Control of Shape Memory Alloy Actuators with a Neuro-fuzzy Feedforward Model Element

This paper describes development of a motion controller for Shape Memory Alloy (SMA) actuators using a dynamic model generated by a neuro-fuzzy inference system. Using SMA actuators, it would be possible to design miniature mechanisms for a variety of applications including miniature robots for micro manufacturing. Today SMA is used for valves, latches, and locks, which are automatically activated by heat. However it has not been used as a motion control device due to difficulty in the treatment of its highly nonlinear strain-stress hysteresis characteristic. In this paper, a dynamic model of a SMA actuator is developed using ANFIS, a neuro-fuzzy inference system provided in MATLAB environment. Using neuro-fuzzy logic, the system identification of the dynamic system is performed by observing the change of state variables (displacement and velocity) responding to a known input (input voltage to the current amplifier for the SMA actuator). Then, using the dynamic model, the estimated input voltage required to follow a desired trajectory is calculated in an open-loop manner. The actual input voltage supplied to the current amplifier is the sum of this open-loop input voltage and an input voltage calculated from an ordinary PD control scheme. This neuro-fuzzy logic-based control scheme is a very generalized scheme that can be used for a variety of SMA actuators. Experimental results are provided to demonstrate the potential for this type of controller to control the motion of the SMA actuator.     

Architecture design of a multiaxis cellular actuator array using segmented binary control of shape memory alloy

A new approach to artificial muscle actuator design is presented, and is implemented using shape memory alloys (SMA). An array of SMA actuators is segmented into many independently controlled, spatially discrete volumes, each contributing a small displacement to create a large motion. The segmented cellular architecture of SMA wires is extended to a multiaxis actuator array by arranging the segments in a two-dimensional (2-D) array. The multiaxis control is streamlined and coordinated using a 2-D segmentation method in order to activate multiple links of a robot mechanism in a coordinated manner. The basic principle of segmented binary control (SBC) is first presented, followed by multiaxis segmentation theory and a design procedure. The method is applied to a five-fingered robotic hand capable of taking a variety of postures. A 10-axis SMA actuator array is built, and SBC is implemented using Peltier-effect thermoelectric devices for selective local heating and cooling. Experiments demonstrate the feasibility and effectiveness of the new method.     

A new method for actuating parallel manipulators

This paper presents a new technique of actuating a parallel platform manipulator using shape memory alloy (SMA). This is a type of smart materials that can attain a high strength-to-weight ratio, which makes them ideal for miniature application. The work is mainly to develop a new SMA actuator and then incorporating the actuator in building the parallel manipulator prototype. The SMA used in this study is a commercial NiTi wire. The SMA wire provides an actuating force that produces a large bending and end displacement. A 3-UPU (universal–prismatic–universal) parallel manipulator using linear SMA actuators was developed. The manipulator consists of a fixed platform, a moving platform and three SMA actuators. The manipulator workspace was specified based on the restrictions due to actuator strokes and joint angle limits. System identification techniques were used to model both heating and cooling processes. An ON/OFF control was performed and the results showed closeness in simulation and experimental results. This study showed that shape memory alloy actuated beam can successfully be used to provide linear displacement. The built prototype indicates the feasibility of using SMA actuators in parallel manipulators.     

Simultaneous On-Chip Sensing and Actuation Using the Thermomechanical In-Plane Microactuator

Many applications in microelectromechanical systems require physical actuation for implementation or operation. On-chip sensors would allow control of these actuators. This paper presents experimental evidence showing that a certain class of thermal actuators can be used simultaneously as an actuator and a sensor to control the actuator's force or displacement output. By measuring the current and voltage supplied to the actuator, a one-to-one correspondence is found between a given voltage and current and a measured displacement or force. This integrated sensor/actuator combination will lead to efficient on-chip control of motion for applications including microsurgery, biological cell handling, and optic positioning.     

