The Viking Lander gas chromatograph/mass spectrometer will analyze pyrolyzed samples of the Martian surface for organic content. The surface-sample loader and pyrolyzer assembly (SSPLA) is described, along with the major problems encountered during design and testing. Three mechanisms were developed to implement the required SSLPA functions: (1) a soil loader that forces soil from a filled rotating funnel into each of three ovens located on a carriage, (2) a Geneva drive for rotating and precisely indexing the ovens to receive sample, and (3) a toggle-clamp mechanism for sealing the ovens by forcing circular double knife edges into gold sealing surfaces

Chase, C. P.

Weilbach, A. O.

English

NASA Technical Reports Server

IVIKIN(; ('.C/MSMECHANISMS DESIGN AND PERFORMANCE
By C. Peter Chase and August O. Weilbach
•
_ _1 _.. Advanced Technology OperationsBeckrmm Instruments, Inc.
ABS rRACT
The Viking l,ander (;as Chromatograph/Mass Spectrometer will analyze pyro-
" lyzed samples of tlleMartian surface for organic content. Tlle Surface-Sample
l.oaderand Pyrolyzer Assembly (SSLPA) Is described, along with tilemajor problems
encountered during design and testing. Three mechanlsms _:eredeveloped to imple-
ment tilerequired SSLPA functions: (I) a soil loader that forc':s soll from a
filled rotating funnel into each of three ovens located on a carriage, (2) a
(,enlva drive for rotating and precisely Indexing tileovens to receive sample,
and (3) a toggle-clamp mechanism for sealing tileovens by forcing clrc,,lar double
knife edges into gold sealing surfaces.
I NTRODUCTl ON
Tile "laboratory" launched the afternoon ,.f 20 August 1975 from Kennedy
Space Center contained sophisticated tnstrumeuts for experiments on Mars. News-
paper accounts of the mission focused on these experiments, especially those for
detecting evidence of llfe. Some accounts mentioned the instruments themselves.
None described the intricate mechanisms that made the experiments po., ,.ble. This
paper describes one such mechanism--the Surface-Sample Loading and Pyrolyztng
Assembly for the gas chromatograph/mass spectrometer (GC/MS) instrument.
THE GC/MS EXPERIMENT
The basic concept of the GC/MS experiment ts to perform analyses on
organic matter in soil samples by pyrolyatng the soil sample (i.e., heating the
sample to a high temperatare, causing the breakdown of organic _atter to volatile
products) and then separating the components of the pyrolyzed organic matter with
a gas chromatograph. As each separated component elutes from the gas chromato-
graph, it is identified by a mass spectrometer using the known fragmentation
pattern of the component. The results of these analyses will be a guide in
determining the presence of biological organic matter. Also, because the pyroly-
sis of each class of biological compounds gives characteristic and predictable
products, it is possible to deduce what classes of organic material were origin-
ally present in the soil and to obtain an Indication ot bow much was present.
The crucial part of the experiment is sample handling and pyrolysis,
which is carried out by the Surface-Sample I.oadtng and Pyrolyzlng Assembly (SSLPA).
Z08 JPL Technical Memorandum 13-777
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https://ntrs.nasa.gov/search.jsp?R=19760021204 2020-03-20T02:33:29+00:00Z
SSLPA FUNCTIONAL DESCRIPTION (Figure 1)
The SSLPA consists of a soll loader, an oven carriage and indexing mecha-
nism, an oven-seal clamping mechanism, two prime movers, and tee _ecessary drive
systems. The principal functions of the SSLPA are to:
• Receive a soil sample from the Viking Lander Sample Processor.
• Load and pack the soll into one of three ovens suspended in the o_en ..
carriage.
• Rotate and _ndex the oven carriage.
• Clamp and seal the oven.
4
• Pyrolyze the soil in the oven at a selected temperature, while flush-
ing the effluents to the gas chromatograph.
• Dump and clean out the excess soil from the soil loader and repeat on
co_mmnd the above functions for the other two ovens.
The soil loader rotates and taps the soil inlet funnel as a reciprocating
plunger directs and packs the soil sample through the funnel exit into one of
three 60-_ ovens. Ali these movements are achieved by an arrangement of a pair
of helical gears, a spur gear pair, a slider crank, and a spring-loaded tapping
follower. The carriage then rotates, indexing the filled oven to the next sta-
tion and bringing a dump cavity under the inlet funnel. The soil loader mecha-
nism cleans out the soil remaining in the funnel, dumpin_ it into the reservoir
in preparation for receiving another soil sample.
o
The ovens and dump cavities are located on a rotating carriage indexed
by a Geneva drive. The carriage has seven stations--six for oven and dump .:
reservoirs and one non-operating station for transit to Mars.
Oven-seal clamping is achieved by a four-bar toggle-clamp mechanism
operated by a motor-driven cam. The linkage forces a set of circular knife edges
into gold discs at both ends of the ovens, plastically deforming the discs to
form a seal with a leak rate less than i x 10-9 scc/s helium at one atmosphere.
Only i.I kg (2.5 ib) input force is required at the cam to impart 20 kg (45 Ib)
force into the gold discs.
The ovens are suspended by triangular stainless steel spring washers, ! _
which thermally isolate the ovens and apply a preload for holding them in place
under vibration and g loads. All power and sensor connections to the ovens are
hard wired usin_ a flexible cable that permits unrestrained rotation of the oven i
carriage through one full cycle. !
All precision mechanzsms are susceptible to failure from particulate
contamination. To prevent failure of the GC/MS mechanisms from sample contamln- _ ¢
ation, the motors are sealed, the one-way clutch uses a labyrinth seal, and
shielded ball bearings are used to minimize friction.
i SOIL LOADER MECHANISM (Figure 2)
_ _ The soil loader receives _oil sample from the Lander Sample Processor
via a simple gravity feed. However, once sample is in the inlet funnel of the
_ soll loader, a mechanism is needed to load the sample into the three ovens and
I clean the funnel between loadings to prevent sample cross contamination in excess
JPL Technical Memorandum 33-777 Z09
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of I0 percent. The main problems encountered were: (I) maintaining sample flow
through the narrow funnel exit opening--l.5 mm (0.058 inch)--and into the ovens
througb their matching 1.5 mm opening without plugging; (2) cleaning the funnel
of residual sample; and (3) loading the ovens with a reproducible amount of sam-
ple. These problems were further complicated by the range of sample particle
size, particle configuration, and moisture content that had to be accommodated.
The original design used an oscillating rod tangential to the inside of
the funnel for poking the soll into the ovens. The funnel was rotated, causing
the rod to scrape the soil from the sides of the funnel. However, even though
the rod was close-fitted to the wall of the funnel, soil packed underneath it,
forcing it away from the wall and defeating its use as a scraper. Additionally,
the 90-degree end surface of the rod caused plugging at the funnel exit opening,
preventing reliable, complete filling of the ovens.
Two simple design changes solved both problems. First, the poker cross
section was c_hanged from circular to triangular, providing a sharp edge for
scraping the inside wall of the rotating funnel and adding rigidity to prevent
any lifting action. Second, the poker end was tapered to a 30-degree angle,
allowing excess sample to flow back over the face of the poker tip as it pene-
trated the narrow exit opening. This design change eliminated plugging and im-
proved oven loading and sample packing consistency.
In its original concept the poker was guided and supported by a complex
linkage. During tests of the soil loader, it became apparent that an improved
mechanism was needed. The new mechanism had to provide better alignment of the
poker rod with the funnel and work with less friction and less effect from soll
induced interference. A small ball bushing was selected as an alternative sup-
port and low friction was obtained. During testing of the engineering model it
became apparent that the bushing idea worked but new soll testing requirements
required more force on the poker mechanism. Within permissible space a next ,_
larger bushing was installed, solving the problem,
Another problem was filling the very narrow 60-pE ovens with a known
and reproducible amount of sample. It was initially believed that this could be
accomplished by sensing the resistance of the poker to sample packing force and
turning the loader mechanism off at a predetermined resistance force. This
approach turned out to be unduly complex and unreliable. The approach finally
selected was simply to load for a fixed time sufficient to guarantee loading a
worst-case sample.
As shown in Figure 3, the soil loader mechanism is driven by a 150-
r/minmotor on whose shaft is attached a helical gear. This gear drives another
helical gear at right angles and at the same speed of 150 r/mln. The second
helical gear is fixed to the crank shaft. The crank drives a connecting rod
which, in turn, drives a shaft guided by a linear bearing. Rigidly attached to
the guided shaft is the poker, which reciprocates at 150 cycles per minute in
the funnel. The first helical gear has, on the same shaft, a spur pinion and a
four-lobed cam. The p_nlon drives another gear attached to the funnel. The
funnel thus rotates at approximately 27 r/min. The funnel tapper is sprlng_
loaded against the cam, causing it to tap the funnel wall as the funnel rotates.
i
glL JPL Technical Memorandum 33-777
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] 97602 ] ] 84-2 ] 7
OVEN CARRIAGE AND INDEXING MECHANISM (F." re 3)
The oven carriage holds three sample ovens and n flight oven, two dump
cavities, and a flush tube--seven stations in all. Indexing precisely positions
the seven stations for each operation.
The primary design feature is the _ndexing mechanism. Initially, it was
conceived as a spring-loaded ratchet and pawl mechanism, but this deslgn was dis-
carded because of its complexity, high power requirements, and inadequate ac-
curacy and reliability. A Geneva drive mechanism was selected since its inherent
characteristics closely matched the design requirements. It is extremely simple,
requires very little power, provides precise indexing, and is highly reliable.
It was the ideal mechanism for the application.
The Ceneva drive is powered by the same motor used for the soil loader.
On command, an electrical clutch engages and a gear set transmits the motion to
the Geneva drive. The actuator drives the Geneva wheel, which is an integral
part of the carriage. It takes two revolutions of the drive wheel to index the
carriage one station. This arrangement, because of the inherently-accurate
position capability of a Geneva drive, assures a smooth and positive transition
from one station to another.
Position status indication is provided by a 3-bit binary code on a thin
disc attached to the carriage. The encoder head is implemented with light-
emitting diodes and photosensitive detectors. The functional sequence (see
Figure 4) is as follows:
• An oven is readied for receiving the sample by a preheat cycle at
500°C for about 1 minute in order to clean out possible residual
contamination.
• With the oven placed under the loader, soil is fed from the loader "_
and compacted by the poker of the soil-loading mechanism.
• The oven is indexed to the pyrolysis station, where it is clamped
and sealed.
• Simultaneously, a dump cavity is positioned to receive the residual
sample from the loadec.
As a loaded oven is indexed to the pyrolysis station, residual soll on
the surface of the oven seal is removed by the wiping action of a set of thin
flexible metal blades. Any remaining fine particles--down in the 5 pm size or
less--do not interfere with hermetic closure of the oven.
OVEN-SEAL CLAMPING MECHANISM (Figures 4 and 5) i
Sealing the ovens to attain a leak rate no greater than 2 x 10-5 scc/s
of helium presents several problems. The seal has to be effective in the presence
of _oil particles up to 5 _m that might not be wiped away by the metal blades, _
it has to be maintained at high temperatures, and there has to be a mechanism
for applying sufficient force to attain the seal and maintain it during analysis.
The initial design approach to sealing was to force circular knife edges
into gold discs brazed to both ends of the ovens. Although a satisfactory seal i
was obtained at the shearing surfaces between the knife edge and the disc under
ideal conditions, when soll particles were present they caused leak rates in
excess of the requirement. Also, the gold tended to cold-flow away from the
knife edge, deteriorating the seal.
SPL Technical Memorandum 33-777 Zll
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1976021184-218
The solution was a two-edged knife (see Figure 4), which solves b_th the
cold-flow and particle problems. The two edges prevent the entrapped gold from
extruding and they provide a redundant seal that increases reliability and re-
duces leakage to an acceptable rate.
There were a numbe_ of possibie implementations explored for applying
a sufficient force to the circular knife edges to obtain the required seal. But
in addition to the force required to make the initial seal, there was also the
problem of maintaining the seal as the gold softened during oven heating. This
required a controlled spring rate that would cause additional knife penetration
as required to maintain the seal.
The toggle-clamp mechanism (Figure 5) selected is ideal for applying
force. It is simple, fits the space available for clamping movement, and attains
its greatest clamping force as it approaches its 180-degree stop position. The
corresponding force imparted to the eccentric is only about 1.1 kg (2.5 lb)--a
force easily achieved with the drive system used. Additionally, by selecting a
