Cyber-physical Systems

ANCS at MIT

Distributed cyber-physical architectures typically consist of multiple control applications mapped onto spatially distributed embedded systems (DES) with sensors and actuators that communicate via shared buses such as CAN and FlexRay. These CPS systems provide considerable flexibility in designing the network in terms of mapping control tasks to processors and determining various scheduling parameters and protocols. We refer to such systems as arbitrated networked control systems (ANCS) to emphasize the fact that the arbitration policy in the network also needs to be determined, and these policies together with the embedded system topology can be co-designed with the control system. By making use of real-time calculus, suitable information can be extracted about the delays due to communication and computation. This information in turn can be incorporated to result in a co-design of both the embedded systems network and the control system. One example of such a co-design is the use of a hybrid protocol and an adaptive control architecture. We design the protocol using the fact that any communication delay needs to be minimal during the transient mode of a control application, but can be larger during its steady-state mode. In particular, a time-triggered protocol is used during the transient mode and an event-triggered protocol during the steady-state mode. An adaptive controller is designed for each mode and suitable switching strategies are imposed to switch between the modes. The resulting multi-modal co-design is shown to be stable, schedulable, and realize efficient performance from multiple control applications.


Arbitrated Network Control Systems

Arbitration is inevitable in any embedded system where multiple applications have to be serviced using shared computational and communication resources. Our research investigations pertain to the analysis and synthesis of Arbitrated Network Control Systems (ANCS) that are capable of obtaining high control performance with limited consumption of resources. Main features of the ANCS include (i) hierarchy and (ii) multi-modality, both of which are necessary components in the design of a DES where advanced controllers need to be implemented. An overview of the ANCS can be found here. Highlights of recent results are listed below.

Hierarchical ANCS

A hierarchical scheduling policy consists of several levels. At each level of the hierarchical scheduler runs a distinct scheduler. For example a hierarchical TDMA/FP scheduler has a TDMA scheduler at its top-level and a fixed priority scheduler at the second level, see Figure 1. In [2], it is shown how the parameters of hierarchical schedules on the communication bus can be optimally chosen and made to satisfy multiple control performance metrics using a schedulability analysis carried out using a Real-time Calculus framework.

In general, a DES consists of several components including sensors, actuators, ECUs and buses, a shared communication medium. As only one entity can send a message over a bus at one time, in a control application, the underlying ANCS needs to arbitrate the scheduling of messages sent by one plant to another. Together with the management of this information as well as that stemming from the underlying physical plants, the ANCS can be used to ensure high performance. In [3], we consider control of plants using a DES with a hierarchical schedule with uncertainties that may be present either in the plant or in the network. An adaptive controller is proposed to accommodate the effect of uncertainties. It is shown that this adaptive controller can accommodate the uncertainties, stabilize the system, make use of the structure of the hierarchical scheduler in its design, and result in improved performance compared to non-adaptive NCS.

Recent Publications

  1. H. Voit and A.M. Annaswamy, “Local Adaptive Controllers for networked cooperative systems,” In Proceedings of the 2010 American Control Conference, Baltimore, MD, USA, Jun. 2010.
  2. H. Voit, R. Schneider, D. Goswami, A.M. Annaswamy, and S. Chakraborty, “Optimizing Hierarchical Schedules for Improved Control Performance,” In Proceedings of the IEEE Symposium on Industrial Embedded Systems, Trento, Italy, Jul. 2010.
  3. H. Voit and A.M. Annaswamy, “Adaptive control of a networked control system with hierarchical scheduling,” In Proceedings of the 2011 American Control Conference, San Francisco, CA, USA, Jun. 2011.

Multi-Modal ANCS

Often, a physical plant to be controlled has inherently different levels of operation called modes (denoted as Mss and Mtr, see Figure 1). The corresponding DES, by virtue of the distinct properties of the constituent protocols may have different modes as well. In addition, any resident non-control applications that need to be serviced by the DES may have their modes as well. This research is focused on a cyber-physical architecture with an ANCS with multiple processing units that communicate via a combination of time-triggered (MTT) and event-triggered (MET) modes and implement multiple control applications. Associated with each of these communication protocols are different set of advantages and disadvantages. The assignment of time-triggered (TT) slots to all control-related signals has the advantage of high quality of control (QoC) due to the possibility of reduced or zero delays, but leads to poor utilization of the communication bandwidth, high cost, overall inflexibility, and infeasibility as the number of control applications increase. On the other hand, event-triggered (ET) schedules often result in poor control performance due to the unpredictable temporal behavior of control messages and the related large delays which occurs due to the lack of availability of the bus. These imply that a hybrid protocol that suitably switches between these two schedules offers the possibility of exploiting their combined advantages of high QoC, efficient resource utilization, and low cost. A control application may have two modes, a transient one caused by external disturbances, and a steady-state one, with more stringent temporal constraints in the first case (see Figure 2). Given the differing properties of the constituent protocols and the distinct modes of a control application, we have developed a co-design of the control and protocol components is proposed that realizes their combined advantages. In particular, a switching controller is designed whose switches are aligned with those in the protocol. An adaptive component is added to the controller so as to accommodate uncertainties in the application as well as unforeseen changes in the protocol. The resulting adaptive switching controller is shown, using a FlexRay-based case study, to result in improved performance while requiring less time-triggered slots compared to non-adaptive co-designs. We use FlexRay as the hybrid protocol as it is the de-facto standard in automotive systems. Stability proofs and schedulability analysis have been derived.

Our next set of steps consists of the development of formal methods that combine control in engineering and real-time systems in computer science, and validation in an automotive system which is a typical example of a distributed embedded system.

