Abstract:
A safety circuit permits the operation of a device when the device is located in a safe operating environment. There is a power supply that supplies power to the sensor and to the logic circuits. The sensor could be one of any number of sensors depending on the environment variable which it is desired to detect or monitor and the size of the space for the detector to fit in. The logic circuits use the signal from the sensor to determine when a safe condition exists. When the logic circuits determine that a safe environment is present, the logic circuits send a signal to the power control circuit, which will permit the operation of the device, which if operated in an unsafe environment could endanger personnel and/or property. The results of operating a device in an unsafe environment could include, but are not limited to, fire; explosion; injury or death of personnel, or any other undesirable event.

Description:
This is a non-provisional application based on an earlier filed provisional application, Ser. No. 60/873,501 filed Jan. 30, 1998. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to devices used to detect the presence of toxic, explosive or otherwise dangerous gases; temperature; humidity; light; particulate; or other environment parameters, and more particularly to devices used to detect the presence of toxic, explosive, or dangerous gases; temperature; humidity; light; or other environment parameters and permit the operation or actuation of a device when the monitored environmental parameter is in the safe range. 
     2. Description of the Related Art 
     Toxic and explosive gas detectors have been used in many different areas. The first gas detector was probably the canary used in mining to indicate toxic atmospheres. Since the use of the canary began there have been many developments in gas detecting technology. Currently gas detectors are used to detect the presence of propane, carbon monoxide, gasoline vapors, hydrogen, oxygen and other gases. Typical sensor or detector locations are: 1) the bilge of a recreational or commercial vessel; 2) the inside of a house; 3) enclosed spaces prior to entry; and 4) areas where dangerous gases may be present due to manufacturing or transportation. 
     The detectors currently on the market merely provide an indication that a hazard is present or a value of the environmental parameter monitored or measured. This indication may be a warning alarm, a warning light, or meter indication. However, these detectors will not take any other action to prevent an explosion, fire, injury, death, or property damage in the event that an unsafe environment exists. The inability of these detectors to either act in the event an unsafe condition is detected or to permit action only when the environment is safe results in unnecessary injuries, deaths, and damage or destruction of valuable property by fire, explosion, toxic gas, or other environmental hazards. 
     SUMMARY OF THE INVENTION 
     The present invention has solved the problems cited above and generally comprises a safety circuit. There is a power supply that supplies power to the sensor and to the logic circuits. The sensor could be one of any number of sensors depending on the environment variable which it is desired to detect or monitor and the size of the space for the detector to fit in. The logic circuits use the signal from the sensor to determine when a safe condition exists. When the logic circuits determine that a safe environment is present, the logic circuits send a signal to the power control circuit which will permit the operation of a device which, if operated in an unsafe environment could endanger personnel and/or property. The results of operating a device in an unsafe environment could include, but are not limited to, fire; explosion; injury or death of personnel, or any other undesirable event. 
     The unsafe environments include, but are not limited to, the presence of hazardous temperature; light; high or low humidity; excessive vibration; smoke; toxic, explosive, flammable, or other dangerous gas; or toxic, explosive, flammable, or other hazardous particulate. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings incorporated in and forming a part of the specification illustrate several aspects of the present invention, and together with the description serve to explain the principles of the invention. In the drawings: 
     FIG. 1 is a functional block diagram in accordance with the present invention. 
     FIG. 2 is an electrical schematic of the preferred embodiment of the present invention. 
     FIG. 3A is a sample response curve for a sensor used to detect combustible gases. 
     FIG. 3B is a sample temperature/humidity dependency curve associated with the sensor response curve shown in FIG.  3 A. 
    
    
     Reference will now be made in detail to the present preferred embodiment of the invention, an example of which is illustrated in the accompanying drawings. 
     DETAILED DESCRIPTION 
     1. Overview 
     The purpose of the safety circuit is to permit operating an electrical or other device when the device is in a safe environment. The environmental parameters that could be monitored include, but are not limited to, pH, temperature, humidity, gas concentration, particulate concentration, conductivity, resistance, electrical charge, light intensity, salinity, radiation, and any other environmental parameter capable of being measured. The sensor will typically be a gas sensor. Although any type of gas may be sensed, typically the gases may be: propane vapor, gasoline vapor, hydrogen, oxygen, other explosive or flammable gases; or carbon monoxide, freon, or other toxic gases. 
