Virtual Fixture

Also known as attractive fixtures, these provide assistive forces or visual overlays that guide an operator or autonomous system toward a desired path or target, without rigidly prohibiting deviation. They function like magnetic guides or haptic tunnels.

• Primary Use: Assisting in precise alignment, tracing contours, or reaching targets in cluttered environments.
• Control Law: Often implemented using potential fields, where an attractive force pulls the end-effector toward the desired trajectory.
• Example: In robotic surgery, a guidance fixture might create a haptic channel leading a tool to a tumor, reducing surgeon tremor while allowing override if necessary.

Also known as repulsive fixtures, these create software-defined boundaries that the robot is not permitted to penetrate. They act as protective shields or keep-out zones.

• Primary Use: Preventing collisions with sensitive anatomy, delicate equipment, or other robots in a shared workspace.
• Control Law: Typically implemented using repulsive potential fields or barrier functions that generate a repelling force or command a velocity of zero at the boundary.
• Example: In an assembly task, a forbidden-region fixture could prevent a drill from contacting a pre-installed electrical component, enforcing a hard safety constraint.

These fixtures are implemented within an admittance control framework. The robot measures external forces (e.g., from a human operator or contact) and uses a virtual model to compute a resulting motion. The fixture modifies this model.

• Key Mechanism: Force in, motion out. The fixture alters the desired admittance—the relationship between applied force and permitted velocity/displacement.
• Behavior: Feels natural for human-in-the-loop teleoperation, as the operator directly feels the fixture's effect through the robot's responsive motion. A forbidden region feels like pushing against a soft or hard virtual wall.

These fixtures are implemented within an impedance control framework. The robot commands forces/torques to achieve a desired dynamic relationship between its motion and the interaction forces with the environment.

• Key Mechanism: Motion in, force out. The fixture defines a virtual mechanical impedance (mass-spring-damper system) that the end-effector should exhibit.
• Behavior: The robot actively generates forces to enforce the fixture. A guidance path feels like moving through a viscous channel; a forbidden region exerts a repulsive force. This is common for autonomous robots or where high stiffness is required.

These are non-haptic overlays that provide sensory guidance through visual or auditory cues. They are critical in systems without force feedback or to augment haptic information.

• Visual Fixtures: Graphical overlays like constrained manipulation handles, semi-transparent volumes, or predicted trajectory lines displayed on a screen or in AR/VR.
• Auditory Fixtures: Spatialized sounds or changing tones that indicate proximity to a target or boundary.
• Application: Used extensively in teleoperation interfaces, surgical navigation systems, and augmented reality guides for manual assembly.

The most advanced fixtures are not static; their geometry, stiffness, or very existence changes based on the system's state, sensor input, or task phase.

• Activation/Deactivation: A fixture may only become active when a specific tool is engaged or when an object is detected within a region.
• Geometry Adaptation: The forbidden region around a moving object (like a beating heart) updates in real-time based on perceptual tracking.
• Stiffness Modulation: A guidance path may start with low stiffness (easily overridden) and increase as the tool nears a critical target for final precision.

A robot control strategy that regulates the dynamic relationship between force and motion at the end-effector, making the robot behave as a programmable mass-spring-damper system. This is foundational for implementing virtual fixtures that feel like soft guides or hard walls.

• Key Mechanism: The controller defines a target mechanical impedance (desired inertia, damping, and stiffness).
• Use Case: Creates compliant virtual surfaces that allow an operator to 'feel' a constraint, such as a forbidden region or a desired path.

A complementary strategy to impedance control where measured external forces are used to compute a desired motion. The robot's end-effector moves in response to contact, effectively implementing a programmable inverse impedance.

• Key Difference: While impedance control commands torque based on position error, admittance control commands position/velocity based on force error.
• Use Case: Often used in cobot applications and for creating guidance virtual fixtures that translate operator force inputs into assisted motion along a predefined trajectory.

A robot control technique that uses real-time visual feedback to directly guide the end-effector's motion. It closes the control loop in image space or Cartesian space.

• Image-Based Visual Servoing (IBVS): Error is computed directly from feature positions in the camera image.
• Pose-Based Visual Servoing (PBVS): Error is computed from the estimated 3D pose of the target.
• Integration with Virtual Fixtures: Visual servoing can be used to dynamically generate or update the reference path or target region for a virtual fixture based on live camera data.

The use of tactile sensations (forces, vibrations, motions) to convey information to a human operator. It is the primary interface for making virtual fixtures perceptible.

• Key Components: Haptic interfaces (e.g., force-feedback joysticks, exoskeletons) apply computed forces to the user's hand.
• Role in Virtual Fixtures: Translates the software-defined constraint (e.g., a forbidden region, an attractive guidance path) into a tangible force field that the operator can feel, enabling intuitive teleoperation or shared control.

A robotic control paradigm where authority is dynamically distributed between a human operator and an autonomous system. Virtual fixtures are a primary technical implementation of shared control.

• Types:
  • Collaborative Control: The human and robot work simultaneously on different aspects of a task.
  • Traded Control: Authority switches between human and robot based on context.
• Mechanism: The autonomous system uses virtual fixtures to assist (by guiding motion) or protect (by constraining motion), while the human provides high-level intent and supervision.

A specific, critical type of virtual fixture that prohibits robot motion into defined volumes of the workspace. This is a safety and precision mechanism.

• Primary Function: Creates invisible barriers to prevent collisions with sensitive anatomy (in surgery) or fragile components (in assembly).
• Implementation: Typically uses a repulsive force field, where the force exerted on the operator's interface increases as the end-effector approaches the forbidden region's boundary.