Compliant Robot Research Articles

A Learning-Based Approach to Sensorize Soft Robots.

Soft actuators and their sensors have always been separate entities with two distinct roles. The omnidirectional compliance of soft robots thus means that multiple sensors have to be used to sense different modalities in the respective planes of motion. With the recent emergence of self-sensing actuators, the two roles have gradually converged to simplify sensing requirements. Self-sensing typically involves embedding a conductive sensing element into the soft actuator and provides multiple state information along the continuum. However, most of these self-sensing actuators are fabricated through manual methods, which results in inconsistent sensing performance. Soft material compliance also imply that both actuator and sensor exhibit nonlinear behaviors during actuation, making sensing more complex. In this regard, machine learning has shown promise in characterizing the nonlinear behavior of soft sensors. Beyond characterization, we show that applying machine learning to soft actuators eliminates the need to implant a sensing element to achieve self-sensing. Fabrication is done using 3D printing, thus ensuring that sensing performance is consistent across the actuators. In addition, our proposed technique is able to estimate the bending curvature of a soft continuum actuator and the external forces applied to the tip of the actuator in real time. Our methodology is generalizable and aims to provide a novel way of multimodal sensing for soft robots across a variety of applications.

Design and implementation of fault-tolerant control strategies for a real underactuated manipulator robot

This paper presents the design and implementation of four control strategies applied to a real underactuated manipulator robot with 3-DOF (Degrees of Freedom). Additionally, an original methodology for controlled oscillatory compensations is designed and implemented to mitigate the effect of a passive joint on the overall performance of this manipulator robot. The objective of this methodology is to create controlled oscillations that allow the faulty link and its (passive) joint to physically align with their adjacent previous link. The implemented control techniques are sinh–cosh, neural compensation, gain scheduling PID, and gain scheduling sinh–cosh. The real robot in which these four control strategies and oscillatory compensation methodology are implemented is a SCARA (Selective Compliant Assembly Robot Arm) robot. To assess controller performance—once the 3-DOF underactuated manipulator robot starts its trajectory—after t = 4.5 s, a fault is activated in its joint No. 2, converting it into a passive joint. The performance indicators IA (index of agreement), RMS (Root Mean Square), and RSD (Residual Standard Deviation) are used to analyze, compare, and evaluate the behavior of the four control strategies and the compensation methodology.

Multifunctional Tactile Feedbacks Towards Compliant Robot Manipulations via 3D-Shaped Electronic Skin

Electronic skin (e-skin) has attracted considerable interests to endow robots the abundant perception abilities like human skin. However, multifunctional tactile feedbacks and poor wearability hinder the practical application towards compliant robot manipulations and friendly human-robot interaction (HRI). Here, a 3D-shaped e-skin integrated with hierarchically distributed flexible pressure, proximity, humidity sensors, and digital conversion circuits has been developed via the parting casting method. It can perfectly cover the non-developable and time-dynamic curved surface of a bionic hand and shows excellent wearability. The sensing data are collected by multichannel acquisition circuits and then sent to processing center to establish the mapping relations between multiple stimuli and movements of the bionic hand and arm. The multifunctional tactile feedbacks have been successfully demonstrated, which include non-contact obstacle avoidance, grasping force control, slip detection, and voice feedback. The e-skin shows great potential in the fields of HRI, artificial limbs, health care robots, and service robots.

UPed: A Quadruped Robot Platform to Study Directional Leg Compliance

Abstract This paper presents UPed, a quadruped robot platform, for studying passive, directional, leg compliance in robots. This work was motivated by the benefits provided by passively and directionally compliant legs in humans and animals. UPed uses under-actuated legs based on the bent-knee model which is capable of changing the direction and magnitude of the leg compliance by adjusting the knee angle and distal leg stiffness. Pneumatic pistons were chosen as variable compliance elements, enabling the leg compliance to be adjusted during locomotion. The platform was validated experimentally by evaluating how the robot’s pitch rate and cost of transport changed with variations in leg compliance and knee angle. Results of these experiments indicated that significant performance improvements are possible by selecting appropriate values of compliance and knee angle. They also demonstrated a unique capability to explore adjustable directional compliance and other parameters in robot legs on a physical robotic system. UPed has the potential to facilitate significant performance improvements of legged robots by enabling additional in-depth studies on directional passive leg compliance.

Preliminary Study of End-Effector Compliance for Reducing Insertion Force in Automated Fluid Coupling for Trains

Robotic assembly of mating parts (peg-in-hole (PiH)) inevitably encounters misalignments. Although passive end-effector compliance is key to successful alignment during the assembly, the literature does not propose many solutions for large misalignments, which is relevant to applications such as compliance of a robot end-effector for train fluid servicing. The results from physical experiments indicate insertion forces that are too large for practical applications, even with small misalignments. This preliminary study applies a hybrid approach combining physical experiments and simulation modelling for large motion PiH coupling with end-effector compliance. This provides a platform for investigating insertion force during misaligned coupling. The simulation model contains configurable parameters for robot compliance and PiH friction which are informed by the physical experiment results. The many robot compliances are lumped as two torsional springs on the pitch and yaw motion axis of the robot arm model. The simulation model is then calibrated using the physical results without having to conduct further intensive physical experiments. The calibrated model represents the physical measurements to a satisfactory degree, however its performance can still be improved.

Workspace Computation of Planar Continuum Parallel Robots

Continuum parallel robots ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">CPRs</i> ) comprise several flexible beams connected in parallel to an end-effector. They combine the inherent compliance of continuum robots with the high payload capacity of parallel robots. Workspace characterization is a crucial point in the performance evaluation of <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">CPRs</i> . In this paper, we propose a methodology for the workspace evaluation of planar continuum parallel robots ( <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">PCPRs</i> ), with focus on the constant-orientation workspace. An explorative algorithm, based on the iterative solution of the inverse geometrico-static problem is proposed for the workspace computation of a generic <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">PCPR</i> . Thanks to an energy-based modelling strategy, and derivative approximation by finite differences, we are able to apply the Kantorovich theorem to certify the existence, uniqueness, and convergence of the solution of the inverse geometrico-static problem at each step of the procedure. Three case studies are shown to demonstrate the effectiveness of the proposed approach.

Variable-Stiffness Control of a Dual-Segment Soft Robot Using Depth Vision

A soft-bodied robot exhibits prominent dexterity due to the soft nature of its material. However, the softness can become a burden when the robot needs to interact with the environment, given that the targeted object is usually much stiffer than the compliant soft robot. A variable-stiffness soft robot, fusing the merits of softness and stiffness, is in favor of many applications, such as robot-assisted minimally invasive surgeries. In this article, we propose a tendon-tensioning method to adaptively control the stiffness of a dual-segment tendon-driven backboneless soft robot based on depth vision. A depth-vision-based closed-loop controller is designed for stiffness compensation when the manipulator is subjected to the external load. Experiments were conducted to examine the feasibility and performance of the proposed method. The results confirm our control scheme on the robot with controllability of stiffness up to 132%. Based on our method, the manipulator with an external payload can follow designated trajectories with positioning errors reduced up to 50% comparing to that with open-loop control. Without quantifying the instantaneous stiffness, this work contributes a generalized method for tuning the stiffness of the tendon-driven soft robots in the presence of external disturbances without onboard sensing.

The Redesigned Serpens, a Low-Cost, Highly Compliant Snake Robot

The term perception-driven obstacle-aided locomotion (POAL) was proposed to describe locomotion in which a snake robot leverages a sensory-perceptual system to exploit the surrounding operational environment and to identify walls, obstacles, or other structures as a means of propulsion. To attain POAL from a control standpoint, the accurate identification of push-points and reliable determination of feasible contact reaction forces are required. This is difficult to achieve with rigidly actuated robots because of the lack of compliance. As a possible solution to this challenge, our research group recently presented Serpens, a low-cost, open-source, and highly compliant multi-purpose modular snake robot with a series elastic actuator (SEA). In this paper, we propose a new prototyping iteration for our snake robot to achieve a more dependable design. The following three contributions are outlined in this work as a whole: the remodelling of the elastic joint with the addition of a damper element; a refreshed design for the screw-less assembly mechanism that can now withstand higher transverse forces; the re-design of the joint module with an improved reorganisation of the internal hardware components to facilitate heat dissipation and to accommodate a larger battery with easier access. The Robot Operating System (ROS) serves as the foundation for the software architecture. The possibility of applying machine learning approaches is considered. The results of preliminary simulations are provided.

CoMet: Modeling Group Cohesion for Socially Compliant Robot Navigation in Crowded Scenes

We present CoMet, a novel approach for computing a group’s cohesion and using that to improve a robot’s navigation in crowded scenes. Our approach uses a novel cohesion-metric that builds on prior work in social psychology. We compute this metric by utilizing various visual features of pedestrians from an RGB-D camera on-board a robot. Specifically, we detect characteristics corresponding to the proximity between people, their relative walking speeds, the group size, and interactions between group members. We use our cohesion-metric to design and improve a navigation scheme that accounts for different levels of group cohesion while a robot moves through a crowd. We evaluate the precision and recall of our cohesion-metric on pedestrian datasets. We highlight the performance of our social navigation algorithm on a Turtlebot robot and demonstrate its benefits in terms of multiple metrics: freezing rate (57% decrease), deviation (35.7% decrease), and path length of the trajectory(23.2% decrease).

