Fail-safe photoelectric detector

A photoelectric detector arrangement comprising:

• a light source;
• a photoelectric receiver substantially sight-aligned with said light source, said receiver having a predetermined drop-in time and a predetermined drop-out time, said receiver drop-in time being substantially shorter than said receiver drop-out time;
• and
• a circuit for testing the detector arrangement, said testing circuit including:
• at least a first switch and a second switch, each said switch having conducting means capable of assuming conducting and non-conducting conditions, and means for controlling said conducting means, the controlling means of said first switch connected to receive a signal from said photoelectric receiver, and each switch having a predetermined switch drop-out time, each said switch drop-out time being substantially longer than said receiver drop-out time;
• signal conditioning means connected between said controlling means of said first switch and the controlling means of said second switch, for preventing long-term passage of steady-state signals between said two controlling means;
• and
• inverting means connected between said controlling means of said second switch and said light source, such that signals received from said controlling means of said second switch are inverted and passed to said light source for controlling whether said light source is energized.

A mobile storage system comprising:

• a. at least one movable storage carriage;
• b. a light source attached to said storage carriage;
• c. a photoelectric receiver attached to said storage carriage, spaced apart from said light source and substantially sight-aligned with said light source, said receiver having a predetermined drop-in time and a predetermined drop-out time, said receiver drop-in time being substantially shorter than said receiver drop-out time;
• d. moving means for moving said storage carriage, said moving means including:
• 1) a prime mover;
• 2) a control circuit for controlling said prime mover to place said moving means in an on condition and an off condition, including at least a first switch and a second switch, each said switch having:
• a) conducting means capable of assuming conducting and non-conducting conditions;
• b) controlling means for controlling the condition of said conducting means;
• and
• c) a predetermined switch dropout out time, each said switch drop-out time being substantially longer than said receiver drop-out time;
• and
• e. a circuit for monitoring the operation of the light source and photoelectric receiver, wherein:
• 1) the controlling means of said first switch is connected to receive a signal from said photoelectric receiver;
• 2) signal conditioning means are connected between said controlling means of said first switch and the controlling means of said second switch, for permitting only transients to pass therebetween;
• and
• 3) inverting means are connected between said controlling means of said second switch and said light source, such that signals received from said controlling means of said second switch are inverted and passed to said light source for controlling whether said light source is energized.

A fail-safe circuit for controlling a pair of switches connected in series, said switches including conducting means capable of assuming conducting and non-conducting conditions, controlling means for controlling the condition of said conducting means, and a predetermined switch drop-out time, said fail-safe circuit comprising:

• a light source;
• a photoelectric receiver substantially sight-aligned with said light source, said receiver having a predetermined drop-in time and a predetermined drop-out time, said receiver drop-in time being substantially shorter than said receiver drop-out time, and said receiver drop-out time being substantially shorter than said switch drop-out time;
• signal conditioning means connected between said controlling means of one of said switches and the controlling means of the other of said switches to provide AC coupling;
• and
• inverting means connected between said controlling means of said other of said switches and said light source, such that signals received from said controlling means of said other of said switches are inverted and passed to said light source;
• the controlling means of said one of said switches being connected to receive a signal from said photoelectric receiver.