Patent Abstract:
A system and control method mitigates hysteresis of an adjustable component in the system. A control module can allow small control changes to be effected to the component within limits of the component&#39;s and/or the system&#39;s normal hysteresis band. The control method can allow finer, more accurate and more aggressive control to be obtained from the component. The system and method can utilize two separate control regimes to control adjustments to the component. The first control regime can control changes larger than a hysteresis band and/or changes in a same direction as the last adjustment that was performed with the first control regime. The first control regime can be a feedback-based adjustment to the component. The second control regime can be utilized to control changes within the hysteresis band. The second control regime can use open-loop based adjustments to the component.

Full Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Application No. 61/043,192, filed on Apr. 8, 2008. The entire disclosure of the above application is incorporated herein by reference. 
    
    
     FIELD 
     The present teachings relates to hysteresis and, more particularly, to mitigation of hysteresis and a control method for same. 
     BACKGROUND 
     The statements in this section merely provide background information related to the present teachings and may not constitute prior art. 
     Moveable components, such as control valves, have performance limitations due to hysteresis. In the case of a valve, the hysteresis is a limitation of the valve to return to a specific control output given an identical control input. For example, the ability of the valve to return to a specific control output, given an identical control input, can vary depending upon whether the control point is approached from below or approached from above. In other words, if a control input to the valve is a command to go 50% open, the valve may go to 48% open when approaching the control point, from say 40% open originally, and may go to 52% open when approaching the control point, from say 60% open originally, giving a hysteresis of ±2%. 
     The hysteresis, in the case of a valve, can be caused by several factors. The factors can include, by way of non-limiting example, mechanical slop in the valve linkages, mechanical slop in the feedback linkages, mechanical or electrical slop in the feedback sensor, mechanical backlash, and imposed software hysteresis bands or “dead band” for a given closeness of the desired control point compared to the feedback signal from the valve, etc., or some combination thereof, depending on the particulars of the valve and control methodology. The “dead band” can be considered to be a region wherein the valve is close enough to the actual/desired value that the control system will not call for any further changes to the valve position. 
     In some applications, depending on the loading conditions and other factors, the hysteresis can prevent proper control of the system, in particular when small changes in control are required to adjust about a setpoint to account for setpoint variations or to mitigate the effects of disturbances. The hysteresis can lead to either no output change for small control input change, or to excessive change for a small control input change. Both situations can lead to the output not matching the desired control point. 
     To reduce the hysteresis for a component, expensive precise control linkages and mechanisms can be utilized to reduce the mechanical slop. Additionally, better performing or more precise sensors can also be utilized to reduce the electrical slop in the feedback loop. The result, however, can be a control arrangement that is expensive and increases the overall costs of the system within which the component and the associated control system are utilized. 
     SUMMARY 
     The present teachings reduce and/or mitigate the effects of hysteresis on the output of a device. The present teachings can overcome some of the limitations of hysteresis in a component using software control improvements to allow small control changes to be effected within the limits of the component&#39;s, such as a valve&#39;s, normal hysteresis band. The present teachings can allow finer, more accurate and more aggressive control to be obtained from the component. The improved control can be achieved without requiring a more expensive component wherein special steps have been taken to remove the mechanical and/or electronic contributions to hysteresis. The present teachings can thereby provide increased performance for a component without requiring the expensive mechanical linkages and/or sensors to reduce the hysteresis. 
     A control system according to the present teachings utilizes two separate control regimes to control changes to the operational state of the component. A first control regime can control changes larger than the hysteresis band and/or changes in the same direction as the last change that was performed with the first control regime. The first control regime can be a feedback-based control regime. The second control regime can be utilized to control changes within the hysteresis band. The second control regime can use open-loop based adjustments, such as time-based movement by way of non-limiting example. In this manner, the present teachings can reduce the effective hysteresis of a component, allowing small, repeatable control changes about a setpoint by dividing the control effort into two regimes. 
     A hysteresis mitigation and control method for a system having an adjustable component according to the present teachings includes determining if a change in an operational state of the component is required. The method also includes ascertaining whether to implement a first control regime or a second control regime when the determination is indicative of a required change. One of the first and second control regimes is implemented based on the ascertainment. The first control regime is a closed-loop, feedback-based adjustment to the component and the second control regime is an open-loop based adjustment to the component. 
     A system according to the present teachings can include a component having an adjustable operational state and a control module that controls adjustment of the component. The control module determines if a change in an operational state of the component is required, ascertains whether to implement a first control regime or a second control regime when it is determined that a change is required, and implements one of the first and second control regimes when adjusting the operational state of the component. The first control regime is a closed-loop, feedback-based adjustment to the component and the second control regime is an open-loop based adjustment to the component. 
     In some embodiments, ascertaining can include comparing the required change to a hysteresis characteristic of at least one of the component and the system. One of the first or second control regimes is implemented based on the comparison to the hysteresis characteristic. 
     Further areas of applicability will become apparent from the description provided herein. It should be understood that the description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the present teachings. 
    
    
     
       DRAWINGS 
       The drawings described herein are for illustration purposes only and are not intended to limit the scope of the present teachings in any way. 
         FIG. 1  is a simplified representation of an exemplary automatically actuated component, in this case in the form of a control valve, that can be controlled by a control system and method according to the present teachings; 
         FIG. 2  is a simplified representation of a control system according to the present teachings; 
         FIG. 3  is a flow chart of the methodology of the control system according to the present teachings; 
         FIG. 4  is a flow chart illustrating a first control regime utilized by the control system according to the present teachings; 
         FIG. 5  is a flow chart illustrating a second control regime utilized by the control system according to the present teachings; and 
         FIG. 6  is a flow chart illustrating a preferred embodiment of the second control regime utilized by the control system according to the present teachings. 
     
    
    
     DETAILED DESCRIPTION 
     The following description is merely exemplary in nature and is not intended to limit the present teachings, applications, or uses. As used herein, the term “module” refers to an application-specific integrated circuit (ASIC), an electronic circuit, a processor (shared, dedicated, or group) and memory that execute one or more software or firmware programs, a combinational logic circuit, or other suitable components that provide the described functionality. 
     Referring to  FIG. 2 , an exemplary control system  20  according to the present teachings is shown. Control system  20  includes a control module  22  that is operable to control an automatically actuated component  24  using the first and second control regimes according to the present teachings. Control module  22  can be a single module operable to perform the described functionality; a plurality of integrated modules, as shown, that can perform the described functionality; a combination of integrated and individual modules that can perform the described functionality; and/or one or more individual modules that can perform the described functionality. Thus, control module  22  shown and described herein is merely exemplary in nature and is not intended to limit the scope of the present teachings. 
     As stated above, control module  22  can include a plurality of integrated modules that perform the described functionality. By way of non-limiting example, control module  22  can include a comparison module  26 , an algorithm module  28 , and a manipulator module  30 . Comparison module  26  can receive a setpoint signal  32  along with a current value signal  34 . The current value signal  34  is indicative of the current value of the primary control target (variable or parameter) for which control system  20  is trying to control. The setpoint signal  32  is indicative of the desired value of the primary control target that control system  20  is trying to achieve. The primary control target can vary, depending upon the application within which control system  20  is utilized. For example, the primary control target can be a reference temperature, a flow rate, a pressure, and the like, by way of non-limiting example. Setpoint signal  32  can, by way of non-limiting example, be provided by a control panel, an operator input, an input device, or another module that is utilized in the system within which component  24  is disposed. The current value signal  34  can be provided by one or more sensors or other modules that can monitor the current value of the primary control target and provide a signal indicative of its value. 
     It should be appreciated that the primary control target can be a target that is directly or indirectly related to component  24 . A direct relation is indicative of a relationship wherein the primary control target is a direct position, level, value or other characteristic of component  24 . An indirect relation is indicative of a relationship wherein the primary control target is affected by a particular characteristic of component  24  but is not a specific characteristic for component  24 . A non-limiting example of an indirect relation can be wherein the primary control target is a temperature and operation of component  24  alters a fluid flow rate that can affect the temperature. As a result, by controlling the position of component  24  and the flow rate therethrough, a desired temperature for the primary control target may be achieved. In this example, the setpoint signal  32  is indicative of the desired temperature for the primary control target and the current value signal  34  is indicative of the actual temperature of the primary control target. Thus, the primary control target can be directly or indirectly related to the characteristics of component  24 . It should also be appreciated that other factors can affect the actual value of the primary control target other than the characteristics of component  24  when there is an indirect relationship. 
