Kopicki, Marek (2010)
Ph.D. thesis, University of Birmingham.
This thesis addresses an important problem in robotic manipulation, which is the ability to predict how objects behave under manipulative actions. This ability is useful for planning of object manipulations. Physics simulators can be used to do this, but they model many kinds of object interactions poorly, and unless there is a precise description of an object’s properties their predictions may be unreliable. An alternative is to learn a model for objects by interacting with them. This thesis specifically addresses the problem of learning to predict the interactions of rigid bodies in a probabilistic framework, and demonstrates results in the domain of robotic push manipulation. During training, a robotic manipulator applies pushes to objects and learns to predict their resulting motions. The learning does not make explicit use of physics knowledge, nor is it restricted to domains with any particular physical properties. The prediction problem is posed in terms of estimating probability densities over the possible rigid body transformations of an entire object as well as parts of an object under a known action. Density estimation is useful in that it enables predictions with multimodal outcomes, but it also enables compromise predictions for multiple combined expert predictors in a product of experts architecture. It is shown that a product of experts architecture can be learned and that it can produce generalization with respect to novel actions and object shapes, outperforming in most cases an approach based on regression. An alternative, non-learning, method of prediction is also presented, in which a simplified physics approach uses the minimum energy principle together with a particle-based representation of the object. A probabilistic formulation enables this simplified physics predictor to be combined with learned predictors in a product of experts. The thesis experimentally compares the performance of product of densities, regression, and simplified physics approaches. Performance is evaluated through a combination of virtual experiments in a physics simulator, and real experiments with a 5-axis arm equipped with a simple, rigid finger and a vision system used for tracking the manipulated object.
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