Actuator Unit Research Articles

Elastic Structure Preserving Control for Compliant Robots Driven by Agonistic-Antagonistic Actuators (ESPaa)

The regulation of the link positions of compliant robots, damping out undesired link oscillations while preserving the system's inherent elasticity is still a challenging task in practical applications. This task becomes even harder to be tackled in the case of compliant robots driven by agonistic-antagonistic variable stiffness actuators in which there are two motors associated with each joint of the system. In this work, leveraging on the physical realization of the elastic mechanism of such actuators, we propose a novel control law able to simultaneously achieve a good regulation performance and a desired damped behavior at the link, while preserving the elastic structure of the system as well as the possibility of adjusting the passive stiffness at the joints. Simulations on the agonistic-antagonistic actuators of the forearm and wrist joints of the Hand Arm System from the German Aerospace Center (DLR) and experiments on a planar platform with an analogous actuation unit, namely qbMove Advanced, validate the proposed method.

Flight safety oriented ice shape modulation using distributed plasma actuator units

Ice accretion on the wings seriously threatens the flight safety of an aircraft. From the perspectives of ensuring flight safety and saving power consumption, the ice shape modulation method using distributed plasma is proposed. Distributed plasma actuator units are designed to modulate the spanwise continuous ice at the leading edge into periodically segmented ice pieces, forming a wavy leading edge. Both airfoil and scaled aircraft model, with continuous and modulated ice, are experimentally investigated and simulated. Compared with the continuous ice, ice shape modulation can significantly improve the aerodynamic performance, flight control characteristics and flight safety. This method can save about half electric power, which is very beneficial for application.

3D printed magnetically-actuating micro-gripper operates in air and water

The emerging trend towards miniaturization of robotic grippers is motivated by the needs for precise manipulation of smaller objects within confined spaces. However, it faces a multitude of challenges in micro-fabricating, assembling, and actuating the grippers with increasingly smaller dimensions. To overcome these challenges, we report here a method to 3D print a magnetically-driven triple-finger micro-gripper for robust micro-manipulation in air and water. Magnetic actuation was chosen for its advantage of untethered operation in complex surroundings. Mitigating the trade-off between mechanical compliance and magnetic actuation forces, the monolithic micro-gripper design comprising compliant mechanical flexures and magnetic force actuation units was rapidly fabricated using micro-continuous liquid interface production (μCLIP) process. Finally, we attached the 3D printed gripper to a robotic arm and demonstrated its ability to manipulate micro objects both air and water. This work may enable potential biological and biomedical applications such as operation of live cells and soft tissues.

Investigation of Coil-Shaped Optical Fiber Displacement Sensor for a Twisted and Coiled Polymer Fiber Actuator Unit

Investigation of Coil-Shaped Optical Fiber Displacement Sensor for a Twisted and Coiled Polymer Fiber Actuator Unit

A Compact Adjustable Stiffness Rotary Actuator Based on Linear Springs: Working Principle, Design, and Experimental Verification

Inspired by improving the adaptive capability of the robot to external impacts or shocks, the adjustable stiffness behavior in joints is investigated to ensure conformity with the safety index. This paper proposes a new soft actuation unit, namely Adjustable Stiffness Rotary Actuator (ASRA), induced by a novel optimization of the elastic energy in an adjusting stiffness mechanism. Specifically, a stiffness transmission is configured by three pairs of antagonistically linear springs with linkage bars. The rotational disk and link bars assist the simplified stiffness control based on a linear transmission. To enhance the elastic energy efficiency, the force compressions of the linear springs are set to be perpendicular to the three-spoke output element, i.e., the output link direction. Besides, the ASRA model is also formed to investigate the theoretical capabilities of the stiffness output and passive energy. As a simulated result, a high passive energy storage ability can be achieved. Then, several experimental scenarios are performed with integral sliding mode controllers to verify the physical characteristics of the ASRA. As trial results, the fast transient response and high accuracy of both the position and stiffness tracking tests are expressed, in turn, independent and simultaneous control cases. Moreover, the real output torque is measured to investigate its reflecting stiffness.

