Patent Publication Number: US-7916019-B2

Title: Device for detecting the relative position of two elements via electric circuit section having mechanical function in at least one of the elements

Description:
1. TECHNICAL FIELD 
     The present invention refers to a device for detecting the position of two movable components relative to each other the integration of which is improved, in particular a sensor to be implemented in assistance or monitoring devices, of the alarm type. 
     2. PRIOR ART 
     The devices for detecting or sensing a relative position of two elements of a known type are interposed between the two elements and detect a modification of the parameters of the environment, for example due to a displacement of a movable part in the sensor. In the case of a refrigerator, the opening of the door causes the illumination of the lamp which is arranged inside the refrigerator. In this case, the sensor is formed by a stud maintained into contact with the door by a spring. The displacement of this stud causes, when the door is closed, the opening of the electric-power supply circuit of the lamp. When opening the door, the stud moves into the opposite position, causing the closing of the electric circuit and the electric-current supply to the lamp and its lighting. The electric circuit is inserted against the structure of the refrigerator. 
     EP 1 014 322 B1 discloses a burglar-proof device for a roller shutter closing an opening. This device includes a sensor on the lower blade of the shutter aimed at informing about the position of the lower blade with respect to the lower frame of the shutter. This sensor is formed by a stud protruding with respect to the lower blade. The latter, when the roller shutter is closed, is into contact with the lower frame of the opening. The stud then adopts an inserted position, and no information is sent by radio wave to a radio-frequency receiver. 
     In the event of breaking-in, i.e. when the roller shutter is lifted, the lower blade is then separated from the lower edge of the opening, and the stud is no longer in an inserted position. This change in position of the stud causes a radio-frequency signal to be transmitted by a transmitter to the radio-frequency receiver, and for example an alarm to be activated. The transmitter is inserted against the roller shutter. 
     These detecting devices have the drawback of requiring to be inserted against at least one of the elements a relative displacement of which one wants to detect, or close to the latter. This addition can sometimes be complex, unaesthetic and increase the cost price of the structure. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of this invention to provide a detecting device integrated into the system that is provided with same, without disturbing its operation and without deteriorating its outer aspect in a substantial way. 
     The above-mentioned object is achieved by a device for detecting the relative position of two elements, one of the two elements fully ensuring a mechanical function, the device including at least one of the elements. 
     In other words, at least one of the elements the position of which with respect to the other element one wants to detect is used to transmit the information about the nearing or the separation of the two elements, while this element ensures its mechanical function. 
     To this end, the sensor according to the present invention includes an electric circuit capable of being opened and closed according to the position of the two elements, at least one of the elements being an electrically conductive part being part of an electric circuit, the nearing or the separation of the other element causing an electric circuit to close or to open. 
     The present invention has the advantage of providing a very robust detecting device, since it is not inserted against the device that is provided with it, but integrated at least partly into one of the elements the relative position of which one wants to detect. 
     Therefore, the present invention has mainly as an object a device for detecting a relative position of a first element, having a main mechanical function, and of a second element, including an electric circuit the opening and closing of which respectively correspond to a first and a second position of the first and second elements with respect to each other, wherein at least the first element is part of the electric circuit. 
     In the present application, by “element having a first main mechanical function” is understood an element that ensures any mechanical function. This can be, for example, a supporting function in the case of a door-frame, of seizing function, in particular in the case of a door handle, a suspension or coupling function, for example in the case of means for coupling an object. The above-mentioned list is not exhaustive, it can also relate to a supporting element, such as a support arm, cleats, etc. 
     In the present application, by first position and second position is also understood a relative arrangement and another relative arrangement, whereby each one of these relative arrangements is not necessarily unique. For example, in the case of a door, the first position can correspond to the fully closed position, in which the leaf is flush with the sash-frame and the second position corresponds to the open position, this open position including all the positions of the leaf of the door with respect to the sash-frame of the door, from slightly opened to wide open. Thus, the second position actually includes a plurality of positions of the leaf with respect to the sash-frame. 
     It could also been foreseen that the closed position of a door corresponds to a minimum closing threshold, for example in the case of a roller shutter. It could be considered that the roller shutter is in closed position when it closes the opening to the extent of ¾ or ⅘. 
     Thus, the first element, in addition to ensuring its first mechanical function, ensures the additional function of transmitting information to a device for detecting the relative position of this first element with respect to the second element. This second element can ensure a mechanical function identical to that of the first element, for example in the case of a handle comprised of a first element and a second element. The second element can also ensure a function complementary to that of the first element, for example in the event the first element is a sash-frame of a door and the second element is a leaf of the door. 
