Robotic Arm Structure
Introduction
A robotic arm (manipulator) is the core execution mechanism for grasping, handling, assembly, and other tasks. This section covers arm structural types, joint design, transmission methods, and engineering practice.
Kinematics and dynamics theory: See Kinematics
Serial Manipulators
Serial manipulators are the most common configuration, consisting of a series of links and joints connected in sequence, forming an open kinematic chain.
Joint Types
| Joint Type | Symbol | DOF | Motion | Typical Application |
|---|---|---|---|---|
| Revolute | R | 1 | Rotation about axis | Most joints |
| Prismatic | P | 1 | Translation along axis | Lifting, extension |
| Spherical | S | 3 | Three-axis rotation | Wrist (theoretical) |
| Universal | U | 2 | Two-axis rotation | Drive shaft |
Common Configurations
| Configuration | Joint Sequence | Workspace | Features |
|---|---|---|---|
| Articulated | RRR...R | Spherical | Most flexible, most common |
| SCARA | RRP | Cylindrical | Fast in horizontal plane, assembly |
| Cartesian | PPP | Rectangular | Intuitive, 3D printers |
| Cylindrical | RPP | Cylindrical | Large range |
| Polar | RRP | Partial sphere | Welding |
6-DOF Articulated Arm
A standard 6-DOF articulated arm can reach any position and orientation within its workspace:
Base → [J1:Yaw] → [J2:Shoulder] → [J3:Elbow] → [J4:Wrist Roll] → [J5:Wrist Pitch] → [J6:Wrist Roll] → End-Effector
- J1–J3 (position joints): Determine end-effector position \((x, y, z)\)
- J4–J6 (orientation joints): Determine end-effector orientation \((roll, pitch, yaw)\)
7-DOF Redundant Arm
Seven degrees of freedom provide one redundant DOF, allowing:
- Adjusting elbow position while keeping end-effector pose constant
- Reaching around obstacles
- Optimizing joint torque distribution
- Examples: Franka Emika Panda, KUKA iiwa
Parallel Manipulators
Parallel mechanisms use multiple kinematic chains simultaneously connecting the base to the moving platform, forming a closed-chain structure.
Common Types
| Type | DOF | Description | Application |
|---|---|---|---|
| Delta | 3 (x,y,z) | Three parallelogram linkages | High-speed pick-and-place |
| Stewart-Gough | 6 | Six-strut platform | Flight simulators, precision machining |
| 3-RRR | 3 | Three revolute chains | Planar motion |
Serial vs. Parallel
| Feature | Serial | Parallel |
|---|---|---|
| Workspace | Large | Small |
| Load capacity | Low-medium | High |
| Speed/acceleration | Medium | High |
| Stiffness | Low | High |
| Accuracy | Medium | High |
| Forward kinematics | Simple | Complex |
| Inverse kinematics | Complex | Simple |
| Cost | Lower | Higher |
Link Design
Link Force Analysis
Links primarily bear bending and torsion loads. Maximum bending moment occurs at joint connections:
\[M_{max} = m_{payload} \cdot g \cdot L_{arm}\]
For a horizontally extended cantilever arm, the required section modulus:
\[W \geq \frac{M_{max}}{\sigma_{allow}}\]
Cross-Section Selection
| Cross-Section | Moment of Inertia \(I\) | Features | Suitable For |
|---|---|---|---|
| Solid circle | \(\frac{\pi d^4}{64}\) | Isotropic | Small arms |
| Hollow tube | \(\frac{\pi(D^4-d^4)}{64}\) | Lightweight, efficient | Main links |
| Rectangular tube | \(\frac{bh^3-b_1h_1^3}{12}\) | Easy to machine and mount | Square arms |
| I-beam | — | Highest bending efficiency | Large industrial arms |
Payload vs. Reach Trade-off
\[\tau_{joint} = m_{payload} \cdot g \cdot L_{reach}\]
Longer reach requires greater joint torque, which means:
- Larger, heavier motors and reducers
- Thicker links (increasing self-weight, further increasing torque demand)
- Typical trade-offs: Desktop-scale reach 0.3–0.5 m / payload 0.5–2 kg, industrial-scale reach 1–2 m / payload 5–50 kg
Reducer Integration
Joint reducers are critical components in robotic arm design, converting the motor's high-speed low-torque output to low-speed high-torque.
Reducer Type Comparison
| Type | Ratio | Precision (arcmin) | Efficiency | Backdrivability | Cost | Suitable For |
|---|---|---|---|---|---|---|
| Harmonic drive | 30–160 | 1–3 | 70–85% | Poor | High | Cobot arm joints |
| Planetary gear | 3–100 | 3–10 | 90–95% | Medium | Medium | General |
| RV reducer | 30–200 | <1 | 75–85% | Poor | Very high | Industrial arm base |
| Worm gear | 10–100 | 5–15 | 40–70% | Very poor (self-locking) | Low | Self-locking needed |
| Quasi-direct drive (low ratio) | 6–10 | — | 95%+ | Excellent | Medium-high | Legged / force-control arms |
Harmonic Drive
Harmonic drives are widely used in robotic arm joints due to their compact size, high reduction ratio, and low backlash:
- Structure: Wave generator (input) + Flexspline (output) + Circular spline (fixed)
- Reduction ratio: \(i = \frac{z_{circular}}{z_{circular} - z_{flex}}\)
- Brands: Harmonic Drive (Japan), Laifual (China), Han's Precision
- Price: ~1,000–5,000+ RMB per unit
Modular Joint Design
Modern collaborative arms trend toward modular joint design (Joint Module), integrating motor, reducer, encoder, and driver into one unit:
graph LR
subgraph Joint Module
M[BLDC Motor] --> G[Reducer<br/>Harmonic/Planetary]
G --> O[Output Flange]
E1[Input Encoder] --> M
E2[Output Encoder] --> O
D[Driver PCB] --> M
T[Torque Sensor] --> O
end
Representative products:
- DJI RoboMaster GM6020: Integrated motor + reducer + driver
- Dynamixel series: XM430, XM540 smart servos
- Unitree A1/GO-M8010: Quasi-direct drive joint modules
Cable Management
Internal Routing
- Cables pass through hollow shafts or inside link cavities
- Pros: Aesthetic, good protection
- Cons: Difficult maintenance, limits joint rotation range
External Routing
- Cables run along the outside of links, secured with cable clips/conduit
- Pros: Easy maintenance
- Cons: May interfere with motion, less aesthetic
Rotary Joint Routing
- Slip ring: Allows unlimited rotation, but has contact noise
- Flexible flat cable (FFC/FPC): Suitable for limited rotation range (+-180 degrees)
- Spring cable: Spiral wound, allows some rotation
End Effectors
| Type | Description | Suitable For |
|---|---|---|
| Two-finger gripper | Parallel / angular open-close | General grasping |
| Three-finger gripper | Adaptive grasping | Irregular objects |
| Suction cup | Vacuum suction | Flat objects |
| Magnetic | Electromagnetic / permanent magnet | Metal objects |
| Dexterous hand | Multi-finger multi-joint | Fine manipulation |
| Tool | Screwdriver / torch, etc. | Dedicated tasks |
6-DOF Robotic Arm Design Example
Parameter Settings
| Parameter | Value |
|---|---|
| DOF | 6 (RRRRRR) |
| Reach | 500 mm |
| Rated payload | 1 kg |
| Repeatability | +-0.5 mm |
| Max joint velocity | 180 deg/s |
Joint Configuration
| Joint | Function | Motor | Reducer | Ratio | Torque (Nm) |
|---|---|---|---|---|---|
| J1 | Base rotation | BLDC 60W | Planetary | 50:1 | 15 |
| J2 | Shoulder | BLDC 100W | Harmonic | 100:1 | 30 |
| J3 | Elbow | BLDC 60W | Harmonic | 80:1 | 20 |
| J4 | Wrist rotation | BLDC 30W | Planetary | 50:1 | 5 |
| J5 | Wrist pitch | BLDC 30W | Planetary | 50:1 | 5 |
| J6 | Wrist roll | BLDC 20W | Planetary | 30:1 | 3 |
Material Selection
- Base: Aluminum alloy 6061, CNC machined
- Upper/lower arm: Aluminum alloy square tube + 3D-printed connectors
- Wrist: Aluminum alloy CNC + 3D-printed shell
Open-Source Robotic Arm References
| Project | DOF | Features | Link |
|---|---|---|---|
| AR4 Robot Arm | 6 | Stepper motors, low cost | GitHub |
| BCN3D Moveo | 5 | 3D printed, educational | GitHub |
| Niryo One/Ned | 6 | ROS integrated, educational/commercial | niryo.com |
| SO-ARM100 | 6 | Low-cost servo solution | GitHub |
References
- Craig: "Introduction to Robotics: Mechanics and Control"
- Siciliano et al.: "Robotics: Modelling, Planning and Control"
- Harmonic Drive Technical Manual
- Kinematics Theory