checkCollision

Check if robot is in collision

Since R2020b

Syntax

[isSelfColliding,selfSeparationDist,selfWitnessPts] = checkCollision(robot,config)

[isColliding,separationDist,witnessPts] = checkCollision(robot,config,worldObjects)

[___] = checkCollision(___,Name,Value)

Description

[isSelfColliding,selfSeparationDist,selfWitnessPts] = checkCollision(robot,config) checks if the specified rigid body tree robot model robot is in self-collision at the specified configuration config. Add collision objects to the rigid body tree robot model using the addCollision function. The checkCollision function also returns the closest separation distance selfSeparationDist and the witness points selfWitnessPts as points on each body.

The function ignores adjacent bodies when checking for self-collisions.

[isColliding,separationDist,witnessPts] = checkCollision(robot,config,worldObjects) checks if the specified rigid body tree robot model is in collision with itself or a specified set of collision objects in the world worldObjects.

[___] = checkCollision(___,Name,Value) specifies additional options using one or more name-value pair arguments in addition to any of argument combinations from previous syntaxes.

Examples

Add Collision Meshes and Check Collisions for Manipulator Robot Arm

Load a robot model and modify the collision meshes. Clear existing collision meshes, add simple collision object primitives, and check whether certain configurations are in collision.

Load Robot Model

Load a preconfigured robot model into the workspace using the loadrobot function. This model already has collision meshes specified for each body. Iterate through all the rigid body elements and clear the existing collision meshes. Confirm that the existing meshes are gone.

robot = loadrobot('kukaIiwa7','DataFormat','column');

for i = 1:robot.NumBodies
    clearCollision(robot.Bodies{i})
end

show(robot,'Collisions','on','Visuals','off');

Add Collision Cylinders

Iteratively add a collision cylinder to each body. Skip some bodies for this specific model, as they overlap and always collide with the end effector (body 10).

collisionObj = collisionCylinder(0.05,0.25);

for i = 1:robot.NumBodies
    if i > 6 && i < 10
        % Skip these bodies.
    else
        addCollision(robot.Bodies{i},collisionObj)
    end
end

show(robot,'Collisions','on','Visuals','off');

Check for Collisions

Generate a series of random configurations. Check whether the robot is in collision at each configuration. Visualize each configuration that has a collision.

figure
rng(0) % Set random seed for repeatability.
for i = 1:20
    config = randomConfiguration(robot);
    isColliding = checkCollision(robot,config,'SkippedSelfCollisions','parent');
    if isColliding
        show(robot,config,'Collisions','on','Visuals','off');
        title('Collision Detected')
    else
        % Skip non-collisions.
    end
end

Change Self-Collision Checking Behavior

This example shows how to change which rigid body pairs are skipped during self-collision checking in rigid body trees using the SkippedSelfCollisions name-value argument for checkCollision.

Serial Manipulator Robot

Load a serial manipulator robot represented as a two joint rigid body tree. Since this robot does not have collision geometries, add some primitive collision geometries.

rbt2j = twoJointRigidBodyTree;
P = [0.05 0.45]; % Geometry parameters for capsules
T = trvec2tform([0.2 0 0]) * eul2tform([0 pi/2 0],"XYZ"); % Transformation parameters for capsules
addCollision(rbt2j.Base,"cylinder",[0.075 0.1],trvec2tform([0 0 0.05]))
addCollision(rbt2j.Bodies{1},"capsule",P,T)
addCollision(rbt2j.Bodies{2},"capsule",P,T)
addCollision(rbt2j.Bodies{3},"box",[0.2 0.05 0.2])

Visualize the robot with collisions on.

show(rbt2j,homeConfiguration(rbt2j),Collisions="on");

By default, SkippedSelfCollisions is "parent", so self-collision checking skips collisions between parent and child bodies. Check the parent and child bodies of body2.

body2 = rbt2j.Bodies{2};
rbt2j.BodyNames

ans = 1x3 cell
    {'body1'}    {'body2'}    {'tool'}

body2.Parent.Name

ans = 
'body1'

body2.Children{1}.Name

ans = 
'tool'

This means that "body2" is not checked for collisions against "body1" or "tool".

List the body names of the robot. This shows that in the cell array, "body2", which is stored at index 2, is adjacent to both "body1" at index 1 and "tool" at index 3. Because the skipped collision pairs have not changed, the SkippedSelfCollisions name-value argument has no effect on the self-collision checking result for this robot.

rbt2j.BodyNames

ans = 1x3 cell
    {'body1'}    {'body2'}    {'tool'}

Run collision checking with both SkippedSelfCollisions options to verify that the SkippedSelfCollisions name-value argument returns the same result for this robot.

checkCollision(rbt2j,homeConfiguration(rbt2j),SkippedSelfCollisions="parent")

ans = logical
   0

checkCollision(rbt2j,homeConfiguration(rbt2j),SkippedSelfCollisions="adjacent")

ans = logical
   0

Parallel Manipulator Robot

Use the exampleHelperCreate2ArmRBT example helper to create a parallel robot comprised of two one-joint arms.

