Collision Detector Node

The Collision Detector is a node similar to the Collision Monitor, so it is recommended to read the Using Collision Monitor tutorial first.

In some cases, the user may want to be informed about the detected obstacles without affecting the robot’s velocity and instead take a different action within an external node. For example, the user may want to blink LEDs or sound an alarm when the robot is close to an obstacle. Another use case could be to detect data points in particular regions (e.g extremely close to the sensor) and warn of malfunctioning sensors. For this purpose, the Collision Detector node was introduced. It works similarly to the Collision Monitor, but does not affect the robot’s velocity. It will only inform that data from the configured sources has been detected within the configured polygons via message to the collision_detector_state topic.

See the package’s README for more information.

Features

Similarly to the Collision Monitor, the Collision Detector uses robot’s relative polygons to define “zones”. However, unlike the Collision Monitor that uses different behavior models, the Collision Detector does not use any of them and therefore the action_type should always be set to none. If set to anything else, it will throw an error

The zones around the robot and the data sources are the same as for the Collision Monitor, with the exception of the footprint polygon, which is not supported by the Collision Detector.

Parameters

frequency

Type	Default
double	10.0

Description:

Frequency of the main loop that checks for detections.

base_frame_id

Type	Default
string	“base_footprint”

Description:

Robot base frame.

odom_frame_id

Type	Default
string	“odom”

Description:

Which frame to use for odometry.

transform_tolerance

Type	Default
double	0.1

Description

Time with which to post-date the transform that is published, to indicate that this transform is valid into the future.

source_timeout

Type	Default
double	2.0

Description:

Maximum time interval in which source data is considered as valid. If no new data is received within this interval, an additional warning will be displayed. Setting source_timeout: 0.0 disables it. This parameter can be overriden per observation source.

base_shift_correction

Type	Default
bool	True

Description:

Whether to correct source data towards to base frame movement, considering the difference between current time and latest source time. If enabled, produces more accurate sources positioning in the robot base frame, at the cost of slower performance. This will cause average delays for ~1/(2*odom_rate) per each cmd_vel calculation cycle. However, disabling this option for better performance is not recommended for the fast moving robots, where during the typical rate of data sources, robot could move unacceptably far. Thus reasonable odometry rates are recommended (~100 hz).

polygons

Type	Default
vector<string>	N/A

Description:

List of zones to check for data points. Causes an error, if not specialized.

observation_sources

Type	Default
vector<string>	N/A

Description:

List of data sources (laser scanners, pointclouds, etc…). Causes an error, if not specialized.

Polygons parameters

<polygon name> is the corresponding polygon name ID selected for this type.

<polygon_name>.type

Type	Default
string	N/A

Description:

Type of polygon shape. Available values are polygon, circle. Causes an error, if not specialized.

<polygon_name>.points

Type	Default
string	N/A

Description:

Polygon vertexes, listed in "[[p1.x, p1.y], [p2.x, p2.y], [p3.x, p3.y], ...]" format (e.g. "[[0.5, 0.25], [0.5, -0.25], [0.0, -0.25], [0.0, 0.25]]" for the square in the front). Used for polygon type. Minimum 3 points for a triangle polygon. If not specified, the collision detector will use dynamic polygon subscription to polygon_sub_topic

<polygon_name>.polygon_sub_topic

Type	Default
string	N/A

Description:

Topic to listen the polygon points from. Causes an error, if not specified and points are also not specified. If both points and polygon_sub_topic are specified, the static points takes priority.

<polygon_name>.radius

Type	Default
double	N/A

Description:

Circle radius. Used for circle type. Causes an error, if not specialized.

<polygon_name>.action_type

Type	Default
string	N/A

Description:

Only none action type is supported (more options available for collision monitor)

<polygon_name>.min_points

Type	Default
int	4

Description:

Minimum number of data readings within a zone to trigger the action. Former max_points parameter for Humble, that meant the maximum number of data readings within a zone to not trigger the action). min_points is equal to max_points + 1 value.

<polygon_name>.visualize

Type	Default
bool	False

Description:

Whether to publish the polygon in a separate topic.

<polygon_name>.polygon_pub_topic

Type	Default
string	<polygon_name>

Description:

Topic name to publish a polygon to. Used only if visualize is true.

<source name>.enabled

Type	Default
bool	True

Description:

Whether to use this source for collision detection. (Can be dynamically set)

Observation sources parameters

<source name> is the corresponding data source name ID selected for this type.

<source name>.type

Type	Default
string	“scan”

Description:

Type of polygon shape. Could be scan, pointcloud or range.

<source name>.topic

Type	Default
string	“scan”

Description:

Topic to listen the source data from.

<source name>.min_height

Type	Default
double	0.05

Description:

Minimum height the PointCloud projection to 2D space started from. Applicable for pointcloud type.

<source name>.max_height

Type	Default
double	0.5

Description:

Maximum height the PointCloud projection to 2D space ended with. Applicable for pointcloud type.

<source name>.obstacles_angle

Type	Default
double	PI / 180 (1 degree)

Description:

Angle increment (in radians) between nearby obstacle points at the range arc. Two outermost points from the field of view are not taken into account (they will always exist regardless of this value). Applicable for range type.

<source name>.enabled

Type	Default
bool	True

Description:

Whether to use this source for collision detection. (Can be dynamically set)

<source name>.source_timeout

Type	Default
double	(node parameter source_timeout value)

Description:

Maximum time interval in which source data is considered as valid. If no new data is received within this interval, an additional warning will be displayed. Setting source_timeout: 0.0 disables it. Overrides node parameter for each source individually, if desired.

Example

Here is an example of configuration YAML for the Collision Detector.

collision_detector:
  ros__parameters:
    base_frame_id: "base_footprint"
    odom_frame_id: "odom"
    transform_tolerance: 0.5
    source_timeout: 5.0
    base_shift_correction: True
    polygons: ["PolygonFront"]
    PolygonFront:
      type: "polygon"
      points: "[[0.3, 0.3], [0.3, -0.3], [0.0, -0.3], [0.0, 0.3]]"
      action_type: "none"
      min_points: 4
      visualize: True
      polygon_pub_topic: "polygon_front"
    observation_sources: ["scan"]
    scan:
      source_timeout: 0.2
      type: "scan"
      topic: "scan"
      enabled: True
    pointcloud:
      type: "pointcloud"
      topic: "/intel_realsense_r200_depth/points"
      min_height: 0.1
      max_height: 0.5
      enabled: True