51 research outputs found
Workshop on "Control issues in the micro / nano - world".
International audienceDuring the last decade, the need of systems with micro/nanometers accuracy and fast dynamics has been growing rapidly. Such systems occur in applications including 1) micromanipulation of biological cells, 2) micrassembly of MEMS/MOEMS, 3) micro/nanosensors for environmental monitoring, 4) nanometer resolution imaging and metrology (AFM and SEM). The scale and requirement of such systems present a number of challenges to the control system design that will be addressed in this workshop. Working in the micro/nano-world involves displacements from nanometers to tens of microns. Because of this precision requirement, environmental conditions such as temperature, humidity, vibration, could generate noise and disturbance that are in the same range as the displacements of interest. The so-called smart materials, e.g., piezoceramics, magnetostrictive, shape memory, electroactive polymer, have been used for actuation or sensing in the micro/nano-world. They allow high resolution positioning as compared to hinges based systems. However, these materials exhibit hysteresis nonlinearity, and in the case of piezoelectric materials, drifts (called creep) in response to constant inputs In the case of oscillating micro/nano-structures (cantilever, tube), these nonlinearities and vibrations strongly decrease their performances. Many MEMS and NEMS applications involve gripping, feeding, or sorting, operations, where sensor feedback is necessary for their execution. Sensors that are readily available, e.g., interferometer, triangulation laser, and machine vision, are bulky and expensive. Sensors that are compact in size and convenient for packaging, e.g., strain gage, piezoceramic charge sensor, etc., have limited performance or robustness. To account for these difficulties, new control oriented techniques are emerging, such as[d the combination of two or more ‘packageable' sensors , the use of feedforward control technique which does not require sensors, and the use of robust controllers which account the sensor characteristics. The aim of this workshop is to provide a forum for specialists to present and overview the different approaches of control system design for the micro/nano-world and to initiate collaborations and joint projects
Vision Based Automatic Calibration of Microrobotic System
During the last decade, the advancement of microrobotics has provided a powerful tool for micromanipulation in various fields including living cell manipulation, MEMS/MOEMS assembly, and micro-/nanoscale material characterization. Several dexterous micromanipulation systems have been developed and demonstrated. Nowadays, the research on micromanipulation has shifted the scope from the conceptual system development to the industrial applications. Consequently, the future development of this field lies on the industrial applicability of systems that aims to convert the micromanipulation technique to the mass manufacturing process. In order to achieve this goal, the automatic microrobotic system, as the core in the process chain, plays a significant role.
This thesis focuses on the calibration procedure of the positioning control, which is one of the fundamental issues during the automatic microrobotic system development. A novel vision based procedure for three dimensional (3D) calibrations of micromanipulators is proposed. Two major issues in the proposed calibration approach - vision system calibration and manipulator kinematic calibration - are investigated in details in this thesis. For the stereo vision measurement system, the calibration principle and algorithm are presented. Additionally, the manipulator kinematic calibration is carried out in four steps: kinematic modeling, data acquisition, parameter estimation, and compensation implementation. The procedures are presented with two typical models: the matrix model and the polynomial model. Finally, verification and evaluation experiments are conducted on the microrobotic fiber characterization platform in the Micro- and Nano Systems Research Group (MST) at Tampere University of Technology.
The results demonstrate that the proposed calibration models are able to reduce the prediction error below 2.59 micrometers. With those models, the pose error, compensated by the feed-forward compensator, can be reduced to be smaller than 5 µm. The proposed approach also demonstrates the feasibility in calibrating the decoupled motions, by reducing the undesired movement from 28 µm to 8 µm (For 4800 µm desired movement)
Vision Based Automatic Calibration of Microrobotic System
During the last decade, the advancement of microrobotics has provided a powerful tool for micromanipulation in various fields including living cell manipulation, MEMS/MOEMS assembly, and micro-/nanoscale material characterization. Several dexterous micromanipulation systems have been developed and demonstrated. Nowadays, the research on micromanipulation has shifted the scope from the conceptual system development to the industrial applications. Consequently, the future development of this field lies on the industrial applicability of systems that aims to convert the micromanipulation technique to the mass manufacturing process. In order to achieve this goal, the automatic microrobotic system, as the core in the process chain, plays a significant role.
This thesis focuses on the calibration procedure of the positioning control, which is one of the fundamental issues during the automatic microrobotic system development. A novel vision based procedure for three dimensional (3D) calibrations of micromanipulators is proposed. Two major issues in the proposed calibration approach - vision system calibration and manipulator kinematic calibration - are investigated in details in this thesis. For the stereo vision measurement system, the calibration principle and algorithm are presented. Additionally, the manipulator kinematic calibration is carried out in four steps: kinematic modeling, data acquisition, parameter estimation, and compensation implementation. The procedures are presented with two typical models: the matrix model and the polynomial model. Finally, verification and evaluation experiments are conducted on the microrobotic fiber characterization platform in the Micro- and Nano Systems Research Group (MST) at Tampere University of Technology.
The results demonstrate that the proposed calibration models are able to reduce the prediction error below 2.59 micrometers. With those models, the pose error, compensated by the feed-forward compensator, can be reduced to be smaller than 5 µm. The proposed approach also demonstrates the feasibility in calibrating the decoupled motions, by reducing the undesired movement from 28 µm to 8 µm (For 4800 µm desired movement)
A steady state tip control strategy for long reach robots
The work presented in this thesis describes the development of a novel strategy for the steady state tip position control of a single link flexible robot arm. Control is based upon a master/slave relationship. Arm trajectory is defined by through 'master' positioning head which moves a laser through a programmed path. Tip position is detected by an optical system which produces an error signal proportional to the displacement of the tip from the demand laser spot position. The error signal and its derivative form inputs to the arm 'slave' controller so enabling direct tip control with simultaneous correction for arm bending. Trajectory definition is not model-based as it is defined optically through movement of the positioning head alone.
A critical investigation of vacuum tube and solid state sensing methods is undertaken leading to the development of a photodiode quadrant detector beam tracking system. The
effect of varying the incident light parameters on the beam tracker performance are examined from which the optimum illumination characteristics are determined.
Operational testing of the system on a dual-axis prototype robot using the purpose-built beam tracker has shown that successful steady state tip control can be achieved through
a PD based slave controller. Errors of less than 0.05 mm and settling times of 0.2 s are obtained. These results compare favourably with those for the model-based tip position
correction strategies where tracking errors of ± 0.6 mm are recorded
Chain of refined perception in self-optimizing assembly of micro-optical systems
Today, the assembly of laser systems requires a large share of manual
operations due to its complexity regarding the optimal alignment of optics.
Although the feasibility of automated alignment of laser optics has been
shown in research labs, the development effort for the automation of
assembly does not meet economic requirements – especially for low-volume
laser production. This paper presents a model-based and sensor-integrated
assembly execution approach for flexible assembly cells consisting of a
macro-positioner covering a large workspace and a compact micromanipulator
with camera attached to the positioner. In order to make full use of
available models from computer-aided design (CAD) and optical simulation, sensor systems at different
levels of accuracy are used for matching perceived information with model
data. This approach is named "chain of refined perception", and it allows for
automated planning of complex assembly tasks along all major phases of
assembly such as collision-free path planning, part feeding, and active and
passive alignment. The focus of the paper is put on the in-process
image-based metrology and information extraction used for identifying and
calibrating local coordinate systems as well as the exploitation of that
information for a part feeding process for micro-optics. Results will be
presented regarding the processes of automated calibration of the robot
camera as well as the local coordinate systems of part feeding area and
robot base
Pseudo-elastic Flexure-Hinges in Robots for Micro Assembly
The increasing tendency of products towards miniaturization makes the substitution of conventional hinges to flexure hinges necessary, since they can be manufactured almost arbitrarily small. On account of their multiple advantages like no backlash, no slip-stick-effects and no friction, their application is especially reasonable in high-precision robots for micro assembly.
Particular pseudo-elastic shape memory alloys offer themselves as material for flexure hinges. Since flexible joints gain their mobility exclusively via the elastic deformation of matter, the attainable angle of rotation is strongly limited when using conventional metallic materials with approximately 0.4% maximal elastic strain. Using pseudo-elastic materials, with up to 15% elastic strain, this serious disadvantage of flexure hinges can be avoided.
A further problem of flexible joints is their kinematic behavior since they do not behave exactly like conventional rotational joints. In order to examine the kinematics of the hinges an experimental set-up was developed whereby good compliance with theoretical computed values could be achieved. A three (+1) degree of freedom parallel robot with integrated flexure hinges is investigated showing its kinematic deviations to its rigid body model. The data of the kinematic model of the flexible joint can then be implemented into the control of this compliant mechanism in order to gain not only a higher repeatability but also a good absolute accuracy over the entire working space
Design, Development and Implementation of the Position Estimator Algorithm for Harmonic Motion on the XY Flexural Mechanism for High Precision Positioning
This article presents a novel concept of the position estimator algorithm for voice coil actuators used in precision scanning applications. Here, a voice coil motor was used as an actuator and a sensor using the position estimator algorithm, which was derived from an electro-mechanical model of a voice coil motor. According to the proposed algorithm, the position of coil relative to the fixed magnet position depends on the current drawn, voltage across coil and motor constant of the voice coil motor. This eliminates the use of a sensor that is an integral part of all feedback control systems. Proposed position estimator was experimentally validated for the voice coil actuator in integration with electro-mechanical modeling of the flexural mechanism. The experimental setup consisted of the flexural mechanism, voice coil actuator, current and voltage monitoring circuitry and its interfacing with PC via a dSPACE DS1104 R&D microcontroller board. Theoretical and experimental results revealed successful implementation of the proposed novel algorithm in the feedback control system with positioning resolution of less than ±5 microns at the scanning speed of more than 5 mm/s. Further, proportional-integral-derivative (PID) control strategy was implemented along with developed algorithm to minimize the error. The position determined by the position estimator algorithm has an accuracy of 99.4% for single direction motion with the experimentally observed position at those instantaneous states
Design, characterisation and testing of SU8 polymer based electrothermal microgrippers
Microassembly systems are designed to combine micro-component parts with high
accuracy. These micro-components are fabricated using different manufacturing
processes in sizes of several micrometers. This technology is essential to produce
miniaturised devices and equipment, especially those built from parts requiring different
fabrication procedures. The most important task in microassembly systems is the
manipulator, which should have the ability to handle and control micro-particles.
Different techniques have been developed to carry out this task depending on the
application, required accuracy, and cost. In this thesis, the most common methods are
identified and briefly presented, and some advantages and disadvantages are outlined.
A microgripper is the most important device utilized to handle micro-objects with
high accuracy. However, it is a device that can be used only in sequential microassembly
techniques. It has the potential to become the most important tool in the field of micro-robotics, research and development, and assembly of parts with custom requirements.
Different actuation mechanisms are employed to design microgrippers such as
electromagnetic force, electrostatic force, piezoelectric effect, and electrothermal
expansions. Also, different materials are used to fabricate these microgrippers, for
example metals, silicon, and polymers such as SU-8.
To investigate the limitation and disadvantages of the conventional SU-8
electrothermal based microgrippers, different devices designed and fabricated at IMT,
Romania, were studied. The results of these tests showed a small end-effector
displacement and short cycling on/off (lifetime). In addition, the actuator part of these
microgrippers was deformed after each operation, which results in reduced displacement
and inconsistent openings at off state every time it was operated in a power ON/OFF
cycle. One of these limitations was caused by the existence of conductors in arms of the
end-effectors. These conductor designs have two disadvantages: firstly, it raises
temperature in the arms and causing an expansion in the opposite direction of the desired
displacement. Secondly, since the conductors pass through the hinges, they should be
designed wide enough to reduce the conductor resistance as much as possible. Therefore,
the wider the hinges are, the higher the in-plane stiffness and the less out of plane
deflection. As a result, it increases the reaction force of the arm reducing the effect of
deformation. Based on these limitations a new actuatorstructure of L-shape was proposed to reduce
the effects of these drawbacks. This actuator has no conductor in the hinges or the arms
of the end-effectors which reduce limitation on the hinge width. . In addition, a further
development of this actuator was proposed to increase the stiffness of the actuator by
doubling its thickness compared with the other parts of the griper. The results of this
actuator proved the improvement in performance and reduction of the actuator
deformation.
This new actuator structure was used to design several different microgrippers with
large displacement and suitable for a wide range of applications. Demonstrations of the
capabilities of the microgrippers to be used in microassembly are presented.
In addition, a novel tri-directional microactuator is proposed in this thesis. This
actuator’s end-effector is capable of displacements in three different directions. This
actuator was used with the other designs to develop a novel three-arm (three fingers)
multidirectional microgripper.
To study the microgripper displacement as a function to the heater temperature, the
TCR of the conductor layer of each device was measured. Because different
configurations of conductor layers were studied, a significant effect of the metal layer
structure on TCR was discovered. The TCR value of gold film is reduced significantly
by adding the chromium layers below and about it which were used to improve the
adhesion between the gold film and the SU layers.
In this thesis, a new method based on a robotic system was developed to characterise
these microgrippers and to study the steady state, dynamic response, and reliability
(lifetime cycling on/off). An electronic interface was developed and integrated to the
robotic system to control and drive the microgrippers. This new system was necessary to
carry out automated testing of the microgrippers with accurate and reliable results.
Four different new groups of microgrippers were designed and studied. The first
group was indirectly actuated using an L-Shaped actuator and two different actuator
widths. The initial opening was 120 μm for both designs. The maximum displacement
was about 140 μm for both designs. However, the actuator in the wider heater width
showed more stable behavior during the cycling and the dynamic tests.
The second group was based on direct actuation approach using the L-Shaped
actuator. There were eight different designs based on this method with different heater
conductor shape, actuator width, and arm thickness. The initial opening was 100 μm and there were different displacements for the eight designs. The study of these microgrippers
proved that the actuator stiffness has a significant effect on the microgripper
displacement. In addition, the shape of the heater conductor has less effect. The largest
displacement achieved using this method of design was about 70 μm.
The third group was designed for dual mode operation and has three different designs.
The initial openings were 90 μm and 250 μm. The displacement was about 170 μm in
both modes. The last microgripper design was a tri-arm design for multi-mode operation.
The lifetime study of SU8 based microgrippers in this thesis was the first time such
an investigation was carried out. The results of IMT designs showed that the larger is the
displacement the less stable is the gripper design because of the high rection force acting
on the actuators. The L-shape based microgrippers had better performance and they did
not break after more than 400 cycles. In addition, the studies of static displacement and
dynamic response of different designed microgripper proved that better performance of
the proposed actuator can be obtained by using double thickness for the actuator as
compared to the arm thickness
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