version: "1.0.1" name: matlab-model-robot-kinematics description: > Use whenever a MATLAB robot model (rigidBodyTree) is needed — whether for simulation, visualization, IK, motion planning, pick-and-place, or trajectory generation. Triggers on: any mention of a manipulator by name (UR3, UR5, UR5e, UR10, UR10e, UR16e, UR20, KUKA iiwa, Fanuc, ABB, Panda, Kinova, Sawyer, Baxter), robot modeling verbs (load, create, build, import, simulate, model), tasks implying a robot model (pick, place, lift, reach, move, grasp, plan motion for, animate), kinematics keywords (IK, FK, inverse kinematics, forward kinematics, joint configuration, end-effector pose, gripper), or working with rigidBodyTree, loadrobot, importrobot, URDF, DH parameters, addVisual, addCollision. license: https://www.mathworks.com/content/dam/mathworks/license/pmrl/license.md metadata: author: MathWorks version: "1.0"

Robot Modeling & Kinematics

Build manipulator models correctly and validate every kinematic solution.

When to Use

• Any task that requires a robot model in MATLAB (even if the goal is downstream: animation, pick-and-place, trajectory, motion planning)
• User mentions a specific manipulator by name (UR5e, KUKA iiwa14, Panda, Kinova Gen3, etc.)
• Loading, building, or importing a robot model
• Simulating, animating, or visualizing a manipulator
• Tasks involving lifting, reaching, picking, placing, or grasping
• Adding visual meshes or collision geometry
• Attaching a gripper to a manipulator
• Choosing between inverseKinematics vs generalizedInverseKinematics
• Validating IK/GIK solutions (exit flags, position error, joint continuity)

When NOT to Use

• Collision-free motion planning (RRT, CHOMP, path planning) — use a motion planning skill
• Trajectory generation (contopptraj, cubicpolytraj, trapveltraj) — use a trajectory skill
• Closed-loop parallel mechanisms (delta, Stewart) — see references/parallel-robot-guidance.md
• Mobile robot navigation or wheeled platforms — not covered here
• Dynamics simulation (inverseDynamics, forwardDynamics) — separate domain

Workflow

Step 1: Determine Model Source

Choose the highest-fidelity source available:

Critical: A robot without collision meshes will cause downstream motion planning to silently report "collision-free" paths that actually collide. Always prefer sources that include meshes.

If the user says "build a robot" but names a known robot (UR5e, KUKA iiwa, Panda, etc.), suggest loadrobot first.

After importing from URDF, Simscape, or CAD: Check if collision meshes are present. Simscape Multibody models and many URDFs/CAD exports only provide visual geometry — no collision meshes. If collision meshes are missing, read references/import-mesh-handling.md for the decision tree on generating them from visuals.

Step 2: Load or Build the Model

Always set DataFormat="row" — this is required for planning and dynamics workflows.

Option A — loadrobot:

robot = loadrobot("universalUR5e", DataFormat="row");

Option B — importrobot (URDF/Xacro/SDF):

robot = importrobot("myRobot.urdf", DataFormat="row");

If the URDF has only visual geometry (no <collision> tags), add VHACD collision decomposition at import time:

opts = vhacdOptions("RigidBodyTree");
opts.SourceMesh = "VisualGeometry";
robot = importrobot("myRobot.urdf", DataFormat="row", ...
    MeshPath="path/to/meshes", CollisionDecomposition=opts);

Option C — Build from DH parameters:

robot = rigidBodyTree(DataFormat="row");
 
body = rigidBody("link1");
jnt = rigidBodyJoint("joint1", "revolute");
setFixedTransform(jnt, [a alpha d 0], "dh");
jnt.PositionLimits = [-pi, pi];
body.Joint = jnt;
 
addVisual(body, "Cylinder", [0.04, 0.3]);
addCollision(body, "Cylinder", [0.05, 0.3]);
 
addBody(robot, body, "base");

Step 3: Ensure tool0 Exists with Z = Outward

Before attaching a gripper, the robot MUST have a tool0 body whose local Z-axis points along the arm's outward direction (away from the wrist). Most loadrobot models satisfy this already — but KUKA iiwa models do not.

Check `references/ee-frame-alignment.md` for the full compatibility table and diagnostic procedure.

If the robot already has `tool0` with Z = outward (all UR, ABB, Techman, FANUC, Kinova, Yaskawa): proceed directly to gripper attachment.

If the robot lacks `tool0` or EE Z is misaligned (KUKA iiwa 7/14): add a corrective tool0 frame:

% KUKA iiwa: iiwa_link_ee Z is perpendicular to arm — fix with Ry(90°)
tool0 = rigidBody("tool0");
tool0Joint = rigidBodyJoint("tool0_joint", "fixed");
R_correction = [0 0 1; 0 1 0; -1 0 0]; % Ry(90°)
setFixedTransform(tool0Joint, rotm2tform(R_correction));
tool0.Joint = tool0Joint;
addBody(robot, tool0, "iiwa_link_ee");

For unknown robots (URDF imports): Run the diagnostic check from references/ee-frame-alignment.md to verify alignment before attaching anything.

Step 4: Configure the Model

Visuals vs collisions — both are needed, for different purposes:

To color a visual: addVisual(body, "Mesh", stlFile, tform, FaceColor=[1 0.8 0])

Robots from loadrobot already have both. When building from scratch, add both explicitly.

Attach a gripper (MANDATORY for manipulation tasks):

If the task involves object interaction (picking, placing, lifting, grasping, manipulating), a gripper is required — do not skip this step. Only omit for pure visualization or reachability studies with no object contact.

Before building ANY custom gripper, you MUST read `references/gripper-models.md`. Available models include both parallel-jaw (robotiq2F85, 85mm opening) AND vacuum grippers (robotiqEPick variants for large/flat objects). Do NOT assume robotiq2F85 is the only option.

1. Read references/gripper-models.md
2. Follow the decision tree in that file (check compatibility → check object size → select grip strategy)
3. Only build custom if no provided model fits (incompatible manipulator AND object unsuitable for vacuum)

gripper = loadrobot("robotiq2F85", DataFormat="row");
addSubtree(robot, "tool0", gripper, ReplaceBase=false);

Critical: Always use `ReplaceBase=false`. The default (true) merges the gripper's base link into the parent body, silently discarding the base visual mesh (the gripper housing/coupling). All provided grippers have a base visual that is lost without this flag.

After attaching a gripper, add a contact frame at the gripper's contact point (fixed joint, zero DOF added), then solve IK to that frame. This avoids confusing tip frame orientations and manual offset math. See references/gripper-models.md for per-gripper offsets and the full pattern.

contactFrame = rigidBody("contact_point");
contactJoint = rigidBodyJoint("contact_joint", "fixed");
setFixedTransform(contactJoint, trvec2tform([0, 0, gripperOffset]));
contactFrame.Joint = contactJoint;
addBody(robot, contactFrame, "tool0");

Add a custom end-effector frame (when no gripper is attached or you need a specific offset):

ee = rigidBody("tool_tip");
eeJoint = rigidBodyJoint("tool_tip_joint", "fixed");
setFixedTransform(eeJoint, trvec2tform([0.1, 0, 0]));
ee.Joint = eeJoint;
addBody(robot, ee, "tool0");

Step 5: Verify the Model with Kinematics

Use FK and IK to confirm the model is built correctly — correct link lengths, joint axes, limits, and end-effector frame placement.

Verify with FK: Move to a known configuration and check the end-effector reaches the expected position.

q = homeConfiguration(robot);
tform = getTransform(robot, q, "tool0");
fprintf("Home position: [%.3f, %.3f, %.3f] m\n", tform(1:3,4));

Visually confirm the pose makes sense:

figure;
show(robot, q, Frames="off");
axis auto;
title("Home configuration");

Verify with IK: Pick a target you know is reachable and confirm the solver converges.

Decision — IK vs GIK:

See references/gik-constraints.md for all available GIK constraint types.

ik = inverseKinematics(RigidBodyTree=robot);
weights = [0.25 0.25 0.25 1 1 1];
targetPose = trvec2tform([0.4, 0.1, 0.3]) * eul2tform([0 pi 0], "ZYX");
[qSol, solnInfo] = ik("tool0", targetPose, weights, homeConfiguration(robot));

Step 6: Validate Every IK/GIK Solution (MANDATORY)

Never skip this step. Agents consistently skip validation and deliver solutions that silently failed.

After `inverseKinematics`:

if solnInfo.ExitFlag <= 0
    warning("IK did not converge. ExitFlag: %d, Status: %s", ...
        solnInfo.ExitFlag, solnInfo.Status);
end
 
tformActual = getTransform(robot, qSol, "tool0");
posError = norm(tformActual(1:3,4)' - targetPose(1:3,4)');
if posError > 1e-3
    warning("IK position error %.4f m exceeds threshold.", posError);
end

After `generalizedInverseKinematics`:

if solInfo.ExitFlag <= 0
    warning("GIK did not converge. ExitFlag: %d, Status: %s", ...
        solInfo.ExitFlag, solInfo.Status);
end
 
for i = 1:numel(solInfo.ConstraintViolations)
    cv = solInfo.ConstraintViolations(i);
    if cv.Violation > 1e-3
        warning("Constraint %d (%s) violated: %.4f", i, cv.Type, cv.Violation);
    end
end

For sequential waypoints — check joint continuity:

for i = 2:size(qAll, 1)
    maxJump = max(abs(qAll(i,:) - qAll(i-1,:)));
    if maxJump > deg2rad(30)
        warning("Joint jump of %.1f deg between waypoints %d and %d.", ...
            rad2deg(maxJump), i-1, i);
    end
end

Key Functions

Patterns

Visualization

figure;
show(robot, q, Frames="off");
axis auto;
view(45, 30);
title("Robot at configuration q");

For multiple configurations, use tiledlayout/nexttile:

figure;
tiledlayout(1, 3);
for i = 1:3
    nexttile;
    show(robot, qAll(i,:), Frames="off", PreservePlot=false);
    axis auto;
    title(sprintf("Waypoint %d", i));
end

Using Initial Guess to Improve IK Convergence

When solving IK for sequential waypoints, use the previous solution as the initial guess:

qPrev = homeConfiguration(robot);
for i = 1:numWaypoints
    [qSol, solnInfo] = ik("tool0", targetPoses(:,:,i), weights, qPrev);
    % ... validate ...
    qPrev = qSol;
end

Conventions

• Always set DataFormat="row" on creation or import
• Joint limits in radians (use deg2rad when specifying in degrees)
• Use Frames="off" and axis auto for clean visualization
• Use tiledlayout/nexttile instead of subplot
• Name end-effector frames descriptively (e.g., "tool_tip", "bucket_tip")
• Weights for inverseKinematics: [orientation(3) position(3)] — set position weights higher for position-priority tasks

Common Mistakes

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