Yi-Hung Wei

RT-WiFi: Real-Time High-Speed Communication Protocol for Wireless Cyber-Physical Control Applications

Applying wireless technologies in control systems can significantly enhance the system mobility and reduce the deployment and maintenance cost. Existing wireless technology standards, however either cannot provide real-time guarantee on packet delivery or are not fast enough to support high-speed control systems which typically require 1kHz or higher sampling rate. Nondeterministic packet transmission and insufficiently high sampling rate will severely hurt the control performance. To address this problem, in this paper, we present our design and implementation of a real-time high-speed wireless communication protocol called RT-WiFi. RT-WiFi is a TDMA data link layer protocol based on IEEE 802.11 physical layer to provide deterministic timing guarantee on packet delivery and high sampling rate up to 6kHz. It incorporates configurable components for adjusting design trade-offs including sampling rate, latency variance, reliability, and compatibility to existing Wi-Fi networks, thus can serve as an ideal communication platform for supporting a wide range of high-speed wireless control systems. We implemented RT-WiFi on commercial off-the-shelf hardware and integrated it into a mobile gait rehabilitation system. Our extensive experiments demonstrate the effectiveness of RT-WiFi in providing deterministic packet delivery in both data link layer and application layer, which further eases the controller design and significantly improve the control performance.

Energy-efficient real-time scheduling of multimedia tasks on multi-core processors

In recent years, various multi-core architectures have become popular selections for the designs of mobile platforms. With the strong computing demands from many multimedia applications, how to energy-efficiently utilize the computing power of mobile platforms without violations of timing constraints has become a critical design problem. In this paper, a data-partitioning-based approach is proposed to explore the parallelism of multimedia workload processing over multiple cores. Dynamic voltage scaling and dynamic power management strategies are both considered in the dynamic scaling of the computing power of cores and the adjustment of the set of active cores, respectively. The practicability and the energy efficiency of the proposed algorithms were evaluated by a series of experiments and simulations, for which we have encouraging results.

RT-WiFi: real-time high speed communication protocol for wireless control systems

Due to their enhanced mobility and reduced configuration and maintenance cost, wireless control systems (WCSs) are widely used in process and vibration control systems, on medical devices, unmanned vehicles and robotics. However, most literatures in WCSs focus on monitoring and low speed control, and less effort has been made on high speed WCSs. It is because most existing wireless communication protocols cannot provide real-time and reliable communication links with preferable high speed by taking energy saving into consideration.

Improving Control Performance by Minimizing Jitter in RT-WiFi Networks

Wireless networked control systems have received significant attention due to their great advantages in enhanced system mobility, and reduced deployment and maintenance cost. To support a wide range of high-speed wireless control applications, we presented in our prior work the design and implementation of a flexible real-time high-speed wireless communication platform called RT-WiFi. RT-WiFi currently provides up to 6kHz sampling rate and deterministic timing guarantee on packet delivery. While guaranteed delivery latency is essential for networked control, control performance is also impacted by communication jitter and other QoS parameters. To reduce jitter, a flexible network manager is needed to control network-wide scheduling of packet transportation. In this paper, we present an RT-WiFi network manager design and propose efficient solutions for two fundamental RT-WiFi network management problems. To improve control performance in networked control systems, our RT-WiFi network manager is designed to generate data link layer communication schedule with minimum jitter under both static and dynamic network topologies. In order to minimize network management overhead, an efficient data structure called S-tree is invented to manage the communication requests to deal with network dynamics. We have implemented the RT-WiFi network manager, and validated its network and control performance through extensive experiments with a real application.

Building wireless embedded internet for industrial automation

The Internet of Things (IoT) is considered to be the biggest challenge and opportunity for the Internet today. The Internet of Things makes it possible to connect embedded devices in physical environments to the Internet and interact with those devices through both IP and web interfaces. As a subset of the Internet of Things, the wireless embedded Internet targets at enabling resource-limited wireless devices with IP functions and connecting them to the Internet through low-power and low-bandwidth wireless networks. In this paper, we describe our design of the network infrastructure of wireless embedded Internet for industrial automation, and present the implementation and demonstration of a prototype system which integrates WirelessHART mesh networks into the Internet and supports web-based monitoring and control services.

