Rolamite Joints for S pacecraft Subsystem Vibration Isolation 

Gregory A. Compton* and Matthew M. Botke* 

Abstract 

A one-de gree of freedom joint with uniqu e performan ce requi rements has be en designed an d built for use   in a passive  isolation sy stem fo r stru cturally-conn ected de ployable sp acecraft subsyste ms. Usi ng a   Rolamite ap proach, a nearly frictionle ss and zero spring rate hin ged joint can b e achi eved tha t is ca pable  of passin g significa nt ele ctrical power and d ata harnessin g. Impl ementin g a Rolamite join t in a sy stem  design with requi reme nts typical of spaceflight m ust be accompli shed in a very  judici ous fashion in order  to maintain t he expe cted perfo rmance benefits. Th e followin g describ es the d esign prog ression, trad es  and le sson s learned in d evelopin g and b uilding thi s mechanism. 

Introduc tion 

Rolamite de scribes a cla ss of singl e degree of free dom (DOF ) hinges with ki nemati cs similar to a gear  pair, zero ba cklash, ideall y zero spring rate an d very low frictio n. Joint s of this type are comprised of two  flexible elem ents (tape s or cable s) arranged  in opposing serpentine fa shion around  a pair of cylinde rs in  rollin g conta ct. Figure 1 ill ustrates the element s of a Rolamite joint . The un strained fre e state of each tape  is straight. T he tape routing enfo rces j oint kin ematics and m aintains constant store d strain energy in th e  system. Th e need fo r a zero-sp ring-rate , low-fri ction hinge had been ide ntified as the f unctional ele ment  of a passive vibration i solator for structurally conn ected spa cecraft subsy stems such as solar arrays a nd  reflectors that  have hi gh m ass properti es relative to the host. The i solatio n system is illustrated in Figure 2.   A chain of six near fri ctionless an d zero-spring-rate joi nts is arranged to yield full relative mo bility between   the su bsystem and host. By eliminatin g sources of energy sto rage an d dissipation, the o nly transmitte d  forces are th ose due to the in ertial fo rces (D’Alam bert force s) of the isolation  system it self. Therefore,  induced jitter on and a gility of the host is de coupled from the sub system mass propertie s and become s  solely de pendant on isolat ion sy stem dynami cs and operation. 

Since the ide al isolation sy stem is ze ro sp ring rate and nearly frict ionless, syst em mass wa s identified as  a sig nificant perfo rmance driver early in  the de sign.  As typical sp acecraft requirements we re impo sed on  the syste m, achi eving zero sp ring rate and mini mizing fricti on b ecame at lea st as im portant as ma ss  propertie s. Impleme nting a Rolamite jo int that is stif f in the remai ning five degrees of free dom, able to be  driven, lo cked for safe m ode, and carry  the requi red electrical harnessin g whil e maintainin g the de sired   perfo rmance presented ma ny desi gn challeng es. A p oint de sign was o ptimized that inclu ded an assume d  subsystem wi th mass p roperties and stif fness to be is olated from a  typical large host bu s. Th e first mode  natural freq uency of the subsystem to be isolated was used to  bracket acceleration s of the isol ation  system withi n an operati ng envel ope. A system mass budg et wa s impo sed and D'Ala mbert forces  calculate d. All other contri butors to  transmitting l oads to th e host such as source s of en ergy storage  (sprin g rate ) and en ergy dissi pation (friction) were lim ited to a fracti on of the pre dicted D'Alam bert force s. 

A Rolamite joint engine ering unit com prised of syn chronizing tapes an d harness sim ulators has be en  fabricated as a pro of of concept an d is curre ntly un der te st. The  desi gn tra des and lesson s lea rned are  presented in this di scussio n. 

                                                            * ABLE Engineeri ng, Golet a, CA 

Procee dings of the 37th Aeros pace Mec hanisms Sy mpos ium, Joh nson Sp ace Ce nter, May 19-2 1, 200 4  163