In this paper, we revisit the artificial potential based approach in the flocking control for multi-agent systems, where our main concerns are migration and trajectory tracking problems. The static destination or the tracking reference point is modeled by a virtual leader, whose information is utilized by some agents, called active agents (AA), for the controller design. We study a controller for the case where the set of AAs is fixed. By introducing dwell time for the topology-varying system, we define the solutions for the closed-loop system equations. The existence and uniqueness of the solution with any given non-singular initial condition is proved; and some results on the velocity consensus, collision avoidance, group configuration and robustness are proposed. Finally, we apply the proposed controllers to the flocking control of a team of nonholonomic mobile robots.