No abstract is provided for this article.
No abstract is provided for this article.
Pneumatic soft robotic systems show remarkable potentials in producing versatile locomotion and manipulations, owing to their flexibility in structural design and material selections. However, fabrication of pneumatic soft robotic systems with complex 3D structures and material distribution still remains elusive. Herein, a mode‐free fabrication approach, called planar laser cutting and stacking fabrication (PLCSF), is proposed to create pneumatic soft robotic systems with multi‐material and complex structure, which involves the following steps: 1) slicing the 3D model of desired pneumatic soft robotic systems into 2D layers; 2) fabricating each layer via laser cutting corresponding 2D membrane; 3) stacking all layers together to finish the fabrication. With the PLCSF approach, various prevalent pneumatic soft actuators are fabricated with complex structures (including fiber‐reinforced and pneu‐net) and different actuation modes (such as bending, elongation, twisting, abduction, contraction, and grabbing). The scalability of the PLCSF approach to fabricate multiple degrees‐of‐freedom (DOFs) pneumatic soft actuators with integrated structures, such as twisting‐bending pneumatic soft actuators for delivering and bending‐elongation‐bending pneumatic soft actuators for crawling, is also demonstrated. It is further demonstrated that the PLCSF approach also enables creating a bio‐inspired soft hand with nine DOFs, capable of various dexterous motions and manipulations.
No abstract is provided for this article.
Dielectric elastomer actuators have shown promising applications in the field of soft robotics. However, due to their rate-dependent viscoelastic hysteresis nonlinearity, it is still challenging to achieve precision tracking control of dielectric elastomer actuators. In this letter, we propose a feedforward control approach that can compensate for the rate-dependent viscoelastic hysteresis nonlinearity with maximum tracking errors of 6.18% and root-meansquare errors of 2.96% when the frequency of the input voltage is between 0.05 Hz and 1.5 Hz. Our control approach consists of two feedforward compensators: 1) for the ease of the hysteresis compensation, a creep compensator is firstly developed to remove the viscoelastic creep nonlinearity; 2) based on a phenomenological mathematical model, an inverse hysteresis compensator is then developed to compensate for the rate-dependent viscoelastic hysteresis nonlinearity. Experimental results of tracking various periodic trajectories demonstrate that: the maximum tracking errors are reduced by 87.17% and the root-mean-square errors are decreased by 89.53%, by comparing the results without the viscoelastic compensation. It is the first time to successfully compensate for both the viscoelastic creep nonlinearity and rate-dependent hysteresis nonlinearity of dielectric elastomer actuators by a feedforward control approach, which may pave the way for further applications in dielectric-elastomer-actuators based soft robotics.
In this paper, we present the development and performance evaluation of dielectric-elastomer actuated XY (DEA-XY) stages with a millimeter range and submicrometer resolution. The design of the DEA-XY stage is based on a circular dielectric-elastomer actuator with two vertically distributed electrodes that can generate a two degree-of-freedom motion. To optimize their performances, various DEAXY stages with different dimensions and pre-stretches are firstly fabricated. Then, both static and dynamic performances of the DEA-XY stages are investigated by experiments, which demonstrate that: i) the workspace of the DEA-XY stage can reach 2.5 mm × 2.5 mm; ii) the cross-coupling effect is less than 6.5%; iii) the dynamic responses of the DEA-XY stage show both viscoelastic creep and hysteresis nonlinearities. To demonstrate the resolution of the DEA-XY stage, we implement a feedback controller to remove the creep effect. By tracking step trajectories, the results show that the DEA-XY stage can achieve the positioning resolution of 100 nm. This work demonstrates the potential application of the DEA-XY stage in the field of microscale manipulators.
This paper proposes a novel high-performance control scheme with hysteresis compensator and disturbance observer for high-precision motion control of a nanopositioning stage driven by a piezoelectric stack actuator (PSA). In the developed control scheme, a real-time inverse hysteresis compensator (IHC) with the modified Prandtl-Ishlinskii model is firstly designed to compensate for the asymmetric hysteresis nonlinearity of the PSA. Due to the imperfect compensation, the dynamics behaviors of the PSA-actuated stage with the IHC can be treated as a linear dynamic system plus a lumped disturbance term. Owing to the unknown nature of this lumped disturbance term, a disturbance observer (DOB) is used as a means for disturbance rejection. With the DOB, a tracking controller is finally designed and implemented to stabilize the position error. To verify the proposed control scheme, a real-time experimental platform with a PSA-actuated nanopositioning stage is built, and extensive experimental tests are performed. The comparative experimental results demonstrate the effectiveness and improved performance of the developed control approach in terms of the maximum-value errors, root-mean-square-value errors and hysteresis compensation.
