The pressure sensor comprises either a door, {HEAD ~ whose movement} is measured without contact, or {TAIL ~ a sensor}, whose movement is measured optically 
The pressure sensor comprises either a door , { HEAD ~ whose movement } is measured without contact , or { TAIL ~ a sensor } , whose movement is measured optically
OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL REL REL OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH