An Angular Accelerometer for Guidance

Abstract

H velocity aircraft and missiles are frequently guided by controlling the angular velocity of the control surfaces. To accomplish this guidance effectively, the primary element of the auto-pilot servo should detect one order higher than angular velocity, that is, angular acceleration. The detection of this angular acceleration is tradionally achieved by employing a rate gyro with a differentiating network in the output of its pick-off. Much simpler is to use an angular accelerometer directly to override the rate gyro, thus eliminating the necessity for a differentiating network; but past experience with conventional angular accelerometers has not been encouraging because of their undesirable sensitivity to linear acceleration and to angular velocity. Both of these handicaps have been overcome in a radically new invention which has been developed at Statham Laboratories. Fig. 1 shows the essentials of a Statham angular accelerometer. The novelty of this angular accelerometer is that its rotor consists of liquid only. When the case is subjected to an angular acceleration, the inertial forces will cause the liquid to rotate and develop a hydrostatic pressure against the paddle as the liquid is forced between the paddle and the baffles. The paddle will deflect angularly until the elastic restraint of the flexure and the pick-off balances the forces on the paddle. This novel construction has been adapted to both unbonded strain gage and inductive pick-offs. The advantages of the Statham liquid rotor angular accelerometer design are apparent after a careful perusal of Fig. 1. Since the liquid mass serves as the rotor, the weight of the paddle which must be supported along the sensitive axis can be made extremely low. In the construction of low range, rigid rotor angular accelerometers, because of the weight of the rotor, ball bearings are frequently employed to contain the rotor spindle. In the liquid rotor design, weak flexures are practical since only a small, rigid mass need be suspended.