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Jun
08

Deadtime’s Secret Identity – Part 2

What steps can be taken to make the real loop deadtime step forward? Last week we found that a step made in the controller setpoint rather than its output for a controller gain less than one increases the deadtime because of the time it takes for the controller output to work through the valve deadband and resolution. Slower tuning makes the deadtime larger. Subsequent increases in Lambda factors for additional robustness can get the user into a downward spiral in terms of loop performance (slower tuning -> larger deadtime -> slower tuning -> larger deadtime).

Additionally, small steps in the signal to control valves, particularly those with pneumatic positioners, have a dramatic effect on valve response time and hence loop deadtime. The following tests show that the response time of positioner can increase from 1 second to 100 seconds when the step size is decreased from 10% to 0.2%. While you may not be making such a small change in controller output, consider that a 1% change in setpoint to a controller with a 0.2 gain translates to a 0.2% step in the signal to the valve.

Effect of Step Size on Positioner Response

For slow loops like tank level and temperature, the time it takes for a change in the process variable to work through the resolution limit or noise band of the measurement creates another increase in deadtime. For a 1980s vintage DCS with 12 bit A/D (one sign bit) wide range thermocouple cards, the resolution limit of about 0.25 degrees adds significant deadtime besides loop A/D noise. The additional deadtime can be estimated as the measurement resolution divided by the rate of change of the process variable. For a temperature loop changing 0.05 degrees per minute from a step change in controller output, a resolution limit of 0.25 degrees can add 5 minutes of loop deadtime.

I remember trying to use an auto tuner on level loops on large tanks and waiting what seemed like forever for the measurement to get out of the noiseband. I quickly realized that I needed to take larger steps to drive the level faster before the auto tuner or my brain timed out.

Where tight control is needed for slow level and temperature loops, the controller is normally tuned with a controller gain much larger than one. This is a tip that the step changes in the controller output should be large so you are not waiting till the cows come home to see the process variable stir. For more Texas talk, see my Control Talk Column “Puzzler Roundup” in the July issue of Control magazine.