Note: Descriptions are shown in the official language in which they were submitted.
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FAULT-TOLERANT ELECTRO-MECHANICAL ACTUATOR
BACKGROUND OF THE INVENTION
This invention relates to actuators. An "actuator" is defined in the
Merriam-Webster's Collegiate Dictionary, Tenth Edition as a mechanical device
for
moving or controlling something. Actuators perform myriad functions and enable
many modem conveniences.
Aircraft for example, require actuators to fly. Flaps, spoilers and
ailerons in each wing, each require an actuator. Actuators in the tail control
the
rudder and elevators. Actuators in the fuselage open and close the doors that
cover
the landing gear bays. Actuators raise and lower the landing gear. Actuators
on each
engine control thrust reversers by which the plane is decelarated.
In addition to uses in aircraft, actuators are used in computer disk
drives to control the location of the read/write head by which data is stored
on and
read from the disk. Actuators are used in robots, i.e., in automated factories
to
assemble products. Actuators operate brakes on vehicles; open and close doors;
raise
and lower railroad gates and perform numerous other tasks of everyday life.
Prior art actuators fall into two general categories: hydraulic and
electric with the difference between the two categories being the motive force
by
which movement or control is accomplished. Hydraulic actuators require a
pressurized, incompressible working fluid, usually oil. Electric actuators use
an
electric motor, the shaft rotation of which is used to generate a linear
displacement
using some sort of transmission.
A problem with hydraulic actuators is the plumbing required to
distribute and control the pressurized working fluid. In an airplane, a pump
that
generates high-pressure working fluid and the plumbing required to route the
working
fluid add weight and increase design complexity because the hydraulic lines
must be
carefully routed.
Electric actuators, which are powered and controlled by electric
energy, require only wires to operate and control but a problem with prior art
electrical actuators is their reliability. Windings of electrical motors are
susceptible to
damage from heat and water. Bearings on motor shafts wear out. The
transmission
between the motor and the load, and which is inherently more complex than the
piston
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and cylinder used in a hydraulic actuator, is also susceptible to failure.
While
electrical actuators have advantages over hydraulic actuators, an electrically-
powered
actuator that provides increased reliability, would be an improvement over the
prior
art. Fault-tolerance, i.e., the ability to sustain one or more component
failures or
faults yet keep working, would provide an improvement over prior art
electrical
actuators.
SUMMARY OF THE INVENTION
A fault-tolerant, electrically-powered actuator uses two or more,
independent integrated motor modules in the same housing to drive an output
ram that
can be extended from and retracted into a housing that encloses the electric
motor
module that drive the ram into and out of the housing. The integrated motor
module
(FIG. 2) is defined as a unit that consist of one or more electric motor
armature/field
unit that provide the driving function for an engaged or active roller scres
nut
assembly. The roller screw nut assembly (FIG. 2A) consists of helical threaded
rollers, nut assembly directly coupled to a common screw shaft. The ram's
outside
surface is threaded. Since the output ram is threaded, the ram can moved into
or out
of the housing by the rotation of one or more "drive nuts" that engage the
threads of
the output ram and which are rotated themselves but laterally fixed in place
such that
the output ram moves laterally on the rotation of the drive nut.
The "drive nut" is provided by roller screws that make up part of the
motor's armature and which engage the threads on the output ram. When this
"drive
nut" rotates, its rotation causes the output ram to translate., i.e., move
into or out of
the housing. Reliability and fault tolerance are provided by the multiple
motors and a
drive nut armature in each motor that enables each motor to be separately
disengagable and/or engagable.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a cross-sectional diagram of a fault-tolerant electrical
actuator.
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FIG. 2 is a perspective view of the roller screw nut to be used as the
armature of an electrical motor used in the fault tolerant electrical actuator
and
showing a releasing clutch.
Figure 2A is a roller screw nut assembly.
Figure 2B is a helical thread roller
FIG. 3 depicts the electrical poles of the stator and armature of an
electric motor, the armature of which engages the output ram of the electrical
actuator.
