Nicholas Kottenstette

Toward a Science of Cyber–Physical System Integration

System integration is the elephant in the china store of large-scale cyber-physical system (CPS) design. It would be hard to find any other technology that is more undervalued scientifically and at the same time has bigger impact on the presence and future of engineered systems. The unique challenges in CPS integration emerge from the heterogeneity of components and interactions. This heterogeneity drives the need for modeling and analyzing cross-domain interactions among physical and computational/networking domains and demands deep understanding of the effects of heterogeneous abstraction layers in the design flow. To address the challenges of CPS integration, significant progress needs to be made toward a new science and technology foundation that is model based, precise, and predictable. This paper presents a theory of composition for heterogeneous systems focusing on stability. Specifically, the paper presents a passivity-based design approach that decouples stability from timing uncertainties caused by networking and computation. In addition, the paper describes cross-domain abstractions that provide effective solution for model-based fully automated software synthesis and high-fidelity performance analysis. The design objectives demonstrated using the techniques presented in the paper are group coordination for networked unmanned air vehicles (UAVs) and high-confidence embedded control software design for a quadrotor UAV. Open problems in the area are also discussed, including the extension of the theory of compositional design to guarantee properties beyond stability, such as safety and performance.

Feedback Thermal Control for Real-time Systems

Thermal control is crucial to real-time systems as excessive processor temperature can cause system failure or unacceptable performance degradation due to hardware throttling. Real-time systems face significant challenges in thermal management as they must avoid processor overheating while still delivering desired real-time performance. Furthermore, many real-time systems must handle a broad range of uncertainties in system and environmental conditions. To address these challenges, this paper presents Thermal Control under Utilization Bound (TCUB), a novel thermal control algorithm specifically designed for real-time systems. TCUB employs a nested feedback loop that dynamically controls both processor temperature and CPU utilization through task rate adaptation. Rigorously modeled and designed based on control theory, TCUB can maintain both desired processor temperature and CPU utilization, thereby avoiding processor overheating and maintaining desired soft real-time performance. A salient feature of TCUB lies on its capability to handle a broad range of uncertainties in terms of processor power consumption, task execution times, ambient temperature, and unexpected thermal faults. The robustness of TCUB makes it particularly suitable for real-time embedded systems that must operate in highly unpredictable environments. The advantages of TCUB are demonstrated through extensive simulations under a broad range of system and environmental uncertainties.

Passivity-Based Design of Wireless Networked Control Systems for Robustness to Time-Varying Delays

Real-life cyber-physical systems, such as automotive vehicles, building automation systems, and groups of unmanned vehicles are monitored and controlled by networked control systems. The overall system dynamics emerges from the interaction among physical dynamics, computational dynamics, and communication networks. Network uncertainties such as time-varying delay and packet loss cause significant challenges. This paper proposes a passive control architecture for designing wireless networked control systems that are insensitive to network uncertainties. We describe the architecture for a system consisting of a robotic manipulator controlled by a digital controller over a wireless network and we show that the system is stable even in the presence of time-varying delays. We present simulation results that demonstrate the advantages of the architecture with respect to stability and performance and show that the system is insensitive to network uncertainties.

Relationships between positive real, passive dissipative, & positive systems

This paper shows how: i) (strongly) positive real; ii) (asymptotically stable) dissipative (strictly-input) passive; and iii) (Lm2-stable strictly) positive; continuous time system definitions are equivalent for linear time invariant (LTI) systems. In parallel this paper shows how: i) (strictly) positive real; ii) (asymptotically stable) dissipative (strictly-input) passive; and iii) (lm2-stable strictly) positive; discrete time system definitions are equivalent for LTI systems. A frequency test is derived to determine if a single input single output LTI system is strictly output passive. Finally, the necessary conditions to synthesize a system which is both passive and stable but neither strictly-input passive nor strictly-output passive are presented.

On relationships among passivity, positive realness, and dissipativity in linear systems ✩

The notions of passivity and positive realness are fundamental concepts in classical control theory, but the use of the terms has varied. For LTI systems, these two concepts capture the same essential property of dynamical systems, that is, a system with this property does not generate its own energy but only stores and dissipates energy supplied by the environment. This paper summarizes the connection between these two concepts for continuous and discrete time LTI systems. Beyond that, relationships are provided between classes of strictly passive systems and classes of positive real systems. The more general framework of dissipativity is introduced to connect passivity and positive realness and also to survey other energy-based results. The frameworks of passivity indices and conic systems are discussed to connect to passivity and dissipativity. After surveying relevant existing results, some clarifying results are presented. These involve connections between classes of passive systems and finite-gain L2L2 stability as well as asymptotic stability. Additional results are given to clarify some of the more subtle conditions between classes of these systems and stability results. This paper surveys existing connections between classes of passive and positive real systems and provides results that clarify more subtle connections between these concepts.

