Leseprobe
I. Introduction
II. Fundamental Concept & Background
III. Different Micro-Actuation System
IV. Piezoelectric Actuators
V. Amplifier Model and Operations:
VI. Applications & Future Works
VIII. Conclusion
REFERENCES
I. Introduction
The last 30 years of Micro-Electro-Mechanical Systems (MEMS) have been seen as an era of exponential development of electronic devices such as sensors and actuators on the order of micro dimension. Throughout these three decades, MEMS becomes the key to make a multibillion-dollar industrial production from the very beginning of the research & experimental outcomes in the laboratories. Besides the electronic devices there are lots of applications of MEMS in small robotics is found today. Different types of MEMS actuators are designed to study and lots of diverse researches have been conducted on those designs to build small microrobots. Some of those robots are made bionic to mimic natural living beings. Being familiar with many types of MEMS devices like sensors and actuators are very much important to make such robots. Sensors generate electrical pulses according to any environmental changes. On the other hand, transforming any energy into mechanical energy is technically made by actuators. Different classifications of actuators are there based on construction as well as operation are available in industrial production.
Miniaturization of devices and assembly needs micro level actuations. Micro actuation becomes a challenging job today in MEMS technology. Several systems out there among almost all devices, this technology is being implemented today. From tiny molecular separation control to millimeter range different actuations can be achieved because of the MEMS enormous development. In many fields of industrial and scientific research we need precise control in hardware systems and MEMS becomes so beneficial to do this with much more accuracy. Various actuation methods are available in MEMS such as electrostatic, electromagnetic, microfluidic, piezoelectric and many more to advance our medical and engineering fields or scientific and industrial researches. Each and every method have some pros and cons as well. Some needs more power to actuate on the other hand some makes small deflection. Our discussion is limited here about such an actuator which consumes less power and deliver large deflection with high force. Some applications of these type of actuations are like artificial muscles, tactile displays. In these actuators, large deformation with high efficiency we need to produce with the help of MEMS somewhere and technology delivers almost desirable results till now. But when we go for light weight low power high deflecting actuation requirement, then MEMS become more challenging and still under research nowadays. Even it is become more difficult to construct when our actuation needs fast response time or it have to oscillate at high frequency with large deformation even with large output force. Bionic robots sometimes need these types of actuators to control itself. We called those actuators as Artificial muscles. Nano-technology and VLSI fabrication are the key topic we have to understand to make this kind of on chip devices. We will recall several fabrication methods of making an on chip artificial muscle throughout the demonstration. Here we present an idea on this artificial muscle and discuss about the simulation results of the experiment.
II. Fundamental Concept & Background
Traditionally, Hydraulic systems are used to produce large deflection and high force actuation converting hydraulic pressure into linear or rotary motion. Physics behind the hydraulics works good also in micro level. Of course, the process integration of hydraulic systems in micro level such as on a Si chip is very difficult. But in these days, it can be manufactured by several advanced fabrication methods. Hydraulic fluids are considered here incompressible and this complete hydrostatic fluid in any section is sealed by Polydimethylsiloxane (PDMS) membrane as any minor leakage of liquid can affect the microlevel actuation. Keeping the basic principle of hydraulics same here it is (depicted in Fig. 1) to visualize a fabricated structure of MEMS hydraulic on a Si substrate. It actually works as a displacement amplifier.
Due to copyright issues, this picture has been removed by the editorial staff.
Fig.1 Schematic of Micro hydraulic actuation (Left) and generic hydraulic principle (right). L stands for large displacement with d or large force with F at suffix similarly S stands for the same parameters with small values. (A) is for displacement amplification and (B) is for force amplification.
If the lower piston with larger area moves very small height due to pressure, the upper piston with small cross section area will move at large distance as the overall volume of the hydraulic liquid is constant at any particular pressure. The thermal expansion coefficient of the liquid should be very small. Also, we consider incompressible liquid here in the chamber.
Various research groups have experimented on micro-scale pneumatic actuators. In this type of actuators, a flexible membrane or chamber is inflated or a micro/meso-scale movable piston is actuated with an external source of pressurized fluid 1. For instance, Jeong et al. used cascaded pneumatic membrane actuators to realize a peristaltic pump for microfluidic applications 2. Konishi et al. utilized inflatable polyimide balloon actuators to make a 2-degree-of-freedom out of plane end-effector 3. Gandhi et al. used a bellow spring to create an in-plane pneumatic actuator 4 and De Volder et al. demonstrated a miniaturized cylindrical pneumatic piston 5 and microfabricated pneumatic actuators 6. All of these require an external source of pressurized fluid. While these pneumatic devices and numerous similar structures 7 – 9 are sometimes referred to as micro-hydraulics, these works depart from the concept of hydraulic systems since they do not amplify deflection or force.