FIG. 4 is an isolated view of one motor in the housing of the electrical
actuator and a portion of the output ram.
FIG. 5 shows a fault-tolerant actuator with crank arm driving a drive
shaft.
FIG. 6 shows a fault-tolerant actuator with a flight control surface of
an aircraft.
FIG. 7 shows a fault-tolerant actuator with a vehicle steering system.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
FIG. 1 is a cross-sectional view of a preferred embodiment of an
electrically-powered and fault tolerant electrical actuator 10.
Briefly, the actuator 10, is comprised of cylindrically-shaped housing
20 that encloses two or more integrated electrical motor modules(three shown)
24, 30
and 34 that can drive an output ram 12, the exterior surface 16 of which is
helically
threaded. A helical thread 18 (also referred to as "threads") on the output
ram 12
surface are threaded into one or more complementary "drive nuts" within the
housing
that engage the threaded output ram 12 and which can rotate about the output
ram 12
but which are laterally fixed in the housing, i.e., they cannot move along the
length of
the output ram 12. When the end of the output ram (not shown in FIG. 1) is
connected to a machine, such as an aircraft's control surface, lateral
movement
motion of the ram 12 operates or controls the machine to which the output ram
12 is
coupled.
The output ram 12 can be extended from and retracted into the housing
simply by controlling the direction of rotation of at least one of the
aforementioned
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"drive nuts" that engages the threaded surface 16. The drive nut rotation
direction is
readily changed by the electrical power provided to the field windings 26 of
the
motors 24, 30 and 34 that drive the output ram 12.
More particularly, the ram 12 has a central axis 14, owing to the fact
that it's cylindrically shaped. Its exterior surface 16 has a helical thread
or thread 18
in its exterior surface 16, such that the ram 12 can be considered to be
"threaded" as is
a bolt or screw. The helical "thread" 18 in the ram 12 exterior surface 16
enables the
ram 12 to be axially moved by engaging the threads 18 of the output ram 12
with a
rotating a "drive nut" in the housing 20, which is structured and arranged to
rotate
about the axis 14 and engaged to the threads 18 but which is laterally fixed
in the
housing 20, i.e., it cannot move along the axis 14 of the output ram 12. The
thread 18
pitch will affect the ram's speed (i.e., the rate at which it travels axially)
as well as the
load "seen" by the drive motors 24, 30 and 34.
As shown in FIG. 1, the housing 20 has at least one opening 22, in one
end of the housing through which the output ram 12 can extend and retract so
as to
impart control or movement to a machine or machine part (not shown in FIG. 1).
In
at least one alternate embodiment shown in FIG. 7, which embodiment requires a
double-acting output ram 12, the housing 20 has a second opening that is
opposite the
first opening 22. A second opening required to implement the embodiment of
FIG. 7
is omitted from FIG. 1 for simplicity.)
Each motor 24, 30 and 34 has a stator 26, also known as a "field" or
"field winding" shown in cross section in FIG. 1 As is well-known, application
of an
electrical current to the field winding 26 will induce one or more magnetic
fields,
which in turn, extend into the motor's armature and cause the armature to
rotate.
Each field winding 26 lies against the inside wall of the cylinder-shaped
housing 20,
which also acts as a heat sink for the motor windings.
The structure and operation of the armature 28 of each motor 24, 30
and 34 is the aforementioned "drive nut" that rotates about the ram 12 and
which is
axially fixed. Threaded roller screws in the armature 28 engage the threads 18
and
can rotate about the output ram 12 but are laterally fixed. In so doing, the
armature 28
of each motor acts as a"drive nut" that drives the output ram 12 but which
also
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provides a capability of being decoupled from (or coupled to) the thread 18 on
the
output ram 12.
The armature 28 includes two or more helical-threaded roller screws,
44-1 and 44-2, evenly spaced around the output ram 12 and which engage the
threads
18 in the output ram 12. The roller screws 44-1 and 44-2 are held in place
laterally,
but are freely rotatable around the ram 12 by way of a cage 50 (Shown in FIG.