     This system uses a regulated power supply to provide power to both the logic circuit and to the sensor. The sensor provides an output signal which will vary depending on the environmental parameter that the sensor is designed to detect. The logic circuit receives the signal from the sensor. When the logic circuit detects a signal indicating that a safe environment is present, the logic circuit will indicate a safe condition to the power control circuitry. A safe environment is an environment where the environmental variable being monitored is safe for both personnel and the equipment (which the safety circuit controls) to operate. Upon receiving an indication of a safe condition, (including proper and safe operation of the safety circuit) the power control circuit will act to permit the operation of the electrical or other device that is controlled by the safety circuit. A safe condition is a safe environment together with the proper and safe operation of the safety circuit. For example, if a safety circuit with an explosive gas sensor was installed in a portable drill, and if the operator inadvertently took the portable electric drill into an area which had an explosive concentration of propane gas, the circuit would prevent the drill from being operated by preventing the electricity from reaching the motor. 
     2. Description of the Functional Block Diagram 
     Referring now to the drawings in detail, wherein like numerals indicate the same elements throughout the views, FIG. 1 shows a block diagram of safety circuit  10 . Safety circuit  10  is comprised of the following functional blocks: power supply  20 , sensor  40 , logic circuit  60 , and control circuit  80 . 
     Power supply  20  supplies the power to both sensor  40  and logic circuit  60 . Power supply  20  typically provides the proper voltage for both logic circuit  60  and sensor  40 . Logic circuits typically operate on between 3 and 5 volts and thus power supply  20  should provide an output at the proper voltage for the logic circuits utilized. Additionally, sensors  40  utilized with this circuit typically have voltage requirements from 5 to 25 volts dc. However some sensors that may be interfaced with this circuit may require different voltages. Therefore, power supply  20  will typically have a second voltage output if the sensor  40  requires a different voltage than the logic circuit  60 . 
     Sensor  40  is any sensor that is required or desired to be used in a specific application. Typically, a single sensor will be used, however, there are safety circuits that can effectively use two or more sensors connected either in series or parallel. When two or more sensors  40  are employed, the sensors  40  may be identical sensors  40  placed in two different locations so that a larger area is monitored. Alternatively, the sensors  40  may monitor two different environmental variables, for example, both a conductivity sensor  40  and a pH sensor  40  could be used to monitor a steam system for proper operation. Typically, the sensor  40  selected will be used to detect an explosive gas mixture in the atmosphere. There are, however, applications for sensors capable of detecting other environmental parameters. For example: using a toxic gas sensor on the safety circuit to prevent inadvertent entry to a room into which a toxic gas has leaked; or using both temperature and humidity sensors in the safety circuit to shut down a steam system on indications of a steam rupture. 
     Logic circuit  60  contains the appropriate circuits necessary to determine when a safe environment is present based on the signal provided by the sensor. Since this is a safety device, it is preferred that the logic circuit use redundant logic subcircuits. Additionally, since this is a safety circuit each logic subcircuit should provide an affirmative signal indicating that the environmental parameter measured is in the safe range. When the environment is safe and all the upstream portions of the circuit are operating properly the output of the logic circuit is a signal which will cause the power control circuit  80  to permit the device to which the safety circuit  10  is attached from operating. Typically the safety circuit  10  will be used in or on an electric device and the power control circuit  80  would permit the electrical power to energize this device. 
     3. Circuit Diagram 
     FIG. 2 provides a circuit diagram for the preferred embodiment of a safety circuit  10  in accordance with the present invention. The safety circuit  10  has the same basic components as shown in the functional block diagram (FIG.  1 ). These components are: power supply  20 , sensor  40 , logic circuit  60 , and power control circuit  80 . 
