Magnetic field is a popular candidate for the actuation and control of small-scale untethered robotic systems, thanks to its unique advantages in convenience, safety, and versatility.
Magnetic field is relatively easy to generate and manipulate. Two common approaches to generate a controllable magnetic field are 1) permanent magnet(s) and 2) electromagnetic coils.
Permanent magnets are made of magnetically “hard” materials that maintains a strong remanent magnetization in the absence of external magnetic field. One representative example is NdFeB. It consumes no energy to maintain the magnetic field, but at the same time, the field cannot be shut down. Its disadvantages in comparison with electromagnetic coils include 1) the need to move/rotate the magnet to change the field, 2) the lack of an emergency shut-down mechanism, and 3) highly nonlinear field distribution.
Electromagnetic coils utilize a running current within a conductor fibre, often copper wires, to generate a magnetic field. When the current is removed, the field also disappears. It consumes power to maintain the field, but at the same time, the field could be swiftly shut down in emergencies. Its disadvantages in comparison with permanent magnets include 1) large volume and weight, 2) power consumption and heat generation, and 3) weak field-generating capability.