flexing arm design and using it as a spring for the top and bottom clamp arms,
the required uniform spring rate is attained to maintain the seal throughout the
oven heating cycle.
PRIME MOVERS
Actuation of the pyrolyzer mechanisms is performed by two geared hystere-
sis synchronous motors enclosed in beryllium housings. One motor, rotating at i
approximately 8 r/min, drives the oven-seal clamp mechanism. The second motor
drives the soil loader at 150 r/min. An electric clutch with a reducing gear
train couples the second motor to the Geneva drive for indexing the oven carriage. °_
The selection of this type of prime mover permits a low profile package
with a relatively high stall and high drive torque; the latter is needed especial-
ly for the clamping mechanism. Reduction of the rotor speed down to the output
shaft is achieved by an integral gear train.
l
Potential contamination from the motor into the pyrolyzer assembly is
prevented by using chevron-type seals at the drive shaft exit. Since this type
of device seals well, balancing of internal to external pressure is achieved
through a 2-pm filter built into the lower end of the motor case.
OTHER PROBLEMS
Contamination control requirements for the final assembly were extremely
stringent. During the cleaning procedure the dry lubricant applied in small
quantity to the gears and ball bearings did not survive the original cleaning
process applied at the White Sands clean room facility. This process was
eventually modified for particular components.
The size and weight requirements for the SSLPA necessitated the use of
such materlals as berylllum, titanium, and magnesium. Custom minlaturization of
electrical connectors, plumbing components, clutches, and motors was also
required.
212 JPL Technical Memorandum 33-777 *
"197602"1"184-2"19
! i I ' t
f
During thermal vacuum testing, Vespel* shrank up to i0 percent and future
design efforts were required to take this unexpected performance into consideration.
CONCLUSION
f
The mechanisms described, although not unique, have been implemented to
meet the system environmental and performance requirements. The problems of
lubrication, material limitations, and severe environments made the design a
challenge. Tile final design takes advantage of these simple mechanisms with
relatively few moving parts to maintain a highly reliable system. The extensive
testing program, which included soil loading with various soil models, oven
" sealing after loading, and environmental testing, has validated this design
approach. There were several significant outcomes of this design and testing
effort. First, the ability to elimil,ate conventional lubricants when compatible
materials are used as wear surfaces was tested. Even though extensive cleaning
was performed on the pieces and the finished assembly, no "cold welding" or
galling problems were encountered during testing. Second, it is possible to
desig[, precision mechanisms that are tolerant of particulate contamination, such
as excess soil. Third, the repeated assembly and disassembly of the unit in the
clean room can be accomplished without impact on the system performance.
*Registered trademark of duPont. !
FUNNEL
{RIAGE ,-:
i
OVEN_
/
!
GENEVAWH _-
TOGGLE-CLAMP
------ MECHANISM
ELECTRICCLUTCH
:- GENEVADRIVER :,
C
--GEARED MOTORASSEMBLIES _,
i 'Figure la. Diagram of Surface-Sa=ple Loading and Pyrolyzlng ,::_
:._ Assembly ($SLPA). _ ;
_ JPL Technical Memorandum 33-'/77 Z13
1976021184-220
l!
It
Figure lb. Complete SSLPA Without Cover
>
°
REPP,X)DUCIBILITY ()pT'r, :
ORI(XNALPAGE IS POi)K '-;
214 JPL Technical Memorandum J'3-777
f-
i
1976021184-221
BEARING /_ HAMMERCAM/
POKERSUPPORT
HAI_MER -,_-__
.\ BALL BUSHING
FUNNEL DRIVE PINION__'C_\,_,._ FIXED POSITION
FUNNEL DRIVE SPUR-----=_ BEARING
POKER DRIVE HELICAL
DRIVE HELICAL CONNECTOR ROD
BEAF BALL BEARING ENDS
CLUTCH ONE _ BEARING
MOTOR
SHAFT
Figure 2. Soil Loader Mechanism
>
} _ JPL Technical Memorandum 33-777 215 ::
i
1976021184-222
i" _() "_4
, _. __
Figure3a. Oven Carriageand IndexingMechanism
OADER WI
PREHEAT
STATION--.,,,,/_
FLOW 2 GAS
_o_l__'-_ RESTRICTOR
(WEN _-
CARRIAGE '_r? rv GAS
_' CHROMATOGRAPH
Figure3b. FunctionalSequenceDiagram _-
Z16 [_:_'RODUCI]_[LI_Y 0_' '_'_ ,] PL Tech,,dcal Memorandum 33-777 *_
t ....... ...... ]
1976021184-223
fI
<N,,_ 1 _<N,,E
GOLOO,sc__. _ __I<,ART,CLE
OVEN_ _ _ --]"-GOLDDISC"
CASE _
Figure4. Oven SealingConcept
TOP
i /PIVOTS CLAMP I
• i- I ! /ECCENTRIC
' _ _LINKAGE
i': _ASSEMBLY
_--: "LOWERCLAMP
_ Figure5. Oven-SealClamping Mechanism
_- JPL TechnicalMemorandum 33-777 217 ;
L _
1976021184-224


Viking GC/MS mechanisms design and performance

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