Abstract:
A surveillance system having two or more monitoring devices moving on a single track. The monitoring devices are suitably video cameras but may also include audio monitoring. Power for the monitoring devices, control signals and signals from the monitoring devices are all transmitted on the track. Power is suitably DC and other signals are suitably RF. The system incorporates collision avoidance means to prevent collision between adjacent moving cameras. The collision avoidance means includes hand-over software so that a person can seamlessly scan a region with camera control being passed to adjacent cameras to avoid collision.

Description:
FIELD OF THE INVENTION 
   The present invention relates to a track mounted multiple mobile camera surveillance system. 
   BACKGROUND OF THE INVENTION 
   Remote cameras to survey an area are known and commonly used. Attaching movable surveillance cameras to a track system to permit viewing of different locations is also known and the subject of U.S. Pat. Nos. 4,656,509 and 4,510,526. These patents describe remote controlled carriage mounted cameras for surveying an area, but do not permit multiple cameras on a single track. 
   A typical video surveillance system is disclosed in Australian patent 659190 and comprises a track assembly which is mounted to a room ceiling. A movable carriage is able to travel repetitively back and forth along the track and is provided with a camera to transmit video images of monitored areas to a remote location. 
   The carriage in AU 659190 comprises two cameras mounted to a single platform, a drive assembly, drive control and video circuit boards. The cameras are mounted to the support platform at different angles in order to observe a wide area. 
   The track includes two conductors of copper tubing suitably mounted and supported within semi-cylindrical grooves of an isolation block made of electrically insulating material. Each conductor is in slidable contact with at least one corresponding isolated slidable electrically conductive brush located on the underside of the carriage. 
   Output signals from the cameras are provided to a video modulator board on the carriage which modulates suitable carrier signals for transmission through the conductors to a demodulator connected at the end of the track. The demodulator demodulates each camera output signal from its respective carrier signal and displays the corresponding image on monitors. 
   Proximity sensors are located along the length of the track and these are hardwired back to a controlling interface system so that the location of the carriage is able to be monitored through the proximity sensors. 
   Power to the carriage is provided through the two conductors, so that the conductors carry both the power, control and video signals received from the cameras. 
   The above system has the drawback that it is not possible to accurately monitor more than one area at a single time because the single carriage carrying the cameras cannot be at two locations along the track simultaneously. Also, the above system requires maintenance of wearable parts such as conductive bushes which contact the conductors. 
   SUMMARY OF THE INVENTION 
   It is an object of the invention to provide an apparatus to overcome one or more of the limitations of, or improve upon, the prior art as discussed above. 
   These and other objects, features and advantages of the present invention will become more apparent in the light of the detailed description of exemplary embodiments thereof, as illustrated by the accompanying drawings. 
   In one form, the invention resides in a surveillance system comprising:
         an electrical conducting track;   two or more carriages movable on the track;   a driving means mounted on each carriage for moving each carriage to different locations along the track;   a power supply providing power to each carriage;   at least one monitoring device mounted on each carriage providing an output signal for a monitored location;   a modulation means receiving the output signal;   a transmission means for transmitting modulated output signals through the track;   a means for receiving and demodulating the transmitted modulated output signals;   a viewing means to view the demodulated output signal at a remote location; and   a control means for controlling movement of each carriage on the track.       

   The track suitably comprises at least one conductor. 
   Preferably the track comprises three conductors, one transferring power, a second transferring video and control signals and a third as a ground conductor. 
   Each carriage comprises a data processor which includes position management software for recording the location of the carriage along the track, storing data on the location of each adjacent carriage, transmitting data relating to its position along the track to an interface translator, receiving and storing data relating to the location of each adjacent carriage from the interface translator and/or each adjacent carriage and controls movement of each carriage whereby collisions between adjacent carriages are avoided. 
   Another form of the invention provides a brushless means for transmitting information from a carriage to a conductor. This information includes data and/or output signals from a monitoring device, such as video, transmitted at radio frequency (RF). 
