Why single-axis motion requires single-actuator control.
The structural requirement that each axis of motion be mechanically isolated and independently controllable, as a direct consequence of rigid-body kinematics.
Independent Degrees of Freedom: Each axis of motion must be controlled by its own dedicated actuator. One rotation happens by itself, not requiring the other rotations to occur in order for it to work.
Failure Consequence: If axes are coupled, the system cannot deliver clean single-axis motion cues, cannot isolate rotational behavior, and cannot achieve true synchronization with visual and vestibular outputs.
In the real world, vehicle motion along each axis occurs independently. When a car rolls during cornering, that motion happens around the longitudinal axis without requiring pitch or yaw to simultaneously occur. Each degree of freedom operates on its own.
For true physics recreation, simulation systems must mirror this independence. Each axis of motion must be controlled by its own dedicated actuator, free from mechanical coupling with other axes.
Rotation around the longitudinal axis. Occurs independently during cornering and does not require yaw or pitch to function.
Rotation around the lateral axis. Happens independently during acceleration and braking, requiring no roll or yaw.
Rotation around the vertical axis. Functions independently when the vehicle changes direction, separate from roll and pitch.
Roll does not need yaw to operate. Pitch does not need roll to occur.
Why would an actuator assigned to roll also deliver pitch?
The answer: coupled architecture is a structural compromise, not a technical solution.
When coupled or dependent degrees of freedom are present, the actuator responsible for roll must also work for pitch, and for yaw if yaw is present on the system.
Result: The system cannot accurately recreate real-world physics because movements are mechanically interdependent.
Each degree of freedom has its own dedicated actuator that operates completely independently from all other axes.
Result: The system accurately recreates real-world physics with mechanically independent motion along each axis.
When actuators are coupled or dependent, fundamental physics laws governing independent-axis motion are violated. Incorrect axis coupling results in inaccurate motion cues, cross-axis contamination, and degraded training validity.
A system that presents roll cues blended with pitch information cannot train correct single-axis perception. The nervous system learns to interpret a geometrically approximate signal rather than a physically valid one. This distinction cannot be corrected at the output layer once introduced at the mechanical level.
Independent degrees of freedom are a structural requirement for physics-accurate simulation. Systems that couple actuators are choosing mechanical simplicity over physical correctness.
This is why hexapod platforms and other coupled-actuator designs, despite moving through three-dimensional space, do not operate in true 3D. They create motion, but that motion is geometrically approximated rather than physically correct.
True simulation fidelity requires independent actuator control for each degree of freedom. One axis of motion requires one actuator. Anything less is a structural compromise that limits training validity regardless of other system parameters.
Yaw emerges as the primary rotational cue when degrees of freedom are implemented correctly. Full treatment is provided on the yaw in simulation page.
The source requirement from which independent DOF follows
The required origin for all vehicle motion and rotation
The dominant motion cue that independent DOF enables
The structural conditions a system must satisfy before measurement