Actuator Overview
The actuator system in MuJoCo provides a unified mechanism for applying forces, torques, or generalized controls to joints and tendons. Actuators are essential for driving motion, simulating motors, springs, muscles, hydraulic cylinders, and a variety of biomechanical or robotic control systems.
This section explains:
How actuators work conceptually
How they apply forces within MuJoCo’s dynamics
The structure and purpose of the main actuator shortcuts: motor, position, velocity, intvelocity, damper, cylinder, muscle, adhesion
No attribute-level details are included—only structural and functional concepts.
1. What an Actuator Does
In MuJoCo, an actuator is a mechanism that maps control signals (inputs) into forces or torques that act on:
a joint, or
a tendon, or
a slider-like constraint such as in hydraulic actuators
Actuators allow you to:
Drive a robot’s joints
Apply force through cables, tendons, or motors
Implement closed-loop or open-loop controllers
Simulate biologically inspired actuators (muscles, springs)
Add damping, stiffness, adhesion, or constraint-driven forces
Core Logic
The simulation receives a scalar control signal (e.g.,
ctrl[i]).The actuator translates this signal into a scalar generalized force.
This force is applied to the controlled joint/tendon in generalized coordinates.
The solver integrates the result into the system dynamics.
Thus, actuators bridge control input → physical force.
2. Actuator Shortcuts Overview
MuJoCo provides a collection of high-level “shortcut” actuator types that correspond to common control behaviors. These are convenience wrappers around the more general actuator model, each configured for a specific dynamic behavior.
Below is an overview of each actuator shortcut and its structural role.
2.1 Motor
A motor actuator applies force or torque proportional directly to the control input.
Represents ideal torque or force motors
Most common actuator for robotic joints
Linear mapping from control → force
Structurally: 👉 The controller directly determines the joint or tendon effort.
2.2 Position
The position actuator acts like a servo that drives a joint toward a target angle or displacement.
Characteristics:
Accepts a desired position as control
Internally generates a force that pulls toward that target
Implements proportional (and often soft) position control
Structurally: 👉 A closed-loop controller hidden inside the actuator.
2.3 Velocity
The velocity actuator tries to regulate joint velocity toward a target.
Control input = desired velocity
Actuator produces force to reduce velocity error
Useful for speed-controlled motors or rotational drives
Structurally: 👉 A damping-like force with velocity tracking.
2.4 IntVelocity (Integrator Velocity)
The intvelocity actuator integrates the control input over time to produce a target velocity.
Control signal is integrated → becomes a velocity target
Smoothly accumulates input
Enables “rate-based” velocity control
Structurally: 👉 Suitable for controllers where velocity should evolve smoothly.
2.5 Damper
A damper actuator resists relative motion by applying force proportional to velocity.
Produces viscous damping
No control input needed
Helps stabilize joints or absorb shocks
Structurally: 👉 Passive element that adds velocity-dependent resistance.
2.6 Cylinder
A cylinder actuator simulates:
Pneumatic cylinders
Hydraulic pistons
Linear actuators with pressure/force based on displacement
Functional characteristics:
Applies force along a defined line between two bodies
Length of the cylinder depends on body positions
Control input often interpreted as pressure or force
Structurally: 👉 Force applied based on geometric extension.
2.7 Muscle
The muscle actuator implements a biomechanical muscle model.
Computes force based on activation dynamics
Includes force–length and force–velocity relationships
Control input often represents muscle activation level
Structurally: 👉 Physiology-inspired actuator for tendons and musculoskeletal systems.
2.8 Adhesion
The adhesion actuator models adhesion or sticking forces between surfaces.
Applies attractive force to keep two sites together
Used for simulating suction, sticky pads, climbing robots
Can act like a controllable “sticky” constraint
Structurally: 👉 Applies a force that resists separation between two points.
3. Actuator System Summary
MuJoCo’s actuator system provides a versatile way to convert control signals into physical forces applied to joints or tendons. Actuators support a diverse range of use cases:
Robotic motors and servos
Damping and stabilization
Hydraulic and pneumatic cylinders
Biomechanical muscle models
Adhesive or suction-like attachment forces
Shortcut types (motor, position, velocity, intvelocity, damper, cylinder, muscle, adhesion) offer convenient configurations for commonly needed actuator behaviors.
Together, they allow MuJoCo to simulate a wide variety of robotic actuators, biological muscles, compliant elements, and force-based mechanisms in a unified dynamic framework.
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