Method and apparatus for suppressing spurious alarms in an electrical protection system

Claim 3

The method set forth in claim 1 in which said step of making a second measurement of the value of said electrical signal and detecting achievement of said second threshold value is performed at a faster rate than said step of making a first measurement of the value of said electrical signal and detecting achievement of said first threshold value.

Claim 4

The method set forth in claim 1 in which said measuring and detecting steps are performed at substantially the same rate and delaying said step of producing an alarm signal for a predetermined time interval after detection of said first threshold value.

Claim 5

The method set forth in claim 1 in which said ambient condition is temperature, said alarm condition is fire and said nonalarm condition is a periodically operated heat source.

Claim 6

The method set forth in claim 1 in which said ambient condition is the light-beam-passing characteristic of a narrow, elongated volume of air, said alarm condition is smoke particles in said volume of air and said nonalarm condition is the presence of an opaque body substantially preventing the passage of a light beam along said volume of air.

Claim 7

The method set forth in claim 1 in which said ambient condition is capacity to ground, said alarm condition is motion of an intruder changing capacity to ground and said nonalarm condition is motion of a body having a much greater effect on capacity to ground than an intruder.

Claim 8

The method set forth in claim 1 comprising the additional step of producing a distinctive trouble signal indication in response to detection of said second threshold value.

Claim 11

Electrical protection apparatus for protecting a place by monitoring the value of an ambient condition at said place and minimizing the occurrence of spurious alarms, an alarm condition at said place producing a relatively small change in the value of said ambient condition and a particular nonalarm condition at said place producing a relatively large change in the value of said ambient condition, said apparatus comprising: a. ambient-condition-sensing means located at the protected place and arranged to monitor the value of said ambient condition and to produce an electrical signal upon detection of a change in the value of said ambient condition, said electrical signal being proportional in value to the magnitude of the detected change, said electrical signal having a first threshold value characteristic of detection of said alarm condition at said place and a second and substantially higher threshold value characteristic of detection of said nonalarm condition at said place; b. first and second signal-responsive elements arranged to operate, respectively, at said first and second threshold value of said electrical signal, said second signal-responsive element being arranged, upon application thereto of said electrical signal having a value equal to said second threshold value, to operate before said first signal-responsive element; c. means intercoupling said condition-sensing means and said signal-responsive elements to apply said electrical signal to the latter; d. alarm signal indication producing means coupled to said first signal-responsive element and arranged, upon operation of the latter, to produce an alarm signal indication; and e. means responsive to operation of said second signal-responsive means to suppress production of an alarm signal indication and to maintain suppression of an alarm signal indication so long as said electrical signal has a value equal to said second threshold value irrespective of the time interval during which said electrical signal has a value equal to said second threshold value.

Claim 12

Electrical protection apparatus for protecting a place by monitoring the value of an ambient condition at said place and minimizing the occurrence of spurious alarms, an alarm condition at said place producing a relatively small change in the value of said ambient condition and a particular nonalarm condition at said place producing a relatively large change in the value of said ambient condition, said apparatus comprising: a. ambient-condition-sensing means located at the protected place and arranged to monitor the value of said ambient condition and to produce an electrical signal upon detection of a change in the value of said ambient condition, said electrical signal being proportional in value to the magnitude of the detected change, said electrical signal having a first threshold value characteristic of detection of said alarm condition at said place and a second and substantially higher threshold value characteristic of detection of said nonalarm condition at said place; b. first and second signal-responsive elements arranged to operate, respectively, at said first and second threshold value of said electrical signal, said second signal-responsive element being arranged, upon application thereto of said electrical signal having a value equal to said second threshold value, to operate before said first signal-responsive element; c. means intercoupling said condition-sensing means and said signal-responsive elements to apply said electrical signal to the latter; d. alarm signal indication producing means coupled to said first signal-responsive element and arranged, upon operation of the latter, to produce an alarm signal indication; and e. means responsive to operation of said second signal-responsive means to suppress production of an alarm signal indication and to maintain suppression of an alarm signal indication so long as said electrical signal has a value equal to said second threshold value irrespective of the time interval during which said electrical signal has a value equal to said second threshold value.

Claim 13

Apparatus as set forth in claim 9 in which said second signal response element has a response time less than that of said first signal response eLement.

Claim 14

Apparatus as set forth in claim 9, comprising delay means coupled to said first signal-responsive element to delay operation thereof for a predetermined time interval after application thereto of said electrical signal having said first threshold value.

Claim 15

Apparatus as set forth in claim 9 in which said first and second signal-responsive elements each comprises an electronic trigger circuit and a relay arranged to have its condition changed in response to operation of the corresponding trigger circuit.

Claim 16

Apparatus as set forth in claim 12 in which a capacitive element is coupled to the electronic trigger circuit of said first signal-responsive means to delay operation thereof for a predetermined time interval after said electrical signal reaches said first threshold value.

