NASA Technical Reports Server (NTRS) 20080009427: Impact micro-positioning actuator

0.94 MB·English

by  NASA Technical Reports Server (NTRS)

United States Patent
(12) (io) Patent No.: US 6,983,667 B2
Cuerden et al. (45) Date of Patent: Jan. 10,2006
(54) IMPACT MICRO-POSITIONINGA CTUATOR 5,410,206 A 411995 Luecke et al.
5,511,438 A * 411996 Aki ........................... 74189.43
(75) Inventors: Brian Cuerden, Tucson, AZ (US); J. 5,522,579 A * 611996 Rock ....................... 2491219.1
Roger P. Angel, Tucson, AZ (US);
FOREIGN PATENT DOCUMENTS
James H. Burge, Tucson, AZ (US);
Scott T. DeRigne, Tucson, AZ (US) DE 25 54 315 * 611976
EP 0 478 928 * 411992
(73) Assignee: University of Arizona, Tucson, AZ
OTHER PUBLICATIONS
(US)
Cuerden, Brian et al., “A Linear Micro-Positioning Actuator
( * ) Notice: Subject to any disclaimer, the term of this for Ambient and Cryogenic Operation,” Steward Observa-
patent is extended or adjusted under 35 tory, The University of Arizona.
U.S.C. 154(b) by 0 days.
* cited by examiner
(21) Appl. No.: 10/202,431 Primary Examiner-William C Joyce
(74) Attorney, Agent, or Firm-Quarles & Brady Streich
(22) Filed: Jul. 23, 2002
Lang LLP
(65) Prior Publication Data
(57) ABSTRACT
US 200210175579 A1 Nov. 28, 2002
Related U.S. Application Data An impact micro-positioning actuator. In one aspect of the
invention, a threaded shaft is threadably received in a nut
(60) Provisional application No. 601307,202, filed on Jul. and the nut is impacted by an impacting device, causing the
23, 2001.
nut first to rotate relative to the shaft by slipping as a result
of shaft inertia and subsequently to stick to the shaft as a
(51) Int. C1.
result of the frictional force therebetween. The nut is
F16H 25/20 (2006.01 )
returned to its initial position by a return force provided by
(52) U.S. C1. ....................................... 74189.23; 74189.1
a return mechanism after impact. The micro-positioning
(58) Field of Classification Search .................... 74/25, actuator is further improved by controlling at least one and
74189.23, 127,89.1 preferably all of the following: the friction, the impact
See application file for complete search history. provided by the impacting device, the return force provided
by the return mechanism, and the inertia of the shaft. In
(56) References Cited
another aspect of the invention, a threaded shaft is thread-
ably received in a nut and the shaft is impacted by an
U.S. PATENT DOCUMENTS
impacting device, causing the shaft to rotate relative to the
1,459,402 A * 611923 Johnson ...................... 1731108 nut.
2,882,772 A * 411959 Potter .......................... 173193
3,207,961 A * 911965 Lohr .......................... 3351255
3,211,028 A * 1011965 Christensson ................ 173193 13 Claims, 5 Drawing Sheets
US. Patent Jan. 10,2006 Sheet 1 of 5 US 6,983,667 B2
US. Patent
Jan. 10,2006 Sheet 2 of 5 US 6,983,667 B2
14
FIG. 3
US. Patent
Jan. 10,2006 Sheet 3 of 5 US 6,983,667 B2
10
FIG. 4
60
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55 54b
b k
57
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J
US. Patent Jan. 10,2006 Sheet 4 of 5 US 6,983,667 B2
70a
FIG.
