Abstract:
Systems and methods for urging a valve plug off a seat of a control valve and toward a set point in the absence of significant delay. Adjustable, user-selected input parameters to the input of a lead-lag filter in communication with the input of a control loop facilitates achieving tunable response by a control valve upon application of small amplitude inputs.

Description:
FIELD OF THE DISCLOSURE 
   This disclosure relates generally to the field of servo controllers for use in logical processes or control loops and, more particularly, to the augmentation of electro-pneumatic control loops and other logical processes for improvement of performance of control valves and pneumatic actuator accessories. 
   BACKGROUND 
   Electro-pneumatic control systems are increasingly being employed with process control devices, such as valve actuators and piston actuators, in order to provide better or more optimal control of fluid within a process plant. Some such electro-pneumatic control systems include one or more accessories for controlling valve and piston actuators such as volume boosters and quick exhaust valves (QEVs). A volume booster, which is typically coupled to a pneumatic actuator for a valve, increases the rate of air supplied to the pneumatic actuator, or increases the rate of air exhausted from the pneumatic actuator. This increased air movement amplifies the actuator stroke speed, thereby increasing the speed at which the actuator is able to stroke the valve plug toward its open or closed position, and thus enables the valve to respond more quickly to process fluctuations. Similar to volume boosters, QEVs increase the speed at which an actuator is able to stroke a valve toward an open or closed position. 
   Currently, volume boosters are utilized with pneumatic actuators in a manner that makes the actuators move very slowly in response to very small set point or control signal changes. In particular, to help stabilize overall loop dynamics, some volume boosters are designed with a built-in dead band that prevents the volume booster from becoming active in response to small amplitude change control signals. While some volume boosters have small dead bands, these volume boosters still move very slowly in response to small amplitude signal changes, becoming fast only in response to larger amplitude input signals. To adjust the sensitivity of the booster and allow actuators to respond to small amplitude signals, bypass restrictions in the form of needle valves are often incorporated in the design of volume boosters. 
   Positioners use travel cutoffs to ensure that full seat load is reached when a reference signal falls below a predefined threshold. When cutoffs are active, the servo is bypassed and the drive signal to the current-to-pressure (I/P) transducer is set to 0% or 100%, depending on the fail state of the given actuator and active cutoff. In order to move the valve off the seat (or off the upper travel stop), back to an active region, the valve positioner must wind or unwind the actuator pneumatics from the cutoff state. 
   When the valve is on the valve seat, and a small amplitude command is sent to the valve, such as a slow ramp or a small step change, servo error signals are small and there can be a significant delay before the valve responds. The delay for the valve to move off the valve seat and reach the set point can be particularly significant for large volume actuators equipped with volume boosters, because the volume boosters may not become active in response to such small signal changes. Volume boosters tend to have about 5% dead band, and rarely activate in response to signals below 5%. If the volume boosters do not activate, the positioner will fill or exhaust air from the actuator through the booster bypass restriction, thereby causing a further delay in the response of the valve. 
   For compressor control systems, the delay in valve response is particularly problematic, because the control logic of a compressor often requires the antisurge valve to lift off the seat quickly in response to small amplitude signals. For example, when the flow rate in a compressor system drops, it is desirable to recirculate flow around the compressor to prevent the compressor from surging. Because volumes downstream of the compressor tend to be large, flow rates generally drop gradually. If the flow rate or equivalent control variable falls below a given threshold, the compressor controller will begin to move the valve off the seat slowly to obtain the required flow rate through the compressor. To maximize compressor efficiency and prevent significant upsets in the system, it is often desirable to operate the antisurge valve in this manner as long as possible before sending a trip signal to the valve that will open it up completely. 

   
     DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
       FIG. 1  is a block diagram of an electro-pneumatic control system augmented with a lead-lag input filter; 
       FIG. 2  is an example screen display generated by a user interface routine of an electro-pneumatic control system, such as that shown schematically in  FIG. 1 , illustrating travel set point plotted against time, and lead-lag filter response plotted against time, when the lead-lag input filter is engaged; 
       FIG. 3  is an example screen display generated by a user interface routine of an electro-pneumatic control system, such as that shown schematically in  FIG. 1 , illustrating travel set point plotted against time, and lead-lag filter response plotted against time, when the lead-lag input filter is disengaged; 
       FIG. 4  is an example screen display of a menu enabling a user to select a stimulus source for the lead/lag filter of the control loop and to input values in data entry fields when such fields are enabled; 
       FIG. 5  is a flow chart diagramming actions performed and information displayed as a result of various inputs in a user interface of an electro-pneumatic control system; 
       FIG. 6  is a flow chart diagramming the status of various input controls of a user interface in response to particular filter type selections; 
       FIG. 7  is an example screen display of a menu enabling a user to select among various instrument control settings, including a setting “Remote Tuning”; 
       FIG. 8  is a block diagram of an electro-pneumatic control system augmented with a switching function directly connected to the I/P drive circuit; 
       FIG. 9  is a block diagram of an electro-pneumatic control system augmented with a switching function directly connected to the forward gain path; 
       FIG. 10  is a plot of travel set point, and travel, against time, graphically depicting a typical initial response of a large volume actuator to small amplitude set point changes from a cutoff state; and 
       FIG. 11  is a plot similar to that shown in  FIG. 10 , but graphically depicting an initial response of the same large volume actuator to the same signal change when a command signal for the servo is overdriven by setting desired initial conditions to the input of a lead-lag filter of the present disclosure, which promotes improved valve response time to small amplitude signal changes. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Generally speaking, a lead-lag input filter is provided ahead of a positioner feedback loop in conjunction with one or more valve accessories, such as a volume booster or a QEV, to overcome slow dynamics experienced by the accessories when receiving low amplitude change control or set point signals. Additionally, a user interface enables an operator or other control personnel to view and change the operating characteristics of the lead-lag input filter to thereby provide the control loop with any of a number of desired response characteristics. Through manipulation of the ratio of lead-to-lag of the lead-lag input filter, a process parameter, such as displacement or travel of a valve stem, may be controlled, and in particular, fine tuned. 
     FIG. 1  illustrates a control loop  40 , such as an electro-pneumatic control loop or other logical process, having a lead-lag filter  20  connected to the input thereof. In particular, a reference control signal  10 , such as a 4-20 mA set point signal or control signal generated by a process controller or user interface, is applied to the input of the lead-lag input filter  20  which operates on the reference signal (which can be a set point or other control signal) to provide a filtered output  50  (also called a travel set point signal) to a summer  30  associated with the electro-pneumatic control loop  40 . As illustrated in  FIG. 1 , the summer  30  compares the valve travel with the travel set point signal  50  to generate an error signal, which is provided to an amplifier or gain unit  90  (called a forward path gain unit) which applies a gain K. The output of the forward path gain unit  90  is provided to a further summer  94  which sums (in this case, subtracts) a velocity feedback gain developed by a gain unit  95  and a minor loop feedback gain developed by a gain unit  105  from the output of the forward path gain unit  90 . The output  110  of the summer  94  is provided to a current-to-pressure (I/P) transducer  80  which develops and provides a pneumatic or pressure signal to a pneumatic relay  85 . As illustrated in  FIG. 1 , a measurement of the relay position  100  is provided to the gain unit  105  and is used to develop the minor loop feedback gain. 
   The pneumatic output of the relay  85  is provided to the volume booster or QEV  65 . This pneumatic signal is used to control the valve actuator of an actuator  55  associated with a valve  60 . As illustrated in  FIG. 1 , the measured valve travel of the valve plug, or the position of the valve stem with which the valve plug is associated, is provided to the summer  30  for comparison to the travel set point signal, as well as to the velocity feedback gain unit  95  to develop the velocity feedback gain. At least one sensor (not shown) is employed to detect the measured valve travel of the valve plug or the position of the valve stem. 
   Generally speaking, the transfer function and operation of lead-lag input filter  20  is configurable via a user interface  107 . In particular, a technician can remotely adjust the travel set point signal  50  for driving the pneumatic actuator  55  and the control valve  60 , or other device controlled by the electro-pneumatic control loop  40 , by adjusting parameters of the lead-lag filter  20 . The user interface  107  may be provided to enable remote monitoring of, control of, or communication with the electro-pneumatic control loop  40  from a remote location or from a location in the immediate vicinity of the control loop  40 . 
   During operation, the lead-lag filter  20  will generally provide a large amplitude, but short duration, spike at the beginning of any step change in the received reference signal  10 , which allows the valve  60  to move in smaller steps. Additionally, a fast decay rate (which translates to a small lag time) is provided in the filter response to mitigate overshoot for larger steps. 
   While a distributed control system (DCS) typically updates at a frequency on the order of 1 Hz or slower, a positioner (within the control loop  40 ) can update at a frequency of 100 Hz or more. As a result, the response time provided by the lead-lag filter  20  in series with the positioner can be on the order of 100 ms, which is much faster than can be provided by the control dynamics of the DCS alone. 
   Additionally, the lead-lag filter  20  can provide inherent protection against over driving the valve plug of the valve  60  into the valve seat or into the upper travel stop. In particular, algorithms or control routines can be implemented within or as part of the filter  20  to clip the valve&#39;s response near a valve seat or a travel stop, and thereby prevent the lead-lag filter  20  from bouncing the valve plug of the valve  60  off of the valve seat or an upper travel stop. 
   Still further, as will be understood with respect to  FIGS. 2 and 3 , the operating characteristics of the lead-lag filter  20  can be easily adjusted using the user interface  107 , which may be stored in a computer and operably coupled to the control loop  40  and one or more display screens. Because many processes that use large actuators with complex accessory configurations generally require complicated and highly customized control algorithms to control the process loop, operators are typically reluctant to modify the process controller by adding dynamics within the control routine. Instead, operators generally prefer to effect or change dynamics at the valve level. The lead-lag filter  20 , which can be modified to vary the process dynamics at the valve or loop level, provides the operator with just such control. 
   As illustrated in  FIG. 1 , the lead-lag input filter  20  is preferably implemented in combination with a user interface  107 , such as a computer program with user-friendly, real-time graphics. One or more routines and one or more processors in operable communication with the user interface  107 , the lead-lag input filter  20 , and one or more devices or components within the control loop  40  may be employed to implement the functionality and features disclosed herein. 
   The user interface  107  is preferably implemented in communication with a graphical user interface (GUI) to facilitate a user&#39;s interaction with the various capabilities provided by the user interface  107  and lead-lag input filter  20 . The GUI may include one or more software routines that are implemented using any suitable programming languages and techniques. Further, the software routines making up the GUI may be stored and processed within a single processing station or unit, such as, for example, a workstation, a controller, etc., such as in a control room within a process control plant or a central control room facility for one or a number of geographically remote process control plants, or, alternatively, the software routines of the GUI may be stored and executed in a distributed manner using a plurality of processing units that are communicatively coupled to each other. 
   Preferably, but not necessarily, the GUI may be implemented using a familiar graphical windows-based structure and appearance, in which a plurality of interlinked graphical views or pages include one or more pull-down menus that enable a user to navigate through the pages in a desired manner to view and/or retrieve a particular type of information. The features and/or capabilities of the user interface  107  described herein may be represented, accessed, invoked, etc. through one or more corresponding pages, views or displays of the GUI. Furthermore, the various displays making up the GUI may be interlinked in a logical manner to facilitate a user&#39;s quick and intuitive navigation through the displays to retrieve a particular type of information or to access and/or invoke a particular capability of the user interface  107  and lead-lag input filter  20 . 
   An example of such a GUI is generally depicted in a display  120  illustrated in  FIG. 2 . As depicted in  FIG. 2 , the display  120  graphically depicts the filter output or travel set point signal  50  and the position feedback, utilizing, for example, data collected from the actuator feedback signal  70  or the relay position feedback signal  100 . The feedback signals  70 ,  100  vary proportionally in response to changes in a process parameter with which they are associated, in this case the position of the actuator  55  or the relay  85 , so graphically depicting changes in the feedback signals  70 ,  100  provides an accurate indication of actual variation in valve stem position. Such real-time graphics allows the control valve  60  to be tuned remotely and provides quantifiable results. Additionally, remote tuning of the control valve loop via the user interface  107  significantly reduces maintenance costs by avoiding physical maintenance visits to individual control valves. 
   A control room with one or more computer terminals for accessing the user interface  107  may be provided in the geographic vicinity of the valves or loops to be controlled. Alternatively, satellite communication, telephone lines, coaxial cable, Ethernet, fiber optic cable connections, an intranet, the Internet, or other long distance communication technology may be employed to provide remote access to the user interface  107  at geographically distant locations. A central control facility may be provided in which one or more computer terminals for accessing the user interfaces  107  associated with valves or loops provided with lead-lag filters  70  in a plurality of locations separated by long distances from the central control facility. As explained in greater detail below, the user interface  107  is provided with a plot allowing the operator or technician to predict or view the filter response when particular settings are selected for various user-adjustable parameters of the lead-lag input filter  20 . 
   While there is inherent delay when signals or data are transmitted via one or a combination of the various communication technologies especially over long distances, the user interface  107  can be employed in a manner to adjust for such delays, provided the extent of the delays are known or can be calculated or determined. For example, the user interface  107  may provide the user or operator with the option of implementing a particular set of adjustments to the user-adjustable parameters of the lead-lag input filter  20  which the user or operator has first plotted using the predicted response capabilities of the user interface  107 , discussed in more detail below. If the new set of adjustments is to be implemented for a valve or loop in a distant location at a time selected by the user or operator, the user interface  107  may factor the delay into a calculation of the timing for sending actual signals to the lead-lag input filter  20  of a particular valve or loop. For instance, if the user or operator wants the new set of adjustments to be implemented in 10 seconds, and there is a known or calculated delay of 0.5 second, the actual signal to the lead-lag input filter  20  may be sent in 9.5 seconds. This assumes the user or operator is receiving and displaying in real time the filter output and travel feedback data concerning the actual control valve or control loop to which the lead-lag input filter  20  has been added. 
   Using a computer software program for the control of parameters associated with a control valve, such as the AMS ValveLink® Software program, available from the Fisher Controls division of Emerson Process Management, the user interface  107  may be configured to display real-time filter output and travel feedback data from the control valve or other device with which the lead-lag input filter  20  is employed. Additional data may also be displayed, such as reference signal to the device. For example, as illustrated in  FIG. 2  by the graph  130 , the user interface  107  may plot on the GUI the real-time travel set point (“Tvl Set Pt”) and travel feedback data (“Tvl”), displayed as percentages (%), against time to enable an operator to easily view the response of the control valve to changes in the reference signal. 
   The improved control achieved by using the lead-lag filter  20  at low amplitudes can be appreciated by comparing the plot  130 , shown in the graphics display  120  shown in  FIG. 2 , reflecting real-time data for the travel set point  50  and the travel feedback  79  collected while the lead-lag filter  20  is engaged, to the plot  135  shown in the graphics  140  displayed in  FIG. 3 , reflecting data collected while the lead-lag filter is turned off or disengaged after the 0:02:12 time mark, where the times displayed on the horizontal axis of the plot are in hours, minutes, and seconds. Here, it can be seen that, without the lead-lag filter  20 , the response of the valve  60  deteriorates in and slows as a result of a simple step change in the travel set point (reference) signal. Real-time graphics, such as those illustrated in  FIGS. 2 and 3 , are particularly advantageous for tuning the lead-lag input filter  20 , given the sensitivity and complexity associated with the valve dynamics, even at low amplitudes. 
