Comparison of Actively Leveled Test Platforms

The most level stable test pads now available utilize a platform which is slaved to leveling accelerometers. If, in addition to accelerometers, gyros are used, the resulting active pad will have performance substantially better than the corresponding semiactive pad. In order to bring out the important characteristics of active pads, they are compared in detail to passive and semiactive pads. This is done by first considering the functional characteristics of the fore-going pad systems. Comparison of the systems is then made in terms of numerical examples. Finally, control loops for active test pads are considered in detail. Considerable attention is given to the problem of platform motion produced by high-frequency gyro noise. As well as treating the problem of gyro noise analytically, it is considered in terms of several numerical examples. These examples pertain to active pads both with and without the incorporation of gyro bypasses. It is shown that gyro bypasses, mechanized using tachometer generators, can significantly improve pad performance with respect to gyro noise.