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Mar
04

A Calculation so Simple Yet so Powerful

The simple creation of a rate of change signal by passing a process variable with noise suppression (new PV) through a deadtime block with a deadtime equal to the process deadtime to create an old PV and then subtracting the old PV from the new PV creates a powerful delta PV. The PV one deadtime into the future is the new PV plus the delta PV. If you want an intelligent PV rate of change, divide the delta PV by the deadtime. Some of the potential uses can be seen in the following:

(1) Short Cut Tuning Method and PID Tuning for Near-Integrating Processes

(2) Full Throttle Batch and Setpoint Response and PID Control of True Integrating Processes

(3) Unlocking the Secret Profiles of Batch Reactors

(4) Dynamic Reset Limit

(5) Rapid Model Developement and Deployment See slides 11 and 12 in WebSeminarDemoLab06.pdf

(6) Feed Flow Calculation from Rate of Change of Level or Weight and Reaction Rate Optimization from Rate of Change of Temperature. Use the loop deadtime in the deadtime block on page 111 of the Online E-book Continuous Control Techniques for Distributed Control Systems

(7) Are We Misleading our Operators?

If a calculation was made periodically after a time interval, the discontinuous update would be delayed by the time interval. By the use of a deadtime block, the rate of change calculation is updated every execution of the deadtime block since the block creates a continuous train of old PV values. Now the delay is just the execution time of the deadtime block. For example, if the loop deadtime and thus the time interval is100 seconds, the discontinuous update is delayed 100 sec whereas the deadtime continuous update is delayed only 1 sec. In this case, the traditional discontinuous update method adds 100 times as much deadtime to the loop.