A two position (0 and 180 deg) actuating mechanism driven by two alternately heated opposing flat NITINOL springs is proposed for rotating the low field triaxial fluxgate magnetometer experiment on the 1977 Mariner Jupiter-Saturn spacecraft to its 0 deg and 180 deg positions. The magnetic field, power requirements, weight, and volume of this device are very restrictive. The problems encountered in design and development are presented

Stange, W. C.

English

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14, A NEW CONCEPT FOR ACTUATING SPACE MECH_'_ISMS
By William C. Stange
NASA Goddard Space Flight C_nter '
' i SUMMARY
: A two-position (0° and 180 °) actuating mechanism driven by two alternately-heated
i opposing fiat NITINOL springs is proposed for rotating the low field tris_i:,J fluxgate
magnetometer experiment on the 1977 Mariner Jupiter-Saturn spacecraft to its 0 ° and
I 180 ° positions. The magnetic field, power requirements, weight and, and volume of this de-
i Vice are very restrictive. The problems encountered in design and development of this
i device are presented.
: l INTRODUC TION
t
_ The purpose of this experiment is to provide precise, accurate, and rapid vector
measurements (from 0.01 T to 20 gauss, 1T = 10 -s gauss) of the magnetic fields of
Jupiter and Saturn in interplanetary space to them and beyond. These data extend in situ
: studies of the solar wind interaction with Jupiter and characteristics of its magnetic field
: and yield first studies of Saturn's field and its interactions if the solar wind extends to
I0 AU.
Performing accurate measurements of magnetic fields on a spacecraft _ot fabricated
magnetically clean is a major problem. A moderately long boom will be used to place
two low-field(_< 6400 T) triaxial fluxgate magnetometers at remote distances from the i
spacecraft. Simultaneous measurements will yield separate estimates of the spacecraft
field and the ambient field.
The purpose of this essentially nonmagnetic actuator is in-flight calibration of the
trtaxial fluxgate magnetometers. This calibration, which determines the sensor zero
point, is accomplished by periodically flipping tb_ magnetometers by 180 degree_.
The advantages of this mechanism are that it satisfies more than any other known
device, the constraints of volume, weight, nonmagnetic materialS, andpower in relation
to the requirements of high torque, fast cycling and long life. These properties are de-
rived solely from the unique mechanical qualities of 55-NITINOL. This actuator will
provide cyclical hi-directional rotary motion under varying environmental temperatures
(-45°C to +40°C) in a vacuum for peric_is up to five years.
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This paper describes the mechanical and electrical functions of the design which
evolved, as well as the problems encountered ._ne objectives achieved are evaluated
and other possible applications are presen_d.
i
OBJECTIVE
The objective was to develop an actuator which would meet the following require-
ments.
1. Rotate 180 degrees ± 15 minutes of arc.
2. Remain at the indexing stop until again actuated.
3. Have a permanent magnetic field (when not being powered) less than 0.1 _ at 2.54 cm (I in}.
4. Have a minimum capability of 300 cycles during a period of five years.
5. Complete the rotational indexing within 30 seconds of initiation.
6. Require not more than 8 watts of power for less than 30 seconds.
7. Weigh less than 0.227 kg (0,5 lb).
8. Fail-safe indexing, i.e., the actuator must no_ stop in any position other than 0 or
180 degrees.
9. Operate within the temperature range of -45°C to +40°C.
10. Operate in a vacuum.
DESIGN :
The selection of a design approach required the consideration of other feasible
concepts. Among those reviewed were bimorph piezoelectric devices, opposing coil
solenoids (without cores), nonmagnetic electric motors, Freon state conversion bellows,
and wax pellet actuators.
A concept utilizing 55-NITINOL was adopted because it appears to most reliably
meet the above design requirements.
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" 55-NITINOL
About ten yoars ago, Buehler and Wiley of the U.S. Naval Ordnance Laboratory
t_ developed a special purpose nickel-titanium alloy (55-NITINOL) that possesses "mechan-
i ical memory" (see reference 1}. The phenomenon is caused by a thermally induced
_ atomic shear transformation which occurs in 55-NITINOL. The salient property of this
alloy is its memory of shapes imposed on it at a characteristic "annealing" temperature
._ T a . This effect is manifested, after return to an arbitrary lower temperature T1 at
_ which plastic deformation (< 8% strain, fig. 1) has been induced, by its forceful return to
; the originally imparted shape (at T_) following heating to the transition temperature
i_ T t (T 1 < T t < T ). Simply stated, this means that any shape formed at the "annealing"
_ temperature can be thermally induced to reoccur in spite of deformations (< 8% strain)
imposed at lower temperatures.
_ The 55-NITINOL used in this mechanism has a T_ = 500°C and T t = 80°C. The
i transition temperature T t is a function of material composition and cold working and
_: can range from -100°C to +300°C. The onset of restoring stress (memory} is not sharply
_ defined (figs. 1 and 2), rather T t is a temperature range (_10 °) over which the restoring
stress increases by a factor of four and the modulus by a factor of four, as illustrat¢d in
fig. 2. If the 55-NITINOL flat springs were um.estrained they would assume the shape of
, a straight strip (memory shape) at 90°C (upper T t range). The spring torque of this