An earthworm-like micro robot using shape memory alloy actuator

A novel bio-mimetic micro robot with wireless control and wireless power supply using shape memory alloy (SMA) actuator is developed. There have been many kinds of mobile micro robot using the micro actuators such as ionic polymer metal composite (IPMC), micro motors and piezo actuators. These actuators generally require electric cable for power supply, which might highly influence the mobility of the micro robot. Therefore, a perfect wireless micro robot comprising telemetry and batteries is realized using only one SMA spring actuator and one silicone bellow. The SMA actuator and bellow play a role in contraction and extension of an earthworm muscle respectively. Based on theoretical analysis, specifications of a SMA actuator and a bellow are properly selected. For temporal stopping, setae of earthworm mimicked claws are employed. On the issue of control, the proposed robot is controlled according to On/Off signal via wireless communication. The operation is customized through tuning of on-/off-time of an actuator and using different type batteries such as a lithium, silver oxide and alkaline battery. After the design and experiment, we find out that the earthworm-like micro robot without wired power supply and control can move freely without limitation of working space and be fabricated easily.     

Swing-arm-type PZT dual actuator with fast seeking for optical disk drive

 In this paper, a swing-arm-type dual positioning mechanism using a voice coil motor (VCM) and bimorph PZT actuators is proposed for the possible application to the future optical disc drive actuator. A VCM is used as a coarse motion actuator, and a set of piezoelectric actuators is used for fine motion. The two pairs of PZT actuators are arranged in parallel and are carefully designed to deflect in `S' shape such that tension and compression forces are generated simultaneously and thus the hysteresis is minimized. The static and dynamic analyses are performed and the parameter studies on the key dimensions of the set of PZT actuators are investigated. For fast seeking motion, time optimal control scheme combined with PD algorithm is adopted for the fast seeking motion of VCM. Positive position feedback (PPF) control is used to suppress residual vibration for the PZT actuator beams by activating it at the end of VCM swing motion. The feasibility of the suggested actuator system and the control scheme is demonstrated through simulations and experiments. Received: 5 July 2001/Accepted: 21 December 2001     

Position control of a flexible gantry robot arm using smart material actuators

This article presents new feedback actuators that achieve accurate position control of a flexible gantry robot arm. Translational motion in the plane is generated by two dc motors and controlled by applying electric fields to electro‐rheological (ER) clutch actuators. On the other hand, during control action of translational motion, a flexible arm attached to the moving part produces undesirable oscillations due to its inherent flexibility. Oscillations are actively suppressed by employing feedback voltage to the piezoceramic actuator attached to the surface of the flexible arm. Consequently, an accurate position control at the end‐point of the flexible arm can be achieved. To accomplish this control goal, governing equations of the proposed system are derived and written as transfer functions. Transfer functions are used in design of a set of robust H_∞ controllers. Electric fields to be applied to ER clutch and control voltage for the piezoceramic actuator are determined via H_∞ methodology which is incorporated with classical loop shaping design technique. To evaluate effectiveness of the proposed control system, experiments for both regulating and tracking controls are undertaken. ©1999 John Wiley & Sons, Inc.     

Development of a shape memory alloy actuator. Improvement of output performance by the introduction of a σ-mechanism

The driving-force-generating principle of an actuator made of shape memory alloy (SMA) is based on the thermal elastic martensitic transformation, a kind of elastic phenomenon. As a result, conventional SMA actuators which use a circular pulley have tended to exhibit undesirable characteristics such as the maximum generable torque being changed depending on the rotating angle when the robot joint was driven by rotating motion transformed from expansion/contraction of the SMA, so that the servo system to support the torque under a certain load could not have an operable range wide enough for practical application. This paper intends to clarify these problems of SMA actuators and proposes a new joint mechanism using a σ-shaped non-circular pulley, called the σ-mechanism, for joint linkage to overcome the problems. This design enables the maximum generated torque to be kept uniform at all times by reducing the torque-arm-length in inverse proportion when the SMA tension increases corresponding to the rotating angle of the joint. Subsequently, a specifically designing algorithm for the proposed σ-mechanism is discussed. The validity of the new mechanism is demonstrated by an experimental model using an SMA actuator with a σpulley.     

