Large Output Force Research Articles

Application of model predictive control scheme for piezoelectric ceramic micro-displacement

Abstract Currently, in the domain of micro-control technology, many operations need to meet the standard of nanometer-level positioning. As a high-precision micro-displacement element, piezoelectric ceramics are widely used in the field of micro-control due to their characteristics such as no need for transmission devices, high resolution, large output force, small size, and fast response speed. However, the inherent hysteresis nonlinearity of piezoelectric ceramics can lead to decreased displacement accuracy, oscillation, and even system instability. Therefore, eliminating the influence of nonlinear characteristics and improving displacement accuracy possess significant practical importance. In this paper, a model predictive control scheme is designed, which achieves faster response speed and higher accuracy compared to traditional control methods such as Proportional Integral Derivative control. Relevant simulations and tests have been conducted. In this scheme, hysteresis nonlinearity is first modeled and then compensated through feedforward, thereby simplifying the displacement control of piezoelectric ceramics into a more manageable linear problem. Subsequently, a linear model predictive control method is adopted to control the entire system. Both simulation and experimental results have verified the effectiveness of this control method.

A novel linear piezoelectric inchworm actuator based on a ratchet mechanism

Abstract A new working principle of inchworm actuator, which converts vibrations of a single piezo actuator into unidirectional step movement of a mover via a ratchet mechanism, was proposed. The proposed working principle has the following priorities: (1) it requires only one piezoelectric actuator which greatly simplifies its driving signals and driving circuits; (2) it can achieve a large driving speed with little compromise of a large force output while maintaining a high positioning precision of piezoelectric actuator and a theoretically unlimited motion range, although it can only achieve unidirectional movement with unidirectional self-locking capability; (3) it could be open-loop controlled with no accumulated step errors. The proposed actuator was designed with compliant mechanism and an analytical model of the design was developed, validated by finite element simulations carried out in Commercial Software ANSYS and used to guide the selection of design parameters. A prototype was fabricated and tested. Experiments show that the proposed actuator achieved a speed larger than 12 mm s−1, a driving load larger than 60 N in the moving direction, a reliable open-loop controllability with no step accumulated errors even under driving load variations of 60 N, and a working range larger than 1 mm with a high positioning precision around 320 nm under closed-loop control, which validated the superiorities of the proposed actuator.

Dielectric Elastomer Network with Large Side Groups Achieves Large Electroactive Deformation for Soft Robotic Grippers

AbstractDielectric elastomer actuators (DEAs) face an acknowledged challenge: On the one hand, the majority of elastomers only achieve small electroactive deformation (<20%) in the absence of prestretch; on the other hand, rare elastomers capable of showing large electroactive deformation require relatively complicated processing and chemistry. This work addresses this challenge by fabricating an elastomer with a network of large side groups, which achieves a very large electroactive deformation (218%) without pre‐stretch. This elastomer can be rapidly and massively fabricated within a few min, by polymerizing a commercial monomer with a large alkyl side group. The large side groups in the polymer network repel each other and extend the load‐bearing strands, which results in a pronounced strain‐hardening behavior. This behavior helps the elastomer to get rid of electromechanical instability during actuation and hence to exhibit a large electro‐active deformation, a high energy density (>> human muscle), and a large output force (≈500 times self‐weight). the elastomer capable of manufacturing a soft electroactive gripper is demonstrated with large deformation, large force, and rapid response, which enables grasping fragile objects of various complex shapes in an agile away.

A compound twisted and coiled actuators with payload-insensitive untwisting characteristics

Compound Twisted and Coiled Actuators (CTCAs) are promising candidates for artificial muscles due to their outstanding competences in producing large output forces and long strokes. However, as their thermal actuation performance depends on not only the driving temperature, but also the spandex deformation caused by the payload, it is rather difficult to obtain accurate actuation models for the conventional CTCAs and thus seriously restricts their control performance and application development. To tackle such difficulties, a CTCA with a novel strategic fabrication method is proposed in this work. Motivated by the kinetostatic performance of a stiffening spring with a high preload, the fiber draw ratio of the CTCA is significantly increased through pre-stretching the fiber during the fabrication process, which makes the thermal actuation performance of the CTCA insensitive to the external payload. The effectiveness of the proposed payload-insensitive characteristics of the CTCA is validated through experimental results, which shows that the CTCA has almost the same untwisting strain–temperature curves under different payloads. Consequently, a simplified yet accurate actuation model is obtained such that the displacement of the CTCA is mainly determined by the driving temperature within a meaningful range of payloads. Experiments are conducted to evaluate the accuracy of the actuation model and the model error is less than 7.6%. Such a simplified actuation model is implemented for the pose control of a 2-DOF CTCA-driven continuum robot joint module, in which the average steady-state error of the bending angle is less than 1.0∘.