   Preferably, the control means includes means for avoiding collision of the carriages. A suitable means for avoiding collision comprises:
         a location means to determine a location of each carriage on the track;   a means for storing the location of each carriage;   a transmission means associated with each carriage for transmitting the carriage location;   a receiving means for receiving and monitoring the locations of each carriage; and   a means for controlling the location of each carriage to avoid collision of any carriages.       

   Preferably, the means for avoiding collisions between adjacent carriages provides a means for transmitting and receiving carriage positional information from each carriage and the interface translator. Each carriage and interface translator is able to monitor and store the locations of each carriage. 
   A suitable location means is a location or position sensor means comprising registration marks or position indicators associated with the track; means on the carriages to read the registration marks; means comprising a rotatable wheel on the carriages whereby wheel rotations represents distance travelled by the carriages; means to calculate a carriage position; means associated with each carriage for transmitting the position; means for receiving the positions of each carriage; and means for controlling the position of each carriage to avoid collision of any carriages. 
   The interface translator is suitably adapted to receive position data from each of the carriages and stores data on the position of each carriage based on the data received from the location means. 
   The collision avoidance means is suitably retrofitable to known surveillance systems. 
   The position management software comprises a means for allocating a priority value to each carriage at a particular time, whereby a carriage allocated a higher priority is commanded by the position management software to move to a predetermined location on the track when the interface translator receives a command signal from a master controller. 
   The interface translator preferably comprises a microprocessor which is controlled by the position management software, memory storage for recording the position of each carriage and the minimum distance between adjacent carriages and a track receiver and transmitter for communicating data between the master controller and the microprocessor. 
   Because a carriage cannot physically overtake another carriage on the single track, a mechanism is provided to automatically transfer control from one carriage to a second carriage, simulating an overtaking process. The interface translator, which is monitoring the positions of all the carriage, provides a means for transferring control information from one carriage to an adjacent carriage as part of a handover process. As an example, if a first carriage is moving along the track and encounters a second carriage, the first carriage is stopped at a minimum buffer distance from the second carriage and the control commands are transferred to the second carriage. When a preset viewing location is requested, the interface translator which stores information relating to viewing locations, instructs the carriage closest to the preset viewing location, thus reducing a response time which is especially important if the preset viewing location is activated by an alarm input. If the carriages are performing tours, which are automated movements of the carriages along the track, the handover process is more complicated. When a first carriage performing a tour encounters a second carriage on the track, tour information which is stored at the interface translator is sent to the second carriage allowing the second carriage to continue the tour. Completion of the handover process occurs as the tour is executed. 
   The position management software of the interface translator polls each carriage at predetermined time intervals or rate to monitor the location of each carriage. The polling rate of the carriages may change according to the number of carriages on the track and the number of active or stationary carriages. 
   According to another form of the invention there is provided a track assembly comprising an insulative insert which engages a conductor at one surface and engages a portion of the track assembly at a second surface whereby the insulative insert function as an insulator of a conductor and a means for attaching a conductor to a track assembly. 
   The insulating insert may have additional insulating members attached to a surface of the insert providing a means for insulating two or more conductors. The additional insulating members are suitably located between two or more conductors. Preferably, the insulation insert contacts the conductor(s) and track assembly at positioned intervals along the track assembly; however, the insulation insert may contact the conductors and track assembly continuously. 
   Another form of the invention provides surveillance method including the steps of:
         locating two or more carriages on a track;   mounting at least one monitoring device on each carriage;   providing power to power movement of each carriage on the track;   transmitting output signals, preferably image or image and audio signals, from the monitoring device to a remote location; and   controlling movement of the carriages on the track.       

   Throughout this specification unless the context requires otherwise, the word “comprise”, and variations such as “comprises” or “comprising”, will be understood to imply the inclusion of the stated integers or group of integers or steps but not the exclusion of any other integer or group of integers. 

