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Chapter 1
Introduction
1.1 Nanotechnologies
The study of systems and structures always smaller and smaller, is stimulated by the reduction of the encumbrances of the new technologies, from the reduction of the energetic consumptions and for the intrinsic tendency of the structures to the progressive reduction of the inside defects. It is properly such reduction that allows to raise the performances of energetic systems and structures.
For instance is well known, how the mechanical characteristics of strength of the glass, in example, is low in comparison to the one of the same material produced as
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fibers as for the realization of composite materials. In such cases the fiber’s structure leads to increase the strength to a dimensional unit’s order.
The discrepancy among the mechanical strength of the materials, defined according to the energetic theory by Hooke’s model and the real one, it is, in fact, around two dimensional orders The effect is properly tied to the presence of inside defects of the material. Such defects involve that stresses concentration that lead to a consistent loss of the mechanical strength.
Besides this, when the dimensional scale decrease to nanometers, there is a radical change of the optic, electric and magnetic properties of the materials. Such effect depends on the substantial reduction of the volume/surface ratio and on the low number of free electrons, that means a discretisation of the energetic levels in the electronic structure. Such defined effect '' Quantum Size Effect '' appears in fact when the diameter of the particle has the same magnitude’s order of the wavelength of the electron’s wave function [1.1]; Insofar the study of forms and base structures of materials is stimulated by the peculiarity of the physical properties that some structures have.
In the case of the Nanowires, for instance, the particular form and atomic disposition, vary according to chosen angle of helix, to make it develops, or it’s decided to wrap a foil of Graphene. This involves the increasing of mechanical strength and notable increasing of the physical properties of the basic material. Such effects are strongly due to the reduction of volume/surface ratio with the consequent substantial increase of the active surface, in comparison to the general volume, of the interfaces.
An example of the importance of the study and the applications of such materials and structures it’s the chemical catalysis of the batteries for which the adoption of carbon’s Nanowires leads to largely increase the efficiency . This is properly due to the high surface/volume ratio and electric conductibility, characteristic of these structures.
Such peculiarity makes the carbon’s Nanowires, incredibly efficient in the role of electrode, allowing an accumulation and an incredibly rapid release of charges; this
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happens without any substantial damages of the structures submerged in the electrolyte.
FIGURE 1.1: Nanotubes filaments on battery’s electrodes [1.2].
Another example of a very interesting and important nano-structure is the Graphene. Its extreme versatility allows, in fact, to get the principal elementary structures of the carbon, starting from an opportunely shaped foil (Figure 1.2).
FIGURE 1.2: Principles structures obtained by Graphene, from left to right Fullerene, Nanotube and Graphite[1.3].
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Even not explaining deeply the peculiarities of the Graphene, that are exceptional as for the electric conductibility properties, the optical ones and the mechanics elevated rigidity; thanks to this it is clear how it could be exploited in very different situations and uses. An example is the study of Graphene for development of the organic photovoltaic cells (OPV) for which it is succeed in exploiting the flexibility and the transparency of the electrode of Graphene to realize transparent electrodes that allow the light to react with the active material, getting high flexible panels capable to suit very well for very different surfaces [1.5].
1.2 Nanorobotics
and
micro/nano-world
theory
The study of nano-structures and the development of nanotechnology, however, presents numerous problems to be addressed. The main problem is manipulating such small objects as micro but especially nano-metric. Special tools are needed to enable the operator to observe, recognize, pick up, move, and properly characterize such small structures.
To treat the problem of manipulation, is not possible through a simple miniaturization of conventional technologies that operate on larger dimensional scale. This is due to the so-called "scale effect" (Table 1.1) it means that reducing the scale the predominance of the gravitational forces effects are equalized and then overcome by those from the adhesion’s forces.
Generally, in fact, on a body act different types of external actions depending on volume’s effects and/or surface’s effects such as gravitational attraction, magnetic attraction or repulsion, electrostatic adhesion, capillarity, etc… However, while the macro-world problems for handling come primarily from the friction on contact’s surfaces depending on the weight of the object to be manipulated, in micro and, especially, in nano-world are most of everything the surface’s forces to be
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considered. It’s immediately comprehensible that the effects of gravity acting on the mass and depending on the volume, decreases with the cube of the characteristic size variation while the superficial actions reduce following a quadratic law. In particular under the characteristic size of about 1[mm] surfaces actions become predominant.
INTERACTION DISTANCE FORCE
To infinit Gravity
From some [nm] to 1[mm] Capillar Forces > 0.3 [nm] Electrostatic Forces > 0.3 [nm] Van der Walls Forces < 0.3 [nm] Molecular interactions
0.1−0.2 [nm] Chemical Interactions
TABLE 1.1:Forces related to their dimensional scale [1.4].
All this means that the techniques for micro and nano-manipulation are studied and planned on the basis of very different physical phenomena most difficult to manage as electrostatic forces, capillary, etc.
1.3 Approch to micro/nano-manipulation
Being known the different typologies to be faced it results clear that the systems of manipulation need to be designed and their movements planned according to different canons to those of the conventional systems of the macro-world. Keeping in mind, most of all, the radical reduction of the weight’s effects and grip. An example is the use of mechanical gripper capable to pick up objects by the only exploitation of the of adhesion’s forces but, that, at the same time, are not able, through equal
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and contrary movements, to release and place the same object because for those same adhesion forces that have allowed the lifting, and then caused the permanence of the object attached to the gripper.
It is also necessary that these systems has a quite high dexterity to be well adapted to different types of structures to manage.
In order to make the manipulation easier it’s necessary to reduce, as much as possible, the effects related to capillary action and to disturbances caused by the instability of the medium in which the structures and objects are; in examples by conditioning the air to be as stable as possible. It is clear, therefore, necessary to operate in controlled environments in order to limit those effects. Working in environments with stringent level of vacuum that can limit disruption and moisture is the most desirable thing.
For the reason above it’s obvious, in order to achieve a high level of repeatability, the necessity to adopt micro-robotic precision, higher than that one of the objects to be manipulated.
1.4 Micro/nano-manipulation techniques
Micro and nano-manipulation system need to be developed by considering all the aspects that in the macro-world can be largely neglected due to their ineffective.There are numerous techniques for handling, made up till today, and which can be divided into several categories, first of all these can be classified in contact-based systems or contact-free relation between the object and the manipulator.This must be combined with a necessary knowledge and study of the characteristics of strength and fragility of the objects that need to be manipulated. In order to work and get the target without damaging or breaking it.
