Abstract:
A passive isolator may fit between two serial network cables to convert a standard serial network to a serial network suitable for use in hazardous environments.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
     BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates to industrial control systems for use in explosive or other hazardous environments and in particular to a barrier for allowing a standard serial electrical network to pass between a hazardous and non-hazardous area.  
         [0002]     Industrial controllers are special purpose computers used for controlling factory automation and the like. Under the direction of a stored program, a processor of the industrial controller examines a series of inputs reflecting the status of a controlled processor or device and changes outputs affecting control of the controlled process or device.  
         [0003]     Generally industrial controllers are constructed in modular fashion to accommodate different applications requiring different numbers and types of input/output (I/O) circuits as is determined by the particular device or process being controlled. In such modular systems, a number of different functional modules connect together through a common backplane in a rack or the like to share data and power.  
         [0004]     Often a controlled process may require control points (e.g., sensors or actuators) in an environment exposed to combustible vapors or airborne particles. In such hazardous environments, electrical arcing or excessive temperature in electrical equipment can cause an explosion. Electrical equipment intended for use in such hazardous environments must conform to certain safety standards intended to reduce the chance or effect of an explosion. Under such standards, the equipment may be placed in a special housing that is flameproof or explosion-proof. The housing may be charged with an inert gas to prevent the infusion of explosive fumes.  
         [0005]     Other methods of protection are also available for use in hazardous environments including the method of intrinsic safety. Equipment designed to be “intrinsically safe” generally limits the electrical energy used by the equipment to a low level to avoid the occurrence of sparks with sufficient energy to ignite a flammable atmosphere during a fault condition, and to avoid surface temperatures above those needed to cause spontaneous ignition. Fault conditions must be considered as well as the energy storage characteristics of the components of the equipment.  
         [0006]     A number of agencies certify equipment to an intrinsic safety standard. See generally, Underwriter&#39;s Laboratories document UL-913,1988, Intrinsically Safe Apparatus And Associated Apparatus For Use In Class I, II, and III, Division I, Hazardous (Classified) Locations. See also, National Electrical Code Handbook, 1993, Article 500, Hazardous (Classify) Locations, Article 504, Intrinsically Safe Systems, Article 505, Class I, Zone 0, 1 and 2 Locations. See also, FM Cl. No. 3600, March 1989, Electrical Equipment for Use in Hazardous (Classified) Locations General Requirements and Cl. No. 3610, October 1988, Intrinsically Safe Apparatus and Associated Apparatus for Use in Class I, II, III, Division 1 Hazardous (Classified) Locations. See also, EN50014:1992, Electrical Apparatus for Potentially Explosive Atmospheres, EN50020:1994, Electrical Apparatus for Potentially Explosive Atmospheres—Intrinsic Safety ‘i’ and EN50039:1980, Electrical Apparatus for Potentially Explosive Atmospheres—Intrinsically Safe Electrical Systems ‘i’. These documents are hereby incorporated by reference.  
         [0007]     The terms “intrinsic safety” and “intrinsically safe” as used herein do not indicate that the equipment presents no danger or that it meets the above standards but only that it is designed to permit use in some hazardous environments without additional precautions such as explosion-proof casings and the like.  
         [0008]     In a typical control system where a portion of the controlled process is in a hazardous area, the industrial controller will be placed a distance away in a “safe” or non-hazardous area free from combustible gases or other combustible elements. Input and output signals to and from the hazardous area are carried by cables leading from the industrial control system to the respective portion of the controlled process. Cables passing into the hazardous area from the non-hazardous area, even for low power components, must first pass through barrier circuits or isolating circuits.  
         [0009]     Barrier circuits shunt hazardous energy to special safety ground connections. In a typical barrier circuit, electrical power passing from the non-hazardous area to the hazardous area will pass through a fuse to the cathodes of one or more voltage limiting zener diodes having their anodes connected to ground. High voltages are thus shunted safely to ground. Current into the hazardous area is limited by a resistor positioned after the voltage limiting zener diodes.  
         [0010]     Isolating circuits work by separating the two halves of a conductor so there is no direct current path for any hazardous energy from the non-hazardous side to the hazardous area. A typical isolator may use transformers, capacitors, or optical-type isolators as its means of separating two halves of a conductor.  
         [0011]     When there are many points of control in a hazardous area, the cost to the control system may be substantial driven by the cost of many barriers or isolation circuits for each control point and the long runs of wiring.  