     Comparison module  26  is operable to compare the current value signal  34  to the setpoint signal  32  and ascertain the difference therebetween. Comparison module  26  can provide the results of the comparison to algorithm module  28 . Algorithm module  28  is operable to use the input from comparison module  26  to implement the appropriate control regime for component  24 . In particular, algorithm module  28  can determine if a change in the operation of component  24  is necessary to achieve the desired setpoint for the primary control target. Additionally, algorithm module  28  can determine what change to make to component  24  in an attempt to achieve the primary target setpoint. Algorithm module  28  will employ either the first or second control regime, depending upon the change in operation of component  24  that is needed to achieve the primary target setpoint, as described below. 
     Algorithm module  28  can receive a feedback signal  40  from component  24 . Feedback signal  40  can be indicative of the current operating condition of component  24 . Algorithm module  28  can utilize feedback signal  40  in conjunction with the control regimes to determine whether an adjustment to the operation of component  24  is needed. Feedback signal  40  can be indicative of a position of component  24 , by way of non-limiting example. Feedback signal  40  can be provided by one or more sensors that provide signals indicative of the current operating condition of component  24 . Feedback signal  40  is subject to the hysteresis described above and, as a result, can provide a signal that is representative of the current operating condition of component  24 , subject to the hysteresis. 
     Algorithm module  28  provides the appropriate signal to manipulator module  30  when a change in the operation of component  24  is desired. Manipulator module  30  utilizes the signal from algorithm module  28  to command a change in the operation of component  24 , as described below. 
     Referring now to  FIG. 1 , control module  22  is shown as being utilized with a component  24  which is in the form of a control valve  60 . Control valve  60  can be used to control a fluid flow  62  through a flow path  64  of control valve  60 . A valve seal  66  is attached to a valve stem  68  and can be moved within flow path  64  to engage and disengage with a valve seat  70 . The spacing between valve seal  66  and valve seat  70  can alter the rate of fluid flow  62  through flow path  64 . 
     Valve stem  68  is coupled to a stem link  72  which, in turn, is coupled to an actuator link  74 . The engagement between stem link  72  and actuator link  74  results in some mechanical slop that can contribute to the overall hysteresis of control valve  60 . 
     Actuator link  74  is coupled to an actuator  76 , such as a linear actuator, by way of non-limiting example. Actuator  76  can move actuator link  74  which, in turn, drives movement of stem link  72 . Movement of stem link  72  drives movement of valve seal  66  within flow path  64 . As a result, actuator  76  can drive movement of valve seal  66  to change the rate of fluid flow  62  through control valve  60 . 
     Control valve  60  can include a feedback sensor  80  that can provide a feedback signal  40  to control module  22 . Feedback sensor  80  can take a variety of forms. By way of non-limiting example, feedback sensor  80  can include a potentiometer, whose resistance changes as a function of the position of valve seal  66 , and a linear variable differential transformer that can provide a signal indicative of the position of valve seal  66 . Feedback sensor  80  can be coupled to actuator link  74 . Feedback sensor  80  can include a pickup  82  which is responsive to movement of actuator link  74  through interaction with projections  84  coupled thereto. As can be seen, some movement of actuator link  74  is possible without one of the projections  84  contacting pickup  82  and results in sensor slop that can contribute to the overall hysteresis of control valve  60 . In other embodiments, feedback sensor  80  can be coupled to linear actuator  76 , stem link  72 , and valve stem  68 , by way of non-limiting example, each of which may have different mechanical or electrical hysteresis constraints and limits. It should be appreciated that the sensor arrangement shown for control valve  60  is merely exemplary in nature and that other sensors and position feedback devices can be utilized. Additionally, it should be appreciated that digital sensor feedback sensors can also be employed along with analog feedback sensors. 
     Control module  22  commands linear actuator  76  to adjust fluid flow  62  through control valve  60 . Control module  22  commands the operation of linear actuator  76  based upon setpoint signal  32 , current value signal  34 , and utilizing one of the first and second control regimes, as discussed below. 
     Referring now to  FIGS. 3-6 , flow charts illustrating the control methodology and control regimes according to the present teachings that may be implemented by control system  20  are shown. Referring first to  FIG. 3 , control includes a main control loop  98  and begins by ascertaining if an operational change is needed, as indicated in decision block  100 . Whether an operational change is needed or not can be ascertained by comparison module  26  based on a difference between setpoint signal  32  and current value signal  34 . In other embodiments, whether an operational change is needed or not can be ascertained by comparison module  26  based on a rate of change of current value signal  34 , time interval of difference between setpoint signal  32  and current value signal  34 , fuzzy logic, or other means to determine if an operational change is necessary, by way of non-limiting example. When an operational change is not needed, control continues to monitor whether an operational change is needed. 
     When an operational change is needed, as determined in decision block  100 , control ascertains if the change is greater than a hysteresis characteristic, such as the hysteresis band threshold, of component  24  and/or control system  20 , as indicated in decision block  102 . If the change is greater than the hysteresis band threshold, control moves to block  104  and implements the first control regime, as described below. The hysteresis band threshold is chosen to be indicative of the hysteresis of component  24  and/or control system  20  and to reflect a magnitude of change within which the first control regime may not be as effective as the second control regime. 
     If the change is less than the hysteresis band threshold, as indicated in decision block  102 , control ascertains if the last change was implemented by the first control regime and if the needed change in component  24  calls for a further change in the same direction as its previous change, as indicated in decision block  106 . If both of these conditions are true, control moves to block  104  and implements the first control regime, as described below. 
     If either of the conditions in decision block  106  is not met, control ascertains if the net open-loop change is greater than a threshold value, as indicated in decision block  108 . The net open-loop change is the change implemented using the second control regime, as described below. The net open-loop change threshold value is selected to be indicative of a value above which the use of the second control regime is no longer to be used. If the net open-loop change is greater than the threshold value, control moves to block  104  and implements the first control regime, as described below. If the net open-loop change is less than the threshold value, as indicated in decision block  108 , control moves to block  110  and implements the second control regime, as described below. 
     Thus, main control loop  98  will only implement the second control regime if the following conditions are met: (1) The needed change is less than the hysteresis band threshold, as indicated in decision block  102 ; (2) The last change was not made with the first control regime or the current needed change is in a different direction than the last change, as indicated in decision block  106 ; and (3) The net open-loop change is less than the threshold value, as indicated in block  108 . When main control loop  98  cannot implement the second control regime, main control loop  98  will implement the first control regime. 
     The first control regime, as indicated in block  104 , is a change using closed-loop feedback-based control. The details of first control regime  104  are shown in  FIG. 4 . The first control regime  104  begins by ascertaining the setpoint for component  24  to achieve the primary control target, as indicated in block  112 . The appropriate setpoint for component  24  to achieve the primary control target can be based upon a relationship (direct or indirect, as described above) between an operational state, such as a position by way of non-limiting example, of component  24  and the value of the primary control target. A variety of methods can be utilized to determine the appropriate setpoint for component  24 . By way of non-limiting example, the relationship can be contained in a look-up table  114  that algorithm module  28  accesses to determine the appropriate new setpoint for component  24  based on the comparison between the primary control target setpoint signal  32  and the primary control target current value signal  34 . The ascertained setpoint is an operational state for component  24  and is not necessarily the same as setpoint signal  32  or current value signal  34 . It should be appreciated that the use of a look-up table  114  is optional and that other methods of ascertaining the setpoint for component  24  can be implemented. By way of non-limiting example, such other methods can include the use of PID control, fuzzy logic, model-based control algorithms, and/or pulse-width-modulation type control/output. 
     Once the setpoint for component  24  has been ascertained, control commands component  24  to adjust toward the setpoint, as indicated in block  116 . Manipulator module  30  can be utilized to command component  24  to make the appropriate change. 
     Control then ascertains the operational state of component  24 , as indicated in block  118 . The ascertained operational state can be provided by feedback signal  40 . 
     Next, control compares the ascertained operational state of component  24  to the setpoint operational state of component  24 , as indicated in block  120 . Control determines if the operational state is acceptable based on the comparison, as indicated in decision block  122 . If the operational state is acceptable, control terminates the implementation of first control regime  104  and returns to main control loop  98 , as described below. 
     If control determines that the operational state is not acceptable, as indicated in decision block  122 , the control then ascertains if a trigger event has occurred that would command the termination of first control regime  104 , as indicated in decision block  124 . The termination trigger events are events that require the current control regime, in this case the first control regime  104 , to be terminated prior to completing their appropriate task. The trigger events, by way of non-limiting example, can include receiving of a new setpoint signal  32  or current value signal  34  that requires control to return to main loop  98 , a time-based limitation, an interrupt signal from the system within which control system  20  is utilized, an over-ride condition, as a result of a fault or error, and the like. 