actuator varies directly with the yield stress of the NITINOL which is solely dependent
on whether its temperature is above or below the transition temperature range. The
springs used in this test model will provide 0.029 l_-m (40 in-oz) of gross torque. After
about 1,000 cycles the gross torque output should level off at about 95% of the initial value.
MECHANICAL OPERATION
: The mechanism as shown in figs. 3 and 4 is simply an experiment container which
can be bi-directionally rotated by two opposing NITINOL fiat springs with bonded heater
strips, whose indexing is biased by two over-center Flexator springs. To operate the
actuator, the appropriate NITINOI spring (fig. 3, section A-A) is heated to its transition
i temperature range (+80°C to +90°C), increasing the modulus of elasticity from
27.6 x 106 N/m 2 to 82.7 x 106 N/m 2 (4,000,000 psi to 12,000,000 psi) (fig. 2), providing
a torque greater than the unheated spring. This action drives the toggled spring crank
(figs. 3 and 4) and the pinned crank arm clockwise from the po_ tion shown. After
90 degrees rotation, the two Flexato: springs drive the crank arm, fail safe, to the
opposing stop, 180 degrees from where it started. The shaft rotates the experiment in
its container and acknowledges the indexing through a cam-actuated microswitch as
: shown in fig. 3. When the heater is turned off by tetemetry and the NITINOL spring falls
! below the transition temperature, the crank arm is held against the indexing stop by the :
i two Flexatorsprings. The rotationisreversedby applyingelectricalpower tothe heater
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1976012084-188
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strip which is bonded to the NITINOL fiat spring shown in section A-A, fig. 3. This
drives the crank arm counter clockwise back to where it started.
m
MATERIALS
The prime considerations in the selection of materials for this mechanism were low
i_ ms_etic permeability (< 1.001), volume and weight. The Flexater springs were made of :
Elglloy, the bushings, housing and experiment container from Delrin, and the shsfts and
other hardware from titanium alloy, beryllium copper, aluminum and brass, i
?
ALTERNATY_'E USES
Other mechanisms utilizing NITINOL in different covfiguratlons are currently being '_
designed and fabricated. Devices utilizing NITINOL can be competitively employed in
many areas requiring relatively quick response and high driving force with severe limi-
= tations on the permanent magnetic field, weight and volume.
; CONC LUSION
The mechanism described in this paper provides positive, cyclical indexing for a
, sensor rotating 180 degrees ±15 minutes; the permanent magnetic field is less than 0.1_'
at 2.54 cm (one in), the power consumed is less than 240 watt-seconds, the weight is
less than 0.168 kg (6 oz), and the volume (less the experiment container) is less than !
3.8 x 105 cubic meters (2.25 cubic inches). Two mechanisms of the type described in
this paper have been fabricated for feasibility testing. Eight additional units will be :
built, two of which it is contemplated will go aboard the 1977 Mariner Jupiter-Saturn
spacecraft as the actuating mechanism for the low field trlaxtal fluxgate m_gnetometer
experiment. _,
REFERENCE _
_J
1. Jackson, C. M., Wagner, H. J., and Wasilewski, R. J., "55-NITINOL -- T_ Alloy :
with a Memory: Its Physical Metallurgy, Properties, and Applications." NASA-
SI_ll0, 1972. _ '_
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ADDENDUM _
The actuator described in the Mechanical Operation
Section of this paper (figure 4) has been modified as
shown in figures 5 and 6. Refinement of the conceptual
design necessary for the development of flight hardware
has wrought most of the changes. However, one basic !
addition has been incorporated into the original concept;
; _ and that is the feature of a position lock.
I This addition was necessitated because of the tendency
of the NITINOL springs, after _ 1,000 flips and especially
at low temperatures, to return to their strained shape upon
cooling and therefore pull the crank arm off its position
stop (figure 5).
• The concept shown in figure 4 was modified to incorporate
a mechanism which locks the crank arm in either its 0° or 180 °
position. As shown in figure 5 the outboard end of either
_ NITINOL spring is reacted by a pivot arm which also carries
two cam springs (tension) 9 a follower and an arm stop.
!_ The two followers interface with a cam which has one slot.
In figure 5 the left follower has engaged the cam slot and
bears against the lower slot face thus eliminating play and
locking the crank arm against its position stop.
When the NITINOL spring on the left is heated, its in-
' creased modulus reacts with the locked toggled spring crank i
on the right and the pivot arm on the left. As the force i
of the two cam springs is overcome the pivot arm and its
slaved follower move counter clockwise, thus ultimately
_ releasing the cam. The toggled spring crank is now free !
to be driven clockwise as in the previous concept (figure 4)
and the crank arm rotates 180 ° to the left stop. The spring
loaded right follower detents with the cam and locks the
crank arm and its associated hardware in this position until
it is again flipped back to the position as shown in figure 5.
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1976012084-191
Flsure 3. Typioal Tempe1"ature Effect on Yield St1"es8 and Modulus of Elastieit_
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A new concept for actuating space mechanisms

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