A sensor drive signal is controllably perturbed and sensor sensitivity monitored by monitoring {HEAD ~ the sensor response} to {TAIL ~ the perturbation} 
to
A sensor drive signal is controllably perturbed and sensor sensitivity monitored by monitoring { HEAD ~ the sensor response } to { TAIL ~ the perturbation }
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL