**1.3. Open problems in multi-robot sensor networks**

In MRSN, when an event occurs, multiple robots in the near area sense the event data and generate an abundance of sensed data; however, many of the data generated in the same area are highly redundant. Hence the transmission and relaying of all generated data caused a big waste of bandwidth and energy; it also causes data collision and congestion so that result in low efficiency of data gathering. On the other hand, similar to wireless sensor network, WRSN could not avoid the shortcoming of lack of continuous energy supplement. One may say robot node can be equipped a large capacity battery, but its energy consumption is also large due to its big size, moving, detecting and transmitting etc. An energy harvesting algorithm (Eu et, al., 2010) is proposed for WSN. According to energy harvesting technique, a robot can absorbs solar energy from sunlight. However, how can the robot manage its task in the night? The vibrational energy get from the environment is too lees to trigger the robot. Therefore, saving energy is the most feasible way in WRSN.

For energy saving, decreasing the redundant data and sending a representative data of the detected area are the most considerable strategy. With a view of reducing the quantity of the transmitted data, the well-known scheme is data aggregation (Rajagopalan &Varshney, 2006). Since a sensor node in WSN waits for a period of time to collect extensive quantity of data to aggregate, data aggregation leads long transmission delay and low data accuracy. Some application, like medical and architectural utilization requires more accurate data while disaster relief requires receiving data as soon as possible. However, energy, delay and accuracy are trading off each other, one can not improve three of them at the same time. Hence, how to control the trade off of energy, delay and accuracy among different applications is the problem we will solve in our work.