2.) that
is laterally restrained in the housing 20 by axial thrust bearings, not shown
in the
figures but well-known to those of ordinary skill in the art.
When the field 26 is energized, it causes the armature structure 28 to
rotate about the output ram 12, in turn causing the roller screws 44 in the
cage 50 and
which engage the threaded output ram 12, to rotate around the ram 12 and exert
a
lateral force on the threads 18. The lateral force on the threads 18 cause the
ram 12 to
move laterally.
For purposes of this disclosure and claim construction, the term
"armature" is used interchangeably with and is considered to be equivalent to
a
"rotor." In other words, a "rotor" is equivalent to an "armature" and vice
versa.
Similarly, the term "stator" is considered to be equivalent to a "field"
winding.
FIG. 2 shows the cage 50 and the included roller screws 44 in greater
detail. The cage 50 radially separates two or more, helically-threaded roller
screws
44-1 and 44-2 that mate with the threads 18 in the output ram 12. The threads
of the
roller screws 44 are sized and shaped to mate with the threads 18 on the
surface 16 of
the output ram 12 such that the roller screws 44 can smoothly rotate about the
output
ram 12. Those of skill in the art will appreciate that roller screw thread
pitch should
match the thread pitch of the output ram 12.
That the cage 50 with the included roller screws 44 functions as an
armature can be seen in the electrical representation of one of the motors
shown in
FIG. 3. In FIG. 3, the armature 28 has six poles 29 around the axis 14, with
each pole
29 being formed by and corresponding to one of the band sections 51 that
extend
between the bearing caps 55-1 and 55-2. Each poll 29 acts to enclose the
roller
screws 44 as well as providing a path for magnetic lines of flux. The armature
structure 28 will rotate in response to the magnetic fields created about the
armature
28 by the stator 26. Rotation of the armature structure 28 causes the roller
screws 44
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to rotate. (Those of skill in the art will recognize that the roller screws 44
will rotate
about their axes of rotation albeit in an opposite direction than the
armature's rotation
28.)
Referring again to FIG. 2, the metal bands 51 that run parallel to the
roller screws 44 carry magnetic flux lines. They also strengthen the cage 50
and
thereby help maintain the radial separation between separate the roller screws
44-1
and 44-2. Journals 45 at the ends of the roller screws 44 (shown in FIG. 2B)
ride in
small bearing holes 53 in the opposing bearing caps 55-1 and 55-2.
As shown in FIG. 2A, the roller screws 44 have a central threaded
section 49. Tapers 43 are just inside the journal bearing sections 45. The
journal
sections 45 ride in the bearing caps 55-1 and 55-2 that are at each end of the
roller
screws 44 and freely rotate about the output ram 12 and which do not engage
the
threads in the output ram 12.
The taper sections 43 in the roller screw 44 provides a structure by
which the roller screws 44 can be disengaged from the helical threads of the
output
ram 12. The roller screws 44 are disengaged using a complementary taper in the
bearing caps 55-1 and 55-2, which can slide "under" the taper section 43,
causing the
roller screw 44 to be lifted upward, disengaging the roller screw 44 from the
output
ram 12. The bearing caps 55-1 and 55-2 are urged toward each other to
disengage the
roller screws 44 when the motor fails.
During operation, the fault-tolerant electro-mechanical actuator or
"EMA" generates signals such as voltage, current, speed and position. A
microprocessor, not shown, monitors voltage, current, speed and position
anddetects
when an excessive torque is being developed by one or more of the driving
motors,
typically by way of an unusually-high current drawn by a motor.
Referring to FIG. 4, upon determining that an excessive torque is being
attempted, the microprocessor or other controller trips a ramp and lock
mechanism
(57) that totally disengages the rollers (44) of the problematic motor from
the screw
(12). The ramp and lock mechanical (57) requires an electromagnetic actuation
device (59) that provides lateral movement of ramp and lock mechanism (57) to
lift
the roller (44) from the screw (12) and lock itself into a segmented roller
cage/nut
(50).
