CollisionCheck¶
Introduction¶
The CollisionCheck component is an optional on-board component of the rc_cube.
Note
The component is optional and requires a separate CollisionCheck license to be purchased.
The component provides an easy way to check if a gripper is in collision with a load carrier. It is integrated with the ItemPick and BoxPick, SilhouetteMatch and CADMatch modules, but can be used as standalone product.
Warning
Collisions are checked only between the load carrier and the gripper, not the robot itself, the flange, other items, or the item located in the robot gripper.
Setting a gripper¶
The gripper is a collision geometry used to determine whether the grasp is in collision with the load carrier. The gripper consists of up to 15 elements connected to each other.
At this point, the gripper can be built of elements of the following types:
BOX, with dimensionsbox.x,box.y,box.z.CYLINDER, with radiuscylinder.radiusand heightcylinder.height.
Additionally, for each gripper the flange radius, and information about the Tool Center Point (TCP) have to be defined.
The configuration of the gripper is normally performed offline during the setup of the desired application. This can be done via the REST-API interface or the rc_cube Web GUI.
Robot flange radius¶
Collisions are checked only between the gripper and the load carrier. The robot body is not considered.
As a safety feature, to prevent collisions between the load carrier and the robot, all grasps having any part of the
robot’s flange inside the load carrier can be designated as colliding (see Fig. 33).
This check is based on the defined gripper geometry and the flange radius value. It is optional to use
this functionality, and it can be turned on and off with the run-time parameter check_flange as described
in Parameter overview.
Fig. 33 Case A would be marked as collision only if check_flange is true, because the robot’s flange (red) is inside the
load carrier. Case B is collision free independent of check_flange.
Creating a gripper via the REST-API¶
When creating a gripper via the REST-API interface, each element of the gripper has a parent element, which defines how they are connected. The gripper is always built in the direction from the robot flange to the TCP, and at least one element must have ‘flange’ as parent. The elements’ IDs must be unique and must not be ‘tcp’ or ‘flange’. The pose of the child element has to be given in the coordinate frame of the parent element. In the REST-API representation, the coordinate frame of an element is always in its geometric center. Accordingly, for a child element to be exactly below the parent element, the position of the child element must be computed from the heights of both parent and child element (see Fig. 34).
Fig. 34 Reference frames for gripper creation via the REST-API
The reference frame for the first element for the gripper creation is always the center of the robot’s flange with the z axis pointing outwards. Via the REST-API it is possible to create a gripper with a tree structure, corresponding to multiple elements having the same parent element, as long as they are all connected.
Creating a gripper in the Web GUI¶
The tab CollisionCheck on the rc_cube Web GUI offers a simplified interface to create grippers. It is possible to select the type, the size, as well as the position of each element. In the Web GUI representation the position of each element originates from the bottom of the parent element. Therefore, a child element with position (0, 0, 0) will always be placed exactly below its parent element, irrespective of the elements’ heights. Grippers which have a tree structure or which have rotated elements cannot be created via the Web GUI.
Calculated TCP position¶
After gripper creation via the set_gripper service call, the TCP position in the flange coordinate
system is calculated and returned as tcp_pose_flange.
It is important to check if this value is the same as the robot’s true TCP position.
Creating rotationally asymmetric grippers¶
For grippers which are not rotationally symmetric around the z axis, it is crucial to ensure that the gripper is properly mounted, so that the representation stored in the CollisionCheck module corresponds to reality.
Collision checking¶
Stand-alone collision checking¶
The check_collision service call triggers collision checking between the chosen gripper and the provided load carriers
for each of the provided grasps.
The CollisionCheck module checks if the chosen gripper is
in collision with at least one of the load carriers, when the TCP of the gripper is positioned in the
grasp position. It is possible to check the collision with multiple load carriers simultaneously. The grasps
which are in collision with any of the defined load carriers will be returned as colliding.
The pre_grasp_offset can be used for additional collision checking.
The pre-grasp offset \(P_{off}\) is the offset between the grasp point \(P_{grasp}\) and the pre-grasp position \(P_{pre}\)
in the grasp’s coordinate frame (see Fig. 35).
If the pre-grasp offset is defined, the grasp will be detected as colliding
if the gripper is in collision with the load carrier at any point
during motion from the pre-grasp position to the grasp position (assuming
a linear movement).
Fig. 35 Illustration of the pre-grasp offset parameter for collision checking. In this case, the pre-grasp position as well as the grasp position are collision free. However, the trajectory between these poses would have collisions. Thus, this grasp pose would be marked as colliding.
Collision checking within other modules¶
Collision checking is integrated in the following modules’ services:
- ItemPick and BoxPick:
compute_grasps(see compute_grasps for ItemPick and compute_grasps for BoxPick) - SilhouetteMatch:
detect_object(see detect_object) - CADMatch:
detect_object(see detect_object)
Each of these services can take a collision_detection argument consisting of the gripper_id of the gripper
and optionally the pre_grasp_offset as described in the previous section
Stand-alone collision checking.
