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Vision-based control of a robot formation is challenging because the on-board sensor (camera) only provides the view-angle to the other moving robots, but not the distance that must be estimated. In order to guarantee a consistent estimate of the distance by knowing the control inputs and the sensor outputs in a given interval, the nonlinear multi-robot system must preserve its observability. Recent theoretical studies on leader-follower robot formation exploit the interesting influence that the control actions have on observability. Based on these results, in this paper we present a switching active control strategy for formation control. Our control strategy is active in the sense that, while asymptotically achieving the formation control tasks, it also guarantees the system observability in those cases in which all the robots tend to move along non-observable paths. As a result, both estimation and formation performances are improved. Extensive simulation results show the effectiveness of the proposed design.