Communication architectures for web-based telerobotic systems

Controlling robotic devices from remote places, using an existing, wide spread network is significantly extend their application. Internet, considering its wide spread and accessibility, is and ideal media for data transmission between a robot and its operator, but the limitation in bandwidth, the notable delay, and the unreliability of the network introduces several problems. Proposals for communication strategies are presented in the paper, for maximizing the usability of a web telerobot system in different network conditions. Results are based on the evaluation of a teleoperated mobile microrobot system, in that a human operator has real-time remote access to a microrobot through a web browser running on any Internet-connected computer. Methods for choosing the appropriate transport protocol, selecting relevant data to be transmitted, and compression of live video for teleoperation are addressed. A combined multi-resolution / JPEG compression image coding technique and its application is described, which offers very effective (up to 1:50-1:80) compression without significant image quality degradation in the region of interest. I. OVERVIEW OF INTERNET TELEROBOTICS A. Internet telerobotics As Internet became a standard medium for digital communication more and more systems used it for exchanging data between a device and its controller. Though Internet has unique strengths (low-cost, widespread), it suffers from the following problems, which make difficult its application for real-time teleoperation: throughput: limited bandwidth, depending on network congestion delay: time-varying (delay jitter), its upper bound depends on the network traffic packet loss: routers may drop packets in case of heavy network traffic. There are efforts to apply traditional control theory principles to packed-switched systems by statistically modeling the network and compensate its shortcomings with different techniques [1]. Others concentrate on providing the real-time visual feedback by developing protocols that support real-time streaming media transmission via Internet (e.g. by inserting timing information, defining QoS requirements, and implementing services that use these information) [2, 3]. Visual feedback plays an extremely important role in telerobotic applications, and delivering it to the user requires significant bandwidth, therefore its efficient transmission has to be investigated with distinguished attention. B. Web telerobotics First interactively accessible robots through the world wide web was devices connected to a server which were controlled by the user through HTML forms and image maps using the common gateway interface (CGI) (e.g. the Mercury Project, 1993; Australia’s telerobot). However the set-and-submit cycle mechanism of CGI provides only very limited interactivity. Later Java-based solutions appeared, with sophisticated user interfaces and improved communication architecture (stream or datagram based), providing easy-to-use graphical user interfaces with instant user feedback and support for augmented and virtual reality (e.g. PumaPaint; Web Interface for Telescience project – WITS). Although web telerobots are went through substantial developments in the last decade they still have serious limitations, and do not utilize the Internet efficiently. Since Internet is a complex and continuously changing network system, its modeling is a very complicated task, Communication architectures for web-based telerobotic systems András Lassó, Tamás Urbancsek Budapest University of Technology and Economics Department of Control Engineering and Information Technology H-1117 Budapest, Pázmány Péter sétány 1/D, Hungary E-mail: [email protected], [email protected] IEEE Mediterranean Conference on Control and Automation DUBROVNIK, CROATIA, June 27-29, 2001. and so realistic testing and evaluation are essential to continuously verify theories. C. Our telerobot system Our system is a fully functional web telerobot system, which enables to observe and control a special 5 DOF microrobot remotely using a Java capable web browser. Graphical user interface and live video is provided for the operator to formulate commands and verify the proper execution. The system is portable and highly reconfigurable in a way that it can use different techniques and multiple protocols for the data transfer through Internet, making it an ideal tool to evaluate different communication methods for Internet telerobotics. II. SYSTEM ARCHITECTURE A. Telerobot The telerobot side consists of the microrobot, its controller circuit, the controller PC connected to the Internet and two intelligent CCD cameras with built-in DSP processor. The first camera takes pictures of the whole workspace, and the second is mounted on a microscope to reach the micrometer resolution in a limited area. The microrobot has an aluminum body of 30mm x 30mm x 60mm size, which makes 3-5 μm long "jumps" on its piezoelectric legs by the stick-and-slip method [4]. Its actuator is a small pipette, which is driven by three other piezoelectric legs. The controller circuit provides the high-voltage signals that move piezoelectric legs, according to the commands of the controller PC. The controller PC is communicating with the clients via an Internet connection, which makes it possible to observe and control the robot remotely using a Java-supporting web browser. To ensure portability and flexibility, the software of the controller PC is based on the objectoriented, open-source Adaptive Communication Environment framework (ACE, [5]) developed at the Washington University, St. Louis. B. Web client The web client is Java applet running in a web browser. It implements the user interface of the system, and manages the communication with the controller PC. The user can give commands, configure the communication parameters, quality preferences, the region of interest (in the multi-resolution pictures [6, 7, 8]), and get real-time visual feedback through the graphical user interface (Figure 1). Figure 1. Screenshot of the web client