Lattice metamaterials exhibit diverse functions and complex spatial deformations by rational structural design. Here, lattice metamaterials are exploited to design pneumatic soft robots with programmable bending, twisting, and elongation deformations. The system comprises an elastomeric tube reinforced by lattice metamaterials. We develop an analytical framework to model the twisting, bending, and elongation finite deformation taking into account the geometric orthotropy and nonlinear elasticity. We experimentally validate our modeling approach and investigate the effects of geometric patterns and input loading on the soft actuators' deformation. Theoretical guided design of lateral-climbing soft robots and exploration soft manipulators are demonstrated. The soft actuator could exhibit a combined twisting–bending–elongation deformation by lattice superimposition. The proposed structural design method paves the way for designing soft robots with complex and dexterous deformations.
In practice, the parameters of the flight controller of the quadrotors are commonly tuned experimentally with respect to a certain type of reference, such as the step reference and the unit-ramp reference. In this way, the performance of the flight controller might be affected by the variations of the references in real-time flights. Besides, real-time dynamic effects such as measure noises, external disturbances and input delays, which are usually neglected in the reported works, could easily deteriorate the performances of the flight controllers. This work is thereby motivated to develop a high-performance flight control approach utilizing a modified disturbance rejection technique for the quadrotors suffering from input delays and external disturbances. This control approach is developed in a cascaded structure and the attitude angles are chosen as the pseudo control inputs of the translational flight of the quadrotors. To facilitate the development, the dynamic model of the quadrotors is firstly formulated by including the effects of input delays, and the dynamics of the pseudo control variables are identified through real-time experiments. Based on the identified model, the flight control approach is proposed with a modified active disturbance rejection technique, which consists of a time optimal tracking differentiator, an extended state observer/predictor, and a nonlinear proportional–derivative controller. The tracking differentiator is designed to generate smooth transient profiles for the references, and the extended state observer/predictor is implemented for lumped disturbance estimation and state estimation considering the input delays. With the aid of the tracking differentiator and the extended state observer/predictor, the nonlinear proportional–derivative controller can thereby establish a fast tracking control and effectively reject the estimated disturbances. To verify the feasibilities of this development, comparative tests are carried out in both simulations and experiments. The results show that in the presence of small lumped disturbances, such as the measurement zero-drift, the steady-state errors of the proposed control approach for the ramp responses are less than 2 cm, and in the tests of sinusoidal trajectory tracking, the cross-tracking errors are less than 0.04 m. When with large disturbance airflow that is equivalent to strong breeze, the steady-state error achieved by the proposed flight controller is also less than 10 cm. All of these facts demonstrate the effectiveness of this development.
This paper presents the design, analysis, and testing of a parallel-kinematic high-bandwidth XY nanopositioning stage driven by piezoelectric stack actuators. The stage is designed with two kinematic chains. In each kinematic chain, the end-effector of the stage is connected to the base by two symmetrically distributed flexure modules, respectively. Each flexure module comprises a fixed-fixed beam and a parallelogram flexure serving as two orthogonal prismatic joints. With the purpose to achieve high resonance frequencies of the stage, a novel center-thickened beam which has large stiffness is proposed to act as the fixed-fixed beam. The center-thickened beam also contributes to reducing cross-coupling and restricting parasitic motion. To decouple the motion in two axes totally, a symmetric configuration is adopted for the parallelogram flexures. Based on the analytical models established in static and dynamic analysis, the dimensions of the stage are optimized in order to maximize the first resonance frequency. Then finite element analysis is utilized to validate the design and a prototype of the stage is fabricated for performance tests. According to the results of static and dynamic tests, the resonance frequencies of the developed stage are over 13.6 kHz and the workspace is 11.2 μm × 11.6 μm with the cross-coupling between two axes less than 0.52%. It is clearly demonstrated that the developed stage has high resonance frequencies, a relatively large travel range, and nearly decoupled performance between two axes. For high-speed tracking performance tests, an inversion-based feedforward controller is implemented for the stage to compensate for the positioning errors caused by mechanical vibration. The experimental results show that good tracking performance at high speed is achieved, which validates the effectiveness of the developed stage.