     According to the invention, the first element can include two electrically isolated conductive parts. 
     The device according to the invention can be used for detecting an opening or closing of a door or a window. The first element can then be a sash-frame of a door or window, and the second element can be a leaf of a door or window, respectively. 
     The sash-frame of a door can advantageously include an electrically conducting outer frame and an electrically conductive inner frame, said outer and inner frames forming said conductive parts, at least one of the frames including an electric discontinuity capable of being filled with an electrically conductive movable part in order to close the electric circuit, said movable part being either carried by the leaf, or arranged in the discontinuity and bringing into contact portions on both sides of the discontinuity during the nearing or the separation of the leaf. 
     The electrically conductive movable part is for example T-shaped, the arms of the T being each aimed at entering into contact with portions on both sides of the discontinuity and the foot of the T acting as actuation means aimed at entering into contact with the leaf, the closing of the door causing a separation of the legs of the T from the portions on both sides of the discontinuity. 
     The electrically conductive movable part can also be formed by a movable keeper capable of being displaced by a bolt. 
     Diode-type illumination means capable of lighting when the electric circuit is closed can also be provided for, mounted between the inner frame and the outer frame. 
     An object of the present invention is also a monitoring installation, including several detecting devices according to the present invention, electrically connected in series. 
     The first elements of each device can then be electrically connected by plinths. 
     An object of the present invention is also a device for detecting the presence or absence of an object, the first element forming a support for coupling a suspension arm for said object. 
     The two electrically conductive parts are then capable of being electrically connected in order to close the electric circuit by the second element formed by an electrically conductive plunger movable through applying a load on the suspension arm. 
     The load is, for example, transmitted by the suspension arm to the plunger. 
     The two electrically conductive parts can be formed by profile bars or cables capable of permitting the suspension and the monitoring of several objects. 
     If the electrically conductive parts are formed by profile bars, a first profile bar includes a tube with a rectangular cross-section provided, on one face, with a groove, a second profile bar includes a tube with rectangular cross-section provided, on one face, with a groove, the two grooves being arranged in front of each other, an electrically insulating material being arranged between the two faces of the tubes including the grooves, the second tube including, on a face opposite the face including the groove, holes for the passing-through of a load-transmitting finger, the movable probe being movably fitted perpendicular to the axis of the tube, a base of the plunger being mounted in the tube of a first element, a probe of the plunger being mounted in the tube of the second element, said probe being in front of a passageway for the finger, the probe being elastically restored towards the passageway for the finger. 
     An object of the present invention is also a monitoring installation, wherein the first element of the device according to the present invention forms a cornice extending along at least part of a wall allowing fixing several objects next to each other and their monitoring. 
     The first and the second element are, for example, two portions of a handle for opening a leaf, said portions being made out of electrically conductive materials, the actuation of the handle causing the first and second elements to be brought into electrical contact and means for assisting the opening to be activated. 
     The two elements of the handle advantageously include an elongated portion with a protrusion in the median portion, the two protrusions being in front of each other and aimed at entering into contact, springy means maintaining the two protrusions separated from each other in resting position. 
     The assisting means can be of the electric motor type. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will be better understood by means of the following description and of the attached drawings, in which: 
         FIG. 1  is a schematic representation of a first embodiment of a detecting device according to the present invention applied to the control of the closing of a door, 
         FIG. 2A  is a cross-sectional view according to a horizontal plane, the door being in closed position, 
         FIG. 2B  is a detail view at the level of a central portion of the door and the sash-frame of the door, the door being in closed position, the leaf of the door being omitted, 
         FIG. 3  is a schematic representation of another example of the detecting device applied to a door, 
         FIGS. 4A and 4B  are perspective views of another exemplary embodiment of a detecting device according to the present invention in two different positions, 
         FIG. 5  is a detail view of a variant embodiment of a device provided with a sensor according to the present invention, 
         FIG. 6  is a schematic representation of an alarm system for doors and windows of a house, 
         FIGS. 7A and 7B  are schematic side representations of another embodiment of a detecting device according to the present invention, 
         FIGS. 8A to 8C  are schematic representations of a first exemplary embodiment of a third embodiment of a detecting device according to the present invention, 
         FIGS. 9A to 9C  are perspective views of the various elements a detecting device according to a second exemplary embodiment of the third embodiment is comprised of, 
         FIGS. 10A to 10C  are cross-sectional representations of the detecting device of the  FIGS. 9A to 9C . 