Y
100 'W1
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I
70b
7B
t
1
115'
7A
US. Patent Jan. 10,2006 Sheet 5 of 5 US 6,983,667 B2
US 6,983,667 B2
1 2
IMPACT MICRO-POSITIONING ACTUATOR rotary motion by moving the jaw relatively slowly in a first
direction such that the coefficient of friction between the
REFERENCE TO RELATED APPLICATIONS shaft and the jaw overcomes the inertia of the shaft, and
moving the jaw relatively fast in a second direction such that
This application claims the benefit of the inventors’ s the inertia of the shaft prevents it from following the jaw, so
provisional application, U.S. Ser. No. 601307,202, filed Jul. that the shaft slips between the jaws to preserve the preced-
23, 2001. ing incremental motion. Accordingly, a cyclic electrical
signal applied to the piezoelectric element that is slowly
FEDERALLY SPONSORED RESEARCH rising and rapidly falling causes the shaft to rotate in one
io direction, and where the signal is rapidly rising but slowly
This application is based on research sponsored by falling, the shaft rotates in the opposite direction.
NASA, Marshal Contract NAS 8-97309. Although the Luecke device is mechanically simple and
therefore can be implemented at lower cost than stepper
BACKGROUND OF THE INVENTION motors, it has serious drawbacks for use at cryogenic tem-
is peratures. Particularly, the piezoelectric effect at cryogenic
The present invention relates to an impact micro-posi- temperatures is an order of magnitude smaller than the effect
tioning actuator providing for precision micro-positioning at at room temperature. Therefore, a complex calibration and
temperatures ranging from ambient to cryogenic. compensation scheme would be required to produce a sys-
Micro-positioning actuators have many applications, and tem that operates repeatably at both temperature extremes.
are used generally for optical or mechanical systems requir- 20 Moreover, piezo-materials that function well at room tem-
ing precise alignment. A most demanding application is in perature do not function well at cryogenic temperatures and
outer space on large devices of lightweight construction. vice versa. For these reasons, it is difficult or impossible to
Such devices must rely on active control to maintain scale a piezoelectric micro-positioned to repeatably position
required dimensional stability. Micro-positioning actuators a range of loads over a range of temperatures.
have been used, for example, to produce large mirrors zs The present inventors have proposed an alternative impact
suitable for use in an orbiting telescope, whereill a thin, micro-positioning concept in a paper entitled “A Linear
flexible mirror element is rigidly attached at many points to Micro-Positioning Actuator for Ambient and Cryogenic
a stiff carbon composite structure through corresponding Operation.” The actuator was to take the form of a shaft
micro-positioning actuators. Such systems must be operable threaded into a nut which is impacted with a small mass to
at the cryogenic temperatures of space, and it is desirable to 30 cause momentary rotation. A torsional spring was proposed
be able to operate the devices at ambient temperature on to restore the nut to its initial position after each impact. The
earth as well, to facilitate testing the systems. Micro-posi- shaft was preloaded to obtain a particular frictional torque
tioning actuators used for one-time or repeated adjustments between the nut and the shaft. The preload was proposed to
of cryostats must also be operable at cryogenic temperatures. set a frictional force between the shaft and nut that would be
It is always important to provide micro-positioning actua- 3s lower than the force required to accelerate the shaft at the
tors with sufficient durability, reliability, and precision for high initial angular acceleration of the nut on impact. As a
the application, and this is especially difficult where the result, the angular position of the shaft would lag behind the
actuators are exposed to extremes of temperature. It is also nut after impact. However, as the nut is thereafter deceler-
important to minimize hysteresis in micro-positioning actua- ated by the torsion spring, a point is reached where the force
tors that must be cycled, and to minimize their cost and 40 required to turn the shaft at the same rate falls to a level at
weight, the latter being especially important in space appli- or below the frictional force, so the shaft and nut move
cations. Another highly desirable feature in a micro-posi- together as the nut returns to its original position. The result
tioning actuator employed in outer space is the ability to of the cycle is that some of the initial advancement of the nut
hold a position without the application of power, to conserve relative to the shaft is preserved.
