BeginnerActuator Fundamentals · Part 1

Series Elastic Actuators: Force Control from First Principles

The single most useful idea in compliant actuation, explained from scratch: insert a spring between motor and load and force becomes something you can measure and command directly.

4:2843K viewsAug 21, 2017#series-elastic-actuators#force-control#compliance#impedance-control

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Timestamps jump straight to the matching moment in the video.

A rigid gear train hides force. You command position, and any collision goes straight into the gearbox as a shock load.

Connect the motor to the load through an elastic element. Now the element deflects in proportion to force, so measuring deflection measures force.

Force fidelity, shock tolerance and compliance all come from the same trick. It is why prosthetics and exoskeletons rely on it.

A softer spring is more sensitive and less stiff. Force response slows down, so effective stiffness becomes a deliberate tuning choice.

Once deflection is your force signal, you can close an impedance loop and make the joint behave like a programmable spring-damper.

Code

def actuator_force(deflection_m: float, stiffness_n_per_m: float) -> float:
    return deflection_m * stiffness_n_per_m


def impedance_command(x, x_des, v, v_des):
    K = 800.0   # N/m
    B = 12.0    # N*s/m
    return K * (x_des - x) + B * (v_des - v)


if __name__ == "__main__":
    print(actuator_force(0.004, 25000))

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