Balloon-borne equipment consisting of gamma ray telescope with array of particle detector

Anderson, G. A.

English

NASA Technical Reports Server

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P 
E Q U l  PMENT 
G e r a l d  A .  A n d e r s o n  
Schobl o f  Phys i  cs a n d  A s t r o n o m y  
Uni v e r s i  t y  o f  M i  n n e s o t a  
June  1967 
(THRU) I 
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FOR FEDERAL SClEN TlFlC ANDTECHNICAL INF~MAnON 
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I M A ,  Anderson 
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Presents t h e  design of a system to or ien ta te  
Pointing accmcy packa~e abut a vcs t ica l  miso 
a bJ2loan-bom 
obtained is + 1,5" a An 
u 
independent pho-topapfic recording system IT~~ZSLDXS the azhuth mgle w5.th 
respect to the star Polaris to  RI + O a 1 0 6  
against  a light ~ ~ 2 i g : ~ t  reaction wheel 
of irier+-Aao 
motor, 
hstrument package t,o be orientated with respec'c t o  the horizontal  campotlent 
of tne earth's f i e ld ,  
ira t he  north-south plaie and is t i l t ed  a t  an angle with the vertical. which 
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The h s t r m t e n t  package is rotated . 
which is designed for mimm mmmt 
A pdsc-width r r i u l a t i m  systmi is used t o  dr3.ve t h e  orimtzition 
A f1wrgat.e mgnetometer, used for  the sensor element, ca,uses the 
The instrument, a g;;ulcm-~-;ay telescope ., is oriented 
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will ccmsporid w L t h  the e l e v a t i m  of t he  suspected garrana-s;.iy source when 
the souxe crosses the Ucal mx5diane / I 
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One r q u i m n t  of many modern ballocn-borne ex;~er~qents  i  that the 
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orientation of the  instmonent package about a vertical axis be fixed with 
respect t o  a pxe-detehned set of cmrdimtesI A typical experbent having 
such a n q u b e n e n t  i s  a search for gamma-raV sources using a telescope 
tieveloped a t  the School of Fhysks and Astronomy, University of Hinnesota, 
Elis te lesape  consists of an array of p w t i c l e  detectors including photo- 
graphic n x l e a r  emulsions, scint i l la t ion counters, s p r k  chanbe r~~  and a 
The physical s i ze  of t h i s  detector a m y  is large canpared t o  the 
electronks pd Mazes * batteries and a u x i l l h r y  equiplent For t h i s  reason 
the en t i re  instnment package , contained i n  a pressurized cylindrical c o n t a h r ,  
appmxirmtely ,915 n diameter and 2,13 m long8 is hung at a Sxed  declination 
and c;rtiented about t he  ver t ical  axis so that  suspected souroes sweep through 
the apmmg angle of the telescope., I n  o w  suspension sy i tm,  the  cvihder 
is supported by a trapeza-type frant3worko with the  capability of t i l t i n g  the 
detector packa,:e at  angles of IOo t o  26O fm the  vertical, The ent i re  
packaye'welchs approximately 250 kgc The physical constraints on the  
orient.-itfor, s y s t a n  are that no p a r t  of the mechanisn is t o  ?rotrude h t o  the 
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field'cf v i a v  of the  detectm, and that the amount of qaterial above the 
bri:m-i:al plane cc,ntZinhg the  uppr end of the  detectors is mhinized,  
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- I  MECIMJICAL DZSICJ . '  
2ic Bbjectives of L t e  mechanical design of our system are t o  couple the . . 
i n s t n m n t  package t o  t5e balloon in a way that allows the bs tmnen t  package 
tc rotate aLcjut: the ver t ica l  axis with respect t o  t h e  balloon, t o  provide 
adequate strength to  withstand the  shocks of lamchhg,  parachute openbg 
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and landing, and t o  provide a mli  I 
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packageo We murited our reaction wheel above the inslrrarrent package and 
horpozlated the drive nrechanism which rotates the instrumnt package 
against the reaction w k l  into the suspension system, 
main elements of the suspension systemo The attachment to the balrocol is 
made by standard flexible steel cables and cable f i t t i ngs ,  
bearing in the top plate  allows the h s t r u m n t  package to rotate with 
, 
Figure 1 sbws the 
. .  