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1.4.1 Contact-based tecniques overwiew
Between contact-based systems exist many different typologies. These are based on the exploitation of different physical phenomena or the management of change in the contact surface areas in order to make quickly and easily pick and place operations.
These can be obtained using gripper of variable geometry, or combining suitably shaped tips, with carefully planned movements.
FIGURE 1.3: Two examples of prehensors based on surface tension effects:(a) variable curvature controlled by liquid injection; (b) variable curvature controlled by an applied voltage [1.4].
Other techniques are those one which use monolithic or assembling several parts, mechanical grippers. These types of grippers can work by directly grasping (Figure 1.4), by the exploitation of surface adhesion forces (Figure 1.5), by using adhesives or by the adhesion due to a liquid when the change of phase is forced.
FIGURE 1.4: Electrostatically-driven microgripper, constructed using micro-fabrication techniques [1.4].
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FIGURE 1.5: Manipulation by static Pick up and Release [1.4].
Among the high number of the other contact-based manipulation techniques, there are those ones in which the pick-up is obtained by exploiting the adhesive forces while the release is obtained by controlling the imposed vibration’s dynamics to the prehensors acting on the frequencies.
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1.4.2 Contact-free tecniques overview
Contrary, these techniques are based on the exploitation of different physical principles such as levitation due to the generation of stationary acoustic waves, due to the magnetic forces or the attraction due to the pressure dictated by photon reflection in crossing a medium of different density. One of these technique that is incredibly effective is the "Optical Tweezer". This system uses the pressure generated by focusing a laser beam, decentralized respect to the center of gravity of a micro or nano-object (Figure 1.6) allowing the lift and drag and then the release.
FIGURE 1.6: Principles of optical Tweezers [1.4].
Other systems exploit rather electrophoretic or electrostatic forces of attraction. By imposing the object, considered as a dipole, a given electric field is possible to apply a force and a torque that allow, if correctly managed, to pick up and move the object.
1.5 Objective of the project
The objective of the project is to develop a system that could permit the micro-robotic system bought by the team MAP of the group "Systemes Interactifs" to operate even outside the SEM, for which it has been specifically designed, but in a more stable and controlled environmental conditions than the atmospheric.
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One of the main reasons that lead to the realization of a system of that kind operating without necessity of a SEM came from the need to solve two problems. The first is dictated by the impossibility to manipulate samples of Graphene, which are studied in the laboratories of the institute, with a SEM, and this because of the substrate on which, Graphene samples, are prepared by a different laboratory. The available samples, are produced by another team that carries out tests on Graphene with the TEM preparing samples directly on Pyrex® glass plates. When this type of material is undergone to electron beam, as in the SEM, not being conductive, means a continuous accumulation of charges. The result is to cause highly distorted images, and thus the impossibility of being able to observe and to operate any type of manipulation.
The second need and challenge is to get a much more practical and economical solution than the use of a SEM. Even if it is clear that the level of control and conditioning of a chamber cannot reach the same accuracy than the one of the SEM, to succeed in making manipulations in a better and more stable environment than the atmospheric one, it would make it extremely simple, cheap and easy to purchase, to use, to manage and to transport.
1.6 Actual system
The existing system (Figure 1.7) is made up of the micro robotic RAITH (Figure 1.8) schematically shown under (Figure 1.9), mounted on a plate fixed on a plate and realized with a manual microstage to be easily aligned under the magnification system by controlling the X and Y position. It is made of a six DC-motors granting to the tips a motion range of 10[mm] than 2 X–Y–Z piezo-stages with a range of 10[μm] for X and Y, and 100[μm] for Z.
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FIGURE 1.7: Mirco/Nano manipulation system bought and developed at the ISIR.
FIGURE 1.8: RAITH microrobotic manipulation’s platform. Lateral view showing the arms actuating the micro tips over the working surface.
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The system is equipped with two arms on which are inserted two bars. At the end of each bar is located the tips. Through these pass and then are tightened the tips made of a tungsten wire with a size at the top about 5[μm]. Every tips is interchangeable and can be replaced with others of different shape and size depending on conditions and needs.
The micromanipulators are driven by an analog power supply box (Figure 1.10) with push-button and speed control for DC-motors and a series of potentiometers for the piezo-actuators. There is also a commutator to operate separately the right or the left turret while DC-motors are being used.
FIGURE 1.10: DC motors and piezo-actuators alimentation and command box.
The system is fixed as a cantilever at the frontal steel plate that constitutes the front and closing of the Plexiglas box in which is expected to realize the vacuum (Figure 1.7).
This plate has all the interfaces for manual positioning controls and the connections for cables. It is connected to the Plexiglas box with four holes that allow the same number of screws to be tighten into the corresponding threaded holes realized directly in the Plexiglas frontal chamber.
On the rear surface, the plate, has an excavated ring profile in which is inserted the seal, that should allow to ensure the vacuum’s integrity of the system.
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Because of the necessity of an external observation of the workspace, that will be explained in next chapters, the vacuum chamber has been purchased made of Plexiglass. For this the supplier ensures the achievement of a "low vacuum” with an internal differential pressure .
The device is also equipped with an external vision and magnification system consisting in a series of converging and diverging lenses and a mirror that allows an horizontal extension of the system. At the end there is a CCD (Charge-Coupled Device) camera for images acquisition. In addition, two supports have been made to keep this system positioned above the box and centered over the workspace. This system is designed for the needs imposed by a low budget that did not allow the purchase of any industrial existing magnification’s system operating with large focal lengths such as those needed; being a gap over 70[mm] between the top surface of the box and the working surface.
A membrane air pump with a hose allows to extract the air from the box using appropriate connections, fixed with clamps, on suction holes drilled on the frontal metal plate. Then a mechanical pressure gauge permits to check the pressure level inside the chamber and to limit the vacuum level.
1.7 System’s problems and objectives
In this section are briefly introduced the issue and the features characterizing the system and that are going to be analyzed and studied case by case in the next chapters.
