Collision Monitor¶

The Collision Monitor is a node providing an additional level of robot safety. It performs several collision avoidance related tasks using incoming data from the sensors, bypassing the costmap and trajectory planners, to monitor for and prevent potential collisions at the emergency-stop level.

This is analogous to safety sensor and hardware features; take in laser scans from a real-time certified safety scanner, detect if there is to be an imminent collision in a configurable bounding box, and either emergency-stop the certified robot controller or slow the robot to avoid such collision. However, this node is done at the CPU level with any form of sensor. As such, this does not provide hard real-time safety certifications, but uses the same types of techniques with the same types of data for users that do not have safety-rated laser sensors, safety-rated controllers, or wish to use any type of data input (e.g. pointclouds from depth or stereo or range sensors).

This is a useful and integral part of large heavy industrial robots, or robots moving with high velocities, around people or other dynamic agents (e.g. other robots) as a safety mechanism for high-response emergency stopping. The costmaps / trajectory planners will handle most situations, but this is to handle obstacles that virtually appear out of no where (from the robot’s perspective) or approach the robot at such high speed it needs to immediately stop to prevent collision.

See the package’s README for more complete information.

Features¶

The Collision Monitor uses polygons relative the robot’s base frame origin to define “zones”. Data that fall into these zones trigger an operation depending on the model being used. A given instance of the Collision Monitor can have many zones with different models at the same time. When multiple zones trigger at once, the most aggressive one is used (e.g. stop > slow 50% > slow 10%).

The following models of safety behaviors are employed by Collision Monitor:

Stop model: Define a zone and a point threshold. If min_points or more obstacle points appear inside this area, stop the robot until the obstacles will disappear.
Slowdown model: Define a zone around the robot and slow the maximum speed for a slowdown_ratio, if min_points or more points will appear inside the area.
Limit model: Define a zone around the robot and restricts the maximum linear and angular velocities to linear_limit and angular_limit values accordingly, if min_points or more points will appear inside the area.
Approach model: Using the current robot speed, estimate the time to collision to sensor data. If the time is less than time_before_collision seconds (0.5, 2, 5, etc…), the robot will slow such that it is now at least time_before_collision seconds to collision. The effect here would be to keep the robot always time_before_collision seconds from any collision.

The zones around the robot can take the following shapes:

Arbitrary user-defined polygon relative to the robot base frame, which can be static in a configuration file or dynamically changing via a topic interface.
Robot footprint polygon, which is used in the approach behavior model only. Will use the static user-defined polygon or the footprint topic to allow it to be dynamically adjusted over time.
Circle: is made for the best performance and could be used in the cases where the zone or robot footprint could be approximated by round shape.

All shapes (Polygon and Circle) are derived from base Polygon class, so without loss of generality they would be called as “polygons”. Subscribed footprint is also having the same properties as other polygons, but it is being obtained a footprint topic for the Approach Model.

The data may be obtained from different data sources:

Laser scanners (sensor_msgs::msg::LaserScan messages)
PointClouds (sensor_msgs::msg::PointCloud2 messages)
IR/Sonars (sensor_msgs::msg::Range messages)

Parameters¶

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.

cmd_vel_in_topic

Type	Default
string	“cmd_vel_raw”

Description:: Input cmd_vel topic with desired robot velocity.

cmd_vel_out_topic

Type	Default
string	“cmd_vel”

Description:: Output cmd_vel topic with output produced by Collision Monitor velocities.

state_topic

Type	Default
string	“”

Description:: Output the currently activated polygon action type and name. Optional parameter. No publisher will be created if it is unspecified.

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.

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).

stop_pub_timeout

Type	Default
double	1.0

Description:: Timeout, after which zero-velocity ceases to be published. It could be used for other overrode systems outside Nav2 are trying to bring the robot out of a state close to a collision, or to allow a standing robot to go into sleep mode.

polygons

Type	Default
vector<string>	N/A

Description:: List of zones (stop/slowdown/limit bounding boxes, footprint, approach circle, etc…). 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
vector<double>	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 monitor will use dynamic polygon subscription to polygon_sub_topic for points in the stop/slowdown/limit action types, or footprint subscriber to footprint_topic for approach action type.

<polygon_name>.polygon_sub_topic

Type	Default
string	N/A

Description:: Topic to listen the polygon points from. Applicable only for polygon type and stop/slowdown/limit action types. 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>.footprint_topic

Type	Default
string	“local_costmap/published_footprint”

Description:: Topic to listen the robot footprint from. Applicable only for polygon type and approach action type. If both points and footprint_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:: Zone behavior model. Available values are stop, slowdown, limit, approach. Causes an error, if not specialized.

<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>.slowdown_ratio

Type	Default
double	0.5

Description:: Robot slowdown (share of its actual speed). Applicable for slowdown action type.

<polygon_name>.linear_limit

Type	Default
double	0.5

Description:: Robot linear speed limit. Applicable for limit action type.

<polygon_name>.angular_limit

Type	Default
double	0.5

Description:: Robot angular speed limit. Applicable for limit action type.

<polygon_name>.time_before_collision

Type	Default
double	2.0

Description:: Time before collision in seconds. Maximum simulation time used in collision prediction. Higher values mean lower performance. Applicable for approach action type.

<polygon_name>.simulation_time_step

Type	Default
double	0.1

Description:: Time iteration step for robot movement simulation during collision prediction. Lower values mean lower prediction accuracy but better performance. Applicable for approach action type.

<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.

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.

Example¶

Here is an example of configuration YAML for the Collision Monitor. For more information how to bring-up your own Collision Monitor node, please refer to the Using Collision Monitor tutorial.

collision_monitor:
  ros__parameters:
    base_frame_id: "base_footprint"
    odom_frame_id: "odom"
    cmd_vel_in_topic: "cmd_vel_raw"
    cmd_vel_out_topic: "cmd_vel"
    state_topic: "collision_monitor_state"
    transform_tolerance: 0.5
    source_timeout: 5.0
    base_shift_correction: True
    stop_pub_timeout: 2.0
    polygons: ["PolygonStop", "PolygonSlow", "FootprintApproach"]
    PolygonStop:
      type: "circle"
      radius: 0.3
      action_type: "stop"
      min_points: 4  # max_points: 3 for Humble
      visualize: True
      polygon_pub_topic: "polygon_stop"
    PolygonSlow:
      type: "polygon"
      points: [1.0, 1.0, 1.0, -1.0, -0.5, -1.0, -0.5, 1.0]
      action_type: "slowdown"
      min_points: 4  # max_points: 3 for Humble
      slowdown_ratio: 0.3
      visualize: True
      polygon_pub_topic: "polygon_slowdown"
    PolygonLimit:
      type: "polygon"
      points: [0.5, 0.5, 0.5, -0.5, -0.5, -0.5, -0.5, 0.5]
      action_type: "limit"
      min_points: 4  # max_points: 3 for Humble
      linear_limit: 0.4
      angular_limit: 0.5
      visualize: True
      polygon_pub_topic: "polygon_limit"
    FootprintApproach:
      type: "polygon"
      action_type: "approach"
      footprint_topic: "/local_costmap/published_footprint"
      time_before_collision: 2.0
      simulation_time_step: 0.02
      min_points: 6  # max_points: 5 for Humble
      visualize: False
    observation_sources: ["scan", "pointcloud"]
    scan:
      type: "scan"
      topic: "/scan"
    pointcloud:
      type: "pointcloud"
      topic: "/intel_realsense_r200_depth/points"
      min_height: 0.1
      max_height: 0.5