Abstract: MEMS is a current research hotspot, and microrobots are of great significance to the development of MEMS and are an indispensable part of MEMS. Micro-motion technology is an important branch of robotics theory and the foundation for the development of microrobots and related micro-technologies. Currently, various new micro-actuators are emerging in an endless stream, greatly promoting the development of microrobot technology. Analyzing the principles of micro-motion and clarifying the mechanism of micro-motion generation can not only enrich robotics theory but also potentially lead to a qualitative leap in micro-motion technology. From this perspective, various micro-motion principles are analyzed and compared in detail to draw meaningful conclusions. Keywords: Microrobot; Actuation mechanism With the continuous development of science and technology towards miniaturization, MEMS technology has become a current research hotspot. The processing and assembly of various micro-motors, micro-pumps, micro-sensors, and micro-parts urgently require the development of corresponding micro-technologies. Micro-motion robots are an important branch of MEMS and a major means of realizing micro-machining, handling, and micro-assembly. Micro-motion technology is fundamental to the design of microrobots. Cell cutting, microsurgery, precision worktables for micromachining, micromanipulators, and even miniature factories of the future all rely on micro-actuators. Microrobots must meet the goals of reducing their mass and size while possessing significant driving force, torque, and working space. Therefore, direct drive should be employed to minimize transmission chains. Many micro-actuators have been developed, such as micromotors, electromagnetic actuators, piezoelectric actuators (PZT), supermagnetostrictive material actuators, shape memory alloy actuators (SMA), and intelligent gel actuators. Since the objects of micromanipulation are mostly microscopic, the motion principles of microrobots differ significantly from those in the macroscopic environment. The movements of microrobots are generally small, mostly at the micrometer or even nanometer scale, thus requiring high precision from the actuators. Analyzing the motion principles of microrobots helps to fundamentally understand the mechanisms of robot motion and ultimately promotes the development of new microrobots and improves their performance. The following section analyzes these driving principles and investigates solutions for achieving omnidirectional motion using these driving methods. 1. Analysis of Motion Principles There are three main driving methods for micro-robots: wheeled, mechanical friction, and legged. Wheeled micro-robots use micro-motors to drive micro-gears to rotate wheels. This method is familiar, but its disadvantage is low motion precision, only reaching the micrometer level. Mechanical friction, based on general physical laws, utilizes certain material properties and the combined effect of friction to convert minute material deformations into micro-displacements of the robot, driving it forward. The main driving methods include vibration, impact, inchworm, elastic deformation, and collision. Legged driving methods use single or multiple legs to vibrate and jump forward or glide. Through speed planning, flexible turning, forward movement, and backward movement can be achieved. Many scholars at home and abroad have conducted extensive research in this area. Japan, in particular, has conducted research and experiments on mechanical friction driving methods. German and European scholars have conducted more research on legged driving, forming another theoretical system. The motion principles of various driving methods are analyzed below. 1.1 Vibration Driving Method The vibration driving method was proposed by Nagoya University in Japan. In 1993, they used the vibration of piezoelectric crystals to create a miniature robot. The principle is that by changing the voltage and frequency of the piezoelectric crystal fixed to the L-shaped frame, the movement speed and direction of the mechanism can be controlled. The speed can reach 100 mm/s, and it can move within a 15° inclined plane. The principle model of the vibration method is shown in Figure 1, which consists of a rectangular frame and a piezoelectric crystal attached to the frame. Its movement follows the law of motion of the center of mass and is subject to friction. Its movement process can be divided into two stages: (1) Rapid movement stage. Assuming that the frame vibrates as shown in Figure 1a, the piezoelectric crystal expands rapidly at this time, pushing the right beam where it is located to bend to the right. Since the beam protrudes outward, the center of gravity of the piezoelectric crystal moves to the right. If the frame and the piezoelectric crystal are regarded as a system, then the force of the piezoelectric crystal acting on the frame is the internal force of the system. Since the bottom beam bends upward at this time, it has little contact with the horizontal surface, and the friction is very small. The external force on the system in the horizontal direction is approximately zero. According to the theorem of motion of the center of mass, the system moves inertial motion in the horizontal direction. Since the center of mass of the system is stationary at the initial instant, the center of mass of the system will remain in the original position. It can be deduced that the frame moves to the left. (2) Friction stagnation stage. As shown in Figure 1b, at this time, the piezoelectric crystal contracts, the beam on the right side of the frame bends inward, and the center of mass of the piezoelectric crystal moves to the left. According to the theorem of motion of the center of mass, the frame will move to the right. However, since the bottom beam of the frame bends downward at this time, it is obstructed by the horizontal surface. The beam overcomes this obstruction and acts on a distributed force system on the horizontal surface. Similarly, the horizontal surface also acts on the beam with an opposite distributed normal force system. As the normal force increases, the friction between the beam and the horizontal surface increases, making the frame unable to move to the right. These two stages form a movement cycle. The alternating expansion and contraction of the piezoelectric crystal causes the frame to move intermittently to the left. The characteristic of this method is that the movement speed is fast, which can reach more than 100 mm/s. However, its motion is unstable, has vibration and noise, and is difficult to control. 