Deadtime sets the ultimate limit to loop performance. Deadtime is any delay in the action needed to exceed the threshold for the start of error correction. The loop deadtime is the sum of pure actuation, correction, or recognition delays and the equivalent deadtime from lags smaller than the largest time constant in the loop. For …
Search results for: deadtime
Dec
30
Universal Concept – Deadtime
The most important parameter for process control is the easiest to identify. The time after a change in the PID output till you see a significant change in the process variable is the total loop deadtime. Deadtime is directly or indirectly a factor in all aspects of control system design and tuning for loop performance. …
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 …
Jun
01
Deadtime’s Secret Identity – Part 1
For a constant flow and set of process operating conditions, is the observed total loop deadtime relatively constant? We know from last week’s blog, the deadtime also depends upon the sensor time constant and hence it’s fouling. Less recognized is that it depends upon whether a step change is made in the controller output versus …
Oct
30
Control of Deadtime Dominant Processes
A small fraction of the control loops in industry are characterized by the process deadtime being dominant i.e. greater than the process time constant. In most cases the source of the process deadtime is associated with transport delay or analyzer sample time for the process measurement. In many cases the loop directly impacts final product …
Aug
05
Fuzzy Logic Control – Implementation
To minimize the time needed to apply fuzzy logic control, most manufacturers offer a version of fuzzy logic control in which the rules and membership functions are pre-defined. As described in Chapter 6 of Advanced Control Foundation – Tools, Techniques, and Applications, , a manufacturer can provide support for auto-tuning by restricting the fuzzy logic …
Jul
29
Fuzzy Logic Control
Fuzzy logic is a relatively new control technology. In the mid-1970s Mamdani began to investigate fuzzy logic as an alternative to the PID controller and sparked the first practical applications of fuzzy logic in the process control industry. The controller followed rules that generated an output by evaluating the error and the change in error. …
Jul
08
Adaptive Tuning – Models Identified
The basis of adaptive tuning is the identification of the process step response based on operator changes in the loop operation. For example, when a control loop is in Automatic mode, changes made in the setpoint can trigger process model identification. If the loop mode has been changed to Manual, then changes by the operator …
Jul
01
Adaptive Tuning – Model Switching
During the development of the DeltaV adaptive tuning capability, we researched and evaluated many of the most common techniques for implementing adaptive tuning. Also, as part of this research we came across a number of technical articles on a relative new adaptive tuning technique known as model switching. This technique depends on the evaluation of …
Jun
24
Adaptive Tuning
A perfectly-tuned PID controller can degrade over time and perform poorly or become oscillatory. There are two main reasons for these changes: The controlled process is non-linear, and the process operation has entered a region with significantly different process parameters than those used during the tuning. The process operating conditions have changed since the auto-tuning …