Numerous are the issues to be solved on the equipment that, until now, has been tried to perform. These issues involve not one but all the components of the system requiring to solve problems even substantial on technical matters related to different domains. These involve a mix of analysis and design in different domains; the system, in fact, has significant problems and complications in almost all its
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parts and components. Mechanical aspects are related to the solution of the Plexiglass chamber’s problems for sealing and maintaining of the vacuum. After the set up of the optical system for magnification and visualization, not efficient yet and still lacking in certain parts needs to be done. Therefore will be necessary to use the principles of geometrical optics in order to obtain a solution as simple and functional as possible. The following step is the modification of the electronics. For that are need to overcome certain shortcomings of the power system dictated by the desire to get a computer-based control for the micro-robotic system. The system is in fact lacking in amplifiers for the X–Y piezoelectric actuators power supply. Finally the last target is going to be to interface the analog components of the system with a computer and write the control program for the system. This could be carried out by means of any software for enabling a fine tuning in positioning of the piezo-actuators and put the basis to design a system for an automated handling.
1.8 References
[1.1] G. Schmid, M. Decker, H. Ernst, H. Fuchs,W. Grünwald, A. Grunwald, H. Hofmann, M. Mayor, W. Rathgeber, U. Simon, D. Wyrwa. Small Dimensions and Material Properties A Definition of Nanotechnology, November 2003 [1.2] R.Signorelli. MIT Nanotube Super Capacitor,
www.peswiki.com/index.php.directory:MIT_Nanotube_Super_Capacitor
[1.3] A.K.Geim and K.S.Novoselov. The rise of graphene. Nat Mater, 6(3) :183 191, March 2007.
[1.4] S.Régnier and N.Chaillet. Microrobotics for Micromanipulation. 2008.
[1.5] Lewis Gomez De Arco, Yi Zhang, Cody W. Schlenker, Koungmin Ryu, Mark E. Thompson, Chongwu Zhou. Continuous, Highly Flexible, and Transparent Graphene Films by Chemical Vapor Deposition for Organic Photovoltaics. ACS Nano, 2010; 4 (5): 2865 DOI
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Chapter 2
Vacuum chamber analysis and
mechanical interface design
2.1 Vacuum chamber
The architecture adopted to obtain a vacuum of low magnitude, as already explained, is shown in the CAD model in Figure 2.1. It is made of Plexiglass with a rectangular opening in the front. The choice of the material come from the original use of that chamber exploited only for the robotic platform protection. It had no other function than protect and make easy its transport and manipulation. Once it has been decided to exploit the chamber to realize the vacuum, the necessity to use
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a material as transparent as possible has been related to assure the vision with external optics. The chamber is directly attached to the frontal plate of the microrobotic platform by 4 threaded holes directly machined in the Plexiglass. The vacuum chamber has been designed and assured by the constructor to resist a differential pressure between outside and inside of [Pa]. It has been modified by inserting a transversal aluminum bar (Figure 2.2) whose function was the only one to reduce the vertical dimension of the openings. Through this, in fact was permitted, to get the continuity of the contacts at the interface, by the elastomeric joint, placed on the rear part of the frontal plate of the manipulation platform, with the pursue to enable the system to reach the desired vacuum level.
FIGURES 2.1: SolidWorks® model of the Plexiglass vacuum chamber.
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2.2 Functionality and considerations
It is essential to state that the project of a box even if for a low vacuum, by adopting a solution with a chamber entirely made of Plexiglass is an inappropriate approach. The necessity to observe manipulations from outside the chamber combined with the choice of material with that structural configuration is a failure. It would have been much more useful to adapt a structure obtained with a metallic material, much more rigid, and then design on its walls just one or more windows using a transparent material to enable the visual control. Otherwise, a cylindrical structure would have assured a higher level of security for obtaining a higher vacuum.
Equally inappropriate, thinking at the structural strength and effectiveness is the choice of the closing system by forcing the locking plate with four screws directly threaded in the Plexiglass. Is well known, in fact, the harmful effect of hardening for this material when mechanically machined .
After all, it is necessary to add that subsequent modifications to the structure, that we are going to explain now haven’t led to any solution of the problem because they weren’t well studied and analyzed.
The operation performed by the insertion of the aluminum bar has proved unsuccessful at the first attempt to reach the actual level of vacuum with the completely assembled system. During the stages to reach the vacuum, just after having reached a depressurization of e u t the occurrence of leaks led to the impossibility in continuing the depressurization.
Furthermore, the presence of the threaded holes passing through the thickness of the front closing door ,tighten in the threaded holes in the Plexiglass, cannot ensure, the sealing of the vacuum. This because, in the absence of an effective local isolation of the closing under the screw head and the absence of seals around the holes and at the interface between the frontal plate surface of the opening of the box.
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2.3 Analysis of the problems
It has been proceeded by disassembling the device to make a complete check-up of the Plexiglass box status.
A careful analysis of the structure has revealed the causes of the failure during the attempts in depressuring operations. Several fractures were found at the bar-chamber interface. Particularly was noted the unsticking between these two parts originated at one end of the surface interface.
The failure of this solution with the addition of the aluminum bar has been mainly attributed to the incapacity of the adhesive to support the force transferred by the vacuum seal directly on the aluminum bar, that has led to the loss of adhesion and the occurrence of leaks .
From the analysis has been revealed also the inability to proceed with the solution of the system by tightening the screws threaded directly into the Plexiglass as a result of the appearing of circumferential cracks between the fillets of each hole. In Figure 2.3 and 2.4 are visible the breaking point of the adhesive and the cracks around the fillets.
FIGURES 2.3, 2.4: details of cracks around the threads and the point of de-cohesion between the bar and the Plexiglass.
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2.4 Study and design of the solution
In order to solve the problem, the study has been carried out starting by the analysis of the deformations of the chamber’s walls by creating a finite element model that would allow to make clear the role of deformations and especially, the field of total displacements of the external surfaces of the Plexiglass structure. A rough FEM model has been built with COMSOL Multiphysics® because of the impossibility to use multiples planes of symmetry and the low computing power of the computers at disposition; given the interest to know the maximum displacements of the chamber’s walls. This analysis has been sufficiently accurate for the intended purposes and has allowed to show how elevates were the displacement field in the lateral zones of the opening. In these areas more than 5[mm] displacements, have been estimated due to the inward flexion of the walls.
FIGURE 5: analisys of total displacements field by COMSOL Multiphisics®
Because of the impossibility to machine the structure of the chamber and being constricted to keep on using the same solution, it has been decided to apply an interface to be inserted by an adhesive and manufactured as an Aluminum plate.
