Response Characteristics of the VACM Compass and Vane Follower

Abstract

Several simple laboratory experiments have been conducted to study the dynamic behavior of the vector-averaging current meter (VACM) compass and vane follower. They demonstrate that the behavior of the compass and vane follower can be modeled as a damped linear harmonic oscillator for small-amplitude forcing. The combined eddy-current and bearing-friction torque nearly critically damps the free oscillation of the compass and vane follower. This frictional torque is proportional to the angular-velocity difference between the instrument magnet assembly and housing. Dynamic experiments on five compasses indicate a mean (undamped) resonant period of 3?5 s at 41°N. Similar experiments on two vane followers indicate a resonant period of 2?3 s. For the VACM dynamic compass experiments, frictional torque allowed an angular oscillation of the compass housing to drive an oscillation of the compass magnet, and at resonant forcing, the compass magnet oscillates exactly in phase with its housing. For the VACM vane-follower experiments, angular motion of the vane magnet directly drove the vane follower. For resonant forcing of the vane magnet, the vane-follower oscillation overshoots the forcing slightly and is 90° out of phase with the forcing. The damped linear harmonic oscillator model suggests that a small-amplitude angular forcing of the compass or vane-follower housing (which may occur in the field due to mooring motion) should not cause any error in the vector-averaged headings. However, periodic angular oscillations near the resonant frequencies of the compass or vane follower could cause an error in the magnitude of the vector-averaged velocity. Forcing at frequencies lower than the well-defined resonant frequencies of the instruments should have little effect since directional errors do not exceed the angular resolution of the instruments at periods of ≥10 s.