Research Output
1 - 7 of 7
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Publication
Wave-Echo Control of Lumped Flexible Systems
2006-12-22, O'Connor, William
An elegant, generic solution is presented to the problem of point-to-point control by a single actuator of a remote load through an intermediate flexible system, modelled by a system of lumped masses and springs. It is based on new ways of looking at the problem that respect and exploit the underlying dynamics. Under wide-ranging conditions the strategy allows rapid, almost-vibrationless, precise position control of the load, independently of the order of the system, without the need for a detailed system model or ideal actuator. During the start-up, the system itself reveals to the controller how to terminate the motion, so that the real system also acts as the model for the controller. The scheme is very robust to modelling, actuator and sensor errors. The strategy is presented, with some of the motivating ideas reviewed.
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Multibody domain decomposition for parallel processing: a wave-based approach to handling interface dynamics
2012-05, Smoothey, Craig, O'Connor, William
For many good reasons there is growing interest in ways to allow parallel processing of multibody dynamics problems. Some recent approaches include “Domain Decomposition” and “Divide and Conquer”. This paper explores a new approach, reported as work in progress, with initial, promising results. The strategy is an extension of work done on wave analysis of lumped systems in another context. In the approach, a larger system is subdivided into smaller subsystems, which are solved in parallel. Interconnection points are boundaries for each. Dynamic coupling across boundaries is handled in terms of transmitted and reflected motion components (or "waves"), in both directions, across the boundaries.
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Wave-based control of under-actuated flexible structures with strong external disturbing forces
2012-05, O'Connor, William, Habibi, Hossein
Wave-based control (WBC) of underactuated,
flexible systems considers actuator motion as launching a mechanical wave into
the flexible system which it then absorbs on its return to the actuator. The launching
and absorbing proceed simultaneously. This simple, intuitive idea leads to
robust, generic, highly efficient, precise, adaptable controllers, allowing
rapid and almost vibrationless re-positioning of the system, using only sensors
colocated at the actuator-system interface. These wave-based ideas have already
been shown to work on simple systems such as mass-spring strings, systems of
Euler-Bernoulli beams, and flexible space structures undergoing slewing motion
(rotation with lateral translation). The current work extends this strategy to
systems experiencing external disturbing forces, whether body forces which
endure over time, such as gravitational effects which change with system
orientation, or transient forces such as from impacts or external viscous
damping. The revised strategy additionally provides robustness to some sensor
errors. The strategy has the controller
learn about the disturbances and compensate for them, yet without needing new
sensors or measurements beyond those of standard WBC.
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Wave-based control of non-linear flexible mechanical systems
2009-07, O'Connor, William, Ramos de la Flor, Francisco, McKeown, David, Feliu, Vicente
The need to achieve rapid and accurate position control of a system end-point by an actuator working through a flexible system arises frequently, in cases from space structures to disk drive heads, from medical mechanisms to long-arm manipulators, from cranes to special robots. The system’s actuator must then attempt to reconcile two, potentially conflicting, demands: position control and active vibration damping. Somehow each must be achieved while respecting the other’s requirements. Wave-based control is a powerful solution with many advantages over previous techniques. The central idea is to consider the actuator motion as launching mechanical waves into the flexible system while simultaneously absorbing returning waves. This simple, intuitive idea leads to robust, generic, highly efficient, adaptable controllers, allowing rapid and almost vibrationless re-positioning of the remote load (tip mass). This gives a generic, high-performance solution to this important problem that does not depend on an accurate system model or near-ideal actuator behaviour. At first sight wave-based control assumes superposition and therefore linearity. This paper shows that wave-based control is also robust (or can easily be made robust) to non-linear behaviour associated with non-linear elasticity and with large-deflection effects.
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Travelling waves in boundary-controlled, non-uniform, cascaded lumped systems
2012-05, O'Connor, William, Zhu, Ming
A companion paper considers travelling and standing waves in cascaded, lumped,
mass-spring systems, controlled by two boundary actuators, one at each end,
when the system is uniform. It first proposes definitions of waves in finite
lumped systems. It then shows how to control the actuators to establish desired
waves from rest, and to maintain them despite disturbances. The present paper
extends this work to the more general, non-uniform case, when mass and spring
values can be arbitrary. A special ¿bi-uniform¿ case is first studied,
consisting of two different uniform cascaded systems in series, with an obvious,
uncontrolled, impedance mismatch where they meet. The paper shows how boundary
actuator control systems can be designed to establish, and robustly maintain,
apparently pure travelling waves of constant amplitude in either the first or
the second uniform section, in each case with an appropriate, partial, standing
wave pattern in the other section. Then a more general non-uniform case is
studied. A definition of a ¿pure travelling wave¿ in non-uniform systems is
proposed. Curiously, it does not imply constant amplitude motion. It does
however yield maximum power transfer between boundary actuators. The definition,
and its implementation in a control system, involves extending the notions of
¿pure¿ travelling waves, of standing waves, and of input and output impedances
of sources and loads, when applied to non-uniform lumped systems. Practical,
robust control strategies are presented for all cases.
Publication
Wave-based control of under-actuated flexible structures with strong external disturbing forces
2015-03-18, O'Connor, William, Habibi, Hossein
Wave-based control of under-actuated, flexible systems has many advantages over other methods. It considers actuator motion as launching a mechanical wave into the flexible system which it absorbs on its return to the actuator. The launching and absorbing proceed simultaneously. This simple, intuitive idea leads to robust, generic, highly efficient, precise, adaptable controllers, allowing rapid and almost vibrationless re-positioning of the system, using only sensors collocated at the actuator-system interface. It has been very successfully applied to simple systems such as mass-spring strings, systems of Euler-Bernoulli beams, planar mass-spring arrays, and flexible three-dimensional space structures undergoing slewing motion. In common with most other approaches, this work also assumed that, during a change of position, the forces from the environment were negligible in comparison with internal forces and torques. This assumption is not always valid. Strong external forces considerably complicate the flexible control problem, especially when unknown, unexpected or unmodelled. The current work extends the wave-based strategy to systems experiencing significant external disturbing forces, whether enduring or transient. The work also provides further robustness to sensor errors. The strategy has the controller learn about the disturbances and compensate for them, yet without needing new sensors, measurements or models beyond those of standard wave-based control.
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Boundary-controlled travelling and standing waves in cascaded lumped systems
2012-05, O'Connor, William, Zhu, Ming
This paper shows how pure travelling waves in cascaded, lumped, uniform, mass-spring systems can be defined, established, and maintained, by controlling two boundary actuators, one at each end. In most cases the control system for each actuator requires identifying and measuring the notional component waves propagating in opposite directions at the actuator-system interfaces. These measured component waves are then used to form the control inputs to the actuators. The paper also shows how the boundaries can be actively controlled to establish and maintain standing waves of arbitrary standing wave ratio, including those corresponding to the classical modes of vibration of such systems with textbook boundary conditions. These vibration modes are achieved and maintained by controlled reflection of the pure travelling wave components. The proposed control systems are also robust to system disturbances: they react to overcome external disturbances quickly and so to re-establish the desired steady motion.