A Short Introduction to Robotics Applications AIRLab - Artificial Intelligence and Robotics Lab Politecnico di Milano http://www.airlab.elet.polimi.it/ February 2007
Introduction to EAP Focus: Emulate the biological muscle Most conventional mechanisms are driven by actuators requiring gears, bearings and other components. EAPs are plastic materials that change size and shape when given some voltage or current. EAPs behave very similarly to biological muscle and mimic their mechanism. EAPs acquired the moniker Artificial Muscle. Development of biologically inspired system(biomimetic) They are: lightweight, low power, inexpensive, resilient, damage tolerant, noiseless, agile. Emulating the muscles can be able various new manipulation capabilities. Muscle is multifunctional, i.e. in locomotion muscle often acts as an energy absorber, variable stiffness suspension element or position sensor
Skeletal muscle: biological linear Electro-Active actuator Molecular motion on the order of nm distances is converted into the macroscopic movements. Structural hierarchy of skeletal muscle Myofibrils are simply a string of sarcomeres: the functional unit of muscle contraction. Muscles also exhibit the property of scale invariance: their mechanism works equally efficiently at all sizes, which is why fundamentally the same muscle tissue powers both insects and elephants.
Part I EAP Classification
Smart Materials EAP Ionic EAP -Ionic polymer metal composites(ipmc) -Carbon nanotubes(cnt) -Ionic polymers Gels(IPG) Conductive polymers(cp) -Electrorheological Fluid Electronic EAP -Piezoelectric polymers -Electro-strictive polymers -Dielectric elastomer -Liquid crystal elastomer(lce) Ferroelectric Polymers Piezo Piezoelectric ceramics Piezolectric composites Other Shape memory metal and alloys Shape memory polymers Magneto and Electro-strictive materials Magneto and Electrorheological fluids
Electronic EAP Advantages Can operate in room condition for a long time Rapid response (msec level) Can hold strain under DC activation Induces relatively large actuation force Disadvantages Requires HV on the order of 150 MV/m (Ferroelectric 20 MV/m) Compromise between strain and stress Glass transition temperature is inadequate for low-temperature actuation task High temperature applications are limited by Curie temperature Mostly, monopolar actuation, independent of the voltage polarity
Ionic EAP Advantages Produce large bending displacements Requires low voltage Natural bi-directional actuation that depends on the voltage polarity Disadvantages Except for CPs and NTs, Ionic do not hold strain under DC voltage Slow Response (fraction of a second) Bending EAPs induce a relatively low actuation force Except for CPs, it is difficult to produce a consistent material (particularly IMPC) In aqueous system the materials sustain electrolysis over 1.23V Need for an electrolyte and encapsulation Low electromechanical coupling efficiency
How Dielectric EAP work The EAP basic architecture is made up of a film of an elastomer dielectric material that is coated on both sides with another expandable film of a conducting electrode. When voltage is applied to the two electrodes a Maxwell pressure is created upon the dielectric layer. The elastic dielectric polymer acts as an incompressible fluid which means that as the electrode pressure causes the dielectric film to become thinner, it expands in the planar directions. Electrical force is converted to mechanical actuation and motion.
How IPMC work Ionomeric polymer-metal composite is an EAP that bends in response to an electrical activation as a result of mobility of cations in the polymer network or negative ions on interconnected clusters. Electrostatic forces and mobile cation are responsible for the bending.
Part II Robotic Application
Significant EAP proprieties Stress (MPa) Strain (%) Drive voltage (V) Bandwidth (Hz) or Response rate (sec) Power density (W /cm 3 ) Efficiency (%) Lifetime (cycles) Density (g/cm 3 ) Operating Environment (Temperature, pressure, humidity, etc...)
Fields of application Mechanisms Robotics, Toys and Animatronics Human-machine Interfaces Planetary applications Medical applications Liquid and Gases Flow Control Control Weaving MEMS EM Polymer Sensor and Transducers
Robotics, Toys and Animatronics Figure: Flex2, robot using rolled DE EAP actuators(eckerle et.al 2000) Figure: Artificial face, mount on Albert Hubo(Korea Advanced Institute of Science and Technology (KAIST),Hanson Robotics)
Human-machine Interfaces Figure: Haptic glove 3D model Figure: Memica: Remote-Manipulator Forces, damping or resistance would be controlled electronically(jpl/caltech, Rutgers University, NASA JSC, Harbor-UCLA Medical Center)
Medical applications Figure: Catheter activation by an IPMC type bending EAP(Osaka National Research Institute) Figure: A photographic view of a human hand and skeleton as well as an emulated structure for which EAP actuators are being sought(graham Whiteley, Sheffield Hallam University, UK)
Part III Research problems
Research problems Developing and applying EAP materials and mechanisms involves interdisciplinary expertise in chemistry, materials science, electronics, computer science and others. It s possible to divide the problems in two group: Mechanism understanding EAP processing
Mechanism understanding Nonlinear electromechanical modeling Materials properties characterization Computational chemistry New materials synthesis
EAP processing Material fabrication techniques Shaping (films, sheet, fibers, etc.) Microlayering (ISAM, ink jet printing) Support processes and integration (conductive and protective coating, bonding, electroding, etc.) Miniaturization techniques.
Find more info http://www.airlab.elet.polimi.it/.../belluco http://eap.jpl.nasa.gov/ Yoseph Bar-Cohen Electroactive Polymer (EAP)Actuators as artificial muscle, reality, potential and challenges, SPIE PRESS belluco@elet.polimi.it