Abstract:
This invention relates to fault detection in electrical circuits, in particular it relates to fault detection for a plurality of adjacent input circuits. The invention provides a method and apparatus for detecting a control or communication fault on an analogue circuit by digitally simulating said analogue circuit; driving the digitally simulated circuit with a control input also used to drive said analogue circuit; and comparing an output of said simulated circuit with a reference output and setting an error condition in the event that the compared outputs differ.

Description:
BACKGROUND  
       [0001]     a. Field of the Invention  
         [0002]     This invention relates to fault detection in electrical circuits, in particular it relates to fault detection for a plurality of adjacent input circuits.  
         [0003]     In safety control systems, fault tolerance is of utmost importance. Fault tolerance is the ability to continue functioning in the event of one or more failures within the system.  
         [0004]     Fault tolerance may be achieved by a number of different techniques, each with its specific advantages and disadvantages. One example of fault tolerance is known as Hardware Implemented Fault Tolerance (HIFT). HIFT means that the system relies on robust hardware circuits (as opposed to complex software algorithms) to perform the fault detection and redundancy management functions. A significant advantage HIFT has over software-implemented fault tolerance is that HIFT eliminates the overhead for communications between processors, leaving more time for controlling the process. This makes HIFT systems significantly faster and more dependable than systems using software-implemented fault tolerance.  
         [0005]     An example of a HIFT system is a system which provides redundancy, in particular Triple Modular Redundancy (TMR). Using TMR, critical circuits are triplicated and perform identical functions simultaneously and independently. The data output from each of the three circuits is voted in a majority-voting circuit, before affecting the system&#39;s outputs. If one of the triplicated circuits fails, its data output is ignored. However, the system continues to output to the process the value (voltage, current level, or discrete output state) that agrees with the majority of the functional circuits. TMR provides continuous, predictable operation.  
         [0006]     HIFT and TMR provides for automatic fault recovery with no disruption to system operation and ensures minimal fault detection periods.  
         [0007]     Another approach to fault tolerance is the use of hot-standby modules. This approach provides a level of fault tolerance whereby the standby module maintains system operation in the event of module failure. With this approach there may be some disruption to system operation during the changeover period if the modules are not themselves fault-tolerant.  
         [0008]     Fault tolerant systems ideally create a Fault Containment Region (FCR) to ensure that a fault within the FCR boundary does not propagate to the remainder of the system. This enables multiple faults to co-exist on different parts of a system without affecting operation.  
         [0009]     Fault tolerant systems generally employ dedicated hardware and software test and diagnostic regimes that provide very fast fault recognition and response times to provide a safer system.  
         [0010]     Commonly, it is possible to repair faults without interrupting system operation (known as hot replacement). For example active and standby module may operate in parallel so that if an active module becomes faulty there is an automatic change over to a standby module.  
         [0011]     Safety control systems are generally designed to be ‘fail-operational/fail-safe’. Fail operational means that when a failure occurs, the system continues to operate: it is in a fail-operational state. The system should continue to operate in this state until the failed module is replaced and the system is returned to a fully operational state.  
         [0012]     An example of fail safe operation occurs, for example if, in a TMR system, a failed module is not replaced before a second failure in a parallel circuit occurs, the second failure should cause the TMR system to shut down to a fail-safe state.  
         [0013]     Typical safety control applications include emergency and safety shutdown systems, process control, reactor control, wellhead control, turbine and compressor control, fire and gas detection and abatement, and are applicable to many industries including oil and gas production and refining, chemical production and processing, power generation, paper and textile mills and sewage treatment plants.  
       SUMMARY OF THE INVENTION  
       [0014]     According to the invention there is provided a method for detecting a control or communication fault on an analogue circuit comprising the steps of: digitally simulating said analogue circuit; driving the digitally simulated circuit with a control input also used to drive said analogue circuit; and comparing an output of said simulated circuit with a reference output and setting an error condition in the event that the compared outputs differ.  
         [0015]     According to another aspect of the invention there is also provided an apparatus for detecting a control or communication fault on an analogue circuit comprising: a digitally simulated circuit driven with a control input also used to drive said analogue circuit; and a comparator for comparing output data from said digitally simulated circuit with a stored reference data; and means for generating an error condition in the event that the compared outputs differ.  
         [0016]     Preferably, said analogue circuit is an analogue to digital input circuit and said control input is a clock signal, and even more preferably said analogue circuit is a sigma-delta analogue to digital converter.  
         [0017]     In a preferred embodiment the digitally simulated circuit receives test input data from the same source as said analogue circuit.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0018]     The invention will now be described, by way of example only, with reference to the accompanying drawings in which:  
         [0019]      FIG. 1  illustrates schematically a fault tolerant architecture;  
         [0020]      FIG. 2  illustrates a sigma-delta input circuit; and  
         [0021]      FIG. 3  illustrates a simulated digital reference sigma-delta input circuit. 
