Using Obelisk - Basic Example

Obelisk supplies libraries in both C++ and Python which can be easily installed on your system and used as normal libraries. Obelisk also provides a default ROS2 workspace that can be an underlay or included as a package dependency directly in your ROS2 workspace. The Obelisk workspace will provide access to all the messages and launch files whereas the libraries will allow you to write custom ObeliskNodes. For concrete examples, see this repository.

The libraries provide five main class interfaces:

ObeliskController
ObeliskEstimator
ObeliskRobot
ObeliskSensor
ObeliskNode

In your code, you can write classes that inherit from these classes and thus gain the benefit of being an ObeliskNode.

Below, we will walk through an example where we write code to control a simple two-link robot with a single actuator.

Controller Code

#include "rclcpp/rclcpp.hpp"

#include "obelisk_controller.h"
#include "obelisk_ros_utils.h"

class PositionSetpointController
  : public obelisk::ObeliskController<obelisk_control_msgs::msg::PositionSetpoint,
                                      obelisk_estimator_msgs::msg::EstimatedState> {
  public:
  PositionSetpointController(const std::string& name)
      : obelisk::ObeliskController<obelisk_control_msgs::msg::PositionSetpoint,
                                  obelisk_estimator_msgs::msg::EstimatedState>(name) {}

  protected:
  void UpdateXHat(__attribute__((unused)) const obelisk_estimator_msgs::msg::EstimatedState& msg) override {}

  obelisk_control_msgs::msg::PositionSetpoint ComputeControl() override {
      obelisk_control_msgs::msg::PositionSetpoint msg;

      msg.u.clear();
      rclcpp::Time time = this->get_clock()->now();
      double time_sec   = time.seconds();

      msg.u.emplace_back(sin(time_sec));

      this->GetPublisher<obelisk_control_msgs::msg::PositionSetpoint>(this->ctrl_key_)->publish(msg);

      return msg;
  };
};

Here we can see that PositionSetpointController inherits from ObeliskController and implements the two abstract methods required. ObeliskController is templated on the control message type and the state estimator message type.

ComputeControl and UpdateXHat are automatically registered as callbacks for the timer and subscriber respectively.

import numpy as np
from obelisk_control_msgs.msg import PositionSetpoint
from rclpy.lifecycle import LifecycleState, TransitionCallbackReturn

from obelisk_py.core.control import ObeliskController
from obelisk_py.core.obelisk_typing import ObeliskControlMsg, ObeliskEstimatorMsg, is_in_bound


class ExamplePositionSetpointController(ObeliskController):
    """Example position setpoint controller."""

    def __init__(self, node_name: str = "example_position_setpoint_controller") -> None:
        """Initialize the example position setpoint controller."""
        super().__init__(node_name)

    def on_configure(self, state: LifecycleState) -> TransitionCallbackReturn:
        """Configure the controller."""
        super().on_configure(state)
        self.x_hat = None
        return TransitionCallbackReturn.SUCCESS

    def update_x_hat(self, x_hat_msg: ObeliskEstimatorMsg) -> None:
        """Update the state estimate.

        Parameters:
            x_hat_msg: The Obelisk message containing the state estimate.
        """
        pass  # do nothing

    def compute_control(self) -> ObeliskControlMsg:
        """Compute the control signal for the dummy 2-link robot.

        Returns:
            obelisk_control_msg: The control message.
        """
        # computing the control input
        u = np.sin(self.t)  # example state-independent control input

        # setting the message
        position_setpoint_msg = PositionSetpoint()
        position_setpoint_msg.u = [u]
        self.obk_publishers["pub_ctrl"].publish(position_setpoint_msg)
        assert is_in_bound(type(position_setpoint_msg), ObeliskControlMsg)
        return position_setpoint_msg  # type: ignore

Estimator Code

#include "rclcpp/rclcpp.hpp"

