Category Archives: Real-time Systems

Heuristic, real-time search with weighted heuristic function

Nicolás Rivera, Jorge A. Baier, Carlos Hernández, Incorporating weights into real-time heuristic search, Artificial Intelligence, Volume 225, August 2015, Pages 1-23, ISSN 0004-3702, DOI: 10.1016/j.artint.2015.03.008.

Multiplying the heuristic function by a weight greater than one is a well-known technique in heuristic search. When this technique is applied to A* with an admissible heuristic it yields substantial runtime savings, at the expense of sacrificing solution optimality. Its applicability to real-time heuristic search, a search approach that builds upon heuristic search, however, has only been explored by a few studies. In this article we present two new approaches to using weights in real-time heuristic search, applicable to a wide range of algorithms. The first one, weighted lookahead, is a variant of an existing approach by Shimbo and Ishida, and utilizes the weight while the algorithm performs lookahead search. The second one, weighted update, incorporates the weight to the edges of the search graph during the learning phase. We implemented both techniques within LSS-LRTA* and evaluated them in path-planning benchmarks. We show that weighted lookahead outperforms an existing approach by Shimbo and Ishida but that it does not improve over existing approaches that do not use weights. Weighted update, on the other hand, yields performance improvements of up to one order of magnitude both in solution cost and total search time. To illustrate further the generality of weighted update, we incorporate the technique in two other well-known real-time heuristic search algorithms: LRTA*-LS and daLSS-LRTA*, and we empirically show significant improvements for LRTA*-LS and modest but still important improvements for daLSS-LRTA*. We analyze the properties of weighted update in depth, showing, among other things, that it guarantees termination. Convergence behavior of LSS-LRTA*, modified to use weighted update, is also analyzed. In such a setting, we prove solutions are w-optimal, and provide additional bounds on solution quality that in practice are tighter than w-optimality.

Abstract data-type for exchanging information in real-time systems, prioritizing the access to newest data rather than to oldest

Dantam, N.T.; Lofaro, D.M.; Hereid, A.; Oh, P.Y.; Ames, A.D.; Stilman, M., The Ach Library: A New Framework for Real-Time Communication, Robotics & Automation Magazine, IEEE , vol.22, no.1, pp.76,85, March 2015, DOI: 10.1109/MRA.2014.2356937.

Correct real-time software is vital for robots in safety-critical roles such as service and disaster response. These systems depend on software for locomotion, navigation, manipulation, and even seemingly innocuous tasks such as safely regulating battery voltage. A multiprocess software design increases robustness by isolating errors to a single process, allowing the rest of the system to continue operation. This approach also assists with modularity and concurrency. For real-time tasks, such as dynamic balance and force control of manipulators, it is critical to communicate the latest data sample with minimum latency. There are many communication approaches intended for both general-purpose and real-time needs [9], [13], [15], [17], [19]. Typical methods focus on reliable communication or network transparency and accept a tradeoff of increased message latency or the potential to discard newer data. By focusing instead on the specific case of real-time communication on a single host, we reduce communication latency and guarantee access to the latest sample. We present a new interprocess communication (IPC) library, Ach which addresses this need, and discuss its application for real-time multiprocess control on three humanoid robots (Figure 1). (Ach is available at The name Ach comes from the common abbreviation for the motor neurotransmitter Acetylcholine and the computer networking term ACK.).

Interesting and gentle introduction to WCET analysis and synchronous design for hard real-time systems

Pascal Raymond, Claire Maiza, Catherine Parent-Vigouroux, Fabienne Carrier, Mihail Asavoae, 2015, Timing analysis enhancement for synchronous program, Real-Time Systems, Volume 51, Issue 2, pp 192-220, DOI: 10.1007/s11241-015-9219-y.

Real-time critical systems can be considered as correct if they compute both right and fast enough. Functionality aspects (computing right) can be addressed using high level design methods, such as the synchronous approach that provides languages, compilers and verification tools. Real-time aspects (computing fast enough) can be addressed with static timing analysis, that aims at discovering safe bounds on the worst-case execution time (WCET) of the binary code. In this paper, we aim at improving the estimated WCET in the case where the binary code comes from a high-level synchronous design. The key idea is that some high-level functional properties may imply that some execution paths of the binary code are actually infeasible, and thus, can be removed from the worst-case candidates. In order to automatize the method, we show (1) how to trace semantic information between the high-level design and the executable code, (2) how to use a model-checker to prove infeasibility of some execution paths, and (3) how to integrate such infeasibility information into an existing timing analysis framework. Based on a realistic example, we show that there is a large possible improvement for a reasonable computation time overhead.