A system is in-the-loop only when motion, timing, and structure are directly governed by vehicle physics.
"In-the-loop" is not a general description of motion or immersion. It is a structural standard. A system must meet defined criteria before it can be considered valid for in-the-loop training.
This page defines what "In-the-Loop" means as a structural standard — the specific criteria a simulator must meet to be classified at the highest fidelity tier. Plain-language summary below.
In a real vehicle, there is a continuous cycle: the vehicle's physics state produces forces, those forces act on the driver's body, the driver's senses detect them, the driver responds, and that response changes the vehicle's physics state. This cycle repeats in real time. An "In-the-Loop" simulator is one whose motion is directly part of this cycle — the physics generate the motion, and that motion reaches the driver's body in the correct sequence. A system that adds motion separately, or approximates it, is outside this cycle.
An in-the-loop simulation system is one in which the vehicle physics model directly governs the motion state experienced by the participant, with independent degrees of freedom resolved at the vehicle's center of mass and synchronized across all sensory systems.
If motion is added after the fact, approximated through unrelated axes, or detached from vehicle state, the system is not in-the-loop.
The motion output must be driven directly by the simulated vehicle state, not layered on top of it afterward.
Rotation and translation must resolve relative to the vehicle's center of mass.
Rotational and translational axes must operate independently rather than as blended or mechanically coupled substitutes.
Motion, visuals, and control systems must remain synchronized within a valid timing relationship.
The system must deliver usable rotational and translational information that supports correct vestibular interpretation.
The system must preserve timing and structure in a way that supports correct learning and transfer.
Failure of any one requirement compromises in-the-loop validity.
These six requirements support three fundamental criteria defined in What Is The Loop? The criteria describe what the loop requires at a conceptual level; the requirements define how those criteria are structurally satisfied. Both documents must be consulted together for a complete in-the-loop determination.
| Fundamental Criterion | Supporting Requirements |
|---|---|
| A — Causative Accuracy | Req. 1: Physics-Driven Motion & Req. 2: Center-of-Mass Reference |
| B — Temporal Coherence | Req. 3: Independent DOF & Req. 4: Real-Time System Coherence |
| C — Human Response Relevance | Req. 5: Vestibular Relevance & Req. 6: Training Validity |
A moving system is not automatically an in-the-loop system.
The term "in-the-loop" is often used loosely. This standard exists to prevent structural differences from being hidden behind general language.
A system cannot qualify as in-the-loop if it fails to preserve early rotational understanding. Yaw is the primary cue for trajectory, instability, and corrective timing.
| Condition | Outcome |
|---|---|
| Yaw is physics-derived and timely | Predictive control is possible |
| Yaw is delayed, blended, or absent | Reaction becomes late and corrective behavior degrades |
If yaw is not real, the control loop is not real.
This standard defines the threshold conceptually. SFR provides the measurement framework used to evaluate whether the threshold is met.
View SFR Metrics →Systems that satisfy the minimum requirements may qualify as in-the-loop. Systems that do not are classified as out-of-the-loop or surface-level depending on their structure and limitations.
View System Classification →The in-the-loop standard is not merely definitional. It can be applied through structured review and evaluation.
If these conditions are not met, the system is not in-the-loop.
The term "in-the-loop" should only be used when a system meets the required structural and timing conditions. Anything less is a different category and should be classified accordingly.
In-the-loop is earned through structure, not claimed through language.
Framework-based interpretations highlight how different system architectures align with or diverge from training-valid standards.
Apply the framework to a real system, environment, or use case through a structured review pathway.
For teams, facilities, researchers, and organizations seeking structured classification or review.