   Referring again to  FIG. 2 , for ease of operation, tuning coefficients associated with the lead-lag input filter  20  may be represented in the display  120  of the user interface routine using a filter response plot  150 . Additionally, the tuning coefficients (and thereby the transfer function) associated with the lead-lag filter  20  may be changed using one or more virtual interface controls  200 , depicted in  FIG. 2  as graphical representations of slider bars  210 ,  220 , and  230 . A control operator or technician may manipulate the slider bars  210 ,  220 , and  230  using, for example, a computer input device (not shown) such as a mouse, knob, trackball, keyboard, touch-screen monitor, voice-activation, or stylus pad to thereby change the transfer function or dynamics of the lead-lag input filter  20 . Of course, this list of computer input devices is intended to be exemplary only, and other input devices may likewise be used to manipulate the sliders  210 ,  220 , and  230 . Also, the virtual interface controls  200  may alternatively be graphically represented by, for example, dials (not shown) or other graphics. Additionally, as illustrated in  FIG. 2  at the areas  205 ,  207 ,  209  to the left of the sliders  210 ,  220 ,  230 , the filter coefficients or ratios selected by the sliders  210 ,  220  and  230  may be displayed in numerical form, and buttons  214  and  216 , shown in the area designated  212  of the display  120 , may be used to apply the current settings or to reset the current setting of the lead-lag filter  20 . 
   Valid values for the lag time filter coefficient  205  include 0.00 (which results in bypassing the filter), and values in a range from 0.10 to 10.00 seconds. Preferably, the range of lag time filter coefficients  205  is shown in a logarithmic scale on the plot  130  of the display  120 , inasmuch as most lag time filter coefficients are selected in a range from 0.10 to 2.00 seconds. 
   Valid values for the lead time to lag time ratio in the opening direction  207 , and lead time to lag time ratio in the closing direction  209 , range from 0.0 to 2.0, and are shown in a linear scale on the display  120 . 
   As illustrated in  FIG. 2 , the slider  210  adjusts the lag time, which determines the decay rate of the filter response. The larger the lag time, the slower the lead-lag input filter  20  returns its output to the reference signal  10 . The slider  220  of  FIG. 2  adjusts the ratio of the lead time to the lag time in the opening direction. The slider  230  of  FIG. 2  adjusts the ratio of the lead time to the lag time in the closing direction. This ratio determines the initial response of the lead-lag input filter  20 . As indicated above, the lead-lag filter  20  is generally configured to provide a large amplitude, but short duration, spike in the travel set point  50 , which allows the valve  60  to move in smaller steps. A fast decay rate (which translates to a small lag time) also mitigates overshoot for larger steps because the valve  60  tends to slew allowing the filter response to decay away completely before the valve  60  gets close to the set point. 
   Additionally, the filter response graph  150  ( FIG. 2 ) provides the operator or technician with the ability to predict or view the filter response when particular settings are selected for the various user-adjustable parameters, such as lag time and ratio of lag time to lead time. The filter response graph  150  of  FIG. 2  illustrates the predicted response of the lead-lag filter  20  to a unit step change before the parameters changes are applied to the lead-lag filter  20  to thereby enable the operator or technician to view a graphical representation of the predicted filter response before the dynamics of the control system are actually adjusted. Thus, there is a virtual ratio of lead-to-lag that an operator may manipulate in order to generate a predicted response of a process parameter to be controlled or tuned, and that predicted response is displayed on a display associated with the user interface  107 . A similar filter response graph  155  in  FIG. 3  displays the response when the lead-lag input filter  20  is turned off or disengaged. 
   Additionally, an operator may use the selection buttons in the area  228  of the user interface display  120  of  FIG. 2  to configure the lead-lag filter  20  to be turned off or disengaged, to adjust just the lag element of the response, to adjust or select both the lag and the lead/lag ratio of the filter response, or to enable asymmetric lead/lag ratios, i.e. where there is a non-zero lag time coefficient, and the coefficients for the lead time to lag time ratio in the opening direction differs from the lead time to lag time ratio in the closing direction. When the lag time coefficient is zero, and there are non-zero, but identical lead time to lag time ratio coefficients, the lead-lag dynamics are symmetrical. 
   By storing collected and predicted data displayed in the plots  130 ,  150  in a buffer or readable memory of or operatively coupled to a computer, the plots  130 ,  150  may be paused, rewound, and replayed at the operator&#39;s or technician&#39;s convenience, or for future quality control, efficiency, and optimization purposes, educational purposes, regulatory compliance purposes, or other purposes. 
   Control mechanisms, such as the graphically depicted buttons  310 ,  315 ,  320  and slider  330  shown at the top of the display  120  of the user interface  107 , may be manipulated with an appropriate computer input device, such as those listed above, to control a latency period, or delay, between the predicted response depicted in the filter response graph  150  and real world application of the settings to effect actual adjustment of the control system dynamics. In the event an operator determined that the predicted response to a particular adjustment or set of adjustments to the tuning coefficients by manipulation of one or more of the virtual interface controls  200  was an undesired result, the operator can manipulate the graphically depicted buttons  310 ,  315 ,  320  or the slider  330  to increase the latency period, and readjust the tuning coefficients until a desired result is depicted in the filter response graph  150 , preventing the undesired result from ever occurring in the actual, real world control system. 
   Other operations, such as printing, may be performed by a technician&#39;s or operator&#39;s selection of other graphically depicted buttons  335 ,  340 ,  345 ,  350 ,  355 ,  360  on the display  120 . 
   The user interface allows the stimulus for tuning the valve  60  to be applied externally (e.g., through a DCS) or “internally” with a computer software program such as ValveLink® configured to send a digital step command to the positioner. Using an external stimulus, the user manipulates the 4 mA-20 mA input signal and the valve responds accordingly. In addition, the lead-lag filter  20  may be implemented either directly in a device, such as in a valve positioner, or in a distributed control system connected to the device, e.g., in a controller. Generally speaking, the lead-lag filter  20  may be implemented as a digital control program or routine stored in a computer readable memory and executed on a processor, but may be implemented as an analog filter as well. 
   The user interface  107  may be provided with an option screen allowing the user to readily select an external stimulus or an internal stimulus. When the external stimulus is selected, operator adjustment of the adjustable interface controls alters at least one tuning coefficient associated with the lead-lag filter to cause modifications to the reference control signal. When the internal stimulus is selected, the adjustable interface controls are at least partially disabled, such that the disabled interface controls no longer alter tuning coefficients associated with the lead-lag filter. Instead, the tuning coefficients of the lead-lag filter are modified in response to a controller including programming adapted to cause predetermined modifications to the reference control signal. 
   For instance, as shown in  FIG. 4 , a menu is provided from which a user may select either “External Stimulus” or “ValveLink Stimulus (Square Wave)”, which will be understood to be an internal stimulus. Selecting the internal stimulus option enables the user to enter values for the data entry fields “Nominal Set Point (%)”, “Step Size (%)”, and “Step Hold Time (sec)”. When “External Stimulus” is selected, these data entry fields become disabled. When the internal stimulus option is selected, the program may be configured to automatically populate the data entry fields with initial default values, such as the following: 
   