     
    
    
     DETAILED PRESENTATION OF PARTICULAR EMBODIMENTS 
     In  FIG. 1 , one can see a first exemplary embodiment of a detecting device according to the present invention for allowing detecting the opening and closing of a door  2 . 
     According to the present invention, the detecting device includes an electric circuit  4  capable of being closed when closing the door  2  and of being opened when opening the door  2 , the opening and closing of the circuit being processed by a data-processing unit  5  that, depending on the passing-through or not of current, emits an information signal related to the closing or opening of the door. 
     The door  2  includes a sash-frame  6  delimiting an opening  8  and a leaf  10  aimed at being applied against the sash-frame  6  in order to close the opening  8 . 
     According to the present invention, the sash-frame  6  includes an outer frame  12  made out of conductive material, an inner frame  14  also made out of conductive material, the first  12  and second  14  frames are electrically connected and connected to an electric-power source  16 , forming the electric circuit  4 . 
     The inner frame  14  includes an electric discontinuity. In the example shown, it includes a first portion  14 . 1  and a second portion  14 . 2  separated by a gap  15  filled either with air or with electrically insulating solid material. 
     The leaf  10  is pivotally mounted, in the example shown, on the sash-frame  6 , but it could also be slidably mounted in the latter. 
     The leaf  10  includes at least one element  18  made out of electrically conductive material arranged so that it brings into electrical contact the first  14 . 1  and second  14 . 2  portions of the inner frame  14  when the leaf  10  is applied against the sash-frame  6 . 
     In  FIG. 2A , one can see the outer frame  12  and the inner frame  14  formed of profile bars. In the example shown, the outer frame  12  has an angle-shaped transversal cross-section and the inner frame has a rectangular transversal cross-section. An electrically insulating material  17  is interposed between the outer frame  12  and the inner frame  14 . 
     In the example shown, the element  18  is formed by a plate mounted on the face of the leaf  10  aimed at entering into contact with the sash-frame  6 . 
     In  FIG. 2B , one can see the plate  18  electrically connecting the first  14 . 1  and the second  14 . 2  portions of the inner frame  14 . In this configuration, the current flows through circuit  4  and the processing unit detects the closing of the door  2 . 
     In the configuration of  FIG. 1 , the plate  18  does not electrically connect the first  14 . 1  and second  14 . 2  portions of the inner frame  14 , the current does not flow through the electric circuit  4 , the processing unit then detects the open position of door  2 . 
     The present invention also applies to windows. In the case of windows with several leaves, in particular with two leaves, it is foreseen to be able to detect the opening of one and/or the other one of the leaves. In the example shown, the element carried by each leaf, and capable of opening the electric circuit when the leaf is closed, is arranged on the side of the window carrying the hinges. This element can also be arranged on one of the other portions of the sash-frame of the window. 
     It is thus possible to perform a monitoring of all or part of the openings of a house, such as schematically shown in  FIG. 6 , in which the inner  14  and outer  12  frames of a door P, the inner  28  and outer  30  frames of a first window F 1 , the inner  32  and outer  34  frames of a second window F 2  are electrically connected in series through conductive plinths  36 ,  38  forming the extension of each of the inner  14 ,  28 ,  32  and outer  12 ,  30 ,  34  frames. In this configuration, one single open leaf closes the electric circuit connecting all the openings of the house and causes, for example, an alarm to be emitted. The reverse configuration, in which the opening of a leaf would cause the circuit to be opened can also be contemplated, in a way comparable to the device of  FIG. 1 . 
     The electrically conductive elements can, for example, be made out of plain conductive material or include a core or a layer of conductive material. 
     In  FIG. 3 , one can see another example of a door, wherein the plate  18  is placed on an edge  20  of the leaf  10 ; in this case, the inner frame  14  is arranged perpendicular to the faces of the leaf  10 . 
     It can also be contemplated to use a bolt of the door to close the electric circuit  4 . 
     It can be contemplated to open the electric circuit  4  when closing the door, instead of closing it. In such case, there can be provided for a keeper with a movable portion, this movable portion closing the electric circuit when the door is opened, and opening the circuit when closing the door, through the pressure of the bolt on the movable portion of the keeper. 
