Gravity Compensation Research Articles

A Multi-Objective Optimal Design Method for Gravity Compensators With Consideration of Minimizing Joint Reaction Forces

Abstract This paper presents a multi-objective optimal design method for gravity compensators with consideration of minimizing the joint reaction forces. High performance of the gravity compensation is achieved while the joint reaction forces are kept to a minimum. In this method, the ratio of the compensated torque to the uncompensated torque and the maximum value of the joint reaction forces are formulated as cost functions in the optimization problem, which is solved by adopting the Pareto front of multiple fitness functions with a genetic algorithm. This work takes a spring four-bar mechanism as a gravity compensator for a case study. The theoretical models of a gravity compensator and a robot manipulator show that the proposed multi-objective optimal design allows for the achievement of smaller joint reaction forces than the original single-objective optimal design, while their gravity compensation performances are relatively the same. Moreover, a prototype of a 0.2-kg gravity compensator realized from the proposed method was also built. An experimental study with this prototype showed that the measured motor torque was reduced by up to 93% within a range of 3π/4.

Design of a wearable shoulder exoskeleton robot with dual-purpose gravity compensation and a compliant misalignment compensation mechanism.

This paper presents the design and validation of a wearable shoulder exoskeleton robot intended to serve as a platform for assistive controllers that can mitigate the risk of musculoskeletal disorders seen in workers. The design features a four-bar mechanism that moves the exoskeleton's center of mass from the upper shoulders to the user's torso, dual-purpose gravity compensation mechanism located inside the four-bar's linkages that supports the full gravitational loading from the exoskeleton with partial user's arm weight compensation, and a novel 6 degree-of-freedom (DoF) compliant misalignment compensation mechanism located between the end effector and the user's arm to allow shoulder translation while maintaining control of the arm's direction. Simulations show the four-bar design lowers the center of mass by cm and the kinematic chain can follow the motion of common upper arm trajectories. Experimental tests show the gravity compensation mechanism compensates gravitational loading within Nm over the range of shoulder motion and the misalignment compensation mechanism has the desired 6 DoF stiffness characteristics and range of motion to adjust for shoulder center translation. Finally, a workspace admittance controller was implemented and evaluated showing the system is capable of accurately reproducing simulated impedance behavior with transparent low-impedance human operation.

Parallel Elastic Self-Alignment Mechanism Enhances Energy Efficiency and Reduces Misalignment in a Powered Knee Exoskeleton.

This paper aims to enhance exoskeleton compliance during locomotion assistance by reducing misalignment and to improve energy efficiency by overcoming the limitations posed by the bulky structure of powered rigid exoskeletons. A novel compliant knee exoskeleton, featuring a parallel elastic self-alignment mechanism, has been developed and structurally optimized. The exoskeleton uses adaptive oscillators to determine the wearer's gait phase and provides real-time assistance to the knee joint. Bench tests demonstrate that the parallel elastic mechanism significantly reduces the driving torque of the knee exoskeleton. Performance evaluations reveal that, compared to a commercial orthosis, the root-mean-square of knee angle error, joint misalignment, and unexpected interaction forces are reduced by 16.5 ± 11.3%, 23.3 ± 4.9%, and 17.7 ± 1.3%, respectively. Gait intervention experiments show reductions in average and maximum muscle activity of the knee joint by 7.6 ± 4.9% and 23.2 ± 5.7%, respectively. Additionally, the exoskeleton decreases negative work performed by the knee joint and the total lower limb by 22.7% and 8.6%, respectively. The parallel elastic self-alignment mechanism effectively mitigates joint misalignment, while the parallel springs offer partial gravity compensation, thereby enhancing both the energy efficiency and locomotion assistance of the exoskeleton. The parallel elastic self-alignment mechanism effectively addresses both misalignment and energy efficiency challenges in powered exoskeletons, providing valuable insights for future design improvements.

Single Spring Gravity Compensator for a Multi-DOF Manipulator

Single Spring Gravity Compensator for a Multi-DOF Manipulator

Iterative Learning for Gravity Compensation in Impedance Control

Iterative Learning for Gravity Compensation in Impedance Control

Helium pool boiling critical heat flux under various magnetically controlled gravity levels

The characterization of Critical Heat Flux (CHF) under magnetic compensation of gravity in liquid helium (LHe) is particularly important for the design of the future very high magnetic field superconducting magnets. Through a series of experimental sessions, the critical heat flux in LHe pool boiling has been determined under various gravity conditions ranging from 0.03g to 2.2g using magnetic forces. It is the largest range of gravity variation, from reduced to hyper-gravity, ever studied in a single facility so far. The study was conducted in a very high magnetic field up to 30 T. The magnetic compensation of gravity does not allow for perfect resulting gravity homogeneity. Nevertheless, for the first time, the magnetic field configuration is considered in the experimental results analysis. The relative resulting gravity inclination with respect to the heat exchange surface is also taken into account. The experimental results are compared with the well-known CHF correlation in LHe pool-boiling and show a very good behavior agreement. For our experimental gravity level, the CHF is ranging from 3000 W/m2 to 12000 W/m2.

Research on Trajectory-Tracking Control System of Tracked Wall-Climbing Robots.

Different from the vehicles and robots that move on the ground, complex and nonlinear track-wall interactions bring considerable difficulties to the accurate control of tracked wall-climbing robots due to the effect of gravity and adsorption. In this article, the authors propose a trajectory-tracking control system for tracked wall-climbing robots based on the fuzzy logic computed-torque control (FLCT) method. A key element in the proposed control strategy is to consider the adsorption force and gravity compensation based on the dynamic model. Validated via numerical simulations and experiments, the results show that the proposed controller can track the reference trajectory quickly, accurately and stably.

Application of the Kuka Kube Test-Bed for the Hardware-in-the-Loop Validation of the Space Manipulator Control System

Abstract The on-ground validation of control systems designed for manipulators working in orbit is very difficult due to the necessity of simulating the microgravity environment on Earth. In this paper, we present the possibilities of utilising the KUKA KUBE test-bed with industrial robots to experimentally verify space systems using hardware-in-the-loop tests. The fixed-base KUKA industrial robot is operated in gravitational environment, while the space system model plant is solved in real time parallel to on-ground experiment. The test-bed measurements are the input of the model plant, and the output of the model is treated as an input for the industrial robot actuation. In the performed experiment, the control system based on the Dynamic Jacobian is validated. The desired point that is reached by the manipulator's end-effector is constant in the simulated environment and moving with respect to the test-bed frame. The position of the space manipulator's end-effector is calculated by evaluating dynamics of the satellite in real-time model. The results show that the control system applied to the KUKA robot works correctly. The measurements from the torque sensors mounted in KUKA robot's joints are in accordance with the simulation results. This fact enhances the possibilities of gravity compensation, thus simulating microgravity environment on the test-bed.

Deployment dynamics of thick panel Miura-origami

Origami and kirigami folding methods applied to deployable structures have been investigated in many studies, and most of them use approximately zero-thickness materials to follow the paper-folding movement. However, the rigidity and thickness of materials cannot be neglected in aerospace engineering applications, resulting in different kinematic and dynamic characteristics of deployment. This work deals with the deployment dynamics of a typical thick panel Miura origami (Miura-ori). The kinematics of the Miura-ori unit and array are discussed considering the tessellated extension of the Miura-ori pattern, which can be described by the recursively relative movement. Based on the Lagrange's equation, the dynamic equation is determined by generalized force and potential energy. Furthermore, prototypes for Miura-ori unit and array are fabricated, respectively. To measure the dynamic behaviours during deployment, a gravity compensation apparatus is designed and constructed, which can effectively reduce the gravity effect of Miura-ori prototypes. The comparison of the numerical and experimental results validates our dynamic model. Our work provides a recursive dynamic modelling method and experiment apparatus for analysing and testing the deploying dynamic behaviours of tessellated thick-panel origami deployable structures, and the findings can be developed in the other thick-panel origami and kirigami concepts of actual engineering applications in the future.

A Three-link Gravity-balanced Mechanism for Operating Room Assistance

The compensation of gravity is not a new problem and gravity-balanced mechanisms have been extensively studied. Some of these studies present practical applications of the gravity-balanced mechanisms, while others focus on the theoretical modelling of the mechanism. In this study, a practical application, a gravity-balanced planar robotic manipulator for operating room assistance is presented. In this design, an auxiliary parallelogram with two linear springs is utilized to accomplish gravity compensation. As a passive gravity-balanced mechanism, it does not have any actuators. The novelty of the mechanism is the exchangeable end-effector and changeable spring attachment location accordingly. The simulation results show that the designed mechanism is gravity-balanced in all configurations. The presented mechanism in this study can also be extended to solve a multi-link gravity balance problem.

The PID in the Control of Bilateral Teleoperation System with Time-Varying Delays: An Experimental Approach

The control of bilateral teleoperation systems with time-varying delays is a challenging problem that is usually dealt with advanced control techniques. The well-known controllers, such as PD or PID, are scarcely employed and usually in conjunction with other approaches or at least with gravity compensation. In this contribution we intend to show, experimentally, that the standard PID controller exhibits good performance and robustness in position control for teleoperators with variable delays. The results obtained are compared with a well-known algorithm used for teleoperator control.

Folding, stowage, and deployment of composite thin-walled lenticular tubes

Composite thin-walled lenticular tube (CTLT) is a lightweight foldable and deployable structural material that enables large-scale deployable mechanisms for various space missions. A key step toward the structural design of CTLT is to understand its folding, stowage, and deployment behaviors. This work presents an integrated experimental and numerical investigation of the dynamic deployment behavior of CTLT that wraps around a central hub, with emphasis on the effect of long-term storage. A two-meter-long CTLT prototype was manufactured, and a gravity compensation system was designed and built for the on-ground dynamic deployment experiments. The deployment experiments were performed on the CTLT prototype both before and after it had been stowed for extended storage periods. The results indicate that after being stowed for 6 and 10.5 months the CTLT is deployed slower and the deployment time increases by 8.2% and 15.0%, respectively. Furthermore, a high-fidelity numerical model was constructed using the explicit dynamic finite element method, where the CTLT was modeled as a deformable part and the folding/deployment mechanisms were modeled as rigid bodies to perform the folding, stowage, and deployment simulations. The long-term storage effect was accounted for in the numerical analyses with the use of a viscoelastic fiber-reinforced composite material model, and a good agreement has been achieved between the experimental and numerical results.

Adaptive Gravity Compensation Framework Based on Human Upper Limb Model for Assistive Robotic Arm Extender.

The Assistive Robotic Arm Extender (ARAE) is an upper limb assistive and rehabilitation robot that belongs to the end-effector type, enabling it to assist patients with upper limb movement disorders in three-dimensional space. However, the problem of gravity compensation for the human upper limb with this type of robot is crucial, which directly affects the deployment of the robot in the assistive or rehabilitation field. This paper presents an adaptive gravity compensation framework that calculates the compensated force based on the estimated human posture in 3D space. First, we estimated the human arm joint angles in real-time without any wearable sensors, such as inertial measurement unit (IMU) or magnetic sensors, only through the kinematic data of the robot and established human model. The performance of the estimation method was evaluated through a motion capture system, which validated the accuracy of joint angle estimation. Second, the estimated human joint angles were input to the rigid link model to demonstrate the support force profile generated by the robot. The force profile showed that the support force provided by the developed ARAE robot could adaptively change with human arm postures in 3D space. The adaptive gravity compensation framework can improve the usability and feasibility of the 3D end-effector rehabilitation or assistive robot.

