Lightweight wireless protocol based on IEEE 802.11 for delay sensitive telerobotic systems

Abstract

In this paper, we consider wireless telerobotic systems and protocol development for low-delay wireless communication. Telerobotics can be defined as the control of robot arms from a remote location. In a telerobotic system, there is a robot arm to be controlled that is identified as the `slave arm', and a remote operator at a distant location using a robotic manipulator that is called the `master arm'. In the control system measurements, actuator delays do degrade the system's performance. Therefore, communication delays between the master and slave arms, and their minimization, are of extreme importance in telerobotics. In this paper, we first develop a new wireless communication protocol, the lightweight wireless protocol (LWP), designed on top of the 802.11 media access control layer. This low-delay wireless LWP protocol is implemented on an embedded system (AirDrop-LWP) without an operating system and its associated overhead. Finally, 2 AirDrop-LWP--embedded systems running this low-delay wireless LWP protocol are used to build a telerobotic system with a Mitsubishi RV-2AJ industrial robot. The LWP protocol is also tested on a robot car controlled by an AirDrop-LWP card as a slave arm and a standard PC as a master arm. The key features of the LWP are a reduced packet size, simple protocol stack, predictive compression of operator movements, and prediction of lost data packets. The LWP protocol is compared with the user datagram protocol and significant performance improvements are observed: a reduced delay of up to 50% and an additional 20% lower delay via compression. Variation in the packet delay times is also an important parameter for the wireless control system. As the standard deviation of the packet delay times increase, and becomes less and less predictable, the resulting telerobotic system will be more and more difficult to operate. We measured the standard deviation of packet delays and observed that it increases with the packet size, and this increase is faster than the increase in mean packet delay.