Patent Publication Number: US-2009230704-A1

Title: Closure retention and release mechanisms

Description:
The present invention relates to a closure retention and release mechanism and in particular, though not exclusively, to a door holder for a fire door. A system for the control and operation of a closure holder is also described. 
     Fire doors are installed in buildings to prevent the spread of smoke and fire through the building in the event of a fire. In order to serve their purpose the doors must be closed, however this may be inconvenient during periods when the building is occupied, for example an office building, or times when there is significant movement of people within the building, for example hospital, nursing home or hotel building during periods when the occupants are awake. A common solution involves the use door holder devices which are operable to maintain the fire doors in an open state but are arranged to release the doors in the event of a fire alarm being raised. The doors thus released are able to close under the influence of a separate or integrated closing mechanisms. In such a system the door holders are typically required to be hard wired to the fire alarm system. 
     Numerous forms of door holder mechanisms presently exist. The most common system includes an electromagnet mounted on the floor, ceiling, or wall adjacent a fire door, and a ferrous plate mounted on the door. The plate, and hence the door, will remain held by the magnet for as long as it is energised. The door is released when the power supply to the electromagnet is cut. A similar system incorporates the combination of a permanent magnet and an electromagnet. The door is retained by the permanent magnet until the electromagnet is energised. The electromagnet is arranged to oppose the filed of the permanent magnet and thereby cause the door to be released. 
     Known door closing mechanisms may include a spring and a hydraulic damper, the hydraulic damper being arranged to control the closing speed of the door. In such an instance a door holder mechanism may be incorporated into the hydraulic damper to block the flow of fluid therethrough and thereby maintain the door in an opened state. 
     According to the first aspect of the present invention there is provided a closure retention and release mechanism comprising a body having spaced apart first and second portions of a ferrous material, a magnet having north and south poles mounted between said portions and actuation means operable to move the magnet relative to said portions, wherein the magnet is movable between a first position where each pole of the magnet is located in a respective portion of the body and a second position where each pole is not located in a respective portion of the body. In one embodiment the second position of the magnet may correspond to a position where each pole of the magnet is located equally in both portions of the body. Alternatively the second position of the magnet may correspond to a position whereupon the magnet is moved away and is spaced from the body. 
     In the first position of the magnet relative to the body portions the magnet is able to act through the body portions to attract and retain an engagement plate of a closure, such as a door. In the second position of the magnet relative to the body portions the magnet is unable to act through the body portions, either as a result of its orientation relative to and/or spacing from the body portions, and hence is unable to attract and retain an engagement plate. 
     The body portions may be spaced from one another by the provision of a non-ferrous spacer positioned therebetween. In an alternative embodiment an air space may be provided between the body portions. In such an embodiment the body portions may be supported in a holder or frame work which maintains the spacing between the blocks. Preferably the body is provided with an engagement face which, in use, is adapted to interface with a portion of a closure. The engagement face may be provided in an extension of one of the body portions. Preferably both body portions may include an extension, with the engagement face being provided partially upon each extension. 
     Preferably the magnet is located in a recess defined between the body portions. The recess may preferably be a through hole of the body. In such an embodiment the magnet may be substantially cylindrical and mounted for rotation in the through hole. 
     Rotation of the magnet may preferably be effected by the provision of a motor arranged to drive a rotation mechanism. The rotation mechanism may comprise a gear train arranged between an output of the motor and the magnet. The gear train may comprise a worm gear connected to an output shaft of the motor, an drive gear connected to the magnet and a reduction gear provided intermediate the worm and drive gears. The gear train may include one or more additional gears. 
     The motor may be reversible so as to permit the magnet to be rotated both clockwise and anticlockwise. Alternatively the motor may operate in a unidirectional manner so as to rotate the magnet in a unidirectional manner. In both embodiment the motor includes a control system operable to ascertain the position of the magnet relative to the body and to cease the operation of the motor when the magnet reaches a desired position. The control system may include one or more limit switches. 
     In an alternative embodiment the magnet may be movable in a linear manner relative to the body. In such an embodiment the mechanism includes a motor and a reciprocating motion mechanism arranged to move the magnet between the first and second positions. 
     In an alternative embodiment the actuation means may include an actuation member contactable by a closure. The actuation member may be movable between a first position corresponding to the first position of the magnet relative to the body portions, and a second position corresponding to the second position of the magnet relative to the body portions. Preferably the movement of the actuation member from the first position to the second position is resisted a resilient member such as, for example, a hairpin spring. The actuation member may be movable linearly between said first and second positions. 
