Abstract An industrial safety system includes an improved fail-safe system in which a TRIAC switch is connected in series with a relay switch and with a load circuit to complete the circuit from a source through the load. A gating circuit for the TRIAC normally provides a closed switch effect so that a normal voltage drop appears across the TRIAC. An amplifier detects this voltage drop and keeps the relay closed but when the voltage drop disappears indicating a short fault in the TRIAC, the relay opens. If an open fault occurs in the TRIAC, the load circuit is not completed. The gating circuit for the TRIAC is one in which the presence of a human body, or a part of the human body, at or near to an industrial machine such as a press causes the TRIAC to be non-conductive and the load circuit is the power control for the machine. The gating circuit is driven by an oscillator and features a pair of parallel LC tuned circuits one of which has a fixed frequency response and the other of which includes an antenna as the capacitive element so as to have a variable frequency response dependent upon the prohibited presence of the human body. The frequency response of the fixed circuit is lower than that of the variable circuit when the latter is unaffected by a human body and both are resonant at frequencies lower than the oscillator frequency. The TRIAC is gated open when the outputs of the two circuits decrease to a predetermined threshold.
A control system for use in combination with an electrical load and a source of potential for said load, said control system comprising: first switch means and second switch means connected in series to each other and to said load for completing the circuit through said load from said supply, said first switch means normally producing a predetermined voltage drop thereacross when closed to complete the circuit through said load; control means for operating said first switch means between open and closed conditions; and means responsive to the existence of said predetermined voltage drop across said first switch means for maintaining said second switch means in closed condition:
Claim 2
Dependent Claim
108 words
A control system for use in combination with an electrical load and a source of potential for said load, said control system comprising: first switch means and second switch means connected in series to each other and to said load for completing the circuit through said load from said supply, said first switch means normally producing a predetermined voltage drop thereacross when closed to complete the circuit through said load; control means for operating said first switch means between open and closed conditions; and means responsive to the existence of said predetermined voltage drop across said first switch means for maintaining said second switch means in closed condition:
Claim 3
Dependent Claim
46 words
A control system as defined in claim 1 wherein said electrical load is the power control for an industrial machine and said control means is a device for detecting the presence of the human body or a portion thereof in a prohibited region of said machine.
Claim 4
Dependent Claim
28 words
A control system as defined in claim 2 wherein said control means includes a capacity tuned circuit for detecting the human body and an oscillator driving such circuit.
Claim 5
Dependent Claim
29 words
A control system as defined in claim 3 wherein said control means also includes a second capacitively tuned circuit driven by said oscillator and having a fixed frequency response.
Claim 6
Dependent Claim
57 words
In a control system as defined in claim 4 wherein the frequency response of said second tuned circuit is different from the frequency response of the first tuned circuit when the latter is not influenced by the human body, and said control means also includes a comparator having the outputs of said tuned circuits as inputs thereto.
Claim 7
Dependent Claim
26 words
In a control system as defined in claim 5 wherein both tuned circuits are initially tuned to the same side of the frequency of said oscillator.
Claim 8
Dependent Claim
35 words
In a control system as defined in claim 6 wherein the presence of a human body tends to cause the frequency response of said first tuned circuit to approach that of said second tuned circuit.