sdf_contact_estimation

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sdf_contact_estimation

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Accurate pose prediction based on Signed Distance Fields for mobile ground robots in rough terrain.

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# sdf_contact_estimation
sdf_contact_estimation is a library for [ROS](https://www.ros.org/) for the accurate and fast pose prediction of mobile ground robots in rough terrain using [signed distance fields](https://github.com/ethz-asl/voxblox). Based on a 2D pose query and the joint configuration, the statically stable 3D pose on the ground as well as contact points and support polygon are computed:

<img src="sdf_contact_estimation_demo/doc/animation.gif" width="50%"/>

With a runtime of about 0.5 ms per predicted pose, this approach is much faster than a traditional physics simulation and suitable for online planning applications. Possible use-cases are:

- Automatic flipper control
- Path planning
- Tip-over prevention

A demo launch configuration is available below.

**Author/Maintainer: Martin Oehler 
Affiliation: [TU Darmstadt, SIM](https://www.sim.informatik.tu-darmstadt.de/) 
License: MIT**

## Citation
Please cite our paper if you use this software as part of your scientific publication:

```
@inproceedings{oehler2023accurate,
  title={Accurate Pose Prediction on Signed Distance Fields for Mobile Ground Robots in Rough Terrain},
  author={Oehler, Martin and von Stryk, Oskar},
  booktitle={2023 IEEE International Symposium on Safety, Security, and Rescue Robotics (SSRR)},
  pages={47--52},
  year={2023},
  organization={IEEE}
}
```

https://github.com/tu-darmstadt-ros-pkg/sdf_contact_estimation/assets/7110154/4043c145-f7cb-41d0-b682-249bc84ad0cf

## Installation
If you haven't already installed it, [install ROS](http://wiki.ros.org/noetic/Installation/Ubuntu). Please use **Desktop-Full Install** to run the demo. Noetic is officially supported, but Melodic and Kinetic should work as well.

[Create a new catkin workspace](https://catkin-tools.readthedocs.io/en/latest/quick_start.html). Skip this step if you use your existing workspace. 
```bash
source /opt/ros/noetic/setup.bash
mkdir -p catkin_ws/src
cd catkin_ws/
catkin init
```

Go into the source directory of your workspace and check out this repository
```bash
cd src/
git clone https://github.com/tu-darmstadt-ros-pkg/sdf_contact_estimation.git
```

Install dependencies via rosdep and wstool (.rosinstall)
```bash
cd sdf_contact_estimation
rosdep install --from-paths . --ignore-src -r -y
wstool init ../.. # not required if already initialized
wstool merge sdf_contact_estimation_https.rosinstall
wstool update
```
Build
```bash
catkin build sdf_contact_estimation_demo
```
and source the workspace
```bash
source ../../devel/setup.bash
```
Verify your installation by launching the demo (see below).

## Getting started

### Launching the demo
Launch the demo with
```bash
roslaunch sdf_contact_estimation_demo demo.launch
```

 <img src="sdf_contact_estimation_demo/doc/demo_annotated.jpg" width="80%"/>

An rviz window will open (see above), showing the robot model and the environment. Using the interactive marker (1), the pose query can be updated. The robot model visualizes the corresponding statically stable pose on the ground. Additionally, a small rqt window (2) will open with sliders for each joint. These can be used to update the joint configuration.

### Configure your own robot
A good way to get started is modyfing the provided [demo launch](sdf_contact_estimation_demo/launch/demo.launch) to use your own robot. The algorithm requires a robot model provided as an [URDF](http://wiki.ros.org/urdf) and a Signed Distance Field (SDF) environment model provided by [Voxblox](https://github.com/ethz-asl/voxblox). Based on the URDF, contact candidate points need to be defined on relevant surfaces such as tracks and the underside of the chassis. These contact candidates are used to check for collisions against the SDF.

