Actuators and Drives
Actuators are the core components through which robots interact with the physical world, directly determining motion performance, force control capability, and safety. This article covers servo motors, quasi-direct drive, harmonic reducers, series elastic actuators, and hydraulic actuators.
Actuator Classification Overview
| Type | Gear Ratio | Backdrivable | Force Control | Power Density | Typical Application |
|---|---|---|---|---|---|
| Direct Drive (DD) | 1:1 | Excellent | Excellent | Low | Research platforms |
| Quasi-Direct Drive (QDD) | 4:1~9:1 | Good | Good | Medium-high | Quadrupeds/humanoids |
| Harmonic Drive | 50:1~160:1 | Poor | Poor | High | Industrial robot arms |
| Planetary Gear | 3:1~100:1 | Medium | Medium | High | General purpose |
| Cycloidal Drive | 30:1~120:1 | Poor | Poor | High | Industrial |
| Series Elastic Actuator (SEA) | Variable | Good | Good | Medium | Human-robot collaboration |
| Hydraulic | — | Good | Excellent | Very high | Heavy-duty/humanoid |
Motor Fundamentals
Brushless DC Motor (BLDC) Principles
Modern robot actuators are almost universally based on BLDC (Brushless DC) motors. Core equations:
Torque equation:
where \(K_t\) is the torque constant (Nm/A) and \(I\) is the current (A).
Output torque after the reducer:
where \(N\) is the gear ratio and \(\eta\) is the transmission efficiency.
Backdrivability depends on:
The higher the gear ratio, the greater the external force needed for backdrive. High gear ratios (e.g., harmonic drive 100:1) are virtually non-backdrivable.
Key Parameters
| Parameter | Symbol | Unit | Description |
|---|---|---|---|
| Torque constant | \(K_t\) | Nm/A | Current-to-torque conversion coefficient |
| Back-EMF constant | \(K_e\) | V/(rad/s) | Equals \(K_t\) (in SI units) |
| No-load speed | \(\omega_{no-load}\) | rpm | Maximum motor speed under no load |
| Stall torque | \(\tau_{stall}\) | Nm | Maximum torque at stall |
| Continuous torque | \(\tau_{cont}\) | Nm | Sustained output torque at thermal steady state |
| Peak torque | \(\tau_{peak}\) | Nm | Maximum torque for short duration |
| Rotor inertia | \(J\) | kg*m^2 | Affects dynamic response |
Dynamixel Series Servos
Dynamixel is a line of smart servo motors produced by ROBOTIS, widely used in educational and research robots.
Series Comparison
| Series | Model Example | Stall Torque | Communication | Reducer | Control Modes | Price |
|---|---|---|---|---|---|---|
| XL | XL330-M288 | 0.52 Nm | TTL | Planetary | Position/Velocity/PWM | ~$25 |
| XC | XC330-T288 | 0.76 Nm | TTL | Planetary | Position/Velocity/PWM | ~$30 |
| XM | XM430-W350 | 4.1 Nm | RS-485/TTL | Planetary | Position/Velocity/Current/Extended Position | ~$220 |
| XH | XH540-W270 | 9.2 Nm | RS-485/TTL | Planetary | All modes | ~$400 |
| XW | XW540-T260 | 9.2 Nm | RS-485 | Planetary | All modes (IP67) | ~$550 |
| PH | PH54-200-S500 | 44.7 Nm | RS-485 | Planetary | All modes | ~$2,800 |
Control Modes
| Mode | Description | Use Case |
|---|---|---|
| Position control | PID position closed-loop | Most scenarios |
| Extended position | Multi-turn absolute position | Continuous rotation |
| Velocity control | PID velocity closed-loop | Wheeled chassis |
| Current control | Current (torque) closed-loop | Force control, compliant manipulation |
| Current-position | Position control with current limiting | Safe grasping |
| PWM | Open-loop | Debugging |
SDK Example
from dynamixel_sdk import *
# Initialize
port = PortHandler("/dev/ttyUSB0")
packet = PacketHandler(2.0) # Protocol 2.0
port.openPort()
port.setBaudRate(1000000)
# Enable torque
TORQUE_ENABLE = 1
packet.write1ByteTxRx(port, DXL_ID, 64, TORQUE_ENABLE)
# Write target position (position mode)
goal_position = 2048 # 0~4095 maps to 0~360 degrees
packet.write4ByteTxRx(port, DXL_ID, 116, goal_position)
# Read current position
present_position, _, _ = packet.read4ByteTxRx(port, DXL_ID, 132)
Usage in open-source robots: Koch v1.1, SO-100, Open Manipulator X, and ALOHA all use Dynamixel series motors.
Quasi-Direct Drive Actuators (QDD)
Quasi-Direct Drive (QDD) actuators use low gear ratios (typically 4:1 ~ 9:1), retaining good backdrivability while providing sufficient torque output.
