Abstract:
A detector of an event includes an electrical energy generator formed by a flexible piezoelectric element with a weight fastened to the flexible piezoelectric element that is biased with the weight in a position with the piezoelectric element flexed. In response to detection of the event, a trigger releases the weight so as to cause a vibration of the piezoelectric element. This vibration is converted by the flexible piezoelectric element into electrical energy. An electronic system is power by the electrical energy and is operable to generate an electrical signal indicative of the detected event.

Description:
PRIORITY CLAIM 
       [0001]    This application claims priority from French Application for Patent No. 1562409 filed Dec. 15, 2015, the disclosure of which is incorporated by reference. 
       TECHNICAL FIELD 
       [0002]    Embodiments of the invention relate to event detectors, for example heat detectors or shock detectors, although these examples are not limiting; and more particularly, to a contactless, batteryless autonomous event detector, in particular one based on mechanical energy storage. 
       BACKGROUND 
       [0003]    The purpose of an event detector is generally to indicate the occurrence of a given event, such as smoke, heat, movement, light or shock. 
         [0004]    Generally, an event detector of this type requires a power source, such as batteries, to supply an electronic module configured to generate an alarm signal, for example an acoustic and/or light signal, in the presence of the event. It is therefore always necessary to manage the life of the power source, which may cause problems. 
       SUMMARY 
       [0005]    In one embodiment, therefore, a batteryless autonomous event detector is proposed, based on mechanical energy storage. Energy generation is performed on a one-off basis by conversion of the stored mechanical energy on the occurrence of the event to be detected. The detector signal is also delivered, wirelessly for example, to a receiving node connected, for example, to a central action network such as a fire brigade center. 
         [0006]    According to one aspect, an event detector is proposed, comprising: an electrical energy generator, including a flexible piezoelectric element and a weight fastened to the element; a trigger having a first configuration in which the trigger is configured to keep the weight in an initial position with the piezoelectric element flexed, and a second configuration in which the trigger, in response to said event, is configured to release the weight so as to cause a vibration of the piezoelectric element and enable electrical energy to be generated; and an electronic system coupled to the generator and configured to convert the electrical energy delivered by the generator into at least one electrical signal. 
         [0007]    According to one embodiment, the event to be detected is the crossing of a temperature threshold in one or other direction. 
         [0008]    The event detector may comprise a chassis, for example, and the trigger may include at least one part which is deformable as a function of the temperature, the deformable part being fixed to the chassis and configured to be sufficiently deformed when the temperature crosses the temperature threshold, so as to place the trigger in its second configuration and enable the weight to be released. 
         [0009]    Additionally, the deformable part may comprise a bimetallic strip, for example. Said temperature threshold may advantageously be determined by the critical temperature of the bimetallic strip. 
         [0010]    This detector may advantageously be used as a fire detector. 
         [0011]    According to another embodiment, the event to be detected is a threshold shock level on the detector. 
         [0012]    The trigger may comprise a chassis, and the trigger may comprise a retaining element fixed to the chassis, configured to retain the weight in said initial position and release it in the presence of a shock on the chassis at the threshold shock level. 
         [0013]    This shock detector may be widely used in home automation, motor vehicle and equipment storage applications, and for detecting an intrusion into a vehicle or room. 
         [0014]    The electronic system may also comprise, for example, a storage means configured to store the electrical energy delivered by the generator and an electronic circuit supplied by the storage means and configured to deliver said at least one electrical signal. 
         [0015]    The electronic system may advantageously comprise an interface, for example a wireless interface (Wi-Fi, or NFC (“Near Field Communication”)) linked to a communications network such as the internet. 
         [0016]    A wireless interface enables event detectors to be moved and installed easily in desired locations. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0017]    Other advantages and characteristics of the invention will be evident from an examination of the detailed description of embodiments, which are not limiting in any way, and the appended drawings, in which: 
           [0018]      FIGS. 1 to 4  show in a schematic way some aspects of the invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0019]      FIG. 1  shows schematically an event detector, in this case a heat detector  1 , comprising an electrical energy generator  2 , a flexible trigger  3 , an electrical system  4  and a chassis  5 . A receiving node  6  for receiving the alarm signal delivered by the detector  1  via a wireless connection is also shown in  FIG. 1 . 
         [0020]    The electrical energy generator  2  comprises a flexible piezoelectric element  21 , one end  21   a  of which is fixed to a rigid block  22  fastened to the chassis  5 , and an oscillating weight  23  fastened to a second end  21   b  located opposite the first end  21   a.    
         [0021]    The piezoelectric element  21  is torsionally deformable to enable the oscillating weight  23  to reach at least one initial position  24  in which the generator  2  can store energy in mechanical form, that is to say potential energy. 
         [0022]    Additionally, given the intrinsic characteristics of the piezoelectric element  21 , the electrical energy generator  2  can be used to convert the stored mechanical energy into electrical energy for delivery to the electrical system  4  when the generator  2  is triggered by the trigger  3 . 