<img src="sdf_contact_estimation_demo/doc/contact_candidates.jpg" width="50%"/>

A simple configuration file could look like this:

```yaml
joints: # Specify joints that effect the collision link transforms
  - flipper_front_joint
  - flipper_back_joint
collision_links: # Specify links that are considered for collision checking
  # Chassis
  - link: chassis_link
    type: body # Optional meta data
    ignore_indices: [1] # Ignore collision bodies of this link for improved performance
  # Main tracks
  - link: main_track_left_link
    type: track
    resolution: &main_track_resolution 0.08 # Sample resolution, edges are always sampled
  - link: main_track_right_link
    type: track
    resolution: *main_track_resolution
  # Flippers
  - link: flipper_front_left_link
    type: track
    resolution: &flipper_track_resolution 0.045
  - link: flipper_front_right_link
    type: track
    resolution: *flipper_track_resolution
  - link: flipper_back_left_link
    type: track
    resolution: *flipper_track_resolution
  - link: flipper_back_right_link
    type: track
    resolution: *flipper_track_resolution
```
Please refer to the [demo configuration](sdf_contact_estimation_demo/config/shape_config.yaml) and the parameter list below for further parameters to tune the behavior and increase performance.

### Voxblox
[Voxblox](https://github.com/ethz-asl/voxblox) is a volumetric mapping library based on Signed Distance Fields (SDF). The demo configuration loads an artificial environment for demonstration purposes. On your real robot, you want to create a live SDF based on pointcloud data using the `voxblox_ros/esdf_server` node instead.  Follow the documentation [here](https://voxblox.readthedocs.io/) and [here](https://voxblox.readthedocs.io/en/latest/pages/Using-Voxblox-for-Planning.html) to set up Voxblox for your robot.

### Using the library
The following code snippet illustrates how the sdf_contact_estimation library could be used as part of your project.
```cpp
ros::NodeHandle nh;
ros::NodeHandle pnh("~");

// Set up ESDF server
ros::NodeHandle esdf_server_pnh(pnh, "esdf_server");
auto esdf_server = std::make_shared<voxblox::EsdfServer>(nh, esdf_server_pnh);
// Load map into model
auto sdf_model = std::make_shared<sdf_contact_estimation::SdfModel>(pnh); 
sdf_model->loadEsdf(esdf_server->getEsdfMapPtr(), esdf_server->getEsdfMaxDistance(), false);

// Create robot model 
ros::NodeHandle shape_model_nh(pnh, "shape_model"); 
auto shape_model = std::make_shared<sdf_contact_estimation::ShapeModel>(shape_model_nh); 
 
// Create sdf contact estimation
auto pose_predictor = std::make_shared<sdf_contact_estimation::SDFContactEstimation>(pnh, shape_model, sdf_model);
pose_predictor->enableVisualisation(false); // only for debugging, visualizations lead to decreased performance
```
This model uses an external ESDF server node to transfer the ESDF map as described [here](https://voxblox.readthedocs.io/en/latest/pages/Using-Voxblox-for-Planning.html).

Now the pose and contact estimation can be used as follows:

```cpp
// Prepare query pose
Eigen::Isometry3D pose_eigen = Eigen::AngleAxisd(M_PI, Eigen::Vector3d::UnitZ());
pose_eigen.translation() = Eigen::Vector3d(1.0, 1.0, 0.0);
hector_math::Pose<double> robot_pose(pose_eigen);

// Example calls with result in-place
// Pose prediction only
pose_predictor->predictPose(robot_pose);

// Pose and support polygon
hector_pose_prediction_interface::SupportPolygon<double> support_polygon;
pose_predictor->predictPoseAndSupportPolygon(robot_pose, support_polygon);

// Pose, support polygon and all ground contact points+normals
hector_pose_prediction_interface::ContactInformation<double> contact_information;
pose_predictor->predictPoseAndContactInformation(robot_pose, support_polygon, contact_information);

// The following calls do not perform pose prediction but instead use the provided pose 
// to estimate the current ground contact
// Support polygon
pose_predictor->estimateSupportPolygon(robot_pose, support_polygon);
// Support polygon and contact information (all ground contacts and normals)
pose_predictor->estimateContactInformation(robot_pose, support_polygon, contact_information);

```

### Parameters
**sdf_contact_estimation::SDFContactEstimation**
* **`~debug`** (bool, default: false)

Controls the output of debug messages.
* **`~stepping`** (bool, default: false)

