Webots Actuators Skill

Use this skill to implement and troubleshoot actuator control in Webots controllers, with emphasis on RotationalMotor and LinearMotor behavior, actuator node configuration, and per-step command semantics.

Scope

• Cover only actuator devices and actuator-related node fields.
• Exclude sensor usage patterns (handled by webots-sensors).
• Exclude basic controller bootstrapping and generic simulation setup.

Core Execution Model

• Treat motor and actuator setter calls as buffered commands applied on the next wb_robot_step.
• Issue all actuator commands for a cycle before stepping.
• Read-back methods (for example, target position or velocity) reflect controller-side state and/or the latest simulation update depending on API semantics.

Motor-First Workflow (RotationalMotor + LinearMotor)

Start from motor control mode, because most actuator behaviors in mobile robots, manipulators, and mechanism-driven assets reduce to motor commands.

1) Position control (default mode)

Python:

motor = robot.getDevice("my_motor")
motor.setPosition(1.57)  # target in radians (rotational) or meters (linear)

C:

WbDeviceTag motor = wb_robot_get_device("my_motor");
wb_motor_set_position(motor, 1.57);  // rad (rotational) or m (linear)

Use for joint angle/extension targets with internal PID position tracking (controlPID and related motor fields on the node).

2) Velocity control (continuous rotation/translation)

Python:

motor = robot.getDevice("my_motor")
motor.setPosition(float('inf'))  # MUST set position to infinity first
motor.setVelocity(6.28)  # rad/s or m/s

C:

#include <math.h>

WbDeviceTag motor = wb_robot_get_device("my_motor");
wb_motor_set_position(motor, INFINITY);  // MUST set infinity first
wb_motor_set_velocity(motor, 6.28);      // rad/s or m/s

Apply this mode to wheel drives, conveyor rollers, and continuous actuators.

3) Torque/force control

Python:

motor = robot.getDevice("my_motor")
motor.setPosition(float('inf'))
motor.setVelocity(float('inf'))  # remove velocity limit
motor.setTorque(0.5)  # N·m (rotational) or N (linear)

C:

#include <math.h>

WbDeviceTag motor = wb_robot_get_device("my_motor");
wb_motor_set_position(motor, INFINITY);
wb_motor_set_velocity(motor, INFINITY);  // remove velocity cap
wb_motor_set_torque(motor, 0.5);         // N·m (rotational) or N (linear)

Use for dynamic interaction, compliant contact behavior, or direct effort-limited control.

4) Motor parameters and limits

• Respect node-defined limits (maxVelocity, maxTorque/maxForce, acceleration/jerk constraints where configured).
• Use runtime caps for safer behavior:
  • Python: setAvailableTorque(), setAvailableForce()
  • C: wb_motor_set_available_torque(), wb_motor_set_available_force()
• Use feedback/introspection methods during control loops:
  • getTargetPosition(), getVelocity(), getMaxVelocity(), getMaxTorque()/getMaxForce()
• Pair motors with PositionSensor for closed-loop application logic and convergence checks.

Differential Drive Pattern (Canonical)

Python:

left_motor = robot.getDevice("left wheel motor")
right_motor = robot.getDevice("right wheel motor")
left_motor.setPosition(float('inf'))
right_motor.setPosition(float('inf'))
left_motor.setVelocity(5.0)
right_motor.setVelocity(5.0)

C:

#include <math.h>

WbDeviceTag left_motor = wb_robot_get_device("left wheel motor");
WbDeviceTag right_motor = wb_robot_get_device("right wheel motor");
wb_motor_set_position(left_motor, INFINITY);
wb_motor_set_position(right_motor, INFINITY);
wb_motor_set_velocity(left_motor, 5.0);
wb_motor_set_velocity(right_motor, 5.0);

Actuator-Specific Patterns

Brake

• Apply damping on the same joint as a motor.
• Use setDampingConstant(value) / wb_brake_set_damping_constant(tag, value).
• Treat as dissipative braking, not commanded position holding.

Python:

brake = robot.getDevice("joint_brake")
brake.setDampingConstant(0.8)

C:

WbDeviceTag brake = wb_robot_get_device("joint_brake");
wb_brake_set_damping_constant(brake, 0.8);

LED

• Use boolean/integer for mono LEDs (0 off, 1 on).
• Use packed RGB integer for color LEDs (for example 0xFF0000 for red).

Python:

led = robot.getDevice("status_led")
led.set(0xFF0000)

C:

WbDeviceTag led = wb_robot_get_device("status_led");
wb_led_set(led, 0xFF0000);

Display

• Use immediate drawing API for HUD/in-robot visualization.
• Common primitives: drawPixel, drawLine, drawRectangle, drawOval, drawText.
• Color/transparency controls: setColor, setAlpha, setOpacity.
• Image workflow: imageNew, imageCopy, imagePaste, imageSave.

Python:

display = robot.getDevice("screen")
display.setColor(0x00FF00)
display.drawText("READY", 4, 14)
display.drawLine(0, 0, 63, 31)

C:

WbDeviceTag display = wb_robot_get_device("screen");
wb_display_set_color(display, 0x00FF00);
wb_display_draw_text(display, "READY", 4, 14);
wb_display_draw_line(display, 0, 0, 63, 31);

Emitter

• Send byte payloads to compatible Receiver devices.
• Use setChannel(channel) for multiplexed links.
• Use setRange(distance) to constrain transmission radius when applicable.
• Account for emitter type (radio, serial, infrared) at node configuration time.

Python:

emitter = robot.getDevice("tx")
emitter.setChannel(2)
emitter.setRange(10.0)
emitter.send(b"go")

C:

const char payload[] = "go";
WbDeviceTag emitter = wb_robot_get_device("tx");
wb_emitter_set_channel(emitter, 2);
wb_emitter_set_range(emitter, 10.0);
wb_emitter_send(emitter, payload, sizeof(payload) - 1);

Speaker

• Use playSound for file/sample playback control (volume, pitch, balance, looping).
• Use speak for text-to-speech.
• Use stop to terminate an active sound.

Python:

speaker = robot.getDevice("speaker")
speaker.playSound(speaker, speaker, "sounds/beep.wav", 1.0, 1.0, 0.0, False)
speaker.speak("dock complete", 1.0)

C:

WbDeviceTag speaker = wb_robot_get_device("speaker");
wb_speaker_play_sound(speaker, speaker, "sounds/beep.wav", 1.0, 1.0, 0.0, false);
wb_speaker_speak(speaker, "dock complete", 1.0);

Pen

• Use write(on) to enable/disable drawing traces.
• Use setInkColor(color, density) for color and opacity-like density control.
• Drawing occurs when pen geometry contacts surfaces.

Python:

pen = robot.getDevice("trail_pen")
pen.setInkColor(0x0000FF, 0.8)
pen.write(True)

C:

WbDeviceTag pen = wb_robot_get_device("trail_pen");
wb_pen_set_ink_color(pen, 0x0000FF, 0.8);
wb_pen_write(pen, true);

Propeller

• Use the joint motor for control; Propeller has no dedicated device API.
• Configure thrustConstants and torqueConstants in the Propeller node.
• Command rotor speed with motor velocity/torque methods.

Python:

rotor = robot.getDevice("propeller motor")
rotor.setPosition(float('inf'))
rotor.setVelocity(120.0)

C:

#include <math.h>

WbDeviceTag rotor = wb_robot_get_device("propeller motor");
wb_motor_set_position(rotor, INFINITY);
wb_motor_set_velocity(rotor, 120.0);

Connector

• Use lock() / unlock() to command docking latches.
• Use isLocked() to check local state.
• Rely on compatible connector alignment/proximity for successful auto-attachment.

Python:

connector = robot.getDevice("dock")
connector.lock()
locked = connector.isLocked()

C:

WbDeviceTag connector = wb_robot_get_device("dock");
wb_connector_lock(connector);
bool locked = wb_connector_is_locked(connector);

Muscle

• Treat as visual-only actuator representation linked to articulated motion.
• Configure attachment points and appearance in node fields.
• Drive visible contraction indirectly via associated joints/motors.

Track

• Use motorized joints to command track speed.
• Configure visual motion with animationStep on Track node.
• Apply to tank treads and conveyor-style systems.

Python:

track_motor = robot.getDevice("left track motor")
track_motor.setPosition(float('inf'))
track_motor.setVelocity(3.0)

C:

#include <math.h>

WbDeviceTag track_motor = wb_robot_get_device("left track motor");
wb_motor_set_position(track_motor, INFINITY);
wb_motor_set_velocity(track_motor, 3.0);

Troubleshooting Checklist

• Verify the commanded control mode: position vs velocity vs torque/force.
• For velocity mode, verify setPosition(INFINITY) is applied before velocity commands.
• For torque/force mode, verify both position and velocity are uncapped (INFINITY) before effort commands.
• Check node-level limits (maxVelocity, maxForce/maxTorque, acceleration) when observed behavior saturates.
• Confirm that command effects are inspected only after stepping the simulation.

Cross-Language Mapping Rule

• Python method names map directly to C API with wb_<device>_<action> style.
• Keep units identical across languages:
  • rotational position: radians
  • linear position: meters
  • rotational velocity: rad/s
  • linear velocity: m/s
  • rotational torque: N·m
  • linear force: N

Reference Material

• Use references/actuators_reference.md for complete API tables across C/C++/Python/Java/MATLAB, full node field definitions, and PID parameter details.