Dielectric elastomers with large actuation strain and high output force

AbstractDielectric elastomer actuator (DEA) is considered as an ideal artificial muscle with soft and light in nature, widespread application in soft robotics and sensors. Dielectric elastomer (DE), the core component of DEA, with high output force and large actuation strain under relatively low driving electric field is critical for high‐performance DEA. However, the widely used VHB series (3M) DE suffered from very large pre‐strain and high driving voltage to get their high actuation performance, limiting their scope of application. In this work, the DEs with good mechanical and dielectric properties were prepared via UV curing method consisting of acrylic acid (AA) with strong polar groups as functional monomer, n‐butyl acrylate (BA) as soft monomer and urethane acrylate as cross‐linker (c‐p(BA‐AA)). The c‐p(BA‐AA) DE films obtained the simultaneous increasing of dielectric constant and breakdown strength with the increasing content of AA, which guaranteed the good actuation performance. The c‐p(BA‐AA) film exhibited excellent actuating performance (44.11% actuation strain at 45 kV/mm) without pre‐strain. Besides, a simple linear actuator based on the c‐p(BA‐AA) film could easily output weight that is tens of times of its own active weight (654.1 mN/g at 30 kV/mm).

Piston-like particle jamming for enhanced stiffness adjustment of soft robotic arm

PurposeStiffness adjusting ability is essential for soft robotic arms to perform complex tasks. A soft state enables dexterous operation and safe interaction, while a rigid state enables large force output or heavy weight carrying. However, making a compact integration of soft actuators with powerful stiffness adjusting mechanisms is challenging. This study aims to develop a piston-like particle jamming mechanism for enhanced stiffness adjustment of a soft robotic arm.Design/methodology/approachThe arm has two pairs of differential tendons for spatial bending, and a jamming core consists of four jamming units with particles sealed inside braided tubes for stiffness adjustment. The jamming core is pushed and pulled smoothly along the tendons by a piston, which is then driven by a motor and a ball screw mechanism.FindingsThe tip displacement of the arm under 150 N jamming force and no more than 0.3 kg load is minimal. The maximum stiffening ratio measured in the experiment under 150 N jamming force is up to 6–25 depends on the bending direction and added load of the arm, which is superior to most of the vacuum powered jamming method.Originality/valueThe proposed robotic arm makes an innovative compact integration of tendon-driven robotic arm and motor-driven piston-like particle jamming mechanism. The jamming force is much larger compared to conventional vacuum-powered systems and results in a superior stiffening ability.

Design and Force/Angle Independent Control of a Bionic Mechanical Ankle Based on an Artificial Muscle Matrix.

Inspired by the natural skeletal muscles, this paper presents a novel shape memory alloy-based artificial muscle matrix (AMM) with advantages of a large output force and displacement, flexibility, and compactness. According to the composition of the AMM, we propose a matrix control strategy to achieve independent control of the output force and displacement of the AMM. Based on the kinematics simulation and experiments, we obtained the output displacement and bearing capacity of the smart digital structure (SDS) and confirmed the effectiveness of the matrix control strategy to achieve force and displacement output independently and controllably. A bionic mechanical ankle actuated by AMM was proposed to demonstrate the actuating capability of the AMM. Experimental results show that the angle and force of the bionic mechanical ankle are output independently and have a significant gradient. In addition, by using a self-sensing method (resistance self-feedback) and PD control strategy, the output angle and force of the bionic mechanical ankle can be maintained for a long time without overheating of the AMM.

A load-adaptive hoisting mechanism based on spring-loaded rope and variable radius reel

This paper proposes a new type of the load-adaptive hoisting mechanism (LAHM), which can automatically adjust transmission ratio based on the payload. The load-adaptive hoisting mechanism can perform high speed under light load condition and it can perform large output force under heavy load condition. Because variable radius reel and spring-load rope are used, it can conduct smooth and continuous adaptive adjustment according to the difference of payload. After the theoretical analysis of the load-adaptive hoisting mechanism proposed in this paper, we discuss the working principle of the mechanism and establish the theoretical relationship model which contains key mechanical parameters of the system. In addition, through the contrast experiment with the traditional reel mechanism, the results prove the superiority and efficiency of this mechanism. Finally, the experiment of glass tube with an explosion is designed, which proves the feasibility and advantages of this mechanism and makes it possible to be used widely.

