I is the motor input cu rrent  Tloss is the s um of all internal moto r, bea ring, drag, etc. torque s. 

The stepp er motor torque cannot be cal culate d simply base d on Kt as with b rushless m otors, so m argin  to compensate for this mus t be im posed in the perfo rman ce analysis. Margin  factors fo r stepper moto rs  are u sed to either in crease the torqu e requirement (internal a nd external ) or to red uce the availabl e  voltage. It is common to ap ply establishe d margin fa ctors to the inte rnal an d external load re quireme nts to  arrive at a se t of speed an d torque conditions that the prop osed stepper d esign must meet (typically by   analysi s). If t he p erformance analy ses cannot show the d esign will comply, t hen it i s deemed to  have  insuf ficient margin to assure satisfactory test a nd fli ght op eration . The  margin  facto rs used are typically   those e stablished by the comp any designing the system or th e motor ma nufacturer supplying the motor  design an d will vary some what. 

The p rocess of derivin g motor requi rements from system or me chanism perfo rmance requi rements and  of  imposi ng sufficient ma rgin for the particula r me chan ism appli cation is esse ntial to prope r stepp er moto r  sizing. A co mpre hensive stepp er m otor sp ecification mu st include a n app roach that correlate s the  specification s, simulatio n and analysi s, and hardware testin g. Th e moto r specification approach o utlined  in Table 1 is suitable fo r guidi ng thi s process. Ho weve r, it wi ll need to b e tailore d to the spe cific  appli cation  and te chnical requireme nts. Table  1 is applicatio ns ori ented, di rected towards the  syste ms or  mech anism d esigner d esiring to utilize a  steppe r for a  space flight mech anism. Ty pically, the  system s or  mech anism engin eer will  requi re assi stance in wo rking throug h the detaile d level of eva luation a nd  simulatio n normally req uired for step per motors in sp ace application s.  

Table 1.  Guidelines for the Stepp er Motor Spe cification Proc ess 

(1.a) T ranslate System   and Me chanism Specs  into Stepper Motor  Requirements 

 DEFINITIO NS 

Key Derived Rating s:  Speed (Pul se Rates)  Load To rques  Inertial Loa ds  Step Angle a nd  Output Re solution  (Trade -off Analyse s)   LIMITATIONS  

Key System Limitations :  Unpowere d Holding  Maximum Output Torque  Pulse Width  Input Powe r  Drive r Circuit ry Scheme  Hard-stops CORRELATI ON 

Margi n Appro ach:  Herita ge Review  Torque Ma rgin  Voltage Marg in  Combi nation   [Ref. Table 3]  

(1.b) E valuat e and Iterate  Motor to Meet  Requirements   Key Motor Paramete rs:  Kt and DCR Ran ges  Detent To rque Ran ge  Pull-in and Out Torques  Rate Capability  Visco us Dam ping  Rotor Inertia     Therm al Limit ations:   Duty Time-On  Ambient Ra nge  Windi ng Tem p. Rise  Cold Start Drag Torque  Installation M ounting  Therm al Mass   Analysis a nd Test:   Test to Spec  with  Margi n only in  Analysis…o r  Includ e Margi n at  Testing 

(1.c) Simulate Dynamic   Perform ance and Confirm   with Ha rdware Testing     Dynami c Mod els:  Assess capability of  motor to meet s pecs ,  margi n, and d uty     Wors t Case Limitations :  Environme ntal Extremes  Electri cal Co mbination s  Mech anical Combinatio ns     Test Confirm ation:  • Motor Only  • Mech anism  • System 

Transla ting Sy stem and Mecha nism Specs into Motor Requi rements 

Translatin g system a nd mechanism specs into  specific motor re quire ment s is highlig hted in T able 1,  section (1.a ), and bro ken up into three parts: Definiti ons – Key Derived Rating s, Key System Limitations,  and Correlati on – Margin Appro ach. Failure to add ress any of these thre e area s duri ng dev elopme nt of  121