the stepper motor sp ecifications will result in pe rformance difficulties or failu res later on i n the project.  Followi ng thi s process is (1.b), Evalu ation an d Itera tion of the stepper moto r to meet the requireme nts  developed in (1.a). Developing the sp ecifications in (1.a) will depend si gnificantly on trade -offs associat ed  with the selection of the mech anical reduction (pla netary ge ars or h armonic drives typically.) Beca use  stepp er moto rs are fixed angle, sp eed sensitive, and torque limit ed, a careful  trade betwe en red uction  ratios a nd mo tor perfo rmance is n ecessa ry.  

It should b e understoo d tha t steppe r mot ors come in spec ific packages th at tend to restrict the step a ngle  size as a fun ction of diam eter. A large  angle stepp er will u sually come in a small di amet er pa ckag e.  Conve rsely, a small angle step moto r usually come s in a larger diameter package. Table 2 indicates fo r  referen ce step motor angl es a nd corresponding motor diam eters. Note that t he ste p angle is al so  depe ndent on the nu mber of motor phases. Th ere is a  style of in dustry stepp er referred to a s “Hybrid ” that  posse sses a small step a ngle (1.8 degrees) in a compar atively smaller diamet er an d so is “out-of-fa mily”  with the typical diamete r versus step a ngle si ze shown in Tabl e 2. This difference makes t he Hybri d 1.8  degree ste pper pop ular for some ap plications. 

Table 2.  Motor Step An gle and T ypical Diameter Relationship s 

Step Angle  (Degrees)     Diamete r Range (mm )   No. Win ding  Phase s  90.0   27.1 – 63.5 2 / 4  45.0   27.1 – 44.6 2 / 4  15.0   27.1 – 63.5 2  3.75   50.8 – 140 3  2.0   117 2  1.8 *   38 – 86.4 2 / 4  1.5   99 – 117 3  *1.8 deg ree stepper shown is of spe cial Hybrid config uration. 

A steppe r motor havin g a 2.0-de gree step an gle would need to  be pul sed at  a rate of 18 0 pulses per  second (pps) to a chieve  one revolution  in one second (2.0 d eg x 180 pps = 360 degree s of rotation per  second). By comp arison, a 90-degree stepp er moto r could  go through one re volution with only a 4 pps  excitation rate (90 deg x 4 pps = 360 degree s of rotation pe r second.) So, th e large r step angle  moto rs  can operate at slo wer rates but achieve gre ater speeds. But, act uator resoluti on is al so affected by the   step motor a ngle chosen, and th e 2.0 -degree stepp er will have  45 times sm aller angula r resolution th an  the 90-deg ree step per. Hence, cle ar trade-offs exist  betwe en actuator resoluti on, motor p ulse rate, and   motor to rque. While th ere are exce ption s, most sp ace-rated stepp er moto rs are  specified to  operate with  a maximum rate betwe en 200 an d 100 0 pulse s per seco nd, dep ending on the size an d voltag e input. 

Becau se changes in gear reduction  affect the  moto r ope ration  rates, u sually a trad e exe rcise occu rs to  determi ne th e optim um to rque and speed maximum s within the capability of the motor. Stepper moto rs  are uniq uely sen sitive to speed and  torque ran ges and do  not have a line ar speed-to rque curve. A si ngle  axis step per motor ha rmonic drive u nit like show n in Figure 2 is b ound ed by a maximum no -load speed   range or pulse rate and a maximum powere d holding to rque (comp arable with “stall torque.”) Thi s is  simila r to a brushle ss dc motor, b ut due to th e absence of commutation control, the perfo rmance curve  betwe en no-load speed and stall to rque is no n-linear, may h ave “drop -out regions”, a nd m ay even diffe r  122