Simon C. O. Grocott

Robust control design and implementation on the Middeck Active Control Experiment

This paper presents a coherent methodology for robust controller synthesis for the Middeck Active Control Experiment (MACE): a Shuttle program scheduled for flight on STS-67 in February 1995. The experiment has been designed to investigate the extent to which the on-orbit behavior of a precision-controlled spacecraft can be predicted and controlled using analysis and ground testing prior to launch. A goal of the flight experiment is to demonstrate good payload pointing performance using active controllers designed based on the predicted structural dynamics. For systems with complicated control topologies and large model uncertainties, this requires a systematic control design methodology. The results from preliminary ground-based control experiments are used in this paper to present such a design technique and to illustrate how it can be applied to future flight experiments. This control design methodology is then used to develop controllers that obtain a 22 dB improvement in the performance metric on the current MACE hardware.

Robust control of the Multiple Mirror Telescope adaptive secondary mirror

For force-actuated, thin facesheet mirrors, structural flexibility within the control bandwidth calls for a new approach to adaptive optics. Dynamic influence functions are used to characterize the influence of each actuator on the entire surface of a deformable mirror. A linearized model of atmospheric distortion is combined with these dynamic influ- ence functions to produce a dynamic reconstructor for providing actuator inputs in response to wavefront sensor measurements. This dynamic reconstructor is recognized as an optimal-control problem. A hierarchic control scheme that seeks to emulate the quasistatic control approach that is generally used in adaptive optics is compared with the dynamic reconstruction technique. Although dynamic reconstruction requires somewhat more computational power to implement, it achieves better performance with less power usage, and is less sensitive to errors than the hierarchic technique because it incorporates a dynamic model of the deformable mirror.

Finite element model-based robust controllers for the middeck active control experiment (MACE)

The middeck active control experiment (MACE) is a space shuttle flight experiment intended to demonstrate high authority active structural control in zero gravity (0-g) conditions based on analysis and ground testing. Finite element structural models are very important for the MACE program because they can be used to predict the on-orbit dynamics of the test article. However, finite element models tend to be inaccurate, requiring the use of parametric robust control techniques to achieve good performance. Several such control techniques and an overall design methodology are discussed in this paper. Experimental results from several tests are used to illustrate the feasibility of achieving good H/sub 2/ performance on the test article with robust controllers based on the finite element model. This demonstration improves confidence in the eventual on-orbit performance of this experiment and other future spacecraft.

Parametric uncertainty model for control design and analysis

An approach is presented that develops an accurate model of parametric uncertainty suitable for structural control design and analysis. This approach requires a number of different data sets to capture the dynamics of the system and its variability. Using an identification technique, the modal parameters of the finite element model are estimated. The parameter estimates are combined into a parametric uncertainty model in the form of mean errors and bounds on the modal parameters, which can then be used for robust control design and analysis. The development and utilization of this uncertainty model is demonstrated using the middeck active control experiment, a shuttle middeck experiment that flew on STS-67 in March 1995.

Verification procedure for on-orbit controllers for the MIT Middeck Active Control Experiment

A perturbation expansion on the structural generalized eigenvalue problem is performed which provides a means by which a finite element model and identified frequency and mode shape information can be converted to mass and stiffness matrix perturbations required to match the identified system. For a modified structure, similar to the original, the mass and stiffness matrix perturbations are assumed constant, and are used in a second perturbation expansion to generate predictions of frequency and mode shape perturbations for the modified structure. The stochastic nature of the structure and identification procedure enable first and second order statistics of the frequency and mode shape perturbations to be generated. These are used in a mixed-/spl mu/ analysis which provides a measure of the stability of the system in feedback with controllers. The mixed-/spl mu/ analysis is used to determined critical uncertain parameters which can be emphasized in robust control design.

Finite element model based robust controllers for the MIT Middeck Active Control Experiment (MACE)

The Middeck Active Control Experiment (MACE) is a space shuttle flight experiment intended to demonstrate high authority active structural control in zero gravity (0-g) conditions based on analysis and ground testing. Finite element structural models are very important for the MACE program because they can be used to predict the on-orbit dynamics of the test article. However, finite element models tend to be inaccurate, requiring the use of parameter robust control techniques to achieve good performance. Several such control techniques and an overall design methodology are discussed in this paper. Experimental results from several tests are used to illustrate the feasibility of achieving good /spl Hscr//sub 2/ performance on the test article with robust controllers based on the finite element model. This demonstration improves confidence in the eventual on-orbit performance of this experiment and other future spacecraft.

Overview of Closed Loop Results for MACE

Abstract This paper presents closed loop results and insight from the on-orbit experiments of the Middeck Active Control Experiment (MACE). MACE was flown in the shuttle middeck on STS-67 in March 1995 to investigate issues associated with a change in operational environment from ground to space based operations of a payload pointing spacecraft that cannot be tested in a realistic ground simulation. These results show that equivalent performance can be obtained using finite element based and measurement based control, the benefits available from testing a structure on the ground, even in a different configuration to that used on-orbit, and the limitations associated with various control topologies. These MACE results provide much insight into how future on-orbit closed loop experiments can be improved.

Experimental Comparison of Robust H Control Techniques for Uncertain Structural Systems

Conclusion This Note proposes a method of path planning for space manipulators that reduces disturbances to the spacecraft attitude. The proposed method uses the EDM for planning the manipulator path in the joint space. The method sequentially determines the direction of small steps of joint movements that compromises the biobjectives of minimizing the disturbance to the spacecraft attitude and realizing the terminal end effector position. Numerical simulations have been made for a space robot with a two-link manipulator. The results of the simulations show the feasibility of the present path planning algorithm.

Flight Results from the Middeck Active Control Experiment (MACE)

The middeck active control experiment (MACE) was designed as a Space Shuttle flight experiment to demonstrate high authority active structural control in zero-gravity (0-g) conditions based on analysis, ground testing, and on-orbit control redesign. MACE is a multidisciplinary project that is at the forefront of flexible structural control. MACE was first flown on STS-67 in March 1995, and a summary of the program objectives and mission experimental results is provided.

On-Orbit Closed-Loop Control Results for the Middeck Active Control Experiment

An in-depth analysis is presented of the closed-loop results and insights from the on-orbit control experiments of the Middeck Active Control Experiment (MACE). MACE was e own in the Shuttle middeck on STS-67 in March 1995 to investigate issues associated with a change in operational environment from ground to space for a payload pointing spacecraft that cannot be tested ina realisticground simulation. Theseresults show 1)equivalent performance can be obtained using enite element-based and measurement-based control design models; 2) the beneets available from testing the structure on the ground prior to eight, even if these tests are in a different cone gurationthanthatusedonorbit;and3)theperformancelimitationsassociatedwithvariouscontroltopologies. Theseclosed-loop results provide insight intohow future on-orbit closed-loop experiments could beimproved, and they also help build conedence in the on-orbit capabilities of future multipayload spacecraft.