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PID Control Algorithm: |
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Chapter 2
The Process Response to the Controller
Steady state relationships:
Relating valve change to measurement change
Steady state relationships:
changing load
When the load changes, either the process value changes or the valve position must be changed to compensate for the load change.
Process Dynamics: Simple lag
Process Dynamics: Dead time
Dead Time: A delay in the loop due to the time it takes material to flow from one point to another
Also called: Distance Velocity Lag
Transportation Lag
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Most loop combine dead time and lag.
Measurement of dynamics
The dynamics differ from one loop to another.
However, they usually result in a response curve like this:
L is Lag—the largest lag in the process loop.
D is "Pseudo Deadtime"—the sum of the deadtime and all lags other than the largest lag.
Disturbances
Almost all processes contain disturbances.
Disturbances can enter anywhere in the process.
The effect of the disturbance can depend on where it enters the loop.
Most disturbances cannot be measured.
go to chapter 3: the PID algorithm