         [0012]     For this reason, it is known to place intrinsically safe I/O circuits in the hazardous area and to communicate with them via a single serial network eliminating the need to pass many conductors into the hazardous area, each with a separate barrier. Isolating a serial network has been handled in two principal ways. The first employs a length of fiber optic cable between fiber end or repeater node. A standard end or repeater node may be used outside of the hazardous area and a special intrinsically safe hub used inside the hazardous area. This approach can be quite costly.  
         [0013]     Alternatively, U.S. Pat. No. 6,037,857 describes a module being part of the industrial control system in the non-hazardous areal. This module receives a source of intrinsically safe power to actively create an intrinsically safe serial network signal that may pass directly into the hazardous area on a standard communications cable.  
       SUMMARY OF THE INVENTION  
       [0014]     The present inventors have developed a passive, in-line barrier for high-speed serial networks greatly simplifying the task of communicating between hazardous and non-hazardous areas by eliminating the need for an intrinsically safe power source and a specialized controller module and a corresponding slot in a controller. The barrier may be easily installed anywhere between standard co-axial cables and is compact and relatively inexpensive.  
         [0015]     Specifically, the present invention provides a barrier for high speed serial networks comprising a housing having a first terminal set receiving a first conductor pair from non-intrinsically safe network and a second terminal set for receiving a second conductor pair from an intrinsically safe network. The housing further contains a fusible link having a first end joined through the first terminal set to a first conductor of the first conductor pair and a bi-polar voltage sensitive conductor shunting a second end of the fusible link and a second conductor of the first conductor pair. A matching network in series between the first and second terminal sets substantially matches the impendence of the barrier at the first and second terminal sets to media of the non-intrinsically safe network and intrinsically safe network, respectively.  
         [0016]     Thus it is one object of the invention to provide a simple method of converting a non-intrinsically safe serial network into an intrinsically safe network without the need for a ready source of intrinsically safe power.  
         [0017]     The barrier may further include a current limiting element in series between a second end of the fusible link and a first conductor of the second conductor pair.  
         [0018]     It is thus another object of the invention to insure intrinsically safe operation of a standard serial network by controlling power transferred through the network media.  
         [0019]     The bipolar voltage sensitive conductor may provide a shunt path for voltages in excess of 5.0 volts.  
         [0020]     Thus it is another object of the invention to prevent the communication of high voltages through the network media from the non-hazardous area to the hazardous area.  
         [0021]     The bipolar voltage sensitive conductor may be a parallel combination of back-to-back zener diodes.  
         [0022]     Thus it is another object of the invention to provide protection against high voltages in either of two polarities.  
         [0023]     Each zener diode may be series connected with a standard diode facing the opposite direction as the zener diode. These diodes provide for the required buffering of the high zener diode capacitance. This capacitive effect would be very detrimental for a high speed network.  
         [0024]     Thus it is another object of the invention to use standard zener diodes for bipolar protection.  
         [0025]     The bipolar voltage sensitive conductor may be a parallel combination of at least four conductive elements, each being series connected zener diodes and an opposed standard diode where at least one conductive element is connected to conduct current in the opposite direction of another conductive element.  
         [0026]     Thus it is another object of the invention to provide redundant voltage limiting devices to reduce the risk of incapacitating failure.  
         [0027]     The junction of the zener diodes and the standard diodes of conductive elements of the same direction may be joined.  
         [0028]     It is thus another object of the invention to decorrelate failure of the zener and standard diodes.  
         [0029]     The isolator may further include a DC blocking element in series between a second end of the fuse and a first conductor of the second conductor pair.  
         [0030]     Thus it is another object of the invention to eliminate direct flow of current from the non-hazardous to the hazardous side such as would create unbounded energy transfer.  
         [0031]     The blocking element may be a capacitor.  
         [0032]     Thus it is another object of the invention to provide a lightweight and simple DC blocking element.  
         [0033]     The blocking element may be at least two series capacitors.  
         [0034]     Thus it is another object of the invention to provided redundancy against the possibility of a capacitor shorting.  
         [0035]     The networks may use coaxial cable and the terminals may be BNC-type connectors held at opposite ends of the housing.  
         [0036]     Thus it is another object of the invention to provide an extremely simple inline barrier whose presence may be readily observed.  
         [0037]     One end of the insulator may be marked with indicia to indicate which side should connect to the intrinsically safe network.  
         [0038]     Thus it is another object of the invention to provide a barrier that may be readily installed in the proper orientation.  
