The drive unit is triggered when {HEAD ~ signals} of {TAIL ~ at least two adjacent sensors} are detected 
of
The drive unit is triggered when { HEAD ~ signals } of { TAIL ~ at least two adjacent sensors } are detected
OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH