Vehicle Angular Rate Research Articles

New Procedure for Deriving Optimized Strapdown Attitude Algorithms

A new procedure for deriving strapdown attitude algorithms is described and justie ed. This procedure allows optimization of the solution when the vehicle angular rate components are known analytically. It is based on Miller’ s approach, but it uses the analytical relationship between angularrate derivatives and does not require the derivation of an analytical expression for the error quaternion. The procedure was tested for both classical and more general conic motion, when vehicle angularrate components are described as theJacobian elliptic functions. It is shown that the coefe cients optimized for classical coning hold true for general coning as well but only if the ratecomponent peak valuesare properly specie ed and the third term of the rotation vector differential equation is taken into account. A new kinematically correct description for the generalized coning is presented, and two variants of the angular rate’ s representation, which meet this criterion, are derived. Finally, a statistical ree nement of the deterministic Miller’ s procedure (Miller, R. B., “ A New Strapdown Attitude Algorithm,”Journal of Guidance, Control, and Dynamics , Vol. 6, No. 4, 1983, pp. 287 ‐291), which imparts smoothing properties to an algorithm, is formulated, and an example of its application is presented.

Similarities Between Classical Celestial Navigation and Electrostatic Gyro Navigation

In the absence of torque, a gyroscope's spin axis will maintain a fixed angular orientation in space. This angular orientation is analogous to the fixed line of sight to a star. In principle then, one should be able to perform celestial navigation using gyroscopes as self-contained stars. Unfortunately, most current inertial auto-navigators apply torques to the gyroscopes. Ideally these torques are of just the right magnitude to cause the stable platform on which the gyros are mounted to stay locally level. In the case of strapdown inertial auto-navigators, the gyroscopes are torqued to follow the host vehicle angular rates. This results in the gyroscope becoming just an angular rate sensor, and the only measure of true angular orientation rests within the system computer. These mechanizations lead to concepts unfamiliar to many navigators familiar with celestial navigation. Electrostatic gyroscope (ESG) inertial navigation systems are unique in that they are normally operated untorqued. As such, the orientation of their gyro spin axes remain fixed in inertial space. This makes it possible to consider their operation in terms of classical celestial navigation concepts. This paper will develop this similarity to the point where one familiar with celestial navigation will be able to understand the alinement problem and the fundamental error sources of ESG inertial systems. A specific type of ESG employed at Rockwell International in both gimbaled and strap-down systems will be discussed. This will provide the basis for relating angle readout errors and drift rate to their counterparts in celestial navigation. The buzz words used in inertial navigation, such as Schuler tuning and Kalman filtering, will be related to concepts familiar to celestial navigators. The paper will be based on straightforward physical principles and the concepts presented without resorting to mathematical development.

Strapdown Gyros in a Back-to-Back Configuration

A back-to-back connection of two gyros per axis is suggested that eliminates some of the errors due to cross coupling between the components of vehicle angular rates and accelerations. In particular, the error due to output axis angular acceleration, the anisoinertia error, and the spin reference axis rectification error theoretically vanish. Use of redundant and identical strapdown systems for the purpose of obtaining higher reliability makes this connection practical.

A GYRO MOMENTUM EXCHANGE DEVICE FOR SPACE VEHICLE ATTITUDE CONTROL

A gyro control device is described which may be used in attitude control systems requiring momentum exchange devices. It has been shown that the use of two single-degree-of-freedom gyros per vehicle axis permits nearly independent control of the three components of vehicle angular rate. The new controller will employ only two gyro wheels and three gimbals to effect control of the three components of vehicle angular rate. A moderate degradation of transient response is experienced, due to increased cross-coupling between vehicle axes; but, on the other hand, significant weight and volume reductions are achieved. It is shown that the gyro controller is an extremely efficient device when its energy requirements are compared to those for a flywheel controller. Digital simulation results are presented to illustrate the time response of an attitude control system employing the advanced gyro controller.

Vapor jet control of space vehicles

This paper describes a reaction jet attitude control technique which affords significant advantages in terms of accuracy, reliability, fuel economy and operational flexibility. These advantages are realized by the use, in combination, of low-thrust vapor jets and time-dependent on-off switching circuits. An accuracy potential comparable to inertia wheel control is thus provided, while the proverbial wheel problems of speed saturation, bearing life, threshold nonlinearities, gyroscopic coupling and vibration excitation are avoided. Very-low thrust magnitudes are attained by simply opening a small orifice to allow fuel to vaporize into the surrounding vacuum. Fuel storage, pressurization, circulation and mixing requirements are thus minimized. By augmenting conventional on-off valve switching circuitry with electronic networks that generate thrust pulses of small but constant time duration, vehicle angular rate can be controlled to a very-low threshold. This minimizes fuel consumption and valve cycling frequency. The capabilities and limitations of this design approach were substantiated by an analog computer program incorporating breadboard switching circuits, and by vacuum chamber testing of critical components. These technique and component developments are applicable to such space missions as astronomical observation, earth reconnaissance and stellar navigation. Design guides are presented for synthesizing a reaction jet system to meet any particular set of performance specifications.