Robot arm motion planning in ROS - Mini Project: Build & Apply

Robot arm motion planning

📖 Scenario: You are programming a robot arm to move safely from one position to another in a factory. You will create a simple motion plan using ROS (Robot Operating System) to control the arm's movements step-by-step.

🎯 Goal: Build a ROS node that sets up a robot arm's initial joint positions, defines a target position, plans a motion path, and executes the movement.

📋 What You'll Learn

Create a dictionary with the robot arm's initial joint positions

Define a target joint position dictionary

Use MoveIt! to plan a motion path from initial to target positions

Execute the planned motion path

💡 Why This Matters

🌍 Real World

Robot arms in factories need precise motion planning to move safely and efficiently between tasks without collisions.

💼 Career

Robotics engineers and automation developers use ROS and MoveIt! to program and control robot arms in manufacturing and research.

1

Set up initial joint positions

Create a dictionary called initial_joint_positions with these exact entries: 'joint_1': 0.0, 'joint_2': -1.57, 'joint_3': 1.57, 'joint_4': 0.0, 'joint_5': 0.0, 'joint_6': 0.0.

ROS

# Your code here

Hint

Use a Python dictionary with joint names as keys and float values for angles in radians.

2

Define target joint positions

Create a dictionary called target_joint_positions with these exact entries: 'joint_1': 0.5, 'joint_2': -1.0, 'joint_3': 1.0, 'joint_4': 0.0, 'joint_5': 0.5, 'joint_6': 0.0.

ROS

initial_joint_positions = {
    'joint_1': 0.0,
    'joint_2': -1.57,
    'joint_3': 1.57,
    'joint_4': 0.0,
    'joint_5': 0.0,
    'joint_6': 0.0
}
# Your code here

Hint

Use a Python dictionary with the exact joint names and target angles in radians.

3

Plan the motion path

Use MoveIt! to create a MoveGroupCommander for the robot arm named arm. Set the current joint values to initial_joint_positions. Then plan a motion path to target_joint_positions using plan = move_group.plan().

ROS

initial_joint_positions = {
    'joint_1': 0.0,
    'joint_2': -1.57,
    'joint_3': 1.57,
    'joint_4': 0.0,
    'joint_5': 0.0,
    'joint_6': 0.0
}
target_joint_positions = {
    'joint_1': 0.5,
    'joint_2': -1.0,
    'joint_3': 1.0,
    'joint_4': 0.0,
    'joint_5': 0.5,
    'joint_6': 0.0
}
# Your code here

Hint

Import moveit_commander, create a MoveGroupCommander for 'arm', set initial joints, move, then set target joints and plan.

4

Execute the planned motion

Use move_group.execute(plan, wait=True) to run the planned motion path on the robot arm.

ROS

import moveit_commander

moveit_commander.roscpp_initialize([])
move_group = moveit_commander.MoveGroupCommander('arm')
move_group.set_joint_value_target(initial_joint_positions)
move_group.go(wait=True)
move_group.set_joint_value_target(target_joint_positions)
plan = move_group.plan()
# Your code here

Hint

Call execute on move_group with the plan and wait=True.

Practice

1. What is the main purpose of robot arm motion planning in ROS?

easy

A. To control the robot's camera settings

B. To calculate safe and smooth paths for the robot arm to move

C. To manage network communication between robots

D. To store sensor data from the robot arm

Solution

1. Step 1: Understand the role of motion planning

   Motion planning calculates paths that avoid obstacles and ensure smooth movement.

2. Step 2: Identify the correct purpose among options

   Only To calculate safe and smooth paths for the robot arm to move describes calculating safe and smooth paths for the robot arm.

3. Final Answer:

   To calculate safe and smooth paths for the robot arm to move -> Option B

4. Quick Check:

   Motion planning = safe, smooth paths [OK]

Hint: Motion planning means planning safe arm movements [OK]

Common Mistakes:

• Confusing motion planning with sensor data handling
• Thinking motion planning controls cameras
• Mixing motion planning with network communication

2. Which of the following is the correct way to create a MoveGroupInterface object for a robot arm named "manipulator" in ROS C++?

easy

A. MoveGroupInterface move_group("robot_arm");

B. MoveGroupInterface move_group = new MoveGroupInterface("manipulator");

C. move_group = MoveGroupInterface("manipulator");

D. moveit::planning_interface::MoveGroupInterface move_group("manipulator");

Solution

1. Step 1: Recall correct MoveGroupInterface syntax

   In ROS C++, the MoveGroupInterface is created with the namespace and constructor: moveit::planning_interface::MoveGroupInterface move_group("group_name");

2. Step 2: Check each option for syntax and group name

   moveit::planning_interface::MoveGroupInterface move_group("manipulator"); uses correct syntax and the correct group name "manipulator". MoveGroupInterface move_group = new MoveGroupInterface("manipulator"); uses incorrect new keyword and assignment. move_group = MoveGroupInterface("manipulator"); lacks type declaration. MoveGroupInterface move_group("robot_arm"); uses wrong group name.

3. Final Answer:

   moveit::planning_interface::MoveGroupInterface move_group("manipulator"); -> Option D

4. Quick Check:

   Correct constructor syntax = moveit::planning_interface::MoveGroupInterface move_group("manipulator"); [OK]

Hint: Use full type and constructor with group name in quotes [OK]

Common Mistakes:

• Using 'new' keyword incorrectly in C++
• Omitting type declaration
• Using wrong group name string

3. Given the following ROS C++ code snippet, what will be the output if planning succeeds?

moveit::planning_interface::MoveGroupInterface move_group("manipulator");
auto success = (move_group.plan(plan) == moveit::planning_interface::MoveItErrorCode::SUCCESS);
if (success) {
  std::cout << "Plan found!" << std::endl;
} else {
  std::cout << "Plan failed." << std::endl;
}

medium

A. Plan found!

B. Plan failed.

C. Compilation error

D. No output

Solution

1. Step 1: Understand the plan() return value check

   The plan() function returns a MoveItErrorCode. Comparing it to SUCCESS means success is true if planning worked.

2. Step 2: Analyze the if condition and output

   If success is true, the code prints "Plan found!"; otherwise, "Plan failed.". Since planning succeeds, output is "Plan found!".

3. Final Answer:

   Plan found! -> Option A

4. Quick Check:

   Plan success prints "Plan found!" [OK]

Hint: Success means plan() returns SUCCESS code [OK]

Common Mistakes:

• Confusing success condition logic
• Assuming plan() returns bool directly
• Ignoring MoveItErrorCode enum comparison

4. Identify the error in this ROS C++ code snippet for robot arm motion planning:

moveit::planning_interface::MoveGroupInterface move_group("manipulator");
move_group.setPoseTarget(target_pose);
move_group.plan(plan);
move_group.execute(plan);

medium

A. Missing check if plan was successful before execute

B. Incorrect method name: should be setTargetPose

C. execute() does not accept a plan argument

D. Missing initialization of MoveGroupInterface

Solution

1. Step 1: Review the planning and execution sequence

   The code sets a target, plans, then executes without checking if planning succeeded.

2. Step 2: Identify the missing safety check

   Best practice is to verify plan success before calling execute to avoid errors or unsafe moves.

3. Final Answer:

   Missing check if plan was successful before execute -> Option A

4. Quick Check:

   Always verify plan success before execute [OK]

Hint: Always check plan success before executing [OK]

Common Mistakes:

• Assuming plan always succeeds
• Confusing method names
• Ignoring return values from plan()

5. You want to plan a motion for a robot arm to reach a target pose but avoid a newly added obstacle in the environment. Which ROS framework feature should you use to update the planning scene before planning?

hard

A. Use RViz to manually move the robot arm

B. Directly modify the MoveGroupInterface target pose to avoid obstacle

C. Use PlanningSceneInterface to add the obstacle and update the scene

D. Restart the ROS node to reload the environment

Solution

1. Step 1: Understand how obstacles affect planning

   Obstacles must be added to the planning scene so the planner knows to avoid them.

2. Step 2: Identify the correct tool to update the planning scene

   PlanningSceneInterface allows adding/removing objects in the environment for planning awareness.

3. Step 3: Evaluate other options

   Modifying target pose doesn't update obstacles. Restarting node is inefficient. RViz manual moves don't affect planning scene programmatically.

4. Final Answer:

   Use PlanningSceneInterface to add the obstacle and update the scene -> Option C

5. Quick Check:

   Update planning scene with PlanningSceneInterface [OK]

Hint: Add obstacles via PlanningSceneInterface before planning [OK]

Common Mistakes:

• Trying to avoid obstacles by changing target pose only
• Restarting nodes unnecessarily
• Using RViz manual moves instead of programmatic updates