SELF ASSEMBLY ROBOTS
Seminar Report Submitted by
Aman kukreja (Roll No. : M140157ME) In partial fulfillment of the requirements for the award of the degree o f
Master of Technology In Manufacturing Technology
DEPARTMENT OF MECHANICAL ENGINEERING
NATIONAL INSTITUTE OF TECHNOLOGY CALICUT CALICUT NIT CAMPUS PO, CALICUT KERALA, INDIA 673601
May 2015
CERTIFICATE This is to certify that the report entitled“ SELF ASSEMBLY ROBOTS” is a bonafide record of the Seminar presented by Aman kukreja( Roll Roll No .: M140157ME), in partial fulfillment of the requirements for the award of the degree of Master Master of Technology in Manufacturing Technology from National Institute of Technology Calicut.
Faculty-in-charge (ME6325 - Seminar) Dept. of Mechanical Engineering
Professor & Head Dept. of Mechanical Engineering Place : NIT Calicut Date : 15 may 2015
ABSTRACT
Self assembly robots have number of small modules(or robots) which which can stick or bond together to perform various funtions. In this report bonding methods between modules of self assembly robots are analysed . Tests are conducted and the strength of the bonds for each method are presented for different modu le styles, bonding bon ding conditions condit ions and breaking conditions in a destructive test, and arecompared with magnetic bonding methods. For 80micrometer modules, bond strengths of up to 500 mN areobserved with thermoplastic bonds, which indicates that theassemblies could be potentially used in high-force structuralapplications of programmable matter, microfluidic channelsor healthcare. And finally simulation was performed using small magnetic modules to show the assembly and d isassembly functions. functions.
CONTENTS List of Abbreviations and Tables
ii
List of Symbols
iii
List of Figures
iv
List of Tables 1
Introduction
1
1.1
Introduction
1
1.2
Problem Definition
1
1.3
Outline of the Report
2
2
The Evolution of Manufacturing Industry
3
3
Actuation and Heating Methods
7
3.1
Actuators
7
3.2
Heating methods
10
4
5
6
7
Experimental setup
14
4.1
Module design and fabrication
14
4.2
Bonding using heating
17
4.3
Motion actuation
19
4.4
Module addressing by magnetic disabling
19
Bond analysis
22
5.1
Bonding types
23
5.2
Bonding face styles
24
5.3
Bonding temperature
25
5.4
Assembly and bonding demonstration
26
Disabling for module addressability
29
6.1
30
Disabling for magnetic disassembly
31
Conclusions
32
References
i
LIST OF ABBREVIATIONS PCM
Photo chemical machining process
LIST OF TABLES Table 1
Evolution of self assembly robots
Table 2
Comparison between conventional and solid state refrigeration
Table 3
comparison of bonding forces
ii
LIST OF SYMBOLS ⃗
total magnetic torque;
⃗
total magnetic force;
⃗
total magnetic field;
⃗
magnetic moment;
µ0 ⃗
permeability of free surface; current through the coil;
iii
LIST OF FIGURES Fig. 1.1 Space molycubes Fig. 1.2 Kilobots Fig. 3.1 Magnetic actuation system Fig. 3.2 Electric actuators Fig. 3.3 Peltier element Fig. 3.4 LASER heating Fig. 3.5 Induction heating Fig. 4.1 Photolithography technique Fig. 4.2 Thermoplastic bonding modules Fig. 4.3 Solder bonding modules Fig. 4.4 Heating of modules using peltier element Fig. 4.5 Focussed laser heating Fig. 4.6 Magnetic coils Fig. 4.7 Magnetisation curve Fig. 5.1 Destructive test setup Fig. 5.2 Change in displacement force with time Fig. 5.3 Bond face style Fig. 5.4 Bonding temperature Fig. 5.5 Assembly demonstration Fig. 6.1 Sequence of self assembly Fig. 6.2 Experimental procedure Fig. 6.3 Magnetic disassembly
iv
CHAPTER 1 INTRODUCTION 1.1
INTRODUCTION
Modular self-assembly robotic systems or self-reconfigurable modular robots are autonomous
kinematic machines with
variable morphology. Beyond
conventional
actuation, sensing and control typically found in fixed-morphology robots, selfreconfiguring robots are also able to deliberately change their own shape by rearranging the connectivity of their parts, in order to adapt to new circumstances, perform new tasks, or recover from damage. For example, a robot made of such components could assume a worm-like shape to move through a narrow pipe, reassemble into something with spider-like legs to cross uneven terrain, then form a third arbitrary object (like a ball or wheel that can spin itself) to move quickly over a fairly flat terrain; it can also be used for making "fixed" objects, such as walls, shelters, or buildings.
Fig. 1.1
1.2
Fig. 1.2
PROBLEM DEFINITION AND CHALLENGES
Recent work in reconfigurable robotics has involved the scaling down of individual robotic modules for increased resolution and access to small spaces. This has brought with it several challenges in actuation, computation, and module bonding . Many largescale modular robotic systems use traditional actuators such as dc motors or shape memory alloys to power the assembly, reconfiguring, motion, and disassembly processes
1
using algorithms or motion primitives. However, as these systems are scaled down below the centimeter-scale, compromises must be made which reduce functionality, resulting in modules with less mobility. Since the early demonstrations of early modular self-reconfiguring systems, the size, robustness and performance has been continuously improving. In parallel, planning and control algorithms have been progressing to handle thousands of units. There are, however, several key steps that are necessary for these systems to realize their promise of adaptability, robustness and low cost. These steps can be broken down into challenges in the hardware design, in planning and control algorithms and in application. These challenges are often intertwined.
1.3
OUTLINE OF THE REPORT
Depending on the application area, desirable capabilities for micro-scale modular robotassemblies could include:
Small module size
Physical presence (large assembly size)
Addressable modules
Creation of arbitrary 2D/3D shapes
Mechanical strength
Electrical conductivity/continuity
Reconfiguration
Disassembly
Here in this report, small size robotic modules are made using photo chemical machining(PCM) process. When assembled together using actuation mechnism it can form large assembly. Each module is addresed separately i.e. we should be able to control motion of each module separately. Here it is controlled using magnetic field in different samples having different composition of magnetic particles. Assembly creates various 2D/3D shapes. Bonding of these small modules can be done using thermal bonding or magnetic attraction. Strength of the bonds are analysed using destructive test. And then assembly and disassembly is shown by performing simulation.
2
CHAPTER 2 THE EVOLUTION OF SELF ASSEMBLY ROBOTS The roots of the concept of modular self-reconfigurable robots can be traced back to the “quick change” end effector and automatic tool changers in computer numerical controlled machining centers in the 1970s. Here, special modules each with a common connection mechanism could be automatically swapped out on the end of a robotic arm. However, taking the basic concept of the common connection mechanism and applying it to the whole robot was introduced by Toshio Fukuda with the CEBOT (short for cellular robot) in the late 1980s. The early 1990s saw further development from Greg Chirikjian, Mark Yim, Joseph Michael, and Satoshi Murata. Chirikjian, Michael, and Murata developed lattice reconfiguration systems and Yim developed a chain based system. While these researchers started with from a mechanical engineering emphasis, designing and building modules then developing code to program them, the work of Daniela Rus and Wei-min Shen developed hardware but had a greater impact on the programming aspects. They started a trend towards provable or verifiable distributed algorithms for the control of large numbers of modules. One of the more interesting hardware platforms recently has been the MTRAN II and III systems developed by Satoshi Murata et al. This system is a hybrid chain and lattice system. It has the advantage of being able to achieve tasks more easily like chain systems, yet reconfigure like a lattice system. More recently new efforts in stochastic self-assembly have been pursued by Hod Lipson and Eric Klavins. A large effort atCarnegie Mellon University headed by Seth Goldstein and Todd Mowry has started looking at issues in developing millions of modules. Many tasks have been shown to be achievable, especially with chain reconfiguration modules. This demonstrates the versatility of these systems however, the other two advantages, robustness and low cost have not been demonstrated. In general the prototype systems developed in the labs have been fragile and expensive as would be expected during any initial development.
3
There is a growing number of research groups actively involved in modular robotics research. To date, about 30 systems have been designed and constructed, some of which are shown below. Table 1: Physical systems created System CEBOT Polypod
Metamorphic
Fracta Fractal Robots Tetrobot
3D Fracta
Molecule
CONRO
PolyBot
TeleCube Vertical Crystalline I-Cube Micro Unit
M-TRAN I
Class,
Author
DOF Mobile chain, 2, 3D lattice, 6, 2D lattice, 3 2D
Fukuda et al. (Tsukuba)
1988
Yim (Stanford)
1993
Chirikjian (Caltech)
1993
Murata (MEL)
1994
lattice, 3D Michael(UK) chain, 1 3D lattice, 6 3D lattice, 4 3D chain, 2 3D chain, 1 3D lattice, 6 3D
2D
1996
Murata et al. (MEL)
1998
Kotay & Rus (Dartmouth)
1998
Will & Shen (USC/ISI)
1998
Yim et al. (PARC)
1998
Suh et al., (PARC)
1998
Vona & Rus, (Dartmouth)
lattice, 3D Unsal, (CMU) lattice, 2 2D hybrid, 2 3D
1995
Hamline et al. (RPI)
lattice, 2D Hosakawa et al., (Riken) lattice, 4
Year
1998 1999 1999
Murata et al.(AIST)
1999
Murata et al.(AIST)
1999
4
Pneumatic Uni Rover
M-TRAN II
Atron
S-bot
Stochastic
Superbot
Y1 Modules
M-TRAN III
AMOEBA-I
Catom
Stochastic-3D
Molecubes
Prog. parts
Miche
GZ-I Modules