Claim 17

A projected beam-type smoke detection system, comprising: a. a source of a light beam; b. a photocell physically spaced from said source and disposed in the path of said beam so that the output of said photocell is changed in response to changes in the intensity of the light impinging on said photocell; c. an amplifier coupled to said photocell and arranged to produce an electrical signal which varies with changes in the intensity of light impinging on said photocell; d. means coupled to the output of said amplifier and arranged to rectify said electrical signal to produce a first direct voltage which varies with the intensity of light impinging on said photocell; e. a first trigger circuit coupled to said last-mentioned means and arranged to operate at a value of said first direct voltage corresponding to a first threshold value of said electrical signal; f. alarm signal indication producing means coupled to said first trigger circuit and arranged upon operation of the latter to produce an alarm signal indication; g. delay means arranged to delay production of said alarm signal indication for a predetermined time interval after said electrical signal reaches said first threshold value thereof; h. an amplifier circuit having an input coupled to the output of said linear amplifier to amplify said electrical signal; i. a rectifying circuit coupled to the output of said amplifier and arranged to produce a second direct voltage proportional in value to the value of said electrical signal; j. a second trigger circuit coupled to said rectifying circuit and arranged to operate at a value of said second direct voltage corresponding to a second threshold value of said electrical signal, said second threshold value of said electrical signal, said second threshold value corresponding to a light intensity impinging on said photocell which is substantially less than the light intensity corresponding to said first threshold value; and k. control means coupled to said second trigger circuit and arranged to be operated upon operation of said second trigger circuit, said control means having a control element arranged upon operation of said control means to prevent production of said alarm signal indication.

Claim 18

A projected beam-type smoke detection system, comprising: a. a source of a light beam; b. a photocell physically spaced from said source and disposed in the path of said beam so that the output of said photocell is changed in response to changes in the intensity of the light impinging on said photocell; c. an amplifier coupled to said photocell and arranged to produce an electrical signal which varies with changes in the intensity of light impinging on said photocell; d. means coupled to the output of said amplifier and arranged to rectify said electrical signal to produce a first direct voltage which varies with the intensity of light impinging on said photocell; e. a first trigger circuit coupled to said last-mentioned means and arranged to operate at a value of said first direct voltage corresponding to a first threshold value of said electrical signal; f. alarm signal indication producing means coupled to said first trigger circuit and arranged upon operation of the latter to produce an alarm signal indication; g. delay means arranged to delay production of said alarm signal indication for a predetermined time interval after said electrical signal reaches said first threshold value thereof; h. an amplifier circuit having an input coupled to the output of said linear amplifier to amplify said electrical signal; i. a rectifying circuit coupled to the output of said amplifier and arranged to produce a second direct voltage proportional in value to the value of said electrical signal; j. a second trigger circuit coupled to said rectifying circuit and arranged to operate at a value of said second direct voltage corresponding to a second threshold value of said electrical signal, said second threshold value of said electrical signal, said second threshold value corresponding to a light intensity impinging on said photocell which is substantially less than the light intensity corresponding to said first threshold value; and k. control means coupled to said second trigger circuit and arranged to be operated upon operation of said second trigger circuit, said control means having a control element arranged upon operation of said control means to prevent production of said alarm signal indication.

Claim 19

A projected beam-type smoke detection system, comprising: a. a source of a modulated light beam; b. a photocell physically spaced from said source and disposed in the path of said beam so that the output of said photocell is changed in response to changes in the intensity of the light impinging on said photocell; c. a linear amplifier coupled to said photocell and arranged to produce an electrical signal which varies linearly over a working range with changes in the intensity of light impinging on said photocell; d. a resistance element; e. means coupled to the output of said linear amplifier and to said resistance element and arranged to rectify said electrical signal to produce across said resistance element a first direct voltage which varies linearly with the intensity of light impinging on said photocell; f. means to amplify said first direct voltage; g. a first electronic trigger circuit coupled to said last-mentioned amplifying means and arranged to operate at a value of said first direct voltage corresponding to a first threshold value of said electrical signal; h. means to delay operation of said first trigger circuit for a predetermined time interval; i. alarm signal indication producing means coupled to said first trigger circuit and arranged upon operation of the latter to produce an alarm signal indication; j. an amplifier circuit having an input coupled to the output of said linear amplifier to amplify said electrical signal; k. a rectifying circuit coupled to the output of said amplifier circuit and arranged to produce a second direct voltage proportional in value to the value of said electrical signal; l. a second electronic trigger circuit coupled to said rectifying circuit and arranged to operate at a value of said second direct voltage corresponding to a second threshold value of said electrical signal, said second threshold value corresponding to a light intensity impinging on said photocell which is substantially less than the light intensity corresponding to said first threshold value; and m. control means coupled to said second trigger circuit and arranged to be operated upon operation of said second trigger circuit, said control means having a control element arranged upon operation of said control means to prevent production of said alarm signal indication.

