Intrusion detection method and apparatus

An improved method for detecting the presence of an intruding object or certain other environmental changes in a protected area, the method comprising the steps of:

• (a) periodically projecting pulses of acoustical energy from a source means to a reference surface;
• (b) detecting a reflection of said pulses of acoustical energy at a receiving means and generating a sequence of electrical signals representative of said detected pulses;
• (c) generating a second sequence of periodically recurring electrical signals having a period substantially equal to an interval of time elapsed during the projection of a pulse of acoustical energy from the source means and the reception of a reflected pulse from the surface at said receiving means;
• (d) detecting a coincidence in time between the occurrence of said first and second signals;
• and,
• (e) generating an alarm indication only in the absence of said coincidence.

An improved apparatus for detecting the presence of an intruding object or certain other environmental changes in a protected area, comprising:

• (a) source means for periodically projecting pulses of accoustical energy toward a reference surface;
• (b) receiving means for detecting a reflection of said pulse of acoustical energy and provide a first indication thereof;
• (c) means for generating an adjustable second indication representative of elapsed time between projection of said pulse and reception of a reflected pulse from said reference surface;
• (d) means for detecting variations occurring between said first and second indication;
• and,
• (e) means for generating an alarm upon detection of a variation.

An improved apparatus for detecting the presence of an intruding object or certain other environmental changes in a protected area comprising:

• (a) source means for periodically projecting pulses of acoustical energy at a reference surface;
• (b) receiving means for detecting a reflection of said pulses of acoustical energy and for generating a first sequence of electrical signals indicative of the reception of reflected pulses;
• (c) adjustable means for generating a second sequence of periodically recurring electrical signals having a period substantially equal to an interval of time elapsed during the projection of a pulse of acoustical energy at said source means and the reception at the receiving means of a pulse reflected from said reference surface;
• (d) means for detecting coincidence in time between said first and said second sequences of signals;
• and
• (e) means for generating an alarm indication only in the absence of said coincidence.