Core Advantages
| Advantage | Reason |
|---|---|
| High-bandwidth force control | Low gear ratio reduces friction and reflected inertia |
| Backdrivable | External forces can backdrive the motor, enabling passive compliance |
| Transparent force sensing | Contact forces estimated directly from current (no extra force sensor needed) |
| Impact resistant | Motor freely rotates on impact, protecting the reducer |
Representative Products
| Product | Peak Torque | Gear Ratio | Weight | Application |
|---|---|---|---|---|
| MIT Mini Cheetah actuator | 17 Nm | 6:1 | 0.5 kg | Mini Cheetah quadruped |
| T-Motor AK80-9 | 18 Nm | 9:1 | 0.5 kg | Quadruped/humanoid |
| Unitree A1 actuator | 33.5 Nm | 9.1:1 | 0.5 kg | Unitree quadrupeds |
| Fourier FSA | Multiple specs | Selectable | Selectable | Fourier GR-1 humanoid |
Fourier Smart Actuator (FSA)
A modular smart actuator developed by Fourier Intelligence for their GR-1 humanoid robot.
| Feature | Description |
|---|---|
| Modular | Motor + reducer + encoder + driver integrated |
| Multiple specs | Different specifications for different joints (shoulder, hip, knee, ankle) |
| EtherCAT | High-bandwidth real-time communication |
| FOC control | Field-Oriented Control, high torque accuracy |
| SDK | Python/C++ SDK provided |
Harmonic Drive
Harmonic drives use elastic deformation of flexible gears to achieve speed reduction, and are the standard choice for industrial robot arms.
Working Principle
Three core components: 1. Wave Generator: Elliptical cam + flexible bearing (input) 2. Flexspline: Thin-walled elastic gear (output) 3. Circular Spline: Internal-tooth rigid ring (fixed)
Gear ratio formula: \(N = \frac{z_{rigid}}{z_{rigid} - z_{flex}}\), where \(z\) is the number of teeth.
Characteristics
| Parameter | Typical Value |
|---|---|
| Gear ratio | 50:1 ~ 160:1 |
| Efficiency | 80-90% |
| Backlash | <1 arcmin |
| Lifespan | Limited under high torque (wear) |
Strengths: Compact, lightweight, high precision, zero backlash.
Weaknesses: Non-backdrivable (high gear ratio + friction), expensive, prone to wear under high torque, flexspline has elastic deformation.
Typical applications: UR, Franka Panda, Kinova and other industrial/collaborative robot arms.
Series Elastic Actuator (SEA)
Series Elastic Actuator (SEA) inserts a spring in series between the motor output and the joint, a key technology for human-robot collaboration.
Design Principle
where \(K_{spring}\) is the spring stiffness and \(\theta_{motor} - \theta_{joint}\) is the spring deflection.
Core Advantages
| Advantage | Description |
|---|---|
| Precise force sensing | Torque calculated directly from spring deflection measurement |
| Passive compliance | Spring absorbs impacts, protecting motor and reducer |
| Energy storage | Spring can store and release energy (e.g., jumping) |
| Safety | Limits peak collision forces |
Disadvantages
| Disadvantage | Description |
|---|---|
| Limited force control bandwidth | Spring introduces resonance frequency |
| Reduced position accuracy | Elastic deformation causes position uncertainty |
| Increased volume | Extra spring occupies space |
Typical applications: Rethink Robotics Baxter/Sawyer, ANYmal quadruped (uses variable-stiffness SEA).
Hydraulic Actuators
Hydraulic actuators provide the highest power density, suitable for heavy-duty scenarios.
| Feature | Value |
|---|---|
| Power density | Far exceeds electric motors (10x+) |
| Force/torque output | Extremely high |
| Bandwidth | High (servo valve control) |
| Compliance | Achievable through pressure control |
Disadvantages: Complex hydraulic lines, high maintenance costs, oil leak risks, high noise.
Typical Applications:
- Boston Dynamics Atlas (early hydraulic version)
- Heavy-duty industrial robots
- Construction machinery (excavators, loaders)
Trend: As electric actuator performance has improved, humanoid robots like Atlas have shifted to fully electric drive designs.
Actuator Selection Matrix
| Application | Recommended Type | Key Considerations |
|---|---|---|
| Industrial robot arms | Harmonic/planetary + BLDC | Precision, repeatability |
| Collaborative robot arms | SEA or current force control + low gear ratio | Safety, force control |
| Quadruped robots | QDD | Backdrivability, impact resistance |
| Humanoid robots | QDD (limbs) + harmonic (dexterous hands) | Balance torque/precision needs |
| Education/research | Dynamixel | Low cost, complete SDK |
| Dexterous hands | Micro servos + tendon/linkage | Compact, high DOF |
| Heavy-duty | Hydraulic | Power density |
Related Links
- Dynamixel Product Page
- Harmonic Drive Official Website
- T-Motor Actuators
- Related notes: Humanoid Robots | Quadruped Robots | Hardware Selection Guide