     
       
             
           
             
             
             
           
         
             
                 
             
             
               DATA ENTRY FIELD DEFAULT VALUES 
             
             
                 
             
           
           
             
                 
             
           
        
         
             
                 
               Nominal Set Point 
               50% 
             
             
                 
               Step Size 
               15% 
             
             
                 
               Step Hold Time 
               8 seconds 
             
             
                 
                 
             
           
        
       
     
   
     FIG. 5  is a flow chart diagramming the results performed and displayed on the user interface, depending on whether an external stimulus or an internal stimulus is selected. Warning messages or other alerts are preferably displayed before initiating control valve operation to remind the user that, in the case of selection of an external stimulus, the valve will track the set point, and in the case of selection of an internal stimulus, the internal stimulus will cause the valve to move. If the internal stimulus option is selected, the set point value preferably ramps to the value entered for the nominal set point at 10% per second before the step sequence is initiated. 
     FIG. 6  is a flow chart diagramming the status of various input controls of the user interface in response to particular filter type selections. For instance, when an asymmetric lead-lag filter type is selected, the user interface is configured to enable the user interface control for manipulating lag time. The user interface also is configured to enable the user interface control for manipulating the opening lead-lag ratio and the closing lead-lag ratio. Conversely, if a symmetric or simple lead-lag filter is selected, an initial value from a database is provided in a data entry field of the user interface for the opening lead/lag ratio, the user interface is configured to enable the user interface controls for manipulating the lag time and opening lead/lag ratio, but the user interface control for setting the closing lead/lag ratio is disabled. 
   Still further, as indicated above, the filter  20  may be provided with an automatic reset of the lead-lag filter dynamics to prevent the filter  20  from inadvertently activating above or below a cutoff. In particular, the lead-lag input filter  20  may, in some situations, have the undesirable capability to bounce the valve plug of the valve  60  off the seat or off of a travel stop. This is a particularly difficult problem because positioners typically have built-in travel cutoffs that fully saturate the I/P transducer  80  when set point approaches 0% or 100%. For a Fisher DVC6000 digital valve controller, the problem associated with the use of lead-lag filters at the high or low range of the valve is avoided by establishing travel cutoffs using default values of 0.5% and 99.5%, meaning that if the reference signal or set point falls below 0.5% or exceeds 99.5%, the servo controller is bypassed and the I/P transducer  80  is either saturated at full supply or vented to the atmosphere, depending on the required saturation state. As a result, during normal throttling operation the lead-lag input filter  20  should not trip a cutoff. 
   A pseudo computer programming code provided below demonstrates an example computer program code implementation that may be used to assure that a controller associated with or that implements the lead-lag input filter  20  prevents cutoffs from being tripped. In this case, the lead-lag input filter  20  is bypassed and the dynamics are reset if the output of the filter  20  exceeds a predefined limit near the cutoff value, such as at 0.5% or 99.5%, although other values can be used as well. 
   