     If control ascertains, in decision block  124 , that a termination trigger event has not occurred, control returns to block  116  and continues to command component  24  to adjust toward the setpoint. Control continues to implement the first control regime  104 , until an acceptable operational state has been achieved, as indicated in decision block  122 , or a termination trigger event has occurred, as indicated in decision block  124 . 
     Thus, first control regime  104  can ascertain a setpoint for component  24  that should achieve the desired value for the primary control target. First control regime  104  commands adjustment of component  24  to change its operation to achieve the primary control target. A feedback loop is utilized to drive adjustment of component  24  to the setpoint. It should be appreciated that the first control regime  104  is subject to the hysteresis of component  24  and/or of control system  20 . 
     When control terminates implementation of first control regime  104 , control moves to block  130 , as shown in  FIG. 3 . In block  130 , control records the feedback operational state and clears all open-loop change statistics that may have been previously generated in the second control regime  110 . The open-loop change statistics that are cleared are utilized in second control regime  110 , described below. Control may also set the ascertained current operational state (based on feedback signal  40 ) as the initial starting state for the second control regime  110 , as described below. After clearing the open-loop change statistics and ascertaining the operational state of component  24 , control ascertains if a system termination event has occurred, as indicated in decision block  132 . The system termination decision is utilized to terminate main control loop  98  and the operation of control system  20 . For example, the system termination decision can be based upon the system within which control system  20  is being utilized being shut down, or the particular functionality of control system  20  no longer being needed at that time. If a system termination event occurs, control ends, as indicated in block  134 . If a system termination event has not occurred, control returns to decision block  100  and ascertains if an operational change is needed, utilizing the known operational state of component  24  as determined in block  130 . 
     Referring now to  FIG. 5 , the second control regime  110  utilizes an open-loop control to change the operational state of component  24 . The open-loop control of the second control regime  110  does not utilize or provide feedback of the actual operational state of component  24 , as is done in first control regime  104 . Rather, second control regime  110  uses an estimated or theoretical operational state of component  24  based on operation of the actuator as commanded by manipulator module  30 . In particular, a variety of different variables may be operable to be adjusted that relate to operation of the actuator and impart a desired operational change in component  24 . The particular variables that are operable to be changed to adjust the operational state of component  24  will vary depending upon the particular type of actuator utilized to adjust the operational position of component  24 . For example, a counter-based variable may be utilized wherein changes in the counter correspond to operational changes in component  24 . As another example, a voltage may be associated with the actuator and a change in the voltage can result in a corresponding change in the operational state of component  24 . In yet another example, a magnetic field may be associated with the actuator such that a change in the magnetic field can cause a change in the operational state of component  24 . In still another example, a time-based variable can be utilized wherein the actuator is operated for a particular period of time and results in an associated change in the operational state of component  24 . It should be appreciated that other types of variables can also be utilized and that the above examples are non-limiting examples. 
     Regardless of the particular variable utilized, there is a theoretical relationship between that variable&#39;s influence on the actuator and the adjustment to the operational state of component  24 . The second control regime  110  utilizes this theoretical relationship in adjusting the operational state of component  24  within the hysteresis band. During an implementation of second control regime  110 , the actual new operational state of component  24  is not determined nor utilized. Thus, second control regime  110  is an open-loop control. 
     Regardless of the particular variable to be changed that is associated with the actuator and/or component  24 , second control regime  110  starts with control ascertaining the change in the variable to achieve the desired operational state for component  24 , as indicated in block  140 . The appropriate change in the variable can be ascertained by algorithm module  28  using a relationship between the variable and the operational state of component  24 . The relationship can be contained in a look-up table  142  that can be accessed by algorithm module  28 . The look-up table  142  can have values that indicate that if X amount of change in the operational state of component  24  is desired, then adjusting the variable to (or by) Y would theoretically result in realizing that operational state. It should be appreciated that algorithm module  28  can use algorithm equations, models, and the like by, way of non-limiting example, to ascertain the appropriate change in the variable to achieve a desired operational state of component  24  in lieu of look-up table  142 . The change in the operational state of component  24  is typically a small change that resides within the hysteresis band of component  24  and/or control system  20 . 
     Control then commands a start of the monitoring of the variable being changed, as indicated in block  144 . The monitoring is utilized to track the change in the variable, such as number of counts, voltage, magnetic field, time, etc., by way of non-limiting example. With the monitoring started, control then commands adjustment of the variable toward the new setting Y, as indicated in block  146 . As a result of the adjustment of the variable to the new setting, the operational state of component  24  will also change. 
     Control ascertains if the new variable setting has been achieved, as indicated in decision block  148 . If the new variable setting has been achieved, monitoring of the variable stops, as indicated in block  150 , operation of the second control regime  110  ends, and control returns to main control loop  98 , as described below. If the new variable setting has not been achieved, as ascertained in decision block  148 , control ascertains if a termination trigger event has occurred, as indicated in decision block  152 . The termination trigger events can be the same as those described above with reference to first control regime  104  and decision block  124 . If a termination trigger event has occurred, monitoring of the variable stops, as indicated in block  150 , implementation of second control regime  110  ends, and control returns to main control loop  98 , as described below. 
     If a termination trigger event has not occurred, as ascertained in decision block  152 , control returns to block  146  and continues to adjust the variable to the new setting and continues changing the operational state of component  24  as long as the new variable setting has not been achieved, as indicated in decision block  148 , and a termination trigger even has not occurred, as indicated in decision block  152 . 
     At the end of second control regime  110 , control returns to main control loop  98  and to block  164 . In block  164 , control calculates the net open-loop adjustment statistics for component  24  and the theoretical operational state of component  24  as if there were no hysteresis. The net open-loop adjustment statistic is a summation of the total amount of adjustments that component  24  has undergone in a particular direction and can be a positive or negative value depending upon the individual directions of the adjustment of component  24  during the adjustment of the variable as a result of implementing second control regime  110 . For example, adjustment of the variable in a first direction can result in a positive change in the operational state of component  24  that is summed together for each time that the variable is moved in the first direction during the second control regime  110 . Moving of the variable in a second, opposite direction can result in a negative operational adjustment to component  24  that is summed together for each adjustment of the variable in the second direction during implementation of the second control regime  110 . The net open-loop adjustment statistic is then a sum of the net adjustment of component  24 , and is directly proportional to the difference of the sums between the adjustments of the variable in the first direction and the adjustments of the variable in the second direction. The net open-loop adjustment statistic is utilized in decision block  108 , as discussed above, to ascertain if the magnitude of the net adjustment to the operational state of component  24  exceeds a threshold. Additionally, the net open-loop adjustment statistics are limited to one or more continuous applications of second control regime  110  and that when first control regime  104  is implemented, the net open-loop adjustment statistics are cleared, as indicated in block  130 , upon the conclusion of implementation of the first control regime  104 . 
     The theoretical operational state of component  24  is the total theoretical change, both positive and negative, that component  24  has undergone, for each adjustment of the variable during implementation of second control regime  110  without being interrupted by an implementation of first control regime  104 . The theoretical operational state calculated in block  164  is the sum of the initial starting operational state of component  24  at the beginning of the first implementation of second control regime  110  plus the theoretical change in the operational state that second control regime  110  has adjusted component  24  without being interrupted by an implementation of first control regime  104 . The initial starting operational state is the last operational state ascertained in block  130  using feedback signal  40 . The theoretical change in operational state is based on the relationship, used in block  140 , between the adjustment to the variable and the resulting change in the operational state of component  24 . Again, the changes in different directions can result in positive or negative additions to the change in the operational state and the total theoretical operational change takes this into account when ascertaining the new theoretical operational state of component  24 . The theoretical operational state and theoretical change in the operational state associated with second control regime  110  are cleared (or maintained clear) after each implementation of first control regime  104 , as indicated in block  130 , described above. 
     After calculating the net open-loop adjustment statistics and theoretical operational state, as indicated in block  164 , control determines if a system termination event has occurred, as indicated in decision block  132 . If a system termination event has not occurred, control then ascertains if an operational change is needed, as indicated in decision block  100 . In ascertaining whether an operational change is needed, control utilizes the theoretical operational state of component  24  as determined in block  164  because control has gone to decision block  100  immediately following an implementation of second control regime  110 . 
     As stated above, implementation of the first and/or second control regimes  104 ,  110  can be terminated in the event of a termination trigger event, as ascertained in decision blocks  124 ,  152 , respectively. Further, as stated above, one of the termination trigger events can be a time-out condition wherein a certain time period has passed prior to starting implementation of main control loop  96  anew. This time period can thus result in starting main control loop  98  anew at a particular frequency. It is anticipated that the loop rates of the first and second control regimes  104 ,  110  will be faster than the frequency at which main control loop  98  restarts. Preferably, first and second control regimes  104 ,  110  operate at loop rates such that at least two iterations of first and second control regimes  104 ,  110  can be implemented within the time period of the time-out termination trigger event. 
     In a preferred embodiment, a change in the operational state of component  24  is achieved by changing a position of component  24 . For example, component  24  can be a control valve such as control valve  60  shown in  FIG. 1 . Additionally, when the adjustment to the operational state of component  24  is achieved by altering its position, the second control regime can preferably be a time-based movement of component  24 . 
     Referring now to  FIG. 6 , an implementation of a second control regime  210  utilizing time-based movement of component  24 , according to the present teachings, is shown. In this embodiment, second control regime  210  initiates position changes of component  24  using time-based movement. The second control regime  210  starts with control ascertaining a time period for movement of component  24 , as indicated in block  240 . The appropriate time period for movement can be ascertained by algorithm module  28  using a rate at which the position of component  24  can change as a function of time within control system  20 . The relationship can be contained in a look-up table  242  that can be accessed by algorithm module  28 . The look-up table  242  can have values that indicate that if X amount of change in the position of component  24  is desired, then operating actuator  76  for Y period of time would result in that movement. It should be appreciated that algorithm module  28  can use algorithm equations, models, and the like, by way of non-limiting example, to ascertain the appropriate time period to achieve a desired movement of component  24  in lieu of look-up table  242 . The movement of component  24  is typically a small movement that resides within the hysteresis band of component  24  and/or control system  20 . 
     Control then commands the start of a timer, as indicated in block  244 . The timer is utilized to track the time period of movement of component  24 . With the timer started, control then commands movement of the position of component  24  toward the setpoint, as indicated in block  246 . 
     Control ascertains if the time period for movement of component  24  has elapsed, as indicated in decision block  248 . If the time period for movement has elapsed, the timer is stopped, as indicated in block  250 , operation of the second control regime  210  ends, and control returns to main control loop  98 , as described below. If the time period for movement has not elapsed, as ascertained in decision block  248 , control ascertains if a termination trigger event has occurred, as indicated in decision block  252 . The termination trigger events can be the same as those described above with reference to first control regime  104  and decision block  124 . If a termination trigger event has occurred, the timer is stopped, as indicated in block  250 , implementation of second control regime  210  ends, and control returns to main control loop  98 , as described below. 
     If a termination trigger event has not occurred, as ascertained in decision block  252 , control returns to block  246  and continues to move the position of component  24  toward the setpoint. Second control regime  210  continues to move the position of component  24  toward the setpoint as long as the time period for movement has not elapsed, as indicated in decision block  248 , and a termination trigger event has not occurred, as indicated block  252 . 
     At the end of second control regime  210 , control returns to main control loop  98  and to block  164 . In block  164 , control calculates the net time-based motion statistics for component  24  and the theoretical position of component  24  as if there were no hysteresis. The net time-based motion statistic is a summation of the total amount of motion that component  24  has been moved in a particular direction and can be a positive or negative value depending upon the individual directions of motion of component  24  and duration of motions as a result of implementing second control regime  210 . For example, motion of component  24  in a first direction can result in positive motion that is summed together for each period of time that component  24  is moved in the first direction during the second control regime  210 . Movement of component  24  in a second, opposite direction can result in negative motion that is summed together for each period of time that component  24  has moved in the second direction during implementation of the second control regime  210 . The net time-based motion statistic is then a sum of the net motion of component  24 , and is directly proportional to the difference of sums between the time periods that component  24  was moved in the first direction and the time periods that component  24  was moved in the second direction. The net time-based motion statistic is utilized in decision block  108 , as discussed above, to ascertain if the magnitude of the net time-based motion exceeds a threshold. Additionally, the net time-based motion statistics are limited to one or more continuous applications of the second control regime  210  and that when first control regime  104  is implemented the net time-based motion statistics are cleared, as indicated in block  130  upon the conclusion of implementation of the first control regime  104 . 
     The theoretical position change is the total theoretical position change, both positive and negative, that component  24  has undergone, for each time period of movement during implementation of second control regime  210  without being interrupted by an implementation of first control regime  104 . The total theoretical position calculated in block  164  is the sum of the initial starting position of component  24  at the beginning of the first implementation of second control regime  210  plus the theoretical position change that second control regime  210  has moved component  24  without being interrupted by an implementation of first control regime  104 . The initial starting position is the position last ascertained in block  130  using feedback signal  40 . The theoretical position change is based on the relationship, used in block  240 , between time and movement of component  24 . Again, the movement in different directions can result in positive or negative additions to the position change and the total theoretical position change takes this into account when ascertaining the new theoretical position of component  24 . The theoretical position and theoretical position change associated with second control regime  210  are cleared (or maintained cleared) after each implementation of first control regime  104 , as indicated in block  130 , described above. 
     Thus, in a preferred embodiment, second control regime  210  can utilize an open-loop control that is time-based. This usage is based upon a relationship between a time period of actuation of the actuator that changes the operational state of component  24  as a function of time. It should be appreciated that the specific steps performed in main control loop  98  and/or first control regime  104  can be altered to be consistent with the particular component and methodology utilized in second control regime  210 . 
     Thus, a control system  20  according to the present teachings can implement various control regimes to adjust an operational state, such as a position by way of non-limiting example, of a component  24  to achieve a desired setpoint for a primary control target. Control system  20  can utilize a first control regime  104  when gross changes (changes greater than the hysteresis band threshold) in the operational state of component  24  are required or when the required change is in the same direction as the previous change and the last change was made with the first control regime  104 . Control system  20  can utilize a second control regime when a required change is less than the hysteresis band threshold of component  24  and/or control system  20 , and either the last change was not made with the first control regime  104  or the needed change is not in the same direction as the previous change. Additionally, implementation of the second control regime  110  can be limited so that it is not continuously implemented in a manner that results in the net open-loop change to exceed a threshold value. By utilizing two separate control regimes, more precise control of the operational state of component  24  can be achieved. This increased control precision can be achieved without requiring expensive linkages to reduce the hysteresis. In particular, the second control regime  110  can be utilized to provide changes within the hysteresis band such that the effects of hysteresis can be mitigated and/or eliminated. 
     Thus, the hysteresis mitigation and control method according to the present teachings utilizes either one of a first or second control regime to obtain a desired adjustment to the operational state of the component. In some situations, a first control regime is utilized which is a feedback-based adjustment to the component. In other situations, a second control regime is utilized which is an open-loop based adjustment to the operational state of the component. The time-based adjustment described above is one example of an open-loop system that can be utilized to implement the second control regime. Other open-loop systems can also be employed which may or may not be time-based. By way of non-limiting examples, other types of open-loop systems include counter-based systems, pulses from various components, drive counts, etc. As used herein, the term “open-loop adjustment” refers to an adjustment to the component wherein a feedback directly related to the component is not received and precise information about the current operational state of the component is not available. Rather, any attempt to ascertain the actual operational state of the component is based upon relationships utilized in the open-loop control. Thus, the open-loop, time-based control of component  24  described above is an exemplary embodiment of an open-loop control regime according to the principles of the present teachings. 
     It should be appreciated that while the preceding description of the present teachings were made by way of specific examples and embodiments, that such embodiments and examples are merely exemplary in nature and that changes thereto can be made and still be within the spirit and scope of the present teachings. For example, while component  24  is shown as being a control valve  60 , other automatically actuated devices can be utilized. Additionally, while the adjustment is described as being linear, non-linear adjustment can also be accommodated. Additionally, while the preferred second control regime  210  is shown as being time-based, it should be appreciated that second control regime  210  could instead be based on any secondary measure associated with movement of the actuator  76  or actuator link  74 , or the like, that provides a measure of the actuator motion, excluding the feedback sensor  80 . By way of non-limiting examples, the linear actuator may be comprised of a stepper motor, and the number of steps or revolutions can be monitored, or the actuator may be comprised of a gear set, and the number of revolutions of a gear may be monitored, or a pulsed signal may be used to drive the actuator, and the number of pulses may be counted. Thus, the specific examples, illustrations, and embodiments disclosed herein are merely representative in nature, and changes and alterations should be considered to be within the scope of the present teachings and the claims.

Technology Classification (CPC): 6