A Trajectory Tracking Control Based on a Terminal Sliding Mode for a Compliant Robot with Nonlinear Stiffness Joints.

A nonlinear stiffness actuator (NSA) can achieve high torque/force resolution in the low stiffness range and high bandwidth in the high stiffness range. However, for the NSA, due to the imperfect performance of the elastic mechanical component such as friction, hysteresis, and unmeasurable energy consumption caused by former factors, it is more difficult to achieve accurate position control compared to the rigid actuator. Moreover, for a compliant robot with multiple degree of freedoms (DOFs) driven by NSAs, the influence of every NSA on the trajectory of the end effector is different and even coupled. Therefore, it is a challenge to implement precise trajectory control on a robot driven by such NSAs. In this paper, a control algorithm based on the Terminal Sliding Mode (TSM) approach is proposed to control the end effector trajectory of the compliant robot with multiple DOFs driven by NSAs. This control algorithm reduces the coupling of the driving torque, and mitigates the influence of parametric variation. The closed-loop system’s finite time convergence and stability are mathematically established via the Lyapunov stability theory. Moreover, under the same experimental conditions, by the comparison between the Proportion Differentiation (PD) controller and the controller using TSM method, the algorithm’s efficacy is experimentally verified on the developed compliant robot. The results show that the trajectory tracking is more accurate for the controller using the TSM method compared to the PD controller.

Control of flexible knee joint exoskeleton robot based on dynamic model

SummaryThe knee joint plays a significant role in ground clearness, which is a crucial subtask of normal walking and avoiding falls. While post-stroke survivors are often faced with muscle weakness during walking, which leads to inadequate knee flexion. The lack of ground clearance caused by inadequate knee flexion will severely impede walking, increase metabolic exertion, and increase the risk of falls. A compliant exoskeleton robot possesses more favorable edges than other rigid ones in lightweight, safety, sense of comfort, and so on. We developed a new type of soft exoskeleton robot to assist the knee joint to achieve desired movements with Bowden cable transmitting force and torque. With the agonist–antagonist driving method, like a group of muscles working, we have explored dual-motors structure to realize the knee flexion function. It has built a standard dynamic model to analyze stability and realize the control law. We have conducted simulation and prototype experiments to verify the feasibility and usefulness of our method. The results show that the device can compensate for the lack of the knee joint driving force and realize the reference movement. Finally, we concluded that our method is a desirable way, and the scheme could improve the knee flexion ability and clearing ground.

Locally active globally stable dynamical systems: Theory, learning, and experiments

State-dependent dynamical systems (DSs) offer adaptivity, reactivity, and robustness to perturbations in motion planning and physical human–robot interaction tasks. Learning DS-based motion plans from non-linear reference trajectories is an active research area in robotics. Most approaches focus on learning DSs that can (i) accurately mimic the demonstrated motion, while (ii) ensuring convergence to the target, i.e., they are globally asymptotically (or exponentially) stable. When subject to perturbations, a compliant robot guided with a DS will continue following the next integral curves of the DS towards the target. If the task requires the robot to track a specific reference trajectory, this approach will fail. To alleviate this shortcoming, we propose the locally active globally stable DS (LAGS-DS), a novel DS formulation that provides both global convergence and stiffness-like symmetric attraction behaviors around a reference trajectory in regions of the state space where trajectory tracking is important. This allows for a unified approach towards motion and impedance encoding in a single DS-based motion model, i.e., stiffness is embedded in the DS. To learn LAGS-DS from demonstrations we propose a learning strategy based on Bayesian non-parametric Gaussian mixture models, Gaussian processes, and a sequence of constrained optimization problems that ensure estimation of stable DS parameters via Lyapunov theory. We experimentally validated LAGS-DS on writing tasks with a KUKA LWR 4+ arm and on navigation and co-manipulation tasks with iCub humanoid robots.

