Conditional behavior trees: Definition, executability, and applications

Behavior Trees (BTs) are gaining acceptance in robotics to specify action policies at the deliberative level. Their advantages include modularity, ease of use and increasing tool support. In this paper, we define Conditional Behavior Trees (CBTs) as an extension of BTs wherein actions are decorated considering pre-and post-conditions. CBTs improve on basic BTs in that they enable monitoring the execution of single actions by checking pre-and post-conditions, respectively. Since there might exist action sequences wherein some preconditions are violated, CBT executability may depend on the success/failure of specific actions. We developed an encoding of CBT executability into satisfiability of propositional formulas to be checked off-line in a publicly-available tool that computes the encoding for generic CBTs. For the kind of application scenarios and related behavior specifications that we consider, we show that our approach is effective and yields formal guarantees about the executability of deliberative policies designed as CBTs.