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When the ramp and lock mechanism (57) is required, i.e., when there is
an excessive amount of current drawn, the microprocessor or other controller
applies
a voltage/current to coil (90). The coil (90) becomes an electromagnet and
produces
magnetic line of flux. This flux is then transferred from the coil (90)
through small
air gaps in the thrust bearing (91) to an opposing magnetic field on the ramp
and lock
mechanism (57). As the current increase the magnetic flux builds causing the
ramp
and lock mechanism (57) to move in such a way that the roller (44) is lifted
upward
and away from contacting the screw threads. Once engaged, the ramp and lock
mechanism (57) is locked in place on the segmented roller cage/nut (50)
totally
eliminating the motor module from any further operation or contact with the
screw
(12). Under normal operations the ramp and lock mechanism (57) is free of any
magnetic contact with the electrical coil and the roller (44) is fully engaged
with the
screw (12) and the motor module is fully functional.
In a preferred embodiment, all of the motors 24, 30 and 34 in the
housing are engaged to the thread 18 in the output ram 12. All of the motors
are
powered and help drive the output ram 12. In such an embodiment, the motors
share
the load presented by the output ram 12. When a motor fails, structure in the
armature
28 disconnect the armature 28 from the thread 18 in the output ram 12 enabling
other
motors to assume the load from the ram 12 without interference from the failed
motor.
In one alternate embodiment, all of the motors are engaged to the
thread 18 in the output ram 12, but one motor is powered and drives the load
presented by the output ram 12. The other motors in the housing 20 "go along
for the
ride" but do not provide any motive assistance. When the driving motor fails,
structure in the failed motor's armature 28 disconnects the armature 28 from
the
thread 18 in the output ram 12 enabling one or more of the other motors to
assume the
load from the ram 12 without interference from the failed motor.
In yet another alternate embodiment, two or more motors are engaged
to the thread 18 in the output ram 12 and are powered to drive the load
presented by
the output ram 12 and thereby share the load between them. A single additional
motor is also engaged but not powered so as to be available as a "back-up" or
redundant motor. When a driving motor fails, structure in the failed motor's
armature
28 disconnects the failed motor's armature 28 from the thread 18 in the output
ram 12
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enabling one or more of the other motors to assume the load from the ram 12
without
interference from the failed motor.
In yet another alternate embodiment, only one motor of multiple
motors is engaged to the thread 18 in the output ram 12 and handles the output
ram 12
load. Other motors in the housing 20 are "back-up" or redundant motors. When
the
driving motor fails, structure in the failed motor's armature 28 disconnects
the failed
motor's armature 28 from the thread 18 in the output ram 12. In this
embodiment,
structure in the armature of the other motors engages one or more armatures of
the
other motors by which it (or they) assume the load from the ram 12 without
interference from the failed motor.
Referring to FIGS. 1, 2, 2A and 4, a motor can be disengaged from the
thread 18 in the output ram 12 when the roller screws 44 are lifted away from
the
threads 18, allowing the cage 50 to rotate freely about the output ram 12. The
roller
screws 44 can be lifted away from the thread 18 using the tapered sections 43
(shown
in FIG. 2) between the straight journal section 45 and the threaded section
49. When
a complementary taper in the bearing caps 55-1 and 55-2 or in a clutch
mechanism, is
forced under and against the tapered sections 43, a taper that slides under
the tapered
sections 43 will cause the roller screws 44 out of engagement with the thread
18.
Many of the aforementioned embodiments are structured so that all of
the motor armatures are engaged to the thread 18 in the output ram 12. In
those
embodiments, a failed motor is disengaged from the output ram 12 when a motor
fails
using the tapered section 43 and a complementary taper in the bearing cap 55-1
and
55-2 or a clutch. A roller screw 44 that is initially disengaged using the
tapered
sections can thereafter be engaged to the thread 18 by backing a complementary
taper
away from the taper section 43 in the roller screw 44. In alternate
embodiments,
wherein not all motors are initially engaged, a failed motor is disengaged as
describe
above with a back-up or motor being engaged to the output ram 12 by lowering
the
rollers so that it can operate the actuator 10.