rbt2arm = exampleHelperCreate2ArmRBT;
show(rbt2arm,homeConfiguration(rbt2arm),Collisions="on");
axis padded

List the body names of the robot. The skipped body pairs formed by bodies of adjacent indices is similar to the serial manipulator but without body, "tool".

rbt2arm.BodyNames

ans = 1x2 cell
    {'body1'}    {'body2'}

Check the parent of body1 and the parent of body2. Each body forms a parent-child relationship with the base, even though they are at adjacent indices.

rbt2arm.Bodies{1}.Parent.Name

ans = 
'base'

rbt2arm.Bodies{2}.Parent.Name

ans = 
'base'

Run collision checking with both SkippedSelfCollisions options.

checkCollision(rbt2arm,homeConfiguration(rbt2arm),SkippedSelfCollisions="parent")

ans = logical
   0

checkCollision(rbt2arm,homeConfiguration(rbt2arm),SkippedSelfCollisions="adjacent")

ans = logical
   1

As expected, when skipping parent-child body pairs during self collision checks, checkCollision finds no self collisions, but does find a self collision between the "base" and "body2" when skipping body pairs of adjacent indices.

Input Arguments

robot — Rigid body tree robot model

rigidBodyTree object

Rigid body tree robot model, specified as a rigidBodyTree object. To use the checkCollision function, the DataFormat property of the rigidBodyTree object must be either 'row' or 'column'.

config — Joint configuration of rigid body tree

n-element numeric vector

Joint configuration of the rigid body tree, specified as an n-element numeric vector, where n is the number of nonfixed joints in the robot model. Each element of the vector is a specific joint position for a joint in the robot model.

Data Types: single | double

worldObjects — List of collision objects in world

{} (default) | cell array of collision objects

List of collision objects in the world, specified as a cell array of collision objects with any combination of collisionBox, collisionCylinder, collisionSphere, and collisionMesh objects. The function assumes that the Pose property of each object is relative to the base of the rigid body tree robot model.

Name-Value Arguments

Specify optional pairs of arguments as Name1=Value1,...,NameN=ValueN, where Name is the argument name and Value is the corresponding value. Name-value arguments must appear after other arguments, but the order of the pairs does not matter.

Before R2021a, use commas to separate each name and value, and enclose Name in quotes.

Example: 'Exhaustive','on' enables exhaustive checking for collisions and causes the function to calculate all separation distances and witness points.

Exhaustive — Check for all collisions

'off' (default) | 'on'

Exhaustively check for all collisions, specified as the comma-separated pair consisting of 'Exhaustive' and 'on' or 'off'. By default, the function finds the first collision and stops, returning the separation distances and witness points for incomplete checks as Inf.

If this name-value pair argument is specified as 'on', the function instead continues checking for collisions until it has exhausted all possibilities.

Data Types: char | string

IgnoreSelfCollision — Skip checking for robot self-collisions

'off' (default) | 'on'

Skip checking for robot self-collisions, specified as the comma-separated pair consisting of 'IgnoreSelfCollision' and 'on' or 'off'. When this argument is enabled, the function ignores collisions between the collision objects of the rigid body tree robot model bodies and other collision objects of the same model or its base.

This name-value pair argument affects the size of the separationDist and witnessPts output arguments.

Data Types: char | string

SkippedSelfCollisions — Body pairs skipped for checking self-collisions

"parent" (default) | "adjacent"

Body pairs skipped for checking self-collisions, specified as either "parent" or "adjacent":

• "parent" — Skip collision checking between child and parent bodies.

• "adjacent" — Skip collision checking between bodies on adjacent indices.

See Change Self-Collision Checking Behavior for more information.

Data Types: char | string

Output Arguments

Self Collisions

isSelfColliding — Robot configuration is in self-collision

0 | 1

Robot configuration is in self-collision returned as a logical 0 (false) or 1 (true). If the function returns a value of true for this argument, that means that one of the rigid body collision objects is touching another collision object in the robot model. Add collision objects to your rigid body tree robot model using the addCollision function.

Data Types: logical

selfSeparationDist — Minimum separation distance between bodies of robot

(m+1) -by-(m+1) matrix

Minimum separation distance between the bodies of the robot, returned as an (m+1) -by-(m+1) matrix, where m is the number of bodies. The final row and column correspond to the robot base. Units are in meters.

If a pair is in collision, the function returns the separation distance for the associated element as NaN.

