Stability margins

Gains that track the setpoint perfectly on paper can ring or run away on the plant, because the model is approximate and the plant drifts. Margins are the cushion between "works now" and "oscillates when something changes". Flintloop tunes to a margin, not to a textbook-perfect response.

Gain margin

How much the loop gain could rise before the loop became unstable, as a multiple. A gain margin of 2.0 means the loop gain would have to double – through a stickier valve, a hotter day, a process that got livelier – before it sang. Below about 1.7 you are tuning for the demo, not the plant.

Phase margin

How much extra phase lag the loop could take before instability, in degrees. 45° is the common target: enough damping that a disturbance settles without a long ring, not so much that the loop is sluggish. Flintloop shows both margins for the gains it proposes and marks them against your target.

Why those defaults

Gain margin ≥ 2.0 and phase margin ≥ 45° are the conservative pair most plant engineers already use; they leave room for the model being rough and the process moving. You can lower them for a well-behaved loop you know, but the program will not do it for you.