Experiment 4B: Joint Space Path Planning for the Cobot-C1

Explore path planning in joint configuration space using simultaneous and sequential motion strategies.

Objectives

  • Investigate robot motion generated from joint space planning strategies
  • Analyze the resulting end-effector trajectories in physical space
  • Develop practical experience with path planning approaches that prioritize joint variables
  • Compare joint-space versus task-space planning approaches

Key Concepts

  • Joint Space Planning: Path planning directly in robot's joint configuration space
  • Synchronous Joint Motion: All joints move in coordinated fashion
  • Sequential Joint Motion: One joint moves at a time (stepwise motion)
  • Configuration Space: Space of all possible joint angle combinations
  • Trade-offs: Predictable joint control vs. precise workspace paths

Theory

What is Joint Space Path Planning?

In joint space planning, the trajectory is defined as a sequence of joint positions (angles) over time. The path is specified and executed directly in the robot's configuration space rather than in the physical workspace. This approach has two primary implementations:

Simultaneous/Synchronous Joint Motion

All joints are moved in a time-coordinated fashion from an initial to a final configuration. The time to complete the move is equal for all joints, and trajectories are planned for each joint to ensure coordinated arrival at the goal configuration.

Characteristics:

  • All joints start and stop simultaneously
  • Smooth, coordinated motion
  • End-effector follows a curved path (not necessarily straight in Cartesian space)
  • Predictable and efficient

Sequential Joint Motion

One joint is moved at a time, so the end-effector follows a "city-block" (piecewise) path, not necessarily straight or smooth in Cartesian space. Each joint completes its motion before the next joint begins.

Characteristics:

  • Joints move one at a time
  • Stepwise, segmented motion
  • End-effector follows piecewise linear paths in workspace
  • Simple to implement but less efficient

Advantages of Joint Space Planning

  • Predictable control: Direct, programmable control in joint coordinates
  • Simpler implementation: No inverse kinematics required
  • Computational efficiency: Often less computational overhead
  • Singularity avoidance: Easier to avoid kinematic singularities

Disadvantages of Joint Space Planning

  • Curved paths: May result in non-intuitive end-effector paths
  • Limited workspace control: Not suitable for tasks demanding precise workspace path control
  • Difficult path prediction: Hard to predict exact Cartesian path from joint trajectories

Prerequisites

  • Verified Cobot-C1 (simulation or hardware) with secure workspace
  • ROS and all necessary packages installed and running
  • Access to joint space planning scripts supporting both synchronous and sequential modes
  • Familiarity with entering joint angles and reading robot motion
  • (Optional) PlotJuggler ready for in-depth data observation

Procedure

Step 1: Setup

Confirm proper initialization of robot and ROS nodes.

Step 2: Select Start and Goal Joint Angles

Choose valid joint angle vectors (in radians or degrees as specified).

Step 3: Launch Joint Space Trajectory Script

cd dev/MRL/cobot-C1/src/cobo_control/src/ python3 linear_experiment.py

Step 4: Choose Path Type

Select the motion strategy:

  • Enter 2 for Joint Space (Simultaneous/Synchronous)
  • Enter 3 for Joint Space (Sequential)

Step 5: Input Angles

As prompted, enter the start and goal joint configurations.

Step 6: Observation

  • For simultaneous mode: Observe the coordinated motion of all joints to the final configuration
  • For sequential mode: Note the "stepwise" motion as each joint moves independently
  • Observe and record how the end-effector path differs for the two joint space strategies

Step 7: Repeat

Run tests for a variety of joint configurations spanning the robot's range.

Step 8: Exit

To quit, use Ctrl+C.

Observations and Analysis

What to Observe

  • End-effector path shape: How does the path differ between synchronous and sequential motion?
  • Motion smoothness: Which method produces smoother motion?
  • Execution time: Which method is faster?
  • Joint coordination: How do the joints work together in each mode?

Comparison Questions

  • How does the end-effector path in joint space planning compare to task space planning?
  • When would you prefer synchronous over sequential motion?
  • What are the practical implications of each strategy for real-world tasks?

Sources of Error and Precautions

Common Sources of Error

  • Joint angles entered outside robot limits
  • Configurations that result in self-collision
  • Exceeding physical boundaries or workspace limits

Safety Precautions

  • Ensure joint angles are within robot limits
  • Confirm joint motion does not result in self-collision or exceed physical boundaries
  • Monitor for unexpected movements and be prepared to stop the robot
  • Start with small angle changes before attempting large motions

Learning Outcomes

  • Recognition of distinct joint space planning strategies and their practical effects
  • Insight into mapping between joint space and workspace trajectories
  • Understanding the trade-offs inherent in choosing a path planning method for various robotics tasks
  • Appreciation for when joint space planning is preferable to task space planning
  • Practical experience with different motion coordination strategies
When to Use Joint Space Planning: Joint space planning is ideal for pick-and-place operations, simple point-to-point motions, and situations where the exact Cartesian path is not critical.
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