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