Design of high performance DC motor actuated cable driving system for compact devices

The cable transmission is widely used in the remote operation or complex geometry with high stiffness and low backlash. Larger drum is required to reduce and error of transmission in long stroke. An error of the desired position occurs due to the fleet angle while cable winding on a drum. Therefore, a new cable driving module which overcomes this problem is proposed. A new cable driving module with a sliding platform is connected to the actuator unit. A motion of the sliding platform is corresponding to a screw rod connected to an actuator. The precision of the driving system is measured by a high-resolution rotatory encoder and high gear ratio actuator. Results are measured by load and error of the system. A load of system shows a performance of overall translation and rotation of the drum at different speeds. An error of the system is measured from forward and reverse direction by increasing and decreasing the number of turns with constant speed. A system has an average load consumption along a long stroke of cable winding which has no significant problem on the screw platform. Multiple turns have low error value in specific and continuous turn in forward and reverse motion. A new cable driving system is proved in precision movement. The fleet angle is eliminated in new mechanism. Along with a constraint motion, there is no significant change in load consumption. An error is low value in a different direction of movement. Hence, a new design of cable transmission can perform in high performance and small size of the system.

Dynamic Modeling, Energy Analysis, and Path Planning of Spherical Robots on Uneven Terrains

Spherical robots are generally comprised of a spherical shell and an internal actuation unit. These robots have a variety of applications ranging from search and rescue to agriculture. Although one of the main advantages of spherical robots is their capability to operate on uneven surfaces, energy analysis and path planning of such systems have been studied only for flat terrains. This work introduces a novel approach to evaluate the dynamic equations, energy consumption, and separation analysis of these robots rolling on uneven terrains. The presented dynamics modeling, separation analysis, and energy analysis allow us to implement path planning algorithms to find an optimal path. One of the advantages of this work is that these algorithms can be used when either the analytical or the empirical information about the terrain is available.

Expandable Polymer Assisted Wearable Personalized Medicinal Platform

AbstractConventional healthcare, thoughts of treatment, and practice of medicine largely rely on the traditional concept of one size fits all. Personalized medicine is an emerging therapeutic approach that aims to develop a therapeutic technique that provides tailor‐made therapy based on everyone’s individual needs by delivering the right drug at the right time with the right amount of dosage. Advancement in technologies such as wearable biosensors, point‐of‐care diagnostics, microfluidics, and artificial intelligence can enable the realization of effective personalized therapy. However, currently, there is a lack of a personalized minimally invasive wearable closed‐loop drug delivery system that is continuous, automated, conformal to the skin, and cost‐effective. Here, design, fabrication, optimization, and application of a personalized medicinal platform augmented with flexible biosensors, heaters, expandable actuator and processing units powered by a lightweight battery are shown. The platform provides precise drug delivery and preparation with spatiotemporal control over the administered dose as a response to real‐time physiological changes of the individual. The system is conformal to the skin, and the drug is transdermally administered through an integrated microneedle. The developed platform is fabricated using rapid, cost‐effective techniques that are independent of advanced microfabrication facilities to expand its applications to low‐resource environments.

On the performance analysis of gas-actuated peristaltic micropumps

In this study, we report on the performance analysis of peristaltic micropumps to understand the impact of affecting parameters on micropump actuation membrane and overall micropump operation. To this end, a thermoplastic micropump consisting of three interconnected actuation units made of Poly(methyl methacrylate) (PMMA) and thermoplastic polyurethane (TPU) film is designed and fabricated. The actuation membrane for liquid pumping is made of TPU film that is located between the liquid and the actuation chambers. The effects of actuation gas pressure and actuation frequency on micropump characteristics in terms of pumping flow rate, volumetric efficiency, and generated liquid pressure are studied. A maximum flow rate of 56.28 ± 1.47 μl/min is obtained using an actuation pressure and frequency of 50 kPa and 15 Hz, respectively. An experimental setup is designed and assembled to measure the actual pressure experienced by the actuation membrane at different actuation frequencies and gas pressures. It is found that the increase of actuation pressure leads to the increment of membrane oscillation amplitude with subsequent increase of pumping flow rate and volumetric efficiency. However, a further increase of actuation pressure beyond a threshold (50 kPa) leads to the decline of membrane oscillation amplitude that decreases the flow rate. Also, an image processing of the actuation membrane reveals that the increase of actuation frequency results in the continuous reduction of the oscillation amplitude of micropump membrane. Similar to actuation pressure, the increase of actuation frequency beyond a critical value (15 Hz) leads to the decline of flow rate. Our findings provide critical insights required for optimal design and operation of micropumps and finding a compromise between optimum pumping flow rates and operating conditions for on-chip pumping.

