Intrinsic Compliance Research Articles

A hybrid continuum robot based on pneumatic muscles with embedded elastic rods

Continuum robots have attracted increasing attention in recent years due to their intrinsic compliance and safety. Nevertheless, the use of structure compliance may lead to reduction of stiffness and positioning precision. This paper presents a novel design of a hybrid continuum robot whose actuators are composed of pneumatic muscles and embedded elastic rods. Such robot can switch drive modes between large-scale movement and fine adjustment of position by employing a locking mechanism to change its stiffness. A three-dimensional static model of the robot is presented using an improved Kirchhoff rod theory, where elastic deformation of the robot is accounted for from an optimal control point of view via minimal total potential energy principle. Experiments were carried out to validate the static model and to test the stiffness and precision of the robot. This work provides a possible way to strengthen the control precision of a continuum robot with compliant structure.

Topology Optimized Design, Fabrication, and Characterization of a Soft Cable-Driven Gripper

Soft-bodied robots, due to their intrinsic compliance, have shown great potential for operating within unstructured environment and interacting with unknown objects. This letter deals with automatic design and fabrication of soft robots. From a structure point of view, we synthesize a soft cable-driven gripper by recasting its mechanical design as a topology optimization problem. Building on previous work on compliant mechanism optimization, we model the interactions between the gripper and objects more practically, in form of pressure loadings and friction tractions, and furthermore, we investigate how the interaction uncertainties affect the optimization solution by varying the contact location and area. The optimized soft fingers were three-dimensionally printed and then assembled to build a gripper. The experiments show that the gripper can handle a large range of unknown objects of different shapes and weights (up to 1 kg), with different grasping modes. This letter represents an important step toward leveraging the full potential of the freeform design space to generate novel soft-bodied robots.

Exploiting Elastic Energy Storage for “Blind” Cyclic Manipulation: Modeling, Stability Analysis, Control, and Experiments for Dribbling

For creating robots that are capable of human-like performance in terms of speed, energetic properties, and robustness, intrinsic compliance is a promising design element. In this paper, we investigate the principle effects of elastic energy storage and release for basketball dribbling in terms of open-loop cycle stability. We base the analysis, which is performed for the 1-degree-of-freedom (DoF) case, on error propagation, peak power performance during hand contact, and robustness with respect to varying hand stiffness. As the ball can only be controlled during contact, an intrinsically elastic hand extends the contact time and improves the energetic characteristics of the process. To back up our basic insights, we extend the 1-DoF controller to 6-DoFs and show how passive compliance can be exploited for a 6-DoF cyclic ball dribbling task with a 7-DoF articulated Cartesian impedance controlled robot. As a human is able to dribble blindly, we decided to focus on the case of contact force sensing only, i.e., no visual information is necessary in our approach. We show via simulation and experiment that it is possible to achieve a stable dynamic cycle based on the 1-DoF analysis for the primary vertical axis together with control strategies for the secondary translations and rotations of the task. The scheme allows also the continuous tracking of a desired dribbling height and horizontal position. The approach is also used to hypothesize about human dribbling and is validated with captured data.

MIMO Sliding Mode Controller for Gait Exoskeleton Driven by Pneumatic Muscles

In the past decade, pneumatic muscle (PM) actuated rehabilitation robotic devices have been widely researched, mainly due to the actuators’ intrinsic compliance and high power to weight ratio. However, the PMs are highly nonlinear and subject to hysteresis behavior. Hence, robust trajectory and compliance control are important to realize different training strategies and modes for improving the effectiveness of the rehabilitation robots. This paper presents a multi-input–multioutput sliding mode controller, which is developed to simultaneously control the angular trajectory and compliance of the knee joint mechanism of a gait rehabilitation exoskeleton. Experimental results indicate good multivariable tracking performance of this controller, which provides a good foundation for the further development of assist-as-needed training strategies in gait rehabilitation.

Design of a Pneumatic Muscle Based Continuum Robot With Embedded Tendons

Continuum robots have attracted increasing focus in recent years due to their intrinsic compliance that allows for dexterous and safe movements. However, the inherent compliance in such systems reduces the structural stiffness, and therefore leads to the issue of reduced positioning accuracy. This paper presents the design of a continuum robot employing tendon embedded pneumatic muscles. The pneumatic muscles are used to achieve large-scale movements for preliminary positioning, while the tendons are used for fine adjustment of position. Such hybrid actuation offers the potential to improve the accuracy of the robotic system, while maintaining large displacement capabilities. A three-dimensional dynamic model of the robot is presented using a mass-damper-spring-based network, in which elastic deformation, actuating forces, and external forces are taken into account. The design and dynamic model of the robot are then validated experimentally with the help of an electromagnetic tracking system.

