How to Select the Right Robot Joint Actuator: A Practical Engineering Guide
TL;DR: A step-by-step framework for engineers selecting robotic joint actuators — covering torque requirements, transmission type, communication protocol, and control interface. Includes downloadable spec comparison template.
The Problem: Too Many Options, Too Little Guidance
If you're building a robot arm, cobot, or humanoid and searching for actuators, you've probably noticed:
- Some suppliers list peak torque, others list continuous torque
- "Backdrivability" means different things to different manufacturers
- Communication protocols range from CANopen to EtherCAT to RS485, with no universal standard
- Price varies by 10x for seemingly similar specs
This guide gives you the decision framework I wish someone had given me when I started.
Step 1: Define Your Torque Requirements
The single most common mistake is using peak torque for sizing. Here's the correct approach:
Continuous Torque (T_cont)
This is what drives sustained operation. For a robot joint:
T_cont = m × g × L / (GR × η)
Where:
m = link mass (kg)
g = 9.81
L = distance from joint to center of mass (m)
GR = gear ratio
η = transmission efficiency (~0.6–0.8 for harmonic drives, ~0.85–0.95 for planetary)
Peak Torque (T_peak)
This matters for acceleration and emergency stops. As a rule of thumb, design for:
T_peak = 1.5× to 2× T_cont
Why This Matters
If you size on peak torque, you'll end up with an actuator that's 2× heavier and 3× more expensive than needed — and the extra mass propagates upstream, requiring larger actuators in the base joints.
Step 2: Choose Your Transmission Type
Harmonic Drive
| Pros | Cons |
|---|---|
| Zero backlash (<20 arcsec) | Lower efficiency (~60-80%) |
| High reduction in single stage (50:1–160:1) | Cannot be backdriven without torque sensor |
| Compact, coaxial design | Higher cost |
| Good for precision positioning | Shock loads can damage flexspline |
Best for: 6-DOF arms, cobots, surgical robots, semiconductor equipment
Planetary Gear
| Pros | Cons |
|---|---|
| Higher efficiency (~85-95%) | Backlash (3-15 arcmin typical) |
| High shock load capacity | Larger diameter for same ratio |
| Lower cost per Nm | Higher noise at high speed |
| Can be backdriven with proper motor sizing | Needs multi-stage for high reduction |
Best for: Mobile robots, AGVs, parallel robots, high-cycle pick-and-place
QDD (Quasi-Direct Drive)
| Pros | Cons |
|---|---|
| Very high backdrivability | Low torque density |
| Excellent for force control | Needs extra brake for safety |
| Low friction, high efficiency | Complex thermal management |
Best for: Humanoid robots, exoskeletons, haptic devices
Step 3: Select Your Communication Protocol
This is often the most underestimated decision.
CANopen (CiA 402)
Maturity: Extremely well-established
Speed: Up to 1 Mbps
Real-time: Good for up to 10 axes
Tooling: Widespread — LinuxCNC, ROS2 CANopen packages
Complexity: Simple wiring, differential pair
EtherCAT
Maturity: Growing rapidly in robotics
Speed: 100 Mbps
Real-time: Sub-μs DC synchronization
Tooling: SOEM, IgH, ROS2 EtherCAT drivers
Complexity: Requires master PC with compatible NIC
RS485 / Modbus
Maturity: Declining for new robotic applications
Speed: Up to 10 Mbps
Synchronization: None
Best for: Simple positioning, not coordinated multi-axis
Recommendation: If your robot has more than 4 joints and requires coordinated motion, choose EtherCAT. For 1-4 axes with simpler profiles, CANopen is sufficient and easier to set up.
Step 4: Evaluate Control Modes
Make sure the actuator supports the control modes your application needs:
| Mode | When to Use |
|---|---|
| Cyclic Synchronous Position (CSP) | Trajectory following, point-to-point |
| Cyclic Synchronous Velocity (CSV) | Conveyor tracking, mobile base |
| Cyclic Synchronous Torque (CST) | Force control, compliant motion |
| Profile Position | Simple indexing with predefined ramps |
| Homing Mode | Power-on reference |
For ROS2 ros2_control integration, ensure CSP and CST modes are available and well-documented.
Step 5: Spec Template
Here's a 10-point spec checklist to send to any actuator supplier:
- Continuous torque (Nm) at rated speed
- Peak torque (Nm), duration limit
- Backlash (arcmin or arcsec)
- Reduction ratio
- Communication protocols supported
- Control modes (CiA 402 support)
- Operating voltage and current (continuous / peak)
- IP rating
- Temperature range
- CAD model availability
If a supplier can't provide clear answers for all 10, that's a red flag.
About the Author
I design and build robotic joint actuators for industrial and collaborative robots. Our ZHR series covers 1–91 Nm continuous torque with CANopen, EtherCAT, and RS485 options. Reach out through the robotics community — I'm happy to discuss your project requirements.
Have questions about selecting an actuator for your specific robot design? Drop them in the comments below.