Alternately, a partially failed side
could "limp" home to the center of its range, and then be locked in
place, so that the remaining operable side could provide fifty
percent of the range capacity about the center position.
[0058] It should be mentioned that in some applications, it would
be useful to provide for consistent lubrication of the actuator.
For example, a low viscosity oil under pressure may be used to
provide a misted atmosphere inside the actuator volume. The
lubricant could be recirculated in a closed circuit and may also be
cooled if the duty cycle demands that heat be removed from the
system. This, then, requires at least about two seals: a first seal
between a smooth surface on the carriage 554 and the outer cylinder
shell 540 and a second seal between a smooth surface on the
actuator screw shaft 534 and an extension of the actuator carriage
540. The other end of actuator 500 is sealed by an end cap 552 on
the outer cylinder shell 540.
[0059] Additional objects, advantages and novel features of the
invention as set forth in the description that follows, will be
apparent to one skilled in the art after reading the foregoing
detailed description or may be learned by practice of the
invention. The objects and advantages of the invention may be
realized and attained by means of the instruments and combinations
particularly pointed out here.
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