The thrust 
respect to the balloons 
the difference h lengths between the cables used to attach the instrunlent 
I f  the top plate is not perfectly hDrim-1 when 
The universal joint is used t o  Canpensate for  
age to  the  balroOn, 
the package is suspended from the balloo2, the  universal joint  w i l l  prevent 
The safety cables and safety p la t e ,  With its thrust bearing, a 
backup system for the universal joint; thev will support the h s t r \ r r r r P m t  
package i f  the shock of parachute 0penb-g breaks the universal joint ,  
The t i c a n  wheel is a li t l L 9  kg structure on the general 
&sign of a bicycle w The rim is made of 2,54 oil square 
h g e  The spokss are made of ,l6 art s t e e l  cdrcraf-t control 
control cable fittings used to seem the ends of 
the c&bs to made of la27 a 
ed by 2Oe3 F O l l O W h g  
bicycle wheel tm3.w spokes arranged so that they ane 
nearly tangentid t o  the hub at the points of attachment, in order t o  
transmit the maxirn LYC? to the J?im for a given tensian in the 
This shaft is threaded 051 both ends; the upper end screws into the bmcnn 
1 joint; a nut QA the 
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ath We top bigme 
Pa,, 1 , constitute a 25:1 speed reduction coupling between the m t c o ?  and the  
shaft, and enable the motor t o  mtate the reaction wheel with respect t o  t he  
hs t ru ren t  package,, Thus, the instrumnt package fm the  balloon and as 
the dr ive shaft for the orientation S y S t a e  
In i t i a l ly ,  several desijps for using the  balloon as a neactim wheel were 
considemd, 
paMChute, by which the instrunant package is lowered t o  earth at the termination 
These designs are i n c q a t i b l e  d t h  the requizxment that the  
of the f l ight ,  is t o  be used as the  coupling be=tween the  balbon and t he  h s  nt 
In t h i s  a m g e m n t ,  the shroud lines of the parachute and the instrument package, 
package consti tute a *multifilar' pendulum whose  free oscil lation frequency in 
. the i mode is given 
l/ 2 
w = W k )  Cg/U 
w h e r e  'd is the natural frequency, 'k' is the radius of gyration of the  
hstmnent  package, 'rr is the effective circle of sepamtion of the shroud 
l h s ,  OlS is the length of the shroud lines, and 'g' is the acceleration of 
gravity,, F'mm the  measured valuss of '1' 'k' , and 'rr the  m u l t i f i l a r  
pendulum fmquency for our system was calculated to  be ,4 radians/secand. 