1.2 Impact-driven method In 1988, Toshiro Kitsuguchi et al. of the University of Tokyo, Japan, first proposed the "impact method", which was later used in micro-machining and electrochemical micro-machining. The driving principle of the impact method is as follows: The inertial body is connected to the moving body through the piezoelectric element. The extension and contraction of the piezoelectric element are controlled by the voltage change of the piezoelectric element, thereby moving the object. Figure 2 is the impact-driven principle model. (1) The piezoelectric element is in a contracted state. When a voltage is applied to the piezoelectric element quickly, the piezoelectric element extends rapidly. The moving body moves to the left. (2) The piezoelectric element contracts slowly, and the inertial body moves to the left. During the return process, the inertial body accelerates continuously to generate inertial force, which is less than the static friction force; otherwise, the moving body will also move to the left. (3) When the piezoelectric element contracts to the initial length, it suddenly stops. It is as if a collision has occurred between the inertial body and the moving body. The whole system begins to overcome the friction force and move to the left until the kinetic energy is exhausted. The process of moving to the right is similar to that of moving to the left, except that the piezoelectric element is initially in an elongated state, then rapidly contracts, and slowly recovers. The motion of the piezoelectric crystal is mainly divided into two parts, according to the law of conservation of momentum: Where: m ———inertial body mass; M ———moving body mass; ΔL ———piezoelectric crystal displacement. During acceleration, it is essential to ensure that the acceleration is very small so that the inertial force is less than the static friction between the moving body and the platform. Therefore, the acceleration should satisfy the following equation: The object does work against friction until the kinetic energy is exhausted. The advantages of impact drive are simple structure, easy miniaturization, and reduced energy consumption due to the absence of a holding mechanism. However, impact drive is also called the Stick-slip effect, so the magnitude of friction directly affects its motion accuracy. In addition, because it is sliding, the motion is more difficult to control. 1.3 Inchworm Drive Method In the early 1990s, Shizuoka University in Japan first proposed the inchworm drive principle and conducted experimental research. They imitated the movement principle of the inchworm and successfully developed a small self-propelled mechanism. Later, the inchworm method was used to drive microrobots to realize the "imprinting" processing and "micro-hole" processing. The principle of the inchworm driving method is to imitate the inchworm's peristaltic method, utilize the deformation of the telescopic element, and combine it with the holding mechanism to achieve micro-displacement. Generally speaking, the telescopic element uses piezoelectric ceramic (PZT), and the holding mechanism uses either an electromagnet or a piezoelectric element. Here, we will use an electromagnet as an example. Its principle is shown in Figure 3. (1) When the electromagnet on the left is energized and attracted, the piezoelectric element is energized and elongated. (2) When the electromagnet on the right is energized and attracted, the electromagnet on the left is de-energized and relaxed, and the piezoelectric element contracts. In this way, a cycle is completed. Repeating the above steps can realize step-by-step movement. The characteristics of the inchworm method are: (1) The static contraction and expansion cycle of the piezoelectric element results in a relatively slow movement speed. (2) The holding mechanism makes it accurately positioned, but it is not conducive to miniaturization. (3) It has a large range of movement, is not limited by space, and has high precision. 1.4 Elastic Deformation Drive Method [align=center](a) Moving Mechanism (b) Schematic Diagram Figure 4 Elastic Deformation Drive Principle Model[/align] Using elastic deformation to drive objects and achieve micro-displacement is also a method currently being studied in the field of micro-robots. In 1997, Hayakawa Kazuaki et al. of Aichi Institute of Technology in Japan developed a micro-moving mechanism based on elastic deformation [5]. This Scratch Drive Actuator (SDA) uses the elastic deformation of an L-shaped plate under periodic voltage to drive the robot body forward. The schematic diagram and moving mechanism of SDA are shown in Figure 4. The elastic potential energy generated by the elastic deformation of the silicon plate drives the mechanism to move forward. The deformation amount of SDA is Δx. The characteristic of the elastic deformation method is that the structure is simple and it is easy to miniaturize. Now, micro-mechanisms with a size of only tens of micrometers square and a height of several micrometers have been made. Since the fabrication and control of SDA are simple and easy, it can be applied to many fields. 