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Fail-safe circuit for a control system

  • US 4,739,469 A
  • Filed: 04/18/1985
  • Issued: 04/19/1988
  • Est. Priority Date: 04/19/1984
  • Status: Expired due to Term
First Claim
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1. A fail-safe circuit for a control system, comprising:

  • (a) a main control circuit which produces and sends a first control signal to a controlled device for controlling the controlled device;

    (b) a back-up control circuit which operatively produces and sends a second control signal to the controlled device in place of the first control signal when the main control circuit fails, the back-up control circuit having first means for recognizing a malfunction of said main control circuit and proper operation of the back-up control circuit and second means for generating a unique switching signal when said first means recognizes the malfunction of said main control circuit and proper operation of the back-up control circuit;

    (c) a switching circuit for switching between the first and second control signals to transmit either of the first and second control signals to the controlled device depending on a presence or absence of the unique signal generated from said second means; and

    (d) a switching control circuit for controlling the switching circuit to enable transmission of the second control signal produced from the back-up control circuit to the controlled device when the unique switching signal is received from the back-up control circuit and for enabling transmission of the first control signal produced from the main control circuit when the unique switching signal is not received from the back-up control circuit, thereby preventing an erroneous switching of the switching circuit when the main control circuit is operating properly and the back-up control circuit generates a faulty switching signal.

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