End Effectors
Introduction
An end effector is the "hand" through which a robot interacts with its environment. Mounted at the tip of a robotic arm or on the robot body, it is responsible for grasping, manipulation, machining, and other tasks. Different application scenarios require different types of end effectors.
Parallel Grippers
Operating Principle
A parallel gripper uses two fingers moving linearly in a parallel direction to clamp objects.
Actuation Methods
| Actuation Method | Characteristics | Typical Applications |
|---|---|---|
| Pneumatic | Fast, high force, simple structure | Industrial production lines |
| Electric (lead screw) | Controllable force, high precision | Collaborative robots |
| Electric (rack and pinion) | Fast speed | General scenarios |
Key Parameters
- Stroke: Maximum finger opening distance (e.g., 50 mm / 85 mm / 140 mm)
- Gripping force: Maximum clamping force (typically 20–250 N)
- Repeatability: Finger position repeatability (<0.05 mm)
- Self-weight: Affects the robotic arm's effective payload
Gripping Force Calculation
For friction-based gripping (the most common method):
\[
F_{grip} \geq \frac{m \cdot g \cdot n_{safety}}{2 \mu}
\]
Where:
- \(m\) — Workpiece mass
- \(g\) — Gravitational acceleration
- \(\mu\) — Friction coefficient (rubber ~0.7, metal ~0.2)
- \(n_{safety}\) — Safety factor (typically 2–3)
Adaptive Grippers
Robotiq 2F-85
Robotiq is the benchmark brand for collaborative robot grippers:
| Parameter | Value |
|---|---|
| Stroke | 85 mm |
| Gripping force | 20–235 N (adjustable) |
| Closing speed | 20–150 mm/s |
| Repeatability | 0.04 mm |
| Weight | 0.9 kg |
| Communication | Modbus RTU / EtherNet/IP |
| Features | Underactuated adaptive, fingers can envelope |
Adaptive Mechanism
Adaptive gripper fingers consist of multiple joints through an underactuated design:
- A single motor drives multiple joints
- Fingers automatically conform to the object shape
- No need to precisely know the object geometry
- Can grasp irregularly shaped objects
Pinch mode: Encompassing mode:
┃ ┃ ╲ ╱
┃ [] ┃ ╲ [] ╱
┃ ┃ ╲ ╱
──┛ ┗── ───┛┗───
Vacuum Suction Cups
Principle
Uses negative pressure to generate suction force:
\[
F_{suction} = \Delta P \times A = (P_{atm} - P_{vacuum}) \times \frac{\pi d^2}{4}
\]
Where \(d\) is the cup diameter and \(\Delta P\) is the pressure differential (typically 60–80 kPa).
Vacuum Generation Methods
| Method | Principle | Features |
|---|---|---|
| Venturi tube | Compressed air through a throat generates vacuum | Fast response, no electricity needed, requires air supply |
| Electric vacuum pump | Vane or diaphragm pump | Independent operation, adjustable |
| Vacuum generator | Multi-stage venturi | High efficiency |
Suction Cup Types
| Type | Suitable For |
|---|---|
| Flat suction cups | Smooth flat surfaces (glass, metal sheets) |
| Bellows suction cups | Curved, uneven surfaces |
| Sponge suction cups | Rough or porous surfaces |
| Oval suction cups | Cardboard boxes, bagged items |
Applications
- Warehouse logistics: carton handling
- Electronics manufacturing: PCB and chip handling
- Glass handling: flat panel suction
Soft Grippers
Pneumatic Soft Grippers
Utilize flexible materials (silicone) that deform under air pressure to grasp objects:
- Positive pressure drive: Inflation causes expansion, fingers curl to wrap the object
- Negative pressure drive (granular jamming): A flexible membrane filled with granules hardens when vacuum is applied
Advantages
- Naturally conforms to object shapes
- Does not damage fragile objects (e.g., fruits, food)
- No need to swap grippers for different objects
- High safety (human-robot interaction friendly)
Limitations
- Limited gripping force
- Slower response speed
- Precise control is difficult
- Material lifespan (silicone aging)
Representative Products
| Product | Type | Features |
|---|---|---|
| Soft Robotics mGrip | Pneumatic fingers | Food-grade, multi-finger configurable |
| Universal gripper (granular jamming) | Vacuum | Adapts to any shape |
| Festo FinGripper | Bionic | Mimics fin motion |
Cleaning Mechanisms
Core actuators for cleaning robots such as robot vacuums:
Main Brush (Roller Brush)
| Type | Structure | Features |
|---|---|---|
| Bristle roller | Bristles in V-pattern | Good for carpet, tangles with hair |
| Rubber roller | Rubber blades | Good for hard floors, doesn't tangle hair |
| Dual roller (counter-rotating) | One bristle, one rubber | Handles both floor types |
Drive method: Brushed DC motor + gear/belt transmission, speed approximately 1000–2000 rpm.
Side Brush
- Structure: 3–5 brush arms arranged radially
- Function: Sweeps dust from walls and edges into the main brush area
- Drive: Small DC motor, speed 200–500 rpm
- Count: Single or dual side brushes
Rubber Roller Extractors
iRobot Roomba's AeroForce extractor design:
- Two bristle-free rubber rollers rotate in opposite directions
- Creates airflow to suck debris into the dustbin
- Does not tangle with hair, easier maintenance
- Works on all floor types
Suction System
\[
\text{Suction} = \Delta P \times A_{nozzle}
\]
- Fan type: Brushless DC fan (high speed, 30,000–100,000 rpm)
- Suction level: 1000–5000 Pa (household robot vacuums)
Tool Changers
Overview
A tool changer (Quick-Change) allows a robot to automatically swap between different end effectors.
Structure
Robot arm flange face
│
┌───┴───┐
│ Master │ ← Fixed to the robot arm
│ Plate │
├───────┤
│ Locking│ ← Pneumatic/electric locking mechanism
│ Mech. │
├───────┤
│ Tool │ ← Fixed to the tool
│ Plate │
└───┬───┘
│
End tool (gripper/torch/suction cup, etc.)
Key Features
| Feature | Description |
|---|---|
| Repeatability | <0.025 mm (precision grade) |
| Load capacity | Depends on model (5–500 kg) |
| Change time | <1 second |
| Pass-through | Air, electrical, and signal connections all go through the quick-change |
Typical Brands
- ATI Industrial Automation: Industry standard, QC series
- Schunk: SWS series
- Zimmer Group: Multiple specifications
Application Scenarios
- Welding/cutting/grinding multi-process switching
- Multi-variety mixed-line production
- Research platforms testing different end effectors
Dexterous Hands
A dexterous hand mimics the human hand structure, with multiple fingers and joint degrees of freedom.
Further Reading
For detailed content on dexterous hands, see Dexterous Hands
Representative Products Overview
| Product | Fingers | DOF | Actuation | Features |
|---|---|---|---|---|
| Shadow Hand | 5 | 24 | Pneumatic tendon | Closest to human hand |
| Allegro Hand | 4 | 16 | Motor direct drive | Common in research |
| LEAP Hand | 4 | 16 | Servo | Open-source, low-cost |
| Inspire Hand | 5 | 6–12 | Motor + lead screw | Domestic commercial |
End Effector Selection
| Application Scenario | Recommended Type | Reason |
|---|---|---|
| Regular rigid parts | Parallel gripper | Precise, reliable |
| Irregular objects | Adaptive gripper | Encompassing grasp |
| Flat part handling | Vacuum suction cup | Fast, leaves no marks |
| Fragile / food items | Soft gripper | Non-damaging |
| Multi-task | Tool changer | Flexible switching |
| Fine manipulation | Dexterous hand | Multiple DOF |
Summary
- Parallel grippers are the most common industrial end effectors, reliable and precise
- Adaptive grippers accommodate different object shapes through underactuated mechanisms
- Vacuum suction cups are suitable for high-speed handling of flat objects
- Soft grippers provide gentle grasping for delicate items
- Cleaning robot actuators include main brushes, side brushes, and suction systems
- Tool changers enable a single robot to perform multiple tasks
- Selection should consider object characteristics, speed requirements, and operating environment holistically