         [0039]     The housing of the barrier may include a third terminal set receiving a third conductor pair from a redundant non-intrinsically safe network and a fourth terminal set for receiving a fourth conductor pair from a redundant intrinsically safe network. A second fusible link may have a first end joined through the third terminal set to a first conductor of the third conductor pair and a second bi-polar voltage sensitive conductor may shunt a second end of the fusible link and a second conductor of the third conductor pair. A second matching network in series between the third and fourth terminal sets to substantially match the impendence of the barrier at the third and fourth terminal sets to media of the redundant non-intrinsically safe network and redundant intrinsically safe network, respectively.  
         [0040]     These particular objects and advantages may apply to only some embodiments falling within the claims and thus do not define the scope of the invention. 
     
    
     BRIEF DESCRIPTION OF THE FIGURES  
       [0041]      FIG. 1  is a perspective view of a controller system employing the present invention to communicate serial data from a first controller to I/O points located within a hazardous area;  
         [0042]      FIG. 2  is a perspective view of the barrier of the present invention providing for isolation between standard and intrinsically safe portions of a redundant serial network; and  
         [0043]      FIG. 3  is a schematic diagram of one half of the circuitry of the barrier of  FIG. 2 . 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0044]     Referring now to  FIG. 1 , a standard industrial control  10 , typically not conforming to intrinsically safe standards, may provide a rack  12  holding a plurality of control modules  14  of types well known in the art. One control module  14  may be a serial communication module  16  allowing communication of data over a standard portion  18  of a high speed serial network such as ControlNet, Ethernet, or the like.  
         [0045]     The rack  12  may be supported on a DIN rail  21  also supporting a network barrier  20  of the present invention. Cabling for the standard portion  18  of the network may connect to one side of network barrier  20  which may, in turn, connect to cabling for an intrinsically safe portion  18 ′ of the network leading to a hazardous area  17 . Generally, the intrinsically safe portion  18 ′ of the network will use the same protocol as the standard portion  18  of the network but will be controlled in power level and spectrum, as will be described, to reduce the possibility of creating an ignition hazard within the hazardous area  17 .  
         [0046]     The intrinsically safe portion  18 ′ of the network may connect to a network card  16 ′ of an intrinsically safe rack  12 ′ in a hazardous area  17 . The intrinsically safe rack  12 ′ may also hold I/O modules  14 ′ which may receive input signals from controlled equipment  22  in the hazardous area  17  and provide output signals to that equipment  22  as is well understood in the art. Construction of the rack  12 ′ and modules  14 ′ is described in U.S. Pat. No. 6,154,683 issued Nov. 28, 2000 assigned to the assignee of the present invention and hereby incorporated by reference.  
         [0047]     Referring now to  FIG. 2 , the network barrier  20  may have a housing  24  constructed of an electrically insulating material, and supporting on its front face, four BNC-type connector plugs  26   a - 26   d . Connector plugs  26   a  and  26   b  are arranged on a first half  24   a  of the housing  24  (which is divided along a vertical line of symmetry) in opposition to connector plugs  26   d  and  26   c , respectively, which are arranged on a second half  24   b  of the housing  24 . A portion of the housing  24  under the connector plugs  26   a  and  26   b  is gray denoting that the connectors  26   a  and  26   b  are to be connected to the standard portion  18  of the network whereas a portion of the housing  24  under the connector plugs  26   d  and  26   c  is blue denoting that the connectors  26   d  and  26   c  are to be connected to the intrinsically safe portion  18 ′ of the network.  
         [0048]     Connector plugs  26   a  and  26   d  provide terminals for one barrier circuit and connector plugs  26   b  and  26   c  provide terminals for a second barrier circuit independent for the first. The two barrier circuits allow for isolation of a redundant cabling for the network. For simplicity, one barrier circuit and its connection to the network will now be described, it being understood that the other barrier circuit and network connection is substantially identical.  
         [0049]     Connector  26   b  may join with coaxial cable  30  forming the media of the standard portion  18  of the network via BNC-type socket  28   b . Socket  26   c  provides similar connection coaxial cable  30 ′ forming the media of the intrinsically safe portion  18 ′ of the network via BNC-type socket  28   c . As is understood in the art, the outer portions of sockets  28   b  and  28   c  are metallic and electrically joined to a shield  32  and  32 ′ of the cable  30  and  30 ′, respectively. The shields  32  and  32 ′ in turn are in a coaxial position around a center conductor  34  and  34 ′ of the cable  30  and  30 ′, respectively. The sockets  26   b  and  26   c  are separated sufficiently so that there is little risk that the connectors  28   b  and  28   c  might inadvertently be shorted together.  
