A signal producing unit is responsive to {HEAD ~ the measuring unit} for producing a control signal for controlling the operation of {TAIL ~ the second drive unit} while the first drive unit is operating 
A signal producing unit is responsive to { HEAD ~ the measuring unit } for producing a control signal for controlling the operation of { TAIL ~ the second drive unit } while the first drive unit is operating
OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH