Abstract:
For use in a digital process controller (DPC), a system for, and method of, balancing a control algorithm processing load and distributing the load over a given time interval and a real time plant control system employing the system or the method. In one embodiment, the system includes: (1) a model for estimating execution time of at least one control algorithm based on configuration data applied to that algorithm, the DPC capable of balancing and distributing the load based on the estimate generated by the model and (2) a data entry process, associated with the model, that allows an empirically-determined execution time to be associated with at least one control algorithm, the DPC capable of balancing and distributing the load based on the time in lieu of the model based estimate and balancing and distributing the load upon configuration of at least one control algorithm.

Description:
TECHNICAL FIELD OF THE INVENTION 
     The present invention is directed, in general, to computing and processing systems and, more specifically, to a system and method for balancing and distributing control algorithm processing load and a real time plant control system that employs the system or the method. 
     BACKGROUND OF THE INVENTION 
     Automated process control systems include a comprehensive set of control algorithms, or software-definable process control routines, to control and monitor various processes within, for instance, a manufacturing facility. The control systems can be tailored to satisfy a wide range of process requirements globally or within specified portions of the facility. Conventionally, the control systems include a variety of modules, each having its own processor or firmware, linked together by communication buses to result in a distributed process control system. The distributed nature of the system affords high performance with the capability to expand the system incrementally to satisfy growth or modifications in the facility. 
     To allow a user to tailor a process control system to a particular processing environment, it is important to provide such systems with highly-configurable and adaptable control systems. Process control systems generally provide a means by which users can create custom control strategies, e.g., software-definable process control routines. In object-oriented programming environments, a complete control strategy may be built from smaller components, such as “blocks” and “parameters.” A block is a software construct, or “object,” used to encapsulate the data and the algorithms of elemental control computations; parameters define the interface to individual pieces of data within a block. 
     Digital process controllers (DPCs) are generally required to meet a series of requirements on the manner in which they process control algorithms. For example, they must execute the control algorithms in a periodic fashion. The control algorithms must be executed at a sample rate, where the sample rate “jitter” is sufficiently low compared to the tolerance of process dynamics. DPCs may be required to execute additional algorithms whose functions are essential for the overall process control mission but are not periodic and predictable, e.g., communication with supervisory display and control stations, communication with peer controllers, event distribution and housekeeping tasks. DPCs must execute non-periodic algorithms in a way which does not interfere with deterministic execution of periodic control algorithms as long as DPCs are configured within a defined operating range. 
     To meet these above-described requirements, DPCs are required to balance the control algorithm processing load and distribute it over the time interval of the DPCs&#39; fundamental repetition rate. Conventional DPCs, such as Honeywell Inc. TDC3000 controllers known as “Process Manager,” “Advanced Process Manager” and “High Performance Process Manager,” pre-qualify a configuration and accept or reject the configuration on the basis of predicted potential for overload. These DPCs, however, do not support manual balancing, balancing a subset of algorithms while other algorithms maintain control, user measurement of execution time for a particular algorithm or associating execution time with an algorithm as a characteristic attribute. 
     On the other hand, other conventional DPCs, such as Honeywell&#39;s “Alcont Process Manager,” provide only for manual balancing of the algorithm processing load. These DPCs, however, do not support qualification of an algorithm load on the basis of a model computation or rejection of a load which is too high. Still other DPCs, such as Honeywell&#39;s “TPS Networks Safety Manager,” provide for computing execution time from characteristics of a particular algorithm configuration, but do not support balancing of subset algorithms while other algorithms maintain control. 
     Therefore, what is needed in the art is an improved method for managing algorithm processing load within a process controller that incorporates all the above-described features into one controller. More particularly, what is needed in the art is a method that provides flexible management of algorithm processing load. 
     SUMMARY OF THE INVENTION 
     To address the above-discussed deficiencies of the prior art, it is a primary object of the present invention to provide a far more flexible and powerful way to balance and distribute control algorithm processing loads. 
     In the attainment of the above primary object, the present invention provides, for use in a digital process controller (DPC), a system for, and method of, balancing control algorithm processing load and distributing the load over a given time interval and a real time plant control system employing the system or the method. In one embodiment, the system includes: (1) a model for estimating execution time of at least one control algorithm based on the configuration data applied to that algorithm, the DPC capable of balancing and distributing said load based on the estimate generated by the model and (2) a data entry process, associated with the algorithm model, that allows an empirically-determined execution time to be associated with at least one control algorithm, the DPC capable of balancing and distributing the load based on the time in lieu of the model based estimate and balancing and distributing the load upon configuration of at least one control algorithm. 
     The present invention therefore introduces a model-based control algorithm balancing and distribution system in which data pertaining to each control algorithm to be balanced are employed to derive an estimated execution time, but to which the user may optionally supply overriding, empirically-determined (perhaps actually measured, perhaps formulaically analyzed, perhaps intuitively guessed) execution times. 
     The system can run the model at the time that particular control algorithms are configured to allow balancing and distributing to occur iteratively and interactively, if desired. Alternatively, in one embodiment of the present invention, the data entry process allows the user to balance and distribute the load under the user own control. 
     In one embodiment of the present invention, the model allows at least one control algorithm to be configured and the load to be balanced and distributed without interrupting operation of other control algorithms. Alternatively, the system can interrupt execution of the other control algorithms to pause or stop while balancing and distribution take place. 
     In one embodiment of the present invention, a selected one of the data or the time is replicated in the model for each instance of at least one control algorithm. Preferably, the user does not have to associate the data or the time with each occurrence of a given control algorithm. In some embodiments, however, the user can perform such manual association. 
     In one embodiment of the present invention, the configuration data includes an algorithm type, an algorithm execution time, an algorithm block count and a data flow connection count. Those skilled in the art are aware that other configuration data types may also be advantageously employed with the present invention. 
     In one embodiment of the present invention, at least one control algorithm forms a portion of a control module having configuration parameters selected from the group consisting of: (1) period, (2) estimated weight and (3) phase. The model employs the configuration parameters to balance and distribute processing load. Of course, additional or other parameters may be desirable in a given application. 
     In one embodiment of the present invention, the model is capable of providing an actual execution time of at least one control algorithm to a user. This allows the user to evaluate at least one control algorithm&#39;s performance and make decisions based thereon. 
     The foregoing has outlined rather broadly the features and technical advantages of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features and advantages of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art should appreciate that they may readily use the conception and the specific embodiment disclosed as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
     FIG. 1 illustrates a functional diagram of an exemplary distributed, real-time process control system with which the present invention may suitably be used; 
     FIG. 2 illustrates a high-level block diagram of an exemplary digital processing system that may be employed to execute software-definable process control routines embodying the principles of the present invention; 
     FIG. 3 illustrates an embodiment of a real-time process control system employing an embodiment of a controller constructed using the principles of the present invention in an object-oriented programming environment; and 
     FIG. 4 illustrates an exemplary Interaction Diagram that summarizes how various subsystems interact within the present invention. 
    
    
     DETAILED DESCRIPTION 
     Before undertaking a description of the exemplary embodiments of the systems and methods of the present invention, it will be helpful to describe a computing or processing system environment in which the present invention may suitably be used or implemented. Referring initially to FIG. 1, illustrated is a functional diagram of an exemplary distributed, real-time process control system (generally designated  100 ) with which the present invention may suitably be used. 
     Process control system  100  illustratively includes a computer network having a server  110  and a controller network  111 . The controller network  111  provides an interface between the server  110  and DPCs (generally designated  121 ); the controller network  111  may, for example, carry supervisory messages between the server  110  and DPCs  121 , and peer-to-peer messages between the DPCs  121 . The DPCs  121  communicate with input/output (I/O) devices  122  via an I/O network  112 . The DPCs  121  are adapted to execute software-definable process control routines for controlling and receiving data from process sensors and actuators  130  via I/O devices  122  and I/O network  112 . Those skilled in the art are familiar with various types of process sensors and actuators  130 , such as electrically-controllable motors, valves or pumps, that may be employed in the manufacture of sundry products; the principles of the present invention are not limited to a specific process or processing system, however, but may readily be employed to advantage in any such system. 
     In one embodiment, the process control system  100  further includes a local area network (LAN)  113  that provides an interface between the server  110  and remote workstations (generally designated  140 ). System operators may employ the remote workstations  140  to control and monitor the operation of the process control system  100 . Although illustrated as a separate network, the LAN  112  and controller network  111  may be the same, i.e., the remote workstations  140  and the DPCs  120  may share the same network transmission medium. Those skilled in the art will recognize, however, that providing separate networks for control systems and operator workstations may enhance the reliability of a distributed, real-time process control system, e.g., network traffic on LAN  112  associated with distributing process-related data from server  110  to operator workstations  140  will not interfere with process control information transmitted between server  110  and remote DPCs  120  via control network  111 . 
     Software-definable process control routines may be executed by any digital processing system, such as server  110 , workstations  140 , or DPCs  121 . FIG. 2 illustrates a high-level block diagram of an exemplary digital processing system  200  that may be employed to execute software-definable process control routines embodying the principles of the present invention. Exemplary digital processing system  200  includes a microprocessor  210 , non-volatile memory  220 , and random access memory (RAM)  230 . Non-volatile memory  220 , which is employed to store software-definable process control routines, may comprise, for example, a programmable read-only memory (PROM), flash ROM, or a magnetic storage medium. The software-definable process control routines stored in non-volatile memory  220  are executed by microprocessor  210 . The microprocessor employs RAM  230  to store all or portions of the process control routines as the routines are executed, as well as storage for process control data associated with process sensors and actuators  130 . The description of exemplary digital processing system  200  is merely illustrative; those skilled in the art will recognize that software-definable process control routines employing the principles of the present invention are not limited to a specific hardware implementation for digital processing system  200 , and that all such systems are within the scope of the claims recited hereinafter. 
     To understand the principles of the present invention, it is necessary to define the “environments” necessary to support the present invention. The first is an Application Builder environment. The Application Builder environment provides services which allow the end user to configure control strategies and load them to the controller. The second environment is the controller itself. The controller receives configurations at the time of load, registers them with internal support services and executes them at run time. Those skilled in the art should readily recognize that the Application Builder does not necessarily have to reside in any one particular location. The Application Builder environment may be resident in a variety of supervisory platforms. 
     Within a DPC employing object-oriented programming, i.e., an “object-oriented controller,” algorithms and their associated data can be implemented as objects called “blocks.” Data within a block can be read from, or written to, named attributes of blocks, called “parameters.” Data flow between different blocks can be accomplished by establishing a “connection” from an output parameter of one block to an input parameter of a second block. 
     Control strategies are created as a set of blocks, each of which encapsulate a necessary constituent of the total control strategy. One block, called a “control module,” serves as a container for more elemental algorithm blocks and for the connections which cause data to flow between them. The overall process of creating a control module consists of using the Application Builder to insert algorithm blocks, make connections between block parameters and assign values to configuration parameters. Once a control module has been created, services of the Application Builder are used to load that control module to the controller. 
     Within the context of the Application Builder and controller environments, the task of managing algorithm processing load becomes synonymous with the task of managing processing load for the control module. Using the principles of the present invention, in advantageous embodiments, the control modules may support the following configuration parameters; e.g., “PERIOD,” “ESTWEIGHT” and “PHASE” where: 
     PERIOD establishes the repetitive cycle time of the control module and all its component algorithm blocks. PERIOD is confined to values which are multiples of the lowest PERIOD supported by the controller, called, for example, BASEPERIOD. 
     ESTWEIGHT holds an estimated value for the execution time of the control module. This value may be explicitly configured by the end user on the basis of direct measurements, or if left unconfigured (null), it is estimated by the Application Builder on the basis of a model. The nature of the model used to compute ESTWEIGHT is not necessary to describe the practice of the present invention. 
     PHASE holds a value which directs the controller to run the control module in a particular phase of the selected PERIOD. Possible values of PHASE are, e.g., from 0 to (PERIOD modulo BASEPERIOD). If the end user wishes to manually balance the load, the end user assigns a specific value to the PHASE parameter, within its allowed range. In an advantageous embodiment, if the user wishes the controller to balance the load automatically, he leaves the PHASE value unassigned (null). 
     When a control module is loaded to the controller, the loaded data include the values of the PERIOD, ESTWEIGHT and PHASE parameters. The controller uses these parameters as inputs to the overall process of load balancing and qualification. The PHASE parameter, however, is key in the way it triggers these operations. 
     The concept of PHASE is closely related to the concept of controller execution “cycles.” Each “cycle” is a time interval of length BASEPERIOD which is configured to execute a specific set of control modules. To illustrate scheduling within cycles, suppose a controller is to support a simple configuration of control modules described by the following Table; assuming that BASEPERIOD is 50 ms. 
     
       
         
               
             
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Configuration 
               
             
          
           
               
                 Control 
                   
                   
               
               
                 Module Name 
                 PERIOD 
                 PHASE 
               
               
                   
               
               
                 CMa 
                 200 ms 
                 0 
               
               
                 CMb 
                 100 ms 
                 0 
               
               
                 CMc 
                  50 ms 
                 0 
               
               
                 CMd 
                 200 ms 
                 1 
               
               
                 CMe 
                  50 ms 
                 0 
               
               
                 CMf 
                 100 ms 
                 1 
               
               
                 CMg 
                 200 ms 
                 3 
               
               
                   
               
             
          
         
       
     
     The configuration described in TABLE 1 assigns a value of PERIOD and PHASE to each control module. These two values completely constrain the manner in which each control module executes within the controller&#39;s cycle scheme. The resulting execution pattern is illustrated below in TABLE 2. 
     
       
         
               
             
               
               
               
               
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                 Execution Pattern 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 PERIOD 
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
                   
               
               
                  50 ms 
                 PHASE 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 . . .  
               
               
                   
                 → 
               
               
                 100 ms 
                 PHASE 
                 0 
                 1 
                 0 
                 1 
                 0 
                 1 
                 0 
                 1 
                 0 
                 . . .  
               
               
                   
                 → 
               
               
                 200 ms 
                 PHASE 
                 0 
                 1 
                 2 
                 3 
                 0 
                 1 
                 2 
                 3 
                 0 
                 . . .  
               
               
                   
                 → 
               
             
          
           
               
                 Cycle → 
                 0 
                 1 
                 2 
                 3 
                 4 
                 5 
                 6 
                 7 
                 8 
                 . . .  
               
               
                 Block 
                 CMe 
                 CMf 
                 CMe 
                 CMg 
                 CMe 
                 CMf 
                 CMe 
                 CMg 
                 CMe 
                 . . .  
               
               
                 Execution 
                 CMc 
                 CMe 
                 CMc 
                 CMf 
                 CMc 
                 CMe 
                 CMc 
                 CMf 
                 CMc 
               
               
                 Sequence → 
                 CMb 
                 CMd 
                 CMb 
                 CMe 
                 CMb 
                 CMd 
                 CMb 
                 CMe 
                 CMb 
               
               
                   
                 CMa 
                 CMc 
                   
                 CMc 
                 CMa 
                 CMc 
                   
                 CMc 
                 CMa 
               
               
                   
               
             
          
         
       
     
     From the patterns in TABLE 2 above, it can be seen that the values of PERIOD and PHASE together completely determine how a control module executes within the controller. PERIOD determines the repetition rate and PHASE determines which of a finite set of available cycles, a control module executes within. For instance, at a PERIOD of 200 ms, CMd has possible PHASE selections of 0, 1, 2 and 3. Since the selected PHASE was 1, CMd executes in cycles 1, 5, 9, 13 and so on. Control modules whose periods are assigned equal to the BASEPERIOD have only one option for PHASE: 0. Since the largest execution period used in the above example is 200 ms, cycles 0-3 are equivalent to cycles 4-7. This pattern repeats indefinitely. 
     The total number of cycles that can supported within a controller is finite. The total number that can be supported is based on the ratio of the largest supported period to the BASEPERIOD. Therefore, if the largest period is one second and the BASEPERIOD is 50 ms, there would be a total of 20 cycles. The practice of the present invention is consistent with any number of schemes for ordering control module execution within a cycle. For example, the order shown in cycle 0 (CMe, CMc, CMb and CMa) could have been chosen differently. The manner in which execution order is selected is not necessary to the understanding of the present invention and will not hereinafter be described. Although arranged in cycles, it should be noted that the execution of control modules occurs sequentially in time as illustrated in the time line below where control modules are indicated simply by their letter subscript. The time between adjacent cycles is used for executing non-periodic algorithms.                           
     Turning now to FIG. 3, illustrated is an embodiment of a real-time process control system  300  employing an embodiment of a controller  330  constructed using the principles of the present invention in an object-oriented programming environment. The process control system  300  includes a controller  330  comprising a plurality of control modules (one is shown and designated  340 ). The control module  340  also includes a plurality of algorithm blocks; three algorithm blocks are shown and are designated as first, second and third blocks  345 ,  355 ,  365 . In the illustrated embodiment, the first and second blocks  345 ,  355  are coupled to, or associated with, a plurality of transducers (generally designated as  370 ), such as temperature or pressure sensors that are used to monitor the process environment, while the third block  365  is coupled to, or associated with, an actuator  380 . The controller  330  is also coupled to an Application Builder  310 , which may be a set of software services executing in a supervisory workstation. The Application Builder  310  is also shown coupled to a data entry and display device  320 , such as a conventional computer and monitor, that provides an end user a means to input and monitor process data. In the illustrated embodiment, the Application Builder  310  also includes first, second and third model components  312 ,  314 ,  316  corresponding to the first, second and third blocks  345 ,  355 ,  365 , respectively. The model, in an advantageous embodiment, contains configuration data which include the algorithm block type, count for each block type, algorithm execution time, data flow connection count (input parameters) and data flow execution time of the block. 
     As described previously, the Application Builder  310  constructs the control module  340  (used to perform algorithm processing) using the first, second and third blocks  345 ,  355 ,  365 . The Application Builder  310  also creates within the control module  340  connections between block parameters and assign values to the process configuration parameters, which include the parameters PERIOD, ESTWEIGHT and PHASE discussed previously. 
     The task of balancing processing load includes assigning a value of PHASE to each Control Module  340 . The PHASE values must be assigned so that each cycle spends about the same amount of time executing the Control Modules  340 . To balance the processing load, information conveyed by parameter ESTWEIGHT is essential. The parameter ESTWEIGHT makes up one element of a Control Module&#39;s  340  configuration data and is typically loaded to the controller  330  when the Control Module  340  is first configured. By summing ESTWEIGHT for each Control Module  340  within a cycle, the controller  330  may determine how much execution time will be required for that cycle. Relative loading of cycles can be then compared or modified as appropriate. 