power as well as to prevent the production of excess heat 45 It was further proposed that the speed and momentum of
which is more difficult to dissipate in outer space due to the the impacting mass could be adjusted to vary the relative
lack of a convective heat loss mechanism. motion of the shaft and nut and hence the step size. Motion
Acommonly used precision actuator is the stepper motor. in two directions was proposed to be provided by two
Stepper motors, however, have significant disadvantages as separate impactors. A prototype was fabricated and was
micro-positioners, especially for applications such as SO reported to provide repeatable and reliable steps of from 10
described above where high precision and low weight are of to 100 nm at the 77 degrees Kelvin.
premium importance. In particular, very small stepper The impact micro-positioning device solved the problems
motors provide a limited number of steps per revolution, inherent in the Luecke piezoelectric actuating device. How-
such as about 16, so that a gear train is also required for ever, subsequent testing revealed that the device did not
micro-positioning. The gear train adds cost and weight, and ss perform reliably over the required ranges of temperature,
introduces random positioning errors, particularly hysteresis stroke and direction required for many applications, includ-
due to backlash. Moreover, a stepper motor adapted for use ing applications in outer space. Accordingly, there is a need
at both ambient and cryogenic temperatures, though pos- to refine the concept of an impact micro-positioning device
sible, would be relatively expensive. in a number of its aspects to realize the full benefit of the
Another strategy for micro-positioning is the piezoelectric 60 concept.
actuator such as described in Luecke, et al., U.S. Pat. No.
5,410,206 (“Luecke”). Apiezoelectric element is mounted in SUMMARY OF THE INVENTION
a frame having a pair of jaws between which is disposed a
threaded output shaft to be driven by the jaws. The piezo- The present invention, in a first aspect thereof, meets the
electric element is operative to effect reciprocating motion 65 challenge of improving the reliability of an impact micro-
of at least one of the jaws. The reciprocating motion of the positioning actuator wherein a threaded shaft is threadably
jaw against the output shaft is converted to an incremental received in a nut and the nut is impacted by an impacting
US 6,983,667 B2
3 4
device, causing the nut first to rotate relative to the shaft by FIG. 7A is a side section of a second embodiment of an
slipping as a result of shaft inertia and subsequently to stick impact micro-positioning actuator according to the present
to the shaft as a result of the frictional force therebetween. invention.
The nut is returned to its initial position by a return force FIG. 7B is a view of the micro-positioning actuator of
provided by a return mechanism after impact. As a result, S FIG. 7A taken along a line 7B-7B thereof.
relative motion between the shaft and the nut is preserved.
FIG. 8 is a schematic view of a portion of the actuator
shown in FIG. 7B.
The micro-positioning actuator may be further improved
according to the present invention by controlling at least one
DETAILED DESCRIPTION OF THE
and preferably all of the following: the friction, the impact 10
PREFERRED EMBODIMENT
provided by the impacting device, the return force provided
by the return mechanism, and the inertia of the shaft.
To control friction, a biasing device is provided that 1. First Embodiment of an Impact Micro-Positioning Actua-
laterally biases the shaft against the nut. The biasing device tor
bears against a selected one of the nut and the shaft with less 1s An impact micro-positioning actuator 10 according to the
friction than is caused to be present between the nut and the present invention is shown in FIG. 1. The impact micro-
shaft, so that the latter is dominant. positioning actuator 10 is used to adjust linear position with
To control the impact, a substantially elastic collision very high precision. For high precision, the actuator 10
between the impacting mechanism and the nut is provided. includes a fine threaded shaft 12 received in a correspond-
To control the nut return force, a return mechanism 20 ingly fine threaded nut 14, though coarser threads may be
preferably includes a plurality of n leaf springs arranged to used without departing from the principles of the invention.
provide a torsional spring return force to the nut. Of the n An end of the shaft is used to position an object, which may
leaf springs, n-1 of the leaf springs are preferably provided be a mirror in an orbiting telescope or any other object, as
with slip joints permitting expansion or contraction of the a result of relative movement of the shaft and nut. To
leaf springs with respect to respective points of attachment 2s produce relative movement of the shaft 12 and nut 14, and
of the leaf springs as a result of increasing or decreasing therefore to produce linear stepping of the shaft for posi-
temperature. tioning the object, an impacting device 16 is positioned to
To control the inertia of the shaft, an inertial adjustment provide an impact force to the nut.