The natural frequency of our orientation system, for a torque gradient of 
,25 newton--m/radian and a. mDment of h&i,a of 3544 kg-m2 is ,26 radians/seCand, 
close enough to  each other t o  create a difficult 
design problem i f  the balloon is us 
problem is  mads more difficult by OW inability t o  predict pax l~ tme te r s  such as 
as the lneaotlon w h e e l ,  The design 
air damping and bearing f r ic t ion  at t he  balloon opepating alt i tude,  By 
r i g i a y  coupling the  reaction wheel P, m0st 
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inherent design d i f f i c u l t i e s  can’hc avoided, 
we found t h a t  it is h p o r t a n t  t h a t  t he  reaction wheel be coupled t o  t h e  
balloon and t h a t  the instrument package be cowled to  the reaction wkel, 
I f  the  reaction wheel is mounted on the  bottom of the instmrme-nt Wckage, 
and the orientat ion svsten attempts to’ or ien t  the i n s t r w - n t  gackage, the  
In  the  develoyrent of OUP &sip 
f r i c t i o n  of t h e  t h r u s t  bearing between the  balloon and t h e  instrument 
package W i l l  cause t h e  parachute shroud l i n e s  to  twist and stsre energv, 
away fmri t h e  proper or ientat ion,  
f r i c t ion le s s  bearing, t h e  system of the  reaction wheel., instrument padkaqe 
an? rmtor would be hcapable of acqdr ing  a v  angular nor;lcntm with resnect 
t o  an external reference, 
w i l l  couple t h e  b s t r u n e n t  -oa&age t o  t he  k l l o o n  which i n  turn 5s cowled t o  
t k  ea r th  throuph aercxl:mamic f r i c t ion ,  
wuld acquire an,?;ul;u? m0r;lentUrn when t he  motor rotates the  reaction r - d - ~ e e l ,  
and the systen could onlv diss inate  t h i s  ?-r.omntun t!iroqqh the  f lex ib le  
If t he  th rus t  hearin2 was an i d e a l  
l i ~ t - ~ ~ e ~ ~  the  nonlinear fri.ction of the hcarin? 
Under these conditioris the svsten 
coupling of the r>arao\ute shroud l ines  t o  the  balloon, 
otbw hand, motor tbrque overcones the f r i c t ion  of the bearins betwen the 
instrument package and the  reaction wheel throuch a r i g i d  C O U ~ ~ F ~  a?d the 
In  our design, on the  
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system doe? not acquire am? angular momentum. 
f r i c t ion  in the  bearin? between t h e  react ion wheel and the bcilloon ~mvides 
I n  our sisten, i n  fact-, the  
a smll anount of cou?liny: t o  t5e balloon, and helqs t o  &.ssi->ate anv 
angular mornenturn t h e  tern m v  have acquired, 
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j -  servo syst~.ji, '(3' is the  e m r  angle, and 'T1 is  t h e  dumtion of t h e  fixed 
where 'k' is a constant dependent on the mdel of t h e  magnetorzeter, ' 0 '  is 
the angle bctmcn t h e  ax i s  of the  sensor and t h e  napet ic  fie16 vector, and 
were enccuntered i n  using t h e  s inz le  axis mgnetomter ,  althr>uy"n the ax i s  of 
the sensor wcis horizontal ,  and therefore t h e  e m r  signal m s  ?roportional t o  
the anl:lc between t h e  axis and the 5orizonta.l comonent of the ea r th ' s  rnqpeti? 
field,  
Because t h e  error s igna l  is a low amplitude dc signal, a Eiffercnt ia l  t r m s k t o r  
p a i r  is used for gmd ternpemture s t ab i l i t y .  In ? r h c b l n ,  seveml  other  inputs 
The cmr signal Fmn t3e ncy,netomtc-r i s  feci i n - b  a rdxn,r <ml i f i e r ,  
could be fed t o  tlie mixer-amplifier, suc'? a s p t h e  out7trts of rate svms, 
f ,  
techorneters, and o ther  velocity-dqendent s ignals ,  t o  provide damping, or 
