Torque-feedback controller

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Torque feedback algorithm

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

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

  • 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
  • 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.