The coverage problem is of great interest for many sensor network applications, for example, detection of intruders in the sensor field. Topological changes in sensor networks may affect qualities of sensor coverage. In this paper, we present two suites of algorithms for dynamically maintaining the coverage and the measures of its qualities. Using only local knowledge, our algorithms capture the dynamic changes of network topology and efficiently maintain the coverage by updating the radii of sensors combined with limited sensor mobility. Our algorithms are fully distributed and have the advantages of low communication complexity with no need of a tight bound on message propagation delay.