  An ensemble of Brownian particles in a feedback controlled flashing ratchet
is studied. The ratchet potential is switched on and off depending on the
position of the particles, with the aim of maximizing the current. We study in
detail a protocol which maximizes the instant velocity of the center of mass of
the ensemble at any time. This protocol is optimal for one particle and
performs better than any periodic flashing for ensembles of moderate size,
whereas is defeated by a random or periodic switching for large ensembles.