When the collision_detection argument is given, these services only return the
grasps at which the gripper is not in collision with the load carrier detected by these services. For this,
a load carrier ID has to be provided to these services as well.
Warning
Collisions are checked only between the load carrier and the gripper, not the robot itself, the flange, other objects or the item located in the robot gripper.
The collision-check results are affected by run-time parameters, which are listed and explained further below.
Parameters¶
The CollisionCheck component is called rc_collision_check
in the REST-API and is represented by the CollisionCheck page in the Modules tab of the
Web GUI.
The user can explore and configure the rc_collision_check
component’s run-time parameters, e.g. for development and testing, using the Web GUI or the
REST-API interface.
Parameter overview¶
This component offers the following run-time parameters:
| Name | Type | Min | Max | Default | Description |
|---|---|---|---|---|---|
check_bottom |
bool | false | true | true | Check collisions with the bottom of the load carrier |
check_flange |
bool | false | true | true | Position is in collision if robot flange is inside the load carrier |
collision_dist |
float64 | 0.0 | 0.1 | 0.01 | Minimal distance in meters between any part of the gripper and any of the load carrier’s walls for grasp to be considered collision free |
Description of run-time parameters¶
Each run-time parameter is represented by a row on the Web GUI’s Module tab. The name in the Web GUI is given in brackets behind the parameter name:
collision_dist(Collision Distance)- Minimal distance in meters between any part of the gripper and any of the load carrier’s walls for a grasp to be considered collision free.
check_flange(Check Flange)- Performs an additional safety check as described in Robot flange radius. If this parameter is set, all positions in which any part of the robot’s flange is inside the load carrier are marked as colliding.
check_bottom(Check Bottom)- When this check is enabled the collisions will be checked not only with the side walls of the load carrier but also with its bottom. It might be necessary to disable this check if the TCP is inside the collision geometry (e.g. is defined inside a suction cup).
Status values¶
The rc_collision_check component reports the following status values:
| Name | Description |
|---|---|
last_evaluated_grasps |
Number of evaluated grasps |
last_collision_free_grasps |
Number of collision-free grasps |
Services¶
The user can explore and call the rc_collision_check component’s services,
e.g. for development and testing, using
REST-API interface or
the rc_cube
Web GUI.
Each service response contains a return_code,
which consists of a value plus an optional message.
A successful service returns with a return_code value of 0.
Negative return_code values indicate that the service failed.
Positive return_code values indicate that the service succeeded with additional information.
The smaller value is selected in case a service has multiple return_code values,
but all messages are appended in the return_code message.
The following table contains a list of common codes:
| Code | Description |
|---|---|
| 0 | Success |
| -1 | An invalid argument was provided |
| -7 | Data could not be read or written to persistent storage |
| -9 | No valid license for the module |
| -10 | New gripper could not be added as the maximum storage capacity of grippers has been exceeded |
| 10 | The maximum storage capacity of grippers has been reached |
| 11 | Existing gripper was overwritten |
The CollisionCheck component offers the following services.
set_gripper¶
Persistently stores a gripper on the rc_cube. All configured grippers are persistent over firmware updates and rollbacks.
Request:
The definition for the request arguments with corresponding datatypes is:
{ "elements": [ { "box": { "x": "float64", "y": "float64", "z": "float64" }, "cylinder": { "height": "float64", "radius": "float64" }, "id": "string", "parent_id": "string", "pose": { "orientation": { "w": "float64", "x": "float64", "y": "float64", "z": "float64" }, "position": { "x": "float64", "y": "float64", "z": "float64" } }, "type": "string" } ], "flange_radius": "float64", "id": "string", "tcp_parent_id": "string", "tcp_pose_parent": { "orientation": { "w": "float64", "x": "float64", "y": "float64", "z": "float64" }, "position": { "x": "float64", "y": "float64", "z": "float64" } } }Required arguments:
elements: list of geometric elements for the gripper. Each element must be oftype‘CYLINDER’ or ‘BOX’ with the corresponding dimensions in thecylinderorboxfield. The pose of each element must be given in the coordinate frame of the parent element (see Setting a gripper for an explanation of the coordinate frames). The element’sidmust be unique and must not be ‘tcp’ or ‘flange’. Theparent_idis the ID of the parent element. It can either be ‘flange’ or it must correspond to another element in list.
flange_radius: radius of the flange used in case thecheck_flangerun-time parameter is active.
id: unique name of the gripper
tcp_parent_id: ID of the element on which the TCP is defined
tcp_pose_parent: The pose of the TCP with respect to the coordinate frame of the element specified intcp_parent_id.Response:
The definition for the response with corresponding datatypes is:
{ "gripper": { "elements": [ { "box": { "x": "float64", "y": "float64", "z": "float64" }, "cylinder": { "height": "float64", "radius": "float64" }, "id": "string", "parent_id": "string", "pose": { "orientation": { "w": "float64", "x": "float64", "y": "float64", "z": "float64" }, "position": { "x": "float64", "y": "float64", "z": "float64" } }, "type": "string" } ], "flange_radius": "float64", "id": "string", "tcp_parent_id": "string", "tcp_pose_flange": { "orientation": { "w": "float64", "x": "float64", "y": "float64", "z": "float64" }, "position": { "x": "float64", "y": "float64", "z": "float64" } }, "tcp_pose_parent": { "orientation": { "w": "float64", "x": "float64", "y": "float64", "z": "float64" }, "position": { "x": "float64", "y": "float64", "z": "float64" } }, "type": "string" }, "return_code": { "message": "string", "value": "int16" } }
gripper: returns the gripper as defined in the request with an additional fieldtcp_pose_flange. This gives the coordinates of the TCP in the flange coordinate frame for comparison with the true settings of the robot’s TCP.
