Abstract: A system has a plurality of transceiver units, including a parent unit and one or more portable and wearable child units, for generating an alarm when a child transceiver unit is more than a predetermined distance away from a parent transceiver unit. The distance is determined by the difference in the phase of a reference signal from the time it is transmitted on an rf signal by a parent unit, received and retransmitted by a child unit, and then received again by the parent unit.

Abstract: A system generates an alarm at one unit of the system when a second unit of the system is more than a predetermined distance from the first unit. The system has a first transceiver unit with a first transmitter for transmitting a first reference signal having a phase with respect to a reference source, and a first receiver for detecting a second reference signal having a phase which bears a relationship to that of the first reference signal. There is a second, portable, transceiver unit with a second transmitter for transmitting the second reference signal after receipt of the first reference signal. The first transceiver unit includes a distance resolver for determining the distance between the two transceiver units from the phases of the first and second reference signals, and an alarm, responsive to the distance resolver, for generating an alarm signal if the distance between the two transceiver units is more than a predetermined amount.

Abstract: A cover for a cabinet of the kind having an exposed portion to which access is to be controlled in which the cover has a panel to be attached to the cabinet; the panel has an opening corresponding to the exposed portion; a movable door is arranged to obstruct access to the exposed portion when the door is in a first location, and to permit access to the portion through the opening when the door is in a second location; and a locking mechanism is provided both for locking the panel to the cabinet and for locking the door in the first location. In another aspect, there are a plurality of doors arranged to obstruct access to the exposed portions; the cover has a first state in which a locking mechanism locks only one door in a first location and another state in which the locking mechanism locks at least another door. In another aspect, the doors, when not obstructing access via openings in the panel, lie in parallel planes adjacent to the panel in an area spaced away from the opening.