1.5 Collision Drive Method The Shenyang Institute of Automation of the Chinese Academy of Sciences proposed the collision method. As shown in Figure 5. An object of mass m (armature) moves along the direction of the arrow under the influence of magnetic force (or other force). After colliding with an object of mass M at a certain speed, object M gains a certain speed, causing M and m to move together. According to the principles of conservation of momentum and energy, the following equation of motion can be obtained: Figure 5 Collision-driven principle model Theoretically, the mass ratio of this method is quadratic, which may have higher feeding accuracy than the impact method. However, its controllability needs further verification. 1.6 Leg-driven method The University of Karlsruhe in Germany proposed the concept of a leg-driven microrobot and made an experimental prototype using this method. Currently, they have applied this principle in a comprehensive microrobot development project MINIMAN and achieved satisfactory experimental results. The principle of leg-driven is shown in Figure 6. The robot's micro-motion platform is driven by three piezoelectric ceramic legs. The piezoelectric ceramic is tubular, and its length changes when a voltage is applied. Each tube is plated with metal electrodes inside and outside. These are used to apply voltage to the piezoelectric tube to change its length. Due to changes in the electric field, the piezoelectric tube either elongates or contracts. To make the piezoelectric ceramic bend, the outer electrode is divided into four parts, distributed at 90° along the axial direction, as shown in Figure 7. The motion process is based on the speed of the piezoelectric ceramic, applying the slip-stick driving principle. First, the piezoelectric tube bends slowly, then moves one step quickly. Due to the inertia of the robot platform and the high speed of the tube, they slide on the glass plate. Because the friction between the ruby ​​ball and the glass is much smaller than the mass of the robot, the platform pulls back slightly, but this is negligible compared to the step length. The piezoelectric tube extends again when it reaches a new position, completing the step. Compared with mechanical friction drive, the leg-driven method can better utilize the flexibility of microrobots. Each actuator can move in any direction in the plane without needing to be combined. Its motion is smooth and can simultaneously achieve rapid movement (coarse motion) and high-precision micro-manipulation (micro-motion). The microrobot has accurate positioning, good controllability, and does not require mechanical guide rails, allowing for an unrestricted range of motion. Figure 7 Piezoelectric ceramic tube actuator [align=center](c) (d) Figure 8 Schematic diagram of omnidirectional micro-movement platform[/align] 2 Realization of planar omnidirectional motion As can be seen from the above analysis, by changing the applied voltage of different electrodes of the piezoelectric tube in the foot-driven principle, the actuator can move in any direction, thus giving the robot high flexibility. The mechanical friction motion principle can usually only achieve linear movement. To achieve flexible planar omnidirectional motion, the above structure must be combined. Here, taking the impact drive method as an example, several specific combination methods are given, as shown in Figure 8. When the actuator can only move in one direction, a symmetrical planar omnidirectional motion platform can be obtained by combining 8 actuators, and its structural layout is shown in Figure 8a. The characteristic of this structure is stable operation and no coupling phenomenon. Obviously, due to the large number of actuators used, it is difficult to miniaturize this structure. A simplified model is shown in Figure 8b, where the number of actuators is reduced to 4. When using piezoelectric element drive, since the motion is bidirectional, at least 3 actuators can be used to achieve omnidirectional motion, as shown in Figure 8c. Figure 8d shows another platform, driven by four dual piezoelectric crystals. Here, abrupt bending replaces abrupt elongation to achieve the driving effect. 3. Improvements in Driving Methods In fact, in research, researchers do not adhere to a single method but rather combine and improve upon multiple methods based on practical needs. Connecting multiple piezoelectric ceramics in parallel to drive inchworms can extend the stroke of micro-motion robots and has been used in STM and precision stages. A bidirectional self-propelled mechanism driven by a single piezoelectric element using an impact-driven method has been used for cell manipulation. The Smooth Impact Drive (SIDM) mechanism, in addition to the self-propelled function of the impact-driven method, can also achieve high-speed coarse motion and high-precision micro-motion. Tetsuro Sakano proposed a friction-driven method, using two identical driving bodies with stacked piezoelectric ceramics. The pressure generated by the minute deformation of the piezoelectric ceramics acts on the clamping mechanism, creating friction between it and the guide rail, driving the robot forward. To meet the needs of microrobots in biomedical technology, Koji Ikuta proposed a controllable micro linear mobile robot combining impact-driven methods with an electromagnetic clamping mechanism. Compared to conventional fixed-friction impact, this friction-controlled impact-driven method can achieve four driving states: release, locking (also known as clamping), enhancement, and reduction. The system can eliminate excessive force through sliding, thus improving the safety of the operator and the mechanism—this is the biggest difference between this system and others. This controllable robot is small in size, lightweight (force/mass ratio reaches 70), high-speed (up to 35 mm/s), and highly efficient, while operating safely and quietly.