The Distributed Flight Array
Evolve
lattice, 2D Inou et al., (TiTech) mobile, 2 2D hybrid, 2 3D lattice, 1 3D mobile, 3 2D lattice, 0 3D hybrid, 3 3D chain, 1 3D hybrid, 2 3D Mobile, 7 3D lattice, 0 2D lattice, 0 3D hybrid, 1 3D lattice, 0 2D lattice, 0 3D
Hirose et al., (TiTech)
2002
Murata et al., (AIST)
2002
Stoy et al., (U.S Denmark)
2003
Mondada et al., (EPFL)
2003
White, Kopanski, Lipson (Cornell)
2004
Shen et al., (USC/ISI)
2004
Gonzalez-Gomez et al., (UAM)
2004
Kurokawa et al., (AIST)
2005
Liu JG et al., (SIA)
2005
Goldstein et al., (CMU)
2005
White, Zykov, Lipson (Cornell)
2005
Zykov, Mytilinaios, Lipson (Cornell)
2005
Klavins, (U. Washington)
2005
Rus et al., (MIT)
2006
chain, 1
Zhang & Gonzalez-Gomez (U. Hamburg,
3D
UAM)
lattice, 6 3D chain, 2 3D
2002
2006
Oung & D'Andrea (ETH Zurich)
2008
Chang Fanxi, Francis (NUS)
2008
5
Odin
EM-Cube
Roombots Programmable Matter by Folding Sambot
Moteins
ModRED
Hybrid, 3
Lyder et al., Modular Robotics Research Lab,
3D
(USD)
Lattice, 2 2D
An, (Dran Computer Science Lab)
Hybrid, 3
Sproewitz, Moeckel, Ijspeert, Biorobotics
3D
Laboratory, (EPFL)
2008
2009
Sheet, 3D Wood, Rus, Demaine et al., (Harvard & MIT) 2010
Hybrid, 3D Chain, 1 3D Chain, 4 3D
HY Li, HX Wei, TM Wang et al., (Beihang University)
Hybrid, 4, 3D
2010
Center for Bits and Atoms, (MIT)
2011
C-MANTIC Lab, (UNO/UNL)
2011
Programmable Smart Sheet Sheet, 3D An & Rus, (MIT) SMORES
2008
Davey, Kwok, Yim (UNSW, UPenn)
Symbrion
Hybrid, 3D EU Projects Symbrion and Replicator
ReBiS - Re-configurable
Chain, 1,
Rohan, Ajinkya, Sachin, S. Chiddarwar, K.
Bipedal Snake
3D
Bhurchandi (VNIT, Nagpur)
6
2011 2012 2013 2014
CHAPTER 3 ACTUATION AND HEATING METHODS 3.1 Actuators
An actuator is a type of motor that is responsible for moving or controlling a mechanism or system. There are several actuation methods available, but in the case of self assembly robots it can mainly be divided into two categories as: i) Actuation through external magnetic field ii) Actuation through electric signals
3.1.1
Actuation using magnetic field
A MEMS
magnetic
actuator is
a
device
that
uses
the microelectromechanical
systems (MEMS) to convert an electrical current into a mechanical output by employing the well-known Lorentz Force Equation or the theory of Magnetism . Motion is accomplished by rolling, direct pulling or by vibration based stick slip crawling. .
Fig. 3.1
7
3.1.1.1 Mathematic analysis of magnetic actuator
Case Magnetic modules can be controlled by the magnetic coilssurrounding the workspace, and also interact with eachother. The total magnetic torque and force thatgovern these interactions are:
(1)
(2)
The magnetic field and its spatial gradients depend linearlyon the currents through the coils, and so the fieldand gradient terms can be expressed as:
(3)
(4)
where each element of I is current through each of the ccoils, H is a 3 ×c matrix mapping these coil currents to themagnetic field vectorand H x , H y , H zare the 3 × cmatrices mapping the coil currents to the magnetic field
spatialgradients in the x, y and z directions, respectively. Thesemapping matrices are calculated for a given coil arrangementby treating the coils as magnetic
dipoles
in
space
andare
measurements.
8
calibrated
through
workspace
Thus, for a desired field and force on a single magneticmicro-robot we arrive atas:
(5)
In this way, we can achieve 5 DOF control of magnetic modules,enabling motion on 2D surfaces for assembly tasks as will be shown in the experimental
demonstrations.
Motion
is
accomplished
by
rolling,
directpulling or by vibration-based stick-slip crawling, methodswhich have been demonstrated previously. Feedbackcontrol of single or teams of microrobots moving in3D are not fully exploited in this work, but the feasibility ofsuch control has been shown in our previous studies.
3.1.2
Actuation using electricity
An electric actuator is powered by a motor that converts electrical energy into mechanical torque. The electrical energy is used to actuate equipment such as multi-turn valves. It is one of the cleanest and most readily available forms of actuator because it does not involve oil.
Fig. 3.2
9
3.2 Heating methods
A heating system is a mechanism for maintaining temperatures at an acceptable level; by using thermal energy. As you will see in the coming chapters, how are these method s are being used to form strong bonds with the neighboring modules. We can heat the small modules of assembly robots using three methods, which have their own advantages and disadvantages. These methods are: i) Heating by Peltier element ii) Heating by focused laser iii) Inductive heating by high power AC fields
3.2.1 Heating by Peltier element
The Peltier effect thermoelectric heat pump is a semiconductor based electronic component that functions as a heat pump. Just by applying a low DC voltage to this module, one surface gets cold and the other surface gets hot. And just by reversing the applied DC voltage, the heat moves to the other direction. Thus this thermoelectric device works as a heater or a cooler. The Peltier thermoelectric heat pumps have been used for medical devices sensor technology, cooling integrated circuits, automotive applications and military applications. 3.2.1.1 Principle and operation
The thermoelectric heat pump was discovered by a French watchmaker during the 19th century. It is described as a solid state method of heat transfer generated primarily through the use of dissimilar semiconductor material (P-type and N-type).
.
Fig. 3.3 10
Figure 3.3 shows a Peltier effect heat pump typical mechanical and electrical installation. Like conventional refrigeration, Peltier modules obey the laws of thermodynamics. Basically the refrigerant in both liquid and vapor form is replaced by two dissimilar conductors. The solid junction (evaporator surface) becomes cold through absorption of energy by the electrons as they pass from the low energy level to the high energy level. The compressor is replaced by a DC power source that pumps the electrons from one semiconductor to another one. A heat sink replaces the conventional condenser fins, discharging the accumulated heat from the system. The following table outlines the differences and similarities between the thermoelectric module and the conventional refrigerator.
Table 2 : Comparison between conventional and solid state refrigeration. The evaporator
Allows
the
pressurized At the cold junction, heat is
refrigerant to expand, boil and absorbed by the electron as evaporate (The heat is absorbed they pass from a low energy during the change of state from level (P-type) to a high energy liquid to gas). The compressor
Acts
on the
level (N-type) refrigerant
and The power supply provides the
recompresses the gas to liquid. energy to move the electrons. The
refrigerant
leaves
the
compressor as a vapor. The condenser
Expels the heat absorbed at the At the hot junction, heat is evaporator to the environment expelled to the heat sink as the plus the heat produced during electrons
move
from
high
compression into the ambient. energy level to the low energy Also, the refrigerant returns to level. theliquid phase.
11
3.2.2 Focused LASER:
n this process the laser is used to heat the surface of materials. Any subsurface heating is accomplished by conduction. For intensity values up to about 1x104 W/cm2, the absorbed power depends on the wavelength ( λ), the material (and its surface condition). Generally, as the material temperature increases, so does the absorption of laser light. The most relevant processing parameters are the laser power and the beam/material interaction time. In metals, local surface heating is very rapid and produces a thin hot layer on a relatively cool bulk material. This conducts heat away from the surface very quickly. Cooling rates of the order of several thousand degrees per second are possible, which can be used to advantage in producing microstructural changes, for example, in transformation hardening which uses IR lasers.
Fig. 3.4
3.2.3 Induction heating:
Induction heating is the process of heating an electrically conducting object (usually a metal) by electromagnetic induction, through heat generated in the object by eddy currents (also called Foucault currents).An induction heater consists of an electromagnet, and an electronic oscillator which passes a high-frequency alternating current (AC) through the electromagnet. The rapidly alternating magnetic fieldpenetrates the object, generating electric currents inside the conductor called eddy currents. The eddy currents flowing
through
the resistance of
the
material
heat
it
by Joule
heating.
In ferromagnetic (andferrimagnetic) materials like iron, heat may also be generated by
12
magnetic hysteresis losses. Thefrequency of current used depends on the object size, material type, coupling (between the work coil and the object to be heated) and the penetration depth.An important feature of the induction heating process is that the heat is generated inside the object itself, instead of by an external heat source via heat conduction. Thus objects can be very rapidly heated. In addition there need not be any external contact, which can be important where contamination is an issue.
Fig. 3.5
13
CHAPTER 4 EXPERIMENTAL SETUP
4.1Module design and fabrication: Modules
were
fabricated
by
using
PCM
technique, Photolithography,
also
termed optical lithography or UV lithography, is a process used in microfabrication to pattern parts of a thin film or the bulk of a substrate. It uses light to transfer a geometric pattern from a photomask to a light-sensitivechemical "photoresist", or simply "resist," on the substrate. A series of chemical treatments then either engraves the exposure pattern into, or enables deposition of a new material in the desired pattern upon, the material underneath the photo resist.
Fig. 4.1
This process is generally used for manufacturing of electronic circuits, about 0.0125 mm tolerance can be achieved with this process.