     Although used by the controller  330 , ESTWEIGHT is not computed by the controller  330 . It may be determined by one of two methods, both of which are outside the scope of the controller  330 . The first method is for ESTWEIGHT to be computed by an end user. This may be done on the basis of statistical timing data provided by the controller  330  itself. For example, the controller  330  may provide measurements of cycle execution time or Control Module  340  execution time from which an end user could formulate a value for ESTWEIGHT. Once obtained, the ESTWEIGHT value can then be attached to the Control Module  340  using a Builder configuration interface (data entry and display device  320 ). 
     Developing a value for ESTWEIGHT may take some effort. However, this effort is required only as often as unique Control Module  340  configurations are created. Those skilled in the art are aware that process control configurations frequently involve multiple copies of identically configured Control Modules  340 . In these cases, the same value of ESTWEIGHT may be used for all copies identically configured Control Modules  340 . 
     The second method for computing ESTWEIGHT is for the Builder  310  to employ a mathematical model of Control Module  340  execution times. The nature of the mathematical model used is not necessary for an understanding of the present invention. It need only satisfy the requirement that, based on the configuration characteristics of the Control Module  340 , the mathematical model computes a fairly accurate estimate of execution time. As one example, a sufficient model might be a set of linear equations which compute ESTWEIGHT by knowing the typical execution time for each type of component algorithm block, the typical connection execution time for an input parameter of each component algorithm block, and the overhead execution time for the Control Module  340  itself. 
     The present invention contemplates means for the end user to indicate whether the end user wishes to supply a value for ESTWEIGHT, or have the Builder  310  automatically compute an estimate. One way to accomplish this is for the end user to leave ESTWEIGHT at a null value to indicate that automatic computation is required, or set it to a non-null value if automatic computation is to be disabled. 
     After creating the Control Module  340 , the Application Builder  310  loads configuration data to the Control Module  340 , using conventional techniques well-known in the art, to the controller  330 . 
     When a Control Module  340  configuration is loaded to the controller  330 , the load balancing algorithm depends on the value of PHASE; PHASE will either be null or non-null (not to be confused with null and non-null values of ESTWEIGHT). A non-null PHASE value indicates that the end user wishes to override the automatic balancing process otherwise conducted by the controller  330 . If the end user wishes to directly control which set of cycles a Control Module  340  will execute on, the end user may directly assigns the desired PHASE value. For example, the end user could have assigned CMd referenced in TABLE 2 above, to run in phase 1. 
     If the received PHASE value is null, the controller  330  interprets this to indicate that automatically load balancing is desired and proceeds with the operations that will hereinafter be described. It should be noted that regardless of which method is used to determine the PHASE value, the PHASE value may be subjected to further validation checks before being accepted by the controller  330  (which will also be described hereinafter). It should also be noted that the use of null and non-null values of PHASE to indicate when automatic load balancing is requested is by convention only. Other methods to indicate this distinction may be employed and are within the broad scope of the present invention. 
     When the controller  330  determines that the loaded value of PHASE is null, i.e., the end user has not preselected a value, the controller  330  assigns a candidate value to PHASE. An exemplary algorithm to assign a value to PHASE is set forth in Table 3. The net effect of this algorithm is to select a PHASE value which will add the load to a particular set of cycles with the lowest maximum load. In an advantageous embodiment, the individual control algorithms in the Control Module  340  may be configured, loaded and balanced without interrupting the operation of other control algorithms. 
     