   
     DESCRIPTION OF THE FIGURES 
     Preferred embodiments of the invention will now be described by way of example only with reference to the accompanying drawings in which: 
       FIG. 1  is a drawing of the general hardware components of a surveillance system; 
       FIG. 2  is a drawing of a carriage or Video Transport Unit (VTU) attached to a track; 
       FIG. 3A  is an end view drawing of a brush bogie of a VTU comprising brushes for power, data and ground and a Radio Frequency (RF) antenna for transmission of video information; 
       FIG. 3B  is an end view of the brush bogie of  FIG. 3A  attached to a three conductor track assembly; 
       FIG. 4A  is an end view drawing of a brush bogie of a VTU comprising brushes for power and ground and an RF antenna for transmission of both data and video information; 
       FIG. 4B  is an end view of the brush bogie of  FIG. 4A  attached to a three conductor track assembly; 
       FIG. 5  is a plan bottom view of a brush bogie of a VTU as shown in  FIG. 3A ; 
       FIG. 6A  shows a three conductor track assembly for the video surveillance system; 
       FIG. 6B  shows a two conductor track assembly for the video surveillance system; 
       FIG. 7  is a diagram of a general overview of the electrical components of the surveillance system; 
       FIG. 8  shows a block diagram of electrical components of a microprocessor and camera sections of a VTU as in  FIG. 7 ; 
       FIG. 9  shows a block diagram of electrical components of an interface translator as in  FIG. 7 ; 
       FIG. 10  shows a block diagram of electrical components of data or control signal flow between a controller and VTU; 
       FIG. 11  is an electrical diagram of the track interface of FIG.  7 ; 
       FIG. 12  is an electrical diagram of the VTU interface of  FIG. 7 ; 
       FIG. 13  shows a diagram of a data polling pattern; and 
       FIG. 14  shows a flow diagram of a controller request. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring to the drawings, wherein like numerals designate like or corresponding parts throughout the several views of a surveillance system. 
     FIG. 1  shows three carriages or Video Transport Units (VTUs)  12 ,  13 ,  14  attached to track  11  in communication with an interface translator  29  via conductors associated with track  11 . Interface translator  29  links with a video monitoring system  100  and controller  34  which may be at a remote location. Interface translator  29  links control information between controller  34  and each VTU; receives video information from VTUs and provides this information to video monitoring system  100  in a form suitable for viewing; and provides a means for position management of VTUs as part of a collision avoidance means. 
     FIG. 2  shows a VTU  14  attached to track  11 . The VTU includes a driver section  73 , a monitoring device section  71  and a microprocessor section  72 . Driver section  73  provides a means for moving VTU  14  on track  11 . Monitoring device section  71  comprises a mounted video camera  46  which is capable of panning continuously through 360°, tilt 120°, zoom and focus. It will be understood that the video camera is merely one example of a monitoring device which may operate in the visible, infrared or ultraviolet spectrum and which may include audio monitoring. Microprocessor section  72  comprises electronic means for controlling VTU  14 , transmitting video information and transmitting and receiving data information to a remote location as shown in  FIG. 1 . 
     FIG. 3A  shows an end view of a brush bogie  15  in isolation and  FIG. 3B  shows an end view of the brush bogie  15  of  FIG. 3A  attached to a track  11  within track assembly  111 . Brush bogie  15  electronically links a VTU  14  to track  11 . Four wheels  23 , composed of plastic or other suitable material, which contact ground conductor  200  at an extended slanted portion enables rolling movement of brush bogie  15  and attaches brush bogie  15  to track  11 . A pair of wheels  24  contact ground conductor  200  at a same surface as carbon brushes  205  and provide a force to assure constant and even contact of wheels  23  with extended slanted portions of ground conductor  200 . Four spring loaded carbon brushes  205  attached to brush bogie  15  contact ground conductor  200  and provide suitable grounding of VTU  14 . A pair of spring loaded carbon brushes  206  transmit power to VTU  14  from power conductor  201 . A pair of spring loaded carbon brushes  207  transmit data and control information between microprocessor section  22  and data and video conductor  202 . An antenna  208  composed of teflon coated tinned copper or other suitable material transmits video signals from microprocessor section  72  to data and video conductor  202  by RF. An optical encoder  209  comprises a means for measuring rotations of a rotatable wheel for determining speed and distance travelled by VTU  14 . 
   Track assembly  111  comprises a base member  112  with slanting sidewalls  113 , a semi-opaque cover  114  which is mountable to slanting side walls  113  and centrally positioned track  11  shown comprising three conductors  200  (ground),  201  (power) and  202  (data and video) and insulator insert  203 . 