Recent Publications

  1. A. Masrur, D. Goswami, R. Schneider, H. Voit, A.M. Annaswamy, and S. Chakraborty, “Schedulability Analysis of Distributed Cyber-Physical Applications on Mixed Time-/Event-Triggered Architectures with Retransmissions,” In Proceedings of the IEEE Symposium on Industrial Embedded Systems, Västerås, Sweden, Jun. 2011.
  2. H. Voit, R. Schneider, D. Goswami, S. Chakraborty, and A.M. Annaswamy, “Adaptive Controllers for Multi-Mode Cyber-Physical Systems with Modeling Uncertainties,” Preprint, 2011.
  3. H. Voit, A. Annaswamy, R. Schneider, D. Goswami, S. Chakraborty , “Adaptive Switching Controllers for Systems with Hybrid Communication Protocols,” Proceedings of the American Control Conference (ACC), Montreal, Canada , 2012.
  4. H. Voit, A. Annaswamy, R. Schneider, D. Goswami, S. Chakraborty , Adaptive Switching Controllers for Tracking with Hybrid Communication Protocols , Proceedings of the 51st Conference on Decision and Control (CDC), Maui, Hawaii , 2012.

Parallelized Model Predictive Control for Distributed Networked Systems

Model predictive control (MPC) has been used in many industrial applications because of its ability to produce optimal performance while accommodating constraints. However, its application on plants with fast time constants is difficult because of its computationally expensive algorithm. In this research, we propose a parallelized MPC that makes use of the structure of the computations and the matrices in the MPC. We show that the computational time of MPC with prediction horizon N can be reduced to O(log(N)) using parallel computing, which is significantly less than that with other available algorithms with similar accuracy.

Recent Publications

  1. Soudbakhsh D., Annaswamy A.M., “Parallelized model predictive control,” American Control Conference, Washington DC, 2013.

Delay based Co-design in ANCS

The domain of networked control systems (NCS) has traditionally been concerned with modeling and designing distributed controllers in the presence of control message loss, varying delay and jitter. Here, the characteristics of the network are assumed to be given and the focus has largely been on the controller. In several cyber physical systems, it is possible to not only design distributed controllers, but also design both the scheduling parameters of the resident processors as well as those for the communication buses. We refer to such systems as arbitrated networked control systems (ANCS), where the parameters of the arbitration policies in the network are co-designed with the controller. Analytical methods from real-time calculus are used to design the former, and delay aware design procedures are used to determine the controller. A case study was performed on the co-design of platform and control of three quadrotors with delays. More details can be found in [1].

We investigated co-design of control and platform in the presence of dropped signals in [2]. In a cyber-physical system, due to increasing complexities such as the simultaneous control of several applications, limited resources, and complex platform architectures, some of the signals transmitted may often be dropped. We address the challenges that arise both from the control design and the platform design point of view. A dynamic model is proposed that accommodates these drops, and a suitable switching control design is proposed. A Multiple Lyapunov functions based approach is used to guarantee the stability of the system with the switching controller. We then present a method for optimizing the amount of platform resource required to ensure stability of the control systems via a buffer control mechanism that exploits the ability to drop signals of the control system and an associated analysis of the drop bound. The results are demonstrated using a case study of a co-designed lane keeping control system in the presence of dropped signals.

In [3], we exploited robustness of the controller and proposed a novel implementation approach to achieve a tighter design to answer the following questions: (i) given a distributed architecture, how to characterize and formally verify the bound on deadline misses, (ii) given such a bound, how to design a controller such that desired stability and Quality of Control (QoC) requirements are met. We addressed (i) by modeling a distributed embedded architecture as a network of Event Count Automata (ECA), and subsequently introducing and formally verifying a property formulation with reduced complexity. Question (ii) was addressed by introducing a novel fault-tolerant control strategy which adjusts the control input at runtime based on the occurrence of fault or drop. Using the proposed fault-tolerant strategy QoC under faulty communication improved significantly.

Recent Publications

  1. Annaswamy A.M., Soudbakhsh D., Schneider R., Goswami D., Chakraborty S., “Arbitrated Network Control Systems: A co-design of control and platform for cyber-physical systems,” Control of Cyber-Physical Systems, Lecture Notes in Control and Information Sciences, Vol. 449, Ed: D.C. Tarraf, Springer Verlag, 2013.
  2. Soudbakhsh D., Phan L.X, Sokolsky O., Lee I., and Annaswamy A.M., “Co-design of Control and Platform with Dropped Signals,” The 4th ACM/IEEE International Conference on Cyber-Physical Systems [ICCPS’13], Philadelphia, PA.
  3. Kauer M., Soudbakhsh D., Goswami D., Chakraborty S., and Annaswamy A.M., “An LMI-based design for distributed embedded control systems via Verification of Firm Deadlines,” Design, Automation, and Test in Europe (DATE2014), to appear.

Tutorial on ANCS

ANCS Tutorial

General Timing Analysis and Schedulability of Multiple Applications with Mixed Priority

Hybrid communication protocols such as FlexRay are often used in DES. We propose a co-design that uses information from closed-loop responses to determine an efficient resource utilization for control systems under hybrid communication protocols.

  • Used multi-mode control structure
  • Analyzed systems with non-monotonic behavior
  • Leads to a less conservative schedulability analysis.
Simulation using Truetime + Simulink for 6 non-monotonic control applications
  • Our approach: 3 slots, Monotonic upper-bound: 5 slots
  • Reduced number of TT slots compared to those based on monotonic upper-bound