ACE: Anomaly clustering ensemble for multi-perspective anomaly detection in robot behaviors

This paper addresses an application of anomaly detection from subsequences of time series (STS) to autonomous robots' behaviors. An important aspect of mining sequential data is selecting the temporal parameters, such as the subsequence length and the degree of smoothing. For example in the task at hand, the patterns of the robot's velocity, which is one of its fundamental features, vary significantly subject to the interval for measuring the displacement. Selecting the time scale and resolution is difficult in unsupervised settings, and is often more critical than the choice of the method. In this paper, we propose an ensemble framework for aggregating anomaly detection from different perspectives, i.e., settings of user-defined, temporal parameters. In the proposed framework, each behavior is labeled whether it is an anomaly in multiple settings. The set of labels are used as meta-features of the respective behaviors. Cluster analysis in a meta-feature space partitions anomalous behaviors pertained to a specific range of parameters. The framework also includes a scalable implementation of the instance-based anomaly detection. We evaluate the proposed framework by ROC analysis, in comparison to conventional ensemble methods for anomaly detection.