     
    
     DETAILED DESCRIPTION  
       [0022]     Referring now to  FIG. 1 , signals  1  from an input field device (not shown) are connected to the system via an input Field Termination Assembly (FTA)  2 . An input module  3  receives, buffers and carries out any necessary conversion of the input data from the field device. The input data is then transmitted over a triplicated Inter-Module Bus (IMB)  5  to a TMR processor  4 . Triplicated microprocessors of the TMR processor receive the input data from each channel of the IMB  5  and carry out a majority vote of the data. During data processing, each of the three processors compares and majority votes its input data against that of the other two processors. The processors synchronously execute the application program, and the derived output data is transmitted back to the IMB. An output module  6  receives, majority votes, and carries out any necessary conversion of the output data from the TMR processor  4 . The output circuits are then driven by the majority-voted command. The output signals are connected to a field device (not shown) via an output FTA  7 . A communications interface  8  provides facilities to allow systems to be networked using peer to peer communications links, the collection of Sequence of Events (SOE) data, and connection to other systems.  
         [0023]     If an internal circuit in the system fails, it is simply voted out. Comprehensive distributed voting both out-votes failures, preventing them from being propagated into the process, and simplifies the process of identifying where faults have occurred by means identifying the source of a discrepant value.  
         [0024]     For safety systems it is beneficial for line monitored analogue and digital input modules to detect open circuit faults between the Field Termination Assembly (FTA) and the input module itself. The requirement also exists that any fault in the components of each input circuit should be detectable.  
         [0025]      FIG. 2  illustrates a sigma-delta analogue to digital (A/D) converter  10  used by the input module  3 . The input module  3  comprises an array of 42×3 triplicated sigma-delta A/D converters  10 . A single array of 42 sigma-delta A/D converters is referred to herein as a ‘slice’. There is a dedicated A/D converter for each of the three input ‘slices’. To maximize fault detection no buffers are inserted in the signal path.  
         [0026]     The sigma-delta A/D converter  10  comprises an integrator  11 , a one-bit A/D converter  13  and a one-bit D/A converter  14 . A test signal D TEST  is converted by a one-bit D/A converter  15  and connected to a first input of the integrator  11  via an RC low pass filter  16 . An input scaling resistor R IN  connects an input V IN  of the module to a second inverting input of the integrator  11 . The output of the integrator  11  is converted to a digital output by the one-bit A/D converter  13 . The one-bit D/A converter  14  feeds back an analogue version of the digital output to the inverting input via a feed back resistor R FB .  
         [0027]     Each sigma-delta A/D receives a global reference clock, and a unique pulse width modulated (PWM) test signal.  
         [0028]     All of the sigma-delta A/D converters for a given input slice are controlled centrally with signals sent via a data transport system  
         [0029]     Certain classes of faults, involving the global reference clock, test signal generation, and data transport, can result in every channel being slightly in error. Such faults can be difficult to detect when testing sigma-delta A/D converters separately or comparing output signals to those of another sigma-delta A/D converter.  
         [0030]     In order to rapidly detect such faults, a digital model of a sigma-delta A/D converter is implemented. This functional model is connected to the global reference clock and data transport subsystem, and receives it&#39;s own uniquely phased test PWM signal. Control software receives and processes the data provided by this functional model in the same way as it does the real A/D converter channels.  
         [0031]     Because the simulated channel is implemented digitally, the simulated D OUT  value should be precise. Small deviations of the computed values for the simulated channel from the established reference values are indicative of a systematic fault which can be quickly detected.  
         [0032]     Referring now to  FIG. 3 , a simulated digital reference channel is shown which mimics the behaviour of an analogue Sigma Delta modulator with an open input connection.  
         [0033]     This simulated channel provides a “perfect” reference channel that responds identically to the analogue sigma-delta input modules to pulse ratio modulation signals and clock parameters, but is uninfluenced by external analogue circuitry or connections.  
         [0034]     The data produced by this simulated channel is used to detect subtle faults in control and communications circuitry by comparing the output data with reference data and setting an error condition in the event that the compared data differ by more than a predetermined threshold.  
         [0035]     A counter  2302  and a register  2303  are controlled by a signal from an edge detector  2301 . Thus the register  2303  provides an accumulated sample value every time a quadrant input signal changes and emulates the RC filter  16  of the analogue circuit.  
         [0036]     Two adders  2309  and  2310 , a multiplexer  2304  feeding a twenty bit integrator  2305  emulate the op amp  11  of the analogue circuit.  
         [0037]     A comparator  2306  and register  2307  simulates the A/D and D/A converters  13  and  14  of the analogue circuit.  
         [0038]     The “Data Out” signal is bundled with the data from the other replicated analogue channels from a slice and sent to an external DSP (not shown) for processing, indistinguishably from them. The test signal amplitude and phase data are extracted and compared to stored reference values, just as for the actual analogue channels.  
         [0039]     It is possible to use very small comparison windows (ie only tolerate very small deviations from stored reference values) for this data because the digital nature of the sigma delta simulated circuit is inherently precise under normal unfaulted operating conditions.  
         [0040]     It is to be recognised that various alterations, modifications, and/or additions may be introduced into the constructions and arrangements of parts described above without departing from the scope of the present invention as defined in the following claims.