Torque-feedback controller

Torque feedback algorithm

• The torque command from the ankle is a function of ankle state. Specifically:

T_out = ES[Kes*Theta + Bes*Thetadot] + LS,LSP[Kls*Theta + Bls*Thetadot] + LSP[PFF/(T_meas/PCI)^Exp]

• • Starts transition using the T_meas ~ (T_meas-T_enter_LSP), then smoothly transitions to just using T_meas
• ES[expression] means the expression is only executed during the specified STATE (ES=early stance, LS=late stance, LSP=late stance power)
• T_out = output torque command
• T_meas = measured torque (sensors.EstAnkleTorque_f)
• Theta, Thetadot = Ankle angle, angular velocity (positive = dorsiflexion)
• Kes, Bes = Virtual spring, damper constants for early stance
• Kls, Bls = Virtual spring, damper constants for late stance
• PFF = Positive force feedback gain
• PCI = Power curve intercept (based on subject's weight (see tuning interface for details on setting weight)
• Exp = Exponent

• T_meas is determined by taking the difference between the motor encoder position and the output ankle joint position, using the measured series spring constant to determine ankle torque.
• T_out is provided using current-control.

• In practice, it is likely that PFF mainly affects the net work done by the ankle, while Exp mainly affects the timing. This has yet to be verified.

Walking parameters

In c code: walking_param

• mom_thresh_power_on = estimated ankle torque at entry to late stance power