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This plate, molded and shaped to have the same external dimensions of the wall and the opening of the box. It allows the reduction of the opening’s area by solving the problem of contact between the seal and the wall. Then it gives significant reduction of the displacements of the lateral walls especially for those parts around the opening. This solutions aims to let the forces transferred to the plate, as result of the deformations induced by the external pressure, being not exclusively borne by the adhesive.
The plate has been designed providing it an optimal shape to pass all around the internal perimeter of the opening. This profile with variable thickness has been designed to work as a support for the side walls of the opening during deformation by increasing the stiffness of the structure. The solution provide both a stiffening of the structure from the inside of the opening and a high reduction of the induced stress to the adhesive. This because the adhesive is applied not only on the frontal surface of the interface but even on the internal perimeter, filling the gap between the plate and the Plexiglass. This has been thought in order to ensure the continuity between of the structural elements to ensure the maintenance of the vacuum. Furthermore, in order to eliminate all possible leaks the use of screws to close the chamber has been eliminated. By this way the use of threaded holes to tight the screws into Plexiglass has been no more necessary. A different locking system has been decided to design and it will be shown and explained later.
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To study the plate by a finite element method, a CAD model, not exactly corresponding to actual one has been created trying to simulate the behavior of the structure as correspondently as possible. This because of the low calculus power of the hardware and because of the actual complexity in modeling the behavior of the adhesive at the interface. It has been preferred to analyze the problem by modeling two different models both concerning a structure made of just one element. This choice comes from the simplification of the analysis without introducing those non-linearity coming from modeling contacts between the surfaces as not fixed constraints.
2.4.1 Model I
The first model, has been thought for being precautionary. In it, 4 lateral cantilevered plates have been designed as a medium to transfer the forces, resulting from pressure, acting on the faces next to the opening of the chamber to the perimeter of the plate. This model simulates the effects of the external actions transfer as a result of deformation of the Plexiglass structure.
FIGURE 2.7: first COMSOL Multiphisics® FEM model of to analyze displacements of the sides of the plate induced by the pressure acting on the cantilever plats.
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In order to model a behavior more close to the reality considering the actual behavior of the structure while it deforms, it has been constrained with a two “double connecting rod”. These allow the slide but not the rotation respectively along the vertical direction for the horizontal plates and along the horizontal direction for vertical ones. Differently than in the real system, the bonded contacts between triangle plates and the main plate, have been modeled as fixed constraints. Even if this model doesn’t correspond exactly to the real one it has been one of the unique possibilities to obtain a simulation with a lower computational power.
With this model, two different simulations have been done for being more cautionary. In particular, the first simulation was conducted by applying a pressure of , on the surface of the triangular plates. In this way is simulated the effect deriving from the differential pressure, equal to the expected one, that acts on chambers walls and that is transferred to the plate’s edges. This has been done considering that only ¼ of the total load of each face is transferred to each side of the opening. The simulation results are shown in Figure 2.8 and Figure 2.9. These show the maximum equivalent stress conditions, according to Tresca, and a maximum total displacement everywhere sufficiently reduced if compared to original situation.; particularly in areas considered most flexible. These displacements, except for the 4 triangular plates, modeled by necessity, result everywhere lower than 0.3 [mm].
FIGURE 2.8, 2.9: Tresca equivalent stress rate (left) and total displacement field (right) applying a differential pressure.
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Contrary to the previous one, the second simulation was conducted by doubling the value of the differential pressure usually acting on the system. By introducing a differential pressure of , the objective is to analyze the problem in a more cautionary way. By this way, the contribution of the lateral walls to the stiffness is considered zero and the externals actions on the side walls are directly transferred only to the front opening and on the rear wall. Practically, the side walls have been considered not involved in supporting stresses.
The simulation as shown in the Figure 2.10 and Figure 2.11 has shown that the equivalent stress’s field has increased, reaching anywhere a value lower than 100[MPa] except in areas where stress is due the superposition effects of curving and simplified modeling of the joints with sharp edges. Obviously the deformation field and thus the total displacement is nearly doubled leading to a final maximum displacement lower than 0.5[mm].
FIGURE 2.10, 2.11: Tresca equivalent stress rate ( left ) and total displacement field (right) applying a differential pressure.
Commenting the results obtained knowing the approximations introduced in the model, the geometry of the plate was considered satisfactory; whereas considering that this model doesn’t consider the support in stiffness given by the Plexiglass structure. In the real system this, means lower effective total displacements; that in this case are due to a direct load along the plate.
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2.4.2 Model II
A second analysis and simulation has been done by creating a different model in which the previous geometry has been substituted with the only one of the plate, without lateral triangular cantilevered plates. This time, the load has been applied directly on the perimeter’s surfaces of the plate, assuming, with precautionary considerations, that the external pressure, was acting only on the first half of each adjacent face to the chamber’s opening. In this case the loads are totally and directly transferred to the main plate. Practically even here, as in the previous model but in a different way, the support of the lateral Plexiglass walls is not considered. Once calculated the resulting forces, given by the pressure acting on each half of the faces, these have been applied as remote forces on the perimeter’s surfaces of the main plate.
The simulation done is much more precautionary than the previous one. Continuing to ignore the contribution to stiffness given by the structure of Plexiglass, once simulated, has shown a maximum total displacement of the walls, in the same areas where previously considered, being next to 3.5[mm].
Although reduced, the field of totals displacements hasn’t been deemed satisfactory.
2.4.3 Final model
Basing on the consideration and results explained above, a further simulation has been carried out, after having proceeded to increase the thickness of the aluminum plate. Both the frontal part and the profile coming inside the opening of the Plexiglass chamber were doubled for being almost 4[mm].
Once updated the geometry, it has been simulated under the same loads and constraints conditions.
The new simulation has shown that the limits of structural strength of the plate where respected everywhere; except for those areas of stress concentration due to the curvatures combined with the effects induced by the necessary fixed constraints
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to simulate the model. The impossibility to use surfaces constraints has led the necessity to adopt those fixed, applied on edges. These have been chosen as small as possible, in order to be more corresponding to the reality of the displacement field. In according to Tresca’s criterion, the simulation has revealed structural tensions generally lower than 100[MPa] (Figure 2.12). The presence of areas with higher stresses is not significant thanks to the ductility effect of the material adopted that leads concentration’s effects induced by geometry being not critics.
The most important thing as shown in Figure 2.13 is the high reduction of maximum displacement in critical areas obtained choosing a higher value of thickness reaching anywhere below 0.95[mm].