#include "obelisk_estimator.h"
#include "obelisk_ros_utils.h"

class JointEncodersPassthroughEstimator
    : public obelisk::ObeliskEstimator<obelisk_estimator_msgs::msg::EstimatedState> {
  public:
  JointEncodersPassthroughEstimator(const std::string& name)
      : obelisk::ObeliskEstimator<obelisk_estimator_msgs::msg::EstimatedState>(name) {

      this->RegisterSubscription<obelisk_sensor_msgs::msg::JointEncoders>(
          "sub_sensor_setting", "sub_sensor",
          std::bind(&JointEncodersPassthroughEstimator::JointEncoderCallback, this, std::placeholders::_1));
  }

  protected:
  void JointEncoderCallback(const obelisk_sensor_msgs::msg::JointEncoders& msg) { joint_encoders_ = msg.y; }

  obelisk_estimator_msgs::msg::EstimatedState ComputeStateEstimate() override {
      obelisk_estimator_msgs::msg::EstimatedState msg;

      msg.x_hat = joint_encoders_;

      this->GetPublisher<obelisk_estimator_msgs::msg::EstimatedState>(this->est_pub_key_)->publish(msg);

      return msg;
  };

  private:
  std::vector<double> joint_encoders_;
};

Here we can see that JointEncodersPassthroughEstimator inherits from ObeliskEstimator and implements the one abstract method required. ObeliskEstimator is templated on the estimated message type.

ComputeStateEstimate is automatically registered as callbacks for timer. JointEncoderCallback is not automatically registered as a callback since it it not a part of a required component in the node. We can see that it is registered with the std::bind() call in the constructor.

from typing import Union

from obelisk_estimator_msgs.msg import EstimatedState
from obelisk_sensor_msgs.msg import JointEncoders
from rclpy.lifecycle import LifecycleState, TransitionCallbackReturn

from obelisk_py.core.estimation import ObeliskEstimator


class JointEncodersPassthroughEstimator(ObeliskEstimator):
    """Passthrough estimator for joint encoder sensors."""

    def __init__(self, node_name: str = "joint_encoders_passthrough_estimator") -> None:
        """Initialize the joint encoders passthrough estimator."""
        super().__init__(node_name)
        self.register_obk_subscription(
            "sub_sensor_setting",
            self.joint_encoder_callback,  # type: ignore
            key="subscriber_sensor",  # key can be specified here or in the config file
            msg_type=JointEncoders,
        )

    def on_configure(self, state: LifecycleState) -> TransitionCallbackReturn:
        """Configure the estimator."""
        super().on_configure(state)
        self.joint_encoder_values = None
        return TransitionCallbackReturn.SUCCESS

    def joint_encoder_callback(self, msg: JointEncoders) -> None:
        """Callback for joint encoder messages."""
        self.joint_encoder_values = msg.y

    def compute_state_estimate(self) -> Union[EstimatedState, None]:
        """Compute the state estimate."""
        estimated_state_msg = EstimatedState()
        if self.joint_encoder_values is not None:
            estimated_state_msg.x_hat = self.joint_encoder_values
            self.obk_publishers["publisher_est"].publish(estimated_state_msg)
            return estimated_state_msg

Spinning all the Nodes

Obelisk comes with a helper function SpinObelisk to make spinning up your nodes easy.

Controller spin up:

#include "obelisk_ros_utils.h"
#include "position_setpoint_controller.h"

int main(int argc, char* argv[]) {
    obelisk::utils::SpinObelisk<PositionSetpointController, rclcpp::executors::MultiThreadedExecutor>(
        argc, argv, "position_setpoint_controller");
}

State estimator spin up:

#include "jointencoders_passthrough_estimator.h"
#include "obelisk_ros_utils.h"

int main(int argc, char* argv[]) {
    obelisk::utils::SpinObelisk<JointEncodersPassthroughEstimator, rclcpp::executors::MultiThreadedExecutor>(
        argc, argv, "passthrough_estimator");
}

Simulation spin up:

#include "rclcpp/rclcpp.hpp"

#include "obelisk_mujoco_sim_robot.h"
#include "obelisk_ros_utils.h"

int main(int argc, char* argv[]) {
    obelisk::utils::SpinObelisk<obelisk::ObeliskMujocoRobot<obelisk_control_msgs::msg::PositionSetpoint>,
                                rclcpp::executors::MultiThreadedExecutor>(argc, argv, "mujoco_sim");
}

Controller spin up:

from typing import List, Optional

from rclpy.executors import SingleThreadedExecutor

from obelisk_py.core.utils.ros import spin_obelisk
from obelisk_py.zoo.control.example.example_position_setpoint_controller import ExamplePositionSetpointController


def main(args: Optional[List] = None) -> None:
    """Main entrypoint."""
    spin_obelisk(args, ExamplePositionSetpointController, SingleThreadedExecutor)


if __name__ == "__main__":
    main()