     FIG. 4A  shows an end view of a brush bogie  15 A with an RF antenna  208  for transmitting both data and video information to a conductor. Accordingly, carbon brushes for transmitting data signals shown as carbon brushes  207  in  FIGS. 3A and 3B  are omitted. 
     FIG. 4B  shows an end view of the brush bogie  15 A of  FIG. 4A  attached to a three conductor track  11  in a similar manner as in  FIG. 3B . 
     FIG. 5  is a bottom plan view of brush bogie  15  as shown in  FIG. 3A . Shown more clearly are RF antenna  208  for transmitting video information, pair of carbon brushes  206  for transmitting power, pair of carbon brushes  207  for transmitting data signals, four carbon brushes  205  for grounding VTU  14  and optical encoder  209 . Also shown more clearly is a pair of wheels  24  and four wheels  23  for attaching VTU  14  to track  11 . 
     FIG. 6A  shows a track assembly  111  for a three conductor track  11  comprising a base member  112  with slanting side walls  113 , a semi-opaque cover  114  which is mountable on to slanting side walls  113  and centrally positioned track  11  comprising conductors  200  (ground),  201  (power) and  202  (data and video) and insulator insert  203 . Insulator insert  203  is a T-shaped clip with two pairs of barbs  210  on top and two pairs of barbs  211  on bottom sides respectively of insulation insert  203 . Perpendicular wall  220  is centrally located along the bottom of insulation insert  203  separating conductors  201  and  202 . The two pairs of barbs  210  engage a central base member of track assembly  111  at recesses  18  and  19 . The two pairs of barbs  211  engage conductors  201  and  202  on a bottom side of conductors  201  and  202 . When in use, insulator insert  203  insulates and attaches conductors  201  and  202  to the central base member of track assembly  111 . Track  11  also comprises two opposed ground conductors  200  which form a cross sectional U-like shape in which conductors  201  and  202  are internally located. 
   Insulation inserts  203  are positioned at intervals along the central base of track assembly  111  to insulate and attach conductors  201  and  202  to track assembly  111 . 
     FIG. 6B  show a track assembly  312  which is similar to the track assembly of  FIG. 6A ; however, track  311  of  FIG. 6B  comprises only two conductors  300  and  301 . Conductor  301  conducts power, video and data information whereas conductor  300  provides grounding. Insulator insert  303  attaches to conductor  301  and central base member of track assembly  311  at raised recess  18  and  19  in a similar manner as in  FIG. 6A  using barb members on top  310  and bottom  311  of insulator  303 . 
     FIG. 7  shows a block diagram overview of the surveillance system. Video and data signals are modulated to different frequencies, combined and transmitted along the track  11  shown as a dual track for power and data and video signals, then separated and processed by either a VTU  14  or interface translator  29 . The signal path is bi-directional; data and control signals are transmitted in both directions between VTU  14  and interface translator  29 . Video signals are transmitted only in one direction, from VTU  14  to interface translator  29 . Provisions for up to eight video signals modulated to different frequencies are provided, but it would be appreciated by one skilled in the art that additional video signals could be used. 
   The VTU  14  receives and transmits signals to track  11  via VTU interface  40 , which may be a mixer and splitter. VTU interface  40  provides a means for receiving and transmitting signals from VTU  14  and track  11 . Power flows through the VTU interface  40  from track  11  to VTU  14  to power devices such as a driver motor on driver section  73  and camera  46  on monitoring device section  71 . Video signals from a camera  46  pass through video transmitter  43  to provide the video signals in a suitable form to be transmitted on to track  11  by VTU interface  40 . VTU interface  40  also receives and transmits data or control signals to and from data transceiver  44 . Data transceiver  44  provides electrical signals in a suitable form for micro-controller  48 . Data transceiver  44  also provides information from micro-controller  48  to VTU interface  40  in a suitable form for transmission on track  11 . Micro-controller  48  is a microprocessor controlling the functions of VTU  14  including position management and camera functions. 