Data Types: double

selfWitnessPts — Witness points between robot bodies

3(m+1) -by-2(m+1) matrix

Witness points between the robot bodies including the base, returned as an 3(m+1)-by-2(m+1) matrix, where m is the number of bodies. Witness points are the points on any two bodies that are closest to one another for a given configuration. The matrix takes the form:

The matrix is divided into 3-by-2 sections that represent the xyz-coordinates of witness point pairs in the form:

$$\left[\begin{array}{cc}{x}_1& x_2\\ y_1& y_2\\ z_1& y_2\end{array}\right]$$

Each section corresponds to a separation distance in the selfSeparationDist output matrix. Use these equations to determine where the section of the selfWitnessPts matrix that corresponds to a specific separation distance begins:

$$W_r=3S_r-2$$

$$W_c=2S_c-1$$

Where (S_r,S_c) is the index of a separation distance in the separation distance matrix and (W_r,W_c) is the index in the witness point matrix at which the corresponding witness points begin.

If a pair is in collision, the function returns each coordinate of the witness points for that element as NaN.

Data Types: double

World Collisions

isColliding — Robot configuration is in collision

two-element logical vector

Robot configuration is in collision, returned as a two-element logical vector. The first element indicates whether the robot is in self-collision. The second element indicates whether the robot model is in collision with any world objects.

Data Types: logical

separationDist — Minimum separation distance between collision objects

(m+1)-by-(m+w+1) matrix

Minimum separation distance between the collision objected, returned as an (m+1)-by-(m+w+1) matrix, where m is the number of bodies and w is the number of world objects. The first m rows correspond to the robot bodies, where the (m+1)^th row or column index corresponds to the base. The remaining w columns correspond to the world objects.

The matrix is divided into 3-by-2 sections that represent the xyz-coordinates of witness point pairs in the form:

$$\left[\begin{array}{cc}{x}_1& x_2\\ y_1& y_2\\ z_1& y_2\end{array}\right]$$

Each section corresponds to a separation distance in the separationDist output matrix. Use these equations to determine where the section of the witnessPts matrix that corresponds to a specific separation distance begins:

$$W_r=3S_r-2$$

$$W_c=2S_c-1$$

Where (S_r,S_c) is the index of a separation distance in the separation distance matrix and (W_r,W_c) is the index in the witness point matrix at which the corresponding witness points begin.

If a pair is in collision, the function returns each coordinate of the witness points for that element as NaN.

If a pair is in collision, the function returns the separation distance as NaN.

Dependencies

If you specify the 'IgnoreSelfCollision' name-value pair argument as 'on', then the matrix does not contain values for the distances between any given body and other bodies in the robot model.

Data Types: double

witnessPts — Witness points between collision objects

3(m+1)-by-2(m+w+1) matrix

Witness points between collision objects, specified as a 3(m+1)-by-2(m+w+1) matrix, where m is the number of bodies and w is the number of world objects. Witness points are the points on any two bodies that are closest to one another for a given configuration. The matrix takes the form:

[Wr1_1       Wr1_2     ...    Wr1_(N+1)     Wo1_1     Wo1_2      ... W1_M;
 Wr2_1       Wr2_2     ...    Wr2_(N+1)     Wo2_1     Wo2_2      ... W2_M;
 .           .         .      .             .         .          .   .
 .           .         .      .             .         .          .   .
 .           .         .      .             .         .          .   .
 Wr(N+1)_1   Wr(N+1)_2 ...    Wr(N+1)_(N+1) Wo(N+1)_1 Wo(N+1)_2  ... W(N+1)_M]

Each element in the above matrix is a 2-by-3 matrix that gives the nearest [x y z] points on the two corresponding bodies or world objects. The final row and column correspond to the robot base.

If a pair are in collision, witness points for that element are returned as NaN(3,2).

Dependencies

If the "IgnoreSelfCollision" name-value pair is set to "on", then the matrix contains no Wr elements.

Data Types: double

Extended Capabilities

C/C++ Code Generation

Generate C and C++ code using MATLAB® Coder™.

Usage notes and limitations:

When creating the rigidBodyTree object, use the syntax that specifies the MaxNumBodies as an upper bound for adding bodies to the robot model. You must also specify the DataFormat property as a name-value pair. For example:

robot = rigidBodyTree("MaxNumBodies",15,"DataFormat","row")

To minimize data usage, limit the upper bound to a number close to the expected number of bodies in the model. All data formats are supported for code generation. To use the dynamics functions, the data format must be set to "row" or "column".

The show and showdetails functions do not support code generation.

Version History

Introduced in R2020b

R2022b: Alter rigid body tree self-collision checking behavior change and new default self-collision checking behavior

You can now specify self-collision checking behavior for a rigid body tree robot model by using the SkippedSelfCollisions name-value argument. Specify SkippedSelfCollisions as "parent" or "adjacent":

• "parent" — Collision checking ignores self-collisions between parent and child rigid bodies.

• "adjacent" — Collision checking ignores self -collisions between rigid bodies of adjacent indices.

As of R2022b, the default behavior of collision checking is to ignore self-collisions between parent and child rigid bodies. In previous releases, the default behavior of self-collision checking was to ignore self-collisions between adjacent rigid bodies. To instead ignore self-collisions between rigid bodies of adjacent indices, specify SkippedSelfCollisions as "adjacent".

See Change Self-Collision Checking Behavior for more information about how to use this name-value argument.