       
         
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
             
           
               
                 TABLE 3 
               
               
                   
               
             
             
               
                 MinPhaseWeight=0; 
               
               
                 MinPhase=0; 
               
               
                 for all PhaseValue consistent with configured PERIOD 
               
             
          
           
               
                   
                 MaxCycleWeight=0; 
               
               
                   
                 for all Cycle consistent with PERIOD and PhaseValue { 
               
             
          
           
               
                   
                 if (CycleWeight [Cycle]&gt;MaxCycleWeight) { 
               
             
          
           
               
                   
                 MaxCycleWeight=CycleWeight[Cycle]; 
               
             
          
           
               
                   
                 } 
               
             
          
           
               
                   
                 } 
               
               
                   
                 if (MinPhaseWeight==0) { 
               
             
          
           
               
                   
                 MinPhaseWeight=MaxCycleWeight; 
               
               
                   
                 MinPhase=PhaseValue; 
               
             
          
           
               
                   
                 } 
               
               
                   
                 if (MaxCycleWeight&lt;MinPhaseWeight) { 
               
             
          
           
               
                   
                 MinPhaseWeight=MaxCycleWeight; 
               
               
                   
                 MinPhase=PhaseValue; 
               
             
          
           
               
                   
                 } 
               
             
          
           
               
                 } 
               
               
                 CandidatePhase=MinPhase 
               
               
                   
               
             
          
         
       
     
     Once the PHASE parameter is determined, either as the outcome of the load balancing selection, i.e., the process described in Table 3, or as an explicit configuration by the end user, the controller  330  may, in an advantageous embodiment, perform a qualification check. An exemplary qualification check algorithm is set forth in Table 4. 
     
       
         
               
             
               
               
             
               
               
             
               
               
             
               
             
               
               
             
               
             
           
               
                 TABLE 4 
               
               
                   
               
             
             
               
                 for all Cycle consistent with PERIOD and CandidatePhase { 
               
             
          
           
               
                   
                 if (CycleWeight [Cycle]&gt;MaxCycleWeight) { 
               
             
          
           
               
                   
                 MaxCycleWeight=CycleWeight[Cycle]; 
               
             
          
           
               
                   
                 } 
               
             
          
           
               
                 } 
               
               
                 if (MaxCycleWeight+ESTWEIGHT&gt;MaxAllowedCycleWeight) { 
               
             
          
           
               
                   
                 take appropriate action 
               
             
          
           
               
                 } 
               
               
                   
               
             
          
         