   At a remote control station, a user may control a VTU by inputing commands at controller  34  which sends signals to interface translator  29  which comprises: track interface  21 , data transceiver  28 , micro-controller  32 , RF splitter  21 A, video receiver  22  and video demodulator  25  which sends a suitable video signal for viewing at video monitoring station  100 .  FIGS. 9 and 10  provide additional information relating to interface translator  29  as discussed below. Micro-controller  32  sends signals to data transceiver  28  which communicates with track  11  through track interface  21 . Track interface  21  is also linked to RF splitter  21 A and DC power supply  70 . RF splitter  21 A is attached to a video receiver  22  which is further linked to a video demodulator  25  and a monitor  100  for viewing images originating from camera  46 . 
   The characteristic impedance of dual track  11  is approximately 22 ohms. Track  11  has a track terminator  11 A at an end opposite to track interface  21  to reduce signal reflections which may cause video picture distortion. VTUs are designed to appear as high impedance to avoid video and data signals from being loaded with multiple VTUs on track  11 . 
     FIG. 8  shows a block diagram of the power and signal processing schema of the invention. A camera  46 , mounted on monitoring device section  71  is controlled by microprocessor  48  for viewing of an area under surveillance. The microprocessor  48  is able to rotate, pivot, zoom and focus the camera  46 . 
   Video signals from the camera  46  are processed by video modulator  45  for transmission by video transmitter  43 . The video modulator encodes the video signals for a suitable carrier frequency. As described below, this may be an RF signal for brushless contacts or a lower frequency in embodiments that use brush contacts. 
   VTU interface  40 , which may comprise a filter and mixer, manages the placing of signals on the track  11  and receiving signals from the track  11 . Power supply  42  is also transferred from track  11  to the VTU via VTU interface  40 . 
   The microprocessor  48  also controls the movement of the VTU  14  by controlling the servo control  49  on the driver section  70 . Control of the VTU may originate from a user at a remote station with a controller  34 . As described below, an interface translator  29  transmits control signals on track  11 . The control signals are picked up by VTU interface  40  and communicated to data transceiver modulator and demodulator  44 . Data transceiver modulator and demodulator  44  is connected to driver  47  which is connected to a microprocessor  48  which controls both the camera  46  and servo control  49 . 
     FIG. 9  shows an interface translator  29  comprising track interface  21  connected directly to track  11 . Track interface  21  is connected to video receivers  22 ,  23 ,  24  which receive video signals originating from each VTU. Video signals are transmitted at different frequencies for each VTU. The video signals are demodulated by a respective demodulator  25 ,  26 ,  27  which is hard wired to monitoring system  100  which includes a video display whereby images from each camera  46  of each VTU can be displayed. 
   A power supply  70  is connected to track interface  21  and may be a stand alone battery unit or connected to a remote power source through conductor  201 . 
   Track interface  21  is also connected through a data transceiver modulator and demodulator  28  and driver  31  to controller  34  for controlling a VTU from a remote location. 
     FIG. 10  shows data and control signal flow between controller  34  to VTU  14  via interface translator  29 . Data and control information flows in both directions between controller  34  and VTU  14 . Interface translator  29  includes a driver  31  connected to a microprocessor  32 . The microprocessor  32  includes a communication and positional manager, protocol translator and memory  33 . Data transceiver  28  shown as “modulated track data” modulates data and sends the modulated data to track interface  21  or receives signals from track interface  21  and demodulates the signals. Signals are transmitted to and received from track  11  by VTU interface  40 , shown as a mixer and launching circuit. Signals received from track  11  by VTU interface  40  are demodulated by data transceiver  44  shown as “modulated track data”. Signal originating from VTU  14  are modulated by data transceiver  44  before placing onto track  11  by VTU interface  40 . Data transceiver  44  is attached to microprocessor  48  which can store data information in memory  48 A. 
     FIG. 11  is an electronic diagram of a track interface  21  as shown in  FIG. 7 . Track  11  is shown as a dual track with DC power and RF signals on separate conductors. At the interface translator  29 , signals from the track  11  are band pass filtered  400  between 7 MHz and 270 MHz to remove as much brush, motor and other interference as possible. The track impedance is transformed back to 75 ohms and video and data signals are separated using directional couplers  401 A and  401 B. Two video paths  402 A and  402 B are provided for multiple VTU applications. These two video paths are split into four thus providing eight video channels. The video channels are band pass filtered  403 A and  403 B between 100 MHz and 275 MHz to cover the eight video channels before passing to a video receiver. The data signal is low pass filtered  404  to 13 MHz to remove video signals before passing to a data transmitter and receiver. A low pass filter  405  filters data and video signals on DC track. 
