Classifications

• • G—PHYSICS
  • G08—SIGNALLING
  • G08B—SIGNALLING OR CALLING SYSTEMS; ORDER TELEGRAPHS; ALARM SYSTEMS
  • G08B17/00—Fire alarms; Alarms responsive to explosion
  • G08B17/12—Actuation by presence of radiation or particles, e.g. of infrared radiation or of ions
• • F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
  • F24—HEATING; RANGES; VENTILATING
  • F24C—DOMESTIC STOVES OR RANGES ; DETAILS OF DOMESTIC STOVES OR RANGES, OF GENERAL APPLICATION
  • F24C7/00—Stoves or ranges heated by electric energy
  • F24C7/08—Arrangement or mounting of control or safety devices
• • G—PHYSICS
  • G08—SIGNALLING
  • G08B—SIGNALLING OR CALLING SYSTEMS; ORDER TELEGRAPHS; ALARM SYSTEMS
  • G08B21/00—Alarms responsive to a single specified undesired or abnormal condition and not otherwise provided for
  • G08B21/18—Status alarms
  • G08B21/22—Status alarms responsive to presence or absence of persons

Definitions

• the present invention relates to an overheating alarm, and in particular to a device, which, located adjacent to a hot object, measures radiated infra-red rays, arranged to transmit a warning signal and/or interrupt the power supply when an object under surveillance becomes exceptionally warm, i.e. when a predetermined temperature level is exceeded.
• the overheating alarm can be arranged in a housing, for example attached to a wall in the premises where surveillance is to be carried out.
• Temperature indication by means of thermo elements, arranged in each monitored hotplate, can obviously also be used, if such monitoring is arranged to result in suitable actions when a predetermined temperature level is exceeded.
• cookers often lack protection against overheating, and often only one hotplate is arranged temperature controlled. Also for other purposes, it is desirable to accomplish surveillance against overheating, for example electrical motors, electrically heated radiators and other types of objects.
• the object of the present invention is to disclose an overheating alarm, which can be arranged at a distance from the object to be placed under surveillance, arranged to transmit alarm and/or interrupt the power supply when the temperature of the object exceeds a predetermined temperature level .
• the overheating alarm according to the present invention is intended for surveillance of heat emitting objects, for example hotplates, electric cookers or similar, and is mainly characterised in that emission of heat from an object under surveillance is being monitored by an IR-detector, arranged located at a distance from the object under surveillance, arranged to transmit a warning signal and/or interrupt the power supply to same when the temperature level for the object under surveillance exceeds a predetermined maximum temperature level.
• the IR- detector is arranged with a highpass filter, which by means of absorption or reflection reduces or prevents radiation having shorter wavelengths than 1 - 3 um from influencing the detector element included in the IR-detector, and that the IR-detector is arranged with a short- pass filter, arranged to reduce radiation having longer wavelengths than 3 ⁇ m.
• Fig. 1 shows a perspective view of a cooker and associated kitchen fan, disclosing an embodiment attached to the lower plane of the kitchen fan, intended to monitor the hotplates of the cooker;
• Fig. 2 shows a diagram with regard to radiated power from a hotplate as a function of the temperature of same
• Fig. 3 shows a diagram with regard to spectral dispersion of radiated effectt different temperatures
• Fig. 4 shows a diagram of the temperature of hotplates as a function of time, indicating with a continous line all hotplates at maximum power
• Fig. 5 shows how the temperature and output signal of a certain detector element varies with time when the hotplates emit power according to Fig. 4, and with the detector located on the wall behind the cooker, and with the output voltage over 1 Mohm from a bridge having 9 V feeding voltage;
• Fig. 6 shows a circuit diagram of an embodiment of an overheating alarm according to the present invention.
• the shown embodiment which is more fully described below, is intended to be used for surveillance of electrical hotplates at a cooker, but the overheating alarm can obviously also be used for other applications.
• the overheating alarm measures IR- radiation from a monitored object, e.g. a hotplate, and warns and/or interrupts the power supply to the monitored object when the temperature of same exceeds a predetermined temperature level.
• a monitored object e.g. a hotplate
• the technical background is well known, and devices for measuring without physical contact have been known for a long period of time. In an ardometer for example, emitted heat is focused by means of a lens against a thermo element, thereby heating same, and resulting voltage is measured by means of a miHivoltmeter.
• IR-target searching devices are also previously known, which can detect warm objects at a distance of several kilometers. However, any embodiment intended for the field of application according to the present invention is previously not known.