disk is preferably provided that is attached to the shaft. The nut 14 is coupled to a return mechanism 22 for
In a second aspect of the invention, an impact micro- 30 decelerating the nut and returning it to its original position
positioning actuator is provided wherein a threaded shaft is after impact. However, the shaft moves relative to the nut
threadably received in a nut and the shaft is impacted by an according to the principles of the invention because of
impacting device, causing the shaft to rotate relative to the friction between the nut and the shaft. For example, if there
nut. Preferably, both impact micro-positioning and continu- were no friction between the shaft 12 and the nut, no
ous micro-positioning is provided. 3s movement of the nut would be transmitted to the shaft. The
Accordingly, it is a primary object of the present invention nut would spin in one direction about the shaft and then be
to provide a novel and improved impact micro-positioning returned in the other direction to its original disposition with
actuator. respect to the shaft. At the other extreme, if the shaft and nut
It is another object of the present invention to provide a are rigidly mounted together, the shaft and nut would always
micro-positioning actuator providing for both impact and 40 move as one, so that no relative movement would be
continuous micro-positioning. possible.
The foregoing and other objectives, features and advan- The first embodiment of the invention is based, in part, on
tages of the invention will be more readily understood upon a recognition of the advantage of providing a "slip-stick"
consideration of the following detailed description of the operation in an impact micro-positioning device. In the
4s
invention, taken in conjunction with the accompanying micro-positioning actuator 10, the nut first slips around the
drawings. shaft in response to the impact due to the inertia of the shaft,
but later sticks to the shaft as a result of friction between the
BRIEF DESCRIPTION OF THE DRAWINGS nut and the shaft as the nut decelerates and, eventually, is
returned by the return mechanism 22.
so
FIG. 1 is a pictorial view of a first embodiment of an As mentioned above, a prototype impact micro-position-
impact micro-positioning actuator according to the present
ing actuator employing the slip-stick concept was previously
invention. proposed by the inventors. However, testing of the prototype
FIG. 2 is a side section of a biasing element of the impact revealed that the device did not perform reliably over the
micro-positioning actuator of FIG. 1 according to the required ranges of temperature, stroke and direction required
5s
present invention. for many applications, including applications in outer space.
FIG. 3 is a cross-section of the nut and shaft of FIG. 2 For example, step size varied when the direction of motion
shown with a biasing force being applied. was reversed, so that after reversal the first steps were
FIG. 4 is a plan view of the micro-positioning actuator of sometimes very large or sometimes even in the wrong
FIG. 1, showing an impacting mechanism according to the direction. It was also recognized that movement of the shaft
60
present invention in section. was not reliable over appreciable stroke lengths.
FIG. 5 is a plan view of a return mechanism for the According to the invention, a number of structures and
micro-positioning actuator of FIG. 1 according to the methods for controlling the friction, the impact provided by
present invention. the impacting device, the return force provided by the return
FIG. 6 is a section of an anchoring member of the return 65 mechanism, and the inertia of the shaft particularly adapted
mechanism of FIG. 5 according to the present invention, for the first embodiment of the invention. Such control
taken along a line 6-6 thereof. aspects of the invention are provided as described below.
