Abstract:
A system and method for limiting energy to an industrial control system is described. The method includes receiving a supply voltage, limiting the supply voltage so as to generate a limited voltage, and providing the limited voltage to a field transducer, the field transducer being disposed to monitor a parameter of the industrial process. A field current, that is generated in response to the limited voltage, is received from the field transducer and the field current is indicative of a magnitude of the parameter under normal operating conditions. During a fault condition, the field current is restricted with a variable resistance that is responsive to an amount of thermal energy generated during the fault condition so as to limit an amount of energy drawn by the control system.

Description:
PRIORITY  
       [0001]     The present application claims priority from to commonly owned and assigned application No. 60/578,808, filed Jun. 10, 2004 Attorney Docket No. TRCX-007/00US, entitled System and Method for Limiting Energy in an Industrial Control System, which is incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates generally to control and monitoring systems, and more specifically to industrial process control and monitoring systems.  
       BACKGROUND OF THE INVENTION  
       [0003]     Modern industrial systems and processes tend to be technically complex, involve substantial energies and monetary interests, and have the potential to inflict serious harm to persons or property during an accident. Although absolute protection may not be possible to achieve, risk can be reduced to an acceptable level using various methods to increase an industrial system&#39;s safety and reliability and mitigate harm if an event, e.g., a failure, does occur.  
         [0004]     Integral with industrial systems are process and safety control systems, which typically include programmable controllers (e.g., programmable logic controllers (PLCs)) and a collection of sensors and actuators for detecting and reacting to events, respectively. Typically, sensors (e.g., temperature, pressure and flow transducers) are coupled to a programmable controller via signal lines that may be hundreds of feet in length. As a consequence, the potential exists for these signal lines to be inadvertently severed or short circuited during an event (e.g., an accidental collision).  
         [0005]     During an event arising in the context of hazardous gas atmospheres (e.g., flammable gas atmospheres), electrical and/or thermal energy released from the signal lines, field devices (e.g., sensors) and/or the programmable controller circuitry may create a spark or generate sufficient temperatures to cause the gas atmosphere to ignite. Recognizing these potential hazards, regulations in the United States and Europe mandate that current, voltage and temperature be limited in electronic circuits including programmable controllers that are operating in the presence of the hazardous atmosphere.  
         [0006]     Consistent with existing, standardized methodology, power ballast resistors are often employed to limit the amount of current drawn by signal lines. Power ballast resisters, however, are often expensive, and are typically bulky and heavy because they are intended to limit temperatures by absorbing heat with a substantial amount of thermally conductive mass and dissipating the heat with a substantial amount of surface area.  
       SUMMARY OF THE INVENTION  
       [0007]     Exemplary embodiments of the present invention that are shown in the drawings are summarized below. These and other embodiments are more fully described in the Detailed Description section. It is to be understood, however, that there is no intention to limit the invention to the forms described in this Summary of the Invention or in the Detailed Description. One skilled in the art can recognize that there are numerous modifications, equivalents and alternative constructions that fall within the spirit and scope of the invention as expressed in the claims.  
         [0008]     In one exemplary embodiment, the present invention may be characterized as a method for limiting energy of an industrial process controller, the method including receiving a supply voltage limiting the supply voltage so as to generate a limited voltage, providing the limited voltage to a field transducer that is disposed to monitor a parameter of the industrial process, receiving, from the field transducer, a field current that is generated in response to the limited voltage. The field current in this embodiment, is indicative of a magnitude of the parameter under normal operating conditions. During a fault condition, the field current is restricted with a resistance so as to limit the field current and the generation of thermal energy.  
         [0009]     According to another embodiment, the invention may be characterized as a programmable controller for controlling an industrial system. The programmable controller in this embodiment includes a plurality of energy-limiting modules, each of the plurality of energy-limiting modules being configured to provide an output signal indicative of a parameter measured by a corresponding one of a plurality of field devices. Each of the plurality of energy-limiting modules is configured to limit a field current of the corresponding one of a plurality of field devices with a thermally responsive current limiter. A processor is also included to process the output signals from the plurality of energy-limiting modules and provide control signals to actuators of the industrial system in accordance with instructions stored in a memory coupled with the processor.  