14
4.1.1Thermoplastic bonding modules:
Modules with bonding sites are fabricated in a multi-step molding process,First, a polyurethanemodule with embedded magnetic composite particles isfabricated in a batch process using a micro-molding techniqueusing SU-8 photolithography. They are composed of a mixture of neodymium-iron-boron(NdFeB) particles (Magnequench
MQP-15-7)
suspended
ina
polyurethane
matrix
(TC-892,
BJB
enterprises). The shapewas chosen as roughly hexagonal to allow tessellation ofmany modules in 2D. Curved edges are used to aid in alignmentof adjacent faces prior to μ m, bonding. The module hasvoids along its ed ges of approximate size several hundred
which will ultimately becomethe binding sites. As a low-temperature thermoplastic forbonding, commercially available Ethylene-vinyl acetatehot-melt adhesive is used. These adhesives are availablewith a variety of properties, including different meltingtemperatures. To fill the binding sites with thermoplasticmaterial in a controlled manner, the module is placed in asoft rubber jig designed to match the shape of the module without voids. The thermoplastic is thendeposited under high temperature (130 ◦C) to melt into thevoids, bubbles are removed by placing in a vacuum chamber,and the remaining material is scraped clean using a sharp edge. After cooling, the moduleis removed from the jig and is ready for bonding withother modules under low heat (approximately 70 ◦C). the whole process is shown in the figures below:
Fig. 4.2
15
4.1.2Solder bonding modules:
Modules with solder bonding sites are fabricated in a similarmanner to the thermoplastic bonding modules, with thethermoplastic replaced by low-melting point metal. Thisprocedure is shown in figure below. To permit the solder to wetthe the module, copper is sputtered onto the top and bottom polyurethane surface. Some copper is deposited into thebinding sites on the side of the modules during this sputtering.The solder is then applied manually to each bonding site using tweezers in an optical microscope at 80 ◦C. Anindium alloy (Field’s metal) is used as low melting temperaturesolder, with a sharp melting point of about 62 ◦C. Thismetal fuses well with itself at high temperature, when in air,water or oil.
Fig. 4.3
4.1.3Magnetic bonding modules:
Bonding by magnetic attraction alone is a simple me thodwhich follows naturally from the use of magneticallyactuatedmodules. This method is widely used in large-scalerobotics, was investigated for micro-scale robotic elementsin detail in our previous work, and has the major advantageof being an easily reversible bond. However, the majorlimitation is that the bonding strength is low, and it requiresspecial out-of-plane module geometry to resist contact slidingand rotation, as the magnetic force provides no interfaceshear strength. In addition, modules can only be bondedin a limited number of magnetically stable configurations.The most stable shapes are long straight chains, althoughsome other geometries can be assembled, albeit with alower bonding strength.
16
4.2Bonding using heating:
Module bonding occurs by heating to temperatures around50–90 ◦C, depending on the bonding material. As manypotential applications of micro-robots occur in liquid environments,all experiments are conducted in silicone oil orwater. One heating approach involves raising the temperatureof the entire liquid volume. This method has theadvantage of simplicity, and does not require precise knowledgeof the module locations. However, it melts all bondingsites in the workspace, and takes a longer time to heat andcool than other methods. It also required mechanical accessto the workspace area. An experimental setup has been createdto heat the workspace liquid by electrothermal heating.This setup, shown in Fig. 4.4 , is based around a well ofwater or oil on a glass slide, which forms the experimentalworkspace. Underneath the glass slide is glued a Peltierheating element, heated by a 1.0A power supply. In theexperiments in this paper, the water temperature is monitoredby a thermocouple, and is assumed constant overthe water volume due to the thin water layer and the highthermal conductivity of water when compared with the air above it.
Fig. 4.4
Once the desired bonding temperature is reached,the heating current is removed or reversed and the systemcools to room temperature, at which point adjacent modules
17
are bonded together. For a thin 2mm water layer, heatingfrom 30 ◦C to 70 ◦C with 1A takes approximately 12 secwhile cooling takes approximately 30 sec with no currentapplied. Larger volumes of liquid take a longer duration toheat and cool. Heating by electrical means is used as the primarymethod in this paper due to its simplicity and precisetemperature control capability. Laser heating is a second possible method which wouldachieve fast, localized bonding. This method could be integratedinto an experimental setup for targeted bonding ofonly a single glue site pair. As a proof-of-concept demonstrationof this method, a commercial CO2 laser (PinnacleV-Series Laser Engraver, 35W) is used to bond two modules, as shown in Fig. 4.5. Here the laser power is approximately 3W with a spot size of approximately 100 μm, andis traced across the module bond at a speed of 0.55 m/sfour times with a spacing of 0.5 s. This power providesenough heat to bond the modules without damaging thepolyurethane module base.
Fig. 4.5
As a third potential method for heating, inductive heatingby high-power AC fields could be used remotely withouta line-of-sight to the modules. While inductive heating ofmagnetic particles and particle suspensions have been studied, achieving the desired temperature rise of over 15 ◦Ccould be a practical challenge with this method, especially as the module size is reduced below hundreds of micronsize. Magnetic modules are fabricatedusing the same methods used to make the thermoplastic andsolder modules, but without the bonding sites. As the bondingstrength is dependent on the magnetic moment of themodules, a strong magnetic moment is necessary.
18
4.3 Motion actuation:
Magnetic modules are actuated by a set of independentelectromagnetic coils, aligned pointing towards a commoncenter point, with an open space of app roximately 10.4 cm. The coils are operated with an air or iron core, dependingon the desired magnetic fields and gradients. The maximumfields produced by the system driven at maximum current (19A each) are 6.6 kA/m using air cores, and 19.4 kA/m using iron cores. Similarly, maximum field spatial gradientsare 271 A/m2 using air cores, and 812 A/m2 usingiron cores. Fields are measured using a Hall effect sensor(Allegro A1321) with an error of about 80 A/m. Control ofthe currents driving the electromagnetic coils are performedby a PC with data acquisition system at a control bandwidthof 20 kHz, and the coils are powered by linear electronicamplifiers (SyRen 25). A photograph of the experimental coil system is shown in Fig. 4.6.
Fig. 4.6
Magnetic influences have already been explained in 3.1.1.1, using the same effect magnetic modules are moved to form different assemblies.
4.4 Module addressing by magnetic disabling:
Remotely and selectively turning on and off the magnetizationof individual modules could
allow
for
addressablecontrol
of
19
each
module.
We
have
developed
a
compositematerial whose net magnetic moment can be selectivelyturned on or off by application of a large magnetic fieldpulse. The material is made from a mixture ofmicronscale neodymium-iron-boron and ferrite particles,and can be formed into arbitrary actuator shapes using thesimple molding procedure discussed before. Themagnetic coercivity and remanence (retained magnetizationvalue when the applied field His reduced to zero)are distinct, which allows for moderately-large fields tore-magnetize the ferrite while maintaining the NdFeB magnetization.By applying a pulse in the desired directiongreater than the coercivity field ( Hc ) of the ferrite, its magnetizationdirection can be switched instantly. In addition,the magnetic states of both NdFeB and ferrite can be preservedwhen driving an actuator using small fields of lessthan 12 kA/m. In general it is difficult to demagnetize a single magnetby applying a single demagnetizing field because theslope of the hysteresis loop (i.e. the magnetic permeability)near the demagnetized state is very steep. Thus, such ademagnetization process must be very precise to accuratelydemagnetize a magnet. While steadily decreasing AC fieldscan be used to demagnetize amagnetic material, thismethoddoes not allow for addressable demagnetization becauseit will disable all magnets in the workspace.
This
motivatesthe
use
of
a
magnetic
composite
to
enable
untetheredaddressable magnetic disabling.
Fig. 4.7
We employ a demagnetization procedure to achieve amore precise demagnetization by employing two materials,both operating near saturation where the permeabilityis relatively low. In this method, an applied switching field H pulsecan be applied to switch only one material’s (ferrite)magnetization without affecting the second material(NdFeB).
20
As the two materials are mixed in one magneticmodule, this switching allows the device to be switchedbetween on and off states as the magnetic moments add inthe on state or cancel in the off state. While the internal fieldof the magnet at any point will not be zero, the net field outsidethe magnet will be nearly zero, resulting in negligiblenet magnetic actuation forces and torques. When fields are applied below the NdFeB coercivity, theNdFeB acts as a permanent magnet, biasing the device magnetization,as shown in Fig. 4.9 for H pulseup to about 240 kA/m. Traversing the hysteresis loop, the device begins inthe off state at point A, where motion actuation fields, indicatedby the 12 kA/m range, only magnetize the device toabout 0.08 μAm2, resulting in minimal motion actuation.To turn the device on, a 240 kA/m pulse is applied in theforward direction, bringing the device to point B. After thepulse, the device returns to point C, in the on state. Here,motion actuation fields vary the device moment betweenabout 1.7 and 1.8 μ Am2. To turn the device off, a pulse inthe backward d irection is applied, traversing point D, andreturning to the o ff state at point A at the conclusion of thepulse. For small motion actuation fields in the lateral direction,the device is expected to show even lower permeabilityin the on or off state due to the shape anisotropy inducedduring the molding process.Thus, modules can be magnetically disabled by a shortfield pulse of high strength. This will allow for multiplemodules to be added to an assembly at a time and sequentiallydisabled. As the process is reversible, the modules canbe re-enabled magnetically once the assembly process iscomplete, allowing for the entire assembly to be actuated.
21
CHAPTER 5 BOND ANALYSIS
Module bonding strength is measured in a destructive testwhich pulls two modules directly apart while measuring theloading force using a load cell, as shown in Fig. 5.1. Themodule pair is glued to a 3-axis motion stage with manuallinear motion. The ‘loading beam’ is placed on the loadcell and a fulcrum, and serves to reduce the force transmittedto the load cell, as shown schematically in the figureinset. The modules are first glued on a removable plastictip by instant adhesive. Then the tip is fixed on the motionstage and aligned manually using the camera such that themodules come into contact with the loading beam, which iscoated in a thin layer of adhesive. When the adhesive hascured, the motion stage is lowered at a constant rate untilthe bond between the modules breaks. The load cell datais acquired by amplifier/conditioner (TMO-2) and DAQ ata rate of 10 kHz. To get the breaking force, the differencebetween the maximum value during the breaking and thevalue of zero load after the break is calculated, as shown inFig. 5.2. Calibration is made by a 20 g proof mass placed atthe same position on the loading beam in a separate measurement.
Fig. 5.1
Fig. 5.2 22
The bond breaking phase is not instantaneous dueto the viscoelastic nature of the thermoplastic at elevatedtemperatures.
5.1 Bonding types
Representative thermoplastic, solder, and magnetic bondingmodule pairs are compared with respect to bonding force,with results shown in Table 3. Module ‘style’ refers to thesize, number and shape of the bonding sites, and can be referencedin Fig. 5.3. This table shows the mean and standarddeviation in force for five module pairs for each type oftest. Modules assembled by hand are pushed together usingtweezers in a microscope and then heated. Those assembledby “coils” are pushed together in the magnetic coil systemusing magnetic force on one module while the secondmodule remains in place. Assembly by hand using tweezersis also directly compared with assembly using magneticforces in the magnetic coil system, showing that while handassembly can result in larger bond force, the coil-assembledpairs maintain an adequate force. Bonding by laser wasonly performed for one module pair as a proof-of-conceptdemonstration due to the difficulty in aligning the laser inthe current setup. Results for solder bonding show moderatebond force, where it is noted that the failure mode is the delamination of the solder with the copper seed layerrather than failure of the bulk solder. As expected, magneticbonding is by far the weakest bonding style, but has theadvantage of easily reversible bonding. Table 3 Comparison of bonding forces
23
5.2 Bonding face style
For thermoplastic bonds, the bonding faces of each modulecontain voids for thermoplastic to reside. By varyingthe number, shape and size of these voids, different bondingstrengths are achieved, with results shown in Fig. 5.3for five samples of each face style. Six styles of modulewith the same outer diameter size of 800 μm but different bonding faces are designed and fabricated for direct comparison.Below the bar graph, the polyurethane module bodyis shown in white while the thermoplastic is shown in grey.To obtain a thin layer of thermoplastic in style ‘A’ with novoids, the 800 μ m module is placed into a jig with diameterof810 μm. All other module styles are prepared in a jigA three-dimensional
system
approach
is
presented in this paper as a decisionsupport framework for environmentally sustainable manufacturing. This system approach considers the three components of
manufacturing:
technology,of
size
800 μm. Results in the bar graph show the advantagesof
using
dedicated
thermoplastic voids over the ‘A’ style. This indicates that the design of bonding faces withvoids has improved the bonding
force
between
modulesfrom
Fig. 5.3
approximately 100mN to several hundred mN. It isseen that all void styles show adequate bond force, but thesingle-void of wide size but shallow depth (style ‘C’) showsthe highest bond force. It is expected that the wider bondingsite increases the bond strength as it should depend primarilyon the cross-sectional bond area, but the depth of style ‘D’prevents it from having high adhesion as the glue needs topenetrate deep into the recess to adhere fully. Thus, whileit is determined that any module style with voids resultsin acceptable bond force, further study could improve thebond force even further through optimized void style. It isthus desired to increase the cross-sectional bond area whilemaintaining full glue penetration.
24
5.3 Bonding temperature:
To determine the required temperature to form a securebond, the breaking force of thermoplastic module pairsbonded under different temperatures is investigated, withresults shown in Fig. 5.4a. Here heat is applied carefully toeach ‘C’style module pair using a heat gun in an air environment.The temperature at the bonding site is monitoredusing a thermocouple. After cooling to room temperature,each module pair is mounted in the destructive test, for breaking at 22 ◦C. Results show that a critical temperatureof about 55 ◦C is
necessary
Modulepairs bonding
to
form
indicated
force
a
bond.
with
achievement
0 no
binding atall, as indicated by them not being able to support their
Fig. 5.4 a, b
ownweight when lifted with tweezers. At temperatures above55 ◦C, high bonding force is relatively consistent (with onefailed bond at 65 ◦C). However, above about 80 ◦C thethermoplastic melts to such a degree that it flows from thebond sites. While this can allow for high bonding force, thegeometry of the module pair is distorted by the pool of thermoplasticwhich forms around the base of the modules. Theduration of heating was not observed to have an effect onbond strength. The strength of a thermoplastic bond broken at high temperature is investigated in Fig. 5.4 b, where identical modulesbonded at 70 ◦C are broken at varying temperatures. Hightemperature during the break test is achieved using a Peltierthermo-electric element, driven by 1A current, and monitoredwith a thermocouple glued to the Peltier elementface near the module pair. The plot shows that for temperatureslower than about 40 ◦C, the bond
force
is
highand
relatively
insensitive
25
to
temperature.
However,
at
highertemperatures the bond force is lower. The bond force neverreduces to zero due to the strong capillary and visco-elasticnature of the melted thermoplastic. The results from bothof these temperature tests are specific to the thermoplasticused.
5.4 Assembling and bonding demonstration:
A demonstration to show the bonding of 2D tessellating shapes is shown in Fig. 8. Here, six ‘F’ thermoplastic modulesare moved in the magnetic coil system for assemblyin Fig. 5.5. Modules are bonded by applying heat using theintegrated resistive heating setup. The insetin Fig. 5.5b shows the capillary drawing action which pullsadjacent modules into intimate contact during the reflowprocess. In this experiment, only one module is magnetic,allowing
for
the
step-by-step
addition of the non-magnetic modules.When the
assembly
is
moved
adjacent
to
a
newmodule, heat is applied to bond it to the assembly. Thisproves that subsequent bonding
Fig. 5.5
cycles can be performed inthe presence of existing bonds at other sites without negativeeffect. The single magnetic module is able to carrythe non-magnetic modules, although when the assemblyreaches a size of five modules, as in Fig. 5.5e, the mobilityis somewhat reduced, making it more difficult to preciselyadd more modules to the assembly in the desired configuration. However, the large assembly is still very mobile usingthe methods of stick-slip crawling or tumbling cartwheellikerolling motion. The bond force is high enough that theassembly is not broken by moving in the coil system undermagnetic torques and forces, even when the temperature iselevated to 70 ◦C.
