Commanding Impedance

This tutorial demonstrates how to implement impedance control with a Dephy actuator using the opensourceleg library. You'll learn how to command position setpoints while maintaining compliant behavior through impedance control.

Overview

Impedance control allows for position control while maintaining a specified dynamic relationship between position and force. This example shows how to:

• Initialize a Dephy actuator in impedance control mode
• Command a position step input
• Monitor position tracking and motor current

Code Structure

The tutorial script is organized into four main sections:

1. Configuration

import time

import numpy as np

from opensourceleg.actuators.base import CONTROL_MODES
from opensourceleg.actuators.dephy import DephyActuator
from opensourceleg.logging.logger import Logger
from opensourceleg.utilities import SoftRealtimeLoop

TIME_TO_STEP = 1.0
FREQUENCY = 1000
DT = 1 / FREQUENCY
GEAR_RATIO = 1.0

Key parameters:

• TIME_TO_STEP: Delay before position step (1.0 second)
• FREQUENCY: Control loop rate (1000 Hz)
• GEAR_RATIO: Actuator gear ratio (1.0)

2. Initialization

def impedance_control():
    impedance_logger = Logger(
        log_path="./logs",
        file_name="impedance_control",
    )
    actpack = DephyActuator(
        port="/dev/ttyACM0",
        gear_ratio=GEAR_RATIO,
        frequency=FREQUENCY,
        debug_level=0,
        dephy_log=False,

This section:

• Creates a data logger for recording measurements
• Initializes the Dephy actuator with specified parameters
• Sets up a real-time loop for consistent timing

3. Control Setup

    clock = SoftRealtimeLoop(dt=DT)

    impedance_logger.set_stream_terminator("\r")

    with actpack:
        actpack.update()
        actpack.set_control_mode(mode=CONTROL_MODES.IMPEDANCE)
        actpack.set_impedance_gains()

        current_position = actpack.output_position
        command_position = current_position

Before the main loop, we:

• Configure the actuator for impedance control mode
• Initialize impedance control gains
• Get initial position and set up command position
• Configure logging variables for position and current

4. Control Loop

        impedance_logger.track_variable(lambda: command_position, "Command Position")
        impedance_logger.track_variable(lambda: actpack.motor_current, "Motor Current")
        impedance_logger.track_variable(lambda: time.time(), "Time")

        for t in clock:
            actpack.update()

            if t > TIME_TO_STEP:
                command_position = current_position + np.pi / 2

            actpack.set_output_position(value=command_position)

            impedance_logger.info(
                f"Time: {t}; "
                f"Command Position: {command_position}; "
                f"Output Position: {actpack.output_position}; "

The main loop:

1. Starts at current position
2. After TIME_TO_STEP, commands a π/2 radian (90 degree) position step
3. Updates actuator state
4. Logs time, positions, and motor current

Running the Example

1. Navigate to the tutorial directory:

   cd tutorials/actuators/dephy
   

2. Run the script:

   python commanding_impedance.py
   

3. Expected behavior:

   • t < 1.0s: Motor maintains initial position
   • t ≥ 1.0s: Motor moves to position + π/2 radians
   • Movement will be compliant due to impedance control
   • Data is continuously logged to ./logs/commanding_impedance.csv

If you have any questions or need further assistance, please post on the Open Source Leg community forum.