Giving Up the Finite State Machine in the Control of Lower-Limb Wearable Robots?

The gait cycle is typically viewed as a periodic sequence of discrete events, starting with heel contact during initial stance and ending with knee extension during late swing. This convention has informed the design of control strategies for powered prostheses and orthoses, which almost universally utilize the concept of a finite state machine (FSM), e.g., the five states in Fig. 1. Each state (or phase) of the FSM has a separate control model that usually enforces joint impedances [1] or tracks patterns of joint angles, velocities, or torques. These prosthetic legs switch between control models based on switching rules or estimates of gait cycle phase that rely on multiple sources of sensory feedback. The higher-level state machines required for multiple ambulation modes (e.g., level ground, ramps, and stairs) can involve hundreds of control and switching parameters, presenting a critical challenge to the clinical viability of powered prostheses and orthoses [2].