Collision Monitor Node

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. For more information how to bring-up your own Collision Monitor node, please refer to the Using Collision Monitor tutorial.

Also, the practical demonstration of Collision Monitor abilities presented at 6th ROS Developers Day 2023, could be found below:

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.

  • VelocityPolygon: allow switching of polygons based on the command velocity. This is useful for robots to set different safety zones based on their velocity (e.g. a robot that has a larger safety zone when moving at 1.0 m/s than when moving at 0.5 m/s).

All shapes (Polygon, Circle and VelocityPolygon) 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_smoothed”
Description:

Input cmd_vel topic with desired robot velocity. Please note, pre-Jazzy this was set to cmd_vel_raw by default.

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. If no new data is received within this interval, the robot will be stopped. Setting source_timeout: 0.0 disables this blocking mechanism. 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).

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.

use_realtime_priority

Type

Default

bool

false
Description

Adds soft real-time priorization to the controller server to better ensure resources to time sensitive portions of the codebase. This will set the controller’s execution thread to a higher priority than the rest of the system (90) to meet scheduling deadlines to have less missed loop rates. To use this feature, you use set the following inside of /etc/security/limits.conf to give userspace access to elevated prioritization permissions: <username> soft rtprio 99 <username> hard rtprio 99

enable_stamped_cmd_vel

Type

Default

bool

false
Description

Whether to use geometry_msgs::msg::Twist or geometry_msgs::msg::TwistStamped velocity data. True uses TwistStamped, false uses Twist.

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 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:

For polygon type, topic to listen the polygon points from. For circle type, topic to listen the circle radius from. Applicable for stop/slowdown/limit action types. Causes an error if not specified and static polygon geometry (using parameter points for polygon type or radius for circle type) is also not specified. If both static polygon geometry and polygon_sub_topic are specified, the static parameter 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>.polygon_subscribe_transient_local

Type

Default

bool

False
Description:

QoS durability setting for the incoming polygon or footprint topic subscription.

<polygon_name>.radius

Type

Default

double

N/A
Description:

Circle radius. Used for circle type. If not specified, the collision monitor will use dynamic polygon subscription to polygon_sub_topic for circle radius in the stop/slowdown/limit action types.

<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. Higher 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.

<polygon_name name>.enabled

Type

Default

bool

True
Description:

Whether to use this polygon for collision monitoring. (Can be dynamically set)

VelocityPolygon parameters

All previous Polygon parameters apply, in addition to the following unique parameters for VelocityPolygon.

<vel_poly>.holonomic

Type

Default

bool

False
Description:

Whether to use holonomic or non-holonomic robot model for collision prediction. For holonomic robot model, the resultant velocity will be used to compare the linear velocity range. Additionally, there will be 2 more parameters, direction_start_angle and direction_end_angle, to specify the resultant velocity direction.

<vel_poly>.velocity_polygons

Type

Default

vector<string>

N/A
Description:

List of sub polygons for switching based on the robot’s current velocity. When velocity is covered by multiple sub polygons, the first sub polygon in the list will be used. Causes an error, if not specified.

<vel_poly>.<subpoly>.points

Type

Default

vector<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. Causes an error, if not specified.

<vel_poly>.<subpoly>.linear_min

Type

Default

double

N/A
Description:

Minimum linear velocity for the sub polygon. In holonomic mode, this is the minimum resultant velocity. Causes an error, if not specified.

<vel_poly>.<subpoly>.linear_max

Type

Default

double

N/A
Description:

Maximum linear velocity for the sub polygon. In holonomic mode, this is the maximum resultant velocity. Causes an error, if not specified.

<vel_poly>.<subpoly>.theta_min

Type

Default

double

N/A
Description:

Minimum angular velocity for the sub polygon. Causes an error, if not specified.

<vel_poly>.<subpoly>.theta_max

Type

Default

double

N/A
Description:

Maximum angular velocity for the sub polygon. Causes an error, if not specified.

<vel_poly>.<subpoly>.direction_start_angle

Type

Default

double

-PI
Description:

Start angle of the movement direction(for holomic robot only). Refer to the Example section for the common configurations. Applicable for holonomic mode only.

<vel_poly>.<subpoly>.direction_end_angle

Type

Default

double

PI
Description:

End angle of the movement direction(for holomic robot only). Refer to the Example section for the common configurations. Applicable for holonomic mode only.

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 monitoring. (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, the robot will be stopped. Setting source_timeout: 0.0 disables this blocking mechanism. Overrides node parameter for each source individually, if desired.

Example

Here is an example illustrating the common configurations for holonomic robots that cover multiple directions of the resultant velocity:

../../../_images/holonomic_examples.png

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

collision_monitor:
  ros__parameters:
    base_frame_id: "base_footprint"
    odom_frame_id: "odom"
    cmd_vel_in_topic: "cmd_vel_smoothed"
    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
    enable_stamped_cmd_vel: False
    use_realtime_priority: false
    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"
      enabled: True
    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"
      enabled: True
    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"
      enabled: True
    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
      enabled: True
    VelocityPolygonStop:
      type: "velocity_polygon"
      action_type: "stop"
      min_points: 6
      visualize: True
      enabled: True
      polygon_pub_topic: "velocity_polygon_stop"
      velocity_polygons: ["rotation", "translation_forward", "translation_backward", "stopped"]
      holonomic: false
      rotation:
        points: "[[0.3, 0.3], [0.3, -0.3], [-0.3, -0.3], [-0.3, 0.3]]"
        linear_min: 0.0
        linear_max: 0.05
        theta_min: -1.0
        theta_max: 1.0
      translation_forward:
        points: "[[0.35, 0.3], [0.35, -0.3], [-0.2, -0.3], [-0.2, 0.3]]"
        linear_min: 0.0
        linear_max: 1.0
        theta_min: -1.0
        theta_max: 1.0
      translation_backward:
        points: "[[0.2, 0.3], [0.2, -0.3], [-0.35, -0.3], [-0.35, 0.3]]"
        linear_min: -1.0
        linear_max: 0.0
        theta_min: -1.0
        theta_max: 1.0
      # This is the last polygon to be checked, it should cover the entire range of robot's velocities
      # It is used as the stopped polygon when the robot is not moving and as a fallback if the velocity
      # is not covered by any of the other sub-polygons
      stopped:
        points: "[[0.25, 0.25], [0.25, -0.25], [-0.25, -0.25], [-0.25, 0.25]]"
        linear_min: -1.0
        linear_max: 1.0
        theta_min: -1.0
        theta_max: 1.0
    observation_sources: ["scan", "pointcloud"]
    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