Robust time delay compensation in a wireless motion control system with double disturbance observers

Time delay is a common phenomenon which can be varying and unknown in many networked motion control systems. Since time delay negatively affects the stability and tracking performance, it needs to be carefully handled in controller design. Moreover, both external disturbance and sensor noise exist in such systems, which makes the controller design more challenging. In this paper, a double disturbance observer (DDOB) structure is proposed to handle time delay, external disturbance, and measurement noise simultaneously. Design of disturbance observers (DOBs) and baseline controllers are elaborated. An RT-WiFi wireless network is developed for high-speed and real-time control applications. The RT-WiFi network is integrated with a DC motor and its performance is examined. Simulation and experimental results are demonstrated to verify the effectiveness of the proposed algorithm.

A Double Disturbance Observer Design for Compensation of Unknown Time Delay in a Wireless Motion Control System

Unknown time delay poses a significant challenge to the design of networked motion control systems. Moreover, modeling uncertainties, mechanical disturbance, and sensor noise coexist with time delay in such systems, which makes the controller synthesis even more challenging. It has been proven effective to model the time delay as fictitious disturbance so that a disturbance observer (DOB) can be employed to cancel the negative effect of time delay. In this brief, a new double DOB (DDOB) design is proposed by adding one more DOB into the control system to handle actual external disturbance and enable satisfactory tracking performance. Design considerations of the baseline controller and the two DOBs are illustrated, and robust stability analysis is provided to handle modeling uncertainties. A real-time wireless communication protocol, RT-WiFi, is integrated with a DC motor to examine the performance of the proposed DDOB by simulations and experiments.

Time delay compensation in a wireless tracking control system with previewed reference

In this paper, a wireless tracking control problem with varying time delay longer than one sampling interval is discussed, and a preview controller is employed for precise motion control. A delay-dependent system model is first introduced and a reference generator is then employed to model the previewed future reference. The system model is augmented with the reference generator and an optimal controller is synthesized to minimize a quadratic cost function of tracking errors and control inputs. A time-varying Kalman filter is designed for state estimation and feedback control. To make the Kalman filter feasible under long time delay, a linear regression model is proposed for delay estimation based on past measurements. A new wireless protocol called RT-WiFi is developed for high-speed and real-time control applications. Using the delay measurement from the RT-WiFi network, simulation studies is conducted to verify the effectiveness of the proposed algorithm.

Schedule Adaptation for Ensuring Reliability in RT-WiFi-Based Networked

With the ever-growing interests in applying wireless technologies for networked embedded systems to serve as the communication fabric, many real-time wireless technologies have been recently developed to support time-critical sensing and control applications. We proposed in previous work the RT-WiFi protocol that provides real-time high-speed predictable data delivery and enables designs to meet time-critical industrial needs. However, without explicit reliability enforcement mechanisms, our previous RT-WiFi design is either subject to uncontrolled packet loss due to noise and other interferences or may suffer from inefficient communication channel usage. In this article, we explicitly consider interference from both Wi-Fi and non-Wi-Fi based interference sources and propose two sets of effective solutions for reliable data transmissions in RT-WiFi-based networked embedded systems. To improve reliability against general non-Wi-Fi based interference, based on rate adaptation and retransmission techniques, we present an optimal real-time rate adaption algorithm together with a communication link scheduler that has low network management overhead. A novel technique called overbooking is introduced to further improve the schedulability of the communication link scheduler while maintaining the required communication reliability. For Wi-Fi-based interference, we present mechanisms that utilize virtual carrier sensing to provide reliable data transmission while co-existing with regular Wi-Fi networks. We have implemented the proposed algorithms in the RT-WiFi network management framework and demonstrated the system performance with a series of experiments.

A high-speed, real-time mobile gait rehabilitation system

Combining advanced technologies in real-time wireless communication, control theory, sensor and actuator design, and rehabilitation science.