• Emitted heat is absorbed and refl ected in a fashion simil ar to visible l ight.
• the human eye can only regi ster radiation wi thin the wavelength range 0,45 - 0,65 yum.
• Shoul d the eye instead act sensitively in the so cal led medium IR-range of 2-6 ⁇ m, the kitchen woul d be l it up by the heated hotpl ates which woul d appear as strong photo l amps.
• FIG. 2 shows the effect of hotplates in three conventional sizes emitted as a function of temperature.
• the spectral dispersion is shown in Fig. 3. As shown, the wavelength in which maximum emission is achieved is reduced with increased temperature, from approximately 10 ⁇ m at room temperature to o approximately 3 yum at 700 C. This fact, in combination with the several times increased power obtained, makes it possible to fulfill the above stipulations.
• the short-pass filter can be omitted, provided that the sensitivity of the detector is reduced at a corresponding wavelength.
• a number of detectors can be used to transfer the incoming radiation into for example an electrical signal. They are divided into two main groups. Photon (photo) or quantum detectors utilize the changes in electrical properties which arise in semiconductor materials during IR-radiation. By a suitable choice of material and degree of doping, maximum sensitivity can be obtained for for a certain wavelength/tem ⁇ perature. Any temperature changes in the detector can be regarded as nonimportant. Amongst others, the following detectors operate according to this principle.
• Photoconductive detectors Incoming photons against a semiconductive material can lift up electrones in the conductive band - the photo ⁇ electric effect. As a result, the resistance of the material is changed. When one applies a voltage over the material, incoming photons are detected by altered current. Mainly used to detect rapid changes.
• Photovoltaic detectors These very sensitive detectors generate a voltage when made subject to photon radiation below a certain restricting wavelength. Bias voltage is not required.
• Thermal detectors are basically temperature indicating and result in a monitorable output signal when the temperature of the detector element is changed.
• Thermistors comprise of semiconductor material, having a resistance strongly dependent of the temperature of the element.
• Thermistors may have a positive temperature coefficient, PTC- thermistors, with a resistive body consisting of sintered oxides.
• NTC-thermistors are manufactured in a basically corresponding way, but from other oxides, which results in a negative temperature coefficient. At room temperature, the change in resistance is 2-5% per degree C.
• thermo electrical voltage source is created feeding a current through the circuit, and having a magnitude in relation to the temperature difference.
• a combination of iron and constantan would for example result in 52 yjV/C at room temperature.
• the voltage can be increased by connection of several elements in series.
• Pyroelectrical detectors A pyroelectrical chrystal has spontaneous polarisation or charge concentration, which is strongly dependent of the temperature. A thin layer of such a material between two electrodes form a condensator, the charge of which is a function of temperature. Heating of the element alters the charge and produces a voltage over the electrodes proportional to the radiation. Pyroelectrical detectors are also very sensitive, but are best suited for monitoring rapid changes.
• Pneumatical detectors Pressure changes in a gasfilled chamber are registered in these.
• Suitable photondetectors are lead sulphide, PbS, and lead selenium, PbSe, both being photo conductive semiconductor detectors influenced by IR-radiation without requiring direct heating. Without additional cooling, their maximum sensitivity o for temperatures resides in the region of 600 C, being rapidly reduced for both higher and lower temperatures, which eliminates the need for filters. Such a warning device could in principle be used anywhere.
• both thermistors As a bandpass filter, conventional glass and a dark coloured plastics film was used. In order to ensure that the temperature difference only is related to differences in incoming radiation, it is important that the glass filter used to screen IR-radiation having low frequencies can be "seen" by both thermistors, in order to compensate for heating of the- glass.
• the best function is achieved utilizing two glass plates, separated from each other by means of an air filled space. The inside glass will thereby screen emitted heat from the outer, the temperature of which is increased due to emission of heat and e.g. steam from the cooking vessels.
• both thermistors must be located within the same housing, which must be manufactured from a material having good heat transmission properties, e.g. aluminium.
• the temperature increase achieved in the detector made subject to radiation becomes a function of the difference between absorbed heat radiation and the heat emitted by convection and the heat conducted by the connection wires.