FIG. 4 better illustrates the ramp and lock mechanism 57 at both ends
of the cage 50, which prevent the cage 50 from moving laterally. As shown in
FIG. 4,
complementary tapers 58 in the ramp and lock mechanism 57 will lift the roller
screw
44 out of engagement if the ramp and lock mechanism 57 is urged toward the
roller
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screw 44. The displacement of the ramp and lock mechanism is controlled by an
electromagnetic actuator and microprocessor.
Whether the ram 12 extends away from the housing 20 or retracts into
the housing 20 is determined by the armature's rotation direction. The
armature's
rotation direction is in turn determined electrically. Therefore, the output
ram 12 of
the actuator 10 can be moved in different directions simply by changing the
electrical
power source.
In addition to changing the output ram's direction electrically, the
output ram 12 speed can be determined electrically. In the case of a D.C.
motor,
rotational speed is determined by the applied voltage amplitude. In the case
of a
synchronous A.C. motor, rotational speed is determined by the frequency of the
applied A.C. voltage. As is well-known, an A.C. induction motor speed can be
varied
somewhat by changing the voltage amplitude but also by changing the A.C. duty
cycle.
For any given motor speed, thread pitch of the ram 12 will affect the
displacement speed of the output ram 12. While a relatively large number of
threads
per inch will require more motor revolutions, per unit of linear displacement,
increasing the threads per inch will also decrease the amount of force that
the driving
motor "sees" from the output ram 12.
The ease with which the output ram 12 direction and speed can be
changed are but two significant advantages that the actuator 10 has over prior
art
hydraulic actuators. Fault-tolerance and hence reliability of the actuator 10
is
achieved by having multiple motors drive the output ram 12, such that the
motors can
be disengaged if and when they fail.
Referring again to FIG. 1, a second motor 30 is shown in the housing
20 immediately to the right of the first motor 24. The structure and operation
of the
second motor 32 is preferably identical to the first motor 24. Alternate
embodiments
include having a third motor 34 as well. Like the armature of the first motor
24, the
armature of the second motor 30 is provided by a cage 50 with two or more
roller
screws 44-1 and 44-2.
As set forth above, fault tolerance is provided by the ability to
disengage one motor when it fails so that another motor can continue to
operate and
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take up the load. As can be seen in FIG. 4, the output ram 12 can be engaged
and
disengaged from the rotor 28 depending on whether the roller screws 44 in the
armature cage are initially engaged or disengaged with the threads of the
output ram
12. The roller screws 44 can be disengaged from the output ram and hence the
motor
24 disengaged from the output ram 12, by sliding the roller engagement thrust
bearing
47 against the tapered sections 43 of the roller screw 44. In so doing, the
roller
screws 44 are lifted out of engagement with the output ram 12, physically
disconnecting the motor from the output ram 12.
The ramp and lock mechanism 57 is forced along the axis of the roller
screw and against the roller screws 44 by either a mechanical or electrical
clutch (not
shown), which will force the thrust bearing 47 against the roller screw taper
43 or pull
the thrust bearing away from the taper section 43, depending on whether the
motor is
to be disengaged or engaged from the output ram 12. By lifting the roller
screw 44 of
one motor out of the threads of the output ram 12 the armature 28 of that
motor can be
disengaged from the ram 12. By lowering the roller screw of a different motor
into
the threads of the output ram 12, the other motor can be engaged with the
output ram
12.
The tapered face on the thrust bearing 43, or any other structure that
lifts the roller screws 44 away from the output ram or that lowers the roller
screws
into engagement with the output ram 12, should be considered to be a roller-
engaging/roller-disengaging mechanism that operably couples and de-couples an
armature/rotor of a motor with the helical thread and the output ram. In so
doing, the
thrust bearing, and/or its tapered surfaces in combination with the tapered
section of
the roller screw act as a mechanism for engaging or disengaging the motors
from the
output ram 12.