Design and control of a tendon-driven continuum robot

Continuum robots are suitable for operating in unstructured environments owing to their intrinsic compliance. This paper presents a novel tendon-driven continuum robot equipped with two modules and a compliant backbone formed by helical springs. Each module is driven by four parallel arranged tendons to implement a redundant actuation system that guarantees dexterous motions of the robot. A position feedback controller for the continuum robot is then developed, and a quadratic programming algorithm is incorporated into the controller to achieve a smooth configuration of the robot. Experiments results show that the control method has good trajectory tracking performance against external disturbances.

Grasping with a soft glove: intrinsic impedance control in pneumatic actuators.

The interaction of a robotic manipulator with unknown soft objects represents a significant challenge for traditional robotic platforms because of the difficulty in controlling the grasping force between a soft object and a stiff manipulator. Soft robotic actuators inspired by elephant trunks, octopus limbs and muscular hydrostats are suggestive of ways to overcome this fundamental difficulty. In particular, the large intrinsic compliance of soft manipulators such as 'pneu-nets'-pneumatically actuated elastomeric structures-makes them ideal for applications that require interactions with an uncertain mechanical and geometrical environment. Using a simple theoretical model, we show how the geometric and material nonlinearities inherent in the passive mechanical response of such devices can be used to grasp soft objects using force control, and stiff objects using position control, without any need for active sensing or feedback control. Our study is suggestive of a general principle for designing actuators with autonomous intrinsic impedance control.

Multi-modal control scheme for rehabilitation robotic exoskeletons

In the past few decades, a variety of rehabilitation robotic exoskeletons have been developed for patients with stroke and traumatic brain injury, which can assist therapists and potentially improve the functional outcomes. Many robotic exoskeleton systems adopted series elastic actuators, which are known to offer a number of advantages over rigid actuators, such as high force/torque fidelity, low output impedance, intrinsic compliance, and tolerance to shocks. While several control schemes have been proposed for robotic exoskeletons driven by series elastic actuators, existing methods are limited at high level, by assuming a perfect inner control loop. Due to the nonlinearity in robot dynamics, changing interaction forces, etc., the stability of the overall system consisting of both the robot dynamics and the actuator dynamics may not be guaranteed. This paper presents a multi-modal control scheme for rehabilitation robotic exoskeletons. Three control modes are smoothly integrated into the controller, where the robot-assisted mode allows the human to exert voluntary efforts within a desired region, the robot-dominant mode is activated to correct the movement of the human when the robot is outside the region, and the safety-stop mode is to stop the robot whenever the interaction force is too large which may result in injuries. By using the regional position and force feedback, the proposed controller achieves the paradigm of “assist-as-needed” and also guarantees the safety of the human. The development of the proposed controller follows the singular perturbation approach, and the stability of the overall system is rigorously proved by using Tikhonov’s theorem. Experimental results in both upper-limb and lower-limb exoskeleton systems are presented to demonstrate the effectiveness of the proposed control method.

Large-Deformation Model of a Soft-Bodied Esophageal Actuator Driven by Air Pressure

To study the effect of food flow rheology on the human esophageal swallowing process, a biologically inspired actuator prototype has been developed. The actuator is made of silicone rubber where smooth and continuous peristaltic motion is generated by inflating air chambers distributed within its body. The soft material gives the actuator intrinsic compliance and infinite degrees of freedom in motion. Thus, it is challenging to model the behavior online by current methods. In order to investigate the large-scale deformation of the soft-bodied actuator in response to the air chamber pressures, a geometrically simplified two-dimensional (2-D) model composed of separated beam-shaped elements is proposed. It considers the mechanical properties of the material, the sophisticated geometry of the actuator as it deforms, and the distributed pressure behavior. Empirical data from the actuator prototype (captured by articulography) and the simulated results are compared to investigate the accuracy of the model. The differences of deformations between the experimental results and the theoretical model are analyzed.

Design, modelling and simulation of soft grippers using new bimorph pneumatic bending actuators

AbstractSoft compliant grasping is essential in delicate manipulation tasks typically required in manufacturing and/or medical applications to prevent stress concentration at the point of contact. In comparison with their rigid counterparts, the intrinsic compliance of soft grippers offers simpler control and planning of the grasping action, especially where robots are faced with a number of objects varying in shape and size. However, quantitative analysis is rarely utilized in the design and fabrication of soft grippers, due to the fact that significant and complex deformation occurs once the soft gripper is in contact with external objects. In this paper, we demonstrate the design of a soft gripper using our novel bimorph-like pneumatic bending actuators. The gripper was modelled through finite element analysis to reflect its gripping capability during interaction with certain targeted objects. The proposed systematic design and analytical model was validated via experiments. The system’s gripping capab...