FIGURE 2.12, 2.13: Tresca equivalent stress rate (left) and total displacement field (right) once increased the thickness.
Based on the results obtained with the last simulation, they have been considered satisfactory enough, because of the high level of precautions introduced choosing a load condition worse than reality. The plate has been quoted and the drawing passed for manufacturing [ANNEX 2.1].
The production has been demanded to a company in according with the institute considering manufacturing’s times that an internal production would have taken.
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2.5 Clamping system
To fix the profiled plate on the box,it has been conveniently chosen to use the same, two-component epoxy glue, "ARALDITE", for metals and plastics, used even previously because of its good adhesion strength and sufficient elasticity remaining once dried.
Because of the necessity to remove the locking system, with the screws, by replacing it with a non-invasive and sufficiently non-harmful system to fix the closing plate of to the chamber. It has been chosen a solution of rapid development, totally non-invasive for the Plexiglass’s structure and extremely cheap.
It has been chosen to tight the closing plate door with a latch toggle clamps of which an example is shown in Figure 2.14. The CAD model is shown in Figure 2.15.
FIGURE 2.14: examples of latch toggle clamps
The solution is to adopt a strip of aluminum, 30[mm] in height and 780 [mm] in length with a 2[mm] thickness. This strip, bended in three long pieces in succession 280[mm], 220[mm] and 280[mm], has to pass around the structure along the left, rear and right walls. At the extremities of the strip, threaded holes, are realized to fix the locking clamps. By adopting a very fine thread pitch, it is possible to tighten harder the locking clamps.
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The same things need to be done on the closing plate door in which the realization of the holes is designed to allow the anchor of the latch closures. In order to prevent the strip to be in direct contact with the walls of the box, it was also planned to use 1[mm] neoprene rectangles as high as the aluminum strip to avoid scratches or damages the surface and edges. These are glued inside the edges of the aluminum strip, at the central part of the 220[mm] section and the ends of the lateral section. By this way, interposing those neoprene’s pieces between the metallic strip and the Plexiglass is prevented direct contact and therefore the damage of the surfaces. A closing system, designed as shown in Figure 2.15, aims to support only the minimum force to ensure the structure to remain compact and maintain the internal vacuum. The task to keep the seal tighten between the plates surfaces is ensured and allowed by the internal vacuum.
FIGURE 2.15: complete CAD model of the closing system with latch toggle clamps fixed on the aluminum bended strip.
To avoid, during the lifting of the equipment, the friction of the seal to keep attached the front plate, it has been chosen to drill on the aluminum plate (Figure 2.16) four short blind holes correspondingly to the holes in the closing plate door, as visible in the final model of the plate in Figure 2.16. By this way is allowed to fix the closing plate door with screws, even with the purpose to work as a reference between the two plates.
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FIGURE 2.16: final CAD model of the aluminum plate to interface the vacuum chamber with frontal plate of the microrobotic platform designed.
2.6 Final assembly and conclusions
The machining of the aluminum plate outside of the center expected an amount cost around 1000€. The excessive costs, have led to be forced to manufacture it at the institute. The manufacturing has been managed by the technician of the institute that has delegated to a third parties the operation.
Once the plate has been delivered, after having cleaned accurately the surfaces of the Plexiglass opening’s chamber and the plate’s rear, the deposition of the adhesive has been done. The procedure of gluing consisted in pouring the whole glue’s components from theirs tubes in a clean plate, mixing them to obtain a mixture as homogenous as possible then deposing the glue on both the interface’s surface and then apply the plate on the vacuum chamber’s openings. The necessity to complete
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the operations to be done in just 4 minutes before the glue starts to dry increased the difficulties of the whole procedure. Once dried, it has been easy to operate a removal of the exceeding glue from the perimeter outside the chamber. Contrarily the excesses in the internal perimeter, being much more complicate to be removed, they have been not treated.
Tests in depressurizing have been carried out by inserting the microrobotic platform in the chamber and just approaching the frontal surface of the plate, now fixed to the Plexiglass’s chamber, to the seal on the rear of the frontal plate of the microrobotic platform. Having already connected the pipe between the pump and the chamber the aspiration has been started. Values of the internal pression and time have been taken during the tests in order to evaluate the efficiency of the system and are shown in Figure 2.17.
FIGURE 2.17: In it there is the comparison between the system left attached to the pump with no other types of closing system (blue dots) and the same test carried out by sealing the exhaust of the
pump once stopped (red dots).
Results obtained shown an excellent behavior of the system in maintaining the vacuum. By this way the solution proposed and carried out has demonstrated its efficacy. However it is clear how the leaks from the valves of the pumps are
0 100 200 300 400 500 600 700 800 900 1000 0 500 1000 1500 2000 2500 3000 Pr e ssu re [ m b ar ] Time [s]
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important. For this it will be necessary to insert an independent closing valve upstream the pump to be closed once reached the minimal level of vacuum.
No furthermore external closing system to keep the system tightly has been adopted to seal the system during the tests meaning effectively its non-necessity.
Clearly since firsts phases has been considered non-useful the choice to use such a structure. This one designed with this with a material of such low stiffness and strength won’t be capable to overcome an higher level of vacuum. In that case, it will be necessary to provide a new integral solution.
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Chapter 3
Magnification system for image
visualization and acquisition
3.1 Introduction
In order to make possible the manipulation’s observation that take place inside the vacuum chamber is necessary to adopt an optical magnification system. Such a system must, however, have some features that require a dedicated design because of the impossibility to buy an industrial system. This is essentially due to the high cost of purchasing an existing commercial solution, around 10,000€ and the budget’s limits.
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3.2 Essential requirements
The system must be integrated with a lighting and image acquisition system that allows visualization using a camera.
It is necessary that the focal distance, to focus the images in the working area is greater than the existing distance between the manipulation surface and the upper surface of the vacuum chamber; meaning 75[mm]
It is also important that the magnification system to be such powerful to make structures and elements, under manipulation, recognizable and easy to be handled.
Concerning the lighting system, between the different lighting methods possible is necessary, to adopt a direct illumination system to be coaxial with the vision system or with a specific incidence’s angle respect to the plane’s normal. This is necessary because the platform for supporting and handling of specimens is not transparent and the machined aluminum is highly reflective.
As an acquisition system for images the easiest solution is to adopt a CCD or CMOS camera to be connected directly to a computer and making immediately visible the images and the ongoing operations.
