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Sep 25, 2025
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doc: minor precision in strictly proper system assumption #263

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13 changes: 9 additions & 4 deletions src/model/linmodel.jl
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Expand Up @@ -135,10 +135,15 @@ LinModel with a sample time Ts = 0.1 s:
state-space representation.

It is assumed that ``\mathbf{D_u=0}`` (or `sys` is strictly proper) since otherwise the
resulting discrete controller is acausal by definition. Indeed, all discrete controllers
(1) sample an output ``\mathbf{y}(k)`` from the plant, (2) computes an action
``\mathbf{u}(k)`` and (3) apply the action on the plant. There is a causality paradox
if ``\mathbf{u}(k)`` impacts ``\mathbf{y}(k)`` even before computing it.
resulting discrete controller would be acausal in its implementation (≠ mathematical
causality). Indeed, at each sampling instant `k`, the predictive controller will:

1. sample an output ``\mathbf{y}(k)`` from the plant
2. computes an action ``\mathbf{u}(k)`` and
3. apply the action on the plant.

There is a causality paradox if ``\mathbf{u}(k)`` impacts ``\mathbf{y}(k)`` even before
computing it.
"""
function LinModel(
sys::StateSpace{E, NT},
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