     Power supply  20  is a regulated power supply that typically supplies a relatively constant voltage to the sensor  40  and logic circuit  60 . The power supply is designed to provide the appropriate power level for the sensor  40 , the logic circuit  60 , and if required, the appropriate voltage for the rest of the electrical circuit; including, the power control circuit  80 . In the preferred embodiment power control circuit  80  does not use any power from the power supply  20 . Control circuit  80  receives its power directly from the same source as the device which safety circuit  10  controls. Sensor  40  will use the output of power supply  20  to provide power for the sensing element and, if required, for a heating or other element of the sensor. Power supply  20  also provides power to the op amps and to the resistors used in a voltage divider to set a “safe” window voltage to which the output of sensor  40  is compared in logic circuit  60 . The design and manufacturing of regulated power supplies providing specific output voltages is well known and thus will not be described in detail. 
     The sensor  40  samples the environment around the sensor and provides a detection signal to the logic circuit  60 . Sensors  40  that are used to detect flammable or explosive atmospheres typically have a heating element which maintains the sensor at a specific temperature and a sensing element whose resistance varies with the concentration of flammable or burnable materials in the atmosphere. FIG. 3A shows a typical response curve for a combustible gas sensor. The resistance of this sensor lowers as the concentration of a combustible gas increases. The resistance of the sensing element of the sensor  40  will determine voltage of the signal that is input to the logic circuit  60 . Additionally, the resistance of the sensing element in combustible gas sensors will vary with the temperature/humidity of the air around the sensor as shown in FIG.  3 B. Thus, the voltage of the output signal from sensor  40  will depend upon the environment around the sensor and the input voltage from power supply  20 . During safe conditions, the voltage of the output signal from sensor  40  stays within a relatively narrow band. 
     Since this is a safety circuit, logic circuit  60  is formed primarily from two identical LM393 window comparators  62 ,  64 . Each window comparator has two op amps that are wired in a logical “or” configuration. The voltage range over which the comparators  62 ,  64  will produce a high output is determined by the values selected for resistors R 7 , R 8 , and R 9  for comparator  64  and resistors R 18 , R 19 , and R 20  for comparator  62 . Some sensors  40  used to measure environmental parameters other than temperature have output voltages that are subject to undesired temperature variations (FIG.  3 B). If the output voltage of sensor  40  is subject to undesired temperature variations, then a thermistor TH 1  is added to resistors R 7 , R 8 , and R 9  to shift the “safe” voltage window for comparator  64  to compensate for the temperature dependence of sensor  40 . Similarly, a thermistor TH 2  is added to resistors R 18 , R 19 , and R 20  for comparator  62 . It is preferred that the temperature response curve of thermistors TH 1  and TH 2  compensate for the temperature dependency of sensor  40  over the expected operating temperatures of safety circuit  10 . When the voltage output of the gas sensor  40  is in the safe range, the output of both window comparators will be high. When the voltage output of the sensor  40  is outside the “safe” window the logic circuit will act as if an unsafe environment existed. Thus, the output of one or both window comparators  62 ,  64  will be low when the voltage output from sensor  40  is outside the “safe” window. For example, in the present circuit the voltage output of a sensor  40  may fall below the safe range either due to a failure of sensor  40  or power supply  20 , or due to a low voltage condition. When the voltage input to window comparators  62 ,  64  is below the safe window the output of op amp U 2 A of comparator  64  and op amp U 3 A of comparator  62  will go low, forcing the output of each window comparator  62 ,  64  to be low. Thus, the output of logic circuit  60  to power control circuit  80  will be low. Alternately, when the sensor  40  is a combustible gas sensor and, senses an unsafe condition, the sensor&#39;s  40  output voltage increases due to the explosive or flammable gas in the atmosphere reducing the resistance of the sensing element in sensor  40 , With the voltage input to comparators  62 ,  64  is above the “safe” window, the output of op amp U 3 B of comparator  62  and U 2 B of comparator  64  will go low with the same result as discussed above when op amps U 2 A and U 3 A go low. 