Because is obligatory to stay, with the magnification and vision system, outside the vacuum chamber, whose upper surface is about 75[mm] from the surface of manipulation, as illustrated by the drawing of Figure 3.1, the system needs to be designed and implemented with a long focal length, greater than that distance.
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FIGURE 3.11: scheme of a view in section of the robotized platform and upper chamber’s wall.
Finally, having an average size of the objects manipulated between 50 [μm] and 5 [μm] the need is to operate with a magnification system able to obtain real images sufficiently large compared to the size of the camera sensor. This means that the real enlarged images, arriving to the camera, have to occupy a consistent portion of the total available surface of the sensor. In fact, a 20X magnification, since the sensor size is about 4.8 x 3.6mm is sufficient to render an object of 50 [μm], in diameter, larger about 1[mm] thereby covering about 6% of the surface of the sensor.
3.3 Actual system
A magnification optical system was already present in the laboratory, but without the lighting system. It was designed as an afocal telescope with a modular structure characterized by four lenses between converging and diverging according to the diagram shown in Figure 3.2. Than a mirror has been adopted to allow an horizontal assembly, rather than vertical one, having to respect a certain optical distance between the lens, that finally have required a linear extension of approximately 700[mm].
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An afocal telescope, is a combination of lenses for which the focal points of two consecutive lenses go coinciding. By way magnification ratio’s calculus is only based on the knowledge of the focal lengths of each lens. Particularly in this case the pair of lenses 1-4 has the unique purpose to make parallel light’s rays and focus them at their focal plane. While it is only the central pair of lenses that generates the magnification.
Taking as reference the optical design in the Figure 3.2 the magnification of the lens’s system depends on the distance between lenses and their focal lengths.
Using the tangent of the angle depending on the ratio between object’s height, and focal length, is:
t obtaining
The image created after the third lens is virtual because the interposition of the third lens. This one positioned between the second lens center and its focus, at a distance from the second lens’s focal plane equal to its own focal length, has the effect to prevent light’s beam to be focused. By this way once light’s rays have crossed the third lens, they continue their way parallelized until the crossing of the 4th and final lens that focuses them at the fourth lens focal plane.
Therefore, being: t
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The final magnification being: ed ; becomes:
However, is not the actual final magnification since then another effect of multiplying acts. It is due to the conversion from CCD pixels to computer’s screen pixels that leads to a further multiplication factor around 50 times.
In fact, since the camera’s sensor is 752 x 582[pixel] of 4.8 x 3.6[mm] and the computer screen being about 1280 x 800[pixel] with an effective size of 19[inch] it results that being the diagonal screen size in pixels equal to:
Then in [mm] equal to:
The diagonal of a pixel is around
Being the diagonal of the sensor in [pixels] equal to:
That expressed in [mm] is:
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The diagonal of a sensor CCD pixel is:
That means an additional multiplying factor
Finally the system designed enables to visualize image with a total linear magnification around 1000 times original image in full screen visualization.
FIGURE 3.2: four lenses magnification system.
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FIGURE 3.4: SolidWorks® CAD model of the optical magnification system
The system, studied with the rules of geometrical optics of thin lenses has never obtained acceptable images. This has been due to the strong geometrical and chromatic aberrations of the lenses that make objects edges and profiles indistinguishable during the testing of the system.
FIGURE 3.5: images examples captured during the tests of the optical magnification system using a transmission lighting system.
The design of the solution to get the desired magnification by the use of a diverging lens with focal length of -25 [mm] has made the combination of lenses, designed according to the criteria of thin lenses, being not entirely appropriate since the bi-concave lens was very thick. This has clearly led to problems of chromatic and geometric aberration such evident as in Figure 3.5. The use of low cost and low quality lenses adopted has shown its limits for high precision imaging system
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3.4 Study of the new solution
As first hypothesis it has been decided to solve the problem by changing the previous system in order to use a diverging lens much thinner to reduce and possibly eliminate the problems of aberration. Thereby maintaining the same previous design of the optical system, has been evaluate how this one would have been changed by eliminating the thick diverging lens and replacing it with a lens characterized by a greater focal length. Because of the greater curvature’s radius ,this one would have had, it’s obvious the thickness would have been much smaller and much more corresponding to criteria of thin lenses geometric optics. Particularly the system would have been designed with no more than 3 lenses. Two of these converging with a focal lengths f = 100 and f = 200 while the third and last one diverging with a focal length f =- 100, would have increased the focus distance and so the magnification.
In figure 6 is shown the display of the study of the matter by the help of a specific software. To respect a 20X magnification this solution would have involved a total length next to 1000[mm], without considering the distance between the first lens and the second on the left. In fact, this dimension can be varied and depending on the circumstances and needs. Between these lenses the beam is in fact parallel as a result of focusing the object by placing it exactly at a distance equal to the focal length of the first lens.
FIGURE 3.6: optical simulation of the new 3 lenses magnification system with.
Such a solution is not effectively the most suitable one to be pursued considering the need to develop a solution that is really compact and easy to use.
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In fact, for such a system based on an architecture similar to the one previously realized, is not respected even the need for an easy managing, Being the requested length for this structure around 1000[mm]. This would have meant a complicated and not rapid handling for the assembly of the system and the realignment of the lenses that would have required times not so brief.
3.5 Final solution
3.5.1 Optical system
The second and final solution designed, is very simple and quick to set. This because the need of a small number of lenses and elements this is made of. Its overall size is therefore smaller than the solutions previously adopted and studied. Moreover, the very simple design makes the solution very versatile with the possibility to adopt different magnification’s ratios.
The basic idea is not to magnify directly the objects and the elements under manipulation, but to enlarge their image translated outside of the workspace, over the upper chamber surface, at a distance variable and dependent on the choice of lenses and their focal length. As comprehensible from the diagram shown in Figure 3.7 in the optical system designed, when the object is coincident to the first lens focal plane, light rays are parallelized between the two lenses and then go refocusing correspondently to the focal plane of the second lens F1 reforming the
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FIGURE 3.7: optical scheme of a two lenses magnification system.
Adopting two converging lenses with focal length f = 100 [mm] and placing the working surface correspondingly to the focal plane of the first lens, it is possible, in fact, to virtually translate the object, under observation, at the focal plane of the second lens.
