Maximum Actuator Force Research Articles

Analysis of nitinol actuator response under controlled conductive heating regimes

In the last few decades, Nitinol (NiTi) actuators have created a massive impact at the commercial level due to their application in various engineering and medical fields. In this paper, an experimental analysis study is presented on commercially manufactured nitinol tubes for performance enhancement. As received tubes were super-elastic at room temperature with Af temperature of 1.7°C. The nitinol tubes were heat treated at 500°C for different time ranging from 30 min to 60 min to raise the Af temperature. Metallography was performed on pristine and heat-treated samples to analyse the changes in the physical properties. XRD analysis revealed the crystalline structure present in the tubes (as received and heat treated) was nitinol cubic (110) while nitinol cubic (211) at room temperature. Moreover, dilatometry was performed which showed thermal expansion coefficients very close as noted in the literature as 11.4x10-6/°C. In the last section of this paper, the actuation force of the tubes was experimentally measured and analysed using different springs attached to the tubes connected to a conductive heating stage. A full factorial Design of Experiments (DoE) was used based on factors of time, temperature, and spring constant. For a surface temperature of 125°C and a spring constant of 2.39 kN/m, 131 N force was attained from the tube. The maximum actuation force of 145 N was obeserved for surface temperature of 145°C at an exposure time of 60 s with k = 2.39 kN/m.

“Water to the ropes”: A predictive model for the supercontraction stress of spider silks

When humidified at different moisture conditions, restrained spider silk fibers can exhibit a very high supercontraction phenomenon. The hydration water molecules induce a Hydrogen-bonds disruption process that, due to entropic effects, decreases the natural – zero force – end-to-end chains length. By considering a bundle of macromolecules, we describe supercontraction as a possible actuation system and determine the maximum actuation force depending on the silk properties at the molecular scale and on the constraining system representing other silk threads or the actuated device. The comparison with experimental results of Argiope trifasciata silk fibers show the effectiveness of the proposed model in quantitatively predicting the experimental actuation properties. The considered historical case study of obelisk rescue in Saint Peter’s Square (Rome) through ropes hydration is discussed evidencing the optimal performances of this natural material adopted as moisture powered actuator: we obtain a work density of 2.19 kJ/m3 making spider silk the most performant hydration driven active material. Moreover we obtain a power density of the order of 730 W/kg about three times the most performant carbon nanotube actuators making such material very competitive as compared with all types of actuators. The analytic description of the macroscopic actuation parameters from microscale properties shows the possibility of adopting our approach also in the field of bioinspired artificial silks design, possibly considering also important non-linear effects in the actuated system.

High-performance low-voltage soft electrothermal actuator with directly printed micro-heater

Soft actuators are widely studied due to their high flexibility and adaptability. In this paper, a soft electrothermal actuator with directly printed microfilament heater was designed, fabricated, and tested. In this soft actuator, a polyimide (PI) film and a polydimethylsiloxane (PDMS) layer were selected as the two structural layers because of their distinct thermal expansion properties. Embedded between these two layers, a metallic microfilament heater that was made of low-melting-point metal alloy (i.e. Bi58/Sn42) was directly printed with custom-designed pattern using an electrohydrodynamic (EHD) printing process. The pattern of the microfilament heater and the thickness of the soft thermoelectric actuator were carefully designed towards the improved actuation capability. When an electric current was applied, the high temperature generated by the resistive heater causes large mismatch in the thermal expansion of the two structural layers, which bent the actuator. The fabricated soft actuator can achieve a maximum bending curvature of 1.0 cm−1 under an extremely low voltage about 3V. The soft actuator has been demonstrated for potential applications in soft robotics as a soft lifter and a soft gripper. At the full capability, the soft lifter provided a maximum actuation force about 3 mN, and was able to fully lift a load that was twice of its own weight. A four-finger soft gripper was designed and fabricated using four soft actuators to mimic humanoid motions and was implemented for pick-and-place operation for delicate objects.

Kinematics and force analysis of a novel offshore crane combined compensation system

Offshore cranes are widely used in offshore industry. However, because of the wave motion of the ship and the flexibility of the rope, the sway of payload is unavoidable and may put relevant operations in danger. Thus, compensation system is an essential part of offshore crane to ensure human safety and increase operation efficiency. In this article, a novel offshore crane combined compensation approach named four-post combined compensation is proposed based on the three-post direct ship motion compensation, aiming to overcome its shortcomings. The structure and principle of four-post combined compensation is introduced. The mathematical models of four-post combined compensation and three-post direct ship motion compensation are established uniformly and based on which we carry out numerical simulations and comparative analysis of three-post and four-post schemes. Numerical simulation results are included to demonstrate that comparing to three-post scheme, four-post scheme has the following advantages: first, the maximum actuator force requirement can be reduced significantly; second, the work load is distributed more evenly among cylinders; third, the main support shaft can prevent the platform from tipping over; the actuators always push the platform and do not have to be bi-directional, which might result in lower cost and risk.

Development of a shape-memory-alloy micromanipulator based on integrated bimorph microactuators

This paper reports a novel structure of a shape-memory-alloy (SMA) micromanipulator with gripping mechanism. A featured integration of multiple SMA bimorph microactuators has been utilized to form a micromanipulator with three degrees of freedom. The design consists of two links (SMA sheets) and a gripper at the end of the second joint. The overall dimensions of the micromanipulator are 33mm×9mm×3mm. The displacement of each actuator is controlled by a heating circuit that generates a pulse-width modulation signal. Theoretical modeling of SMA actuators is studied and verified with simulation. The SMA micromanipulator is able to move in the x- and y-axis by 7.1mm and 5.2mm, respectively, resulting in a maximum displacement of 8.9mm. The micro-gripper has a maximum opening gap between its fingers of 1.15mm. The micromanipulator has a temporal response of 7.5s and 9s for its x- and y-axis. The maximum actuation force generated by the x- and y-axis was around 100mN and 130mN, respectively. The developed micromanipulator has been successfully used to move a small object.

Efficiency Mapping for a Linear Hydraulic-Actuator

This research is aimed at generating an efficiency map for a linear hydraulic-actuator that is controlled using an open-centered 4-way valve. Using the basic configuration of a double rod, double-acting linear actuator, equations for describing the input and output power of the actuator are written and nondimensionalized to produce a nondimensional map for the efficiency of the actuator itself. In conclusion, the actuator is shown to operate below a wide open valve (WOV) line with efficiencies that are typically less than 50%. Note that the WOV line describes the maximum actuator-force that may be achieved at a given actuator-velocity. While the actuator chosen for this paper is common, extensions of the methods used in this study may be applied to double-acting, single-rod designs or single-acting, single-rod designs. Furthermore, it is anticipated that this type of analysis may also be used to study the effects of power regenerating valves and for predicting the efficiency improvements that may be gained for linear hydraulic-actuators that use this type of valve design.

Optimization of Pathway Pattern Size for Programmable Biomolecule Actuation

We generalize a method for optimization of pattern size for an programmable actuation of magnetic beads, as biomolecule carriers, by analytically formulating the governing forces in terms of bead and pattern sizes. There is a good agreement between analytically calculated and numerically obtained forces, giving a validity of the analytical actuation force for the optimization of pattern size. The maximum actuation force is given at the phase angle of π/4 between the direction of magnetic field and the bead position, and the optimum radius of disk pattern is in the range of 2-5 times of bead radius, depending on the magnetization of the disk pattern under an external magnetic field.

Computational analyses of pinching dynamics of a finger exoskeleton composed of IPMC actuators

There is an increased demand for hand exoskeletons that are light weight, low profile, and flexible, and that consume less power. In order to replace the rigid actuators such as motors and pneumatic cylinders, an ionic polymer metal composite (IPMC) actuator may be a good candidate. Because of the limited forces generated by IPMC actuators, prediction of the IPMC actuation force for the required fingertip force is important in designing and improving the performance of a finger exoskeleton. Anthropomorphic data on index fingers and the stiffness of a finger joint were measured, and a standard index finger model was established. Electromechanical characteristics of IPMC actuators were experimentally measured and mathematically modeled. These were incorporated into the dynamics of an index finger actuated by IPMC, and the dynamics of tip pinching were simulated. The arrangement of IPMC actuators at the initial position significantly affected the required actuator force to hold a given load. The maximum actuator force required to hold 0.98 N of fingertip load decreased by 50% for IPMC actuators that were arranged straight at the initial pinching position as compared to straight at the initial open position.

Electrothermally Actuated Lens Scanner and Latching Brake for Free-Space Board-to-Board Optical Interconnects

The design, fabrication, and characterization of an electrothermally actuated lens scanner with bistable mechanical brakes, for the application of free-space board-to-board optical interconnects, are presented. An electrothermally actuated stepper motor is used to scan a 2.8-mm-diameter lens shuttle by a maximum of ±170 μm (±1.57° scanning angle). Bistable mechanical brakes, toggled with U-shaped thermal actuators, will grip and hold the lens shuttle in place while dissipating zero power. The minimum frictional braking force and maximum actuator force are both measured to be 0.75 mN. A position sensing detector is used to accurately measure the dynamics of the stepper motor and lens system, from which we verify our analytical model. Long-term testing results and solutions are also presented. We demonstrate a robust 10-Gb/s optical link capable of maintaining connection despite a board tilting of 0.45° while dissipating zero power.

Fault-tolerant H∞ control for active suspension vehicle systems with actuator faults

This paper investigates the problem of fault-tolerant H∞ control for a class of quarter-car active suspension systems with actuator faults. The corresponding state-space form is established via taking into account the active suspension system performance: namely, ride comfort, road holding, suspension deflection, and maximum actuator force limitation for control strategy. Different from the traditional actuator model, in this paper, the actuator failures are modelled as random variables governed by a homogeneous Markov chain. A fault-tolerant H∞ controller is designed such that the resulting control system is reliable since it has the capability of guaranteeing asymptotic stability and H∞ performance, and simultaneously satisfying the constraint performance in the scenarios of three types of actuator failures. Simulation results are provided to illustrate the effectiveness of the proposed controller design methodology.

Thermo-mechanical Behavior of (Meth)Acrylate Shape-Memory Polymer Networks

ABSTRACTOur overall approach is based on developing a photocrosslinkable polymer network with a favorable shape-memory response, using polymer chemistry and crosslinking density to control thermo-mechanical properties. Three polymer networks were created and thermo-mechanically tested, each from tert-Butyl acrylate linear builder co-polymerized with a poly(ethylene glycol) dimethacrylate cross-linker. By systematically altering the molecular weight and the weight fraction of the cross-linker, it was possible to create three polymers that exhibited the same glass transition temperature, but varied by almost an order of magnitude in rubbery modulus. Therefore, the mechanical stiffness could be tailored to suit a given application. Recovery behavior of the polymers was characterized over a range of deformation temperatures. It has been implicitly assumed a linear relationship between Free-Strain (i.e. no actuation force) and Fixed-Stress (i.e. maximum actuation force), however, this has never been confirmed experimentally. The energy per unit volume performed by the shape-memory polymer was quantified, and observed to be a function of strain recovered. The maximum recoverable work was shown to increase with cross-linking density, although the overall efficiency is similar for all materials tested.

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A self-retracting fully compliant bistable micromechanism

A new class of fully compliant bistable mechanisms with the added benefit of integrated self-retraction has been developed (hereafter identified as Self-Retracting Fully compliant Bistable Mechanism or SRFBM). A technique using tensural pivots to manage compressive loading in compliant mechanisms is introduced and implemented in the SRFBM. The elimination of traditional kinematic joints and their associated clearance allows a total displacement between stable positions of 8.5 μm, and the mechanism size is less than 300 μm square when using 2.0 μm minimum line widths. Maximum actuation force is approximately 500 μN. The SRFBM's small linear displacement and reasonable actuation force facilitate integration with efficient thermal actuators. Furthermore, fully compliant mechanisms allow greater freedom in fabrication as only one mechanical layer is needed. Systems with on-chip actuation have been fabricated and tested, demonstrating bistability and on-chip actuation, which requires approximately 150 mW. A single fatigue test has been completed, during which the SRFBM endured approximately 2 million duty cycles without failure.

Design Issues of a High-Stroke, On-Blade Piezostack Actuator for a Helicopter Rotor with Trailing-Edge Flaps

A compact actuator to oscillate blade trailing-edge flap on a full-scale helicopter has been investigated for vibration suppression. The actuator consists of piezoelectric stacks and a dual-stage mechanical stroke amplifier. It is recommended that commercially available piezostack be used for timely manufacturing and maintenance rather than a custom-designed piezostack. An analytical approach to estimate the maximum actuator force requirement for the rotor in hover condition is discussed first. A piezostack down-selection through the experimental evaluation is presented, which is to select a piezostack with high stiffness as well as good strain/block force output among twelve commercial piezostacks. The design requirements of on-blade piezoelectric actuator are discussed, and one-dimensional stiffness modeling and finite element beam modeling are presented to evaluate the analytical/numerical actuator performance. The prototype actuator with a double-lever stroke amplifier was fabricated and tested on bench-top as well as in vacuum chamber. An amplification factor of 19.4 and the consistent actuator output up to 600 g of centrifugal loading were obtained. An improved actuator has been designed with a design refinement to reduce structural losses.

Thermal bubble powered microactuators

Thermal bubble powered micro mechanical actuators have been successfully demonstrated in working liquids. Micro mechanical plates which function as the mechanical actuators are 70×60×2 μm3 in size. They have been fabricated by surface-micromachining technology and suspended 2 μm above the substrate by the supports of cantilever beams. Micro thermal bubbles which are generated by heavily phosphorus doped polysilicon line resistive heaters have been used to lift the mechanical plates in a controllable manner. The typical current required to generate a single, spherical thermal bubble as the actuation source on top of the micro line resistors (60×2×0.3 μm3 in size) is 8.4 mA (80 m Watt) in FC 43 liquid (an inert, dielectrical fluid available from the 3M company). The thermal bubbles have been demonstrated to actuate the mechanical plate perpendicularly to the substrate with a maximum elevation distance of 140 μm and a maximum actuation force of 2 μN. This new actuation mechanism is expected to find applications for micro fluidic devices.

STRUCTURAL CONTROL OF THERMOELASTIC BEAMS FOR VIBRATION SUPPRESSION

The vibrations of a thermoelastic beam subject to a sudden heat input are damped by active control. The control objective is specified as the minimization of a quadratic functional in terms of displacement and velocity at a given time. The control is exercised by means of point actuators which are placed so as to minimize the maximum actuator force. The solution of the control problem is obtained by means of eigenfunction expansions. The physical problem corresponds to the suppression of vibrations of a structure suddenly entering into the daylight zone. Numerical results indicate that the displacement and velocity of the beam can be effectively reduced by means of active control exercised by point actuators.