Experimental evaluation of a Dielectric Elastomer robotic arm for space applications

A growing interest within the space community focuses on robotics due to the large number of possible applications in many mission scenarios. On-Orbit Servicing (OOS) is arguably the most appealing implementation of space automatic systems. In several cases, OOS requires the capture of orbital objects, which is a complex and risky operation that can be successfully performed by robotic manipulators. Soft robotics, in particular, seems to be suitable for such applications given its intrinsic compliance to the operative environment. Devices based on Dielectric Elastomers (DE) can be employed for the implementation of soft robotic systems and showed promising performances. The introduction of DEs to orbital systems would represent a breakthrough in space technologies. In addition, space conditions could further advantage DE robotics, given the reduced environmental loads experienced and the longer times for operations. Nevertheless, Dielectric Elastomer Actuators (DEA) are a low-TRL (Technology Readiness Level) technology that needs to prove its maturity and suitability to space implementation. In this work, the performances of a redundant manipulator based on DEAs are presented in terms of numerical and experimental results. A 4-DoF planar manipulator has been tested in a gravity-compensated setup. The system is composed by two double-cone actuators mounted in series, each of them providing actuation of two DoF. The end-effector is an optical marker whose position is detected by a vision system. The system has a total of four joint DoF and operates in the xy horizontal plane; only the x and y positions of the end-effector are controlled. Two degrees of redundancy are obtained and exploited for the optimization of joint torques to avoid the saturation of actuators. Numerical simulations have been conducted to predict the system behaviour. The laboratory facility emulates the zero-gravity orbital environment by means of a suspending cable. Detailed experimental results are presented and exploited for the validation of control algorithm and numerical models.

Green tea extract catechin improves internal cardiac muscle relaxation in RCM mice.

BackgroundDiastolic dysfunction refers to an impaired relaxation and an abnormality in a heart’s filling during diastole while left ventricular systolic function is preserved. Diastolic dysfunction is commonly observed in patients with primary hypertension, diabetes and cardiomyopathies such as hypertrophic cardiomyopathy or restrictive cardiomyopathy. We have generated a restrictive cardiomyopathy (RCM) mouse model with troponin mutations in the heart to mimic the human RCM patients carrying the same mutations.ResultsIn the present study, we have investigated the ventricular muscle internal dynamics and pressure developed during systole and diastole by inserting a micro-catheter into the left ventricle of the RCM mice with or without treatment of desensitizer green tea extracts catechins. Our results demonstrate that green tea catechin is able to correct diastolic dysfunction in RCM mainly by improving ventricular compliance and reducing the internal muscle rigidity caused by myofibril hypersensitivity to Ca2+.ConclusionGreen tea extract catechin is effective in correcting diastolic dysfunction and improving ventricular muscle intrinsic compliance in RCM caused by troponin mutations.

Mechanical design and evaluation of a compact portable knee–ankle–foot robot for gait rehabilitation

This paper presents the mechanical design and evaluation of a knee–ankle–foot robot, which is compact, modular and portable for stroke patients to carry out overground gait training at outpatient and home settings. A novel compact series elastic actuator (SEA) is developed for safe human–robot interactions. As a solution to the limitation of conventional SEA designs, one low-stiffness translational spring and a high-stiffness torsion spring are placed in series for force transmission. The springs are selected based on gait biomechanics to maintain a high intrinsic compliance for most period of a gait cycle, while retaining the capacity to provide the peak force. To achieve portability, the robotic joint mechanism is optimized based on gait biomechanics, and the mechanical structure is built with lightweight materials. This robot demonstrates stable and accurate force control in experiments conducted on healthy subjects with overground walking. The activation level of the major leg muscles of subjects is reduced as indicated by the EMG signals and the normal gait pattern is maintained during the test, which demonstrates that the robot can provide effective assistive force to the subjects during overground walking.

Stabilization of bipedal walking based on compliance control

The embodiment of physical compliance in humanoid robots, inspired by biology, improves the robustness of locomotion in unknown environments. The mechanical implementation using elastic materials demands a further combination together with controlled compliance to make the intrinsic compliance more effective. We hereby present an active compliance control to stabilize the humanoid robots for standing and walking tasks. Our actively controlled compliance is achieved via admittance control using closed-loop feedback of the six axis force/torque sensors in the feet. The modeling and theoretical formulation are presented, followed by the simulation study. Further, the control algorithms were validated on a real humanoid robot COMAN with inherent compliance. A series of experimental comparisons were studied, including standing balancing against impacts, straight walking, and omni-directional walking, to demonstrate the necessity and the effectiveness of applying controlled compliance on the basis of physical elasticity to enhance compliant foot-ground interaction for the successful locomotion. All data from simulations and experiments related with the proposed controller and the performance are presented, analyzed, and discussed.