Normally the effect of the distance between the two lenses d is to vary the effective magnification depending on the relative position. If the distance between the two lenses is less than the sum of focal lengths, then we have that the virtual image of the object is reduced. If contrarily that distance is higher, then we have a magnification. Finally if the distance is exactly equal to the sum of the two focal lengths, then no one magnification is generated, and the virtual image has the same size as that observed one. In the case the object is placed exactly at the first lens focal plane the size of the refocused image remains the same, once crossed the last lens. Not depending to the distance d. Referring to Figure 9, through this system the virtual image produced can be observed and magnified by an objective for optical microscope (4) as powerful as desired and then let be passed by the semi-reflective cube of the “Infinity Tube” (2), to the CCD sensor (3) which is located along an axis orthogonal to the lenses system one.
The advantages obtained through this system is allowing to magnify the image of the virtual object overcoming limits to access the workspace imposed by the presence of the vacuum chamber. It is in fact only necessary to position the objective at a distance equal to its focal length over the plane on which the system of lenses focus the virtual image; being this one the focal plane of the second lens.
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Taking advantage of the elements and the components available in the laboratory it has been decided to adopt a simple system with a lighting system coaxial to the magnification one. The "Infinity Tube" from Thorlabs® equipment, shown in Figure 3.8, which has a cube with a semi-reflective mirror has been used to let the light pass from the lamp to the object under analysis and then divert the beam coming back to 90° to the CCD sensor.
FIGURE 3.8: “Infinity Tube” example designed by Thorlabs.
The scheme of the designed solution is shown in here under (Figure 3.9). In it can be distinguished the various elements and their relative position.
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In Figure 3.10 is shown the field of view actually achievable. It has been measured by means of a micrometer calibration TEM grid in which the internal side of each square is about 60[μm]. By counting the number of grid squares visible is obtained that the field of view turns out to be about 300 x 230[μm]. Then performing the measurement of the actual images obtained on the screen the size is resulted around 160 x 120[mm]. This means an actual linear magnification of 500 times, that becomes at least 1000 times if displayed in full screen .
FIGURE 3.10: field of view achievable through the proposed optical system in which is visible the presence of a micromanipulator tip not focused .
3.5.2 Lighting system
For the lighting system, shown in the figure 9, has been built a prototype lamp using an high-power LED, of which an example is shown in Figure 11, of around 1[W] with an alimentation at 4.4[V]. It has been inserted and attached, by the rear surface of the Aluminum support, on the bottom of a plastic container for camera films. This one, had in fact the perfect diameter to be housed in the upper part of the "Infinity Tube".
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FIGURE 3.11: example of white light LED with Aluminum star support.
The light of the LED being highly diverging means that when out of the system of lenses, goes to focus at a point farther than the focal length of the last lens. Even if it makes it more difficult to understand where actually is the focus of the lens’s system, it anyway brings benefits, because of the otherwise excessive power of the LED. In fact the light being in theory focused under working plane, let the light’s spot be larger and the light intensity lower. This means the reduction of excessive reflections coming from the smooth metallic surface that cause the inefficiency of the vision system, unless the use of a special camera that can reduce exposure through a dedicated software. Despite this effect, has not been possible to reduce the light intensity to an optimal level that could have enabled the CCD, at disposition, to operate an accomplice to the impossibility to supply the LED with a voltage lower than 4 volts. The solution has been to operate by moving and fixing the LED-lamp in an higher position respect to previous insertion on the “Infinity Tube”. An higher placement in fact reduce the part of light’s beam that pass through the system; by this way it is even reduced the amount of reflected light.
3.5.3 Focus adjusting
In the system proposed all the elements are fixed. This in order to focus the working surface and then the manipulations it has been connected to the support structure by interposing between them two a micrometric translation platform with regulating screw. This visible at the top of the system (Figures 3.12) is directly adjustable from the top of the system and easily accessible.
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3.6 Conclusions
The adoption of lenses of the same quality and type of those adopted in the previous system (Figure 3.4) has not lead to optimal results (Figures 3.12). Firstly due to the problems coming from the low quality of the lenses as already said and shown. These although suitable for an optical system dedicated to a laser, are totally unsuitable for a system concepted for images rendering. Is most of all because the chromatic aberration problems that the quality of the image has not been sufficient to confirm and assure the validity to the system proposed.
However, the main purpose was not to have an efficient system but rather to find an affordable solution, easy to handle and transport as well as to disassemble and reassemble. The solution proposed and tested (Figure 3.13) is thought will allow these needs not being the main purpose of the project the study of an optical system highly performing it has been decided not to continue its development and optimization, as the results obtained have been considered sufficient.
To obtain an optimal viewing system capable of sharp images the proposed optical system should therefore be tested with lenses of a strongly higher quality.
FIGURES 3.12: images of Nylon spheres between 30 to 50[µm] taken during 100[mm] focal length optical system tests.
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FIGURE 3.13: actual designed and tested system for magnification and acquisition of manipulating operation images.
3.7 References
[3.1] A. Maurel, J-M. Malbec. Optique géométrique, 2002
[3.2] V. Greco. Ottica Geometrica, dispensations of the course, 2011 [3.3] M. V. Klein and T. E. Furtak, Optics. John Wiley, New York, 1986
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Chapter 4
Modifications of the actuator power
supply and control unit
4.1 System configuration
Every micro-robotic system for micro and nano-manipulation needs, naturally, a control system and a power supply for the whole compound of DC-motors and Piezo-actuators used for the end effector’s motion. Generally, when a system is designed to manipulate objects for the industry, it needs a complex electronic control system to assure a proper handling and movement in order to obtain an actuation as fast as possible. Considering this, electronics system should be designed taking in account
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all the dynamic effects of electric motors and the typical hysteresis of piezoelectric actuators. For this reason, in addition to the increase of the accuracy and repeatability, it is essential, to combine the system with sensors and closed loop control for position and speed.
For the system under development whose configuration is the one shown in Figure 4.1, the objective is to obtain a first compact and non-expensive prototype for laboratory’s occasional operations. The system is based on a direct visual control of the manipulation task by the operator. It manages the manipulation through the vision system implemented with suitable methods of magnification and vision. For this reason, the system was conceived having an "open loop" configuration without of any output signal’s conditioning to the actuators. It is the operator the missing link in the system to obtain a "closed loop".