     The actuation means may also include a drive member coupled to the magnet, the drive member being arranged to transmit movement of the actuation member to the magnet. The drive member may be coupled to the magnet via an intermediate member. The drive member is movable between a first position corresponding to the first position of the magnet relative to the body portions, and a second position corresponding to the second position of the magnet relative to the body portions. Movement of the drive member from the first position to the second position may be resisted a resilient member. In a preferred embodiment the drive member is rotatable between said first and second positions. 
     The actuation means may includes a latch arrangement operable to releasably retain the drive member in the second position. The latch arrangement may include a latch arm and a latch arm actuator. 
     According to a second aspect of the present invention there is provided a method of controlling a closure retention and release mechanism the method comprising the steps of:
         providing a master controller having a transmitter;   providing a slave unit having a receiver;   providing a closure retention and release mechanism associated with the slave unit, the slave unit being operable to switch the closure retention and release mechanism between closure retention and release states;   transmitting a control signal from the master controller to the slave unit; wherein the slave unit is operable to switch the state of the closure retention and release mechanism in the absence of receiving the control signal from the master controller.       

     In one embodiment the control signal may comprise a repetitive signal. The signal may comprise a series of pulses. The slave unit may be operable to switch the state of the closure retention and release mechanism in the absence of receiving one or more pulses of the control signal. 
     The slave unit is preferably switchable between an active state and an inactive state. In the active state the slave unit is configured so as to receive the control signal. In the inactive state the slave unit is not able to receive the control signal. Switching of the slave unit from the active state to the inactive state is preferably triggered by receipt of the control signal by the slave unit. Switching of the slave unit subsequently back to the active state occurs after a predetermined time period has elapsed after the slave unit has entered the inactive state. The predetermined time period is synchronised with the period of the control signal pulses. 
     The slave unit may act to switch the state of the closure retention and release mechanism in response to the absence of receiving a single pulse of the control signal. In an alternative embodiment the slave unit may act to switch the state of the closure retention and release mechanism after failing to receive a plurality of pulses of the control signal. In such an embodiment the slave unit may enter an alert state after failing to receive a first pulse of the control signal whereupon the slave unit remains in the active state in anticipation of receiving a subsequent pulse of the control signal. If a pulse of the control signal is subsequently received within a predetermined time period then the slave unit reenters the inactive state. If a pulse of the control signal is not received within a predetermined time period then the slave unit acts to switch the state of the closure retention and release mechanism. The predetermined time period during which the slave unit remains in the alert state preferably corresponds to a multiple of the time period between the scheduled pulses of the control signal. 
     The method may include the additional step of;
         incorporating a command into the control signal, the command prompting the slave unit to switch the state of the closure retention and release mechanism.       

     The method may include the additional steps of;
         providing the slave unit with a transmitter;   providing the master controller with a receiver;   incorporating an interrogation command into the control signal;   sending the interrogation command to the slave unit to prompt the slave unit to respond to the master controller.       

     According to a third aspect of the present invention there is provided a control system for a closure retention and release mechanism, the control system comprising a master controller having a transmitter, a slave unit having a receiver, and a closure retention and release mechanism associated with the controller, the slave unit being operable to switch the closure retention and release mechanism between closure retention ad release states in the absence of receiving a control signal from the master controller. Optionally the slave unit may be provided with a transmitter and the master controller with a receiver to enable the slave unit to correspond with the master controller. 
    
    
     
       Embodiments of the present invention will now be described with reference to the accompanying drawings in which: 
         FIG. 1  is a side view of a mechanism according to a first embodiment the present invention; 
         FIG. 2  is a further side view of the mechanism; 
         FIG. 3  is an other side view of the mechanism; 
         FIG. 4  is a schematic view of a control system according to an embodiment of the present invention; 
         FIG. 5  is a schematic representation of the interaction between a controller and an actuator of the system; 
         FIG. 6  is a schematic representation of the interaction between a controller and multiple actuators of the system; 
         FIG. 7  is a schematic representation of the interaction between multiple controllers; 
         FIG. 8  is a side view of a mechanism according to a further embodiment of the present invention; 
         FIG. 9  is a side view of the other side of the mechanism of  FIG. 8 ; 
         FIG. 10  is an end view of the mechanism of  FIG. 8 ; 
         FIG. 11  is a side view of the mechanism of  FIG. 8 ; and 
         FIG. 12  is a side view of the other side of the mechanism of  FIG. 8 . 