Claim 20

A projected beam-type smoke detection system, comprising: a. a source of a modulated light beam; b. a photocell physically spaced from said source and disposed in the path of said beam so that the output of said photocell is changed in response to changes in the intensity of the light impinging on said photocell; c. a linear amplifier coupled to said photocell and arranged to produce an electrical signal which varies linearly over a working range with changes in the intensity of light impinging on said photocell; d. a resistance element; e. means coupled to the output of said linear amplifier and to said resistance element and arranged to rectify said electrical signal to produce across said resistance element a first direct voltage which varies linearly with the intensity of light impinging on said photocell; f. means to amplify said first direct voltage; g. a first electronic trigger circuit coupled to said last-mentioned amplifying means and arranged to operate at a value of said first direct voltage corresponding to a first threshold value of said electrical signal; h. means to delay operation of said first trigger circuit for a predetermined time interval; i. alarm signal indication producing means coupled to said first trigger circuit and arranged upon operation of the latter to produce an alarm signal indication; j. an amplifier circuit having an input coupled to the output of said linear amplifier to amplify said electrical signal; k. a rectifying circuit coupled to the output of said amplifier circuit and arranged to produce a second direct voltage proportional in value to the value of said electrical signal; l. a second electronic trigger circuit coupled to said rectifying circuit and arranged to operate at a value of said second direct voltage corresponding to a second threshold value of said electrical signal, said second threshold value corresponding to a light intensity impinging on said photocell which is substantially less than the light intensity corresponding to said first threshold value; and m. control means coupled to said second trigger circuit and arranged to be operated upon operation of said second trigger circuit, said control means having a control element arranged upon operation of said control means to prevent production of said alarm signal indication.

Claim 21

A projected beam-type smoke detection system, comprising: a. a source of a modulated light beam; b. a photocell physically spaced from said source and disposed in the path of said beam so that the output of said photocell is changed in response to changes in the intensity of the light impinging on said photocell; c. a linear amplifier coupled to said photocell and arranged to produce an electrical signal which varies linearly over a working range with changes in the intensity of light impinging on said photocell; d. a first manually adjustable variable resistance element; e. means coupled to the output of said linear amplifier and to said first resistance element and arranged to rectify said electrical signal to produce across said first resistance element a first direct voltage which varies linearly with the intensity of light impinging on said photocell; f. means to amplify said first direction voltage; g. a first trigger circuit coupled to said last-mentioned amplifying means and arranged to operate at a value of said first direct voltage corresponding to a first threshold value of said electrical signal; h. means to delay operation of said trigger circuit for a predetermined time interval; i. alarm signal indication producing means coupled to said trigger circuit and arranged upon operation of the latter to produce an alarm signal indication; j. an amplifier circuit having an input coupled to the output of said linear amplifier to amplify said electrical signal; k. a rectifying circuit coupled to the output of said amplifier circuit and arranged to produce a second direct voltage proportional in value to the value of said electrical signal; l. a second trigger circuit coupled to said rectifying circuit and arranged to operate at a value of said second direct voltage corresponding to a second threshold value of said electrical signal, said second threshold value corresponding to a light intensity impinging on said photocell which is substantially less than the light intensity corresponding to said first threshold value; m. control means coupled to said second trigger circuit and arranged to be operated upon operation of said second trigger circuit, said control means having a control element arranged upon operation of said control means to prevent production of said alarm signal indication; and n. means including a secondary manually adjustable variable resistance element interposed between said photocell and the input to said linear amplifier to reduce the magnitude of said electricaL signal by a controlled amount to simulate the presence of smoke in said light beam.

Claim 22

A projected beam-type smoke detection system, comprising: a. a source of a modulated light beam; b. a photocell physically spaced from said source and disposed in the path of said beam so that the output of said photocell is changed in response to changes in the intensity of the light impinging on said photocell; c. a linear amplifier coupled to said photocell and arranged to produce an electrical signal which varies linearly over a working range with changes in the intensity of light impinging on said photocell; d. a first manually adjustable variable resistance element; e. means coupled to the output of said linear amplifier and to said first resistance element and arranged to rectify said electrical signal to produce across said first resistance element a first direct voltage which varies linearly with the intensity of light impinging on said photocell; f. means to amplify said first direction voltage; g. a first trigger circuit coupled to said last-mentioned amplifying means and arranged to operate at a value of said first direct voltage corresponding to a first threshold value of said electrical signal; h. means to delay operation of said trigger circuit for a predetermined time interval; i. alarm signal indication producing means coupled to said trigger circuit and arranged upon operation of the latter to produce an alarm signal indication; j. an amplifier circuit having an input coupled to the output of said linear amplifier to amplify said electrical signal; k. a rectifying circuit coupled to the output of said amplifier circuit and arranged to produce a second direct voltage proportional in value to the value of said electrical signal; l. a second trigger circuit coupled to said rectifying circuit and arranged to operate at a value of said second direct voltage corresponding to a second threshold value of said electrical signal, said second threshold value corresponding to a light intensity impinging on said photocell which is substantially less than the light intensity corresponding to said first threshold value; m. control means coupled to said second trigger circuit and arranged to be operated upon operation of said second trigger circuit, said control means having a control element arranged upon operation of said control means to prevent production of said alarm signal indication; and n. means including a secondary manually adjustable variable resistance element interposed between said photocell and the input to said linear amplifier to reduce the magnitude of said electricaL signal by a controlled amount to simulate the presence of smoke in said light beam.