Human–Robot Interaction Control of Rehabilitation Robots With Series Elastic Actuators

Rehabilitation robots, by necessity, have direct physical interaction with humans. Physical interaction affects the controlled variables and may even cause system instability. Thus, human–robot interaction control design is critical in rehabilitation robotics research. This paper presents an interaction control strategy for a gait rehabilitation robot. The robot is driven by a novel compact series elastic actuator, which provides intrinsic compliance and backdrivablility for safe human–robot interaction. The control design is based on the actuator model with consideration of interaction dynamics. It consists mainly of human interaction compensation, friction compensation, and is enhanced with a disturbance observer. Such a control scheme enables the robot to achieve low output impedance when operating in human-in-charge mode and achieve accurate force tracking when operating in force control mode. Due to the direct physical interaction with humans, the controller design must also meet the stability requirement. A theoretical proof is provided to show the guaranteed stability of the closed-loop system under the proposed controller. The proposed design is verified with an ankle robot in walking experiments. The results can be readily extended to other rehabilitation and assistive robots driven with compliant actuators without much difficulty.

Non-back-drivable rotary mechanism with intrinsic compliance for robotic thumb abduction/adduction

Duplicating the complexity of the human thumb with a robotic one is a difficult task for engineers, when taking into consideration the necessary miniaturization and robustness requirements. Miniaturization is required to fit all the components within the size of a human hand; however, the reduction in size of the components affects the strength of the transmission. Several robotic hands with active thumbs were designed. Most of these had the abduction/adduction joint rigidly connected to the electrical motor and gearhead. This is a non-optimal solution because although miniaturized, the transmission is back-drivable and thus the motor gearhead is exposed to shocks. In this paper, we present the design of a non-back-drivable rotary mechanism with intrinsic compliance for a robotic thumb ab/adduction joint. The mechanism enables switch off of the power supply once a desired posture is stable, thus avoiding accidental dangerous releases of the grasped object, absorbs the impact forces generated at the instant of grasping and dissipates the repeated strains that are induced during manipulation. The mechanism was developed and tested. Detailed kinematic, static and stiffness analyses are presented as well as experimental measurements which demonstrate suitable performance for our application (max efficiency ~0.85, critical load 1.2 Nm, energy absorbed 1.5 J).

A Review of Fish Behavior and Hydrodynamic Aspects

The goal of this review is to highlight key studies that have contributed to a mechanistic and behavioural understanding of how perturbing flows affect fish. Most of our understanding of fish behaviour in turbulence comes from observations of natural conditions in the field and laboratory studies employing controlled perturbations, such as vortices generated in the wake behind simple geometric objects. Laboratory studies have employed motion analysis, flow visualization, electromyography, respirometry and sensory deprecation techniques to evaluate the mechanisms and physiological costs of swimming in altered flows. Studies show that flows which display chaotic and wide fluctuations in velocity can repel fishes, while flows that have a component of predictability can attract fishes. The ability to maintain stability in three-dimensional flows, either actively with powered movements or passively using the posture and intrinsic compliance of the body and fins, plays a large role in whether fish seek out or avoid turbulence.

Towards accurate robot-assisted neuroendoscopy using an ergonomic handling interface and a lightweight robot.

We considered a robot-assisted neuroendoscopy, and we developed a handling interface for linking a clinically-used endoscope to a lightweight robot (tool holder) with 7 DoFs. Such a robot holds potential for soft interaction with the surgeon, yet its intrinsic compliance must be suitably tamed not to lose tool targeting accuracy. Starting from practical specifications by neurosurgeons, we designed, fabricated and preliminarily assessed a compact and ergonomic handling interface. Such an interface permitted to easily insert/retract the tool (the measured force was 2 N), and to accurately hit a predefined target (the mean targeting error was below 0.5 mm, within the accuracy level of the optical tracker used for tool localization and pose). The feedback by neurosurgeons was very positive, thus encouraging further developments.

Pleated Pneumatic Artificial Muscle-Based Actuator System as a Torque Source for Compliant Lower Limb Exoskeletons

Powered lower limb exoskeletons require high-performance actuator systems, capable of producing zero to high-assistive torque and at the same time yielding to human interaction torques. Such variable impedance can be achieved by the means of compliant actuators. Because of their intrinsic compliance and high force-to-weight ratio pneumatic muscles are a viable option. However, previous pneumatic muscle powered exoskeleton designs either used them as a position source or failed to meet the high-dynamic torque requirements when using them as a torque source. This paper contributes to the improvement of pneumatic muscle-based actuator systems as a torque source for exoskeleton-type robots. The use of pleated pneumatic artificial muscles in a novel actuator system design allows for a higher torque range in a larger range of motion. Performance evaluation results are given for a 1 DOF test setup and a powered knee exoskeleton. The proposed torque controller achieves the dynamic torques required for zero to full assistance at moderate walking speeds.

A Portable Powered Knee-Ankle- Foot Orthosis1

A Portable Powered Knee-Ankle- Foot Orthosis1