The use of retrieval filters to localize information in hierarchically tagged text-dominated database

A coil current control device for e.g. a microstepping motor includes a coil (L), switching means (S), control means including a comparator (C) and a current measuring device (R). The coil can be alternately connected to a positive first voltage source (U1) or to a negative second voltage source (U2) by the switching means, under the control of a binary control signal (b), supplied by the control means. In the control means a reference signal (r) is compared with a measuring signal (m), proportional to the actual coil current (i) and supplied by the current measuring device. The binary control signal has a first level if the measuring signal is smaller than the reference signal or a second level if the measuring signal is larger than the reference signal, respectively. The first level of the control signal activates the switching means to connect the coil to the first voltage source, causing the coil current to increase; the second level of the control signal activates the switching means to connect the coil to the second voltage source, causing the coil current to decrease. To avoid undesired oscillations in the control circuit the precise moment of changing the first level into the second level and v.v. can be controlled by means of a clock pulse (p), supplied by a clock pulse generator (I).