     
    
    
     Referring firstly to  FIGS. 1 to 3  there is shown a closure holding device generally designated  10 . The device comprises a plate  12  and a magnetic retention/release mechanism  14 . The plate  12  is attachable to a door by any appropriate means such as, for example screws or like fasteners. The plate  12  comprises a base  16 , an engagement face portion  18  and a flexible neck  20  connecting the base  16  to the face portion  18 . 
     The flexibility of the neck  20  ensures that minor misalignment between the plate  12  and mechanism  14  can be accommodated. The face portion  16  of the plate  12  at least is comprised of a ferrous metal such as steel. The ferrous metal may be provided in an insert mounted to the face portion  16 . Alternatively the plate  12  in its entirety may be comprised of a ferrous metal. 
     The mechanism  14  comprises body  22 , a permanent magnet  24  and an actuation mechanism  26 . The body  22  is comprised of two blocks  28 , 30  of ferrous metal such as, for example, steel. The blocks  28 , 30  sandwich, and hence are separated by, a member  32  comprised of a non-magnetic material such as, for example, aluminium alloy. In an alternative embodiment the blocks  28 , 30  may be mounted relative to one another such that an air gap instead of the non-magnetic material is provided therebetween. In such an embodiment the blocks  28 , 30  may be mounted in a non-magnetic holder The blocks  28 , 30  include an engagement face  34 . In the embodiment shown the engagement face  34  is provided on a projection  36  of each block  28  either side of the non-magnetic member  32 . 
     The permanent magnet  24  is positioned in a through hole  38  provided in the blocks  28 , 30  and non-magnetic member  32 . The through hole  38  is positioned such that equal portions thereof are provided in each block  28 , 30 . The magnet  24  has both north and south poles  40 , 42  and is rotatable in the through hole between a first position where the poles  40 , 42  are aligned with the non-magnetic member  32  ( FIG. 1 ) and a second position where the poles  40 , 42  are aligned transverse to the  32  such that each pole  40 , 42  is positioned in a respective block  28 , 30  ( FIG. 3 ). 
     The release/retention mechanism  14  comprises a motor  44  having a drive shaft  46  and a gear arrangement  48 . The gear arrangement  48  comprises a worm gear  50  mounted on the drive shaft  46 , a reduction gear  52  and a drive gear  54 . The reduction gear  52  comprises a large diameter portion  56  and a concentric smaller diameter portion  58 . The periphery of each portion  56 , 58  is toothed such that the portions  56 , 58  mate respectively with the worm gear  50  and drive gear  54 . The drive gear  54  is connected to the permanent magnet  24  via a spindle  60 . It will thus be understood that operation of the motor  44  causes the drive shaft  46  and worm gear  50  to rotate. Rotation of the worm gear  50  causes the reduction gear  52  to rotate which in turn causes the drive gear  54  to rotate. Rotation of the drive gear results in rotation of the magnet  24  within the through hole  38 . The arrangement of the gears  50 , 52 , 54  is such that the rotational speed of the drive gear  54  is less than that of the drive shaft/worm gear  46 , 50 , while the torque applied to the permanent magnet  24  is greater than that applied to the drive shaft  46  by the motor  26 . The embodiment shown in  FIGS. 1 to 3  the gear arrangement includes a single reduction gear  52 . It will be appreciated that one or more additional gears may be incorporated in the gear train between the drive shaft  46  and drive gear  54  to achieve the desired rotation characteristics for the magnet  24 . 
     The motor  26  may be reversible so as to permit the magnet  24  to rotated both clockwise and counter-clockwise. Alternatively the motor  26  may rotate in a unidirectional manner. In such an embodiment the motor  26  will include appropriately configured limit switches to halt the rotation of the magnet  24  substantially every 90 degrees. 
     Operation of the device  10  will now be described. During normal operation the magnet  24  is aligned with the blocks  28 , 30  as shown in  FIG. 3 . The position of the magnet  24  in  FIG. 3  may be referred to as the retention position. As described above the poles  40 , 42  of the magnet  24  are each positioned in a respective block  28 , 30 . As such, each block  28 , 30  effectively becomes an extension of the respective pole  40 , 42  with the result that magnetic lines of force, indicated by broken lines  62 , extend from the north pole  40  to the south pole  42 . The magnet  24  and block  28 , 30  arrangement is thus able to attach ferrous metal, such as the plate  12  thereto. As described above the plate  12  is connected to a closure such as a door, while the release mechanism  14  is mounted to a wall behind the closure. The strength of the magnet  24  is such that the plate  12  and closure is held securely thereto. The strength of the magnet  24  is selected such that the attractive force experienced by the plate  12  is greater than any force applied to the door by a spring biased or hydraulic closing mechanism. 
