Force-based stability margin for multi-legged robots

• FFSM stands for Foot Force Stability Margin, a new force-based stability criterion for legged robots.
• FFSM has been modified (MFFSM) to adjust stability sensitivity of the robot.
• FFSM/MFFSM can be used to determine system stability in real-time usable for stability control.
• FFSM and MFFSM are compared with the Force Angle Stability Margin, and the Zero Moment Point.
• Both simulation and physical experiments validate the proposed stability margin (FFSM and MFFSM).

Stability analysis of multi-legged robots is necessary for control especially under dynamic situations. This paper presents the Foot Force Stability Margin, a force-based stability margin that utilizes measured contact normal foot forces as the stability metric, simplifying data and computational requirements. The Foot Force Stability Margin assumes tumbling instability and sufficient friction to prevent slippage. A modified extension of the Foot Force Stability Margin is provided to enhance stability sensitivity to desired robot characteristics. The Foot Force Stability Margin and its modified variant were validated through numerical and physical experiments. The numerical simulations compare the Foot Force Stability Margin with its modified variant, the Force Angle Stability Margin, and the Zero Moment Point. The physical experiments were conducted using a Lynxmotion hexapod robot retrofitted with a Gumstix and force sensitive resistors. The experimental results confirm that the Foot Force Stability Margin and its modified variant are accurate, simple with regard to computational cost, sensitive to robot characteristics, and applicable to flat and uneven terrain, making it practical for use within on-line controllers.