A SURVEY OF DIFFERENT ACTUATOR TECHNOLOGIES

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1 A SURVEY OF DIFFERENT ACTUATOR TECHNOLOGIES S. R. Kumbhar 1, S. S. Gawade 2 1 Department of Mechanical Engineering, M.E. Student, Rajarambapu Institute of Technology, Sakharale, Dist. Sangli, PIN , India. 2 Department of Mechanical Engineering, Professor, Rajarambapu Institute of Technology, Sakharale, Dist. Sangli, PIN , India. 1,2 Phone : , Fax: , 1,2 kumbharsr@rediffmail.com, sanjaykumar_gawade@ rediffmail.com. Abstract Actuator in its broadest definition is a device that produces linear or rotary motion from a source of power under the action of a source of control. These are the final elements in a control system. They receive a low power command signal and energy input to amplify the command signal as appropriate to produce the required output. Applications range from simple low power switches to high power hydraulic devices operating flaps and control surfaces on aircraft; valves; car steering, etc. The constant attempt to increase the productivity or efficiency of machines leads, in almost every field of mechanical and automotive engineering, to problems that cannot be solved by a mere adjustment of the construction parameters. Improvements of already-existing machines and vast innovations in new developments will be made possible by the use of electronics in combination with control techniques. The heart of such new applications is the actuator. Depending on the objectives, they control the energy center of the system. In order to achieve this, some promising actuator concepts along with their strong and weak points are discussed with regard to their industrial applications. This paper reviews different actuator technologies in terms of power/mass, torque/mass, and individual characteristics including the functions and working principles of each type of actuator. The objective of this paper is to give hints to help practically oriented engineers choose the appropriate actuators for their specific application. Keywords: Piezoelectric, Shape memory actuator, ER & MR fluid actuators, Magnetostrictive actuators 1.0 Introduction Actuators are applied in all field of technology. The mechanical state of a system can be defined in terms of the energy level it has at a given moment. One possible way of altering the mechanical state of a system is through an effective exchange of energy with its surroundings. This exchange of energy can be accomplished either by passive mechanisms, for example, the typical decaying energy mechanism through friction, or by active interaction with other systems. An actuator is a device that modifies the mechanical state of a system to which it is coupled. Actuators convert some form of input energy (typically electrical energy) into mechanical energy. The final goal of this exchange of energy may be either to effectively dissipate the net mechanical energy of the system, for example, like a decaying passive frictional mechanism, or to increase the energy level of the system [1]. Actuators provide the driving force and motion for a variety of natural and manmade requirements; typical examples are listed in Table No. I. In each case a mechanical action is activated in response to a control signal. Naturally occurring actuators include the muscles of animals and plants, and man-made actuators include hydraulics, pneumatics and solenoids. Other man-made actuators, such as piezoelectric, shape memory alloy and magnetostrictive devices, are based on shape-changing materials; these are used increasingly in novel applications. For example, piezoelectric actuators are used in precision positioning devices such as the reading heads in videocassette recorders and compact disc players. They have been proposed for active materials and structures, adjustable aerodynamic surfaces, vibration damping and noise cancellation. Shape memory alloys have found applications mainly where a single contracting stroke is required, such as in pipe couplings and orthodontic wires; cyclic applications include actuators in robot end effectors and satellite structure deployment. Magnetostrictive actuators have found relatively few applications; suggested uses include vibration isolation and active aerodynamic surfaces [8]. 1

2 Flight control surface Landing gear movement Nose wheel steering Air brakes Power doors/hatches Table No. I Applications of Actuators Aerospace Automotive Industrial equipment Braking Tappets Active suspension Active engine mounts Airbag deployment Automation equipment Numerically controlled machines Presses Lifting equipments Electrical goods Developing technologies Instrumentation Automatic switches / thermostats Video/Compact disc reading head Camera auto-focus Active control of structures Vibration suppression Active materials Surgical equipments Robotics Space structure deployment Atomic space microscope 2.0 Classification of Actuators Actuators can be classified according to a variety of criteria. Classification can be based on the physical laws governing their operation, on the application or on some other convenient distinction between them. There is no single method of classification general enough to include all types and therefore various classifications are used for various purposes. However, certain distinctions between classes of actuators can be useful [6,12]. 2.1 According to advanced technologies Traditional actuators (Conventional) These have been employed extensively during the last century in all application domains. The category of traditional actuators includes three main technologies, namely, Electromagnetic motors, pneumatic actuators and hydraulic actuators [1] Emerging actuators (Unconventional) These are driving technologies developed from novel (or when old, newly developed) transducer materials [1]. 2.2 According to input energy domain Thermo-mechanical actuators In this energy conversion process, the input energy is in the thermal domain and the output energy in the mechanical domain [14]. Several actuators can be developed by following this conversion scheme: (a) Shape memory alloy (SMA) actuators: In this type of actuators, the input thermal energy triggers a phase transition in the alloy, which results in the shape recovery of a previously deformed state. (b) Thermal actuators : In this type of actuators, the different thermal expansion coefficients of two thin metallic laminas cause a bending of the composite structure upon heating and cooling. (c) Thermally active polymer gels: Some polymer gel actuators respond to thermal stimuli. Thermal expansion actuators. It is well-known that temperature changes cause expansion contraction of all materials. Thermal expansion can be considered a direct thermo-mechanical transfer process Magneto-mechanical actuators These actuators establish an energy flow from the magnetic domain to the mechanical domain and vice versa. Again, several actuators can be developed, depending on various different transfer phenomena [1,3]: (a) Magnetostrictive actuators: Magnetostrictive actuators exhibit a reorientation of magnetic dipoles in the presence of an externally imposed magnetic field. Magnetic domain reorientation results in extension contraction in the dominant direction. (b) Magnetorheological fluid (MRF) actuators: MRFs exhibit changes in their rheological properties when subjected to external magnetic fields. The apparent viscosity of these materials is thus modified according to the magnetic field. They are semi active actuators: that is, they can only dissipate energy [4,5,7]. (c) Magnetic shape memory alloy (MSMA) actuators: In most instances, MSMAs are considered a subclass of magnetostrictive actuators. However, they exhibit very different actuator characteristics and are evolving into an independent new class of actuators. 2

3 2.2.3 Electromechanical actuator The energy in the input electrical domain is transformed into mechanical energy. In most of the following actuator technologies, the transfer process is reversible. Some of the technologies listed below are used concomitantly with the converse transfer process in what are known as smart actuators [15]. (a) Electromagnetic actuators: The Lorentz interaction between a flowing electrical charge and a magnetic field is exploited to supply either translational or rotational mechanical energy to the coil. The magnetic field can be established either by means of permanent magnets or by a second coil [11,13]. (b) Piezoelectric actuators: The converse piezoelectric effect resulting from the interaction of an imposed electric field and electrical dipoles in a material results in a deformation. This deformation is used to drive the plant. The converse piezoelectric effect can be used directly or through geometrical transducer concepts [12]. Shape memory alloy (SMA) actuators: These actuators have already been mentioned in connection with thermo-mechanical transduction. Thermal energy is usually supplied through resistive heating (Joule effect), and, hence, these can also be considered electromechanical transducers. In the context of smart actuators, a linear relationship between the electrical resistance and the displacement is used to establish a sensor model [13]. (c) Electro active polymer (EAP) actuators: Within the broad family of EAP actuators, dry type polymers directly exploit Maxwell forces or the electrostrictive phenomenon to obtain mechanical energy from electrical input energy. In addition, some ionic EAPs are triggered by small electric fields. (d) Electrorheological fluid (ERF) actuators: Like MRF actuators, the rheological properties of ERF actuators are altered when an electric field is applied. Again, these are semi active actuators and so can only dissipate the energy of the plant [5, 7] Fluid-mechanical actuators Some traditional actuators (pneumatic and hydraulic actuators) convert the pressure of a fluid into mechanical energy, either rotational or translational. 2.3 According to flow of energy between an input domain and the output domain (a) Active actuators: These actuators can either increase or decrease the energy level of the controlled system. The work exchange can only be negative [9,10]. (b) Semi-active actuators: These actuators can only dissipate energy as a consequence of mechanical interaction with the controlled system. The work exchange can take any positive or negative value [10]. 2.4 According to based on transducing materials Soft actuators (Pulling actuators) These actuators are based on transducing materials configured in thin sheets or wires so that they can only withstand traction forces. Soft actuators are inherently unidirectional actuators but can be configured in antagonistic pairs to provide two-way actuation [9] Hard actuators (Push- pull actuators) These actuators have the ability to sustain both traction and compression forces: Hard actuators are inherently two-directional actuators. 3.0 Selection of Actuators Given the wide variety of existing applications and actuators, some means of matching the requirements of an application to the performance characteristics of an actuator is desirable. The mechanical requirements of an application can be expressed in terms of force, displacement, stiffness, size, mass, response time (or operating frequency), power, efficiency and resolution. These must be matched to the performance characteristics of an actuator in order to determine whether the actuator can give the performance required for the application. Requirements such as cost, durability, maintenance, and environmental impact are less precisely defined and are not considered here. This article provides an overview of the range of actuation systems, gives a quantitative comparison of their performances, and presents examples of a systematic selection procedure for actuators. The mechanical performances of man-made actuators are compared with the corresponding performances of naturally occurring systems; this comparison is relevant to the design of prosthetic devices, where it is necessary to match the performance of a man-made system with that of a natural system [8]. 3

4 4.0 Performance Characteristics of Actuators The maximum actuation stress, σ max, and maximum actuation strain, are basic characteristics of an actuator. For a given size of actuator they limit the force and displacement. Alternatively, given the design values for the required forces and displacements, the size and shape of a suitable actuator may be estimated. The stress versus strain (σ - ) characteristic of an actuator is not a single curve; it is a family of curves, which depend on the control signal and the external constraints. The product σ max. max is an estimate of the maximum work per unit volume in a single stroke. More precisely, a dimensionless stroke work coefficient Cs can be defined as the ratio of the maximum work done in a single stroke to the product of σ max. max [8]. 4.1 Actuator property charts A systematic procedure for the selection of materials in engineering design, employing performance indices and material property charts, has been demonstrated by Ashby and co-workers (Ashby 1989, 1992; Ashby & Cebon 1993). Database of actuator characteristics is summarized in Table No. II. Actuator Type Table II Approximate ranges for the characteristics of Actuators Actuation strain max. [-] Actuation Stress σ max (MPa) Modulus E (GPa) Low strain piezoelectric 5 x x High strain piezoelectric 5 x x Piezoelectric polymer 2 x x Thermal expansion (10 K) 9 x x Thermal expansion (100 K) 9 x x Magnetostrictive 6 x x Shape memory alloy 7 x x Moving coil transducer 1 x x x x x x 10-3 Solenoid 1 x x x x x x 10-3 Muscle 3 x x x x 10-2 Pneumatic 1 x x x x 10-4 Hydraulic 1 x x Actuator Type frequency f max (s -1 ) power density p max. (W m -3 ) Density ρ (kg m -3 ) Low strain piezoelectric 5 x x x x High strain piezoelectric 5 x x x x Piezoelectric polymer 1 x x x Thermal expansion (10 K) 4 x x x Thermal expansion(100 K) 4 x x x Magnetostrictive 3 x x x Shape memory alloy 2 x x x x Moving coil transducer 2 x x x x Solenoid 5 x x x x Muscle 3 x x x Pneumatic 5 x x x Hydraulic 5 x x x Low strain piezoelectric > High strain piezoelectric Piezoelectric polymer Thermal expansion (10 K) Thermal expansion (100 K) Magnetostrictive Shape memory alloy Moving coil transducer Solenoid Muscle Pneumatic Hydraulic

5 When the characteristics of actuators are displayed on property charts, certain relationships between the different classes of actuators become evident [8]. Consider, for example, a chart which displays the feasible combinations of actuation stress, σ, and actuation strain,, as shown in graph no. I. Graph No. I Actuation strain v/s Actuation stress The values of σ and range over several decades, so the axes of the chart are logarithmic. Heavy lines show the locus of the values of maximum actuation stress versus actuation strain for each class of actuator. At low values of actuation strain, this locus follows the highest value of σ max within the class. In some classes of actuator (shape memory alloys are an example) the highest values of σ max correspond to smaller values of and there is a boundary of approximately constant product which is finally cut off by the highest value of σ max in the class. Consequently, the heavy lines in graph no. I marks the upper right hand corner of the envelope of performance of each class of actuator. Actuators which give significant displacement per unit length lie towards the right of graph no. I; they are naturally suited to applications where high stroke is required, as in the moving parts of plants, animals and machines. The actuators towards the top of graph no. I are suited to high force applications: hydraulic rams are used as presses in deformation processing, and shape memory alloy wires are used to press teeth into place and to seal vacuum pipe-work. Presenting this information on logarithmic scales allows more to be shown. A straight line of slope 1 in graph no. I links points of constant (σ - ) product. Now, the stroke work available per unit volume has the form and the values of Cs vary by less than a factor of four. Consequently, lines of slope 1 link classes of actuators with approximately the same volumetric stroke work. The sloping boundary of performance of classes of actuators such as shape memory alloys can be interpreted as a limitation on the available volumetric stroke work from that class. Shape memory alloy actuators operating at high values of actuation strain achieve a reduced actuation stress because there is a constant quantity of energy per unit volume available from the martensitic transformation which drives the actuator. From actuators property chart, one can draw the graphs for various actuators like : Specific actuation stress, σ / ρ, versus actuation strain, Strain resolution min versus actuation strain Volumetric power, p, versus frequency, f Volumetric power, p, versus efficiency,η Stress-strain product (σ - ) versus frequency f 5

6 With the help of actuator property charts and various graphs one could select the actuator accordingly [8]. Also one can take the help of various properties of actuators given in the Table I IV. Table III Rating of different actuator concepts Actuator Method & Device Electromagnetic (Theoretical) Electrostatic (Theoretical) Thermochemical Micro Valve Phase change Micro valve Piezoelectric Micro valve Piezoelectric Meander line actuator Shape Memory Micro valve Voltage Work per Unit Vol. Typical Response Temperature Sensitive? Power Use 100 volts 0.9 J/cm 3 << 1 ms No Very Low 100 volts 0.4 J/cm 3 << 1 ms No Very Low 12 volts 0.02 J/cm ms Yes Medium 15 volts 4 J/cm ms Yes Medium 90 volts 0.02 J/cm 3 < 20 ms No Low 2 volts 0.01 J/cm 3 < 1 ms No Low 5 volts 6 J/cm 3 30 ms Yes Medium Actuator Type Table IV Rating of different actuator concepts Strain Pressure Efficiency Relative Speed (Full Cycle) Power Density Shape Memory Alloy (TiNi) > 5 >> 200 < 10 Slow Very High Electromagnetic (Voice Coil) > 90 Fast High Piezoelectric Ceramic (PZT) Single Crystal (PZN-PT) Polymer (PVDF) Electrostatic Devices (Integrated force array) > 90 > 90 n/a Fast Fast Fast High > 90 Fast Low Shape Memory Polymer < 10 Slow Medium Thermal (expansion) 1 78 < 10 Slow Medium Magnetostrictive (Terfenol-D, Etrema Products) Fast Very High 5.0 Conclusion Considerable improvement of machine can be expected in the near future through the use of electronics in combination with control techniques. A key position will be held by the actuator systems that finally have to accomplish the controlled energy transfer to the objectives. To realize the regulating actions multiple physical effects can be employed. This paper will give guideline for selecting an actuator for particular application. References 1. CONSTANTINOS MAVROIDS, CHARLES PFEIFFER AND MICHAL MOSLEY, Conventional Actuators, Shape Memory Alloys, and Electrorheological Fluids, Dept. of Mech. & Aerospace Engg., Rutgers University, The State University of New Jersey. 2. D. ELATA, 2005, On the Static and Dynamic Response of Electrostatic Actuators, Bulletin of The Polish Academy of Sciences, Technical Sciences, Vol. 53, No G. MAGNAC, P. MENEROUD, M.F. SIX, G. PATIENT, R. LELETTY, F. CLAEYSSEN, 2006, Characterization of Magneto-Rheological Fluids for Actuators Applications, Pro. of ACTUATOR 2006, 10 th International Conference on New Actuators, June, pp HENRI GAVIN, JESSE HOAGG AND MARK DOBOSSY, 2001, Optimal Design Of MR Dampers, Pro. of U.S. - Japan Workshop on Smart Structures for Improved Seismic Performance in Urban Regions, 6

7 Seattle WA, ed. K. Kawashima, B.F. Spencer, and Y. Suzuki, 14 August pp ROLF ISERMANN, 2000, Mechatronic Systems Fundamentals, Springer, pp H. BOSE & A. TRENDLER, 2003, Smart Fluids Properties & Benefits for New Electromechanical Devices, Pro. of AMAS Workshop on Smart Materials & Structures, SMART 03, Jadwisin, September 2 5, pp J. L. PONS, 1987, Emerging Actuator Technologies: A Micromechatronic Approach, John Wiley & Sons, Ltd. 7. J. DAVID CARLSON, 2001, What Makes A Good MR Fluid?, Pro. of 8th International Conference on Electrorheological (ER) Fluids and Magneto-Rheological (MR) Suspensions, July 9-13, 2001, Nice, USA. 8. J. E. HUBER, N. A. FLECK & M. F. ASHBY, 1997, The Selection of Mechanical Actuators Based on Performance Indices, Proc. R. Soc. Lond, A 453,. pp JÖRG PASCHEDAG, GUIDO KOCH, 2006, Comparison Of Different Actuator Configurations For Active Isolation Of Car Engine Induced Vibration Regarding Power Consumption, Pro. of ICSV13 Vienna, 13 th International Congress on Sound & Vibration, July 2-6, 2006., Vienna, Austria. 10. K. YK, J.K. HEDRICK, 1989, Active & Semi-active Heavy Truck Suspensions to Reduce Pavement Damage, SAE Technical Paper Series, Paper No pp N. LHERMET, F. CLAEYSSEN, H. FABBRO, 2004, Electro-Fluidic Components Based On Smart Materials For Aircraft Electro-Hydraulic Actuators, Pro. of ACTUATOR 2004, 9th International Conference on New Actuators, June 2004, Bremen, Germany. pp ROGER G. GILBERTSON AND JOHN D. BUSCH, 1996, A Survey Of Micro-Actuator Technologies For Future Spacecraft Missions, The Journal of The British Interplanetary Society, Vol. 49, pp SMARANDA NITU, CONSTANTIN NITU, GHEORGHE TULUCA, GHEORGHE DUMITRESCU, 2006, Electromagnetic Actuator with Magnetic Stored Energy, Journal of Materials Processing Technology, 181. pp SIYOUL JANG, JOHN A. TICHY, 1997, Internal Damper Characteristics of Rotor System with Submerged ER Fluid Journal Bearing, International Journal of Rotating Machinery, 1997, Vol. 3. pp V.A.W. HILLIER, 1987, Fundamentals of Automotive Electronics, Hutchinson Education, Longwood Publishing Group, New Hampshire.pp