     Typically the strength of the magnet  24  may be chosen such that attractive force applied to the plate is greater than any externally applied forces, such as for example wind loading, which may reasonably be expected to experienced by the closure. As a safety precaution however, the strength of the magnet  24  may be such that the attractive force experienced by the plate  12  may be overcome in the event of an emergency by a person applying sufficient force to the closure 
     In the event that it is desired to separate the plate  12  from the mechanism  14  in a controlled manner the motor  26  can be operated to rotate the magnet  24  through substantially 90 degrees to the position shown in  FIG. 1 . The position of the magnet  24  in  FIG. 1  may be referred to as the release position. With, the magnet  24  in this position both of the poles  40 , 42  provided partially in each block  28 , 30  and as such the magnetic lines of force are fully contained within the blocks  28   30 . The magnet  24  is therefore unable to attract ferrous metal at or near the external faces of the blocks  28 , 30 . The mechanism  14  is thus unable to retain the plate  12  and closure to which the plate  12  is attached. The closure is thus able to be closed either manually or more preferably under the influence of a closing mechanism associated with the closure. When it is desired to reactivate the mechanism  14 , the motor  26  may be operated to rotate the magnet to the position shown in  FIG. 3 . This may be achieved either by reversing the motor  26  or continuing the rotation of the motor  26  in the same direction as before. 
     The mechanism  14  is preferably integrated into a control system which controls the operation of the motor  26 . For example the control system may be configured to move the magnet  24  at predetermined time intervals. Such a system may be particularly useful in a building which is substantially unoccupied during the night. In such a circumstance the control system may be configured to move the magnet  24  to the release position at a predetermined time of the evening to enable a closure to close. At a predetermined time of the morning the magnet  24  may be moved to the retention position ready to retain the closure when it is opened for the first time. The control system is preferably also able to move the magnet  24  to the release position in the event of an emergency such as a fire alarm being raised. 
     Referring now to  FIGS. 8 to 12  there is shown an alternative embodiment of a magnetic retention/release mechanism generally designated  90 . Features common to the embodiment described with reference to  FIGS. 1 to 3  are identified with like reference numerals. The mechanism  90  of  FIGS. 8 to 12  differs from the mechanism  14  of  FIGS. 1 to 3  in that the motor  44  and gear arrangement  48  are replaced by spring operated arrangement for the movement of the permanent magnet  24 . 
     The mechanism  90  includes a mounting plate  92  which has mounted on one side  94  the magnet  24  and the blocks of ferromagnetic material  28 ,  30 , and on the other side  96  an actuation mechanism generally designated  98 . The actuation mechanism  98  includes a slider bar  100 , a latch wheel  102 , a latch lever  104  and a micro switch  106 . The slider bar  100  is movable between an extended position shown in  FIGS. 8 and 9 , and a retracted position shown in  FIGS. 11 and 12 . The bar  100  is movable from the extended position to the retracted position against a spring  108  which provides a restorative force urging the bar  100  towards the extended position. The spring  108  is mounted to a projection  110  of the mounting plate  92 . 
     The latch wheel  102  is connected to a shaft  112  which extends through an aperture of the mounting plate  92 . The shaft  112  is connected to the magnet  24  such that rotation of the latch wheel  102  results in rotation of the magnet  24  relative to the blocks  28 , 30 . The latch wheel  102  is rotatable between a release position, shown in  FIG. 8  and a retention position shown in  FIG. 11 . The latch wheel  102  is movable from the release position to the retention position against a spring  114  which provides a restorative force urging the latch wheel  102  to the release position. The spring  114  is mounted about the shaft  112 . A ratchet pawl  116  is provided between the slider bar  100  and the latch wheel  102  such that movement of the slider bar  100  from the extended position to the retracted position results in rotational movement of the latch wheel  102  from the release position to the retention position. The shaft  112  extends through an elongate slot  113  of the slider bar  100 . 
     The slider bar  100  is provided with a projection  118  which, in use, interact with the microswitch  106 . The microswitch  106  is positioned such that movement of the slider bar  100  between the extended and retracted positions causes the projection to contact, and hence operate, the microswitch  106 . The position of the projection is such that the microswitch is activated towards the end of the slider bar travel in the direction from the extended to the retracted position. 