propmminp; signals t o  cornpensate for the change i n  mgnct ic  vzriatiori, 
basic ti.?ing c i r c u i t  for the  systen is a sirmle mijunct ion re lxmt ion  
oscillator, fo l lomd  bv pulse-shaping c i r c u i t s  which arovide Imth posi t ive and 
negative puls2s a t  500 Hz, 
The 
The timer pulses charge the  integmtinE capacitor 
i n  the llillcr integrntor  m p  generator* The out.;)ut of the  !?iller integrator 
is fed t o  a phase-spli t ter ,  whkh provides twc t - u ~ ~ s  of naarlv equal an@i.tuck 
and opposite polar i ty ,  An atljustable section of the  enitter resistor i n  the 
phase s p l i t t c r  controls the sa twu t ion  voltaze of t h e  ?has? mlit ter ,  md 
also the ef fec t ive  deadbcmd of t?i? ws t enc  
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The rarnp voltages an2 compared t o  the e m r  sip31 i n  two pwal l e9  
nators, The C?d d i s c r i n b a t o r  w i l l  pmvide an output pulse i f  the 
s igna l  4s pcjsitive, and t h e  GCbl c3kximinator will provide an output ;>ulse 
i f  t??e error s igna l  i s  negative, Althmgh one discriminator could hzve been 
used with a t r iangular  wave reference, t he  maxir;lu;n duty cycle ?ossi.ble m u l d  
have been 50 percent; o 
100 percent, 
is fed i n t o  a Schmitt trigger circuit; the regenerative feedhack and hvstercsis 
of t h i s  circuit- ensures tha t  t h e  output s igna l  w i l l  defini te ly  be i n  either 
t h e  $1' (posit ive) state, when t h e  rump voltage is less than the  e w r  
signal;  or the 
t he  e m r  v o l t q e ;  with a minirnUm of switching time, 
c i r c u i t  i s  fed to  a phase s p l i t t e r ,  v7hich nrovides b t h  the s i p d  and i t s  
corqfl.er,Qnt t o  t h e  various logic eimuits, 
crimj.nator pulse t r i pge r s  t h e  Eixed ?ulse generator, 
cliscri.minator, t he  fixed pulse generator, and a prntecthm or enable f l ip -  
flop circuit are comhined i n  a log ic  c i r c u i t  to  .lirnduce the  input pulse t o  
t he  mtor  dr iver  circuit, 
system can a t t a i n  a =imm C I U ~ V  wcle of neaply 
';tie output of the d i f f e ren t i a l  amplifier i n  the discriminator 
(negative) state, when the  mm? voltage i s  g e a t e r  than 
The output of this  
The t r a i l i n g  edge of the  dis- 
&e outputs of the 
The Boolean function of t h e  logic circuit is, ( f o r  a posit ive ermr signal): 
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By I)e!llorgan's theorern, t h i s  expression can be simplified to:  
FIXCD f Cvl DISCRIIItiATOR) c;J EIdAE3I-E 
Thus, for a pulse to  t h e  mtor driver to  be Zenerated, t h e  enable f l j p f l o ?  m u s t  
be i n  the  proper state ilnd e i t h e r  t he  &scrimhator oy the ?%xed D u l s e  generator 
must'be in the $1' s tape  
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The enable f l ip-f lop has t h e  inGortant fundtion of ensuring t h a t  o d v  / 
one 'polar i ty '  of drive pulse to the motor drive c i r c u i t  i s  pmduced i n  any 
one cycle, That is, i f  noise causes the  discriminators and the  fixed pulse 
generators t o  nroduce simultaneous pulses which muld  drive the  m t o r  i n  hotb 
directions at the same the, t h e  enable f l ip-f lop allows onlv one direction of 
roto? drive at  any one t i n e ,  
by the  negative-going t r a i l i n g  eqTe of t he  d i s c d e n a t o r  pulse which has the 
The state of the  enable fl ip-flo? i s  changed 
greater  length and, thexefol-e, more e m r  signal  component. 
The motor dr iver  circuit consists of  a simple bridge c i r C G i t  usinp, EJ? 
