6-Axis Articulated Industrial Robot (Payload 20 kg)
This application case details the multi-axis servo synchronization, safety integration, and control loop tuning for a 6-DOF articulated robot arm used in high-speed material handling and precision path interpolation.
System Architecture Overview
[ EtherCAT Bus / CiA 402 ]
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[Axes 1–3 Drives] [Axes 4–6 Drives] [Safety Controller]
high-inertia joints wrist joints (compact) STO / ST1 / SBC
(2.0–3.5 kW) (200–750 W)
Joint Axis | Motor Rating | Brake | Encoder System | Primary Dynamic Requirement |
Axis 1 (Waist) | 3.5 kW | Mechanical holding | 23-bit Absolute (Dual Loop option) | High base torque, torsional rigidity |
Axis 2 (Shoulder) | 3.5 kW | Mechanical holding | 23-bit Absolute | Gravity load compensation, high stiffness |
Axis 3 (Elbow) | 2.0 kW | Mechanical holding | 23-bit Absolute | Over-hung payload compensation |
Axes 4–6 (Wrist) | 200W – 750 W | Mechanical holding | 23-bit Absolute | Compact size, high speed, dynamic path accuracy |
Pre-Commissioning & Safety Interlocks
1. Hardware & Safety Loop Wiring
Safe Torque Off (STO): Wire dual-channel STO inputs from the main robot safety controller to all six drive axes.
Safe Brake Control (SBC): Integrate dedicated 24V brake relays with embedded flyback suppression circuits. Axes 2 and 3 require fail-safe brake sequencing during emergency stops to prevent arm drop.
DC Bus Distribution: Connect drives via a shared DC bus to capture regenerative energy from braking outer joints during fast decelerations.
2. Absolute Encoder Offsets (Zero Calibration / Mastering)
Move the arm manually or using jog mode to the mechanical zero-reference alignment pins.
Read raw encoder multi-turn / single-turn counts via the motion configuration software.
Write offset values directly into drive non-volatile memory (EEPROM) to establish joint kinematic zero positions.
Servo Drive Parameter Configuration (CiA 402 / EtherCAT)
Set up all drives in Cyclic Synchronous Position Mode (CSP) under EtherCAT with a bus cycle time.
Parameter | Recommended Setting | Rationale |
Operation Mode | CSP (Mode 8) | Upper-level robot motion controller calculates 6 DOF forward/inverse kinematics |
Electronic Gear Ratio | 1:1 (Direct pulse mapping) | Raw encoder counts map straight to controller trajectory planner |
Motor Invariant Control | Field-Oriented Control (FOC) | Low current ripple and optimal torque per ampere |
Torque Limit | Axis-dependent (250%-300% max) | Prevents mechanical gear damage on harmonic drives during accidental collisions |
Brake Release Delay | 100ms - 250ms | Drive applies full holding torque before brake mechanically releases to stop droop |
Multi-Axis Tuning Protocol
Rigid joint motion demands high loop gains, but low mechanical stiffness in harmonic or RV reducers introduces low-frequency resonance.
[Inertia / Load Estimation]
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[Gain Auto-Tuning (Kvp, Kpp, Tvi)]
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[Resonance FFT & Adaptive Notch Filters]
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[Feedforward & Acceleration Compensation]
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[Gravity Loop Compensation (Axes 2 & 3)]
Step 1: Dynamic Load & Inertia Identification
Run automated identification routines across varying joint angles.
Calculate the inertia ratio. The ratio for Axis 1 and Axis 2 varies significantly depending on arm extension; gains must adapt dynamically based on kinematic pose.
Step 2: Feedforward & Cross-Coupling Compensation
Apply 100% Acceleration Feedforward and Velocity Feedforward at the drive or trajectory generator level to keep following error under 10m during path interpolation.
Coriolis & Gravity Compensation: Torque offsets computed by the robot controller are pushed directly to the drive's Torque Offset Object (0x60B2) to offload static gravity torque from the velocity loop.
Step 3: Mechanical Resonance Suppression
Perform a frequency response scan (bode plot) from 10Hz to 1000Hz on each joint.
Harmonic reducers typical on wrist axes exhibit flexible modes between 120Hz and 350Hz. Engage up to 3 adaptive notch filters per drive axis to suppress mechanical chatter.
System Validation & Trajectory Performance Check
1. Path Interpolation Check (TCP Accuracy): Program a Tool Center Point (TCP) straight-line diagonal motion at 2000s. Monitor individual axis position errors (Target Position 0x607A - Actual Position 0x6064). The tracking error on each wrist axis must remain under 5 encoder counts.
2. Payload Step Load Test: Attach the rated 20kg payload to the end-effector. Move Axis 2 through a step change; verify that overshoot is <2% and settling time is <50ms.
3. Emergency Brake Drop Test: Execute an E-Stop while moving Axis 2 downward at max speed. Verify that the SBC sequence locks the mechanical brake without axis drop or structural shock.