     The latch lever  104  is pivotable connected at one end  119  to the mounting plate  92 . The opposing end  120  of the latch lever  104  is connected to a solenoid  122 . The solenoid is operable to move the latch lever  104  between a latching position, shown in  FIG. 11 , and a release position shown in  FIG. 8 . The latch lever  104  is provided with a pawl  124  which, in the latching position, engages a latch surface  126  of the latch wheel  102 . It will be understood that when the latch lever  104  is in the latching position, and the pawl  124  is engaged with the latch surface  126 , the latch wheel  102  is retained in the retention position. 
     Operation of the mechanism  90  will now be described. The mechanism  90  is initially in the configuration shown in  FIGS. 8 and 9 , which is to say the slider bar  100  is in the extended position, the latch wheel  102  in the release position, and the latch lever  104  in the release position. The magnet  24  is positioned such that each pole  40 , 42  thereof is aligned partially within each block  28 ,  30 . As a metal plate of a door (both not shown) is moved towards the mechanism  90 , the plate contacts the slider bar  100  and thereby causes the slider bar to commence movement from the extended to the retracted position. This movement causes the latch wheel  102  to commence rotation from the release position to the retention position. The movement of the slider bar  100  further activates the micro switch  106  with a first projection  118  thereof. 
     The microswitch  106  corresponds with a controller to indicate that the door associated with the mechanism  90  is approaching an open condition. Should the controller determine that it is safe to hold the door open, a command is send to the latch lever solenoid  122  to move from the release position to the latch position and thereby hold the latch wheel  102 . This arrangement can be readily seen in  FIG. 11 . With the latch wheel  102  in the retention position the poles  40 , 42  of the magnet  24  are each disposed fully within separate blocks  28 , 30 . The blocks  28 , 30  are thus magnetized and hence the plate, and the door associated therewith, are retained by the mechanism  90 . When it is required to close the door, the controller sends an appropriate signal to the solenoid  122  to cause the latch lever  104  to move to the release position. The latch wheel  102  is thus able to return to the release position under the influence of the spring  114 . The poles  40 , 42  of the magnet are moved back to the position shown in  FIG. 9  and the blocks  28 , 30  are demagnetized. As the door plate moves away from the mechanism  90  the slider bar  100  is able to return to the extended position under the influence of the spring  108 . 
     According to a further aspect of the invention there is provided a control system for one or more closure mechanisms which will now be described with reference to  FIGS. 4 to 7 . The system may be utilised with devices  10  of the type described above or with other know types of closure retention devices. 
     Referring firstly to  FIG. 4  there is shown a schematic representation of a control system. The control system comprises a master controller  70  which is in wireless communication with a plurality of slave units  72 . Three slave units  72  are shown for the sake of simplicity, however it will be understood that the system may operate with a greater or lesser number of slave units  72 . The master controller  70  is connected to a fire alarm control system  74 . The master controller  72  may be connected to the fire alarm control system via a hardwired connection or a wireless link. The slave units  72  are each connected to a closure retention and release mechanism which in turn is used to hold a fire door in an open position. The slave units  72  are connected to the closure retention and release mechanisms via a hardwired link. 
     The master controller  70  includes a transmitter operable to send information to the slave units  72  and a receiver operable to receive information transmitted by the slave units  72 . The master controller  70  is further provided with a keypad interface  76  and a display  78 . Each of the slave units  72  includes a transmitter operable to send information to the master controllers  70  and a receiver operable to receive information transmitted by the master controller  70 . Each slave unit  72  further includes means to operate the closure retention and release mechanism with which it is associated. Taking the example of the device  10  described with reference to  FIGS. 1 to 3 , the means may comprise a power source such as a battery for the motor  26  and a switching means operable to selectively connect the power source to the motor  26 . 
     In use, the master controller  70  emits a periodic control signal  80  which is intended to be received by the slave units  72 . As illustrated in  FIG. 5  the signal  80  comprises a plurality of equally spaced pulses  82 . Upon initial start up of a slave unit  72 , the slave unit  72  “listens” for a predetermined number of repetitions of the control signal  80 . In doing so the slave unit  72  is able to determine the average time t between each pulse. Once the period t has been determined, the slave unit  72  enters a hibernation mode whereupon it alternates between an active state whereupon it is awake and able to receive the control signal  80  and an inactive state whereupon it is not able to receive the control signal  80 . The switching of the slave unit  72  from the inactive to the active state is synchronised with the period t of the control signal  80 . Thus the slave unit  72  enters the active state shortly before it expects to receive a pulse  82  of the control signal  80 . Upon receiving the control signal  80  the slave unit  72  re-enters the inactive state. During this normal mode of operation wherein the slave unit  72  cycles between the active and inactive states, the closure retention and release mechanism associated with the slave unit  72  remains in its original operational state holding the fire door open. 