State estimator spin up:

from typing import List, Optional

from rclpy.executors import SingleThreadedExecutor

from obelisk_py.core.utils.ros import spin_obelisk
from obelisk_py.zoo.estimation.jointencoders_passthrough_estimator import JointEncodersPassthroughEstimator


def main(args: Optional[List] = None) -> None:
    """Main entrypoint."""
    spin_obelisk(args, JointEncodersPassthroughEstimator, SingleThreadedExecutor)


if __name__ == "__main__":
    main()

Simulation spin up:

from typing import List, Optional

from rclpy.executors import MultiThreadedExecutor

from obelisk_py.core.sim.mujoco import ObeliskMujocoRobot
from obelisk_py.core.utils.ros import spin_obelisk


def main(args: Optional[List] = None) -> None:
    """Main entrypoint."""
    spin_obelisk(args, ObeliskMujocoRobot, MultiThreadedExecutor)


if __name__ == "__main__":
    main()

Obelisk Configuration File

Obelisk nodes can be easily configured via a Obelisk configuration (yaml) file. An example Obelisk configuration file is given here.

config: dummy
onboard:
  control:
    - pkg: obelisk_control_cpp
      executable: example_position_setpoint_controller
      params_path: /obelisk_ws/src/obelisk_ros/config/dummy_params.txt
      # callback_groups:
      publishers:
        - ros_parameter: pub_ctrl_setting
          # key: pub1
          topic: /obelisk/dummy/ctrl
          msg_type: PositionSetpoint
          history_depth: 10
          callback_group: None
          non_obelisk: False
      subscribers:
        - ros_parameter: sub_est_setting
          # key: sub1
          topic: /obelisk/dummy/est
          msg_type: EstimatedState
          history_depth: 10
          # callback_key: sub_callback1
          callback_group: None
          non_obelisk: False
      timers:
        - ros_parameter: timer_ctrl_setting
          # key: timer1
          timer_period_sec: 0.001
          callback_group: None
          # callback_key: timer_callback1
  estimation:
    - pkg: obelisk_estimation_cpp
      executable: jointencoders_passthrough_estimator
      # callback_groups:
      publishers:
        - ros_parameter: pub_est_setting
          # key: pub1
          topic: /obelisk/dummy/est
          msg_type: EstimatedState
          history_depth: 10
          callback_group: None
          non_obelisk: False
      subscribers:
        - ros_parameter: sub_sensor_setting
          # key: sub1
          topic: /obelisk/dummy/sensor
          msg_type: JointEncoders
          history_depth: 10
          # callback_key: sub_callback1
          callback_group: None
          non_obelisk: False
      timers:
        - ros_parameter: timer_est_setting
          # key: timer1
          timer_period_sec: 0.001
          callback_group: None
          # callback_key: timer_callback1
  # sensing:
  robot:
    - is_simulated: True
      pkg: obelisk_sim_cpp
      executable: obelisk_mujoco_robot
      # callback_groups:
      # publishers:
      subscribers:
        - ros_parameter: sub_ctrl_setting
          # key: sub1
          topic: /obelisk/dummy/ctrl
          msg_type: PositionSetpoint
          history_depth: 10
          # callback_key: sub_callback1
          callback_group: None
          non_obelisk: False
      sim:
        - ros_parameter: mujoco_setting
          model_xml_path: dummy/dummy.xml
          n_u: 1
          time_step: 0.002
          num_steps_per_viz: 5
          sensor_settings:
          - topic: /obelisk/dummy/sensor
            dt: 0.001
            sensor_type: jointpos
            sensor_names:
            - sensor_joint1

Breaking down the configuration file

config: dummy
onboard:

First we give the name of this configuration (dummy), and which device this is running on.

control:
  pkg: obelisk_control_py
  executable: example_position_setpoint_controller
  params_path: /obelisk_ws/src/obelisk_ros/config/dummy_params.txt
  publishers:
    - ros_parameter: pub_ctrl_setting
      topic: /obelisk/dummy/ctrl
      msg_type: PositionSetpoint
      key: "asdf"
      history_depth: 10
      callback_group: None
      non_obelisk: False
  subscribers:
    - ros_parameter: sub_est_setting
      topic: /obelisk/dummy/est
      msg_type: EstimatedState
      history_depth: 10
      callback_group: None
      non_obelisk: False
  timers:
    - ros_parameter: timer_ctrl_setting
      timer_period_sec: 0.001
      callback_group: None