         [0023]    The trigger  3  comprises a catch  31  and a deformable part  32  coupled between a rigid base  33  fastened to the chassis  5  and the catch  31 . 
         [0024]    Here, in the case of the heat detector, the deformable part  32  comprises a bimetallic strip  34  including two different metal plates, each having a different coefficient of expansion, glued to one another. The bimetallic strip  34  is therefore deformed with a variation in temperature. 
         [0025]    It should be noted that the temperature detection threshold of the detector  1  is necessarily determined by the critical temperature of the bimetallic strip  34 . 
         [0026]    The trigger  3  has a first configuration and a second configuration. 
         [0027]    In the first configuration, the trigger  3  is configured to retain the oscillating weight  23  in its initial position  24  by means of the catch  31 . 
         [0028]    In the second configuration, the trigger  3  is configured to release the oscillating weight  23  from the initial position  24 , when the temperature threshold is crossed, so as to initiate a cycle of mechanical vibration which can be converted to electrical energy by the piezoelectric element  21 . 
         [0029]    The electronic system  4  of the detector  1  is fixed to the rigid block  22  and coupled electrically to the piezoelectric element  21  so as to receive and use the electrical energy converted by the electrical energy generator  2 . 
         [0030]    As described in greater detail below, the electronic system  4  is configured to convert the electrical energy delivered by the generator  2  into at least one electrical signal SE, and then, in this example, into a radio frequency alarm signal SR. 
         [0031]    Advantageously, the electrical signal SE delivered by the electronic system  4  comprises a data frame intended to be captured by an external receiving node  6  which is permanently operational and connected to an alarm network, so that action can be taken in response to the triggering of the alarm, including, for example, a warning on a mobile phone, or a call to a fire brigade, specifying full details of the position of the room in which the detector  1  is installed. 
         [0032]    The detector  1  further comprises a resetter  7 , located for example above the oscillating weight  23 , and configured to replace the weight  23  in the initial position  24  after each triggering of the generator  2 . 
         [0033]    Reference will now be made to  FIG. 2 , to illustrate an example of the internal structure of the electronic system  4 . This comprises: 
         [0034]    a storage means  41  comprising at least one storage capacitor connected to the piezoelectric element and configured to store the electrical energy EE delivered by the generator  2 ; 
         [0035]    an electronic circuit  42  supplied by the storage means  41  and configured to compare the voltage TE of said at least one storage capacitor with a threshold input voltage so as to deliver at least an electrical alarm signal SE if the voltage TE of said at least one capacitor is greater than the threshold input voltage; the electronic circuit  42  may comprise a microcontroller intended to deliver said at least one electrical signal SE which comprises said frame intended to be captured by the receiving node  6 ; and 
         [0036]    an interface  43  supplied by the electronic circuit  42  and configured to deliver said at least one electrical alarm signal SR to the outside of the detector  1 . 
         [0037]    For greater simplicity and freedom of installation of the detector  1 , the interface  43  is advantageously a wireless interface, for example one of the Wi-Fi, IEEE 802.15.14, BLE (“Bluetooth Low Energy”), or other type, capable of communicating with the receiving node  6  and thus connecting to the internet via the receiving node  6 . The interface may also be of the contactless type known as NFC (for “Near Field Communication”). 
         [0038]    The signal delivered by the interface  43  at the output of the electronic system  4  is then a radio frequency signal SR. Evidently, a wired interface could also be used to deliver the electrical alarm signal SE. 
         [0039]    It should be noted that the electronic circuit  42  requires no power supply other than the voltage TE delivered by said at least one storage capacitor  41  for its operation, once this voltage TE has exceeded said threshold voltage. 
         [0040]    Additionally, the electrical energy EE delivered at the end of a mechanical vibration cycle of the piezoelectric element  21  after the triggering of the detector  1  is sufficient to reach said voltage threshold. 
         [0041]    For this purpose, the electronic circuit  42  may comprise, for example, a circuit for comparing a voltage with a threshold, of the type described in French Application for Patent No. 1462427, an embodiment of which is recalled here with reference to  FIG. 3 . 
         [0042]    The comparison circuit  100  of  FIG. 3  comprises a first branch including a resistor R 1  and a transistor T 1  in series between the nodes A and B to which is applied an input voltage, in this case the voltage TE delivered by said at least one storage capacitor  41 . More particularly, in the illustrated example, the resistor R 1  has a first end connected to the node A and a second end connected to a node C, and the transistor Ti has a first conduction node connected to the node C and a second conduction node connected to the node B. In the illustrated example, the transistor T 1  is an NPN-type bipolar transistor whose collector (c) is connected to the node C and whose emitter (e) is connected to the node B. 
         [0043]    The circuit  100  further comprises a second branch, parallel to the first branch, including two resistors R 2  and R 3  in series between the nodes A and B. More particularly, in the illustrated example, the resistor R 2  has a first end connected to the node A and a second end connected to a node D, and the resistor R 3  has a first end connected to the node D and a second end connected to the node B. The resistors R 2  and R 3  form a voltage divider bridge. The node D, or the mid-point of the divider bridge, is connected to a control node of the transistor T 1 , namely the base (b) of the transistor T 1  in the illustrated example. 