     
       
             
           
             
             
           
             
           
             
             
           
             
             
           
             
             
           
             
           
         
             
                 
             
           
           
             
               //------------------------------------------------------------------------------------ 
             
             
               // Begin lead-lag filter 
             
             
               //------------------------------------------------------------------------------------ 
             
             
               //--- Prefilter stage --- 
             
             
               if((r &gt;= filter_limit_high) ∥(r &lt;= filter_limit_low) ∥ (lag_time = 0.0)) 
             
             
               { 
             
           
        
         
             
                 
               x = r; // bypass filter when in or near cutoffs 
             
           
        
         
             
               } 
             
           
        
         
             
               else 
               // --- Filter stage --- 
             
             
               { 
             
           
        
         
             
                 
               x = a * (r_old − x_old) + x_old + b * (r − r_old); 
             
             
                 
               // check filter output to make sure we do not bump into cutoffs 
             
             
                 
               if(x &gt;= filter_limit_high) 
             
             
                 
               x = filter_limit_high; 
             
             
                 
               else if (x&lt;= filter_limit_low) 
             
           
        
         
             
                 
               x = filter_limit_low; 
             
           
        
         
             
               } 
             
             
               // --- Post filter stage --- 
             
             
               x_old = x; // update old values 
             
             
               r_old = r; 
             
             
               //------------------------------------------------------------------------------------ 
             
             
               // End lead-lag filter 
             
             
               //------------------------------------------------------------------------------------ 
             
             
                 
             
           
        
       
     
   
   In one embodiment, the lead-lag input filter  20  may be implemented with four states, or stages, of execution including a prefilter stage, a filter stage, a post-filter stage, and an initial condition stage. In the prefilter stage, the filter  20  checks to determine if the reference signal  10  has exceeded a predefined upper limit, has dropped below a predefined lower limit, or if the filter  20  has been turned off altogether. When the reference signal  10  exceeds the predefined upper limit or drops below the predefined lower limit (or the filter  20  is turned off or disengaged via the user interface  107 ), the lead-lag input filter  20  bypasses processing of the reference signal and, instead, provides the reference signal  10  directly to the input  30  of the servo-loop. As indicated above, the predefined upper and lower limits are preferably set so that output of the lead-lag input filter  20  will not trip a cutoff or hit a hard stop in the actuator. 
   The following pseudo computer programming code demonstrates one manner in which a controller associated with the lead-lag input filter  20  may be programmed so as to set the upper and lower filter limits to desirable threshold levels: 
   
     
       
             
           
         
             
                 
             
           
           
             
               Filter_limit_high = min((ivp_cutoff_high − 
             
             
               high_cutoff_deadband), (100% − high_cutoff_deadband)) 
             
             
               Filter_limit_low = max((ivp_cutoff_low + low_cutoff_deadband), 
             
             
               (0% + low_cutoff_deadband)) 
             
             
                 
             
           
        
       
     
   
   These limits may be calculated in firmware and are calculated every time the input characteristic, lower travel cutoff, or upper travel cutoff values are changed. Moreover, because the cutoff processing algorithm is downstream of the characterizer, these limits are passed through an inverse characteristic (with x- and y-data vectors reversed) so that the characterized limits are below the cutoff thresholds. 
   In the filter stage, the lead-lag input filter  20  operates as a standard discrete time filter. Generally speaking, the lead-lag input filter  20  may be represented as having two coefficients, “a” and “b.” Coefficient “a” is the coefficient for the lag contribution and coefficient “b” is the coefficient for the ratio of the lead time to lag time, which may be expressed formulaically as: τ lead/τ lag. To prevent the lead-lag input filter  20  from activating a cutoff or hitting a hard travel stop, the output of the filter  20  is preferably reset to the same upper and lower values used in the prefilter stage. During the filter stage or state, the filter  20  applies the filter coefficients (ratio) to the reference signal in any known or desired manner to create the filtered input signal for the servo-loop. 
   During the post-filter stage, the previous values used in the filter calculations are updated based on new inputs from the user interface or from the servo-loop. Finally, during the initial conditions stage, which occurs for example when an instrument is started up, the initial conditions of the lead-lag input filter  20  are set to the present input reference value. Of course, in order to provide inverse dynamics to nonlinearities in the pneumatics, filter coefficients may be separately adjusted for the opening direction and the closing direction of a control valve  60 . 
   In a preferred embodiment, the lead-lag input filter&#39;s result, i.e. the effect of the lead-lag input filter  20  on the set point or the valve input signal, is given by the formula:
 