US 6,983,667 B2
5 6
2. Control of Friction The amount of lateral movement of the elongate axis “A” of
It is an outstanding recognition according to the present the shaft with respect to the axis of rotation “R’ is shown
invention that friction between the nut and the shaft should highly exaggerated. There are two reaction forces Fr, and
be controlling. This friction is controlled according to the Fr2a t the nut 14. The required balancing of the forces and the
invention by providing a number of features. First, it is s torques implies that the reaction force Fr, must be greater
recognized that the surface finishes of the mating threads of than the applied force Fa, due to the presence of the reaction
the nut and the shaft should be carefully treated. Preferably, force Fr,. The force of friction between the nut and the shaft
fine, cooperating surface finishes are provided by lapping is proportional to these reaction forces, while the force of
the threads of the nut and shaft together, i.e., the shaft is friction between the biasing member and the shaft is pro-
inserted into the nut and the nut is run up and down the shaft io portional to the applied force, so that the friction force
repeatedly, with successively finer polishing grits between between the biasing member 18 and the shaft will, all else
the threads. This method produces surface finishes and a being equal, be less than the friction force between the shaft
clearance between the nut and the shaft providing that and the nut as a result of employing the biasing member.
friction between these parts is substantially independent of However, it is desirable, to ensure that friction between
relative rotation between the nut and the shaft. Other surface is the nut and the shaft is controlling, that friction between the
treating means providing the same or similar result may be biasing member 28 and the shaft 12 is made at least about
employed without departing from the principles of the three times less than the friction between the nut and the
invention. shaft. It has been found that mechanical grade polytet-
Second, a dry lubricant is preferably provided between rafluroethylene (“PTFE’) is a superior material for this
the nut 14 and the shaft 12; particularly, a dry lubricant such 20 purpose, not only because it provides low friction but also
as tungsten disulphide or molybdenum disulphide to prevent because it minimizes cold flow into the threads of the shaft.
freezing of the lubricant at cryogenic temperatures. The By contrast, virgin PTFE has been found to cold flow and
lubricant is suspended in a carrier solvent. The lubricant is defeat the friction control provided by the features men-
applied preferably by dipping the threaded shaft in the tioned above, especially after the device is cooled to cryo-
lubricant. The shaft is maintained in a vertical orientation zs genic temperatures where the bearing member may stick to
until the lubricant dries, providing for a uniform thickness the shaft and prevent relative rotation of the shaft and the nut
circumferentially. The coating is then baked to drive off altogether.
residual solvent. The shaft is then run in and out of the nut The magnitude of the frictional force is controlled by the
a number of times, e.g., about ten times, to “wear in” the materials and surface finishes used for the threads of the nut
coating. The aforementioned lapping operation is preferably 30 14 and shaft 12, and by the amount of force applied by the
carried out so that the desired clearance between the nut and biasing element 30. The shaft is preferably formed of
the shaft is achieved considering the amount of lubricant that stainless steel, such as 400 series steel, and the nut is
is to be applied. This method for applying the lubricant preferably formed of a low alloy steel such as 4130 or 4340.
produces a lubricating layer of substantially constant thick- It is believed that the particular selection of these materials
ness, which preserves the constancy of the friction between 35 also contributes to controlling friction, and the materials
the nut and the shaft provided by the surface finishes. Other desirably provide substantially matched thermal expansion
methods producing the same or similar result may be characteristics so that the nut and the shaft do not have
employed without departing from the principles of the excessive play at one extreme of temperature, and do not
invention. bind at the other extreme.
