Safety Architecture: Three-Layer Protection System

1. Safety as an Architecture, Not a Feature

Most robotic mower manufacturers describe safety as a feature list: bump sensor, lift detection, tilt detection. These are individual mechanisms, not an architecture. A safety architecture differs from a feature list in three ways:

1. Defense in depth — multiple independent systems such that single-point failures do not compromise overall safety
2. Temporal coverage — different systems address different phases of a safety event (before, during, after)
3. Independence — each layer operates autonomously, not dependent on the others

2. Temporal Phases of Safety Events

Safety events involving autonomous lawn mowers have three temporal phases:

Conventional mowers typically address only the reactive phase (post-event). Volta's architecture addresses all three.

3. Layer 1: EN 60335-2-107 Compliance

EN 60335-2-107 is the international safety standard for robotic lawn mowers. It defines mandatory safety requirements including:

• Lift detection: Blade motor stops when the unit is lifted above the ground surface
• Tilt detection: Blade motor stops when the unit tilts beyond safe operating angles
• Blade halt mechanisms: Mechanical and electrical cutoff systems for blade motor
• Emergency stop: Manual and automated emergency stop capability

The Lawn Companion complies with EN 60335-2-107 as its foundational safety layer. This layer is reactive — it responds to abnormal states after they occur.

4. Layer 2: Privacy-Preserving Predictive Vision

The downward-facing camera provides a second safety function beyond agronomic perception: predictive path analysis. By analyzing the visual field ahead of the robot's trajectory, the system can detect obstacles before physical contact occurs.

Key characteristics:

• Temporal advantage: Detection occurs seconds before potential contact, enabling path alteration
• Privacy-preserving: The downward-facing camera does not capture personal data (see Privacy Architecture whitepaper)
• Complementary to contact detection: Vision provides pre-contact awareness; the hexoskeleton provides contact-level response

This layer addresses the predictive phase — identifying and avoiding safety events before they begin.

5. Layer 3: Floating Hexoskeleton

The hexagonal outer shell is a structural safety system. The "floating" design means the shell has slight displacement capability relative to the chassis — when external pressure is applied from any direction, the displacement is detected and triggers immediate blade motor shutdown.

5.1 Omnidirectional Sensing

The hexagonal geometry provides a critical property: uniform sensing in all directions. Unlike rectangular or elongated robot bodies that have different sensitivity profiles front-to-back vs. side-to-side, the hexagonal shell provides approximately equal contact detection sensitivity at every point around its perimeter.

5.2 Hexagonal Venation

The shell's hexagonal surface pattern distributes impact forces across the structure rather than concentrating them at a single point. This both improves contact detection (force distribution triggers sensors more reliably) and improves structural durability.

5.3 Enclosed Wheels

The wheels are fully enclosed within the hexagonal shell. This design means no exposed rotating components on the exterior, no gap where limbs could be trapped, and a continuous external surface with no openings.

5.4 Contact Response

Upon detecting contact through the hexoskeleton, the system:

1. Immediately halts the blade motor
2. Stops wheel motors
3. Logs the contact event
4. Resumes only after the contact condition is cleared

6. Layer Interaction and Independence

Each layer operates on different sensor modalities. A failure in the camera system does not affect the hexoskeleton. A failure in contact detection does not affect lift/tilt sensors. This independence is the core architectural property.

7. Geometric Safety: The Centered Blade

The blade is positioned at the geometric center of the hexagonal shell. This creates a critical property: equal safety margin in all directions.

In conventional robotic mowers with offset or front-mounted blades, the distance from the blade to the edge of the robot varies by direction. This means some approach angles have less safety margin than others. Volta's centered blade creates a uniform safety perimeter: the distance from blade to shell edge is identical regardless of the direction of approach.

8. Comparison with Conventional Designs

9. Limitations and Residual Risk

No safety system eliminates all risk. Documented limitations:

• Vision range: The downward-facing camera has a limited predictive range compared to forward-facing systems
• Small ground-level objects: Very small objects may fall below the detection threshold of both vision and contact systems
• Active interference: The system is designed for passive obstacles; an object actively inserted beneath the shell may bypass contact detection timing
• Supervision recommendation: Volta recommends supervision when the robot operates near small children

10. Evidence Registry