Before tuning calculations are applied in a control loop, it is good to consider how much the process gain and deadtime can change before the loop becomes unstable. In the DeltaV control system, this information is provided using a robustness plot that shows gain margin and phase margin as illustrated below.
Information on this type of robustness plot is provided in Chapter 4 of Advanced Control Foundation – Tools, Techniques, and Applications. In this chapter, gain and phase margin are described in the following manner:
- Gain margin simply means how many times loop gain may be increased before reaching a stability limit
- Phase margin is the phase shift that is added to the loop phase shift at the gain crossover frequency to make a total phase shift of 180 degrees.
The robustness map presents a range of reasonable tuning parameters from which the usercan select. Users can simply click a point to see a set of tuning parameters and view the responses. Significant variation in the setpoint overshoot in shown below for different tuning points on the robustness map.
The Solution for the On-demand workshop in Chapter 4 may be accessed at the ControlLoopFoundation web site. The solution is a YouTube video that include a demonstation that show how the Robustness map is used in tuning a control loop.