Third, as seen in FIG. 2, a biasing device 18 is provided 40 It should be noted that the aforementioned features each
for biasing the nut 14 and the shaft 12 against one another, contribute to controlling the frictional force between the
to increase the friction between these parts to a desired level shaft and the nut and may be employed separately as well as
consistent with providing a desired amount of rotation at in within the principles of the invention,
which slip ceases between the nut and the shaft and the parts
stick together. 45 3. Control of Impact
In a preferred embodiment of the device 18 there is a It is recognized according to the present invention that
housing 24 having a guide bore 26. Abearing member 28 fits reliable operation of the device 10 to control the impact
closely into the guide bore 26 to prevent wind up. In the provided by the impacting device 16 and the nut 14. To
example shown, the housing 24 is mounted to the nut and the provide this control, the impacting device is simply and
bearing member 28 presses on the shaft; however, this so preferably implemented as two solenoids 32 and 34 having
configuration may be reversed as well. respective reciprocating drive hammers 36 and 38 as shown
A biasing element 30 such as a compression spring as in FIG. 4. One of the solenoids is used to rotate the nut in
shown biases the bearing member 28 to cause the bearing one direction, and the other solenoid is used to rotate the nut
member 28 to exert a predetermined force on the shaft 12 to in the opposite direction, to provide for stepping of the shaft
bias the shaft against the nut. 5s in two directions. A current input to a selected one of the
It is desired that the biasing element 30 provide a sub- solenoids produces a magnetic field therein causing the
stantially constant force during rotation of the nut or shaft. corresponding hammer to be thrust outwardly in the direc-
Therefore, the shaft 12 should be radially symmetric about tion of the arrow toward the nut. The flow of current is
its axis of rotation “R,” so that the compression of the spring preferably stopped prior to impact so that a simple elastic
30 is not changed substantially as a result of relative rotation 60 collision between the hammer and nut results. It is prefer-
between the nut and the shaft. Accordingly, the biasing ably provided that the hammer is significantly less massive
element need not have any particular force-displacement than the nut, SO the hammer rebounds from the nut; the nut
characteristic, and may be, for example, a plastic material, a is in any case provided with an initial angular velocity of
compressible fluid or in terrestrial applications, a simple rotation about an axis of rotation “R’ as a result of the
weight. 65 impact.
Referring to FIG. 3, the shaft 12 is shown biased against Hammer returns 46 and 48, located on the solenoids,
the nut 14 by the biasing device 18 with a biasing force Fa. return the respective hammers to their original positions
US 6,983,667 B2
7 8
after impact. The hammer returns can be springs or other Therefore, while the shaft may be machined very carefully
devices or members adapted to retract the hammers within to provide a desired inertia, the inertia cannot easily be
the time necessary to prevent a secondary collision with the controlled in this manner, and cannot easily be varied as may
nut. However, it is recognized that the return force provided be desired.
to the hammers should be no more than is required, because
To solve this problem according to the present invention,
additional force causes positioning irregularities. To appre-
the shaft 12 preferably includes an inertial adjustment disk
ciate this, it should be noted that light weight solenoids have
20 which may be attached to an end of the shaft, or anywhere
non-linear accelerating characteristics. Particularly, accel-
along the shaft. The inertial disk 20 is preferably balanced
eration of the drive hammers increases toward the end of
about the axis “R’ of rotation of the shaft and has a
their travel before the nut 14 is impacted. On the other hand,
predetermined diameter “D” and thickness ‘‘tl,” depending
the force provided by the hammer returns in a spring
embodiment thereof is linear with travel. A stronger spring upon the amount of adjustment to shaft inertia that is
must be overcome with a larger solenoid current, which desired. These dimensions may be provided as manufac-
increases the rate of change in acceleration at the end of tured or may be adjusted to suit a particular shaft.
travel, leading to a more variable impact velocity. 15 Because the diameter of the disk is greater than its
A control system 44 for controlling the impacting device thickness, fine adjustments to inertia can be made relatively
16 provides a predetermined magnitude and duration of the easily and economically. Moreover, the inertia can be varied
current that is applied to the solenoids to accelerate the drive over a relatively wide range. The disk 20 may in fact provide
hammers. The strength and duration of the pulse provided by most of the inertia of the shaft and, therefore, may be used
the control system is optimized to achieve uniformity in step 2o to control the inertia of the shaft even without measuring the
size. Particularly, pulse width and step size are determined inertia of the shaft and configuring the disk accordingly as
for each actuator by testing. The current required to yield a
might otherwise be required. In any event, the amount and
desired relative rotation of the nut 14 and the shaft 12 (“step
ease of control of shaft inertia is increased substantially by
size”) is determined for several pulse widths, for each
use of the disk 20.