FIGURE 4.1: Micro-robotic system configuration composed by 2 robotic arms , each one with 3 DC-motors and 3 Piezo-actuators each one for coarse and fine object’s manipulation respectively
The microrobotic platform for manipulations, as said above, is equipped with 6 DC-motors and 6 piezoelectric actuators in total. These are managed by the control and power supply unit shown in Figure 4.2.
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FIGURE 4.2: DC motors and piezo-actuators alimentation and control unit.
The control panel of Figure 4.2 can be detailed as follows:
1 potentiometer located on the upper-left side has the function to regulate the speed of the DC-motors are driven working as a speed controller Figure 4.2 (a).
The 6 buttons switch when held down, allow to close the electronic circuit to supply the power the DC-motors for a selected way between A and B; according to the different directions XYZ Figure 4.2 (a)
6 potentiometer are positioned in two columns, 3 elements each one, on the right hand side. These are intended to adjust the input voltage of the piezo actuators of the two arms of the robotic system in order to set the position Figure 4.2 (c)(d).
The centrally located four-way selector is designed to allow the selection between line A and B, although it has four ways, only two of these are active and exploited. When placed on A, the right arm’s DC-motors motion is enabled. When placed on B it supplies the DC-motors of the left hand side arm, farthest from the plate where the platform is anchored Figure 4.2 (b).
The control system for the piezoelectric actuators consisting of 6 potentiometers described above, is designed so that the output voltage’s adjusting knobs for the
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actuators along Z are supplied with a voltage up to 5[V] (Figure 4.3) [ANNEX 4.3]. Every output of these is connected to a PI (Physik Instruments) amplifier, that multiply the voltage to a maximum output of 750[V] permitting a maximal elongation about 100[μm]. Contrarily those along X and Y are not connected with an amplyfier and work directly providing an output voltage ranging from 0[V] to 104.4[V] (Figure 4.3) [ANNEX 4.3].
FIGURE 4.3: original piezo-actuator’s control circuit for Z piezo-actuator (on the right) and for X-Y piezo-actuators (on the left)
Operations subsequent the original implementation of the circuit, have required new measurements of voltages and currents for every individual control element to for the design of a new amplifier circuit. The following Table 4.1 is filled with the measurements of all the control’s elements of the circuit.
Elements Voltage [V] Current [A]
Speed Potentiometer from -1.8 to 2.9 0
Actuation Buttons -7.9 or 8 from to
Z Potentiometer from 0 to 1.9 from 0 to
X Y Potentiometer from 0 to 104.4 from to
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Even if the electric circuit’s plan [ANNEX 4.3] shows that the reference voltage input to the amplifier of the piezo actuators along Z is 5[V] from direct measurements has been noted a 60% lower value because additional resistances were inserted to reduced the maximum voltage, and were not shown in the circuit design.
Being a large difference in range of elongation between the piezo-actuators along X,Y equal to 10[μm] compared to the ones along Z about 100[μm], what has been supposed to be, is that the action was probably done to reduce the motion range in order to increase the accuracy of the potentiometer by an acceptable reduction of Z maximal extension. So the control scheme for Z piezo-actuator is the one in Figure 4.4 in which appears real voltages.
FIGURE 4.4: actual piezo-actuator’s control circuit for Z piezo-actuator
4.2 Objectives
In particular it was desired to obtain a system not precluding the use of manual a control of the power supply unit; rather a system that allows to choose the use a manual control or the computer-based one. This would give the possibility to choose
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from a wide range of control methods making even possible to interface the system with a joystick or any haptic interface much more suitable for handling.
The aim is to get a numerical control system with user interface for the piezoelectric actuators that will be discussed in the next chapter.
Being in fact the ultimate target a high precision control of the nano/micro-manipulation, has been decided to operate changes only to the circuit arranged to supply and control the piezo-electric actuators. This Because it is properly on the dimensional scales similar to the range of the piezo-electric actuators that occur all the micro-manipulation’s operations.
To make these actions possible, it has been necessary to modify the power supply and control unit to enable the coupling with a data acquisition card (DAQ). Being impossible with a common DAQ supply a voltage as high as the 104[V] needed to supply the X–Y piezo-actuators has been necessary to design an amplification circuit to amplify the output.
4.3 Strategies for the amplifier design
The first step consists in sectioning the circuit correspondently to the output of the 6 piezoelectric actuator’s potentiometer and inserting an equivalent number of switches as shown in Figure 4.5 and Figure 4.6. In this way, it will be possible to select one of the supply circuit between the potentiometer one and the computer-based one through the DAQ card in order to operate the subsequent manipulations. The following step is to make the connections between the cable from DAQ card and those deriving from the circuit.
The most important and significant phase is the design of the amplifier circuit for the X and Y piezo-actuators depicted in Figure 4.6. It is necessary to design and implement 4 independent amplifier circuits since it is essential to maintain the ways separated of each other. Once the amplifier circuit has been assembled it will
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be necessary to adjust the existing circuit to derive the reference voltages and power supply. It will be even necessary to operate the insertion of the switches and then the connection, via pin set, between the access ports of the operational amplifiers and the DAQ card output port.
FIGURE 4.5: operation to be done on the original Z piezo-actuator’s control circuit to provide the same power supply by DAQ card
FIGURE 4.6: operation to be done on the original X-Y piezo-actuator’s control circuit to provide the same power supply by DAQ card. To supply the same voltage is necessary an amplification system.
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4.4 Selection of the DAQ card
To achieve the goals it has been necessary the choice of a DAQ card suitable to provide an analog voltage output signal, to the amplifiers of piezo-actuators. This needed a resolution higher than the piezo-electric actuator system. Considering those ones along X and Y directions, they have a resolution around 0.1[μm] and an elongation range about 10[μm]. Corresponding to that displacement range the voltage range is about 0-104[V]. For this reason the minimum output’s accuracy , is.
In particular what was needed was the a DAQ card with a resolution equal or higher than 7 bits, meaning 128 resolution’s steps. Meaning that an 8 bit DAQ card would have been enough.
A large research has been conducted among the various solutions available that could have met our needs according with the maximum supply current values, the voltage limits and the number of output ports possessed; necessary to be higher than 6. In order to allow possible future actions to be performed for a control on remaining actuators and in accordance with the overall budget constraints AD-Link card PCI-6216-V-GL purchased. The detailed technical specifications are shown in [ANNEX 4.5], from which the main features have been extracted and listed in the Table 4.2 below.
AO Channels # Resolution [Bit] Voltage range [V] Current Output [A]
16 16 ±10 ±