• the wires leading away from the thermistors must have small cross-section and be heat insulated. Concentration of radiation, e.g. by means of a semi-spherical glass member, may be required if the detector is to be located at a longer distance away from the hotplate. Also in this case, the glass will serve as a shortpass filter, screening radiation from bodies having a lower temperature.
• a so called industrial circuit from National Semiconductor denominated 1801 was been used, which was primarily designed for use in fire warning devices of ionization type.
• the electrical circuit is shown in Fig. 6.
• the circuit is connected to a 9 V battery and includes two voltage stabilizers of which one at pin two, having an output level of 5,9 V, is used to feed the bridge circuit. Same comprises of thermistors Tl and T2, and resistors RI and R2.
• the last mentioned is connected in series with a variable resistor R3, which is used to unbalance the bridge, whereby the output voltage obtained via the resistor R4 to the comparator can be unbalanced in the region of 100 V.
• the comparator will monitor when the voltage at the negative input, pin 4, becomes higher than the negative, pin 5, and then trigger via an OR-circuit the final stage, driving a suitable buzzer.
• the circuit also includes a voltage sensing device, causing a short alarm signal each other minute when the battery is becoming discharged and should be replaced.
• the level is decided by means of the resistors R10 and Rll and should be choosen in such a way, that a warning signal is obtained when the voltage becomes lower than approximately 8 V. This voltage is sufficient for continous operation of the warning device during several months.
• the capacitor Cl When an overheating alarm occurs, the capacitor Cl is discharged via R8 and the diode Dl, thereby offsetting the bridge and reducing the voltage applied to pin 4, and interrupting the alarm signal. With the components choosen, a short signal is achieved with intervals of 15 seconds. However, this interval is reduced in proportion to absorbed radiation, i.e. the alarm frequency is successively increased if the hotplate is not disconnected.
• the relatively large capacitors C2 and C3 reduce sensitivity for disturbances, for instance as occuring when the fan is switched on or off.
• NS 1801 is replaced by a more modern circuit, e.g. CA3164E from RCA.
• This circuit also includes an output facilitating connection in parallel of up to 20 overheating alarms.
• most battery operated fire warning alarms today being installed in most homes. This is of particular value in service apartments, since a large number of warning devices of both types can be connected in parallel to a common warner, e.g. located with the supervisor, or connected to an external alarm.
• the overheating alarm can also be arranged including a smoke detector, in order to accomplish alarm during smoke formation, since formation of smoke in certain cases preceed exceeding of a predetermined temperature level. Detectors of standard type can be integrated with the same bridge coupling, thus resulting in alarm both for smoke generation and overheating.
• the alarm apparatus can either receive a voltage supply from a battery, or alternatively be connected to a suitable electrical circuit at the monitored object, e.g. a signal lamp which is lit when the monitored object is switched on. It is obviously also possible to perform the connection in such a way, that surveillance is maintained continously, independent of whether the monitored object s switched on or not.
• hotplates have been used as heat emitting and monitored objects, and with the detector located at a distance of 0,36 from the hotplates. Good temperature monitoring has been confirmed, and the detector was not noticably influenced by heat emitted from the hotplates. Said distance can be further varied within broad limits, while maintaining the good functional properties.
• the detector with suitable logic functions, e.g. by connection of a micro processor, in order to accomplish an intermittent warning signal related to earlier temperature changes and the temperature derivate.

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• Engineering & Computer Science (AREA)
• Emergency Management (AREA)
• Physics & Mathematics (AREA)
• General Physics & Mathematics (AREA)
• Business, Economics & Management (AREA)
• Combustion & Propulsion (AREA)
• Chemical & Material Sciences (AREA)
• Mechanical Engineering (AREA)
• General Engineering & Computer Science (AREA)
• Fire-Detection Mechanisms (AREA)
• Electric Stoves And Ranges (AREA)
• Control Of Resistance Heating (AREA)
• Cookers (AREA)

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• 1984
  • 1984-04-30 SE SE8402344A patent/SE8402344L/ unknown
• 1985
  • 1985-06-07 EP EP85903086A patent/EP0215793A1/en not_active Withdrawn
  • 1985-06-07 JP JP60502792A patent/JPS61502372A/ja active Pending
  • 1985-06-07 WO PCT/SE1985/000240 patent/WO1986000179A1/en not_active Ceased
• 1986
  • 1986-01-21 NO NO86860209A patent/NO860209L/no unknown
  • 1986-02-05 DK DK56986A patent/DK56986A/da not_active Application Discontinuation

Non-Patent Citations (1)

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