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Chapter 1
Design of Integrated Flexible Structures for Micromanipulation
The design of robotic micromanipulators relies on flexible mechanical structures. These are increasingly being used due to their integration of actuator and measuring functions. The general design context for these integrated systems has resulted in a complex and multi-disciplinary approach to the problem. This design exploits structuresâ flexibility to respond to the challenges of robotic manipulation on a microworld scale. The design analysis approach is used in fields ranging from material sciences to advanced automatic control and topological structural optimization. In this chapter, the need for optimal design aid tools for these systems will be clearly highlighted and a range of existing optimization strategies will be examined. Finally, an illustrative example that focuses on the development of an optimal design software tool for flexible monolithic structures, will conclude the chapter. These structures are capable of maintaining actuator and sensor functions in a distributed and integrated form using piezoelectric materials.
1.1. Design and control problems for flexible structures in micromanipulation
The efforts made in Japan and the United States at the beginning of the 1990s in system miniaturization and integration have resulted in the concept of the micro. Whether in electronic devices for use by the general public or microcomputers or pioneering devices in mini invasive surgery, all these systems integrate several functional components (mechanical, optical, electric, etc.) in a more or less restricted space to create a microsystem, also known as a Micro-Electro-Mechanical System (MEMS) or Micro-Opto-Electro-Mechanical-System (MOEMS) if it features optical functions. The concept of a microbot is the result of a combination of microsystems and robotics. Its principle aim is to cause necessary movements to move and direct one or several tools to carry out a task in the microworld, a world of objects on a micrometric scale.
Microsystems carry out technological functions of different kinds, whether mechanical, thermal, electrical or optical. They cover a vast sphere of applications in a number of domains (biomedical, automobile, optical, micromanipulation, etc.). As with robots, the microbot is a mechatronic system controlled on site which is reprogrammable, able to make movements in relation to a specific environment and even interact with it. It includes perceptive, environmental action and information processing functions. In addition, it adopts its dimension and resolution specifications from this microsystem. In its strictest sense, the use of the micro prefix refers to a micron (10â6 m), although the dimensional objective often lies between a millimeter and a centimeter [BOU 02]. If not strictly micrometric in size, a microbot can be qualified as such when it has at least one of the following characteristics [REG 10]:
â It uses micrometric components (microsensors, micro-actuators, etc.).
â It uses micrometric objects or, more generally, carries out tasks within the microscale, that is the world of micro-objects.
â It has high positioning resolutions of less than a micron (the study and creation of high-resolution robots, of the order of 100 nm or less, has often resulted in robots characterized by their small size and a reduced final size due to the constructive principle used).
As a result, the definition of microbots and, more generally, microsystems gives them a wide applicable field. The design and control of microbotic devices dedicated to micromanipulation tasks constitute the core of this chapter.
Micromanipulation relates to the use of an external force to carry out tasks such as picking up and dropping, pushing, cutting and assembling objects whose dimensions range from the micrometer to a millimeter. The creation of the robotic micromanipulation, devices by miniaturizing robotic manipulators, as known on the microscopic scale, is often not possible because the reduction of the scale applied to robotsâ functional components faces technological barriers. Miniaturization attempts must, therefore, take place in several fields:
â micromechanics, as well as the study of manufacturing and micro-assembly procedures dedicated to microworld scales;
â actuators (for the application of forces and movements in volumes in the order of cubic centimeters, cm3), notably in strength and position sensors (small in size but with a high-resolution); control and implementation within computational parts.
Miniaturization cannot be reduced to a simple reduction in scale of existing components, but demands the complete reconsideration of robotsâ major functions and technological means to implement them. In particular, other means of actuating and measuring must be studied with regard to their physical principle as well as their good adaptability to the microworld in terms of movement, forces, mechanical force, output, controllability, observability, etc.
1.1.1. Characteristics of manipulation on the microscale
In the context of microbotics, measuring information from the micrometric world is a problem that raises a number of challenges. Indeed, due to the scale factor, the dynamic behavior of micro-objects is no longer governed by their mass (which is an effect of volume), but surface effects that correspond to adhesive forces (surface tension, electrostatic and van der Waals forces). The dynamic of this kind of micrometric environment differs completely from that of the standard metric world. In addition, these adhesive forces are generally dependent on the type of context (dry or liquid environment) which are variable over time (tribo-electrification, modification of environmental conditions, humidity and temperature) and in space (types of materials in contact, geometry and local roughness). In these conditions, the understanding and prediction of micro-objectsâ dynamic behavior requires at least knowing their position in the microworld and, in addition, knowing the amplitude and gradient of the forces being exerted on them.
The notion of the microworld is currently used to define a space (world) with specific characteristics. This is a world where objects with sizes ranging from 1 ”m to 1 mm evolve. In comparison, the âmicroworldâ is the term adopted to indicate a world of objects that exceeds a millimeter in size. Interactions between objects in the microworld are governed by the laws of âmicrophysicsâ . This term indicates that the laws governing the behavior of objects in the microworld are different to those in the macroworld. This is not the case in reality and the difference lies in the fact that forces, which are completely unnoticeable on the macroscopic scale, become paramount due to the objectsâ reduced size. The surface effects, therefore, play a more important role than volume effects. To highlight this difference, we will take two spheres with a diameter of 20 ”m and 20 mm, respectively. The calculation of the ratio between the surface and volume, which is equal to 3/r1, is 300,000 and 300 for each sphere. As a result, surface forces (surface tension and electrostatic forces) are much more significant in relation to volume-related forces (weight) on the sphere with a diameter of 20 ”m compared with 20 mm.
In everyday life, a large number of examples attest to the influence of surface forces in the microworld. The most obvious example is that of a mosquito which can rest on the ceiling. This is possible when the adhesive forces (surface forces) between the insectâs feet and the ceiling are significant enough to counter balance its own weight (volume force). A second example is that of a human who wants to pick up a small object (i.e. a needle). Very often, and sometimes unconsciously, s/he moistens her/his finger to pick up the object more easily using adhesion by surface tension as this process increases the adhesive forces between the needle and her/his finger.
1.1.2. Reliability and positioning precision
Regardless of the performance in terms of the precision and resolution inherent to the technological choice of actuator and sensor or, more generally, the chain of control, the absolute precision of positioning in robotic systems is generally limited by its mechanical structure, manufacturing faults and potential mechanical challenges in managing mechanical play or backlash in joints that introduce systematic errors. If the precision of the positioning of such systems can generally result in the precise manipulation of millimetric objects, it becomes unacceptable in the more constrained context of micromanipulation, where the resolution required is sub-micrometric.
To overcome this lack of precision, compliant structures are of particular interest. This shape can prove undesirable on the macroscopic scale because it adds small, unpredicted and difficult-to-control movements which can nevertheless be used to an advantage on a micromanipulation scale. The use of the mechanical structureâs shape can be interesting from the perspective of a precise position and guiding. This is the object of study in compliant structures. These structures are often composed of a single compliant body (without any kinematic connection) and are known as monolithic. The coexistence of varied components such as actuators and sensors in the microsystem requires managing micrometric scale assembly technologies to allow a superior final design in the robotic system. Functional challenges and surface states demand manufacturing tolerances which become increasingly difficult to respect on the mesoscopic scale. Whether serial or parallel, it has been shown that microassembly is the most costly aspect in the production of microsystems (up to 80%) [KOE 99]. As a result, the reduction, o...
