An apparatus for controlling droplets allows the phase difference between
a calibration signal and its signature signal on a detector to be
minimized, or the amplitude of the signature signal to be maximized, by
adjusting the droplet charging means of the device, or the droplet
generation means, and the signals on either. The apparatus converts a
stream of fluid into a stream of droplets under the influence of a
droplet stimulation signal imposed onto the droplet generating means.
Droplets are subsequently signal-wise charged under the influence of a
droplet charging signal imposed on the droplet charging means. The
charged droplets are then deflected. The calibration signal is imposed
onto the stream of droplets. The calibration signal has characteristics
that do not appreciably affect the trajectory of the stream of droplets,
thereby ensuring that the placement accuracy of the individual droplets
is a maintained. The calibration signal further has a signal phase that
is independent of the droplet charging signal. A charge detection means
is used to extract a charge detection signal from the at least a part of
the droplets. The charge detection signal is filtered to extract a
signature signal of the calibration signal. The phase control system then
varies at least one of the droplet generation means, droplet stimulation
signal, droplet charging means and droplet charging signal until the
phase between the signature signal and the calibration signal is
minimized. A plurality of streams of droplets may be controlled by the
method of the invention.