         [0010]     In another variation, the invention may be characterized as an energy-limiting module for an industrial controller. The energy-limiting module including a first and a second signal lines disposed to provide a voltage to a field transducer, which is associated with a process parameter. A temperature-dependent current limiter is coupled between a supply voltage and the first signal line so as to limit electrical and thermal energy in the event of a fault. A voltage limiter is coupled to the first signal line so as to limit the voltage provided to the field transducer, and a current to voltage translator is coupled to the second signal line to provide an output voltage to the industrial controller that is indicative of the process parameter.  
         [0011]     In yet another embodiment, the invention may be characterized as an intelligent field device that includes a transducer configured to draw a field current that is a function of a field parameter, an energy-limiting module configured to limit an amount of energy imparted to the transducer and provide, in response to the field current, an output signal indicative of the field parameter. A processor is also included to process the output signal and to control an actuator that affects the field parameter.  
         [0012]     As previously stated, the above-described embodiments and implementations are for illustration purposes only. Numerous other embodiments, implementations, and details of the invention are easily recognized by those of skill in the art from the following descriptions and claims. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]     The aspects, features and advantages of the present invention will be more apparent from the following more particular description thereof, presented in conjunction with the following drawings wherein:  
         [0014]      FIG. 1  is a is a block diagram of an exemplary industrial system in which an energy-limiting system according to one embodiment of the present invention is implemented;  
         [0015]      FIG. 2  is a block diagram of an exemplary embodiment of the energy-limiting portion of  FIG. 1 ;  
         [0016]      FIG. 3  is a block diagram of an exemplary embodiment of the energy-limiting module of  FIG. 2 ;  
         [0017]      FIG. 4  depicts a schematic diagram of one embodiment of the energy-limiting module of  FIG. 3 ; and  
         [0018]      FIG. 5  is a flow chart illustrating steps carried out by the energy-limiting modules of  FIGS. 2, 3  and  4  according to several embodiments of the present invention.  
     
    
     DETAILED DESCRIPTION  
       [0019]     In one aspect, the present invention is directed to an energy-limiting system, which limits both the electrical and thermal energy released by a programmable controller, signal lines and field devices in an industrial control application. Specifically, the present invention according to several embodiments employs a combination of voltage, current and temperature control techniques to limit electrical and thermal energy and thereby mitigate risks associated with operating a programmable controller in a flammable gas atmosphere.  
         [0020]     Unlike current approaches, which use large, expensive power ballast resistors which are difficult to thermally regulate, several embodiments of the present invention utilize a current limiting technique, which simultaneously limits thermal energy in a well controlled manner without passive or active heat exchange. In addition, voltage control techniques are utilized in conjunction with the current and thermal regulation to mitigate the potential for both overheating and sparks, which may cause a flammable atmosphere to ignite.  
         [0021]     Referring first to  FIG. 1  shown is a block diagram of an exemplary industrial system  100  in which an energy-limiting system according to an embodiment of the present invention is implemented. As shown, the system  100  includes a programmable controller  102  within a flammable gas atmosphere that is in communication, via a collection of signal lines  104 , with field devices  106  (e.g., actuators and sensors). The signal lines  104  are coupled to the programmable controller  102  via an energy-limiting portion  108  of a termination panel  110 . As shown, the energy-limiting portion  108  also provides output signals  112  that are utilized by the programmable controller  102  to control one or more aspects of the system  100 .  
         [0022]     The programmable controller  102  may be realized using any one of a variety of devices that have input/output (I/O) functionality and processing capability (not shown). The programmable controller  102  may be, for example and without limitation, a safety controller, a programmable logic controller (PLC), a general purpose computer, or potentially any other device that includes a processor, memory and input/output capability.  
         [0023]     Although the programmable controller  102  is depicted in  FIG. 1  as a separate unit from the field devices  106 , it is contemplated that each field device may be integrated with a programmable controller so as to realize separate intelligent field devices with the energy limiting functionality described herein.  
         [0024]     In the present embodiment, the energy-limiting portion  108  of the termination panel  110  limits energy to the signal lines  104 , the field devices  106  and a portion of the programmable controller  102  so as to reduce the likelihood of a spark or over temperature condition occurring when there are faults in the field devices  106  and/or signal lines  104 .  
         [0025]     Referring next to  FIG. 2 , shown is a block diagram of an exemplary embodiment of the energy-limiting portion of  FIG. 1 . As shown, the energy-limiting portion  200  in the present embodiment includes N energy-limiting modules  202   1-N , which are coupled to a supply voltage  204 . In addition, each of the N energy-limiting modules  202   1-N  is shown coupled, via a corresponding pair of signal lines  206   1-N , to a corresponding one of N field devices  208   1-N , and each energy-limiting module provides one of N output signals  210   1-N  shown extending from the energy-limiting portion  200 .  