Crab-inspired compliant leg design method for adaptive locomotion of a multi-legged robot

Chinese mitten crab has unique limb structures composed of a hard exoskeleton and flexible muscles. They enable the crab to locomote adaptively and safely on various terrains. In this work, we investigated the limb structures, motion principle, and gaits of the crab using a high-speed camera and a press machine. Then, a novel compliant robot leg design method is proposed, inspired by the crab limb. The leg comprises six hard scleromeres and a flexible thin-wall spring steel sheet (FSSS) mimicking the exoskeleton and muscle. The scleromeres connected one by one with rotational joints are designed with slots. The front end of the FSSS is fixed on the scleromere close to the ground. The rear end crosses the slots and is mounted at the shaft of a linear actuator installed at the rear scleromere. The leg bends and stretches when the actuator pushes and pulls the FSSS, respectively. The kinematic modeling, rigid-flexible coupling dynamic simulations, and leg prototype tests are conducted, which verify the leg design approach. Thirdly, we put forward a multi-legged robot with eight compliant legs and design its gait using the gaits of the crab. Finally, the robot’s performance is evaluated, including the capabilities of walking on different terrains at adjustable speeds and body heights, traversing low channels, walking on slopes, and carrying loads. The results prove that the single-motor-actuated compliant legs and their dynamic coupling with the rigid robot body frame can enable them to have the ground clearance ability and realize the adaptive walking of the robot. The leg design methodology can be used to design multi-legged robots with the merits of compact, light, low mechanical complexity, high safety, and easy to control, for many applications, such as environmental monitoring, search and rescue.

Parametric joint compliance analysis of a 3-UPU parallel robot

Sufficiently low compliance is required for robot end-effectors to perform various tasks with high static and dynamic accuracy. The joint compliance and link compliance of robots are the two major factors for the determination of overall end-effector compliance. Unlike serial robots that can allow larger joint sizes to improve the joint compliance, the joints of parallel robots are usually closely located and hence the joint size cannot be easily increased. Compared with the link compliance, the joint compliance of a parallel robot contributes more to the end-effector compliance, but its importance is often overlooked. To reduce the computational intensity of robot compliance design, analytical joint compliance models are proposed for the class of 3-UPU parallel robots. These joint compliance models can be used to quickly evaluate the compliance contribution of each revolute joint at various parallel robot configurations. Numerical and experimental verifications are provided to demonstrate these models. We expect that the parametric analysis of 3-UPU parallel robot compliance can be facilitated by using the proposed joint compliance models.

The Architecture, Methodology and Implementation of STEP-NC Compliant Closed-Loop Robot Machining System

Industrial robots are gradually being employed in machining processes, particularly the cutting process, owing to their flexibility, mobility, and economic efficiency. However, it is difficult to make the manufacturing process intelligent owing to the complexity of robot machining process information handling and programming. In this paper, the architecture of a STEP-NC compliant closed-loop robot machining system was designed, including its function model and information stream. A methodology based on STEP-NC was established to enable the analysis of high-level information directly and automatically generating robot program according to the actual machining conditions. The STEP-NC Application Activity Model (AAM) and Application Reference Model (ARM) of closed-loop robot machining system is built to integrate the machining process data, monitoring and inspection data, mechanical equipment data, machining status data and inspection result data within a unified data flow, making it possible to realize intelligent manufacturing and adaptively adjusting the robot machining process. The proposed closed-loop robot machining system was implemented based on an open STEP-NC interpreter that interprets the high level information in STEP-NC directly to reduce machining robot programming time. An industrial camera was integrated with the robot for rawpiece positioning, then the STEP-NC interpreter can generate robot path rapidly according to the parameters of manufacturing features and position of rawpiece. The STEP-NC interpreter can generate a robot control program or communicate with a software controller using an application program interface, so it can be integrated with both existing industrial robot controllers and future open robot controllers. Finally, case studies are conducted for the functional verification of the proposed STEP-NC compliant closed-loop robot machining system.

Design and Hierarchical Force-Position Control of Redundant Pneumatic Muscles-Cable-Driven Ankle Rehabilitation Robot

Ankle dysfunction is common in the public following injuries, especially for stroke patients. Most of the current robotic ankle rehabilitation devices are driven by rigid actuators and have problems such as limited degrees of freedom, lack of safety and compliance, and poor flexibility. In this letter, we design a new type of compliant ankle rehabilitation robot redundantly driven by pneumatic muscles (PMs) and cables to provide full range of motion and torque ability for the human ankle with enhanced safety and adaptability, attributing to the PM's high power/mass ratio, good flexibility and lightweight advantages. The ankle joint can be compliantly driven by the robot with full three degrees of freedom to perform the dorsiflexion/plantarflexion, inversion/ eversion, and adduction/abduction training. In order to keep all PMs and cables in tension which is essential to ensure the robot's controllability and patient's safety, Karush-Kuhn-Tucker (KKT) theorem and analytic-iterative algorithm are utilized to realize a hierarchical force-position control (HFPC) scheme with optimal force distribution for the redundant compliant robot. Experiment results demonstrate that all PMs are kept in tension during the control while the position tracking accuracy of the robot is acceptable, which ensures controllability and stability throughout the compliant robot-assisted rehabilitation training.

