Electro-Permanent Magnet
- Also called: EPM, electropermanent magnet, switchable permanent magnet
- Category: Actuation & drivetrain › Motors
- Tags: magnetic-actuation, grippers, modular-robotics, low-power, swarm-robotics
A magnet assembly that can be switched on or off with a brief current pulse and then holds its state with no power, used in grippers, latches and modular robot connectors.
Definition
Electro-permanent magnets (EPMs) are magnetic devices that can be switched between holding and releasing with a short electrical pulse, then stay in that state without any continuous power. They combine the convenience of an electromagnet, which can be turned on and off, with the zero-power holding of a permanent magnet.
A typical EPM pairs two kinds of permanent magnet material inside a coil. One is a hard magnet, such as neodymium iron boron (NdFeB), whose magnetization stays fixed. The other is a semi-hard magnet, such as alnico, whose magnetization can be reversed by a strong enough field. A current pulse through the coil flips the semi-hard magnet. When both magnets point the same way, their combined flux passes out through the pole pieces into a ferromagnetic surface or a mating magnet, and the device holds. When they point opposite ways, the flux circulates inside the device and little reaches the outside, so it releases.
Because energy is only used during switching, EPMs suit battery-powered and small robots. They have been used in modular and self-reconfigurable robots as connectors that latch modules together, in grippers and perching mechanisms for drones, and in small robots that attach to steel surfaces. Large electro-permanent lifting magnets are also used industrially to move steel plate.
EPMs have practical constraints. Switching requires a high current pulse for a short time, so the drive electronics usually include an H-bridge and a capacitor or other pulse source. Holding force falls sharply with any air gap and depends on the target's material, thickness and surface finish, and the device only grips ferromagnetic material or another magnet.
Why it matters
Holding something without consuming power is valuable for small, battery-powered and modular robots. Electro-permanent magnets give switchable attachment without the constant energy draw and heat of an electromagnet.
Key applications
- Connectors in modular and self-reconfigurable robots
- Grippers and perching mechanisms for drones and small robots
- Robots that attach to and move across steel structures
- Industrial lifting of steel plates and parts
In practice
A team designing a modular robot or swarm platform might use EPMs as connectors between units. They select magnet materials and coil windings for the required holding force, design a pulse driver that briefly delivers the switching current from the robot's battery through a capacitor, and test holding force at different air gaps and surface conditions. The control firmware sends a pulse in one direction to latch and in the opposite direction to release, and the magnets draw no current in between.
Common misconceptions
- An EPM needs power to keep holding — it only draws current while switching, then holds with the magnets alone
- EPMs work on any material — they hold only ferromagnetic surfaces or a matching magnet
- The rated holding force applies in all conditions — force drops sharply with air gap, paint, rough surfaces or thin material
Who should know this
- Hardware engineers designing grippers and latching mechanisms
- Modular and swarm robotics researchers
- Drone engineers working on perching and payload release
Related technologies
- Electromagnet
- Neodymium Magnet
- Magnetic Gripper
- H-Bridge Driver
- Modular Robotics
Related terms
Related projects
- Aerostack2: ROS 2 framework for autonomous multi-drone systems and swarm robotics