     FIG. 12  is an electronic diagram of a VTU interface  40  as shown in  FIG. 7 . The output impedance of the video transmitter is 75 ohms. The video signal passes through a high pass filter  500  to an antenna loop which is terminated in 75 ohms. The antenna loop directionally launches a signal onto the track  11 , sending most of the signal towards the interface translator  29 . The data transmitter modulates data to 10.7 MHz. The transmitted data passes though a band pass filter between 7 and 13 MHz, shown as filters  501  and  502 , to remove the video signals and other interference from track  11 . The signal passes through a matching transformer  503  which raises the impedance to approximately 320 ohms. This reduces the loading of multiple VTUs on track  11 . 
   Track  11  is shown as a dual track with three conductors for DC power, ground and RF signals on separate conductors. The DC track has a low pass filter  504 . 
   Collision Avoidance Means 
   In one form of the invention, track  11  has multiple VTUs  12 ,  13 ,  14  which are movable along overlapping locations of track  11 . 
   Accordingly, it is important to prevent collisions between adjacent VTUs. 
   A positional management system provides a means to prevent collisions between adjacent VTUs. Two means of collision avoidance are provided, a first main or master level controlled by an interface translator  29  and a second means managed by each VTU. 
     FIGS. 7 ,  9  and  10  show interface translator  29  comprising a microprocessor  32  which provides an interface between master controller  34  and data transceiver  28 . Hardware and software within interface translator  29  control movement of each VTU  12 ,  13 ,  14 . Interface translator  29  is located in a Power Supply Interface enclosure mounted at the start of each track  11 . 
   The interface translator  29  interprets and processes commands received from master controller  34  and forwards the commands to an appropriate VTU  12 ,  13 ,  14 . Interface translator  29  manages positional and movement commands of each VTU and allocates priority to an appropriate VTU in response to an alarm activated preset or other command. 
   Presets are a mechanism where positional information about a camera view is automatically stored so a VTU can return to that position later. Presets can be used with alarms to view particular areas where activity has set off the alarm. The interface translator  29  monitoring the system decides which VTU  12 ,  13 ,  14  is closest to the preset position requested and enables the shortest response time and enures movement of a VTU is unimpeded by other VTUs on track  11 . 
   Interface translator  29  also functions as a main positional manager for a track  11  to which it is attached. Positional information is provided from data sent by each VTU  12 ,  13 ,  14  on track  11  and this information is mapped in memory  33 . Each VTU  12 ,  13 ,  14  comprises an optical encoder  209  (shown in  FIG. 3 ) which measures distance by wheel rotations as the VTU moves along track  11 . Position indicators or registration marks, such as bar codes, along track  11  and markings on the measurement wheel are monitored by a sensor which transmits signals to microprocessor  48  which is then able to store data on the position of each VTU  12 ,  13 ,  14 . As VTU  12 ,  13 ,  14  moves along track  11 , memory in microprocessor  48  is updated so that microprocessor  48  is continually aware of each VTU  12 ,  13 ,  14  location on track  11 . Interface translator  29  acts as an arbitrator if there is conflict of position requests and ensures that adjacent VTUs maintain a minimal distance. A suitable minimal distance is 2.4 meters or 8 feet which forms a buffer zone between any adjacent VTUs. 
   Referring to  FIGS. 7 ,  8  and  10 , while microprocessor  48  records the position of its own VTU  14 , it is also transmitting data on its location through driver  47 , data transceiver  44  and VTU interface  41  along a common communication conductor  202  of track  11  where interface translator  29  is able to receive this positional data through track interface  21  and data transceiver  28  for processing by its microprocessor  32 . 