         [0026]     In the exemplary embodiment, each of the energy-limiting modules  202   1-N  has two signal lines  206   1-N , (a supply line  206   1-N  and a return line  214   1-N ), which set up a voltage across each respective field device  208   1-N , and in response, each field device  208   1-N  generates a field current indicative of a field parameter (e.g., environmental or process condition) that each field device is monitoring. For example, the field devices  208   1-N  may be temperature, pressure or flow transducers that generate respective field currents that are proportional to monitored temperature, pressure and flow rates.  
         [0027]     In operation, each energy-limiting module  202   1-N  receives the supply voltage  204  and implements voltage, current and thermal limiting techniques to limit the amount of energy released in the flammable gas environment in the event of one or more faults among the signal lines  206   1-N  and/or field devices  208   1-N  of the industrial system  100 . In this way, a potentially unlimited amount of energy from the supply voltage  204  is electrically and thermally limited.  
         [0028]     For example, the voltage to the field devices  208   1-N  is limited to reduce the likelihood that a spark ignition-level energy will be generated if a signal line  206   1-N  brushes against a conductive element (e.g., a ladder or a portion of a misguided fork lift). In addition, current to the field devices  208   1-N  is limited so that if the supply signal line  212   1-N  experiences a ground fault or there is a short circuit between the signal lines  206   1-N , dangerous levels of heat energy are not generated in the flammable atmosphere. Moreover, each energy-limiting module  202   1-N  in the exemplary embodiment includes thermal limiting capability so as to prevent the terminal panel  110  itself from reaching a flash point level.  
         [0029]     Referring next to  FIG. 3 , shown is an energy limiting module  300 , which is an exemplary embodiment of one or more of the energy-limiting modules  202   1-N  described with reference to  FIG. 2 . As shown, a temperature controlled current limiter  302  of the energy-limiting module  300  receives a supply voltage  304  from a voltage source  306 . The temperature controlled current limiter  302  is coupled to a voltage controller  308 , which is coupled via a supply signal line  310  to a field device  312 . A surge protection portion  314  of the energy-limiting module  300  is also coupled to the field device  312  via a return signal line  316 , and an output of the surge protector  314  is provide to a current to voltage translator  318 .  
         [0030]     In operation, the energy-limiting module  300  receives the supply voltage  304  from the voltage source  306 , and the temperature controlled current limiter  302  functions to limit the amount of current drawn from the voltage source  306  while simultaneously limiting the amount of thermal energy dissipated. The voltage controller  308  operates to provide a limited voltage via the supply line  310  to the field device  312  so as to reduce the likelihood that any sparks generated during an event will have sufficient energy to ignite the surrounding flammable atmosphere.  
         [0031]     As shown, when the limited voltage is applied to the field device  312 , the field device  312  generates a field current  316  that is returned to the surge protection portion  314 , which protects the current to voltage translator  318  from power surges. The current to voltage translator  318  then converts the field current to an output voltage  320 , which is provided to the programmable controller  102 .  
         [0032]     Referring next to  FIG. 4 , shown is a schematic view of an energy limiting module  400 , which is one embodiment of the energy-limiting module  300  of  FIG. 3 . As shown, in the present embodiment the energy-limiting module  400  includes two inputs (i.e., V 1  and V 2 ), which are disposed to receive a supply voltage  402   1-N  from respective redundant voltage sources  404   1-N  and provide the supply voltage  402   1-N  to a temperature controlled current limiter, which in the present embodiment includes a positive temperature coefficient (PTC) resistor  406 .  
         [0033]     Under normal operating conditions, the PTC resistor  406  operates at 250 ohms with a current that varies between 0 to 20 mA depending upon the parameter monitored by the field device (e.g., field devices  208   1-N ). In the event the supply signal  408  is shorted to either the return signal line  416  or ground, the current through the PTC resistor  406  will quickly rise due to its relatively low resistance.  