     In the exemplary embodiments described, the closing of the electric circuit occurs substantially at the level of a door handle (not shown), but the plate  18  as well as the discontinuity of the inner frame  14  can be provided at a different place, for example on a higher portion of the door or on the side of the hinges, so as to further improve the aesthetics of the door. 
     In  FIGS. 4A and 4B , one can see another exemplary embodiment of a detecting device applied to a door, in this case, the opening or closing of the electric circuit is achieved through a movable conductive part in the sash-frame, the displacement of which is controlled by the leaf of the door. 
     The same reference numerals as those used for  FIGS. 1 to 3  will be used for the description of  FIGS. 4A and 4B . 
     In the example shown, the outer frame  12  is discontinuous and includes a first  12 . 1  and a second  12 . 2  portion separated by a non electrically conductive gap  15 ; the portions  12 . 1  and  12 . 2  are aimed at being brought into electrical contact by filling the gap  15  or bridging the gap  15  by means of an electrically conductive part. 
     A conductive part  22  is movably mounted in gap  15 . It can for example be T-shaped, the two legs of the T  22 . 1 ,  22 . 2  entering respectively into contact with the portions  12 . 1 ,  12 . 2  and allowing the passing-through of the current. 
     The foot  22 . 3  of the T  22  is mounted in the gap  15  and is aimed at entering into contact with the leaf  10 . The T  22  is elastically restored towards the outside of the outer frame  12 , in the direction of the leaf  10 . 
     Thus, when the door is open ( FIG. 4B ), the legs of the T electrically connecting the first  22 . 1  and the second  22 . 2  portions of the outer frame  12 , causing the current to pass through the circuit, the processing unit detects the opening of the door. 
     When the door is closed ( FIG. 4A ), the leaf  10  is resting against the foot  22 . 3  of the T  22  and separates the branches  22 . 1 ,  22 . 2  from the first  12 . 1  and second  12 . 2  portions of the inner frame  12 , respectively, which opens the electric circuit  4 . The processing unit then detects the closing of the door. 
     In contrast to the exemplary embodiments of  FIGS. 1 to 3 , the current flows through the electric circuit when the door is open. 
     The detecting device can, for example, allow activating the illumination inside the room or the space closed by the door. 
     In  FIG. 5 , one can see lamps  24 , of the LED type wired in parallel on the inner frame  14  and the outer frame  12 , as shown in  FIG. 5 . 
     Thus, when the current flows through the electric circuit  4 , the diodes  24  turn on. 
     Therefore, when the door is open, in the example of  FIG. 4B , the diodes illuminate the interior of the space. 
     The detecting device according to the present invention can thus be integrated into a piece of furniture made of electrically conductive material, for example of aluminum, in order to illuminate the interior of the piece of furniture, when a user opens the door. When opening the leaf, the pressure on the movable part  22  is released, which then allows the current to circulate through the inner  14  and outer  12  frames and the diodes. 
     In the case of a sash-frame made according to  FIG. 5 , it can allow signaling an emergency exit. 
     It can be contemplated to have a monitoring installation for a house including several devices according to the present invention, wherein the detecting devices can all be identical or have different structures, for example a door or several doors can be provided with the device according to the second exemplary embodiment and the windows can be provided with the device according to the first exemplary embodiment. 
     In  FIGS. 7A and 7B , one can see another embodiment of a detecting device according to the present invention, namely adapted to the windows in the roofs, of the Velux® type. 
     This kind of window is provided with a leaf rotationally mounted about an axis passing substantially at the level of its median portion. 
     The opening of this kind of window occurs by pulling on a handle arranged along a post of the leaf, the displacement of this handle causes the displacement of the bolt with respect to the keeper, and the unlocking of the leaf with respect to the sash-frame. 
     This kind of window can be difficult to be handled, namely in the case of large-size windows. Indeed, because of the weight of the leaf, the latter can difficultly be retained by a person and is likely to be pulled by the opening motion. In addition, the closing of this kind of window requires lifting the leaf, which can also be problematic. 
     Therefore, there has been contemplated to assist the opening and the closing of Velux® by means of an electric motor, which, when actuating the handle, starts assisting the rotation in a direction of opening or closing the window. The motor can then exert a torque on the leaf at the level of its axis, or exert a pulling or pushing force on the leaf through a toothed-rack screw system mounted at an end of the leaf. 
     Usually, a device for detecting the displacement inserted against the leaf detects the displacement of the handle with respect to the leaf. 