Design and Experiments of Electro-Hydrostatic Actuator for Wheel-Legged Robot with Fast Force Control Response

The wheel-legged robot combines the functions of wheeled vehicles and legged robots: high speed and high passability. However, the limited performance of existing joint actuators has always been the bottleneck in the actual applications of large wheel-legged robots. This paper proposed a highly integrated electro-hydrostatic actuator (EHA) to enable high-dynamic performance in giant wheel-legged robots (>200 kg). A prototype with a high force-to-weight ratio was developed by integrating a micropump, a miniature spring accumulator, and a micro-symmetrical cylinder. The prototype achieves a large output force of more than 9400 N and a high force-to-weight ratio of more than 2518 N/kg. Compared with existing EHA-based robots, it has a higher force-to-weight ratio and can bear larger loads. A detailed EHA model was presented, and controllers were designed based on sliding mode control and PID methods to control the output position and force of the piston. The model’s accuracy is improved by identifying uncertain parameters such as friction and leakage coefficient. Finally, both simulations and experiments were carried out. The results verified the fast response of force control (step response within 50 ms, the force tracking control frequency about 6.7 Hz) and the developed EHA’s good potential for future large wheel-legged robots.

Evolution from Telescoping to Bending: An Origami-Inspired Flexible Bending Actuator

Soft actuators have great potential in human–machine interaction and soft robotics innovation. Origami exhibiting outstanding structural and topological properties can be a paradigm for people to design various soft robots. Inspired by origami, we have previously designed a telescopic actuator with excellent performance, mainly large force output, and two-way working. Although significant advances have been made in soft bending actuators, their further study and applications are limited due to small force output in a monotonous work style. In this paper, we design a series of novel bending actuators that inherit our prior telescopic actuator’s excellent characteristics to diversify soft actuators’ motion forms. Several actuators of different sizes are fabricated using three different materials and evaluated on a designed test platform. The test results show that actuators of different sizes using different materials perform differently. Namely, the maximum tip force produced by an actuator reaches 9.6 N, and the maximum bending angle is achieved by another one up to 138°. Finally, extensive demonstrations and tests include wriggling, gripping, and bidirectional motion in the water. They show our flexible bending actuators’ distinguishing characteristics of large output force and two-way working.

The Development of Piezoelectric Inchworm Actuator Clamped With Magnetorheological Elastomer and Its Potential Application in Brain–Computer Interface Implantation

The piezoelectric inchworm actuator with high-positioning accuracy and large output force is a potential device to implant microelectrode in the invasive brain–computer interface (BCI). However, the traditional piezoelectric inchworm actuator adopts multichannel electrical control signals from external equipment and rigid-to-rigid friction contact to drive, resulting in complex control and uneven friction. To solve the above problems, in this article, magnetorheological elastomer (MRE) is first proposed as the clamping material of the piezoelectric inchworm actuator. An MRE–capillary–cover sandwich structure is adopted to achieve rigid-to-elastomeric clamping. The driving and clamping actions of the actuator are controlled by two cam mechanisms mounted on the same shaft, and a battery can feed the actuator. A kinetic model is established to analyze and predict the actuator motion. Several crucial clamping parameters are explored, and the performance experiments are conducted to evaluate actuator characteristics. A verification experiment is carried out to prove the feasibility of the microelectrode implantation in the pig brain (0.6% agarose gel). The actuator can reach the maximum velocity of 1250.4 <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">μ</i> m/s and the maximum output force of 420 mN; in the forward/backward motion, the minimum step is 0.5837/0.5912 <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">μ</i> m, and the repeatability is 0.0465/0.0469 <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">μ</i> m; thus, the actuator has great application potential in BCI.

An Origami-Inspired Negative Pressure Folding Actuator Coupling Hardness with Softness

Soft actuators have a high potential for the creative design of flexible robots and safe human–robot interaction. So far, significant progress has been made in soft actuators’ flexibility, deformation amplitude, and variable stiffness. However, there are still deficiencies in output force and force retention. This paper presents a new negative pressure-driven folding flexible actuator inspired by origami. First, we establish a theoretical model to predict such an actuator’s output force and displacement under given pressures. Next, five actuators are fabricated using three different materials and evaluated on a test platform. The test results reveal that one actuator generates a maximum pull force of 1125.9 N and the maximum push force of 818.2 N, and another outputs a full force reaching 600 times its weight. Finally, demonstrative experiments are conducted extensively, including stretching, contracting, clamping, single-arm power assistance, and underwater movement. They show our actuators’ performance and feature coupling hardness with softness, e.g., large force output, strong force retention, two-way working, and even muscle-like explosive strength gaining. The existing soft actuators desire these valuable properties.

