The project will address the fundamental problem of how to control the motion of a robot so that it can cooperatively work with humans to assist them in repetitive tasks. Oscillatory body movements constitute an elementary means for various tasks in human living. Such repetitive movements include essential life functions such as heart beat, breathing, eating (chewing), walking; basic daily tasks such as brushing teeth, washing face; house-hold chores such as cleaning windows, sweeping floor; health/entertainment activities such as dancing, swimming, cycling, rowing; and manufacturing labors such as moving objects in factory assembly lines. Robots and mechanical devices that assist such human movements would be found useful in a number of contexts. A robotic manipulator and a human arm may grab a common tool to work together on repetitive tasks where the former assists the latter by providing force and stability to reduce burden on the human. An exoskeleton may be worn to complement reduced capability of, or provide rehabilitations for, elderly people and patients with neurological disorders or physical disabilities. Thus, well-designed assistive devices for oscillatory movements would significantly contribute to improving quality of human life. Design of robotic mechanisms for such assistive devices is surely a challenging task. Equally challenging is the design of control algorithms that command the actuators and govern the motion of the robotic device. The state-of-the-art control technologies allow a designer to program a robot to achieve prescribed motion with speed, precision, and robustness, as seen for instance in industrial manipulators. However, if such robots interact with humans, they would be perceived as stiff, stubborn, or even dangerous, and are therefore not suitable as co-robots in direct support of humans. What is needed is control algorithms that make robots understand human intentions, cooperate with humans without insisting on their preprogramed operations, and assist with human tasks. Development of such algorithms will be the focus of this project.
This basic research aims to establish a systematic method for designing a feedback controller for a general robotic system interacting with a human to stabilize the oscillation intended by the human and to reduce the burden on the human by providing assistive forces. The control architecture is inspired by the central pattern generator (CPG) -- neuronal circuits that command muscle contractions to achieve rhythmic body movements during animal locomotion. CPGs are attractive for engineering applications due to its ability to conform their oscillations to natural dynamics of a varying environment through sensory feedback. This exploratory research will investigate the potential of the CPG architecture to provide a viable foundation for a new system design for achieving co-robots that assist humans to execute oscillation tasks. The controller is realized as an interconnection of identical units, emulating neuronal dynamics. The problem is formulated as the search for the interconnection such that the robot-human-CPG system has a stable limit cycle in which human decides an appropriate oscillation and CPG-controlled robot assists. The method of multivariable harmonic balance will be employed to obtain a convex characterization of feasible interconnection matrices that meet oscillation specifications. The approximate nature of the design method will be complemented by extensive simulations as well as physical experiments on robotic manipulators. While the central theme of control theory has been the regulation around an equilibrium point of a dynamical system, capability of generating coordinated autonomous oscillations can be extremely useful in many engineering applications. The basic research proposed here will provide an initial stepping stone toward a new paradigm for cooperative pattern generations by feedback control.