     Thanks to the present invention, this detecting device is directly integrated into the handle and does not require any external detecting device. 
     It is then foreseen to manufacture a composite handle formed of electrically conductive parts, which are, in the rest position, separated from each other and which, when actuating the handle, enter into contact and close an electric circuit connected to a processing unit, which will send an activation signal to the electric motor. 
     In  FIGS. 7A and 7B , one can see an exemplary embodiment of such a handle  40  including a first portion  42  and a second portion  44  substantially parallel and movable with respect to each other. The first portion  42  is fixed to the frame of the leaf (not shown), and the second portion  44  is mounted articulated, for example on the first portion or the sash-frame of the window. 
     Each portion  42 ,  44  has an elongated shape, and includes a protrusion  46 ,  48  substantially in its central portion, in front of the protrusions  48 ,  46  of the other portion  44 ,  42 . 
     Both portions  42 ,  44  are connected to a processing unit  5 . 
     The handle portions  42 ,  44  are elastically separated from each other by springy means  50 . Thus, the portions are separated from each other and the electric circuit is opened without any effort opposing the efforts exerted by the springy means. 
     In the event of actuating the handle, the portions  42 ,  44  are brought close to each other, the protrusions  46 ,  48  enter into contact as shown in  FIG. 7B , which causes the electric circuit to be closed and the passage of the current to be interrupted, which is detected by the processing unit  5 . The processing unit  5  regards the closing of the circuit as a desire to open the window and sends an adequate signal to the electric motor or a current-supply source for the motor. 
     Thus, the actuation of the handle is detected without the use of an external detecting device, the handle itself is part of the detecting device. The system is then simplified and more esthetical. 
     The present invention can also apply to the opening of large-size doors, the manual displacement of which can be difficult, or to the opening of a cover plate or trunk of a motor vehicle. 
     The shape of both parts  42 ,  44  is not limited to the shape described, protrusions can for example be provided for at each end of the elongated body. 
     The handles, in the case of the Velux, are covered with an electrically insulating coating, such as paint. In addition, the circuit operates at low voltage. Thus, the users are not likely to be electrocuted. 
     This observation applies to any implementation of the invention in which electric protection of the users and/or the external environment is required. 
     In  FIGS. 8A to 8C , one can see a schematic representation of a first example of a third embodiment of a device for detecting a displacement according to the present invention applied to the monitoring of objects, namely paintings. 
     A painting can, for example, be fixed by a rod to a cable extending along a wall. This cable can extend all around the room and support several paintings. 
     According to the present invention, the detecting device includes two parallel electrically conductive parts  52 ,  54  aimed at supporting at least one painting  56 , these electrically conductive parts  52 ,  54 , for example of a cable type, are part of an electric circuit and are connected to a processing unit (not shown). 
     The painting  56  is coupled to the cables  52 ,  54  through a rod  58  fixed with a first end  60  to the painting  56  and with a second end  62  to the cables  52 ,  54 . The second end  62  is in the form of a hook. 
     The detecting device according to the present invention also includes a contactor  64  interposed between the rod  58  and the cables  52 ,  54 , this contactor  64  is movable with respect to the electrically conductive parts  52 ,  54 . 
     The contactor  64  includes a body  65  provided, on one side, with two grooves  66 ,  68  each receiving a cable  52 ,  54 , a stud  70  movable in the body  65  and a metallic electrically conductive element  72  aimed at electrically connecting the two cables  52 ,  54 . 
     The electrically conductive element  72  is maintained into contact with the stud  70  by a spring  74 . 
     In  FIG. 8C , the stud  70  is, in resting position, protruding out of the body  65  under the action of the spring  74 . 
     In  FIG. 8B , a painting  56  is coupled to the cables, the hook  62  presses on the stud  70 , which moves, the displacement of stud  70  results into the displacement of the element  72 , which moves away from the cables  52 ,  54 . The electric circuit is open. 
     If the painting  56  is removed, the stud  70 , because of the force exerted by spring  74 , moves towards the outside of the body  65 , the electrically conductive element  72  enters into electrical contact with the two cables  52 ,  54  and closes the electric circuit. The current can then flow, the processing unit then sends a signal of absence of the painting. 
     All the paintings coupled to the cables  52 ,  54 , can be provided with such a detecting device. 
     In  FIGS. 9A to 9C  and  10 A to  10 C, one can see a second exemplary embodiment of the third embodiment of a detecting device according to the present invention, wherein the cables  52 ,  54  are replaced by a cornice comprised of two electrically insulated profile bars made out of electrically conductive materials. 
