# checkCollision

## Syntax

## Description

`[`

checks if the specified rigid body tree robot model `isSelfColliding`

,`selfSeparationDist`

,`selfWitnessPts`

] = checkCollision(`robot`

,`config`

)`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.

`[`

checks if the specified rigid body tree robot model is in collision with itself or a
specified set of collision objects in the world `isColliding`

,`separationDist`

,`witnessPts`

] = checkCollision(`robot`

,`config`

,`worldObjects`

)`worldObjects`

.

`[___] = checkCollision(___,`

specifies additional options using one or more name-value pair arguments in addition to
any of argument combinations from previous syntaxes.`Name,Value`

)

## 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

### Skip Self-Collision Checking Between Parent and Adjacent Bodies

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.

### Skip Self-Collision Checking Between Specific Body Pairs

When you want to resolve all self-collisions in a robot model without checking them between bodies with a parent-child relationship or at adjacent indices, you can use the `SkippedSelfCollisions`

name-value argument with the `"parent"`

and `"adjacent"`

values, respectively. However, if your robot model has one or more self-collisions that you can not resolve when using either `"parent"`

or `"adjacent"`

, you can specify additional body pairs between which to skip collision checking.

Load an ABB IRB 1600 robot model into the workspace, with a data format of `"row"`

. This is an example of a robot model that contains an additional self-collision when you specify the `SkippedSelfCollision`

name-value argument as `"adjacent"`

.

rbt = loadrobot("abbIrb1600",DataFormat="row"); config = homeConfiguration(rbt)

`config = `*1×6*
0 0 0 0 0 0

Perform an exhaustive self-collision check, and skip self-collisions between bodies at adjacent indices

[isSelfColl1,sepDist1] = checkCollision(rbt,config,SkippedSelfCollisions="adjacent",Exhaustive="on");

You can determine which pairs of bodies are in collision by checking the separation distance matrix `sepDist1`

for `NaN`

values. To make the separation distance matrix easier to read, convert the separation distance matrix to a table. Use the body names and base link name to label the rows and columns. The `NaN`

values indicate that the self-collision is between `link_4`

and `link_6`

..

bodynames = [rbt.BodyNames rbt.Base.Name]; collTable = array2table(sepDist1,VariableNames=bodynames,RowNames=bodynames)

`collTable=`*8×8 table*
link_1 link_2 link_3 link_4 link_5 link_6 tool0 base_link
_______ _______ _______ _______ _______ _______ _____ _________
link_1 Inf Inf 0.176 0.36201 0.55837 0.59557 Inf Inf
link_2 Inf Inf Inf 0.25321 0.49887 0.54104 Inf 0.25599
link_3 0.176 Inf Inf Inf 0.244 0.286 Inf 0.65488
link_4 0.36201 0.25321 Inf Inf Inf NaN Inf 0.7801
link_5 0.55837 0.49887 0.244 Inf Inf Inf Inf 0.91534
link_6 0.59557 0.54104 0.286 NaN Inf Inf Inf 0.95033
tool0 Inf Inf Inf Inf Inf Inf Inf Inf
base_link Inf 0.25599 0.65488 0.7801 0.91534 0.95033 Inf Inf

Check the value of `isSelfColl1`

. The output argument returns `1`

(`true`

), indicating that the robot model has a self-collision between a pair of bodies that are not adjacent.

isSelfColl1

`isSelfColl1 = `*logical*
1

To skip checking for self-collision between `link_4`

and `link_6`

, you must manually specify all of the body pairs for which to skip checking for self-collision. First, create a list of all the body names in the model, in order from the base link to the tool. Then, by combining the ordered list of rigid body names with an offset version of itself, create a two-column cell array that specifies each adjacent rigid body pair. Then add `link_4`

and `link_6`

as an additional skipped body pair.

adjbodynames = [rbt.Base.Name rbt.BodyNames]

`adjbodynames = `*1x8 cell*
{'base_link'} {'link_1'} {'link_2'} {'link_3'} {'link_4'} {'link_5'} {'link_6'} {'tool0'}

skiplist = cell(rbt.NumBodies,2); for i = 1:rbt.NumBodies skiplist(i,:) = {adjbodynames{i}, adjbodynames{i+1}}; end skiplist = [skiplist; {'link_4','link_6'}]

`skiplist = `*8x2 cell*
{'base_link'} {'link_1'}
{'link_1' } {'link_2'}
{'link_2' } {'link_3'}
{'link_3' } {'link_4'}
{'link_4' } {'link_5'}
{'link_5' } {'link_6'}
{'link_6' } {'tool0' }
{'link_4' } {'link_6'}

Perform self-collision checking, and note that `checkCollision`

no longer indicates self-collision between `link_4`

and `link_6`

.

```
[isSelfColl2,sepDist2] = checkCollision(rbt,config,SkippedSelfCollisions=skiplist,Exhaustive="on");
isSelfColl2
```

`isSelfColl2 = `*logical*
0

array2table(sepDist2,VariableNames=bodynames,RowNames=bodynames)

`ans=`*8×8 table*
link_1 link_2 link_3 link_4 link_5 link_6 tool0 base_link
_______ _______ _______ _______ _______ _______ _____ _________
link_1 Inf Inf 0.176 0.36201 0.55837 0.59557 Inf Inf
link_2 Inf Inf Inf 0.25321 0.49887 0.54104 Inf 0.25599
link_3 0.176 Inf Inf Inf 0.244 0.286 Inf 0.65488
link_4 0.36201 0.25321 Inf Inf Inf Inf Inf 0.7801
link_5 0.55837 0.49887 0.244 Inf Inf Inf Inf 0.91534
link_6 0.59557 0.54104 0.286 Inf Inf Inf Inf 0.95033
tool0 Inf Inf Inf Inf Inf Inf Inf Inf
base_link Inf 0.25599 0.65488 0.7801 0.91534 0.95033 Inf Inf

## 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"`

| *p*-by-2 cell array of character vectors

Body pairs skipped for checking self-collisions, specified as either
`"parent"`

, `"adjacent"`

or as a
*p*-by-2 cell array of character vectors:

`"parent"`

— Skip collision checking between child and parent bodies. See Skip Self-Collision Checking Between Parent and Adjacent Bodies for more information.`"adjacent"`

— Skip collision checking between bodies on adjacent indices. See Skip Self-Collision Checking Between Parent and Adjacent Bodies for more information.*p*-by-2 cell array of character vectors — Skip collision checking between specific body pairs. Each row specifies a pair of body names between which to skip self-collision checking.*p*is the number of body pairs to skip for self-collision checking. For more information, see Skip Self-Collision Checking Between Specific Body Pairs.

**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:

$$\left[\begin{array}{cc}{x}_{1}& {x}_{2}\\ {y}_{1}& {y}_{2}\\ {z}_{1}& {z}_{2}\end{array}\right]$$

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:

$${W}_{r}=3{S}_{r}-2$$

$${W}_{c}=2{S}_{c}-1$$

Where
(*S _{r}*,

*S*) is the index of a separation distance in the separation distance matrix and (

_{c}*W*,

_{r}*W*) is the index in the witness point matrix at which the corresponding witness points begin.

_{c}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:

$$\left[\begin{array}{cc}{x}_{1}& {x}_{2}\\ {y}_{1}& {y}_{2}\\ {z}_{1}& {z}_{2}\end{array}\right]$$

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:

$${W}_{r}=3{S}_{r}-2$$

$${W}_{c}=2{S}_{c}-1$$

Where
(*S _{r}*,

*S*) is the index of a separation distance in the separation distance matrix and (

_{c}*W*,

_{r}*W*) is the index in the witness point matrix at which the corresponding witness points begin.

_{c}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 3-by-2 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.

`Wr`

elements represent the witness points between the robot
bodies, `selfWitnessPts`

. `Woi_j`

elements represent
the witness points between the robot bodies and base, and the world objects. The
`i`

indices correspond to the robot bodies, and the
`j`

indices correspond to collision objects from the world object
list `worldObjects`

.

If a pair is 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**

### R2023b: Specify each body pair to skip for self-collision checking

You can now select which body pairs to skip in self-collision checks by specifying the
`SkippedSelfCollisions`

name-value argument as a cell array of body
pairs. For more information, see the `SkippedSelfCollisions`

name-value argument.

### 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 Skip Self-Collision Checking Between Parent and Adjacent Bodies for more information about how to use this name-value argument.

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