Hybrid Adaptive Controller Design with Hysteresis Compensator for a Piezo-Actuated Stage

Piezo-actuated stage (P-AS) has become the topic of considerable interest in the realm of micro/nanopositioning technology in the recent years owing to its advantages, such as high positioning accuracy, high response speed, and large output force. However, rate-dependent (RD) hysteresis, which is an inherent nonlinear property of piezoelectric materials, considerably restricts the application of P-AS. In this research paper, we develop a Hammerstein model to depict the RD hysteresis of P-AS. An improved differential evolution algorithm and a least-squares algorithm are used to identify the static hysteresis sub-model and the dynamic linear sub-model for the Hammerstein model, respectively. Then, a hysteresis compensator based on the inverse Bouc–Wen model is designed to compensate for the static hysteresis of the P-AS. However, the inevitable modeling error presents a hurdle to the hysteresis compensation. In addition, external factors, such as environmental noise and measurement errors, affect the control accuracy. To overcome these complications, a hybrid adaptive control approach based on the hysteresis compensator is adopted to increase the control accuracy. The closed-loop system stability is analyzed with the help of the Lyapunov stability theory. Finally, experimental results indicate that the raised control approach is effective for the accurate positioning of P-AS.

Double-Acting Soft Actuator for Soft Robotic Hand: A Bellow Pumping and Contraction Approach.

When compressing a soft bellow, the bellow will contract and pump out the fluid inside the bellow. Utilizing this property, we propose a novel actuation method called compressing bellow actuation (CBA), which can output fluidic power and tendon-driven force simultaneously. Based on the CBA method, a double-acting soft actuator (DASA) combining fluidic elastomer actuator (FEA) and tendon-driven metacarpophalangeal (MCP) joint is proposed for robotic finger design. The proposed DASA exhibits both compliance and adaptiveness of FEAs, and controllability and large output force of the tendon-driven methods. The fluid in the bellow can be either air or water or even integration of the two, thus constituting three different actuation modes. Mathematical modeling of the relationship between bellow compression displacement and DASA’s bending angle is developed. Furthermore, experimental characterizations of DASA’s bending angle and blocking force are conducted at different actuation modes. The double-acting method can availably promote the bending angle of an FEA by up to 155%, and the blocking force by up to 132% when the FEA is water-filled. A soft robotic hand with a forearm prototype based on the DASA fingers is fabricated for the demonstration of finger motion and gripping applications.

Influence of Multidirectional Oscillations on Output Characteristics of Inertial Piezoelectric Platform

A piezoelectric platform driven by a multifunctional piezoelectric actuator is used to investigate the influence of multidirectional oscillations on the output characteristics of an inertial piezoelectric platform. Multidirectional oscillations can be generated by the vibration PZT (lead zirconate titanate piezoelectric ceramics) element of the actuator while the platform is driven by the actuation PZT element. The output characteristics are tested with the changing amplitude and frequency of multidirectional oscillations. An interesting phenomenon is observed when the amplitude or frequency is larger than specific values: The sticking and sliding displacements decrease quickly, and the step distance drops to zero rapidly; in other words, the oscillations determine the displacement response. The oscillation perpendicular to the contact plane is selected to switch friction and alleviate the inherent defects of the inertial platform. The response displacement is measured under a voltage of 600 V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p-p</sub> and a frequency of 1 Hz; the linear fitting is carried out, and the calculated correlation coefficient is improved from 0.987 to 0.995 by using friction switching; and a larger output force is also obtained. The results indicate that friction switching can be applied to obtain a high linearity displacement output and a large output force for the inertial platform.

A self-sensing soft pneumatic actuator with closed-Loop control for haptic feedback wearable devices

The intuitive and effective haptic feedback interaction is essential for human–machine interfaces. However, most current commercial haptic feedback solutions are rigid, simplex, and insufficient to fully immerse users in virtual and teleoperated environments. Here, we report the design, fabrication and performance of a low-cost and lightweight self-sensing actuator (SenAct) that provides accurate force and vibration feedback to recreate the tactile perception. The SenAct shows good performance such as fast response (10 ms), high robustness (>10,000 cycles), and large output force (up to 1.55 N) with high controllable resolution (up to 0.02 N) based on the real-time closed-loop control. It is capable of providing the corresponding haptic feedback to the wearer through external force or vibration signal input. With this prototype, we presented a haptic feedback glove that supports precise operation by enhancing immersion and comprehensive sensation. Furthermore, we demonstrated it could shape appropriate interaction in different scenarios, including touching or grasping objects in virtual reality and teleoperation, illustrating its potential applications in human-in-loop interaction system and metaverse.

A Novel Self-Actuated Linear Drive for Long-Range-of-Motion Electromechanical Systems

Obtaining powered linear movement over a long range of motion is a common yet challenging task, as the majority of linear actuators have limited ranges of motion as determined by their functioning mechanisms. In this paper, the authors present a novel belt-based self-actuated linear drive (B-SALD), in which a self-powered moving platform slides on a slotted track with essentially unlimited range of motion (only limited by the length of the track). Unlike the traditional rack-and-pinion mechanism, the B-SALD system uses a double-sided timing belt as the power-transmitting element. With the teeth on its inner surface, the belt interacts with a timing pulley for its own circulation; with the teeth on its outer surface, the belt interacts with a linear rail with parallel slots and drives the translation of the moving platform. The unique functioning mechanism generates multiple distinct advantages: no lubrication is required; the slotted track is simple and inexpensive to manufacture; and it provides an inherent compliance to buffer shock loading. With the experiments conducted on a preliminary prototype, it has been demonstrated that the B-SALD is able to provide accurate positioning and continuous motion control, an overall mechanical efficiency of 70% over the majority of the load range, and the capability of generating large force output in the desired manner.

A compact inchworm piezoelectric actuator with high speed: Design, modeling, and experimental evaluation

Inchworm piezoelectric actuators are usually composed of three or more piezoelectric stacks, which makes their structures and circuits sophisticated. Although a variety of inchworm piezoelectric actuators driven by fewer piezoelectric stacks have been proposed to realize the simplification of the traditional structures. It is still difficult to inherit the advantage of large output force and achieve high output speed without backward motion for a simplified inchworm actuator. Here, we present a compact inchworm piezoelectric actuator with two multi-function driving feet, in which only two piezoelectric stacks are utilized. The deformations of the driving feet are utilized efficiently by the proposed working principle, and four steps in each cycle are realized, which improve the output speed of the proposed actuator. The stiffness optimization is carried out to obtain a stable step characteristic and a high output force. Under a voltage of 150 V and a frequency of 50 Hz, the actuator achieved a maximum speed of 2081.16 μm/s, a carrying load larger than 6.442 kg and an output force of 14 N. Furthermore, this actuator also achieved a stable step process without backward motion under a carrying load of 6.442 kg.

Bioinspired Structures for Soft Actuators

AbstractBiological organisms present marvelous morphing behaviors from the quiescent blooming of flowers to the energetic wing‐flapping of birds that have always inspired humans to design better‐engineered products. The diversity of natural motion is attributed primarily to the intricate and hierarchical structure of actuators that are self‐assembled from nanoscale structures to superstructures. Compared to the biological actuators, their manmade counterparts, often with significantly limited capabilities, are fabricated from various materials with relatively simple structures using limited fabrication techniques. With the rapid developments in technologies that require soft robotics and human‐machine interfaces, there is increasing demand for soft actuators with improved capabilities such as larger output force, repeatability, and a more comprehensive range of motion. Biological actuators provide critical insights into the structure‐function relationship and offer exciting concepts to advance the science and technology of artificial soft actuators. Here, the design approaches found in natural actuation systems are discussed from the nanoscale to the highest levels in the structural hierarchy and the physical principles involved in their diverse actuation capabilities. In that context, finally, the fabrication techniques that have been utilized for manmade soft actuators, with a focus on the advantages, challenges, and concepts for potential future developments are reviewed.

A Force-Controlled Parallel Robot for Large-Range Stiffness Rendering in Three Dimensions

A haptic device is used to transmit impedance to a human user to mimic the impedance of a virtual or real environment. Existing haptic devices use serial or parallel robots to deliver impedance in multiple dimensions. These robots usually have nonconstant Jacobian matrices that result in poor dynamic properties and low impedance stability limits in certain regions within the workspace. To account for these regions, the range of stiffness rendering is limited. This letter presents a three-degrees-of-freedom (DoFs) translational parallel robot with a constant Jacobian matrix in the entire workspace. The consistent dynamic parameters allow a large-range virtual stiffness to be rendered. To provide the accurate and large output force required for high-stiffness rendering, series elastic actuators (SEAs) are used as the input for the parallel robot. SEAs can be used to minimize the geartrain friction and effective inertia to control the output force and impedance more accurately. Design, modeling, and three-dimensional impedance control of the haptic device are presented in this work. Multi-dimensional impedance and virtual-wall control experiments are illustrated to demonstrate the accuracy and rendering range of the haptic device. Since the stable range of virtual stiffness is much larger than existing ones, it is expected that this novel device can be used to render accurate stiffness for both soft and stiff environments.