III. Different Micro-Actuation System
In the laboratory research field, there are several types of microhydraulic systems are developed for necessary applications. Such microhydraulic actuator classifications are shown in Table 1. with all the advantages and disadvantages depending on specific requirements.
TABLE 1
Classifications with pros and cons of Several Actuation Methods
Abbildung in dieser Leseprobe nicht enthalten
Hydraulic systems (Fig. 1) are frequently used in many macro-scale systems where amplification of force or deflection is needed. The principle of operation is based on uniform distribution of pressure over an incompressible liquid, filling two connected chambers 10 – 12. Amplification of either deflection or force is made possible by making the surface areas unequal of two mobile pistons/membranes enclosing the chambers. For instance, if a force is applied to the large membrane, and the fluid is incompressible, the same pressure is applied on the small membrane. Therefore, by conservation of work, the ratio of membrane deflection should be inversely proportional to their surface area ratio or the applied force.
Among the above listed all types of actuation we used here the piezoelectric actuation for large displacement actuation as it consumes less power. Piezoelectric actuators have many applications in aerospace industries. NASA, experimented on these types of actuators for their cryogenics. Amplified piezoelectric actuators are lubrication free because of the flexural hinges and important so for cryogenics. On the other hand, piezoelectric actuators can provide low power consumption with high power density, high precision positioning accuracy in quasi-static operation. Although TABLE 1. shows the displacement for piezoelectric actuation is literally small. We then researched for a particular design to get the large displacement investing very low potential as power, simulating over many designs for an efficient amplified piezoelectric actuator. Here we are discussing about such a design which we can implement in the micro-hydraulic piezoelectric displacement amplifier system.
IV. Piezoelectric Actuators
Piezo materials are a special type of ceramic that expands or contracts when an electrical charge is applied, generating motion and force. (Conversely, piezo materials will also generate energy when a mechanical stress is applied.) Piezo actuators harness this motion to provide very short strokes with high frequency and fast response times. They also generate high forces relative to their small size, giving them a significant power-to-size ratio. There are several types of piezoelectric actuators are used today. In many cases we are familiar with stack piezo actuators, disc piezo actuators, tube shaped piezo actuators and cantilever shaped piezo actuators. These actuators have different efficiency with actuation displacement. Here is our objective is to find much higher displacement with low input voltage.
Different Shapes and Actuation
Applying electric potential across a piezo ceramic material we can have some crystal deformation due to its intermolecular reorganization along the electric field. Sometimes if the electric potential is applied not only along the crystal polarization axis but also along any other principle or transverse axes of the crystal, we can also have different structural deformation. Based on which deformations the actuators can be designed as shown below in Fig.2. Although these applications are in macro dimensions but we will apply some application in the MEMS based on-chip actuator IC. Our fascination here to make large displacement by the actuator which are fabricated on an on-board IC. Our objective here is to design an artificial muscle with high deflection applying low potential difference.
Due to copyright issues, this picture has been removed by the editorial staff.
Fig. 2. Different kinds of piezoelectric actuation techniques on large scale.
Cantilever Actuation with Large Deflection
Many applications in MEMS technology of cantilever beam shaped piezo actuators are found todays such as in microvalve systems, drug delivery systems even in many micro actuators like micro grippers depending on the requirements. Fig. 3 shows such examples of cantilever fabrications in MEMS.
Due to copyright issues, these pictures have been removed by the editorial staff.
Fig.3 Some fabrications of cantilever piezo actuators in MEMS.
Mathematical models are frequently reported after simulating in almost real time conditions in some Finite Element Method (FEM) solver software like COMSOL Multiphysics, ABACUS, DSS-Solidworks or ANSYS Workbench. Doing some thorough surveys on several designs we found that cantilever shaped piezo electric actuators shows high tip deflection according to simulation results.
Proposed Shapes of Cantilever Beam and Internal Construction
In our hydraulic amplifier system, we will use such an actuator designed by the piezoelectric cantilever to produce a minute deformation due to applied low potential difference and then will make the actuator displacement some hundred times to initial. As far the information still there are some displacement amplifiers 1, 2, 3 are used as shown in Fig.1. we here advanced our design to the technology of micro fluidic hydraulic amplifier based on sealed by thin polymer film which can be deformed by the pressure but cannot be displaced as sliding motion may causes the leakage of fluid from the piston chamber.
Our design includes a quad-V-shaped triangular cantilever beam piezoelectric bender to produce upward and downward motion in the lower base piston as shown in the below design in Fig. 4.
Due to copyright issues, this picture has been removed by the editorial staff.
Fig. 4. The schematic diagram of the internal bender motion due to applied alternating voltage.
The 3D CAD design of this configuration is done in Autodesk INVENTOR 2017 version. Here it is.
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