Abstract
Classifying objects in complex unknown environments is a challenging problem in robotics and is fundamental in many applications. Modern sensors and sophisticated perception algorithms extract rich 3D textured information, but are limited to the data that are collected from a given location or path. We are interested in closing the loop around perception and planning, in particular to plan paths for better perceptual data, and focus on the problem of planning scanning sequences to improve object classification from range data. We formulate a novel time-constrained active classification problem and propose solution algorithms that employ a variation of Monte Carlo tree search to plan non-myopically. Our algorithms use a particle filter combined with Gaussian process regression to estimate joint distributions of object class and pose. This estimator is used in planning to generate a probabilistic belief about the state of objects in a scene, and also to generate beliefs for predicted sensor observations from future viewpoints. These predictions consider occlusions arising from predicted object positions and shapes. We evaluate our algorithms in simulation, in comparison to passive and greedy strategies. We also describe similar experiments where the algorithms are implemented online, using a mobile ground robot in a farm environment. Results indicate that our non-myopic approach outperforms both passive and myopic strategies, and clearly show the benefit of active perception for outdoor object classification.
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t-Tests with respect to balanced MCAP.
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Acknowledgements
This research is supported in part by the Australian Centre for Field Robotics, the New South Wales State Government, the Australian Research Council’s Discovery Projects funding scheme (Project Number DP140104203), and the Faculty of Engineering and Information Technologies at The University of Sydney under the Faculty Research Cluster Program. We thank Joel Veness, Oliver Cliff, and Graeme Best for helpful discussions. Thanks to Andrew Bate, Jocie Bate, Lasitha Piyathilaka, and Grant Louat at SwarmFarm Robotics for use of the robot and assistance with the hardware experiments. Thanks also to James Underwood and Alen Alempijevic for assistance with sensor calibration.
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This is one of several papers published in Autonomous Robots comprising the Special Issue on Active Perception.
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Appendix 1: Calculating entropy
Appendix 1: Calculating entropy
The state vector of an object is given by \(b = (\mathcal {N}({\varvec{\mu }}, \varSigma ), {\varvec{p}})\) where the components of the pose are assumed to be normally distributed with mean vector \({\varvec{\mu }} = [\mu _x, \mu _y, \mu _{\theta }]\) and covariance matrix \(\varSigma = \text {diag}(\sigma _x, \sigma _y, \sigma _{\theta })\) for the x location , y location, and orientation angle. The class of the object is represented by the probability vector \({\varvec{p}} = [p_{\ell }]_{\ell =1}^{N_L}\).
For the state vector of an object, the entropy of the joint state can be expressed as
which can be decomposed up into the continuous variable (pose) and the discrete class label to give
where \({\varvec{x}} = (x,y,\theta )\).
From the definition of conditional entropy
where \({\varvec{X}} = [X, Y, \varTheta ]\) is a continuous random vector for the pose components, and L is a discrete random variable for the class label. The first term is the probability over the classes which is simply given by
Expanding the second term yields
For the estimation method described in this paper, the conditional entropy \(H({\varvec{X}}|L = \ell )\) is computed from the particles with class label \(\ell \) and calculating the entropy of a multivariate Gaussian distribution
where \(|\cdot |\) is the determinant of the matrix, and the power 3 comes from the dimension of \({\varvec{x}}\). We simplify this expression and assume each dimension \((x,y,\theta )\) to be independent, therefore, \(|\varSigma | = \sigma _{x}^2 \sigma _{y}^2 \sigma _{\theta }^2\).
1.1 Appendix 2: Proof of Lemma 1
In this proof we show that the recursive reward value for a node is equivalent to the empirical average of all rollout reward values for all simulations beginning at the node.
Let \(T = \tau _{\text {max}}\) represent the maximum depth of the tree. For a leaf node, that has no children or rollout reward, the average reward is given by \(\bar{Q}_T = \eta ^T R_T = \eta ^T \frac{1}{W_T} \sum _{i=1}^{W_T} R^i_T\).
Now consider a node one level above the leaf nodes at depth \(T-1\). The immediate reward is the average of all sample rewards \(Q_{T-1} = \frac{1}{W_{T-1}} \sum _{i=1}^{W_{T-1}} r^i_{T-1}\). The rollout reward consists of one step such that \(r_{T-1} = \eta ^{T} r^r_T\). Expanding the recursive definition gives
where the cumulative reward \(R^i_{T-1} = \sum _{j=T-1}^{T} \eta ^j r^i_j = \eta ^{T-1} r^i_{T-1} + \eta ^T r^i_T\) is the sum of the immediate reward and the immediate reward of the leaf node. The third line is obtained by moving the rollout reward into the last summation and using the definition of MCTS that the visit count of a parent equals the sum of the visit counts of its children plus one for the rollout. In other words, \(W_{T-1} = 1 + \sum \limits _{{{v_c \in \textsc {Children}(v)}}} W_{v_c}\). By induction, the result holds for all higher-level nodes. \(\square \)
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Patten, T., Martens, W. & Fitch, R. Monte Carlo planning for active object classification. Auton Robot 42, 391–421 (2018). https://doi.org/10.1007/s10514-017-9626-0
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DOI: https://doi.org/10.1007/s10514-017-9626-0