     Power control circuit  80  is also constructed in a redundant fashion. Power circuit  80  has two switch circuits  82 ,  84 ; two triac pulse detection circuits  86 ,  88 ; two over current protection circuits  90 ,  92 ; an one IDEC RSSAN relay R 1 . Only one relay R 1  is used, since a failure of relay R 1  would cause the circuit to fail in a safe manner by preventing the operation of the equipment attached to or controlled by safety circuit  10 . Switch circuit  82  is coupled to and receives an input from window comparator  62  and switch circuit  84  is coupled to and receives an input from window comparator  64 . When there are no faults within power control circuit  80 , and a “safe” condition exists, a high output (safe condition) from the comparator  62  will actuate switch circuit  82  and a high output (safe condition) from comparator  64  will actuate switch circuit  84 . Both switch circuits  82  and  84  are coupled to and provide a low resistance current path to relay R 1 . When both switch circuits  82  and  84  are triggered, current will flow to relay R 1  causing relay R 1  to energize, closing contacts  94  that will permit the electric or other device to which safety circuit  10  is connected to operate. Additionally, the preferred embodiment has an ARTISAN 436 U.S.A. time delay relay (not shown). This relay typically has a one minute time delay upon energizing the circuit  10  and time delay relay. This one minute time delay will prevent erroneous response of safety circuit  10  while circuit  10  is warming up. Additionally there is a two minute time delay after safety circuit  10  removes power from the device due to the detection of an unsafe condition. 
     Switch circuits  82  and  84  are triggered by high outputs from window comparators  62 ,  64  of logic circuit  10 . For example, a high output form window comparator  62  will cause current to flow through a H11J3 opto-isolator U 6  provided that pulse detection circuit  88  is sensing pulses across triac Q 2 . Thus, a voltage will be applied to diac CR 8 , when the voltage applied to diac CR 8  reaches diac&#39;s CR 8  break over voltage, diac CR 8  will allow current to flow through diac CR 8  and trigger triac Q 2 . Diac means either a diac or an assembly of diodes or other devices that will permit a large enough voltage to develop across the triac, during the portion of the AC cycle when the opto-isolator is forward biased, to trigger the opto-isolator before the triac is triggered. When triac Q 2  is triggered, triac Q 2  will permit current flow through triac Q 2 . Since this circuit uses an AC power source, triac Q 2  will pulse because diac CR 8  will not constantly trigger triac Q 2 . 
     As a further safety feature there are two triac pulse detection circuits  86 ,  88 . These circuits sense the voltage across the triac in each switch circuit  82 ,  84 . The pulse detection circuit  86  senses the voltage across triac Q 2  in switch circuit  82  and pulse detection circuit  88  senses the voltage across triac Q 1  in switch circuit  84 . When switch circuit  82  is activated the voltage across the triac Q 2  will pulse, indicating that the triac Q 2  has been triggered and is functioning properly. The triac Q 1  in switch circuit  84  will behave in a similar manner. When detection circuit  86  detects that triac Q 2  of switch circuit  82  is turned on and functioning properly, the detection circuit  86  will permit switch circuit  84  to be activated. Similarly, when detection circuit  88  detects that triac Q 1  of switch circuit  84  is triggered and functioning properly, the detection circuit  88  will permit switch circuit  82  to be activated. 
     For example, when triac Q 1  pulses there is a time period where triac Q 1  has a voltage difference and a time period when triac Q 1  does not have a voltage difference across triac Q 1 . When there is a voltage difference across triac Q 1 , a 4933 opto-isolator IS 02  will permit current flow. Thus, a 1RE capacitor C 1  will discharge and the voltage between the base of and the collector of a 2N3906 transistor Q 3  will permit current to flow through transistor Q 3 . With current flowing through transistor Q 3 , current will flow through opto-isolator U 6  to ground. When triac Q 1  is permitting current to flow, there will not be a voltage difference across triac Q 1 . Thus, opto-isolator tor IS 02  will prevent current to flow through opto-isolator IS 02  to ground and capacitor C 1  will recharge. During the initial portion of the capacitor&#39;s C 1  recharge the voltage between the base and the collector of transistor Q 3  will be low enough that transistor Q 3  will continue to permit current to flow through transistor Q 3 . Capacitor C 1  is sized to accommodate the pulse length of the triac Q 1  selected, so that before the voltage rise across capacitor C 1  is sufficient to turn off transistor Q 1 , the triac Q 1  has a voltage across the triac Q 1  and capacitor C 1  is discharged. 