(τ 1 s+1)/(τ 2 s+1)
 
   By adjusting the values of τ 1  and τ 2 , the ratio is changed, effecting pure lag, pure lead, or some combination of lead and lag. When applied to a control valve, the resulting ratio correlates to the amount of overshoot that the lead-lag filter will provide. Thus, in different valve performance scenarios, the operator may use the user interface  107  to adjust the ratio to achieve desired alterations. For instance, if it is desired for the lead-lag input filter  20  to produce pure lag, then τ 1 s is set to zero, producing a result of 1/(τ 2 s+1). In a control loop, when pure lag is generated by the lead-lag input filter  20 , error is driven towards zero. As a result, the position of the control valve stem with which the lead-lag filter  20  is employed, or other process variable being controlled, will creep to the travel set point  50 . 
   If it is desired for the lead-lag input filter  20  to produce pure lead, then τ 2 s is set to zero, producing a result of (τ 1 s+1)/1. In a control loop, this provides anticipatory control, by correcting for error prior to occurrence of such error. When plotted, the operator of the user interface  107  would see positive phase with respect to the controlled element. 
   So long as the lead-lag ratio is greater than 1.0, the initial lead response will dominate. If the lead-lag ratio is 2, there is an initial lead response of 2.0, as a result of which any correction in the position of the control valve stem or shaft is substantially reduced, prior to error propagating through the control circuit, and will then gradually move the control valve stem position, or other process variable being controlled, to the travel set point  50 . If the lead-lag ratio is less than 1.0, then the lag correction will dominate. 
   By recognizing the change in performance in various valve performance scenarios resulting from various possible lead-lag ratios, operators may become easily adept at fine tuning process parameters and correcting for errors, and may easily optimize control valve performance. 
   It will be recognized that additional components may advantageously be provided that benefit from the use of a lead-lag filter  20 . For example, feedforward components may be provided which are adapted to respond to data including the reference signal  10 , velocity of the reference signal  10 , and acceleration of the reference signal  10 . 
   The display  120  of the user interface  107  is preferably accessed through one or more menu screens, such as a pull-down menu screen captioned “Instrument Setup” as shown in  FIG. 7 . The menu screen(s) preferably provide adequate indicia to inform the user that the control valve loop may be remotely tuned. For instance, a menu option in  FIG. 5  reads “Remote Tuning”. When selected, the user may select “Enhanced Stabilize/Optimize Lead-Lag Input Filter”. 
   The lead-lag input filter  20  may be implemented in any number of different types of servo-loops. Thus, while the lead-lag input filter  20  is illustrated in  FIG. 1  as being used in one type of electro-pneumatic control system comprising a high-gain, closed-loop servo controller used to set stem or shaft position on control valves, it could be used in other control systems or control loops as well. For example, another application in which a lead-lag filter associated with a set point is effective is in combination with ball valves where shaft windup between the actuator and the plug introduces dead band in flow control. Shaft windup may be overcome by briefly over driving the actuator and allowing the ball to move to the desired location. Because this is an open-loop technique, the response is not perfect, but a considerably better response is obtained than without a lead-lag filter. 
   Still further, there are various techniques available to improve performance by driving the servo to set point faster than what would normally be achieved by closed loop compensation alone, without changing closed loop dynamics. Augmenting the feedback controller with a lead-lag filter on the set point is one such technique, while other techniques involve augmenting the controller with set point velocity feedforward elements. The lead-lag filter could be used in these situations as well. 
   The technique in which a feedback controller is augmented with a lead-lag input filter is particularly useful in applications in which accessories for increasing actuator stroke speed, such as volume boosters and QEVs, are used. In order to compensate for slow dynamics at low amplitude changes, a lead-lag filter may be used to over drive the set point for a brief amount of time, so as to engage volume boosters even at lower amplitudes, such as amplitudes at which conventional volume booster arrangements would not be effectively activated due to low dead bands. 
   While the lead-lag filter  20  may be implemented in a desired manner, including in software and hardware or firmware, when implemented in software, the software routines discussed herein may be stored in any computer readable memory such as on a magnetic disk, a laser disk, or other storage medium, in a RAM or ROM of a computer or processor, such as an application specific integrated circuit (ASIC), a standard multi-purpose CPU or other hard-wired device, etc. Likewise, the software may be delivered to a user or a process control system via any known or desired delivery method including, for example, on a computer readable disk or other transportable computer storage mechanism or over a communication channel such as a telephone line, the Internet, etc. (which are viewed as being the same as or interchangeable with providing such software via a transportable storage medium). 
   One method of getting a valve plug to move quickly relative to the valve seat, toward a set point, in response to small amplitude input, is to overdrive the servo input control signal or set point when the valve is transitioning from the cutoff state to active throttling control. A switching function can be used to switch from one cutoff state to another for a short duration of time, which is typically less than one second, such as 50 ms, in order to overdrive the servo during the transition. As shown in  FIG. 8 , a switch to rapidly transition from a cutoff of 0% to 100% or from a cutoff of 100% to 0% can be implemented by manipulating the I/P drive explicitly through a switching function  95  directly connected to the I/P drive circuit or by manipulating the I/P drive indirectly through a switch function  96  in the forward path gain as shown in  FIG. 9 . One of ordinary skill in the art can appreciate that the switching function may be implemented in a control system with or without a lead-lag filter at the input to the servo. 
   However, such a switching function requires a relatively large amount software overhead to implement and additional protective logic to prevent “chattering” or rapid, undesirable deviations when switching. As described below, a lead-lag input filter provided ahead of a positioner feedback loop, with user-variable input conditions, offers an alternative solution, which utilizes significantly less system memory than the aforementioned switching function. 
   As an alternative to using a switching function to overdrive a servo, such as in a compressor control system in which the antisurge valve must lift off the seat quickly in response to small amplitude signals, the same effect can be achieved by setting the initial conditions on the input of the lead-lag filter of the present disclosure differently from the initial conditions on the output of the lead-lag filter. In order to set the initial conditions on the input differently from the initial conditions on the output of the lead-lag filter, the initial conditions on the numerator dynamics are set differently from the initial conditions on the denominator dynamics. The effect is that a “spike” or a momentary deviation in the travel set point is created, which has the desired result of getting the valve off the seat quickly. 
     FIG. 11  shows an example of an overdriven set point in the transition region from cutoff to active control, when the servo command signal is overdriven by setting the initial conditions on a lead-lag input filter of the present disclosure.  FIG. 10  illustrates the typical response of a large volume actuator to small amplitude set point changes from a cutoff state. In the example shown in  FIG. 10 , there was a delay of over two (2) minutes before the valve reached the travel set point. By comparison, the valve reached the travel set point in  FIG. 11  almost instantaneously. One of ordinary skill in the art can appreciate that the absolute time to move the valve plug off of the valve seat is proportionally related to the volume of the actuator. That is, even under the present example embodiment, the artificial conditions that momentarily override the servo input can yield a valve plug movement time of up to two minutes for large actuator volumes. In this instance, conventional systems can take several minutes beyond this time. 
   One can appreciate that the lead-lag filter may be expressed as:
 