If enabled, the pose prediction halts after each iteration until enter is pressed on the console.
* **`~final_contact_threshold`** (double, default: 0.05)

Distance of a point to the ground to be still considered in contact with the ground. This value will be used to compute the final support polygon after the pose prediction is finished. This value should be should be equal or greater than `iteration_contact_threshold`.
* **`~chassis_contact_threshold`** (double, default: `final_contact_threshold`)

Distance of a point to the ground to be still considered in contact with the ground for the chassis. Greater values can be chosen to account for safety padding. This value should be equal or greater than `iteration_contact_threshold`.
* **`~iteration_contact_threshold`** (double, default: `final_contact_threshold`)

Distance of a point to the ground to be still considered in contact with the ground. This value will be used between iterations of the pose prediction to determine the next rotation axis. This value should be less than `final_contact_threshold`.
* **`~convexity_threshold`** (double, default: 0.0)

Positive values make the requirements for convexity less strict and lead to simpler support polygons. In practice, this makes stability margin values more meaningful by preventing very small support polygon edges.
* **`~max_iterations`** (int, default: 5)

Maximum outer iterations of the algorithm before terminating with a failure.
* **`~robot_fall_limit`** (double, default: M_PI/3.0)

If the angle between robot base link z-axis and world z-axis exceeds this value after an iteration of the pose prediction, it is assumed that the robot fell over and the prediction stops prematurely with a failure.
    
**sdf_contact_estimation::ShapeModel**
* **`/robot_description`** (string, mandatory)

URDF of the robot.
* **`/robot_description_semantic`** (string, default: "")

Optional MoveIt SRDF file. If found and visualizations are turned on, the result robot state is published as [moveit_msgs/DisplayRobotState](https://docs.ros.org/en/noetic/api/moveit_msgs/html/msg/DisplayRobotState.html).
* **`~default_resolution`** (double, default: 0.1)

Default sampling resolution for contact candidate points on all collision geometries.
* **`~/shape_model/collision_links`** (array, mandatory)

Array of dictionaries for each collision link (see the following parameters).
* **`~/shape_model/collision_links[]/link`** (string)

Name of the robot link to consider for collision checking.
* **`~/shape_model/collision_links[]/type`** (string, default: "default")

Optional meta-data that assigns a type to the link. Valid values are: "default", "body", "track".
* **`~/shape_model/collision_links[]/ignore_indices`** (int[], default: [])

Indices of collision geometries to ignore for this link.
* **`~/shape_model/collision_links[]/include_indices`** (int[], default: [])

Only consider collision geometries with the given indices for this link.
* **`~/shape_model/collision_links[]/resolution`** (double, default: `default_resolution`)
	
	If set, overwrites the `default_resolution` for all collision geometries of this link.
* **`~/shape_model/collision_links[]/cylinder_angle_min`** (double, default: 0.0)

For cylinder collision geometries, only place contact candidate points between `cylinder_angle_min` and `cylinder_angle_max`.
* **`~/shape_model/collision_links[]/cylinder_angle_max`** (double, default: 2*M_PI)

For cylinder collision geometries, only place contact candidate points between `cylinder_angle_min` and `cylinder_angle_max`.

**sdf_contact_estimation::SdfModel**

These parameters are only loaded when `SdfModel::loadFromServer(nh)` is called.
* **`~sdf_file_path`** (string, optional)

Load a TSDF (suffix `.tsdf`) or ESDF (suffix `.esdf`) file from this path. Either this or `scenario` has to be set.
* **`~scenario`** (string, optional)

Create an artificial scenario for testing. Valid values are: "flat", "ramp", "step", "obstacle" and "hole". Either this or `sdf_file_path` has to be set.
* **`~truncation_distance`** (double, default: 0.4)

TSDF/ESDF truncation distance of the loaded file or artificial scenario.
* **`~voxel_size`** (double, default: 0.05)

Sets the voxel size for artificial scenarios.
* **`~use_esdf`** (bool, default: false)

If set, an ESDF is computed from TSDFs.

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