     The detecting device according to the present invention includes a support  76  for coupling a painting  56  forming a cornice along a wall  78 . 
     Said support is comprised of a lower element  80  and an upper element  82 , such as metal profile bars arranged above each other, aimed at being fixed to the wall  78 . 
     The lower element  80  includes a tube  84  with an axis X 1 , with a rectangular cross-section, including an axial groove  86  in one side  90  of the tube  84 . 
     A side  92  orthogonal to the side  90  including the groove  86  is extended by a bearing plate  94 , so as to form a base plate aimed at entering into flat contact with the wall  78 . 
     The upper element  82  has substantially the same shape as the lower element  80 . The reference numerals used to designate the portions of the element  80  will be used with the addition “′” to designate the similar portions of the upper element  82 . 
     The upper element  82  is arranged on the lower element  80 , so that the groove  86 ′ is in front of the groove  86 . 
     The upper element includes, parallel to the bearing plate  94 ′, a plate  96 ′ extending on one side  100 ′ parallel to the side  921 . 
     Holes  102 ′ are provided for in a side  104 ′ opposite the side  901  including the groove  86 ′, in order to allow the passing-through of a finger  200 . 
     The plates  94 ,  94 ′ include holes  106 ,  106 ′ for fixing the upper  82  and lower  80  elements to the wall  78 . 
     A plunger  202  is movably mounted in the tube  84 , orthogonally to its axis, and protrudes into the tube  84 ′ through the grooves  86 ,  86 ′. 
     The plunger  202  capable of transporting the electrical current includes, at a lower end, a base plate  204  arranged in the tube  84  and, at an upper end, a probe  206  in the tube  84 ′, in front of an opening  102 ′, the probe  206  being aimed at entering into contact with the finger  200 . 
     The plunger  202  is elastically restored in the direction of the opening  1021  by elastic means  208 , of the helical-spring type mounted in reacting between a bottom of a bore provided for in the base plate  204  of the plunger  202  and the side  104 . 
     The finger  200  forms the end of the coupling rod of the painting. 
     Two strips  210  of electrically insulating material are arranged along the grooves  86 ,  86 ′ so as to insulate the two upper  82  and lower  80  elements. 
     We will now explain the operation of the detecting device. 
     Both profile bars  80 ,  82  as well as the plunger  202  are part of an electric circuit supplied by a processing unit (not shown). 
     When a downward force is applied on the plunger  202  by the finger  200 , as shown in  FIG. 10A , which means that a load is applied on the coupling rod by a coupled painting  56 , the plunger  202  is in the lower position, the electric circuit is then open. The processing unit then detects the presence of the painting. 
     On the other hand, in the absence of the painting, as shown in  FIG. 10B , no force is applied on the plunger  202  against the spring  208 , the plunger  202  is then in the upper position, its base plate enters into contact with the edges of the groove  86  of the profile bar  80  and the probe  206  enters into contact with the contours of the opening  102 ′. The electric circuit is then closed, the processing unit deduces the absence of the painting and then generates an alarm. 
     The profile bars  80 ,  82  are aimed at extending along a wall or several walls of a room, in order to support several paintings. In the upper profile bar  82  are then provided openings at the locations where one wants to couple the paintings. 
     It can also be contemplated to use a support  76  for one single painting. 
     The devices for detecting of presence of the paintings are electrically connected in series. Thus, in the absence of a painting, the processing unit will send an alarm, while this absence can be due to the desire not to couple a painting at that location. There is then advantageously provided for a device for exerting a force on the plunger so as to maintain it in the lower position, an exemplary embodiment of which is shown in  FIG. 10C . 
     The latter is comprised, in the example shown, of a body  212  provided with a protruding stud  214 , with a shape and size similar to those of the finger  200  of the coupling rod and a pin  216  aimed at penetrating into a bore  218  provided for in the body  212  and a passageway  220  provided for in the plate  96 ′. 
     The stud  214  is then maintained applied against the probe  206  by mounting the pin  216  in the bore  218  and the passageway  220 . 
     This device thus constitutes a decoy for the detecting device. 
     It can be advantageous to foresee that the rod  58  is integral with the painting  56 . 
     The devices implementing the present invention advantageously operate at low voltage, for example lower than 5V and a current intensity ranging from 1 μA and a few amperes.