Claim 23

A system as set forth in claim 16 including means to measure the voltage across said first resistance element.

Claim 24

A system as set forth in claim 15 in which said control means includes a relay arranged to have the normal condition thereof changed upon operation of said second trigger circuit, said relay having front and back contacts one of which forms said control element and the other of which is operatively connected to said first trigger circuit and is arranged to prevent operation of the latter until said relay has been restored to its normal condition after said electrical signal rises above said second threshold value thereof.

Claim 25

A system as set forth in claim 15 comprising means operated by said control means and arranged to produce a distinctive trouble signal indication upon operation of said second trigger circuit.

Claim 26

A system as set forth in claim 19 comprising means response to a drop in system supply voltage below a predetermined level to operate said second trigger circuit and thereby produce said distinctive trouble signal.

Claim 27

A system as set forth in claim 15 in which said source and the remainder of said system share a common power supply and in which means are provided to vary the gain of said linear amplifier in a sense to compensate for changes in the intensity of said light beam resulting from changes in the power supply voltage.

Claim 28

A smoke detection method in which the incidence of spurious alarms is minimized, comprising the steps: a. passing a modulated light beam along a path in the place to be protected; b. impinging said light beam on a photocell device located at one end of said path to produce an electrical signal varying in magnitude with the intensity of the light impinging on said photocell device; c. amplifying said electrical signal to produce an amplified signal which varies linearly over a working range with changes in said electrical signal; d. rectifying said amplifier signal to produce a first direct voltage which varies linearly with the intensity of light impinging on said photocell; e. generating an alarm signal indication in response to said first direct voltage decreasing to a first threshold value corresponding to a diminution in the light intensity impinging on said photocell as the result of smoke particles of given density being in said beam; f. delaying generation of said alarm signal indication for a predetermined time interval after said direct voltage reaches said first threshold value; g. separately rectifying said amplifier signal to produce a second direct voltage which varies with the intensity of light impinging on said photocell; h. actuating an electronic trigger in response to said second direct voltage decreasing to a second threshold value corresponding to a diminution in the light intensity impinging on said photocell as a result of substantial blockage of said beam; i. suppressing production of said alarm signal indication in response to actuation of said electronic trigger; and j. generating a distinctive trouble signal indication in response to actuation of said electronic trigger.

Claim 29

A smoke detection method in which the incidence of spurious alarms is minimized, comprising the steps: a. passing a modulated light beam along a path in the place to be protected; b. impinging said light beam on a photocell device located at one end of said path to produce an electrical signal varying in magnitude with the intensity of the light impinging on said photocell device; c. amplifying said electrical signal to produce an amplified signal which varies linearly over a working range with changes in said electrical signal; d. rectifying said amplifier signal to produce a first direct voltage which varies linearly with the intensity of light impinging on said photocell; e. generating an alarm signal indication in response to said first direct voltage decreasing to a first threshold value corresponding to a diminution in the light intensity impinging on said photocell as the result of smoke particles of given density being in said beam; f. delaying generation of said alarm signal indication for a predetermined time interval after said direct voltage reaches said first threshold value; g. separately rectifying said amplifier signal to produce a second direct voltage which varies with the intensity of light impinging on said photocell; h. actuating an electronic trigger in response to said second direct voltage decreasing to a second threshold value corresponding to a diminution in the light intensity impinging on said photocell as a result of substantial blockage of said beam; i. suppressing production of said alarm signal indication in response to actuation of said electronic trigger; and j. generating a distinctive trouble signal indication in response to actuation of said electronic trigger.

Claim 30

In the method of claim 22, the additional steps of measuring the value of said first direct voltage, gradually decreasing the magnitude of said electrical signal, and observing the decrease in value of said first direct voltage between the value thereof before the magnitude of said direct voltage is reduced and the value thereof at which said alarm signal indication is produce as a measure of the sensitivity of said method.