A programmable and self-adaptive dynamic pressure delivery and feedback system for efficient intermittent pneumatic compression therapy

Controlled dynamic pressure delivery is essential for efficiently promoting hemodynamics of the lower extremities in pneumatic compression therapy. Pressure dosage control is challenging because of diverse influencing factors, such as irregular interface contact geometries, different stiffnesses of body tissues and pressure materials, and customized pressure recipes and individual patient demands. To address these challenges, a programmable and self-adaptive dynamic pressure delivery and feedback system for enhanced compression therapy were developed in this study. This system comprised a soft pneumatic actuator (SPA) unit in a stocking laminated with multi-bladder, a programmable pressure control unit for pneumatic and interface pressure control, matrix soft sensors for real-time interface pressure detection, and differential pressure sensors for pneumatic pressure monitoring as well as connective accessories (air tubes and valves). A fluid–structure interaction model was developed to analyse the interaction between the bladder and lower limb by considering essential factors (bladder and soft tissue stiffness, contact area, and bladder-lower limb interface characteristics). An improved proportional–integral–derivative controller was designed to regulate dynamic interface pressure and personalized pneumatic compression treatment. Dedicated software was developed to monitor, process, and display the tested pressure data. In vitro and in vivo experiments were performed to validate the developed system. This study integrated theoretical and experimental approaches to a novel solution for customized dynamic pressure therapy. The findings enhanced our understanding of the working mechanisms and characteristics of the SPA–lower limb system. Moreover, the results serve as a foundation for the design and innovation of advanced programmable units for efficient compression treatment in various applications in healthcare, medicine, and rehabilitation.

Low-Cost Coil-Shaped Optical Fiber Displacement Sensor for a Twisted and Coiled Polymer Fiber Actuator Unit

This study proposes a low-cost coil-shaped optical fiber (COF) sensor that can be used in the twisted and coiled polymer fiber (TCPF) actuator unit. The TCPF is a thermal-driven artificial muscle with large stroke and large power ratio. To improve its response, several actuator units, which combine the TCPF with a cooling system, were developed. However, the displacement sensors for these units were not established, unlike the encoder for the motor. Although several methods are available to estimate the TCPF displacement, they are based on the resistance of the heating wire and are affected by TCPF actuation and load change. Therefore, to accurately measure displacement, in this study, we focus on the bending loss of optical fiber, which is often used in soft robotics. Because the bending loss of COFs is caused by the change in length of the coil, the displacement can be obtained by measuring the light intensity through the optical fiber. This study experimentally demonstrates that the COF is available, even when driving the TCPF actuator and changing the load.

Optimal Selection of Motors and Transmissions in Back-Support Exoskeleton Applications

Because of the central role the actuator unit (electrical motors and transmission parts) has in wearable robots, improving the performance of the torque/force control system is vital, particularly for exoskeletons. This paper proposes an optimal approach to the selection of the main components of an actuation system (brushless DC motor and gearbox transmission) to be used in a back-support exoskeleton, but the principles can be extended and applied to other types of exoskeletons. To perform the optimization, an analytical model based on the dynamics of human–robot interaction has been developed. Moreover, to incorporate the weight of the actuator in the optimization framework, a mathematical relation between the weight and technical characteristics of the components, based on the polynomial regression technique using the low-discrepancy sequences method are developed. Consequently, the optimization criteria in terms of the closed-loop system frequency bandwidth, system power consumption and the weight of the components are formulated by imposing technical constraints on simulation parameters. The optimization results demonstrate two possible actuator combinations. Subsequently, the selected actuator components are evaluated in a lifting scenario by means of a linear quadratic regulator (LQR) controller with double integral action. Extensive simulation results in terms of the control frequency bandwidth, torque tracking control, current produced by motors and system robustness with respect to external disturbances are presented and discussed to make comparisons between the possible combinations of the components and their feasibility in the back-support exoskeleton applications.

Development of a Quadruped Robot System With Torque-Controllable Modular Actuator Unit

This article presents an overview of <bold xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">A</b> rt <bold xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">i</b> ficial <bold xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">DI</b> gitigrade for <bold xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</b> atural Environments (AiDIN-VI), a force-controllable quadruped robot system with incorporated mandatory abilities of speed, efficiency, and mobility for providing real-world services. This article describes the design methodologies and principles employed to implement these requisite capabilities on a single robot platform; particularly, the torque sensing method, along with the components and modularization method of the torque-controllable actuator unit are elucidated herein. The developed robot platform is equipped with all necessary components, including onboard PCs (for motion generation and vision mapping), a battery, modular actuator units, a wireless network router, and a remote e-stop controller for autonomous and manual locomotion control. The robot is subdivided into joints, legs, and robots, and its performance is experimentally tested. The capability of the robot with regard to joint torque control ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\pm$</tex-math></inline-formula> 70 Nm), leg force control (350 N along the <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">z</i> -axis), and robot (vertical loading 25 kg, pulling force 200 N) was experimentally verified, and locomotive performances (walk, pace, trot, and jump) on various terrains were executed. A maximum trot gait speed of 1.2 m/s was recorded, along with minimum costs of transport of 1.187 and 1.15 at a speed of 1.0 m/s and under loading conditions, respectively.

Designing Ordered Structure with Piezoceramic Actuation Units (OSPAU) for Generating Continual Nanostep Motion.

Continual precision actuations with nanoscale resolution over large ranges have extensive requirements in advanced intelligent manufacturing and precise surgical robots. To produce continual nanostep motion, conventionally, multiple pairs of piezo‐actuators are employed to operate in inchworm principle under complex three‐ or four‐phase timing signal drive. Inspired by the idea of ordered structures with functional units, a much simpler nanostep piezoelectric actuator consisting of (2 × 2) arrayed, cofired multilayer piezoceramic actuation units is developed, which operates in an artificially generated quasi shear mode (AGQSM) that is missing in natural piezoelectric ceramics. Under only one‐phase square‐wave voltage drive, the actuator can produce a stable, continual nanostep motion in two ways at nonresonant frequencies, and the obtained minimum step displacement is as low as 7 nm in open control, indicating its potential application as a precise finger or knife actuator in surgical robots. This work is of great guiding significance for future actuator designs using the methodology of ordered structure with piezoceramic actuation units and AGQSM.

The influence of energy recovery on the overall efficiency of acoustic sources at low frequencies

This paper defines a criterion called the overall efficiency (OE) for evaluating the efficiency of acoustic sources. In practice, the conventional efficiency (CE), is not capable of giving a comprehensive overview of the power consumption, especially in the connected amplifier drivers. The reason for this is that the CE is defined based on the nominal input electrical power. The nominal input electrical power is defined as the real part of the power that is delivered by the electrical amplifier to the actuation unit of an acoustic source. Therefore, power loss in the amplifier unit is not included in the CE definition. With the aid of the OE criterion, the effect of power loss in the connected amplifier units is taken into account. In particular, the application of the OE is crucial for acoustic sources that operate in a low frequency range. This is due to the reactive nature of power in both actuators and connected amplifiers. In the current paper, various combinations of amplifiers and actuators are studied. In particular, voice coil actuators and piezoelectric stack elements including both Lead Zirconate Titanate (PZT) ceramics and single crystal Lead Zirconate Niobate-Lead Titanate (PZN-PT) piezoelectric materials are investigated. In addition, the effect of a connected power driver is investigated. A class AB and a class D amplifier are studied respectively as analogue and switching amplifiers. Unlike a class AB amplifier, a class D amplifier is capable of energy recovery. A perforated flat acoustic source is examined in this paper as a practical example to verify the OE criterion. The numerical simulation on a thin acoustic source shows that for a single actuator, thanks to energy recovery, the OE is higher in a class D amplifier than that in a class AB amplifier. This study reveals that if a class D amplifier is the driver, using piezoelectric actuators results in a higher OE compared to using a voice coil actuator. Measurements on the sample acoustic source verify the numerical results presented in the current study.

Battery-Less Soft Millirobot That Can Move, Sense, and Communicate Remotely by Coupling the Magnetic and Piezoelectric Effects.

The soft millirobot is a promising candidate for emerging applications in various in‐vivo/vitro biomedical settings. Despite recent success in its design and actuation, the absence of sensing ability makes it still far from being a reality. Here, a radio frequency identification (RFID) based battery‐less soft millirobot that can move, sense, and communicate remotely by coupling the magnetic and piezoelectric effects is reported. This design integrates the robot actuation and power generation units within a thin multilayer film (<0.5 mm), i.e., a lower magnetic composite limb decorated with multiple feet imparts locomotion and a flexible piezoceramic composite film recovers energy simultaneously. Under a trigger of external magnetic guidance, the millirobot can achieve remote locomotion, environment monitoring, and wireless communication with no requirement of any on‐board battery or external wired power supply. Furthermore, this robot demonstrates the sensing capability in measuring environment temperature and contact interface by two different sensing models, i.e., carried‐on and build‐in sensing mode, respectively. This research represents a remarkable advance in the emerging area of untethered soft robotics, benefiting a broad spectrum of promising applications, such as in‐body monitoring, diagnosis, and drug delivery.

Analytical modelling and numerical simulation of 0–3 PZT/PVDF composite actuated micropump

ABSTRACT In this paper the flow characteristic of laminar flow through planar diffuser was studied for different flow regime and its geometry, including angle, length and width were optimized. The diffuser shows flow recirculation and its efficiency is negative for diffuser angle larger than 30°. The analytical modelling of the micropump is solved by sectioning it as two separate units, an actuator unit and a flow unit. The actuator unit is a circular plate made of lead zirconate titanate (PZT) filler in polyvinyldene fluoride (PVDF) matrix composite material. The deflection of the actuator unit for bilaminar plate is analytically solved using theory of bending plates. The deflection profile of the actuator for the applied voltage is used to determine the stroke volume. The stroke volume from the actuator unit is used to solve the flow rate of the micropump analytically in the flow unit using mass balance equation. The flow rate of the micropump is numerically simulated in 3D for varying geometric and electrical parameters. The results obtained were compared and the numerical procedure was found to have less percentage error in the actuator deflection and flow rate up to a chamber depth of 250 µm.

Uncertainty and Resolution of Speckle Photography on Micro Samples

The efficient development of new materials with defined properties requires fast methods of testing low volumes of material, such as a high-throughput investigation of spherical metallic micro samples with varying compositions and structuring treatments. A classical material testing method for macro samples, the tensile test cannot be employed for analyzing the mechanical properties of spherical samples with diameters below 1 mm since there are currently no methods for holding and stretching them. A combination between the incremental electrohydraulic extrusion as stress actuation unit and the speckle photography as strain measuring method is presented for obtaining the required mechanical characteristics. Positive longitudinal strain is generated at stepwise extrusion through < 1 mm wide forming channels using a liquid punch and the deformation is observed in situ after each forming step at the interface between the micro sample and a transparent window integrated into the forming die. The occurring local strain fields with a lateral extension down to 100 µm and high gradients require displacement measurements with high lateral resolution over a large range of local dislocations between 0.1 and > 10 µm. It is unknown, whether the speckle strain measuring method is able to provide the necessary low uncertainty for the required resolution in the whole measuring range. Therefore, theoretical and experimental investigations of the deformation measurability using the speckle correlation method are presented, showing that local displacements up to 10 µm can be measured with a spatial resolution between 3 and 10 µm depending on the displacement size. The dominant effect influencing the measurement uncertainty for displacements at this high spatial resolution is the speckle noise, resulting into measurement uncertainties of less than 100 nm. Hence, speckle photography is shown to be applicable for tensile test on micro samples.

A Model-Based Method for Predicting the Shapes of Planar Single-Segment Continuum Manipulators With Consideration of Friction and External Force

Abstract The shape prediction of tendon-driven continuum manipulators is a challenging problem due to the effect of inner friction and external force. Many researchers use actuation displacement or actuation force as model input to predict the shapes of manipulators, but very few consider their relations and models able to predict the status of friction. This paper proposes a model-based method that combines the mechanics model with the kinematic model to predict the shapes of planar single-segment manipulators with consideration of external force and friction. Finally, the shape prediction of manipulators is converted to an optimization problem with actuation displacement and actuation force as the inputs of our algorithm. The distribution of tendon force and the situation of friction can be calculated by using the feedback data of the actuation unit even when actuation direction changes and hysteresis occurs. Experimental results indicate that the method has good performance in predicting the manipulator’s shapes.

Varying mechanical compliance benefits energy efficiency of a knee joint actuator

In the field of wearable robots, actuator efficiency and user safety are frequently addressed by intentionally adding compliance to the actuation unit. However, the implications compliance has on the actuator’s overall performance in different conditions and activities are not fully understood, largely due to single task-focused experimental evaluations of these devices. To overcome this, our paper analyzes the effects that changing mechanical compliance has on the actuator’s overall performance in different ideal conditions in an experimental test setup. The torque performance and electrical energy consumption of an orthotic, adjustable-compliance knee joint actuator are evaluated during emulated walking and sit-to-stand-to-sit movements. Furthermore, the feasibility of combined operation of a dual mechanical compliance configuration during walking is investigated, and its outcomes reported in this work. The results demonstrate that varying mechanical compliance can lead up to 50% energy savings compared to a no-compliance configuration and show that, in general, changing compliance level leads to either energy-optimal or power-optimal actuator performance, but not both.