   Because each VTU is effectively identical, they are each able to store data on the location of their own VTU along track  11  and are equally able to transmit this data on the common communication conductor  202  of track  11  where this data can be received by the interface translator  29 . During installation, VTUs are configured with appropriate positional information of other VTUs. This information is continually updated during VTU movement on track  11 . An active VTU broadcasts its changing location which allows other VTUs to update a last known location of adjacent VTUs. The active VTU will immediately stop if conflict arises with an adjacent VTU location. 
   Transmission of data from each VTU occurs on a common frequency. Accordingly, because there is only one communication line  202  which is used by all VTUs, interface translator  29  operates to ensure that only one VTU transmits data at any one time. 
   Position management software of interface translator  29  sequentially polls each VTU. The polling process occurs continually even if there is no movement or activity of any one VTU  12 ,  13 ,  14  on track  11 . Each VTU  12 ,  13 ,  14  has a unique address. Interface translator  29  addresses each VTU  12 ,  13 ,  14  in turn and requests the positional information and status of each VTU. The VTU  12 ,  13 ,  14  which recognises this unique address is the only VTU which responds to a command from interface translator  29 . The response from the commanded VTU  12 ,  13 ,  14  contains its current positional information and status. The rate at which each VTU and its unique address is polled is dependent upon the number of VTUs on track  11 . This polling rate is typically one to two per second. If the information translator  29  detects an active status on any VTU during the normal polling routine, it immediately increase the polling rate to the active VTU. This process is called refresh. The rate at which refresh occurs is directly related to the number of active or moving VTUs on track  11  at any one time. A typical refresh rate is 6 to 10 per second. 
   Information which is returned by a particular VTU  12 ,  13 ,  14  during refresh allows the interface translator  29  to update the last known location of the active VTU at a more frequent rate. 
     FIG. 13  shows a typical data polling pattern  600  comprising poll  601  and reply data  602 . The poll data  601  comprises a preamble  607  followed by  13  characters  603  of information. Each character comprises a start  604 , stop  605  and eight bits of data  606 . Time from start to stop is approximately 1.0 msec. 
   As shown in  FIGS. 7 to 10 , when it is desired to monitor a particular location, master controller  34  transmits command signals to the interface translator  29 . The interface translator  29  has a record of the location of each VTU stored in its memory  33 . Consequently when one particular area must be monitored its selects the closest VTU to move to a location on track  11  where monitoring can occur. Accordingly, as part of the process of commanding one of the VTUs to move, it must first allocate priority to each VTU  12 ,  13 ,  14  according to its distance along track  11  from the final destination along track  11  where monitoring is to occur. If there is any positional conflict between VTUs because they are both within a similar distance from the desired destination, the interface translator  29  ensures that adjacent VTUs do not encroach on a buffer zone which has been preselected and stored in memory. It follows therefore that if two VTUs move to a desired destination and cannot reach the final destination because they both would enter the buffer zone between them, the VTU with the higher priority would be instructed to move to the desired destination while the adjacent VTU with the lower priority would move away from the desired destination to ensure the buffer zone is maintained. 
   In addition to the above, each VTU and its microprocessor  48  stores data in memory  48 A, including the address on the position of each other VTUs  12 ,  13 ,  14  on track  11 . Microprocessor  48  is also able to receive data transmitted by other VTUs  12 ,  13 ,  14  along the common communication conductor  202  of track  11 . Because each VTU  12 ,  13 ,  14  is aware of the location of other VTUs on track  11  an active VTU  12 ,  13 ,  14  will immediately stop if the positional data it has stored on adjacent VTUs indicates that a conflict has arisen because the moving VTU  12 ,  13 ,  14  has entered a buffer zone. Accordingly, if the moving VTU finds that it is within a buffer zone of an adjacent VTU it immediately stops. The interface translator  29  then is able to issue commands to the conflicting VTUs  12 ,  13 ,  14  so that the VTU  12 ,  13 ,  14  allocated the highest priority can move to the desired destination while the other VTU  12 ,  13 ,  14  moves far enough away so that the buffer zone is maintained. 
   When a VTU approaches a buffer zone between itself and a second VTU, information and commands may be transferred from one VTU to a second VTU as part of a handover or swapping procedure. Handover procedures are controlled by interface translator  29 . 
