Method of and system for controlling the ratio of a variable lead parameter and an adjustable lag parameter for a lag-lead process

Abstract

In a method of and system for controlling the air/gas ratio in a lag-lead combustion plant the lead and lag parameters are monitored to provide lead and lag signals representative of the values of the parameters. These are compared to provide an error signal representative of the deviation of the ratio of the lead and lag parameters from a preselected ratio. The lag parameter is then adjusted to reduce the deviation in response to the deviation exceeding a preselected deviation.

1 . A method of controlling the ratio of a variable lead parameter and an adjustable lag parameter for a lag-lead process, the method comprising: 
 monitoring said lead parameter and providing a lead signal representative of the value of said lead parameter;    monitoring said lag parameter and providing a lag signal representative of the value of said lag parameter;    comparing said lead and lag signals and providing an error signal representative of the deviation of the ratio of said lead and lag parameters from a preselected ratio;    and adjusting said lag parameter to reduce said deviation in response to said deviation exceeding a preselected deviation.    
   
   
       2 . A method as claimed in  claim 1  comprising comparing said error signal with a preselected threshold value and adjusting said lag parameter in response to said error signal exceeding said preselected threshold value.  
   
   
       3 . A method as claimed in  claim 1  comprising comparing said error signal with an error range defined by a first, upper preselected threshold value and a second, lower preselected threshold value and adjusting said lag parameter in response to said error signal falling outside said error range.  
   
   
       4 . A method as claimed in  claim 3  comprising: 
 adjusting said lag parameter to reduce said ratio in response to said error signal being above said error range;    and adjusting said lag parameter to increase said ratio in response to said error signal being below said error range.    
   
   
       5 . A method as claimed in  claim 4  wherein: 
 said first, upper preselected threshold value is a positive value and said second, lower preselected threshold value is a negative value;    a positive error signal indicates an increase in said ratio from said preselected ratio and a negative error signal indicates a decrease in said ratio from said preselected ratio;    and the method comprises:    adjusting said lag parameter to reduce said ratio in response to said error signal being positive and exceeding said first, upper preselected threshold value;    and adjusting said lag parameter to increase said ratio in response to said error signal being negative and exceeding said second, lower preselected threshold value.    
   
   
       6 . A method as claimed in  claim 2  wherein the or each said threshold value is a fixed value.  
   
   
       7 . A method as claimed in  claim 2  wherein the or each said threshold value is a function of the value of said lag parameter.  
   
   
       8 . A method as claimed in  claim 7  wherein the step of comparing said error signal with the or each threshold value comprises adjusting said threshold value in dependence on the value of said lag parameter and comparing said error signal with said adjusted threshold level.  
   
   
       9 . A method as claimed in  claim 8  comprising: 
 providing a look up table for storing a plurality of threshold values;    and the step of adjusting said threshold value comprising selecting one of said threshold values in dependence on the value of said lag parameter.    
   
   
       10 . A method as claimed in  claim 7  wherein: 
 the or each threshold value comprises a plurality of threshold levels;    and the step of comparing said error signal with the or each threshold value comprises selecting a threshold level for said threshold value in dependence on the value of said lag parameter and comparing said error signal with said selected threshold level.    
   
   
       11 . A method as claimed in  claim 10  comprising: 
 providing a look up table for storing said plurality of threshold levels;    and the step of adjusting said threshold value comprising selecting one of said threshold levels in dependence on the value of said lag parameter.    
   
   
       12 . A method as claimed in  claim 1  wherein: 
 said lead and lag parameters are lead and lag fluid flow rates for said industrial process; 
 and said method comprises:  
 providing lag valve means for controlling flow of said lag fluid;  
 monitoring the position of said lag valve means during opening or closing of said valve means;  
 monitoring the flow rate of said lag fluid during opening or closing of said valve means;  
 comparing the change in position of said valve means with the change in said flow rate of said lag fluid;  
 and adjusting said threshold value in response to said comparison indicating a change in the characteristic of said valve means.  
   
   
   
       13 . A method as claimed in  claim 29  wherein the step of adjusting said threshold value comprises adjusting the threshold values or levels in said look up table.  
   
   
       14 . A method as claimed in  claim 1  wherein: 
 said lead parameter is the flow rate of a combustion gas;    said lag parameter is the flow rate of air;    and said process is a combustion process.    
   
   
       15 . A control system for providing lag-lead control of a process having a variable lead parameter and an adjustable lag parameter, the system comprising: 
 lead monitoring means for monitoring said lead parameter and providing a lead signal representative of the value of said lead parameter;    lag monitoring means for monitoring said lag parameter and providing a lag signal representative of the value of said lag parameter;    comparator means for comparing said lead and lag signals and providing an error signal representative of the deviation of the ratio of said lead and lag parameters from a preselected ratio;    and adjusting means for adjusting said lag parameter to reduce said deviation in response to said deviation exceeding a preselected deviation.    
   
   
       16 . A control system as claimed in  claim 15  further comprising: 
 threshold value means for providing a preselectable threshold value;    comparator means for comparing said error signal with said preselectable threshold value;    and wherein said adjusting means is operable to adjust said lag parameter in response to said error signal exceeding said preselectable threshold value.    
   