         [0034]     As a consequence, the amount of thermal energy dissipated by the PTC resistor  406  will quickly increase until the temperature of the PTC resistor  406  reaches 120° C. Once the temperature of the PTC resistor reaches 120° C., the resistance of the PTC resistor  406  rapidly increases in response to any further increases in temperature over 120° C. In turn, the rapid increase in resistance limits the current flowing through the PTC resistor  406  so as to prevent a further increase in the temperature of the PTC resistor  406 . In this way, the PTC resistor  406  limits current to the signal lines  206   1-N ,  310 ,  316 ,  408 ,  416  and field devices  208   1-N ,  312 ,  410  while simultaneously limiting the amount of thermal energy generated in the energy-limiting module.  
         [0035]     It should be recognized that PTC resistors with various operating characteristics may be implemented in accordance with the particular operating environment. For example, a PTC resistor may be implemented that increases resistance at a temperature lower than 120° C. if the energy-limiting module  400  is employed in a gaseous environment having a relatively low flash point.  
         [0036]     As shown, the voltage controller  308  in the present embodiment is realized by a 28V zenor diode  412 , which limits the field voltage to 28 volts in the event a user applies a supply voltage  402  that is greater than 28 volts. It should be recognized that the 28 volt field voltage is merely exemplary and that other voltages may be utilized depending upon the field devices and the particular type of atmosphere in which the energy-limiting module  400  is employed.  
         [0037]     As shown, the current to voltage translator in the present embodiment is realized by a 250 ohm resistor  414 , and a 5.6 volt zenor diode  416  is employed as the surge protector to prevent damage to the resistor  414  in the event of a fault.  
         [0038]     Although the energy-limiting module  400  depicted in  FIG. 4  is shown as an analog module, it should be recognized that several embodiments of the present invention extend to digital applications as well. For example, to provide a digital output to a programmable controller (e.g., the programmable controller  102 ), the resistance R 2  of the current to voltage translator  414  may be changed (e.g., to 2K ohms). In such an embodiment, the surge protection portion  416  is less important and D 2  may be removed from the embodiment shown in  FIG. 4 .  
         [0039]     Referring next to  FIG. 5 , shown is a flow chart depicting steps carried out by the energy-limiting modules of  FIGS. 2, 3  and  4  in accordance with an exemplary embodiment of the present invention. As shown, the energy-limiting module  202 ,  300 ,  400  initially receives a supply voltage  204 ,  304 ,  402  (Step  502 ), and limits the supply voltage  204 ,  304 ,  402  so as to generate a limited voltage  212 ,  310 ,  408  (Step  504 ), which is provided to a field device  208 ,  312 ,  410  (Step  506 ).  
         [0040]     In response, the field device  208 ,  312 ,  410  draws a field current (e.g., in proportion to a monitored parameter), which is then received by the energy-limiting module  202 ,  300 ,  400  (Step  508 ) on the return signal line  214 ,  316 ,  416 . If the received field current is less than a threshold (e.g., 20 mA) (Step  510 ), the field current is converted to an output voltage  210 ,  320 ,  420 , which is a function of the monitored parameter (Step  512 ), and the output voltage  210 ,  320 ,  420  is provided to the programmable controller  102  (Step  514 ). The threshold in several embodiments depends upon an upper range of current normally drawn by the field devices  106 ,  208 ,  312 ,  410 . For example, if the field devices  106 ,  208 ,  312 ,  410  normally draw a maximum of 20 mA, then field currents above 20 mA are likely due to a fault situation.  
         [0041]     In the event of a fault (e.g., the supply signal line  212 ,  310 ,  408  is shorted with the return signal line  214 ,  316 ,  416  or the supply signal line  212 ,  310 ,  408  is grounded), the field current in the return signal line  214 ,  316 ,  416  will rise beyond the threshold (e.g., 20 mA)(Step  510 ), and the temperature controlled current limiter  302 ,  406  will restrict the field current with its resistance while releasing thermal energy (Step  516 ). As the temperature controlled current limiter  302 ,  406  releases thermal energy, its resistance increases as a function of the amount of thermal energy so as to further restrict the field current, and hence, the amount of thermal energy generated (Step  518 ).  
         [0042]     As the thermal energy of the temperature controlled current limiter  302 ,  406  decreases, its resistance also decreases (Step  520 ), and once the field current is below a threshold (e.g., because the fault condition is no longer present) (Step  522 ), the output voltage  210 ,  320 ,  420  provided to the programmable controller  102  is again a function of a monitored parameter (Steps  512 ,  514 ).  
         [0043]     Although the invention herein disclosed has been described by means of specific embodiments and applications thereof, numerous modifications and variations could be made thereto by those skilled in the art without departing from the principles of the invention.