Wireless Sensor and Actor Networks (WSAN)




 
 
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Overview

Wireless sensor and actor networks (WSANs) refer to a group of sensors and actors linked by wireless medium to perform distributed sensing and actuation tasks. The physical architecture of the WSAN is given in Figure 1. In such a network, sensors gather information about the physical world, while actors take decisions and then perform appropriate actions upon the environment, which allows remote, automated interaction with the environment.

Why Actors?

In this context, the meaning of the term actor differs from the more conventional notion of actuator. An actuator is a device to convert an electrical control signal to a physical action, and constitutes the mechanism by which an agent acts upon the physical environment. From the perspective considered in this project, however, an actor, besides being able to act on the environment by means of one or several actuators, is also a network entity that performs networking-related functionalities, i.e., receive, transmit, process, and relay data. For example, a robot may interact with the physical environment by means of several motors and servo-mechanisms (actuators). However, from a networking perspective, the robot constitutes a single entity, which is referred to as actor. Hence, the term actor embraces heterogeneous devices including robots, unmanned aerial vehicles (UAVs), and networked actuators such as water sprinklers, pan/tilt cameras, robotic arms, etc. Applications of wireless sensor and actor networks may include team of mobile robots that perceive the environment from multiple disparate viewpoints based on the data gathered by a sensor network, a smart parking system that redirects drivers to available parking spots, or a distributed heating, ventilating, and air conditioning (HVAC) system based on wireless sensors.

Peculiarities of Wireless Sensor and Actor Networks

However, due to the presence of actors, WSANs have some differences from wireless sensor networks (WSNs) as outlined below:
 

  • While sensor nodes are small, inexpensive devices with limited sensing, computation and wireless communication capabilities, actors are usually resource-rich devices equipped with better processing capabilities, stronger transmission powers and longer battery life.
  • In WSANs, depending on the application there may be a need to rapidly respond to sensor input. Moreover, to provide right actions, sensor data must still be valid at the time of acting. Therefore, the issue of real-time communication is very important in WSANs since actions are performed on the environment after sensing occurs.
  • The number of sensor nodes deployed in studying a phenomenon may be in the order of hundreds or thousands. However, such a dense deployment is not necessary for actor nodes due to the different coverage requirements and physical interaction methods of acting task. Hence, in WSANs the number of actors is much lower than the number of sensors.
  • In order to provide effective sensing and acting, a distributed local coordination mechanism is necessary among sensors and actors.

     

                 Figure 1. WSAN physical architecture.



In many real applications, robots are used as actor nodes. The robots designed by several Robotics Research Laboratories are shown in Fig. 2-5. Low-flying helicopter platform shown in Fig. 2 provides ground mapping, and air-to-ground cooperation of autonomous robotic vehicles. However, it is likely that in the near future more several actuation functionalities such as water sprinkling or disposing of a gas can be supported by this helicopter platform, which will make WSANs much more efficient than today. Possibly the world's smallest autonomous untethered robot (1/4 cubic inch and weighing less than an ounce) being developed in Sandia National Laboratories is given in Fig. 3. Although it is not capable of performing difficult tasks that are done with much larger robots yet, it is very likely that it will be the robot of the future. An example of Robotic Mule which is called autonomous battlefield robot designed for the Army is given in Fig. 4. There are several autonomous battlefield robot projects sponsored by Space and Naval Warfare Systems Command and Defense Advanced Research Projects Agency (DARPA). These developed battlefield robots can detect and mark mines, carry weapons, function as tanks or maybe in the future totally replace soldiers in the battlefield. Finally, sub-kilogram intelligent tele-robots shown in Fig. 5 are networked tele-robots having a radio turret which enables communication over UHF frequencies at 4800 kbits/sec. These robots can coordinate with each other by exploiting their wireless communication capabilities and perform the tasks determined by the application.

     



       

                   Figure 2. Aerial mapping helicopter .



       

                   Figure 3. Mini-robot.



       

                   Figure 4. Robotic Mule.



       

                   Figure 5. Sub-kilogram intelligent tele-robots.