     In the embodiment illustrated in  FIG. 5  the slave unit  72  is set up to monitor four repetitions of the control signal  80  in order to determine the period t. It will be appreciated that the slave unit  72  may be set up to monitor a greater or lesser number of control signal  80  repetitions in order to determine the period t. 
     In the event that the slave unit  72  does not receive a pulse  82  of the control signal  80  as expected, then the slave unit  72  enters an alert state whereupon it remains active and awaits the next scheduled pulse  82  of the control signal  80 . If the next scheduled pulse  82  is received then the slave unit  72  reverts to cycling between the active and inactive states as described above. In the event that a predefined number of pulses  82  are not received by the slave unit  72 , then the slave unit  72  acts to change the operative state of the closure retention and release mechanism to which it is connected. In an alternative embodiment the slave unit  72  may be arranged such that it acts to change the operative state of the closure retention and release mechanism if a single pulse  82  of the control signal  80  is not received. It will be understood that in such an embodiment the slave unit  72  does not enter the alert state described above. 
     In the event of a fire alarm being raised the master controller  70  may be instructed by the fire alarm control system  74  to close the fire doors retained in an open state by the retention and release mechanisms. In such a circumstance the master controller  70  stops transmitting the control signal  80  thereby promoting the slave units  72  to change the operative state of the retention and release mechanisms. It will be appreciated that the control signal may be stopped in alternative circumstances. For example, the control signal  80  may be stopped in the event of a power failure at either the master controller  70  or the fire alarm control system  74 . In such an event the fire doors are permitted to close as a precaution. In yet an alternative embodiment the control signal  80  may be stopped at a predetermined time of the day, for example after working hours, to permit closure of the fire doors. 
     Referring now to  FIG. 6  there is illustrated a slave unit interrogation protocol. In order to check the status of the slave units  72  the master controller  70  may send with a pulse  82  of the control signal  80  an interrogation command  84  for a particular slave unit  72 . The interrogation command  84  may, for example, instruct a slave unit  72  to confirm that it is still active and advise on the status of an internal power source used to change the state of an associated retention and release mechanism. Upon receiving the interrogation command  84  the slave unit  72  sends an appropriate response  86  which is received by the master controller  70 . In the event that no response is received, or a response is received which indicates that the slave unit  72  requires attention, the master controller  70  may act to draw this to the attention of a person responsible for the maintenance of the system. In addition or as an alternative to the interrogation command  84  a pulse  82  of the control signal  80  may also include a command to the slave unit  72  to change the state of the closure retention and release mechanism. 
     There may exist circumstances where a building may require more than one master controller  70 . Such a circumstance may arise where the building has multiple floors and it is desired to control the fire doors of each floor independently. Each floor may therefore be provided with a separate master controller  70   a , 70   b , 70   c  as illustrated in  FIG. 7 . So as to prevent the control signals  80   a , 80   b , 80   c  from each controller  70   a , 70   b , 70   c  being received by slave units not on the same floor as the master controller, the control signals  80   a , 80   b , 80   c  are staggered as indicated in  FIG. 7 . As the slave units only enter the active mode shortly before they expect to receive a control signal pulse  82   a , 82   b , 82   c , then the staggering of the control signals  80   a , 80   b , 80   c  ensures that the slave units only receive the control signal  80   a , 80   b , 80   c  from the controller  70   a , 70   b , 70   c  on the correct floor. As indicated on  FIG. 6  the master controller  70  is able to vary the time period t between pulses. Where a plurality of master controllers  70   a , 70   b , 70   c  are provided in close proximity, then during the initial set-up of the controllers  70   a , 70   b , 70   c  the respective time periods t can be selected so as to ensure that the respective control signals  80   a , 80   b , 80   c  are not in conflict with one another. 
     An additional feature of the master controller  70  during the initial set-up thereof is the ability to select a frequency for the control signal which is free of interference from, for example, other radio frequency transmissions. Upon initial set-up the master controller scans a range of frequencies between upper and lower limits. The controller  70  selects a “free” frequency band and commences the broadcast of the control signal. Upon initial set-up of the slave units, they too scan the frequency range until they tune into the control signal and lock on to the frequency of transmission thereof.