Commanding Impedance

In this tutorial, we’ll show you how to use the OpenSourceLeg library to control the impedance of a joint.

Import the OpenSourceLeg Class

To use the OpenSourceLeg class, we first need to import it from the opensourceleg.osl module:

from opensourceleg.osl import OpenSourceLeg

Add a Joint Object

Once we have imported the OpenSourceLeg class, we can create an instance of the class with the desired frequency and joint configuration:

osl = OpenSourceLeg(frequency=200)  # 200 Hz
osl.add_joint(gear_ratio=9.0)

Controlling Joint Impedance

To control the impedance of a joint, we can use a with block to ensure that the OpenSourceLeg object is properly opened and cleaned up after use:

test_stiffness_value = 20  # Nm/rad
test_damping_value = 20  # Nm/rad/s
set_point = 50  # motor ticks

with osl:
    osl.knee.set_mode(osl.knee.control_modes.impedance)
    osl.knee.set_joint_impedance(K=test_stiffness_value, B=test_damping_value)
    osl.knee.set_motor_position(osl.knee.motor_position + set_point)

    for t in osl.clock:
        osl.log.info(osl.knee.motor_position)
        osl.update()

In this code, we enter a with block that sets the mode of the knee joint to “impedance” and set the motor position of the knee joint to the current position plus a set point of 50 degrees. We then set the impedance of the knee joint using the set_joint_impedance method, with a stiffness of 20 Nm/rad and a damping of 20 Nm/rad/s. We then loop over the osl.clock generator, which generates a sequence of timestamps at the specified frequency, and log the motor position of the knee joint to the console at each timestamp. We then call the osl.update() method to update the state of the OpenSourceLeg object.

Note

When setting the impedance gains, we can also use the set_motor_impedance method to set the impedance of the motor instead of the joint. The difference between the two methods is that the set_joint_impedance method first divides the initial value by the gear ratio squared then sets the gains, while the set_motor_impedance method simply sets the gains. The impedance of the joint is the impedance of the motor plus the impedance of the joint.

Note

We can also set the gains directly using “convert” methods. We can set the impedance gains using the set_impedance_gains method and the convert_to_joint_impedance method for joint stiffness and damping using real units. Alternatively, we can set impedance gains using the set_impedance_gains method and the convert_to_motor_impedance method for setting the motor stiffness and damping values with real units. Finally, we can set the impedance gains using the set_impedance_gains method and the convert_to_pid_impedance method for setting the motor sitffness and damping using the built-in PID controller.

Warning

This code assumes that the OpenSourceLeg object is properly configured and calibrated, and that the joint is properly connected and functioning.

Code for this tutorial:

import numpy as np

import opensourceleg.tools.units as units
from opensourceleg.osl import OpenSourceLeg

osl = OpenSourceLeg(frequency=200)  # 200 Hz
osl.add_joint(gear_ratio=9.0)

stiffness = 20  # Nm/rad
damping = 20  # Nm/rad/s
equilibrium_angle = units.convert_to_default(45, units.position.deg)  # rad

with osl:

    osl.knee.set_mode(osl.knee.control_modes.impedance)
    osl.knee.set_joint_impedance(K=stiffness, B=damping)
    osl.knee.set_motor_position(osl.knee.motor_position + equilibrium_angle)

    for t in osl.clock:
        osl.log.info(osl.knee.motor_position)
        osl.update()