Micro motors are motors that are millimeter-scale or smaller. They are generally characterized as small brushed DC motors or coreless motors. Some key features of nano motors include their tiny size, low power consumption and precise control capabilities. Over the years, advances in micro- electromechanical systems (MEMS) technology have enabled continuous miniaturization of these motors, allowing their use in a wide variety of applications.
Rise of MEMS Technology
The development of Micro Motor can be directly attributed to the rise of MEMS technology in the late 20th century. MEMS involve the integration of mechanical elements, sensors, actuators and electronics on a common silicon substrate using microfabrication technology. This allowed creation of miniaturized machines and systems with features ranging from 1-100 nano meters. MEMS technology enabled micro-scale parts and structures to be fabricated with precision. This led to the development of nano motors whose core, coil and assembled parts could fit inside millimeter sized packages. MEMS tech revolutionized nano motor design by incorporating micro-scale recesses, cavities and three-dimensional structures during fabrication.
DC nano motors – The First Generation
Some of the earliest practical micro motors developed were simple direct current (DC) brushed motors. These consisted of a permanent magnet rotor surrounded by stator coils and commutator brushes to direct current through the winding. DC nano motors could be as small as 1mm diameter and provided rotational torque in the range of 1-100μNm. They found usage in applications like micro fluid pumps, hands-free razors and miniature medical devices that required basic rotational motion. However, DC nano motors suffered from relatively low energy efficiency and generated electrostatic noise during commutation.
Coreless Motors – The Next Generation
To address the limitations of DC nano motors, MEMS technologists developed coreless nano motors that eliminated the magnetic core. Coreless motors had an etched copper coil deposited directly on the silicon substrate. When powered, the coil interacting with a permanent magnet produced rotational torque without commutation. Being simpler in design, coreless motors were more energy efficient, near- silent in operation and amenable to batch fabrication. They ranged in size from 0.5-3mm and torque from 1-10μNm. This motor type went on to dominate applications such as medical diagnostics equipment, lab-on-chip systems and micro-robotics.
Advances in Materials and Fabrication
Over the last two decades, advances in materials science complemented MEMS technology to further miniaturization of nano motors. Magnetic materials with high coercivity like neodymium-iron-boron (NdFeB) allowed construction of more compact permanent magnet components. Thermosetting polymers like polyimide and epoxy enabled fabrication of high-aspect ratio 3D nano motor coils. New dry and wet etching techniques refined nano motor designs down to 500μm scale. Self-assembly methods led to creation of motor components with nanoscale features. These developments extended operational capabilities to micro-Newton torque ranges while reducing motor sizes.
Widespread Applications
The confluence of MEMS, materials science and process engineering has resulted in micro motors being used across diverse applications today. In electronics, nano motors enable autofocus and stabilizing mechanisms in smartphone and DSLR cameras. The biomedical field uses nano motors to propel drug capsules, actuate medical instruments and power lab-on-chip diagnostic systems. Microrobotics researchers employ fleets of synchronized coreless nano motors to perform micro-assembly and material transport tasks. Micromotors also find use in microfluidics to precisely manipulate liquid volumes, in data storage to position read/write heads and in automation equipment for micro-parts handling. Going forward, further device miniaturization coupled with energy harvesting promises to expand nano motor applications into new territories.