Actuators and End Effectors

Opening challenge: A gripper closes on a glass beaker. What must the robot know before “close” becomes a safe physical action?

Mental Model: A Robot Acts Through a Chain

An actuator converts supplied energy and a command into physical effort and motion. An end effector is the task-facing device at the end of the chain: a gripper, suction cup, tool, wheel, foot, or other interface with the world.

command → drive → actuator → transmission → joint → end effector → contact
   ↑                                                                    ↓
   └──────────── measured position, speed, effort, current, contact ─────┘

The chain matters because a correct high-level command can still fail through insufficient torque, excessive speed, backlash, wrong payload data, a poor tool, uncertain contact, or stale feedback.

Actuator Is a Role, Not a Synonym for Motor

ROS 2 control represents robot hardware with command interfaces and state interfaces. A controller may command position, velocity, or effort while reading the resulting state. “Command sent” is therefore not evidence that motion or contact succeeded.

Transmission Changes the Trade-off

A transmission maps actuator motion and effort into joint motion and effort. Ideal mechanical power is approximately conserved:

$P = \tau\omega$

where $\tau$ is torque and $\omega$ is angular speed. Gearing can raise output torque while reducing output speed. Real transmissions also add friction, compliance, backlash, heat, and efficiency losses.

Worked example

A motor can supply $0.40\ \text{N·m}$ through a 20:1 reduction. Ignoring losses, the output torque is about:

$0.40 \times 20 = 8.0\ \text{N·m}$

At 80% efficiency, a better estimate is $6.4\ \text{N·m}$. The joint rotates about twenty times slower than the motor. This may help lift a load but may make the action too slow—or amplify collision forces if limits and feedback are poorly designed.

End Effector Selection Starts With the Task

Choose from evidence, not appearance:

• object geometry, material, fragility, surface, and variability;
• required force, speed, accuracy, and cycle time;
• payload mass, center of gravity, and inertia;
• contact sensing and slip detection;
• failure behavior if power, pressure, vacuum, or communication is lost;
• cleaning, contamination, maintenance, and human proximity.

Universal Robots requires payload, center of gravity, and inertia information for optimal robot behavior. Picking and releasing an object changes the active payload. A tool that can hold the mass may still overload the arm when the center of gravity is far from the flange.

Safe Action Requires Feedback and Limits

A decision-ready action contract includes command mode, valid range, maximum speed/effort, expected state update rate, stale-state threshold, contact criterion, stop behavior, and reset conditions. ROS 2 hardware lifecycle states separate configuration, activation, deactivation, shutdown, and error handling; movement interfaces should not be treated as always available.

End-effector safety is also an integration problem, not merely a component rating. ISO/TR 20218-1 provides specific guidance for designing and integrating robot-system end effectors, complementing the broader ISO 10218 robot and application safety requirements. This lesson teaches the engineering evidence needed for reasoning; it does not replace a task-specific risk assessment or the applicable standards.

Retrieval Pause

1. Why can gearing increase torque but not create free power?
2. Why does a gripper need payload and center-of-gravity information?
3. What evidence distinguishes “command delivered” from “task succeeded”?

Misconception Checks

“The motor is the actuator, transmission, and joint.” These are separate roles even when packaged together.

“More grip force is always safer.” Excess force can crush, deform, launch, or damage an object.

For example, Robotiq's 2F gripper control interface treats requested position, speed, and force as separate settings. Its force setting limits motor current, and exceeding that limit stops the fingers and triggers object-detection status. That is concrete evidence that “grip harder” is not a complete control strategy: the robot must also observe state and contact outcome.

“Rated payload proves every pose is safe.” Reach, center-of-gravity offset, inertia, acceleration, tool mass, and orientation all matter.

“Position control guarantees safe contact.” Contact also depends on force, compliance, geometry, sensing, and stopping behavior.

Summary

Robots act through a coupled chain. Select the actuator technology, transmission, joint, and end effector together; verify payload and contact assumptions; read state rather than trusting commands; and define limits and safe stopping before physical execution.

Sources and Further Learning

• ROS 2 Control, hardware interface types, command/state interfaces and limits; retrieved 2026-08-18; official documentation, linked and paraphrased.
• ROS 2 Control, joint and transmission interfaces, power-variable mappings; retrieved 2026-08-18; official documentation.
• Universal Robots, Payload and Center of Gravity, payload/CoG setup and performance; retrieved 2026-08-18; manufacturer documentation.
• International Federation of Robotics, Industrial Robots, ISO 8373-based manipulator terminology; retrieved 2026-08-18; official industry body.
• Robotiq, 2F-85/2F-140 Gripper Control, independent position/speed/force requests, motor-current limiting, and object-detection status; retrieved 2026-08-18; manufacturer documentation, linked and paraphrased.
• ISO, ISO/TR 20218-1:2018 — Safety design for industrial robot systems: End-effectors, scope and integration-safety framing; retrieved 2026-08-18; official standards catalogue abstract, linked and paraphrased.