Now we configure our Controller node. pkg gives the name of the package containing the Obelisk node, and executable tells us what the name is of the executable with main in it. params_path (optional) is a string parameter that allows you to specify a file path that can be accessed within your code. This is useful for things like accessing controller gains that are specfied through a seperate yaml file. Note that there is no convention on how the file path is processed as that is up to you as the user. Now we need to configure all of the Components in this node. Publishers and subscribers have the following options.

ros_parameter gives the string name of the ros parameter declared in the code. This is how the launch file gets these options to the correct node.
topic gives the string topic name that will either be published or subscribed to.
msg_type gives the type of message we want to publish or subscribe to. Note this is only ever used in the Python implementation. In C++ the message type must be specified in the code as a templated parameter.
key gives the string key associated with the component if not already specified in the code implementation. Note this is only ever used in the Python implementation. In C++, the key must be specified during component declaration time.
history_depth (optional) gives the number of messages to hold in the queue before deleting additional messages. If this not set we the use the default value of 10.
callback_group (optional) gives the string name of the callback group to use. The callback groups can be configured within this configuration file. If no value is specified, then the node’s default callback group is used.
non_obelisk (optional) determine whether this node can publish non-Obelisk messages. Note if this is set to true, then this may cause problems with the interfaces, so only use if you are sure this is what you need. If no value is specified, then the default value of False is used.

Timers have the following options.

ros_parameter gives the string name of the ros parameter declared in the code.
timer_period_sec gives the period of the timer in seconds.
callback_group (optional) gives the string name of the callback group to use. The callback groups can be configured within this configuration file. If no value is specified, then the node’s default callback group is used.

This is repeated for every non-system node in the block diagram, which in this case is just an additional estimator.

estimation:
    pkg: obelisk_estimation_py
    executable: jointencoders_passthrough_estimator
    publishers:
      - ros_parameter: pub_est_setting
        topic: /obelisk/dummy/est
        msg_type: EstimatedState
        history_depth: 10
        callback_group: None
        non_obelisk: False
    subscribers:
      - ros_parameter: sub_sensor_setting
        # key: sub1
        topic: /obelisk/dummy/sensor
        msg_type: JointEncoders
        history_depth: 10
        callback_group: None
        non_obelisk: False
    timers:
      - ros_parameter: timer_est_setting
        timer_period_sec: 0.001
        callback_group: None

Lastly, we need to configure the robot (aka, the system).

robot:
  is_simulated: True
  pkg: obelisk_sim_py
  executable: obelisk_mujoco_robot
  # callback_groups:
  # publishers:
  subscribers:
    - ros_parameter: sub_ctrl_setting
      # key: sub1
      topic: /obelisk/dummy/ctrl
      msg_type: PositionSetpoint
      history_depth: 10
      callback_group: None
      non_obelisk: False
  sim:
    - ros_parameter: mujoco_setting
      model_xml_path: dummy/dummy.xml
      n_u: 1
      time_step: 0.002
      num_steps_per_viz: 5
      sensor_settings:
      - topic: /obelisk/dummy/sensor
        dt: 0.001
        sensor_type: jointpos
        sensor_names:
        - sensor_joint1

is_simulated marks if we are running on hardware or in simulation. pkg and executable are as before.

Now, we must configure the Components of the node, which in this example is just a subscriber. These Components have all the same options as the non-system Components given above.

Lastly, since this is a simulation, we must provide the simulator with all relevant information. Here, we are using the Mujoco simulation interface. The new settings here are:

n_u gives the number of control inputs (i.e., the number of scalars)
time_step (optional) gives the length of a simulation time step. If no value is provided, the default value of 0.002 seconds will be used.
num_steps_per_viz (optional) gives the number of steps to use between simulation rendering. If no value is provided, the default value of 8 steps will be used.

sensor_settings is how we can specify what sensors our robot has. Within sensor_settings we have the following new options:

dt gives the sensor publishing period in seconds.
sensor_type gives the Mujoco sensor type. Each sensor type corresponds to a specific Obelisk message type. Note that the Mujoco XML must have all the sensors listed in the Obelisk configuration file, if you request a sensor here that is not available in Mujoco, there will be an error. All supported Mujoco sensors and corresponding Obelisk messages are listed below.

Mujoco sensor type	Obelisk Message Type
`jointpos`	`JointEncoders`

sensor_names gives the names of the sensors to query, as given in the Mujoco XML.

Thats it! Now we have configured our Obelisk nodes.