Feedback thermal control of real-time systems on multicore processors

Embedded real-time systems face significant challenges in thermal management. While earlier research on feedback thermal control has shown promise in dealing with the uncertainty in thermal characteristics, multicore processors introduce new challenges that cannot be handled by previous solutions designed for single-core processors. Multicore processors require the temperature and real-time performance of multiple cores be controlled simultaneously, leading to multi-input-multi-output control problems with inter-core thermal coupling. Furthermore, current Dynamic Voltage and Frequency Scaling (DVFS) mechanisms only support a finite set of states, leading to discrete control variables that cannot be handled by standard linear control techniques. This paper presents Real-Time Multicore Thermal Control (RT-MTC), a novel feedback thermal control framework pecifically designed for multicore real-time systems. RT-MTC dynamically enforces both the desired temperature set point and the schedulable CPU utilization bound of a multicore processor through DVFS. RT-MTC employs a rigorously designed, efficient controller that can achieve effective thermal control with the small number of frequencies commonly supported by current processors. The robustness and advantages of RT-MTC over existing thermal control approaches are demonstrated through both experiments on an Intel Core 2 Duo processor and simulations under a wide range of uncertainties in power consumption.

Design of Networked Control Systems Using Passivity

Real-life cyber physical systems, such as automotive vehicles, building automation systems, and groups of unmanned vehicles are monitored and controlled by networked control systems (NCS). The overall system dynamics emerges from the interaction among physical dynamics, computational dynamics, and communication networks. Network uncertainties such as time-varying delay and packet loss cause significant challenges. This paper proposes a passive control architecture for designing NCS that are insensitive to network uncertainties. We describe the architecture for a system consisting of a robotic manipulator controlled by a digital controller over a wireless network and show that the system is stable even in the presence of time-varying delays. Experimental results demonstrate the advantages of the passivity-based architecture with respect to stability and performance and show that the system is insensitive to network uncertainties.

Digital control of multiple discrete passive plants over networks

This paper provides a passivity based framework to synthesise l m 2 -stable digital control networks in which m strictly-output passive controllers can control n – m strictly-output passive plants. The communication between the plants and controllers can tolerate time varying delay and data dropouts. In particular, we introduce a power-junction-network, a general class of input-output-wave-variable-network which allows even a single controller (typically designed to control a single plant) to accurately control the output of multiple plants even if the corresponding dynamics of each plant is different. In addition to the power-junction-network we also introduce a Passive Downsampler (PDS) and Passive Upsampler (PUS) in order to further reduce networking traffic while maintaining stability and tracking properties. A detailed (soft real-time) set of examples shows the tracking performance of the networked control system.

Digital passive attitude and altitude control schemes for quadrotor aircraft

This paper presents a formal method to design a digital inertial control system for quad-rotor aircraft. In particular, it formalizes how to use approximate passive models in order to justify the initial design of passive controllers. Fundamental limits are discussed with this approach — in particular, how it relates to the control of systems consisting of cascades of three or more integrators in which input actuator saturation is present. Ultimately, two linear proportional derivative (PD) passive controllers are proposed to be combined with a nonlinear saturation element. It is also shown that yaw control can be performed independently of the inertial controller, providing a great deal of maneuverability for quad-rotor aircraft. A corollary, based on the sector stability theorem provided by Zames and later generalized for the multiple-input-output case by Willems, provides the allowable range of k for the linear negative feedback controller KI in which the dynamic system H1 : x1 → y1 is inside the sector [a1, b1], in which −∞ < a1, 0 < b1 ≤ ∞, and b1 > a1. This corollary provides a formal method to verify stability, both in simulation and in operation for a given family of inertial set-points given to the quad-rotor inertial controller. The controller is shown to perform exceptionally well when simulated with a detailed model of the STARMAC, which includes blade flapping dynamics.

A passivity-based framework for resilient cyber physical systems

Resilient control systems play a special role in the area of cyber-physical systems, where the design must address the question how complex dynamic plants are to be controlled safely and reliably when a control system is under a cyber attack. In this paper we describe a control theoretical framework based on the concept of passivity for designing a control network which can tolerate, for instance, denial-of-service attacks on networks used in the closed loop. In particular, we demonstrate how the resilient power junction structure could be applied, and provide simulated results.