The forcer is {HEAD ~ a periodic device} for incrementally moving in {TAIL ~ a normally open loop configuration} over a toothed base element or platen, and can be a linear or other stepper motor system 
The forcer is { HEAD ~ a periodic device } for incrementally moving in { TAIL ~ a normally open loop configuration } over a toothed base element or platen , and can be a linear or other stepper motor system
OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL REL REL REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH