Abstract:
A system and method for controlling transportation traffic signal beacons including powering a signal, encoding a signal state, and decoding the signal state at the beacon is presented. Furthermore, traffic signal beacons may be placed along the length of a bridge to warn of a bridge collapse. The metallic cable that powers the beacons may also function as a bridge collapse detection sensor.

Description:
TECHNICAL FIELD OF THE INVENTION 
   This invention relates generally to the field of transportation signaling and, more specifically, to a system and method of powering and controlling traffic signals. The invention is further expanded to include a bridge collapse detection system 
   BACKGROUND OF THE INVENTION 
   Traffic flow control allows safe and efficient travel for motorists. At a typical automotive intersection, motorists traveling in opposing directions are given alternating rights-of-way via a set of standardized traffic signal beacons. Each beacon consists of a recognizable combination of green, yellow, and/or red electric signal lamps enclosed in a standard housing. These beacons face the desired directions of travel and are controlled from a common point with one traffic signal controller. All lamps are electrically home-run to the controller using 120VAC or other high-voltage AC power. The controller selects which lamps to illuminate at any given time while a conflict monitor prevents unsafe combinations of lamp illumination. The lamps themselves may be incandescent with filters or one of a variety of LED styles. Common patterns include round balls, arrows, and “Xs.” A UPS including a rectifier, battery, and inverter may be included at critical locations. 
   Another application of traffic flow control is a bridge collapse motorist warning system. While bridges are generally safe, they can fail. When they do, frequently motorists that were not even on the bridge at the time of collapse drive over the edge. This is because by the time motorists become aware of the hazard, they may no longer have adequate stopping distance. Bridge curvature can limit visibility of a hazard even under otherwise ideal visibility conditions. To limit this unnecessary loss of life and property, a series of flashing red traffic signal beacons may be spaced along the length of the bridge and driven by 120VAC line power, as further described in Mercier, J. J. and Marshall, R. A., “Bridge Collapse Detection and Motorist Warning System,” IEEE ITS newsletter Vol 7, No 3, September 2005. There is one important drawback to this system. It is possible to short the beacon power to ground or to water, tripping a circuit breaker and causing all the bridge&#39;s beacons to go dark at the only time they are needed. While there is a method of mechanical disconnection of the damaged cable section which may reduce this risk, it does not guarantee critical operation. 
   A bridge collapse motorist warning system must be activated by a bridge collapse detection system, a means to detect the failure of the bridge. Frequently, a cable is run the entire length of the bridge with a break in the cable indicating a structural failure. U.S. Pat. No. 6,972,687, Marshall, et al., “System and Method for Detecting A Structure Failure” illustrates such a system using a fiber optic cable sensor. Unfortunately, fiber optic cable is difficult to grip and attach to fixed points on a bridge. Also, optical fibers and high voltage power are run in separate conduits or cables, which are both very costly and difficult to install. 
   SUMMARY OF THE INVENTION 
   In accordance with the present invention, a system and method to multiplex traffic signals with minimal conductor usage is disclosed that addresses disadvantages and problems associated with other systems and methods. The invention is further expanded to include a bridge collapse detection and motorist warning system. 
   A system and method of multiplexing traffic signals includes encoding a desired state of light illumination, where the encoded state cannot produce conflicting signal illumination; and providing power to the signals. Only a single pair of copper wires is required to control an entire intersection. All traffic signals are electrically in parallel on the single pair of control and power wires. 
   LED signals already employ active controls; each signal lamp includes a power supply to provide the proper illumination. In a beacon with one to three lamps, since only one signal is ever illuminated at any given time, a power supply in each signal lamp is wasteful. In accordance with the present invention, the output of the beacon&#39;s only power supply is directed to the appropriate signal lamps with a single decoded command from the traffic signal controller. The command is included on the same pair of wires as power, and only a single pair of wires is required to operate an entire intersection with turn lanes in all directions. For a four-way intersection with turn lanes, the thirty-five useful states of signal illumination are encoded, for example, with the polarity of the applied power and a single Dual Tone Multi Frequency digit. DTMF encoding takes advantage of very low-cost ICs from the telecom industry. Also, with LED signals, a low-voltage DC may be supplied to the signals, eliminating the inherent hazards with high-voltage AC and allowing the use of cheaper low-voltage wiring, such as telephone drop wire. This also allows a UPS to be replaced with a simpler rectifier and battery charger, eliminating the cost, power loss, and possible failure of an inverter. 
   This multiplexing method is also particularly useful in a bridge collapse detection and motorist warning system. Unnecessary loss of life and property can be significantly reduced with a reliable system to immediately and effectively detect and warn of a failed structure. A series of flashing red traffic signal beacons are spaced along a bridge to warn any motorist approaching or on the bridge of the impending peril. If all of the beacons are simply wired in parallel, any electrical fault on the wire will disable all beacons and fail to notify motorists. A collapsing bridge has the potential to fault the wiring during collapse. Home-run wiring eliminates this concern, as a short on any one beacon will not affect the others. This system only requires flashing red signals. Only three states are needed, so only polarity encoding is adequate. A single pair of wires may be routed to each beacon, containing two signal lamps. This saves one pair of wires per beacon, which can become very significant on mile-long causeways. The use of low-voltage DC allows the use of telephone drop wire, which does not need to be run in conduit, and eliminates the hazard of accidentally coming into contact with shredded power conductors after a collapse. 
   The same pairs of wire used to control the signals may also be used as a collapse detection sensor. This same or a separate pair of wires is monitored for the presence of an applied very low voltage at the opposite end of the bridge. Electrical continuity indicates that the bridge is still intact. This applied voltage must be low enough to not illuminate the signals on this same pair of wires. Alternately, a pair of wires may transmit a data signal, with loss of data indicating a bridge collapse. The cable must be periodically anchored to the bridge in a manner to ensure that the falling bridge will sever the cable, not simply allow it to stretch without breaking. This is of particular concern on low-rise causeways. Metallic cable is ductile and is manufactured by drawing, a controlled stretching process. At each anchor point, the metallic cable is strain-relieved and loosely spindled around a dull edge. The strain relief prevents any wear on the cable from the dull edge. In the event of a collapse, there is enough force on the metallic cable to pull out of the strain relief and be severed against the dull edge that it is now in full contact with. 
   Other technical advantages are readily apparent to one skilled in the art from the following figures, descriptions, and claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the invention, and for further features and advantages, reference is now made to the following description, taken in conjunction with the accompanying drawings, in which: 
       FIG. 1  shows a diagram of a system for controlling traffic signal beacons in accordance with the present invention; 
       FIG. 2  is an illustration of a motorist warning and bridge collapse detection system in accordance with the present invention; 
       FIG. 3  is an electrical schematic showing traffic signal beacon connection of a motorist warning and bridge collapse detection system in accordance with the present invention; 
       FIG. 4  illustrates a cable anchor/breaker installed periodically along the length of the bridge in accordance with the present invention; 
       FIG. 5  is a flowchart demonstrating one method of detecting and warning of a structural failure in accordance with the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Embodiments of the present invention and their advantages are best understood by referring to  FIGS. 1 through 5  of the drawings, in which like numerals refer to like parts. 
     FIG. 1  shows a diagram of one embodiment of a system for controlling traffic signal beacons. A traffic signal controller  105  contains a processor  190  which selects a desired state of a multitude of traffic signal lamps  140  to control the flow of motor vehicles in a desired fashion. A single logical state transmitted over a single pair of wires  160  describes the desired traffic flow for the entire intersection. Power supply  185  provides low-voltage DC power to operate traffic lights  140   a - b . Power supply  185  may also include a backup battery. Encoder  180  encodes the desired state and also couples DC power to cable  160 . Encoding of three states (top lamp on, bottom lamp on, and no lamp on) is possible using only the polarity of the applied power (positive, negative, and off). Any other encoding method is possible. A single pair of wires  160  may connect controller  105  to a multitude of beacons  100 . Use of DC power allows elimination of an inverter associated with battery backup, allowing a little longer battery run-time. 
   At beacon  100 , decoder  110  decodes the state transmitted by encoder  180  and controls selector  130  to apply power to the correct combination of lamps  140 . The number of lamps  140  is determined by the characteristics of the traffic. Lamps  140  are LED lamps. However, incandescent lamps are not precluded. Decoder  110  also separates power from pair of wires  160  to couple to a constant current DC/DC converter  120 . DC/DC converter  120  may or may not sense the temperature of the illuminated lamp  140  and temperature—compensate the current accordingly. 
   For example, a 4-way intersection may flash yellow in the main two directions of traffic flow (E/W) and flash red in the two crossing directions of travel (N/S). Each of the four directions require at least one flashing traffic signal beacon, each with two appropriately-colored traffic signal lamps. Only one pair of wires  160  are required to control the entire intersection as demonstrated by the following encoding table. Single pair of wires  160  is run in parallel to each beacon. Each beacon uses the polarity of single pair of wires  160  to determine which lamp (top or bottom) to illuminate. 
   
     
       
             
           
             
             
           
             
             
             
           
             
             
             
             
             
           
         
             
                 
             
             
               Lamp Illumination Encoding Table 
             
           
        
         
             
                 
               Beacons 
             
           
        
         
             
                 
               Crossing - Red Flash 
               Main - Yellow Flash 
             
           
        
         
             
               Pair Polarity 
               Northbound 
               Southbound 
               Eastbound 
               Westbound 
             
             
                 
             
             
               Positive 
               Top 
               Top 
               Top 
               Top 
             
             
               Negative 
               Bottom 
               Bottom 
               Bottom 
               Bottom 
             
             
               OFF 
               — 
               — 
               — 
               — 
             
             
                 
             
           
        
       
     
   
   Alternatively, a 4-way intersection with left turn lanes in all directions may also be easily controlled with a single pair of wires  160 . Additional states are required due to the complexity of the intersection. These states are added by sending a single DTMF encoded digit. DTMF encoders and decoders are inexpensive due to their widespread use in POTS telephony dialing. However, any other well-known encoding method and state assignment may be used. Decoder  110  must be operable to decode red, yellow, green, red arrow, yellow arrow, and green arrow lamp states. The encoded states are selected to use the same DTMF digits for N/S and E/W directions; this is accomplished with only a polarity reversal of pair of wires  160  to some beacons  100 . The combination of the polarity and DTMF inputs determine the encoded state as shown in the following table. Addition of jumper  111  allows identification of N or E from S or W, thus allowing a single version of decoder  110  to be used in every beacon  100  in the intersection. 
   
     
       
             
           
             
             
           
             
             
             
             
             
           
             
             
             
             
             
             
             
             
             
             
           
         
             
                 
             
             
               Lamp Illumination Encoding Table 
             
           
        
         
             
                 
               Beacons 
             
           
        
         
             
                 
               Northbound 
               Southbound 
               Eastbound 
               Westbound 
             
           
        
         
             
               Pair Polarity 
               DTMF 
               Straight 
               Left Turn 
               Straight 
               Left Turn 
               Straight 
               Left Turn 
               Straight 
               Left Turn 
             
             
                 
             
             
               Positive 
               0 
               G 
               R 
               G 
               R 
               R 
               R 
               R 
               R 
             
             
               Positive 
               1 
               Y 
               R 
               Y 
               R 
               R 
               R 
               R 
               R 
             
             
               Positive 
               2 
               G 
               G 
               R 
               R 
               R 
               R 
               R 
               R 
             
             
               Positive 
               3 
               G 
               Y 
               R 
               R 
               R 
               R 
               R 
               R 
             
             
               Positive 
               4 
               G 
               R 
               R 
               R 
               R 
               R 
               R 
               R 
             
             
               Positive 
               5 
               Y 
               G 
               R 
               R 
               R 
               R 
               R 
               R 
             
             
               Positive 
               6 
               Y 
               Y 
               R 
               R 
               R 
               R 
               R 
               R 
             
             
               Positive 
               7 
               Y 
               R 
               R 
               R 
               R 
               R 
               R 
               R 
             
             
               Positive 
               8 
               R 
               R 
               G 
               G 
               R 
               R 
               R 
               R 
             
             
               Positive 
               9 
               R 
               R 
               G 
               Y 
               R 
               R 
               R 
               R 
             
             
               Positive 
               A 
               R 
               R 
               G 
               R 
               R 
               R 
               R 
               R 
             
             
               Positive 
               B 
               R 
               R 
               Y 
               G 
               R 
               R 
               R 
               R 
             
             
               Positive 
               C 
               R 
               R 
               Y 
               Y 
               R 
               R 
               R 
               R 
             
             
               Positive 
               D 
               R 
               R 
               Y 
               R 
               R 
               R 
               R 
               R 
             
             
               Positive 
               # 
               R 
               G 
               R 
               G 
               R 
               R 
               R 
               R 
             
             
               Positive 
               * 
               R 
               Y 
               R 
               Y 
               R 
               R 
               R 
               R 
             
             
               Positive 
               NO TONE 
               R 
               R 
               R 
               R 
               R 
               R 
               R 
               R 
             
             
               OFF 
               — 
               — 
               — 
               — 
               — 
               — 
               — 
               — 
               — 
             
             
               Negative 
               0 
               R 
               R 
               R 
               R 
               G 
               R 
               G 
               R 
             
             
               Negative 
               1 
               R 
               R 
               R 
               R 
               Y 
               R 
               Y 
               R 
             
             
               Negative 
               2 
               R 
               R 
               R 
               R 
               G 
               G 
               R 
               R 
             
             
               Negative 
               3 
               R 
               R 
               R 
               R 
               G 
               Y 
               R 
               R 
             
             
               Negative 
               4 
               R 
               R 
               R 
               R 
               G 
               R 
               R 
               R 
             
             
               Negative 
               5 
               R 
               R 
               R 
               R 
               Y 
               G 
               R 
               R 
             
             
               Negative 
               6 
               R 
               R 
               R 
               R 
               Y 
               Y 
               R 
               R 
             
             
               Negative 
               7 
               R 
               R 
               R 
               R 
               Y 
               R 
               R 
               R 
             
             
               Negative 
               8 
               R 
               R 
               R 
               R 
               R 
               R 
               G 
               G 
             
             
               Negative 
               9 
               R 
               R 
               R 
               R 
               R 
               R 
               G 
               Y 
             
             
               Negative 
               A 
               R 
               R 
               R 
               R 
               R 
               R 
               G 
               R 
             
             
               Negative 
               B 
               R 
               R 
               R 
               R 
               R 
               R 
               Y 
               G 
             
             
               Negative 
               C 
               R 
               R 
               R 
               R 
               R 
               R 
               Y 
               Y 
             
             
               Negative 
               D 
               R 
               R 
               R 
               R 
               R 
               R 
               Y 
               R 
             
             
               Negative 
               # 
               R 
               R 
               R 
               R 
               R 
               G 
               R 
               G 
             
             
               Negative 
               * 
               R 
               R 
               R 
               R 
               R 
               Y 
               R 
               Y 
             
             
               Negative 
               NO TONE 
               R 
               R 
               R 
               R 
               R 
               R 
               R 
               R 
             
             
                 
             
           
        
       
     
   
   For this selection of encoding method and states, decoder  110  needs only to decode the following states, as the beacon  100  location within the intersection is determined by jumper  111  and the polarity of wires  160 . 
   
     
       
             
           
             
             
             
           
             
             
             
             
             
           
             
             
             
             
             
             
           
             
             
             
             
             
             
           
             
             
             
             
             
             
           
             
             
             
             
             
             
           
         
             
                 
             
             
               Lamp Illumination Decoding Table 
             
           
        
         
             
                 
               DTMF 
                 
             
           
        
         
             
                 
               Jumper 
               Jumper 
               Beacon 
                 
             
           
        
         
             
                 
               Pair Polarity 
               ON 
               OFF 
               Straight 
               Left Turn 
             
             
                 
                 
             
           
        
         
             
                 
               Positive 
               0 
                 
               G 
               R 
             
             
                 
               Positive 
               1 
                 
               Y 
               R 
             
           
        
         
             
                 
               Positive 
               2 
               8 
               G 
               G 
             
             
                 
               Positive 
               3 
               9 
               G 
               Y 
             
             
                 
               Positive 
               4 
               A 
               G 
               R 
             
             
                 
               Positive 
               5 
               B 
               Y 
               G 
             
             
                 
               Positive 
               6 
               C 
               Y 
               Y 
             
             
                 
               Positive 
               7 
               D 
               Y 
               R 
             
             
                 
               Positive 
               8 
               2 
               R 
               R 
             
             
                 
               Positive 
               9 
               3 
               R 
               R 
             
             
                 
               Positive 
               A 
               4 
               R 
               R 
             
             
                 
               Positive 
               B 
               5 
               R 
               R 
             
             
                 
               Positive 
               C 
               6 
               R 
               R 
             
             
                 
               Positive 
               D 
               7 
               R 
               R 
             
           
        
         
             
                 
               Positive 
               * 
                 
               R 
               G 
             
             
                 
               Positive 
               # 
                 
               R 
               Y 
             
             
                 
               Negative 
               OFF 
                 
               R 
               R 
             
             
                 
                 
             
           
        
       
     
   
   With both examples, encoder  180  sends the state of the entire intersection over a single pair of wires  160 . The type of intersection and sequence of progression through each beacon state is determined by the traffic engineer. Use of polarity coding reduces complexity of decoder  110 . Also, a power supply is no longer required in each lamp, but only one per each beacon. 
   Additionally, processor  190  may monitor the current consumed in each state. Any significant variation in this current may indicate a failed lamp  140 . Optionally, resistor  150  may provide a signature impedance for identification of beacon  100 . Optionally, resistors  170  in conjunction with pair  161  may be added to allow for cable continuity checking of pair  160 , without adding an additional opportunity for failure of pair  161  to disrupt the system. 
     FIG. 2  is an illustration of one embodiment of a motorist warning and bridge collapse detection system. Such a system for controlling the flow of traffic may be installed on a bridge, causeway, or other transportation structure to stop the flow of traffic in the event of a collapse of the structure. A metallic cable  260  is run underneath bridge  210  for the length of bridge  210 . A collapsed bridge section  215  will result in a parted cable  265 . Controllers  270  monitor the integrity of cable  260  and upon loss of continuity, activate beacons  100 . Only beacons  100   a - c , located before collapsed section  215 , should be activated to stop motorist  220   a  from plummeting off the end of the bridge. Frequently, without a warning system, by the time motorist  220   a  becomes aware of collapsed section  215 , adequate stopping distance is no longer available, and motorist  220   a  will fall into the water. Beacon  100   f , located after collapsed section  215 , should not be activated to allow motorist  220   b  to exit normally. Beacons  100  are flashing alternating red balls periodically spaced along the structure. Beacons  100  are shown on every span for illustrative purposes. They need not be placed every span and should be spaced according to stopping distance at highway speed, visibility, bridge geometry, and the type of bridge structure. For example, spacing can be on the order of 500 feet for highway speeds. 
   Key to creating parted cable  265  is cable anchor  400 . Without a reliable way of attaching cable  260  to bridge  210 , the collapsed section  215  may not actually break cable  260 , but may simply stretch cable  260 , especially if bridge  210  has a low rise. Metallic cables are subject to drawing, thus a cable anchor  400  is placed at each end of each span. In alternate embodiments, cable anchor  400  is placed less than every span, based on the bridge height and elastic modulus of the cable. Any other spacing is readily envisioned. 
   The same layout is repeated for each direction of traffic flow. Some elements of the system may be combined to service the entire bridge. 
     FIG. 3  is an electrical one-line diagram showing beacon connection. Controller  270   a  contains a processor  190   a  and a DC power supply  185   a  with an optional battery backup. Processor  190   a  determines if cable  260  has been broken using impedance monitor  360   a  to monitor for the presence of signature impedance  360   b  located in remote controller  270   b . Modems  350   a - b  form a datalink between processors  190   a  and  190   b  to allow for passage of diagnostic and health information. Alternatively, loss of signal between modems  350  may be used in addition to or as a replacement to monitoring for a signature impedance  360   b  to determine if cable  260  has been broken. Any other means of detecting a cable break may be employed. 
   When parted cable  265  has been detected, processor  190   a  changes the state of beacons from off to alternating between top beacon on and bottom beacon on with about a one-second interval. Encoder  180   a  encodes this state for transmission to beacons  100   a - f . Encoding the state onto a single pair of wires  160  per beacon  100  is especially important when bridge  210  is over a mile long. Other well-known methods suffer from requiring an additional wire for the each lamp within each beacon, which is only utilized half of the time in the case of a flashing beacon. Also important is the use of low-voltage power as this allows a safe voltage to be used in case of accidental human contact. Allowing DC/DC converter  120  to accept a wide input voltage range allows use of smaller conductors. 
   Each beacon  100  is wired to controller  270   a  with a dedicated pair of wires  160   a - f  within multi-pair cable  260 . Each X in the diagram shows how each pair of wires  160  is cut immediately after each beacon  100 . This allows any short across one or more pairs  160  which may likely develop during the collapse and creation of parted cable  265  from disabling any beacon  100   a - c  located before the collapse. Such an event would fail to warn motorist  220   a  of the impending danger. Connections on pairs of wire  160   d - f  to disabled beacons  100   d - f  may be either open or short due to parted cable  265 . PTCs  330   a - f  limit any fault current associated with each respective beacon  100   a - f  to a value which is easily tolerated by the system. Alternatively, fuses or other current limit means may be incorporated. Short wires  342   a - f  may connect each beacon  100   a - f  to its respective pair of wires  160   a - f  in cable  260 . For pairs of wire  160   e - f , which must be run the entire length of the bridge to allow for collapse detection, the pair is cut and run to beacons  100   e - f , which provide resistors  170  for connection to controller  270   b  via pairs of wire  161   e - f , such that a collapse after cable anchor  400   h  would not disrupt the operation of beacons  100   e - f.    
     FIG. 4  illustrates a cable anchor/breaker located at each beacon  100 . Cable anchor  400  securely attaches cable  260  to bridge  210  via bolt  420 . Cable  260  is a standard multi-pair aerial telecommunications drop wire. The tension of cable  260  from running between other cable anchors  400  is relieved with standard P-clamps  430   a - b  commonly used with drop wire  260 . P-clamp  430  is attached to unistrut  410 . Cable  260  wraps around from the inside of the unistrut channel to the outside of the channel, and back to the inside of the channel. Unistrut U bracket  440  prevents cable  260  from falling out of unistrut  410  even under adverse conditions. A cable anchor  400  is placed on each side of each expansion joint of bridge  210 . This gives a reasonable span of cable  260  between cable anchors  400 , and a small displacement across an expansion joint can produce enough force to break cable  260 . A multitude of other varieties of cables, clamps, and brackets are readily envisioned. 
   Upon collapse of bridge  210 , a large displacement occurs between two adjacent cable anchors  400 . The resulting force exceeds the breaking tension of either P-clamp  430   a  or P-clamp  430   b . Cable  260  is no longer strain-relieved and now is under the large force associated with the falling bridge. Cable  260  is pulled into contact with the edge of unistrut  410 . This large force applied to a relatively sharp bend breaks cable  260  at the bend, thus individually severing each pair of wires. The edges of Unistrut  410  need not be specifically sharpened. The breaking tension of a six-pair drop wire is on the order of 1 000 lbs. This is quite sufficient to prevent accidental breakage and is easily overcome by a falling bridge. Normally, cable  260  does not come in contact with the edge of unistrut  410 , and therefore experiences no wear. Significant advantages of low-voltage power are the use of standard telephone drop wire as cable  260  in conjunction with cable anchor  400 , and that cable  260  need not be run in conduit. An advantage of using of telephone drop wire is the need for only two bolts per protected section of bridge, one at each end, need be drilled into bridge  210 . One well-known system installed on the Queen Isabella Memorial Causeway leading to Port Isabel, TX, requires three conduits run the entire length of the bridge along with 96 holes per 80 foot span drilled into the concrete. 
     FIG. 5  is a flowchart demonstrating one method of detecting a structural collapse. A signal is sent in step  510  and is monitored for in step  520 . If the signal is received, cable  260  is intact along with bridge  210 . This process is repeated indefinitely. If the signal is not received, the bridge has collapsed and an immediate warning to multiple motorists  220   a  already on the bridge and to those not yet on the bridge is provided. Sending any signal capable of easy detection of the signal&#39;s presence or absence is suitable. 
   A command to turn on a lamp  140  in all beacons  100  is issued. This command is encoded into a desired state in step  530 . Positive DC is applied to each pair in cable  260  in step  531 , which is decoded to illuminate top lamp  140   a  in all beacons. After about 1 second, the desired state is inverted in step  540 , which sends a negative DC to cable  260  in step  532 , illuminating bottom lamp  140   b.    
   Although embodiments of the invention and their advantages are described in detail, a person skilled in the art could make various alterations, additions, and omissions without departing from the spirit and scope of the present invention as defined by the appended claims.