   As part of the collision avoidance system, position indicators provided as bar codes are located along the track at intervals to provide reference points for correcting any discrepancies which may occur due to loss of power or optical encoder  209  inaccuracies. The bar codes may provide an absolute measurement of distance along track  11 . These bar codes are sensed by bar code detectors located on each VTU  12 ,  13 ,  14 . Initially, each VTU is moved along track  11  at a slow speed to set bar code locations into memory. Initial referencing of bar code locations at slow speed assures more accurate position identification as wheels  23  and  24  of the VTUs are less likely to slip when compared to the higher speed of movement which is typically 3.3 meters/second to 4.5 meters/second, although speed of movement may be faster or slower than this range. The bar code detectors and the optical encoder  209  together provide positional data to the microprocessor  48  of its VTU  12 ,  13 ,  14 . 
   With the collision avoidance system described above, a fail safe positional management system is achieved for each of VTUs  12 ,  13 ,  14 , whereby the interface translator  29  is able to manage position and movement commands from the master controller  34  and allocate priority to the appropriate VTU  12 ,  13 ,  14  to satisfy such requests as alarm activated presets and tours. The interface translator  29  is able to act as an arbitrator if there is a conflict of position requests received from the master controller  34 . The interface translator  29  ensures that adjacent VTUs  12 ,  13 ,  14  do not encroach on the buffer zone. If for some reason this policing action by interface translator  29  is interrupted there is still a second level of collision avoidance provided by VTUs  12 ,  13 ,  14  monitoring the position of other VTUs  12 ,  13 ,  14  on track  11 . 
   Positional data on each VTU  12 ,  13 ,  14  is mapped into separate memory locations allocated with the specific address of a respective VTU  12 ,  13 ,  14 . A similar mapping process occurs in storage locations of the microprocessor  48  of each of the VTUs  12 ,  13 ,  14 . Each of the microprocessors  48  of the VTUs  12 ,  13 ,  14  also stores data on the buffer zone with the result that each microprocessor  48  can determine when its VTU  12 ,  13 ,  14  is in conflict with an adjacent VTU  12 ,  13 ,  14 . Unlike the interface translator  29 , however, microprocessor  48  of each VTU  12 ,  13 ,  14  does not have the ability to solve a conflict with an adjacent VTU  12 ,  13 ,  14 . 
     FIG. 14  shows a typical flow diagram of a controller request received from the master controller  34 . Master controller  34  requests a VTU  12 ,  13 ,  14  to move forward by sending a command signal  50  to interface translator  29  which receives the request  51 . The interface translator  29  then interprets and processes the request  52 . The interface translator  29  then determines whether the command is valid  53 . If the command is not valid then further action is stopped  54 . 
   If all the position parameters of each VTU is maintained, the interface translator  29  transmits a forward command  55  to one of the VTUs. 
   A VTU receives a forward command, acknowledges and processes the command  56  from the interface translator  29  and is also constantly listening for updated positional data broadcast  57  transmitted from other VTUs on track  11 . 
   Movement of the VTU is then initiated and positional updates are continuously broadcast  58  from the moving VTU to the common communication conductor  202  of track  11 . 
   Adjacent VTUs receive the broadcast  59  from the moving VTUs and thus update their own records to maintain the most recent data on the position of each VTU on track  11 . 
   The microprocessor  48  of the moving VTUs constantly monitor whether the command from the information translator  29  is current  60 . 
   If the command is current, the VTU repeats the step  56 . If the command is not current then the VTU stops  61 . 
   The surveillance system described above provides cameras  46  of a VTU  12 ,  13 ,  14  capable of panning continuously through 360°, tilt 240°, zoom and focus. VTU  12 ,  13 ,  14  provides linear movement for the camera  21  along track  11  and continuous monitoring of multiple areas within a monitored zone is possible using the unique collision avoidance system described above. A wireless radio frequency antenna  208  is capable of transmitting video and/or data information to a conductor extending along track  11 . The above surveillance system provides a means to view multiple areas simultaneously. 
   It is understood that the invention described in detail herein is susceptible to modification and variation, such that embodiments other than those described herein are contemplated which nevertheless falls within the broad spirt and scope of the invention.