Magnetic actuation platforms are composed of several coils used to produce a magnetic field. The magnetic field gradient induces a force. Ferromagnetic objects are attracted toward the highest magnetic field gradients. By controlling the current inside the coils it is possible to control the trajectory of the objects. A camera monitors the position of the object in its environment in real time.
Most of the works on the state of the art focuses on actuation inside liquids. However the viscosity of the liquid induces a drag force which decreases the velocity that can be reached. The MiNaRoB team studies actuation in different environments, such as in air or at the interface between air and liquid to increase the velocity of the displacements. The MiNaRoB team is also interested in increasing the mobility of helicoidal micro swimmers.
Magnetic Actuation in Air
As a proof of concept, FEMTO-ST, jointly with ISIR, developed MagPieR, a magnetic microrobot to participate to the NIST mobile microrobot challenge. This robot won the 2 mm dash and became the fastest magnetically actuated microrobot of less than 500µm. The CNRS French team, which then included LPN, confirmed this leadership in 2011 and 2012 and became three-time world champion.
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Despite these good results, precision is still a challenge for actuation in air. Surface forces are predominant compared to volume ones. Overcoming adhesion between the substrate and the microobobject is thus a challenge. Several directions have been investigated, among which the use of piezoelectric actuation to induce a vertical movement of the object; the structuration of the substrate using microfabrication techniques and the use of the magnetic torque to overcome adhesion. In addition control laws taking into account the two available inputs, the intensity of the current inside the coils and the duration of this signal have been proposed.
Magnetic actuation at the interface between air and liquid
To avoid adhesion issues we have investigated magnetic actuation of objects placed at the interface between a liquid and air. Due to surface tension the objects are placed on the surface, without being immerged. The velocity is not restrained by the viscosity of water. This approach arise original problematics due to the presence of a meniscus and to the high velocities that are reached. Specific control laws must be implemented to ensure high speed trajectory tracking despite the non-linearity of the force fields. Modeling and control have been investigated and tested experimentally. Velocities up to 6mm/s have been reached for objects of less than 100 µm.
Magnetic actuation in liquid
Helicoidal swimmers of less than 2mm have been made using 3D printing technology. Several techniques have been investigated to magnetize the swimmers, and they should be compared in terms of swimming performances. Specific control laws will also be developed to control these swimmers – on going works.