This paper presents a novel sliding mode control scheme for robust tracking control of a nanopositioning stage composed of piezoceramic stack actuators (PSAs) and compliant flexure mechanisms. The developed controller is distinguished by using a proportional-integral-derivative (PID) type sliding surface rather than the traditional proportionalderivative (PD) type sliding surface, which is more effective to reduce the tracking error. The stability of the developed control law is proved by Lyapnuov analysis. Finally, comparative studies are performed on a custom-built PSA-actuated stage to demonstrate the effectiveness of the developed controller. The major advantages of the developed control scheme lie in that: i) both the nominal system model and hysteresis model are not required in the control law; ii) it is ease of real-time implementation just like the PID control but with the improved tracking performance.
This paper presents a modified rate-dependent Prandtl–Ishlinskii (MRPI) model for the description and compensation of the rate-dependent asymmetric hysteresis in piezoelectric actuators. Different from the commonly used approach with dynamic weights or dynamic thresholds, the MRPI model is formulated by employing dynamic envelope functions into the play operators, while the weights and thresholds of the play operators are still static. By this way, the developed MRPI model has a relatively simple mathematic format with fewer parameters and easier parameter identification process. The benefit for the developed MRPI model also lies in the fact that the existing control approaches can be directly adopted with the MRPI model for hysteresis compensation in real-time applications. To validate the proposed model, an open-loop tracking controller and a closed-loop tracking controller are developed based on a dynamic hysteresis compensator, which is directly constructed with the MRPI model. Comparative experiments are carried out on a piezo-actuated nanopositioning stage. The experimental results demonstrate the effectiveness and superiority of the controllers based on the developed MRPI model compared to the controllers based on the rate-independent P–I model and the rate-dependent P–I model with dynamic weighting functions.
This paper proposes a new damping control approach with positive acceleration, velocity and position feedback (PAVPF) scheme for piezo-actuated nanopositioning stages to implement high-bandwidth operation. To achieve this objective, the intrinsic hysteresis nonlinearity of the piezoelectric actuator is firstly handled by a feedforward compensator with a modified Prandtl–Ishlinskii model. Afterwards, the PAVPF controller with the pole-placement method is implemented to suppress the lightly damped resonant mode of the hysteresis compensated system. With the PAVPF controller, the poles of the damped system in a third-model can be placed to arbitrary positions with an analytical method. Finally, for accurately tracking a predefined trajectory, a high-gain proportional-integral (PI) controller is designed, which could deal with the disturbance and the unmodeled dynamics. For verifying the proposed PAVPF-based control approach, comparative experiments with positive velocity and position feedback controller and with PI controller are conducted on a piezo-actuated nanopositioning stage. Experimental results demonstrate that the developed control approach with PAVPF controller is effective on damping control and improves the control bandwidth of the conventional PI controller from 111 Hz to 766 Hz, which leads to the significant increase of the tracking speed.
No abstract is provided for this article.
Dielectric elastomer actuators with a minimum energy structure (DEAs-MES) have been widely used in developing different soft robotics, owing to their large strain and simple structure. However, there is rare study on dynamic modeling of DEAs-MES because of both geometric and viscoelastic nonlinearities. In this work, we present a dynamic modeling approach for DEAs-MES by using an equivalent slider-crank mechanism, where geometric nonlinearity is simplified for calculating the stress distribution on DEAs-MES and the viscoelastic nonlinearity is represented by a series of viscoelastic units. In this sense, the Lagrange equation can be utilized to obtain the analytical dynamic model of DEAs-MES. The quantitative comparisons between experimental data and predicted results well demonstrate the effectiveness of the development, where the maximum root-mean-square errors are less than 10.78%. This work presents the early attempt to analytically characterize the dynamic response of DEAs-MES, which will be necessary for further dynamic-model based control design in the field of soft robotics.
