Decentralized H∞ Structural Control with Experimental Validation using Multi-subnet Wireless Sensing Feedback

This study investigates the feasibility of deploying wireless communication and embedded computing technologies for structural control applications. A feedback control system involves a network of sensors and control devices. As control devices are becoming smaller, more cost effective and reliable, opportunities are now available to instrument a structure with large number of control devices. However, instrumenting a large scale centralized control system with cables can be time consuming and labor intensive. This study explores decentralized feedback control using wireless sensors incorporated with computational core and signal generation module. Decentralized control schemes are designed to make decisions based on data acquired from sensors located in the vicinity of a control device. Specifically, this paper describes an experimental study of a time-delayed decentralized structural control strategy that aims to minimize the H∞ norm of a closed-loop control system. The decentralized controller design employs a homotopy method that gradually transforms a centralized controller into multiple decentralized controllers. Linear matrix inequality constraints are included in the homotopic transformation to ensure optimal control performance. Different decentralized H∞ control architectures are implemented with a network of wireless sensing and control devices instrumented on a six-story scaled steel frame structure. The sensor network supports simultaneous communication within multiple wireless subnets. Experimental tests are conducted to demonstrate the performance of the wireless decentralized control schemes. INTRODUCTION Utilizing a network of sensors, controllers and control devices, feedback control systems can potentially mitigate excessive dynamic responses of a structure subjected to strong dynamic loads, such as earthquakes or typhoons (Housner et al. 1997; Spencer and Nagarajaiah 2003). For a large structure, the instrumentation of a cable-based communication network that connects large number of sensors, control devices, and controllers can be quite costly. Furthermore, maintaining the reliability and performance of a large-scale inter-connected real-time system can be challenging. This study 1 Assistant Professor 2 Professor investigates the feasibility of deploying wireless communication and embedded computing technologies for structural control applications. As control devices are becoming smaller, more cost effective and reliable, opportunities are now available to instrument a structure with large number of control devices. Scalability of current sensing and control devices for large-scale control systems are hindered by their dependence on centralized control strategy where a central controller is responsible for acquiring data and making control decisions. To mitigate some of the difficulties with centralized feedback control systems, decentralized control strategies can be explored (Lunze 1992). Decentralized control schemes can be designed to make decisions based on data acquired from sensors located in the vicinity of a control device. Furthermore, decentralized feedback control can take advantage of wireless sensors incorporated with computational core and signal generation module (Wang et al. 2007). For a wireless sensing and control system, the wireless sensors can not only collect and communicate sensor data, but also make optimal control decisions and directly command control devices in real-time. With the wireless sensor devices, decentralized control algorithms can be embedded and performed in a parallel and distributive manner. This paper describes an experimental study of a time-delayed decentralized structural control strategy that aims to minimize the H∞ norm of a closed-loop control system. ∞ H control can offer excellent control performance particularly when “worst-case” external disturbances are encountered. Centralized ∞ H controller design in the continuous-time domain for structural control has been studied by many researchers (Johnson et al. 1998; Lin et al. 2006; Yang et al. 2004). Their studies have shown the effectiveness of centralized ∞ H control for civil structures. For example, it has been shown that ∞ H control design can achieve excellent performance in attenuating transient vibrations of structures (Chase et al. 1996). We have also conducted numerical simulations to evaluate the performance of time-delayed decentralized ∞ H control (Wang 2009; Wang et al. 2009). The decentralized controller design employs a homotopy method that gradually transforms a centralized controller into multiple decentralized controllers. Linear matrix inequality constraints are included in the homotopic transformation to ensure optimal control performance. This paper presents the results of an experimental study on the time-delayed decentralized ∞ H controller design. Different decentralized H∞ control architectures are implemented with a network of wireless sensing and control devices instrumented on a six-story scaled steel frame structure. Shake table experiments are conducted to examine the performance of different decentralized control architectures. The paper is organized as follows. First, the formulation for decentralized ∞ H controller design is summarized. The experimental setup of the six-story steel frame structure instrumented with wireless sensing and control units is then described. Experimental results will then be presented to evaluate the effectiveness of the wireless decentralized H∞ control strategies. Finally, this paper is concluded with a brief summary and discussion. FORMULATION FOR TIME-DELAYED DECENTRALIZED STRUCTURAL CONTROL Fig. 1 depicts the schematics of a structural control system defined for this study. For a structural model with n degrees-of-freedom (DOF) and instrumented with nu control devices, the discrete-time system dynamics can be written as (Wang et al. 2009): [ ] [ ] [ ] [ ] [ ] [ ] [ ] [ ] [ ] [ ] [ ] [ ] 1