IntermediateBuild a Robot Actuator · Part 1

Build a Ball-screw Actuator for a Leg Joint

Assemble a linear actuator in the same class as the joints inside Spot-style quadrupeds, using a ball screw, a brushless motor, an ODrive and a magnetic encoder, then measure its real force output.

10:0076K viewsApr 19, 2023#actuators#odrive#ball-screw#force-testing

Build along

Timestamps jump straight to the matching moment in the video.

A ball screw trades rotary motion for linear force at high efficiency, which is why it appears in quadruped legs alongside harmonic drives.

Couple the brushless motor to the screw with a rigid coupler and support the screw at both ends - side loading kills these joints quickly.

Power the ODrive from a 6S pack, connect the motor phases and encoder, and calibrate on the bench before attaching any load.

Use the magnetic encoder for position and let the ODrive close torque. Measured output exceeded 20 kg, roughly 60 Nm.

Pull against a known spring or counterweight to sanity-check the force estimate before trusting it in a control loop.

Code

import odrive
from odrive.enums import (
    AXIS_STATE_CLOSED_LOOP_CONTROL,
    CONTROL_MODE_TORQUE_CONTROL,
    INPUT_MODE_PASSTHROUGH,
)

odrv0 = odrive.find_any()
axis = odrv0.axis0
try:
    axis.requested_state = AXIS_STATE_CLOSED_LOOP_CONTROL
    axis.controller.config.control_mode = CONTROL_MODE_TORQUE_CONTROL
    axis.controller.config.input_mode = INPUT_MODE_PASSTHROUGH
    axis.controller.input_torque = 0.0
except Exception as e:
    print("setup failed:", e)

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