As set forth above in the background of the invention, hydraulic and
electrical actuators perform myriad tasks. The electrically powered linear
actuator
described above and depicted in FIGS. 1-4 can be used in a variety of
applications.
As is well-known, a"journal" is a spindle or shaft that turns in a
bearing. In its most general application, and as depicted in FIG. 5, the
output or distal
end 62 of the electrically powered linear actuator 10 is attached to a journal
60 of a
crank arm 68. The journal 60 accommodated by an opening in the ram 12 at its
distal
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end 62 and in which the journal 60 pivots as the output ram 12 reciprocate as
shown
by reference numeral 64.
As shown, the displacement of the journal 60 at the end of the crank
arm 68 will in turn cause the drive shaft 70 to oscillate about its axis of
rotation, as
indicated by reference number 72. In rotating machines, the actuator 10 can
effectuate rotation by its reciprocating displacement using structure shown in
FIG. 5.
Aircraft are well-known to have wings that are attached to a fuselage.
Control surfaces in the wings control the rate of climb and descent, among
other
things. The tail section attached to the rear of the fuselage provides
steering and
maneuverability. An engine provides thrust and can be attached to the plane at
the
wings, in the tail or to the fuselage. Inasmuch as aircraft structures are
well-known,
their illustration is omitted here from for simplicity.
As set forth above, prior art actuators controlled the movement of
flight control surfaces in the wings, tail, landing gear, landing gear bay
doors and as
well as engine thrust reversers. In yet another embodiment of the invention
shown in
FIG. 6, the output end 62 of the output ram 12 is coupled to a pivot point 74
of a
control surface 76 of an aircraft (not shown for clarity, but well known in
the art).
Translation (movement) of the output ram 12 in the direction indicated by the
arrows
64 is but one way that the actuator 10 can cause a control surface, e.g.,
spoilers, flaps,
elevators, rudder or ailerons, to move and thereby control the aircraft.
Similar
translation can control other flight control surfaces, fuselage doors, landing
gear
and/or thrust reverses.
Those of ordinary skill in the art will recognize aircraft as having the
aforementioned well-known prior art structure, but also including a fault-
tolerant
electrical actuator 10 as described herein and as shown in FIG. 6. The safety
and
reliability of an aircraft might therefore be improved by using the actuator
10 within a
wing, fuselage or tail section as needed to operate flight control surfaces,
landing
gear, landing gear doors as well as an engine thrust reverser.
In yet another embodiment shown in FIG. 7, the output ram 12 extends
through both ends of the actuator housing 20. One side or end of the output
ram 12-1
is connected to a first steerable wheel 80 of a vehicle. The other side or end
12-2 is
connected to a steering linkage for another steerable wheel 82. As the output
ram 12
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translates in the direction indicated in the reference number 64, the
steerable wheels
80 and 82 rotate upon the pivot points or axes 86, 88 by which the steerable
wheels
80, 82 are controlled.
Inasmuch as automobiles and trucks are well-known to have at least
one steerable wheel (in the case of a three-wheeler), a chassis or frame to
which the
wheel is rotatably coupled, a body with doors, an engine and a transmission,
and
brakes, all of which are well-known and not requiring depiction, a significant
weight
reduction might be possible by replacing a hydraulic actuator used to control
steering
with a high-reliability, fault-tolerant actuator as described above.
Other embodiments of the electrically powered linear actuator would
include use as a power source for a lift for a door by appropriately coupling
the output
ram to the mechanisms to which loads could be lifted and doors opened.
The preferred embodiment of the electrically powered actuator
disclosed and claimed herein employed DC motors because they are readily
reversible
and their output speed easily controllable. Alternate embodiments would
include
reversible AC motors as well as stepper motors however. Those of skill in the
art will
recognize however that stepper motors require more complex electronics than
those of
DC or AC motors.
By providing two or more motors that are fixed in a housing, each of
which is independently coupled to or releasable from, a helically threaded
output
shaft, an electrically powered, fault-tolerant linear actuator can be
realized.