         [0044]    The circuit  100  further comprises a third branch, comprising a transistor T 2  in series with a resistive element Rf between the node A and the node D. More particularly, in the illustrated example, the resistor Rf has a first end connected to the node D and a second end connected to a node E, and the transistor T 2  has a first conduction node connected to the node E and a second conduction node connected to the node A. In the illustrated example, the transistor T 2  is a P-channel MOS transistor whose source (s) is connected to the node A and whose drain (d) is connected to the node E. A control node of the transistor T 2 , namely its gate (g) in this example, is connected to the node C. 
         [0045]    The nodes E and B are nodes for supplying an output voltage Vs of the circuit  100 . A load LD to be supplied, for example a wireless connection interface, such as said interface  43 , is connected between the nodes E and B of the circuit  100 . 
         [0046]    The operation of the circuit  100  is as follows. When the input voltage TE is low, the voltage at the terminals of the resistor R 3  of the divider bridge is insufficient to make the transistor T 1  conduct. The transistor T 1  is therefore turned off. The voltage at the terminals of the resistor R 1 , corresponding to the gate-source voltage of the transistor T 2  in this example, is then substantially zero. The transistor T 2  is therefore turned off. The current flowing through the load LD is then substantially zero, and the load LD is not supplied. The output voltage Vs of the circuit  100  is then approximately zero. 
         [0047]    The resistive element Rf then contributes to the lowering of the potential of the node D, bringing it closer to that of the node E (which is then substantially equal to that of the node B), and therefore reinforces the non-conducting state of the transistor T 1 . 
         [0048]    If the voltage TE increases until it crosses a threshold V SH , the voltage at the terminals of the resistor R 3  reaches the threshold for making the transistor T 1  conduct. The transistor T 1  then becomes conducting, and a current flows in the branch comprising the resistor R 1  and the transistor T 1 . The voltage at the terminals of the resistor R 1 , or the gate-source voltage of the transistor T 2  in this example, then increases until it reaches the threshold for making the transistor T 2  conduct. The transistor T 2  therefore also becomes conducting. The load LD is then supplied, and the output voltage Vs of the circuit  100  becomes substantially equal to the input voltage TE (with the deduction of the voltage drop of the transistor T 2 ). The resistive element Rf then tends to raise the potential of the node D again, bringing it closer to that of the node E (which is then substantially equal to that of the node A), and therefore helps to maintain the conductive state of the transistor T 1 . 
         [0049]    If the voltage TE falls back below a threshold V SB  which is lower than the threshold V SH , the voltage at the terminals of the resistor R 3  is no longer sufficient to keep the transistor T 1  in the conductive state. The transistor T 1  therefore becomes non-conducting, and the voltage at the terminals of the resistor R 1  becomes substantially zero, causing the transistor T 2  to become non-conducting. Consequently the load LD is no longer supplied, and the output voltage Vs of the circuit  100  becomes substantially zero. 
         [0050]    The presence of the resistor Rf, or feedback resistor, between the nodes D and E causes the circuit  100  to operate with hysteresis; that is to say, its threshold V SH  for switching from the non-conducting state (Vs˜0) to the conducting state (Vs˜Ve) is above the threshold V SB  for switching from the conducting state to the non-conducting state. This makes it possible to avoid undesirable oscillation between the supplied and non-supplied modes of the load LD, notably in the case where switching from the non-conducting to the conducting state is accompanied by a high current demand in the load LD. 
         [0051]    Reference will now be made more particularly to  FIG. 4 , for the description of another embodiment in which the event to be detected is a threshold shock level on the shock detector  10 . 
         [0052]    The shock detector  10  comprises the same electrical energy generator  2 , the same electrical system  4  and the same chassis  5  as the heat detector  1  described in greater detail above. 
         [0053]    In this variant, the shock detector  10  comprises a trigger  30  including a retaining element  35  fixed to the chassis  5  and configured to retain the oscillating weight  23  of the generator  2  in said initial position  24 . 
         [0054]    The shape of the retaining element  35  is specially designed, for example in the form of a cone as shown in  FIG. 3 , as is its positioning on the weight  23 , so as to retain the weight  23  while also being sensitive to shocks on the shock detector  10 . 
         [0055]    If the shock applied to the shock detector  10  crosses a threshold shock level, the retaining element  35  moves and/or is deformed so as to release the oscillating weight  23 , to deliver the radio frequency alarm signal SR to a receiving node  6 , as described above for the case of the heat detector  1 . 
         [0056]    Thus it is possible to produce, for example, an autonomous batteryless event detector ( 1 ,  10 ) having a one-off power source based on storage of mechanical energy, and making it possible to deliver, in response to said event, an electrical alarm signal SE, which if appropriate may be a radio frequency signal SR captured by a receiving node  6  and triggering a warning on a connected network.