and I"4R.J t r ans i s to r s  i n  complemntary syrrmetq, Reversed-bias diodes are 
connected across cadi t r ans i s to r ,  col lector  t o  emit ter ,  t o  clamp any voltage 
t ransient  generated by switching off the current through the  r o t o r  wind<ng,s6 
I n  such a bridge, great  care must be taken tha t  only those two  transisto13 on 
opposite s ides  o f t h e  motor conduct a t  any one tine, 
sa thf ied  bv the e n a b k  fl ip-flo?,  and bv the  design tha t  requi res  p o s i t i w  
base drive f r o m  an external  s o m e r  This desipn also ensures that t h e  bric,ye 
w i l l  not destroy itself i f  the regulated sup3ly voltages t o  the  logic  are not 
This ?,recaution is  
present 
The operation of t he  pulse-width mdulcation can best be understood by 
t racing the signals through a cycle With t h e  &d of t h e  tirxinp &%?ran 
(Figdm 3) 6 
I n i t i a l l y ,  assune a negative e h r  s i p a l  wxch is inverted bv the  r 5 x e p  
The t h i n g  o s c i l l a t o r  pulses and resets the rtm-ps t o  zero, aq l i f i e r ,  
the nvnp voltage is locxtr i n  amplitude than t h e  e m r  voltage, t h e  Cf dis- 
Because 
crb5:rator i s  i n  the *l* state, 
equals the arnplitutle of the error signal the &,scr$nrhator siqitclEs t o  the  
Assu&ny! 
:3-en the  m$5.-tude of thy ram? voltage 
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state 'and, i n  sv i tchhR,  t r i gge r s  the fi.xed pulse generator. 
tlie enable f l i p -  flop is i n  the' p r o p r  state, voltage w i l l  5e a?plied across 
the mtor windings as long as e i t h e r  the discriminator or the fixed pulse 
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TZXGIK RESUZITS 
The 1hiversrit-y of 1-liraedota g m - m y  telescope wad f low fmz the NCA.' 
fsci-l i tv a t  Palestine,  Texas, on 16  October, 1966 and again OQ 1 2  bcenbn,r, 
19G6, The orientat ion system was t m e d  on hv ground comm.d Vhen t h e  balloon 
reached i ts  ce i l ing  of 140,000 Et, Orientation tias achieved t7ith.b 33 seconds 
and held to  within - + 1&5* 
opientzti.on &rection was monitored by t m  35 inn merzs b7hiCh were munted 
on the ir,strmmt packace so tha t  when orientation was a.chieved the s t a r  
i)ol;iris was i n  the  field of view of The caierasI 
careras the fi lm tn-iveled continuously, i n  a horizontal direction, to  record 
4 horuls, t he  duration of the  f l i ph t ,  The 
I n  one of these shuttet.fcsr; 
Yir? horizontal scale i n  F j q R  4 is the actual f<eltf of v k w  of the  
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I camra, 
e m p  a@es and low p,eogra?hical la t i tudes  , aprroxirwtelv 
The a c t u d  azimuth emr fmn the center of t h e  film is, for s ix11 
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A f i e ld  of view 
A a z h u t h  = a 
film, w5kh t rave ls  a t  a l i nea r  velocitv of 4,7 crzlciin, I i s  Koclak Linacmph 
Shellburst, Estar Base, 
The University of Ximesota ~~WIEL- 
!JASA Contmct SC24-005-050(122) I )  
0:3R cont-nwt 7?onr-710(60), 
t o  Captai-n 'I, C, Flay, IJSAF, for his assistance i n  the bes ip  of the orientation 
system; t o  Professor C, J, !.laddh@m, for his  crj-ticism of t h i s  nanuscrint; 
md t o  the shop personnel of the  School of Physics and Astmnomr who 
Balloon f l igh t  servicks ~7eyy: funded by 
The authop wishee t o  express his apnreciation 
constructed the electronic and mechanical components of the or ientat ion 
system, 
'r 
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Figure 1: 
FQyre 2 :  Orientatim circuit, 
Ilechmical d e s i q  of suspension system, 
for the orientation circuit a 
f r o i n  azimuth mnitorinp, camrar 
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m 
m 
I- ._, ,.. - .- ... .- _. . - .- .. - - . . - .. . _ _ _  . . :. -. .__ 
OSC 
ERROR 
LLATOR 
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S I GNAL 
FIGURE 3 
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Design for orientation of balloon-borne equipment

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