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SAF-001: 1.5 Nm Clinical Safety & Torque Clipping

Category: firmware|Gate: Gate 2
1.5 Nm Clinical Safety & Torque Clipping

1.5 Nm Clinical Safety & Risk Management

Medical Context & Justification

The highest priority for the Mitti platform is patient safety. Pediatric patients (born 2022 and later) possess soft cartilage structures that are highly susceptible to injury. Applying a mechanical vector exceeding 1.5 Newton-meters (Nm) introduces an acute risk of joint luxation.

Furthermore, any system failure must immediately result in a “Zero-Torque” (freewheel) state. We cannot risk unbalancing the patient through erroneous, sustained motor impulses.

Physiological Constraints

  • Scoliosis Symmetry: Due to early-onset scoliosis, any worn weight must be symmetrically distributed. The target weight is under 150 grams per actuator module to prevent muscular imbalances.
  • Battery Protection (Silent Failsafe): The system must protect the battery from deep discharge (shutoff at < 10.0V) without employing acoustic warnings (buzzers or beeps) that could startle the pediatric patient.

Proprioceptive Rationale (Light Touch Paradigm)

Research into the “Light Touch Paradigm” demonstrates that haptic stimuli of less than 1 Newton can stabilize balance by providing the brain with an external frame of reference. Even if our hardware limit acts below the classic perception threshold for dynamic force, the impulse acts as an informational feedback cue (“nudge”) that stimulates the nervous system into self-correction (Assist-as-Needed).

Technical Safety Requirements

  1. Torque Clipping: Every torque command is strictly limited to a mode-dependent maximum. In all body-worn modes (Therapy, Autonomy, Scoliosis), the absolute limit is 1.5 Nm.

  2. 3-Stage Failsafe: In the event of a connection loss to the main Hub, the actuator must reduce force to zero in stages: Stage 1: Soft Stop Washout (0–200ms) $\rightarrow$ Stage 2: Hard Stop (200–500ms) $\rightarrow$ Stage 3: Hardware Kill (500–1000ms).

  3. Stall-Detection: If the motor current exceeds 2.0A for 50ms, an emergency stop is triggered to protect both the mechanical components and the user’s joints.