ACTIVE DAMPING APPLICATIONS TO THE SHUTTLE RMS

Ccmtrol-Structure Interaction (CSI) is a relatively new technology developed over the last 10 to 15 years for application to large flexible space vehicles. The central issue is recognition that high performance control systems necessary for gotxt spacecraft performance may adversely interact with the dynamics of the spacecraft structure, a problem increasingly aggravated by the large size and reduced stiffness of modern spacecraft structural designs. CSI analysis and design methtxls have been developed to avoid interactions while maintaining spacecraft performance without exceeding structural capabilities, but they remain largely unvalidated by hardware experiments or demonstrations, particularly in-space flight demonstrations. One recent proposal for a low cost flight validation of CSI technology is to demonstrate active damping augmentation of the Space Shuttle Remote Manipulator System (RMS). This paper describes an analytical effort to define the potential for such an active damping augmentation demonstration to improve the structural dynamic response of the RMS following payload maneuvers. I1 is hoped that this study will lead to an actual inflight CSI test with the RMS using existing Shuttle hardware to the maximum extent possible. By using the existing hardware, the flight demonstration results may eventually be of direct benefit to actual Space Shuttle RMS operations, especially during the construction of Space Station Freedom. A sumnlary of the motivation for the proposed flight test is given along with the task relationships between NASA Langley Research Center, NASA Johnson Space Center, and Charles Stark Draper I,aboratories. The current approach to the active damping augmentation feasibility study tasks are summarized, and results from the initial linear analyses are presented. The results form the basis of the preliminary conclusions that the RMS could be used for an in-flight active damping demonstration using the SPAS payload, and that the only additional hardware needed on the RMS would be a small number of feedback accelerometers. Plans for continued analyses and verification of results using a nonlinear simulation of the RMS, which includes nonlinear joint gearboxes and Space Shuttle computer hardware and software models, are given. ACTIVELY AUGMENTRMSDAMPING TheControl-Structures Interaction(CSI)programatNASALangleyResearchCenterCLaRC) is dedicated to the development, application, and validation of new technologies for the control of large spacecraft systems which have significant structural flexibility. An important goal of this program is in-space flight tests to demonstrate quantitatively the benefits of CSI technology. One such proposed inflight demonstration is to actively augment the structural dynamic damping of the Space Shuttle Remote Manipulator System (RMS) arm, which currently exhibits low damping and long periods of oscillatory motion following routine operational maneuvers. This demonstration would provide a direct quantitative measure of the benefit of CSI technology as a part of the CSI program, while also measuring potential performance improvements in the current RMS which could ultimately have a significant impact on the assembly of Space Station Freedom (SSF). This paper will describe an ongoing analysis effort at LaRC to determine the feasibility of providing active damping augmentation of the RMS following normal payload handling operations. The flight demonstration effort is motivated by a study completed by Charles Stark Draper Laboratory (CSDL) [1-2], which proposed using the Shuttle RMS for a CSI flight experiment. The flight experiment study proposed adding additional sensors to the arm, the installation of a flight experiment computer and hardware in the Shuttle cargo bay, and the use of an instrumented payload at the end of the arm to measure performance. However, the current flight demonstration feasibility study is restricted to the use of existing RMS hardware only if possible, and the minimal addition of new sensor hardware only if necessary. The use of an instrumented payload would be retained, but the flight experiment computer and hardware would be eliminated in favor of using the existing Shuttle General Purpose Computers (GPC's) for control law implementation. The demonstration feasibility study is considering active damping control laws for use in the time period following the end of arm-move commands and the beginning of the normal arm position-hold function, although active damping of arm motion following Shuttle thruster firings is also a possibility. ACTIVELY AUGMENT RMS DAMPING Proposed inflight demonstration of CSI technology: • Quantitative measurement of CSI technology benefits • Improve current RMS operations • Potential benefits for SSF assembly E;caled-down version of CSDL experiment definition • Use existing hardware if possible, minimal additional hardware if necessary • Actively damp between move command and position-hold functions, possibly after STS thruster firings • Cast as development test to improve existing Shuttle hardware RMS-BASED CSI FLIGHT DEMONSTRATION The chart below summarizes the history of the proposed RMS CSI flight demonstration efforts. The original CSDL study [1-21 was completed in the period of December 1988 to June 1989. A study of CSI technology benefits for the assembly of Space Station Freedom was conducted by McDonnell Douglas Space Systems Co. from April to October 1989 13]. This study determined that approximately 10 hours of cumulative time would be spent over 15 SSF-assembly Shuttle flights waiting for arm motions to damp down to + 1 inch amplitudes following maneuvers with SSF components. The study also showed that a simple increase of two in the inherently small level of damping or the arm could reduce the cumulative settling time to 4 hours, a reduction in time approximately equal to the programed arm-operation time on a single assembly flight. This study became a Fine motivator for the proposed flight demonstration. Also during 1989, LaRC consulted with the NASA Johnson Space Center (JSC) about a potential RMS-based flight demonstration, and following the McDonnell Douglas study results, a joint LaRC/JSC planning effort leffto the cmrent effort. The feasibility study has been ongoing since April 1990 and is scheduled to last until April 1991, at which time a decision to proceed to an actual flight test will be made. RMS-BASED CSI FLIGHT DEMONSTRATION History Dec '88 June '89 Draper Lab hardware implementation and feasibility study (oA) April 'g9 Oct '89 Mc3onneU Douglas SSF assembly benefits study Spring '89 Nov '89 Mar '90 ====== JSC consultation JSC/LaRC joint planning April '90 April '91 LaRC controller design feasibility LaRC/JSC BRIDGING PROGRAM The joint LaRC-JSC RMS flight demonstration effort, referred to as a "bridging program", is divided into four tasks as shown below. The first two tasks, determination of feasibility using existing hardware and, if not feasible, the definition of the minimal set of additional needed hardware, is an LaRC activity. The third and fourth tasks, ground-based evaluations and the actual flight test, are JSC responsibilities. The decision to proceed with the flight demonstration will be made jointly. The Charles Stark Draper Laboratory, under contract to JSC, is and will be assisting with all tasks in the program. LaRC / JSC BRIDGING PROGRAM TASK 1 : Determine active damping control feasibility using EXISTING hardware TASK 2: Active damping controller design with MINIMUM hardware changes TASK 3: Ground evaluation of active damping control TASK 4: On-Orbit Demonstration