Robot Arm

Restoration and software integration of a 6-axis robot arm.

Introduction

In 2025, I acquired a used 6-axis robot arm from a Procter & Gamble facility in Ohio through an industrial equipment auction. The arm, which was previously used in a manufacturing line, was purchased for $500. After winning the auction, I drove to the P&G facility to pick up the equipment, which included the robot arm, its controller, and various accessories. The project involves restoring the robot arm to operational condition and developing custom software to control it.

Robot Arm

Setting Up

The first step was to create a dedicated workspace and gather all necessary tools. I cleared a large workbench, ensuring adequate lighting and power access, and assembled essential equipment including a digital multimeter, oscilloscope, various sizes of hex keys, torque wrenches, and specialized lubricants. I carefully unboxed the arm, photographing each step of the process and creating detailed documentation of its initial state. The robot showed signs of extended storage—there was light surface corrosion on some exposed metal parts, and the cables had become stiff from age.

I methodically inspected and documented every visible component. The arm’s six joints were manually articulated to assess their range of motion and identify any concerning sounds or resistance. I tested each servo motor’s electrical connections, checking for continuity and proper insulation. The controller box was opened and examined for any obvious damage or loose connections. Throughout this process, I maintained a detailed log of observations, creating a comprehensive checklist of items that needed attention. My goal was to systematically inspect every component—from mechanical parts to electrical connections—and determine the root cause of its malfunction before attempting any repairs.

Troubleshooting and Disassembly

Early diagnostics pointed to issues with the J2 axis, specifically irregular motion and concerning vibrations during operation. I began by disassembling this section of the robot, which led me to focus on the harmonic drive and the AC servo motor. The harmonic drive showed signs of wear and contamination—the wave generator exhibited minor scoring marks, while the flex spline contained accumulated debris that impeded smooth operation. I carefully took apart the J2 axis, cleaning the harmonic drive thoroughly using specialized solvents to remove debris and hardened grease, followed by applying fresh SHF-32 lubricant to ensure proper operation. The circular spline teeth required particular attention, as they showed signs of uneven wear that needed careful cleaning and inspection.

Similarly, I disassembled and cleaned the AC servo motor, discovering that the bearings had accumulated significant debris and the encoder disk showed signs of contamination. I methodically addressed each component—cleaning the stator windings, replacing the bearings, and carefully realigning the encoder disk to ensure accurate position feedback. The process of reassembly required precise torque specifications for the mounting bolts and careful alignment of the motor shaft with the harmonic drive input.

This restoration process involved a fair amount of trial and error, particularly in achieving the correct preload on the harmonic drive and proper encoder alignment. I developed a systematic testing procedure, gradually increasing the range of motion while monitoring current draw and vibration levels. Through iterative adjustments and careful calibration of each component, I was able to restore the axis to smooth, reliable operation with minimal backlash and proper position accuracy.

Communication with the Controller

After resolving the mechanical issues, I turned my attention to the robot’s control systems. The controller uses a proprietary communication protocol, requiring careful reverse engineering to establish reliable connectivity. I began by analyzing the electrical signals using an oscilloscope to understand the timing and voltage levels of the communication interface.

My current efforts are focused on establishing reliable communication with the controller through multiple approaches:

  • TCP/IP Communication: Implemented a custom socket server listening on port 502 (Modbus/TCP), and a Python client for sending movement commands and receiving position feedback.
  • Serial Communication: Experimented with RS‑232 at 115200 baud, 8N1, tuning baud rates and framing to achieve stable data exchange.
  • RT Toolbox2 Integration:
    • Configured Mitsubishi’s RT Toolbox2 software to communicate over TCP/IP with the CR2DA‑700 controller at IP 192.168.0.20.
    • Developed MELFA BASIC scripts and uploaded them via RT Toolbox2’s project manager. These scripts include custom movement routines, homing sequences, and I/O diagnostics.
    • Automated the deployment process: on startup, the host PC launches an RT Toolbox2 API session, loads the latest MELFA BASIC program, and triggers a self‑test routine on each axis.
    • Utilized RT Toolbox2’s online monitor to log real‑time joint angles, motor currents, and error codes—feeding this telemetry back into my Python dashboard for visualization and alerting.
  • Error Handling: Added CRC checksums, timeout/retry mechanisms, and a state machine to manage the connection lifecycle.

While the integration poses challenges—such as configuring network settings, handling packet loss, and ensuring consistent data transmission—I’ve made substantial progress toward a high‑level API that abstracts these complexities.

Achievements & Technical Specs

Key Achievements to Date:

  • Mechanical Restoration: Complete refurbishment of all six axes, especially the J2 harmonic drive and AC servo motor.
  • Software Integration: Functional TCP/IP & RS‑232 clients with robust error handling and calibration routines.
  • Testing & Calibration: Iterative motion tests, current monitoring, and vibration analysis for reliable, backlash‑free operation.

Technical Specifications:

  • 6‑Axis Articulation: Independently driven joints for full 3D workspace coverage.
  • Harmonic Drive: High‑precision wave generator and flex spline in the J2 axis.
  • Servo Motors: AC servos with custom bearing replacements and encoder realignment.
  • Controller Interfaces: Dual-mode Ethernet (Modbus/TCP) and RS‑232 serial connectivity.
  • Documentation: Detailed logs, calibration checklists, and step-by-step photographs.

Future Directions

  1. Custom Control Interface
    • Web‑based dashboard and manual joystick control.
  2. High‑Level API
    • RESTful endpoints for motion commands, sensor data, and status monitoring.
  3. Path Planning & Collision Detection
    • Integration of motion planning libraries (e.g., MoveIt!) for safe, autonomous trajectories.
  4. Computer Vision Integration
    • Mounting an Intel RealSense camera for object detection, human tracking, and adaptive task execution.
  5. Telemetry & Analytics
    • Real‑time data logging of position, torque, and error states for predictive maintenance and performance tuning.
  6. Community Engagement
    • Publishing tutorials, code samples, and collaborating with other robotics enthusiasts.

Conclusion

Restoring and integrating this robot arm has been an in‑depth exploration of mechanical engineering, electronics diagnostics, and software development. From harmonic drive calibration to protocol reverse engineering, every phase has deepened my expertise in industrial robotics. As the project evolves, I aim to transform this platform into a versatile, intelligent automation system—stay tuned for more updates and demonstrations!