Simplex Architecture for CPS
Simplex is highly reliable and dependable software architecture for process control system. The architecture consists of two versions of a controller, called the advanced controller (AC) and baseline controller (BC), and a physical plant. The advanced controller is designed for high performance and is in control of the plant in normal operating conditions. However, certification that AC keeps the plant state within a prescribed safety region may be infeasible, due to its complexity or, because an accurate model of it is unavailable for analysis. In contrast, the baseline controller, also known as recovery controller, is certified. When the plant is under the control of AC, a decision module (DM) periodically, monitors the state of the plant and switches the control of the plant to BC, if the controller plant is in immediate danger of entering a state that might lead to a safety violation.
Adapting simplex architecture for CPS is nontrivial. In particular, designing the decision module of simplex architecture for CPS is the most challenging part. The earliest methodology applies Lyapunov stability theory by reducing the problem into linear matrix inequality, which is applicable only for linear systems. Approaches for nonlinear systems were later developed based on state-space exploration, also called reachability analysis. However, the reachability-based approach is computationally intensive, particularly is not amenable for high dimensional systems. In , we present an alternative approach to design DM based on barrier certificate (BaC) a methodology developed for safety verification of hybrid systems, and reachability analysis. In particular, a safety envelop, a conservative region that ensures safety, is computed offline using BaC and when a plant state goes beyond this envelop we apply online reachability analysis. This combined approach is proved to be more efficient than existing techniques, as it allows high performance controller to be operated in larger region of the state-space.
Current Activity: Resilient CPS using Simplex
CPSs are monitored, controlled and coordinated by a set of sophisticated computing algorithms and communications infrastructure such as IoT. Such infrastructure are vulnerable not only to serious cyber-attacks, but also, in certain cases, to physical disturbances due to natural disasters, actions by malicious actors and other unforeseen anomalies. Currently, I am working on designing resilient CPSs that can recover from high impact disturbances with minimal performance degradation. Underlying approaches will combine simplex architecture, reachability analysis and domain specific big data analytics.
 J. Yang, M. A. Islam, A. Murthy, S. Smolka, and S. Stoller. A simplex architecture for hybrid systems using barrier certificates. The 36th International Conference on Computer Safety, Reliability and Security, 2017.
 J. Yang, M. A. Islam, A. Murthy, S. Stoller, and S. Smolka. Simplex architectures for hybrid systems based on barrier certificates.In Elsevier journal on Reliability Engineering and System Safety, 2018. (under-review).