return_code: holds possible warnings or error codes and messages.
get_grippers¶
Returns the configured grippers with the requested
gripper_ids. If nogripper_idsare provided, all configured grippers are returned.The definition for the request arguments with corresponding datatypes is:
{ "gripper_ids": [ "string" ] }The definition for the response with corresponding datatypes is:
{ "grippers": [ { "elements": [ { "box": { "x": "float64", "y": "float64", "z": "float64" }, "cylinder": { "height": "float64", "radius": "float64" }, "id": "string", "parent_id": "string", "pose": { "orientation": { "w": "float64", "x": "float64", "y": "float64", "z": "float64" }, "position": { "x": "float64", "y": "float64", "z": "float64" } }, "type": "string" } ], "flange_radius": "float64", "id": "string", "tcp_parent_id": "string", "tcp_pose_flange": { "orientation": { "w": "float64", "x": "float64", "y": "float64", "z": "float64" }, "position": { "x": "float64", "y": "float64", "z": "float64" } }, "tcp_pose_parent": { "orientation": { "w": "float64", "x": "float64", "y": "float64", "z": "float64" }, "position": { "x": "float64", "y": "float64", "z": "float64" } }, "type": "string" } ], "return_code": { "message": "string", "value": "int16" } }
delete_grippers¶
Deletes the configured grippers with the requested
gripper_ids. All grippers to be deleted must be explicitly stated ingripper_ids.The definition for the request arguments with corresponding datatypes is:
{ "gripper_ids": [ "string" ] }The definition for the response with corresponding datatypes is:
{ "return_code": { "message": "string", "value": "int16" } }
check_collision¶
Triggers a collision check.
Request:
The definition for the request arguments with corresponding datatypes is:
{ "grasps": [ { "pose": { "orientation": { "w": "float64", "x": "float64", "y": "float64", "z": "float64" }, "position": { "x": "float64", "y": "float64", "z": "float64" } }, "pose_frame": "string", "uuid": "string" } ], "gripper_id": "string", "load_carriers": [ { "id": "string", "inner_dimensions": { "x": "float64", "y": "float64", "z": "float64" }, "outer_dimensions": { "x": "float64", "y": "float64", "z": "float64" }, "pose": { "orientation": { "w": "float64", "x": "float64", "y": "float64", "z": "float64" }, "position": { "x": "float64", "y": "float64", "z": "float64" } }, "pose_frame": "string", "rim_thickness": { "x": "float64", "y": "float64" } } ], "pre_grasp_offset": { "x": "float64", "y": "float64", "z": "float64" } }Required arguments:
grasps: list of grasps that should be checked.
load_carriers: list of load carriers against which the collision should be checked. The fields of the load carrier definition are described in Detection of load carriers. The position frame of the grasps and load carriers has to be the same.
gripper_id: the id of the gripper that is used to check the collisions. The gripper has to be configured beforehand.Optional arguments:
pre_grasp_offset: the offset in meters from the grasp position to the pre-grasp position in the grasp frame. If this argument is set, the collisions will not only be checked in the grasp point, but also on the path from the pre-grasp position to the grasp position (assuming a linear movement).Response:
The definition for the response with corresponding datatypes is:
{ "colliding_grasps": [ { "pose": { "orientation": { "w": "float64", "x": "float64", "y": "float64", "z": "float64" }, "position": { "x": "float64", "y": "float64", "z": "float64" } }, "pose_frame": "string", "uuid": "string" } ], "collision_free_grasps": [ { "pose": { "orientation": { "w": "float64", "x": "float64", "y": "float64", "z": "float64" }, "position": { "x": "float64", "y": "float64", "z": "float64" } }, "pose_frame": "string", "uuid": "string" } ], "return_code": { "message": "string", "value": "int16" } }
colliding_grasps: list of grasps in collision with one or more load carriers.
collision_free_grasps: list of collision-free grasps.
return_code: holds possible warnings or error codes and messages.
save_parameters¶
This service saves the currently set parameters persistently. Thereby, the same parameters will still apply after a reboot of the rc_cube. The node parameters are not persistent over firmware updates and rollbacks.
This service has no arguments.
The definition for the response with corresponding datatypes is:
{ "return_code": { "message": "string", "value": "int16" } }
reset_defaults¶
This service resets all parameters of the component to its default values, as listed in above table. The reset does not apply to grippers.
This service has no arguments.
The definition for the response with corresponding datatypes is:
{ "return_code": { "message": "string", "value": "int16" } }