 x[k]=a*x[k− 1 ]+b*r[k]+ (1 −b−a )* r[k− 1]
 
where
         x[k]=filter output at index k   r[k]=filter input at index k   a=exp(−Ts/tau_lag)   b=tau_lead/tau_lag   Ts=servo sample time   tau_lag=lag time constant (which may be defined by a user)   tau_lead=lead time constant (which may be defined by a user)       

   When the valve is on the valve seat, the value for x[k−1] is typically set to 0%, and the value for r[k−1] is also set to 0%. By manually setting the value for r[k−1] to an artificially low initial condition, below zero %, such as −10%, when the valve is cutoff, the difference between r[k] (i.e., the filter input) and r[k−1] becomes larger than normal. That is, if r[k−1] is at 0% and the initial condition on the lead-lag set to −10%, a 2% step change would produce a relative change of +12% at the output of the filter. This increased difference causes the output of the filter, x[k], to become overdriven. Alternatively, if the value for x[k−1] is at 100%, the value for r[k−1] may be set to an artificially high initial condition, such as 110%, so that a step change to 98% would produce a change of −12% to the at the output of the filter. Essentially, in the event of active cutoffs, the lead-lag filter is initialized so that small changes in the command signal are interpreted as large momentary changes in the output, having the effect of causing a spike in the current-to-pressure drive signal which moves the valve off the valve seat and drives the valve toward the set point in the same fashion as would occur with a switching function. 
   In adjusting the initial conditions to the input of the lead-lag filter, an adjustable interface control associated with the user interface may be employed. For example, the user interface may be provided with a variable, user-selected level to which the initial condition of the input of the lead-lag filter may be set. A first user-selected level of adjustment, having a designation such as “low lead-lag boost”, may correspond to a small negative value, such as −2%. An intermediate user-selected level, having a designation such as “medium lead-lag boost”, may correspond to a relatively larger negative value, such as −10%. A third user-selected level of adjustment, having a designation such as “high lead-lag boost”, may correspond to an even relatively larger negative value, such as −15%. 
   While certain embodiments have been described herein, claims to the disclosed invention are not intended to be limited to these specific embodiments.