   
       17 . A control system as claimed in  claim 16  wherein: 
 said threshold value means comprises a first, upper threshold value means for providing a first, upper preselected threshold value and a second, lower threshold value means for providing a second, lower preselectable threshold value, thereby to define an error range;    said comparator means is operable to compare said error signal with said upper and lower preselectable threshold values;    and said adjusting means is operable to adjust said lag parameter in response to said error signal falling outside said error range.    
   
   
       18 . A control system as claimed in  claim 17  wherein: 
 said adjusting means is operable to adjust said lag parameter to reduce said ratio in response to said error signal being above said error range and to adjust said lag parameter to increase said ratio in response to said error signal being below said error range.    
   
   
       19 . A control system as claimed in  claim 18  wherein: 
 said first, upper preselected threshold value is a positive value and said second, lower preselected threshold value is a negative value;    a positive error signal indicates an increase in said ratio from said preselected ratio and a negative error signal indicates a decrease in said ratio from said preselected ratio;    and said adjusting means is operable reduce said ratio in response to said error signal being positive and exceeding said first, upper preselected threshold value and to increase said ratio in response to said error signal being negative and exceeding said second, lower preselected threshold value.    
   
   
       20 . A control system as claimed in  claim 16  wherein the or each said threshold value is a fixed value.  
   
   
       21 . A control system as claimed in  claim 16  wherein the or each said threshold value is a function of the value of said lag parameter.  
   
   
       22 . A control system as claimed in  claim 21  further comprising adjusting means for adjusting said threshold value in dependence on the value of said lag parameter and wherein said comparator means is operable to compare said error signal with said adjusted threshold value.  
   
   
       23 . A control system as claimed in  claim 22  wherein: 
 said threshold value means comprises a look up table for storing a plurality of threshold values;    and said threshold value adjusting means is operable to adjust said threshold value by selecting one of said threshold values in dependence on the value of said lag parameter.    
   
   
       24 . A control system as claimed in  claim 21  further comprising: 
 adjusting means for adjusting said threshold value in dependence on the value of said lag parameter;    and wherein:    the or each threshold value comprises a plurality of threshold levels;    said threshold value adjusting means is operable to adjust said threshold value by selecting one of said threshold levels in dependence on the value of said lag parameter;    and said comparator means is operable to compare said error signal with said selected threshold level.    
   
   
       25 . A control system as claimed in  claim 24  wherein: 
 said threshold value means comprises a look up table for storing a plurality of threshold values;    and said threshold value adjusting means is operable to adjust said threshold value by selecting one of said threshold values in dependence on the value of said lag parameter.    
   
   
       26 . A control system as claimed in  claim 15  wherein: 
 said lead and lag parameters are lead and lag fluid flow rates for said process;    and said control system comprises:    lag valve means for controlling the flow of said lag fluid;    position monitoring means for monitoring the position of said lag valve means during opening or closing of said lag valve means and providing a position signal representative thereof;    wherein:    said lag monitoring means is operable to monitor the flow rate of said lag fluid during opening or closing of said lag valve means and provide a flow rate signal representative thereof;    and said control system further comprises:    storage means for storing sampled values of said position and flow rate signals representing a preselected characteristic of said lag valve means;    second comparator means for comparing the valve position of said lag valve means and lag fluid flow rate during movement of said lag valve means with the stored values;    and second adjusting means for adjusting said threshold value in response to said comparison indicating a change in the characteristic of said lag valve means.    
   
   
       27 . A control system as claimed in  claim 15  wherein: 
 said lead and lag parameters are lead and lag fluid flow rates for said process;    and said control system comprises:    lag valve means for controlling the flow of said lag fluid;    position monitoring means for monitoring the position of said lag valve means during opening or closing of said lag valve means and providing a position signal representative thereof;    wherein:    said lag monitoring means is operable to monitor the flow rate of said lag fluid during opening or closing of said valve means and provide a flow rate signal representative thereof;    and said control system further comprises:    storage means for storing sampled values of said position and flow rate signals representing a preselected characteristic of said lag valve means;    second comparator means for comparing the lag fluid flow rate during movement of said valve means with stored value corresponding to the monitored position of said lag valve means;    and second adjusting means for adjusting said threshold value in response to said comparison indicating a change in the characteristic of said valve means.    
   
   
       28 . A control system as claimed in  claim 15  wherein: 
 said lead parameter is the flow rate of a combustion gas;    said lag parameter is the flow rate of air;    and said process is a combustion process.    
   
   
       29 . A method as claimed in  claim 8  wherein: 
 said lead and lag parameters are lead and lag fluid flow rates for said industrial process;    and said method comprises:    providing lag valve means for controlling flow of said lag fluid;    monitoring the position of said lag valve means during opening or closing of said valve means;    monitoring the flow rate of said lag fluid during opening or closing of said valve means;    comparing the change in position of said valve means with the change in said flow rate of said lag fluid;    and adjusting said threshold value in response to said comparison indicating a change in the characteristic of said valve means.