Mechanics of flexible interconnect in lateral tape open drives

A mechanics modeling is presented in this paper with focus given to understanding effect of flexible printed circuit cable’s dynamic effect on tape head actuator’s lateral motion in advanced, high capacity tape drives. The flexible printed circuit cable which connects the actuator to printed circuit board is first examined through establishment of analytical model to predict its profile with considerations of boundary conditions and mechanical design parameters. Secondly, equivalent stiffness produced by the flexible printed circuit cable when the linear tape head actuator is positioned along its lateral positions is examined. Finally, effect of tape head actuator’s voice coil motor is studied and modeled as a magnetic suspension, contributing to stability and controllability of the actuator lateral motion dynamics. Validated by calibrated laboratory experiments, the work presented in this paper can add to the literature regarding dynamics and control of LTM in modern LTO drives.     

Distributed structural dynamics control of flexible manipulators —I. Structural dynamics and distributed viscoelastic actuator

Reducing structural dead weight has become of increasing importance in the design of new generation lightweight and high-speed robot manipulators. However, due to the nature of structural flexibility, the dynamic oscillation associated with robot structures can affect the operation accuracy and precision. This work, in two parts, presents a study on the vibration control of elastic or flexible robot structures. Effects of distributed passive (Part I) and active (in Part II) actuators on elastic robot structures are studied. The proposed distributed passive viscoelastic actuator (in Part I) is a layer (or layers) of viscoelastic polymer directly attached to the flexible robot element, the oscillation of which is to be controlled. The passive actuator is activated by the oscillation of the robot structure and it automatically dissipates vibration energy and constrains the undesirable motion to eliminate the disturbance and to maintain a precise robot trajectory. A finite element program capable of analyzing flexible links is developed. Results obtained from the finite element simulation are presented.     

Design and testing of a microgripper with SMA actuator for manipulation of micro components

The extremely high work-to-volume ratios of the SMA actuators make them suitable to be used as a powerful actuator in micro-scale manipulation. In this study, an easily manufacturable micro-gripper with shape memory alloy (SMA) wire actuator was designed and manufactured. The concept of the designed micro-gripper was based on flexible hinge structures to increase the deflection efficiency and strength. The size of microgripper was a significant criterion in our design. Also, innovative layout in locating of SMA wire caused the microgripper to gripe and manipulate a boarder range of objects. The finite element method was used to analyze and calculate the stress distribution and jaw’s deflection in the gripper. In order to verify the modeling results, an experimental analysis by building a set-up for micro-gripper and running tests were implemented and it showed a good agreement with the modeling results. The approximate size of the micro-gripper is about 12 × 10 mm and its maximum achievable deflection is 200 μm which is perceptibly higher than SMA actuated micro-grippers with the same size.

     

A hybrid actuator scheme for robust position control of a flexible single-link manipulator

This article presents a novel hybrid actuator scheme to actively and robustly control the endpoint position of a very flexible single-link manipulator. The control scheme consists of two actuators; a motor mounted at the beam hub and a piezoceramic bonded to the surface of the flexible link. The control torque of the motor, which produces a desired angular motion, is determined by employing the sliding mode control theory on the equation of motion of the rigid link having the same mass as that of the proposed flexible link. The torque is then applied to the flexible manipulator to activate the commanded motion. During the motion, the undesirable oscillation caused by the torque, based on the rigid link dynamics, is actively suppressed by applying a feedback control voltage to the piezoceramic actuator. Consequently, desired tip motion is achieved. Both regulating and tracking control responses are analyzed through experimental implementation to demonstrate high performance characteristics to be accrued from the proposed methodology. © 1996 John Wiley & Sons, Inc.     

Micro actuators on the basis of thin SMA foils

Three innovative micro actuator concepts on the basis of the differential SMA principle are presented in this paper: a high adaptive multi-actuator system, which is driven by numerous identical single actuators connected in parallel and in series, a micro gripper for handling and assembling of complex hybrid micro systems and a micro actuator system in medical tools for the percutaneous resection of aortic valves. The SMA material is used in the form of 50 μm thin NiTi foils because of their well-defined properties and high strength. In order to integrate them into micro systems, different manufacturing methods have been applied and improved at the Institute for Microtechnology. Laser cutting and wet chemical etching are used for example to microstructure the actuator elements. Different methods for electrical and mechanical connections of the actuators are employed like soldering by the use of an additional gold layer. A batch fabrication process of SMA actuators is realized by embedding NiTi foil elements into SU-8 structures. To optimize the design of SMA actuator elements according to its application different simulation procedures are used.     

On the correlation between the structure and one stepping characteristic of a piezo-driven rotary actuator

The parasitic motion principle was recently proposed to design piezo-driven actuators. Based on this principle, linear and rotary actuators with features of compact structure, large motion range, high velocity and high positioning resolution have been reported. However, some shortcomings such as the backward motion and the nonlinearity still exist and their causes are not clarified. To find potential reasons leading to these shortcomings, one stepping characteristic of a piezo-driven rotary actuator designed by the parasitic motion principle was investigated in this paper, and the correlation between the structure and one stepping characteristic was attempted to be established by theoretical analysis and simulations. Analysis results indicated that the nonlinearity in one stepping characteristic of the rotary actuator was correlated to the small gap between the inner and outer rings of the bearing as well as the self-deformation of the flexible gripper after the contact stiffness between the flexible gripper and the rotor reaches to the maximum value. The backward motion could be mainly resulted from the non-ideal driving wave. These analysis results enhance the understanding of the parasitic motion principle, which are meaningful for design and performance improvement of the parasitic motion principle linear or rotary actuators.     

A multilink active catheter with polyimide-based integrated CMOSinterface circuits

This paper describes an active catheter with flexible polyimide-based integrated CMOS interface circuits for communication and control (C&C IC) to be used for applications in biomedicine. The active catheter has a multilink structure. Distributed micro shape memory alloy (SMA) coils are utilized as actuators for multidegree of freedom movement. The C&C IC's, which are incorporated on the links, require three common lead wires to address all the links and control the selected SMA actuators in the active catheter. An MOS transistor with large channel width is used for switching the SMA actuator. To reduce the system size and simplify the assembly work, the C&C circuit and three lead wires are fabricated on the same substrate using CMOS-compatible polyimide-based process. The outer diameter of the fabricated active catheter is approximately 2.0 mm. The fabricated active catheter has a four-link structure and six degrees of freedom of movement per one link. A simple bending model of one unit is presented and compared with the experiments. The fabricated C&C IC measures 1.0 mm×3.35 mm×0.2 mm. The link addressing and the actuator switching functions of the fabricated chip were confirmed. The minimum access time for addressing and actuating a single unit was 6.4 μs. The active catheter was actuated by the fabricated C&C IC chip with flexible interconnect leads in response to the C&C signals from the outside controller     

Modeling of pneumatic muscle with shape memory alloy and braided sleeve

Pneumatic muscle (PM) of flexible actuators used in bionic robot is an active area of recent research. A novel PM with shape memory alloy (SMA) braided sleeve is proposed in this paper, and SMA is used to improve PM working characteristics. Based on the principle of virtual work, output force model of PM and relationship with braided wire inner-stress are established, and analysis of PM deformation has shown that braided wire length is the key factor of output force characteristic. Based on the crystal structure transitions, the relationship of temperature with wire shrinkage is derived. Then, the synthetic dynamics of novel PM is established. A physical prototype of PM with SMA braided sleeve is developed, and test platform that is built for the experiment. Experiment and simulation test of static isometric-length, static isobaric-pressure, and dynamic characteristics are done. The experimental results are compared with the simulation of theoretical model. Moreover, based on experiment, model of output force was improved by adding a correction factor to deal with the elastic force of rubber tube. The results analysis demonstrates that the established models are correct, and SMA wires can reinforce PM and make PM working characteristics adjustable. PM proposed in this paper has greater output force and is beneficial to achieve more accurate control that is useful for manipulating fragile things.     

形状记忆合金驱动器的自适应滑模反步控制

形状记忆合金(SMA)作为一种智能材料,相对于传统的驱动器具有功率重量比大、驱动电压低、质量轻、干净、无噪音等特性,被广泛应用在各领域.然而,非线性、迟滞、时变等因素影响了形状记忆合金的控制精度,限制了它的应用.为此,本文提出了自适应滑模反步控制方法,用于解决精确控制问题.文中首先搭建了实验装置,建立了形状记忆合金的机理模型;然后,采用最小二乘算法辨识了模型参数;最后,基于机理模型设计了自适应滑模反步控制器.实验结果表明,本文提出的方法具有动态响应快、跟踪精度高和抗干扰能力强的特性.     

An Operator-based Nonlinear Vibration Control System Using a Flexible Arm with Shape Memory Alloy

In the past, arms used in the fields of industry and robotics have been designed not to vibrate by increasing their mass and stiffness. The weight of arms has tended to be reduced to improve speed of operation, and decrease the cost of their production. Since the weight saving makes the arms lose their stiffness and therefore vibrate more easily, the vibration suppression control is needed for realizing the above purpose. Incidentally, the use of various smart materials in actuators has grown. In particular, a shape memory alloy (SMA) is applied widely and has several advantages: light weight, large displacement by temperature change, and large force to mass ratio. However, the SMA actuators possess hysteresis nonlinearity between their own temperature and displacement obtained by the temperature. The hysteretic behavior of the SMA actuators affects their control performance. In previous research, an operator-based control system including a hysteresis compensator has been proposed. The vibration of a flexible arm is dealt with as the controlled object; one end of the arm is clamped and the other end is free. The effectiveness of the hysteresis compensator has been confirmed by simulations and experiments. Nevertheless, the feedback signal of the previous designed system has increased exponentially. It is difficult to use the system in the long-term because of the phenomenon. Additionally, the SMA actuator generates and radiates heat because electric current passing through the SMA actuator provides heat, and strain on the SMA actuator is generated. With long-time use of the SMA actuator, the environmental temperature around the SMA actuator varies through radiation of the heat. There exists a risk that the ambient temperature change dealt with as disturbance affects the temperature and strain of the SMA actuator. In this research, a design method of the operator-based control system is proposed considering the long-term use of the system. In the method, the hysteresis characteristics of the SMA actuator and the temperature change around the actuator are considered. The effectiveness of the proposed method is verified by simulations and experiments.

     

A piezoelectric model based multi-objective optimization of robot gripper design

The field of robotics is evolving at a very high pace and with its increasing applicability in varied fields, the need to incorporate optimization analysis in robot system design is becoming more prominent. The present work deals with the optimization of the design of a 7-link gripper. As actuators play a crucial role in functioning of the gripper, the actuation system (piezoelectric (PZ), in this case) is also taken into consideration while performing the optimization study. A minimalistic model of PZ actuator, consisting different series and parallel assembly arrangements for both mechanical and electrical parts of the PZ actuators, is proposed. To include the effects of connector spring, the relationship of force with actuator displacement is replaced by the relation between force and the displacement of point of actuation at the physical system. The design optimization problem of the gripper is a non-linear, multi modal optimization problem, which was originally formulated by Osyczka (2002). In the original work, however, the actuator was a ‘constant output-force actuator model’ providing a constant output without describing the internal structure. Thus, the actuator model was not integrated in the optimization study. Four different cases of the PZ modelling have been solved using multi-objective evolutionary algorithm (MOEA). Relationship between force and actuator displacement is obtained using each set of non-dominated solutions. These relationships can provide a better insight to the end user to select the appropriate voltage and gripper design for specific application.     

Miniature Pneumatic Curling Rubber Actuator Generating Bidirectional Motion with One Air-Supply Tube

Soft actuators driven by pneumatic pressure are promising actuators for mechanical systems in medical, biological, agriculture, welfare fields and so on, because they can ensure high safety for fragile objects from their low mechanical impedance. In this study, a new rubber pneumatic actuator made from silicone rubber was developed. Composed of one chamber and one air-supply tube, it can generate curling motion in two directions by using positive and negative pneumatic pressure. The rubber actuator, for generating bidirectional motion, was designed to achieve an efficient shape by nonlinear finite element method analysis, and was fabricated by a molding and rubber bonding process using excimer light. The fabricated actuator was able to generate curling motion in two directions successfully. The displacement and force characteristics of the actuator were measured by using a motion capture system and a load cell. As an example application of the actuator, a robotic soft hand with three actuators was constructed and its effectiveness was confirmed by experiments.