Commanding Position¶

This tutorial demonstrates how to command position to a Dephy actuator using the opensourceleg library. You'll learn how to implement a basic step response test by commanding a position setpoint.

Overview¶

Position control allows direct control of the actuator's angular position. This example shows how to:

Initialize a Dephy actuator in position control mode
Command a position step input
Log and monitor the actuator's response

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 position_control():
    position_logger = Logger(
        log_path="./logs",
        file_name="position_control",
    )
    actpack = DephyActuator(
        port="/dev/ttyACM0", gear_ratio=GEAR_RATIO, frequency=FREQUENCY, debug_level=0, dephy_log=False
    )
    clock = SoftRealtimeLoop(dt=DT)

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¶

    with actpack:
        actpack.set_control_mode(mode=CONTROL_MODES.POSITION)
        actpack.set_position_gains()

        actpack.update()
        current_position = actpack.output_position
        command_position = current_position

        position_logger.track_variable(lambda: actpack.output_position, "Output Position")
        position_logger.track_variable(lambda: command_position, "Command Position")
        position_logger.track_variable(lambda: time.time(), "Time")

Before the main loop, we:

Configure the actuator for position control mode
Initialize position control gains
Get initial position and set up command position
Configure logging variables for tracking positions

4. Control Loop¶

        for t in clock:
            if t > TIME_TO_STEP:
                command_position = current_position + (1 / 2) * np.pi
                actpack.set_output_position(value=command_position)

            actpack.update()

            position_logger.info(f"Time: {t}; \
                                 Command Position: {command_position}; \

The main loop:

Starts at current position
After TIME_TO_STEP, commands a π/2 radian (90 degree) position step
Updates actuator state
Logs time and position data

Running the Example¶

Navigate to the tutorial directory:
```
cd tutorials/actuators/dephy
```
Run the script:
```
python commanding_position.py
```
Expected behavior:
- t < 1.0s: Motor maintains initial position
- t ≥ 1.0s: Motor moves to position + π/2 radians (90 degrees)
- Data is continuously logged to ./logs/commanding_position.csv

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