         [0050]     The housing  24  includes a slot  29  for receiving the DIN rail  21  and provides a safety ground lead  25  that is attached to a safety ground, being a ground point reference to an earth ground as defined by intrinsically safe standards.  
         [0051]     Referring now to  FIG. 3 , the conductor  34  of cable  30 , the standard portion  18  of the network, is received barrier circuitry  31  held within the housing  24  and specifically by one terminal of a fuse  36  providing a maximum current limit of approximately 63 mA. The second terminal of the fusible element  36  is received by the cathode of diodes  38   a  and  38   b  and the anode of diodes  38   c  and  38   d . The anodes of diodes  38   a  and  38   b  are joined together and in turn, also connect to the anodes of zener diodes  40   a  and  40   b , while the cathodes of zener diodes  40   a  and  40   b  are connected to shield  32 , which is joined by a separate conductor to shield  32 ′ and also to the safety ground lead  25 .  
         [0052]     Similarly, cathodes of diodes  38   c  and  38   d  are joined together and also connected to the cathodes of zener diodes  40   c  and  40   d . The anodes of zener diodes  40   c  and  40   d  are connected again to shields  32 ′ and safety ground lead  25 .  
         [0053]     The second terminal of the fuse  36  communicates (via other elements to be described) with conductor  34 ′ hence together diodes  38   a - 38   d  and zener diodes  40   a - 40   d  comprise a bipolar voltage limiting conductor  42  triggering at the zener diode voltage of 5.6 V plus the forward drop voltage of diodes  38  to short conductors  34  to ground for voltages over approximately 5 volts (6.3 V in the preferred embodiment) in either of two polarities thereby limiting transmission of high voltages from the standard portion  18  of the network to the intrinsically safe portion  18 ′ of the network. Diodes  38   a - 38   d  are used to compensate or buffer the capacitance of the zener diodes  40   a - 40   d  from appearing in parallel with the transmission line formed by conductors  34 - 34 ′ and  32 - 32 ′. Proper selection of these are critical to proper high speed performance of the transmission line.  
         [0054]     This conduction of the diodes  38   a - 38   d  and zener diodes  40   a - 40   d  forms a crowbar circuit with fuse  36  to hasten the opening of fuse  36  under sustained high voltage conditions. Under brief duration high voltage conditions, the shunting is sufficient together with the natural inductance of the cable  30  to prevent further transmission of the high-voltage pulses through to conductors  34 ′ without opening the fuse  36 .  
         [0055]     A current limiting resistor  44  is connected in series between the second terminal of fuse  36  and with the conductor  34  provides a current limiting element  43  to limit the current flow passing through the barrier circuit  31 .  
         [0056]     Two series connected capacitors  46  (also connected in series with the resistor  44  between the second terminal of fuse  36  and with the conductor  34 ) form a direct current blocking element  45  blocking direct current flowing between conductor  34  and  34 ′. Redundant capacitors  46  are used so that an internal short in either capacitor  46  will not cause a loss of DC blocking.  
         [0057]     An impedance matching element  48  (also connected in series with the resistor  44  and capacitors  46  between the second terminal of fuse  36  and with the conductor  34 ) provide a matching between the impedance of the barrier circuitry  31  as seen at the connectors  26   b  and  26   c  equal to the characteristic impedance of the cables  30  and  30 ′ to which they connect. The impendence-matching element  48  compensates for the resistance of the fuse  36  and resistor  44  to maximize energy transfer through the barrier circuitry  31  eliminating the need for active circuitry to boost the signal through the barrier circuitry  31 . The impedance matching element  48  in the preferred embodiment is a simple T network having two series connected resistors  50  (generally in series between conductors  34  and conductors  34 ′) and shunting resistor  52  leading from the junction of resistors  50  to the shield  32 ,  32 ′, and safety ground lead  25 .  
         [0058]     It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein, but include modified forms of those embodiments including portions of the embodiments and combinations of elements of different embodiments as come within the scope of the following claims. In the claims, per common usage, the statement that a particular circuit element is connected in “series” between two points does not preclude other components also being in series with the particular circuit element between the two points, including previously recited elements, and does not require that the particular circuit element be directly attached to either or both of the two points between which it is in series.