     However, if triac Q 1  stops pulsing but does not have a voltage drop across the triac Q 5 , then the capacitor C 1  will continue to charge and the voltage across capacitor C 1  and across the base and collector of transistor Q 3  will increase until transistor Q 3  turns off. With no current passing through transistor Q 3 , no current will flow through opto-isol ator U 6  resulting in switch circuit  82  turning off or preventing switch circuit  82  from turning on. Pulse detection circuit  86  will operate in a similar fashion to that described above. If the detection circuit  86  does not detect a pulsing voltage across triac Q 2 , then the pulse detection circuit  86  would prevent switch circuit  84  from accuating or turn off switch circuit  84  if this circuit was already operating. 
     If there is a short or fault within power control circuit  80  which causes a high current within control circuit  80 , then either or both current protection circuits  90 ,  92  will operate to protect power control circuit  80 . Protection circuit  90  protects power control circuit  80  by shunting the output from window comparator  62  to ground. The shunting of the output from window comparator  62  to ground will cause switch circuit  82  to see a low input, which results in switch circuit  82  turning off. Similarly, protection circuit  92  will cause switch circuit  84  to turn off. 
     For example, the current protection circuit  90  operates by using the voltage developed across resistor R 22  to trigger a H11J3 opto-isolator U 7 . Resistor R 22  is selected so that when the current through resistor R 22  exceeds safe levels then the voltage across resistor R 22  will trigger opto-isolator U 7 . When U 7  is triggered the output of window comparator  62  of logic circuit  60  is stunted to ground with the result described above 
     4. Operation 
     a. Normal Operation 
     The power supply  20  provides power to gas sensor  40  and to logic circuit  60 . Sensor  40  will provide a steady or relatively steady output signal to logic circuit  60 . This signal will fall within the “safe” voltage window of the window comparators  62 ,  64  of logic circuit  60 . The window comparators  62 ,  64  will produce a high output which accuates switch circuits  82 ,  84  of power control circuit  80 . Upon accuation of both switch circuits  82 ,  84  relay R 1  is energized. Energizing relay R 1  will permit the device to which the circuit is attached to function. 
     b. Low Voltage 
     When there is a low voltage supplied to power supply  20 , the voltage regulator VR 1  fails to provide a high enough voltage, or sensor  40  fails to send an output signal, then the voltage input to window comparators  62 ,  64  of logic circuit  60  will be below the “safe” voltage window. This input to window comparators  62 ,  64  will result in an overall low output from window comparators  62 ,  64  resulting in a low signal to switch circuits  82  and  84  of power control circuit  80 . A low input to switch circuits  82  and  84  will prevent these circuit from operating or if operating to turn off. When switch circuits  82  or  84  are off relay RI will be deenergized and the contacts in the motor controller for the electric device will remain open and the device will not start. 
     c. High Current in Power Control Circuit  80   
     When over-current protection circuit  90  detects an over current condition it shunts the output from the window comparator  62  to ground. As a result of this shunt switch circuit  82  will see a low input and will turn off. When switch circuit  82  is off relay R 1  will be deenergized with the results as described above Over protection circuit  92  will function in a similar manner to that described above. High current in power control circuit  80  would typically be caused by a short circuit or a fault to ground within the circuit. 
     d. Dangerous Concentration of Gas Present 
     In the event that there is an unsafe environment detected by that gas sensor  40 , sensor  40  will typically produce a high voltage output that will be above the “safe” voltage window of window comparators  62 ,  64 . A voltage input to logic circuit  60  above the “safe” voltage window for comparators  62 ,  64  will cause window comparators  62 ,  64  to have a low output with the results described above. 
     e. Short Across a Triac 
     If a short develops across triac Q 2  of switch circuit  82  either due to a failure or due to an over voltage condition, then the pulse detection circuit  86  will not detect the pulsing of the triac Q 2 . When detection circuit  86  no longer detects the pulsing of the triac Q 2 , then the detection circuit  86  will prevent switch circuit  84  from operating. Without both switch circuits  82 ,  84  operating, relay R 1  will be de-energized and, as a result, the attached electrical device will either shut down or not be permitted to start. A short across triac Q 1  of switch circuit  84  would cause detection circuit  88  to act in a similar fashion and produce similar results. 
     In summary, numerous benefits have been described which result from employing the concepts of the invention. The foregoing description of a preferred embodiment of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Obvious modifications or variations are possible in light of the above teachings. The embodiment was chosen and described in order to best illustrate the principles of the invention and its practical application to thereby enable one of ordinary skill in the art to best utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto.