Magnetic actuation

• Also called: magnetic manipulation, magnetic navigation
• Category: Actuation & drivetrain › Motors
• Tags: magnetics, microrobotics, medical-robotics, untethered-robots, actuation

Moving a magnetic object with forces and torques from externally generated magnetic fields, used to steer untethered capsules, catheters and microrobots without wires or onboard motors.

Definition

Magnetic actuation drives a body that contains a permanent magnet or magnetizable material by applying a magnetic field from outside it. The field acts on the body's magnetic moment in two ways. A torque, equal to the cross product of the moment and the field, rotates the moment toward the field direction and needs only a field, even a uniform one. A force requires a spatial gradient of the field: the body is pulled toward regions where the field component along its moment is stronger. A uniform field therefore turns a magnet but does not pull it, and systems that need to push or pull an object must create gradients.

The field can come from several sources. Permanent magnets carried by a robot arm give strong fields at low cost but must be moved to change the field. Electromagnetic coil systems, from simple Helmholtz pairs that create uniform fields for torque to multi-coil arrangements that shape both field and gradient, allow fast electrical control. Clinical MRI scanners can serve as actuators: the strong main field magnetizes a ferromagnetic body to saturation, and the scanner's gradient coils then apply a force proportional to the body's magnetic moment and the gradient, usually interleaved with imaging because the same coils are needed for both. Rotating fields can also spin helical microrobots so that they swim like a corkscrew.

The physics sets hard limits. Force scales with the volume of magnetic material, so small robots receive small forces, and the field of a magnet falls off steeply with distance, its gradient even faster, so actuating deep inside the body or across a large workspace is difficult. Earnshaw's theorem rules out stable levitation with static fields alone, so holding a body in place generally needs feedback control. Magnetic actuation is distinct from electric motors, which also rely on magnetic forces but inside a single device: here the field source and the moving body are separate and the body is untethered.

Why it matters

Magnetic fields pass through tissue, plastic and many other materials, so magnetic actuation can move devices where wires, motors or batteries do not fit. It is the basis of much of medical microrobotics, steerable catheters and capsule endoscopy, and of magnetically programmed soft robots.

Key applications

• Steering capsule endoscopes and drug-delivery capsules
• Magnetic navigation of catheters and guidewires
• Untethered microrobots, including helical swimmers
• Magnetically programmed soft robots and grippers
• MRI-based propulsion and tracking of small devices

In practice

A research team steers a millimeter-scale capsule through a fluid-filled channel standing in for a blood vessel. They use an arrangement of electromagnetic coils around the workspace, calibrated by mapping the field at many points with a magnetometer. A camera or imaging system tracks the capsule, and a controller computes the coil currents that produce the gradient needed to push the capsule toward the next waypoint, while a uniform component keeps its magnet aligned with the channel. Because the available force falls off quickly with distance from the coils, they check that drag and gravity on the capsule stay below the force the coils can deliver everywhere along the planned route.

Common misconceptions

• A strong uniform field pulls a magnet toward the source — a uniform field only produces torque; a net force requires a field gradient
• Field strength can make up for distance — fields and especially gradients fall off steeply with distance, which limits force deep inside a body or far from the coils
• Fixed magnets can hold an object stably in midair — Earnshaw's theorem rules this out for static fields, so stable positioning needs feedback control or another stabilizing effect

Who should know this

• Medical robotics and microrobotics researchers
• Engineers designing coil systems and magnetic manipulators
• Soft robotics researchers working with magnetic materials
• Biomedical engineers working on capsule and catheter devices

Related technologies

• Helmholtz Coils
• Electromagnetic Navigation Systems
• MRI Gradient Coils
• Neodymium Permanent Magnets
• Hall-Effect Magnetometers

Related terms

• Electro-Permanent Magnet
• Actuator
• Soft Robotics
• Dielectric Elastomer Actuator
• Wrench

Related projects

• da Vinci Research Kit (dVRK): Open electronics, firmware and software to control retired da Vinci surgical robots