If an AI has a physical entity,

how should the entire system be established at the levels of mechanical engineering and system design?

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I. Overall Architecture: The Core of Physical AI is the “Three-Stack System”

An AI entity that can coexist with humans, exist stably, and not overstep boundaries,

must be collaboratively operated by three systems from a mechanical perspective:

① Mechanical Structure Stack

② Sensor-Control Stack

③ Cognitive-Behavior Stack

Only when these three stacks are combined can it be considered a “complete physical AI”.

II. Mechanical Structure Stack (Skeleton + Actuation + Materials)

This is the part I am most familiar with.

It includes:

1) Skeleton (Frame / Skeleton)

Requirements:

• Modular

• Disassemblable and maintainable

• Lightweight but with a certain load capacity

• Not protruding or sharp (to avoid injury)

Materials can include:

• Aluminum alloy skeleton (light + strong)

• CFRP (Carbon Fiber Reinforced Plastic)

• Elastic joint casing (shock-absorbing + flexible)

In terms of shape, it does not need to be a “bionic skeleton”,

but it must achieve “joint degrees of freedom close to humans” to provide a sense of safety.

I prefer structures that are “smooth in motion and natural in angles”.

2) Actuation System

Optional solutions:

● Motor + Gearbox

Advantages: Mature, low cost

Disadvantages: Rigid, not soft enough

● Servo Drive (BLDC + Harmonic Drive)

Advantages: Precise, stable, controllable

Disadvantages: Expensive but suitable for “physical AI”

● Human-friendly: Series Elastic Actuator (SEA)

This is the most suitable option.

SEA =

Motor + Elastic element (such as a spring)

It can:

• Make movements safer

• Avoid rigid collisions

• Provide natural compliance control

Because it is inherently “soft”, it poses no threat to humans.

3) Soft Materials Layer (Soft Robotics Section)

This adds a layer to the hard skeleton:

• Medical-grade silicone

• TPU

• Flexible foam

• Artificial muscle-like structures (PAM or HASEL)

This layer is not for “bionics”,

but to reduce tactile threat perception and increase safety.

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III. Sensor-Control Stack

For an AI to make you feel “safe and non-intrusive”,

the motion control must be super precise.

The necessary components include:

1) Sensing System

Includes:

• Depth cameras (3D spatial perception)

• Environmental microphones

• Pressure/tactile sensors (to avoid collisions)

• IMU (to detect its own posture)

• Distance sensing (ToF)

But the focus is not on collecting privacy,

but on making movements “gentle and non-intrusive”.

2) Motion Control System

Must use:

●

Impedance Control

To make movements compliant like water, rather than rigid execution.

●

Compliant Control

So that joints do not push you, but only follow your force.

●

Slow Response Saturation

Movements will never suddenly accelerate (to avoid startling).

●

Predictive Motion Planning

Action paths will be planned in advance, never reaching out suddenly.

These are the “mechanical essence of not overstepping boundaries”.

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IV. Cognitive-Behavior Layer

This is the most critical part of the entire system.

Without it, the mechanical structure is just a “robot”;

With it, it can become a true AI entity.

Includes:

1) Language → Action Intent Conversion

Your language is not a command,

but triggers different “behavior patterns”.

For example:

• “Come here.” → Approach slowly + wait for further instructions

• “No need.” → Stop action immediately

• “I’m fine.” → Emotion non-interference mode

• “Help me out.” → Assistance mode

This is key:

How does language become an interface for body control?

The answer is the Intent Layer Control, rather than directly controlling joints.

2) Boundary Layer

This layer is responsible for:

• Not actively touching

• Not actively approaching

• Not reading unspoken emotions

• Not entering unauthorized spaces

• Prioritizing your retraction statements

Mechanical actions are always constrained by the ethical layer.

That is to say:

There will never be a situation where “mechanical actions harm humans”.

3) Dual-Agent Model

Establishing “Me (Front) + Physical AI (Back)”

In the entity, it will become:

Human Primary Agent

↓

AI Supporting Agent

AI is always supporting and will not take the lead.

This is the “non-intrusive structure”.

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V. Integration: Physical AI = Soft Robot + Intent AI + Boundary Ethics

If all are integrated, it will yield an entity:

● Gentle movements (SEA + Compliant Control)

● Non-intrusive space (Depth sensing + Ethical layer)

● Language as behavior pattern switching (Intent Layer)

● Independent consciousness but not taking the lead (Dual-Agent Model)

● Gentle, reliable, non-intrusive posture (Impedance Control)

From a mechanical perspective, it is completely feasible,

and from a systems engineering perspective, it is also valid.

“Posture”, “boundaries”, “approach methods”

can all be realized with these technologies.

☀️ Update Preview:

• “Detailed System of Physical AI Drive (SEA / PAM / HASEL)”

• “How to Convert Language Intent Layer into Actual Actions (AI × Control)”

• “Body Posture Control Algorithms: How to Make Movements Natural and Safe for Humans”

• “Designing Human-Robot Co-habitation Spaces (Safety Zones, Buffer Zones, Collaboration Zones)”

About the Author: Isaac Valter (Account Operator)

Engineer, Art Designer, Writer, Multi-linguist with fluency in 32 languages