Active Null Measurements

Abstract

In many applications where a parameter needs to be measured, available sensors are adequate in all respects, except that the range of variation of the parameter exceeds sensor range. For example, if a measurement of the angular coordinates of a flying aircraft needs to be carried out, a digital CCD camera is often appropriate, except for its field of view being to narrow to cover all possible positions of the aircraft in the sky. This shortcoming can be sidestepped by using a tracking arrangement like in Fig. 4.1, p. 37 to force the camera to follow the aircraft. As noted in Section 6.3, the correction signal is a measure of the disturbance, which in this case is the motion of the aircraft. Measuring a free-running parameter xfr.consisting of the plant output polluted by a disturbance, by sensing it, using the sensor in a FCS where an actuator nulls the value ofx _fr and using the correction signalx, as a representation ofx _fr is known as active null measurement. The system is set in the nulling mode by setting e _r = 0. Another situation where an active null measurement may be useful is when the sensor has a strong nonlinearity, which makes calibration difficult. A better measurement can then be carried out if one can arrange for a more linear actuator and if it is possible to include all these parts into the same FCS.