Creating autonomous robots that can learn to assist humans in situations of daily life is a fascinating challenge for machine learning. While this aim has been a long-standing vision of artificial intelligence and the cognitive sciences, we have yet to achieve the first step of creating robots that can learn to accomplish many different tasks triggered by environmental context or higher-level instruction. The goal of our robot learning laboratory is the investigation of the ingredients for such a general approach to motor skill learning, to get closer towards human-like performance in robotics. We thus focus on the solution of basic problems in robotics while developing domain-appropriate machine-learning methods. Starting from theoretically well-founded approaches to representing the required control structures for task representation and execution, we replace the analytically derived modules by more flexible, learned ones.

An essential problem in robotics is the accurate execution of desired movements using only low-gain controls such that the robot will accomplish the desired task while not harming human beings in its environment. Following a trajectory with little feedback requires the accurate prediction of the needed torques, which cannot be achieved using classical methods for sufficiently complex robots. However, learning such models is hard as the joint-space can never be fully explored and the learning algorithm has to cope with a never-ending data stream in real time. We have developed learning methods both for accomplishing tasks represented in operational space as well as in joint-space.

While learning to execute tasks is a component essential to a framework for motor skill learning, learning the actual task is of even higher importance as discussed in here. We focus on the learning of elementary tasks or movement primitives, which are parameterized task representations based on nonlinear differential equations with desired attractor properties. We mimic how children learn new motor tasks using imitation learning for initializing these movement primitives while employing reinforcement learning to subsequently improve the task performance. We have learned tasks such as Ball-in-a-Cup or bouncing a ball on a string using this approach.

Efficient reinforcement learning for continuous states and actions is essential for robotics and control. We follow two approaches depending on the dimensionality of the domain. For high-dimensional state and action spaces, it is often easier to directly learn policies without estimating accurate system models. The resulting algorithms are parametric policy search algorithms inspired by expectation-maximization methods and can be employed for motor primitive learning. For lower-dimensional systems, Bayesian approaches to control can be shown to be able to cope with the optimization bias introduced by the model errors in model-based reinforcement learning. As a result, these methods can learn good policies at a rapid pace based on only little interaction of the system. Supervised learning is not always sufficient for motor learning problems, partly because often an expert teacher or idealized version of the behavior is not available. Because of that, one of our goals it the development reinforcement learning methods which scale into the dimensionality of humanoid robots and can generate actions for seven or more degrees of freedom.

In hitting and batting tasks, movement templates with a learned global shape need to be adapted during the execution so that the racket reaches a target position and velocity that will return the ball over to the other side of the net or court. This requires a reformulation of motor primitives to hitting primitives. A key motor skill for manipulating the environment is grasping, which is why one of our research goals is adapting machine learning algorithms to make them applicable in the robot grasping task domain. For grasping or hitting, several alternative motor primitives might be available. Yet other tasks might require several motor primitives to be executed in parallel or in sequence. The selection and composition of motor primitives requires a perceptuo-motor persprective, and is the necessary for learning complex tasks. An example of a complex tasks which requires motor primitive selection and hitting primitives is the task of learning to play ping-pong.

An different research goal focuses on human motion. Learning brain-computer interfaces might help stroke patients to rehabilitate.

Currently, we are moving towards learning complex tasks, requiring the solution of a variety of hard problems. Among these are the decomposition of large tasks into movement primitives (MP), the acquisition and self-improvement of MPs, the determination of the number of MPs in a data set (see Number of Motor Primitives? for some early steps in this direction), the determination of the relevant task-space, perceptual context estimation and goal learning for MPs, as well as the composition of MPs for new complex tasks. These questions are tackled in order to make progress towards fast and general motor skill learning for robotics.


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