Contributions to Date
- In 1982, the ARL was the site of the first-ever experimental
demonstration of quick, precise end-point control of a very flexible
structure. This landmark research by Eric Schmitz [CannonS:84]
has been an invaluable reference for all other research that has
followed in this exciting field.
- The control of a very flexible two-link structure is a complex
problem, extending the challenges addressed by Schmitz to include
non-linear geometric relationships. Once again, the ARL was the
site of the first-ever experimental demonstration of quick, precise
end-point control of a very flexible two-link
structure [OakleyB:90][Oakley:91][OakleyC:90][OakleyC:89].
- One key to the successful end-point control of very flexible
structures is a model that accurately describes the dynamics of the
structure. However, this is a very difficult task for a system as
complex as a flexible structure. The ARL has demonstrated
particular success at creating such a system model through the use
of physical insight and understanding of the plant as can be gained
only through interaction with an experimental
system [OakleyC:89a][OakleyC:89b].
A next logical concern in the control of flexible structures is to
consider the dependence of a closed-loop controller on the system
model. Research in the ARL has methodically addressed the problems
related to having various unknown system parameters, and the resulting
effects which that has on closed-loop system performance.
- Dan Rosenthal studied the sensitivity of non-colocated system
models for a representative class of flexible structures to
uncertainty in various physical parameters, and performed a
comparison of robust control design techniques. The results of this
work were demonstrated experimentally using a rotary
four-disk-and-torsional-spring system[Rosenthal:84][CannonR:84].
- Michael Sidman used this same rotary four-disk system to
demonstrate high-performance adaptive control in the face of a
destabilizing change in disk inertia. This work led to the
development of a ``bumpless transfer'' technique that allows on-line
swapping of feedback controllers without unwanted excitation of the
plant dynamics [Sidman:86].
- Dan Rovner explored the sensitivity to an unknown tip mass. His
experimental work led to fundamental advances in adaptive control
theory, including important new insight into the basic interaction
between adaptation and the realities of experimental applications,
such as sensor noise [Rovner:87][RovnerF:87][RovnerC:87].
- Lawrence Alder continued research in this area by studying the
sensitivity to an unknown dynamic payload. This work incorporated a
novel signal-processing technique in a first-ever experimental
application, and demonstrated, for the first time, the ability to
adapt to a payload that had unknown dynamics of its
own [AlderR:94][Alder:93][Alder:91].
- Due to the importance of having an accurate system model, the
ARL has focused a research program on a systematic means to model
arbitrary flexible structures. This work, begun by William
Ballhaus, employs a global understanding of the system to coordinate
the connection of modular subsystem models in a manner that leads
directly to the design of a high-performance end-point control
system. The technique developed has been demonstrated
experimentally on a two-link very flexible
structure [BallhausR:92a][BallhausR:92b]