Design and Characterization of a 3D-Printed Pneumatically-Driven Bistable Valve With Tunable Characteristics

Although research studies in pneumatic soft robots develop rapidly, most pneumatic actuators are still controlled by rigid valves and conventional electronics. The existence of these rigid, electronic components sacrifices the compliance and adaptability of soft robots.} Current electronics-free valve designs based on soft materials are facing challenges in behaviour consistency, design flexibility, and fabrication complexity. Taking advantages of soft material 3D printing, this paper presents a new design of a bi-stable pneumatic valve, which utilises two soft, pneumatically-driven, and symmetrically-oriented conical shells with structural bistability to stabilise and regulate the airflow. The critical pressure required to operate the valve can be adjusted by changing the design features of the soft bi-stable structure. Multi-material printing simplifies the valve fabrication, enhances the flexibility in design feature optimisations, and improves the system repeatability. In this work, both a theoretical model and physical experiments are introduced to examine the relationships between the critical operating pressure and the key design features. Results with valve characteristic tuning via material stiffness changing show better effectiveness compared to the change of geometry design features (demonstrated largest tunable critical pressure range from 15.3 to 65.2 kPa and fastest response time $\leq$ 1.8 s.

Design of a Fuzzy Fractional Order Adaptive Impedance Controller with Integer Order Approximation for Stable Robotic Contact Force Tracking in Uncertain Environment

Abstract Current research in robot compliance control is unable to take both transient contact force overshoots and steady-state force tracking problems into account. To address this problem, we propose a fuzzy fractional order (FO) adaptive impedance controller to avoid the force overshoots in the contact stage while keeping force error in the dynamic tracking stage, where traditional control algorithms are not competent. A percentage gain is adopted to map FO parameters to integer order (IO) parameters by their natural properties, and a fuzzy logical controller is introduced to improve the system stability. The simulation results indicate that the proposed controller can be made more stable than and superior to the general impedance controller, and the force tracking results also have been compared with the previous control methods.

Fin Ray Crossbeam Angles for Efficient Foot Design for Energy‐Efficient Robot Locomotion

Robot foot and gripper structures with compliancy using different mechanical solutions have been developed to enhance proper contact formations and gripping on various substrates. The Fin Ray structure is one of the solutions. Although the Fin Ray effect has been proposed and exploited, no detailed investigation has been conducted on the effect of different crossbeam angles inside its frame. Thus, herein, an integrative approach is used, combining 3D printing with soft material, finite element modeling, and neural control to 1) manufacture the Fin Ray structure with compliancy; 2) investigate the effect of different crossbeam angles under different loads and cylindrical substrates; and 3) finally apply it as an efficient compliant robot foot structure for energy‐efficient on‐pipe locomotion. Considering the factors of a large contact area, high energy efficiency, and better durability, the Fin Ray model with nonstandard 10°‐inclined crossbeams provides the best compromise in comparison with other models, within the constraints of the defined geometric parameters.

Fully-Printable Soft Actuator with Variable Stiffness by Phase Transition and Hydraulic Regulations

Actuators with variable stiffness have vast potential in the field of compliant robotics. Morphological shape changes in the actuators are possible, while they retain their structural strength. They can shift between a rigid load-carrying state and a soft flexible state in a short transition period. This work presents a hydraulically actuated soft actuator fabricated by a fully 3D printing of shape memory polymer (SMP). The actuator shows a stiffness of 519 mN/mm at 20 ∘C and 45 mN/mm at 50 ∘C at the same pressure (0.2 MPa). This actuator demonstrates a high stiffness variation of 474 mN/mm (10 times the baseline stiffness) for a temperature change of 30 ∘C and a large variation (≈1150%) in average stiffness. A combined variation of both temperature (20–50 ∘C) and pressure (0–0.2 MPa) displays a stiffness variation of 501 mN/mm. The pressure variation (0–0.2 MPa) in the actuator also shows a large variation in the output force (1.46 N) at 50 ∘C compared to the output force variation (0.16 N) at 20 ∘C. The pressure variation is further utilized for bending the actuator. Varying the pressure (0–0.2 MPa) at 20 ∘C displayed no bending in the actuator. In contrast, the same variation of pressure at 50 ∘C displayed a bending angle of 80∘. A combined variation of both temperature (20–50 ∘C) and pressure (0–0.2 MPa) shows the ability to bend 80∘. At the same time, an additional weight (300 g) suspended to the actuator could increase its bending capability to 160∘. We demonstrated a soft robotic gripper varying its stiffness to carry various objects.