Dead time – the quiet one

Of the three model parameters, dead time is the one that decides how aggressive you are allowed to be, and it is the one people underestimate. A loop with a long flat delay before it responds cannot be tuned tight no matter how good the controller – the controller is always reacting to old news.

The ratio that matters

What governs tuning is the ratio of dead time to time constant, L/T. A small ratio (delay short compared with the lag) is an easy loop you can tune briskly. A ratio near or above one – delay as long as the lag – is a hard loop where tight gains guarantee oscillation; the honest move is to loosen the target, not to fight it.

Where dead time hides

Transport lag in a long pipe, a sample-and-hold analyser, a controller scan slower than the loop, a sensor with its own filter. Flintloop measures L from the flat stretch before the response and shows it; if L jumps between captures, something in the sampling chain changed, not the process.

What Flintloop does about it

The tuning rules take L directly: a larger dead time pushes the gains down and the closed-loop time constant up on its own. You do not compensate by hand – but seeing L lets you recognise a loop that no gains will fix, which is worth more than a number.