Introduction

The cliff detector node uses scans from a lidar tilted downwards to detect cliffs.

Dependencies

• ddynamic_reconfigure

Explanation

First, a lookup table of laser scan values are generated, each of these values correspond to the distance travelled by an unobstructed ray from the lidar to the ground plane (at z=0).

The values of the incoming laser scans are compared to the lookup value and the parameter cliff_threshold_constant will determine the extra distance travelled by the scan before it is considered a cliff.

IF the scan range is considered a cliff, then it's value will be set as the lookup value, ELSE it will be assigned a value of infinity. This scan is then used to generate the costmap layer cliff_layer. As can be seen in the 2 figures below, the cliff_layer is being generated by the green laser scan points. The blue laser visualization in gazebo is that of the cliff lidar tilted downwards.

There is also a failsafe added to warn the user and stop the robot in the event that the lidar is obstructed by a foreign object up close. Should a percentage of the scans (failsafe_threshold_percentage) have a value below a distance of failsafe_threshold_dist, the failsafe will be activated.

Quick start

To test the costmap generation from the cliff detection node, simply run:

roslaunch cliff_detector cliff_detector_node.launch

Subscribe to "/scan_cliff" topic of type [sensor_msgs/laserscan] to form the cliff obstacle costmap. And make sure that the frame_id "cliff_lidar" is given a static transform to the robot via TF Publisher.

Parameters

Physical parameters

• lidar_height: [float/meters] Height from base_link
• lidar_pitch: [float/degrees] Lidar pitch (along y axis)
• lidar_resolution: [float/degrees] Lidar resolution (along y axis)

Failsafe params

• failsafe: [bool] whether to turn on failsafe. This will issue a /cmd_vel command that stops the robot should more than a user-specified percentage of the lidar be obstructed.
• failsafe_threshold_dist: [double] (meters) ranges less than this distance is obstructing the lidar
• failsafe_threshold_percentage: [float] (percentage) if the percentage of ranges with value less than "failsafe_threshold_dist" is exceeded, then lidar is obstructed.
• failsafe_timeout: [double] (seconds) Time out from when the lidar is obstructed before failsafe is activated

Cliff detector params

• cliff_threshold_constant: [double] (meters) Distance along the ray below the ground plane to detect as a cliff area

Segmentation

• min_seg: Minimum points required to comprise a segment of cliff points

Usage

Topics subscribed

1. scan_cliff_raw [sensor_msgs/laserscan]: Raw lidar scans from the cliff lidar

Topics published

1. cliff_thresh_values [sensor_msgs/LaserScan]: "Fake" laser scan used as a visual indication of the ranges (which when exceeded), are considered as negative obstacles.
2. scan_cliff [sensor_msgs/LaserScan]: The laser scans that show the cliff obstacles and that are used to build the cliff_layer costmap
3. cmd_vel_safety [geometry_msgs/Twist]: To stop the robot when the failsafe is activated.

To Do

• There is a need to handle the failsafe when the lidar loses connection to the computer. This could be done in the form of listening and parsing the diagnostics message.