       
     
     It should be noted that the qualification check is not necessary to the practice of the present invention and may be eliminated. If the qualification check is employed, the “appropriate action” in Table 4, in the event of a predicted cycle overload, may vary depending on the design philosophy of the controller  330 . Possible actions include rejecting the Control Module  340  load or to accept the Control Module  340  load with a warning displayed to the end user using the data entry and display device  320 . 
     Once the Control Module  340  has been accepted and loaded to the controller  330 , the controller  330  assigns a value to PHASE and updates its record of total weight for each cycle using an exemplary process set forth in Table 5. The process in Table 5 maintains this record within an array named CycleWeight[ ]. 
     
       
         
               
             
               
               
             
               
             
           
               
                 TABLE 5 
               
               
                   
               
             
             
               
                 PHASE=CandidatePhase; 
               
               
                 for all Cycle consistent with PERIOD and PHASE{ 
               
             
          
           
               
                   
                 CycleWeight [Cycle]=Cycleweight[Cycle]+ESTWEIGHT; 
               
             
          
           
               
                 } 
               
               
                   
               
             
          
         
       
     
     Turning now to FIG. 4, illustrated is an exemplary Interaction Diagram  400  that summarizes how various subsystems interact within the present invention. The Interaction Diagram  400  includes the following “objects”: an User  410 , a Builder Human Interface  420 , a Control Module Execution Time Model  430 , a Controller Communication Interface  440  and a Load Manager  450 . 
     The User  410  corresponds to an end user, who is configuring a Control Module called CMa. The Builder Human Interface  420  corresponds to a computer (analogous to the data entry and display device  320  illustrated in FIG. 3) through which the User  410  inputs his configuration specifications to the Builder. The Control Module Execution Time Model  430  corresponds to the “intelligence” within the Builder which holds a model of how to compute Control Modules execution times from known configuration data. The Builder Human Interface  420  and the Control Module Execution Time Model  430  may be both thought of as part of the Builder. The Controller Communication Interface  440  corresponds to the subsystem within the controller which communicates with the Application Builder and the Load Manager  450  may be though of as the subsystem within the controller which manages Control Module processing load. 
     The Interaction Diagram  400  shows a sequence of messages passed between subsystems at the time that a control module is configured and loaded. The process starts with a set of interactions between the User  410  and the Builder  420 , in which the User  410  enters configuration data and, when entry is complete, commands load. The User&#39;s  410  load command triggers the Builder  420  to access the Model  430  and then initiate the load to the Controller Communication Interface  440 . The Load Manager  450 , within the controller, receives the load data from the Controller Communication Interface  440  and processes the data received. The Load Manager  450  then triggers execution of the pseudo-code described previously. For more information on how to read diagrams of the type illustrated in FIG. 4, see “Object Oriented Analysis and Design with Applications,” by Grady Booch, Benjamin/Cummings (1994), which is herein incorporated by reference. 
     From the above, it is apparent that the present invention allows load balancing to occur through the use of a model computed execution time or through the use of an execution time measured directly by the end user. The model computation uses data which may vary with type and count of algorithm blocks and with count of data flow connections. The present invention also allows a user measured execution time to be captured within the control algorithm and replicated when new copies of that algorithm are created. Additionally, the present invention permits balancing to occur automatically upon configuration of the control algorithm or manually at the selection of the end user. Furthermore, the present invention allows new control algorithms to be configured, tested for potential overload and balanced, without requiring the shutdown of other algorithms running within the controller. 
     The present invention provides the end user with the benefits of flexibility, determinism and usability within the same controller. Flexibility is provided through multiple period options and the ability to select manual or automatic balancing. Determinism is provided through the cycle and phase structure of the design. In one embodiment of the present invention, once a phase has been assigned within a controller, the algorithms are executed within a narrow, periodic band of time. Usability is provided through the ability of having configurations qualified and balanced for a load without the whole controller shutting down; through the ability to avoid manual load balancing, if desired; and through the ability to associate a measured execution time with a particular control configuration. 
     Although the present invention and its advantages have been described in detail, those skilled in the art should understand that they can make various changes, substitutions and alterations herein without departing from the spirit and scope of the invention in its broadest form.