impacting device 16. A pulse having a desired magnitude 25
It is also desirable to provide a sufficiently high torsional
and width is selected that provides the most consistent step
size for the respective impacting device, providing for stiffness in the shaft 12 so that the friction between the nut
optimized control of the impacting device 16 corresponding and the shaft does not permit the nut to “wind up” the shaft
to each direction of rotation. relative to the inertial disk 20 as the nut accelerates. Accord-
The nut 14 includes shoulder portions 40 and 42, which 30 ingly, a suitable diameter ‘‘#’ of the shaft is maintained even
receive the impact of the respective hammers 36 and 38. At with the inertial disk, in consideration of the modulus of
elasticity of the material used.
least the tips of the hammer and at least the surface of the
shoulder portions 40 and 42 are formed of materials having
5. Control of Return of the Nut
a high elastic modulus to provide for the desired elastic
The shaft 12 must be able to move as well as support an
collision between the hammers and the nut. To the extent the 35
object, and the nut 14 should be able to support the shaft
collision is not elastic, kinetic energy will be lost in the
along with this load while, at the same time, permitting the
collision, decreasing the initial velocity of the nut and
shaft to rotate substantially freely about the nut except as
thereby lowering the efficiency of the device. Moreover,
provided by the biasing device 18. Accomplishing these
deformations in either the drive hammers or the nut that
would otherwise result from one or a series of inelastic 40 objectives places severe requirements on the support mecha-
nism. It is an outstanding feature of the present invention to
collisions uncontrollably change the amount of impact
provide a return mechanism 22 that is able to support large
velocity imparted to the nut. Tempered or hardened steel is
axial loads in addition to providing a returning feature.
an exemplary material for providing the desired elastic
Turning to FIG. 5, in a preferred embodiment of the inven-
collisions, so at least the ends of the drive hammers are
preferably formed of a hardened steel, and where the nut is 45 tion, the return mechanism 22 comprises a plurality of leaf
springs such as 52, 54, and 56 radially extending from the
formed of low alloy steel it is also preferably tempered or
nut and anchored respectively by corresponding anchoring
strain hardened at least at the shoulder portions.
members 53, 55, and 57. By virtue of the linearity of the
It should be noted that the aforementioned features each
force deflection characteristic of the leaf springs, the return
contribute to controlling the impact with the nut and may be
so mechanism applies a torsional return force to the nut about
employed separately as well as in combination within the
the axis “R’ that is substantially proportional to the angle of
principles of the invention. Moreover, while a preferred
rotation of the nut caused by the impact; however, this is not
impacting mechanism 16 has been shown and described as
essential to the invention. By virtue of the leaf springs, the
preferred, any impacting mechanism that results in a con-
return mechanism 22 applies a greater return force when the
trolled initial angular velocity of the nut may be used
55 nut is maximally deflected from its initial angular position,
without departing from the principles of the invention.
and the return force is decreased as the nut accelerates back
4. Control of Shaft Inertia to its initial angular position. This action is desirable to
As indicated above, the nut 14 slips around the shaft 12 minimize acceleration during return, and therefore to mini-
in response to impact because the frictional force produced mize the potential for the nut to slip with respect to the shaft
by the biasing device 18 is too low to prevent slippage, at 60 during return. This is not essential to the invention either,
least initially. More particularly, the frictional force is too though it enhances repeatability of the actuator 10 and is
low to overcome the resistance to acceleration put up by the therefore preferred.
shaft 12 as a result of its inertia. It is recognized as being With additional reference to FIG. 1, where the return
desirable to control this inertia, particularly, the moment of mechanism 22 employs leaf springs as described above, the
inertia of the shaft about its axis of rotation “R.” Returning 65 leaf springs are substantially wider (dimension “w”) than
to FIG. 2, the shaft is typically elongate, having a diameter they are thick (dimension ‘‘ti’), to provide for a high
“d” that is perpendicular to and smaller than its length “1.” stiffness in the direction of axial loading while at the same