26
CHAPTER 6 DISABLING FOR MODULE ADDRESSABILITY To add more magnetic modules to an assembly, each moduleis magnetically disabled after assembly, as shown schematicallyin Fig. 6.1. This process allows individual modulesto be added to the assembly one at a time into arbitrarylocations on the assembly without regard for the magneticattraction or repulsion associated with that assembly location.Compared with bonding by magnetic attraction, thisallows for a much wider range of assembly morphologiesto be made. New modules can be added
to
the
assemblyin
any
position or orientation. However, it is desirable tobond all modules with the same orientation so that they canbe disabled and enabled as a group using a single globalpulse. Heat
bonding
of
modules
is
accomplished by individualheating (i.e. by laser or inductive heating), or by globalheating (i.e. heat conduction
through
the
entire
medium).Once the assembly is
Fig. 6.1
completed, it is re-enabled magnetically and is free to move through the workspace as a singleunit. As further demonstration of the assembly plus heat activated bonding, a 2D ship-in-bottle morphology is createdin a microfluidic chip environment, as shown in Fig. 6.2.Creating a ship in a bottle requires individual modules topass through a small opening (the bottleneck) and assembleinto a ship shape one at a time. The simple 2D ship shapemade here consists of nine modules, and consists of a hull,mast and sail. Such a demonstration shows the capability ofthe presented addressability, bonding and control method toachieve the creation of arbitrary shapes in remote inaccessibleareas. The assembled
27
ship is enabled magnetically bya magnetic pulse for actuation as a rigid body. The strengthof the assembly is demonstrated by fast actuation after it is assembled, and is promising for future physical interactionswith the environment.
Fig. 6.2
6.1 Disabling for magnetic disassembly:
A final study is conducted using magnetic disabling to show that magnetic module bonds can be broken directly. Whilewe have demonstrated that magnetic bonds aremuch weakerthan the new thermally-activated bonds, they could be usefulfor reversible bonding capability. Our previous workshowed such magnetic bonds can be broken by anchoringand pulling modules apart. However, such a methodrequired electrostatic anchoring to apply pulling forces. Wenow introduce the concept of magnetic disabling to such magnetic bonds for reversible control without the needfor electrostatic anchoring. In this method, a magneticallystableassembly is created, with the restriction that allmagnetic modules must be assembled oriented in the samedirection. To disassemble the group, all modules are magneticallydisabled using a magnetic field pulse, in themanner of the previous section. Thus, the magnetic bondingforce reduces to near-zero and the assembly
28
is brokenapart by small magnetic actuation forces and torques. Atthis point, individual modules can be re-enabled for re-usein further assemblies. The process is thus completely repeatable.A demonstration of this type of disassembly is shownin Fig. 6.3. Assembly in this case was performed by handusing tweezers. Magnetic modules in this study are identicalto those used in the heat-assisted bonding study in thischapter, but without heat activation. This demonstrates thatthe principle could be used to create easily disassembledassemblies
for
reconfigurable
magnetic
micro-robotic
tasks.Further
investigations of the limits and usefulness of thistechnique could lead to a robust and simple disassemblymethod for magnetically-bonded assemblies.
Fig. 6.3
29
CHAPTER 7 CONCLUSIONS In this report, a comparison of severalbonding methods for micro-scale modular robotics is shown. As areversible bonding method, we investigated magnetic attractionbetween modules, and as a stronger but non-reversiblebonding method,heat-activated metal solder and
thermoplastic
bonding
methods
were
shown.We
showed
solder
and
thermoplasticbonds activated by increasing the temperaturethrough heating of the entire workspace, or as a proof-ofconceptdemonstration, through focused laser heating forfaster heating/cooling in environments which may not beconducive to large heating elements. As a potential methodfor use in applications which lack line-of-sight view, weproposed the use of inductive heating to perform bondingwith additional study required. The temperature increaserequired for bonding could be reduced through the use ofdifferent temperature sensitive materials with lower meltingpoints. Bond strength was tested for a variety of differentmodule styles for the thermoplastic bonds, and thetemperatures required for solid bonding and cooling wereinvestigated. As a demonstration, the magnetic
moduleswere
moved
in
a
magnetic
coil
system
as
addressable
microroboticagents to remotely form a target structure from upto nine independent modules in a fluid environment accessiblefrom only a small opening. Such an assembly methodcould potentially be used to form complex desired shapes ininaccessible small spaces for microfluidic manipulation orhealthcare applications. As an additional bonding technique,we also demonstrated reversible magnetic bonding using amagnetic disabling technique for easy bond reversal. Futurework will involve developing reversible adhesive bonds,and creating larger complex assemblies for tasks insidefluid channels. In add ition, the formation of out-of-plane3D assemblies will be investigated for applications suchas in-situ heterogeneous tissue scaffold construction. Suchapplications may require reduced scale using fabricationmethods such as flip-chip assembly and biocompatibilitythrough proper choice of materials and coatings.
30