A backdrivable 6-dof parallel robot for sensorless dynamically interactive tasks

In order to combine low mechanical impedance and sufficient interaction forces, a six-degree-of-freedom robot featuring a mechanically backdrivable transmission with a low reduction ratio is proposed in this paper. Also, a parallel architecture is used in order to allow the use of relatively large and high-torque actuators. The architecture of the robot is first presented. Then, the geometry of the robot and the ratio of the transmissions are analysed in order to ensure that the selected design parameters yield the desired properties. Shock absorbers are also included in the legs of the robot so that impacts can be applied, e.g., for the assembly of some components. Then, algorithms are presented for Cartesian stiffness control and gravity compensation of the robot. Finally, a prototype is presented and validated experimentally for position control, impedance control and impact generation. The parallel robot presented in this paper is of particular interest for industrial assembly tasks.

Lightweight Force-controlled Device for Freehand Ultrasound Acquisition.

This study investigates a force-controlled auxiliary device for freehand ultrasound (US) examinations. The designed device allows sonographers to maintain a steady target pressure on the US probe, thereby improving the US image quality and reproducibility. The use of a screw motor to power the device and a Raspberry Pi as the system controller results in a lightweight and portable device, while a screen enhances user-interactivity. Using gravity compensation, error compensation, an adaptive proportional-integral-derivative algorithm, and low-pass signal filtering, the designed device provides highly accurate force control. Several experiments using the developed device, including clinical trials relating to the jugular and superficial femoral veins, validate its utility in ensuring the desired pressure in response to varying environments and prolonged US examinations, enabling low or high pressures to be maintained and lowering the threshold of clinical experience. Moreover, the experimental results show that the designed device effectively relieves the stress on the sonographer's hand joints during US examinations and enables rapid assessment of the tissue elasticity characteristics. With automatic pressure tracking between probe and patient, the proposed device offers potentially significant benefits for the reproducibility and stability of US images and the health of sonographers.

Robot-Assisted Gravity Offloading Testing of Aerospace Structures

Abstract The demand for testing aerospace structures on Earth before they are launched into space has led to the development of equipment that is able to simulate orbit conditions, namely zero gravity. Several passive solutions have been proposed to perform offloading testing on Earth. However, they present limitations and lack the flexibility normally required by the complexity of the pathways. Active zero gravity emulation systems have been developed to mitigate the difficulties of the passive ones. Moreover, the emergence of robotic arms with the ability to perform complex and easily reprogrammable motion and force-controlled trajectories has opened the possibility of creating robot-based gravity compensation systems. This paper proposes and evaluates a solution for the gravity offloading testing of space devices based on currently available industrial robots. This solution takes advantage of the functionalities of collaborative robots, namely built-in force controllers, together with custom auxiliary subsystems. A setup was arranged to allow the communication between the robot controller and a computer running an internet of things (IoT) platform based on Node-RED so as to connect and control all components of the offloading system. Multiple robot control techniques were designed and tested based on several approaches employing impedance control functionalities and sensing data to create a closed-loop system. The results obtained are within the validation criteria, creating conditions to affirm for the application in question that the gravity compensation was achieved with success using the robot.

Adaptive gravity-balancing mechanism using adjustable spring for a 6-DoF serial cooperative manipulator

Cumbersome high-power joint actuators are not suitable for lightweight cooperative manipulators, which require flexibility and high energy efficiency while maintaining moderate payload capacity. Thus, gravity compensation is proposed in this work to drive cooperative manipulators by small joint actuators. Most existing gravity compensation methods are focused on fixed load (i.e. dead-weight), while in this study, a variable-payload balanced 6-DoF (degrees of freedom) serial manipulator is realized by adaptive gravity-balancing mechanism. Since joint actuators can be small enough to withstand only joint friction and inertia, this manipulator is inherently safe for man-robot interaction. The adaptive gravity-balancing mechanism is composed of ZFL (zero-free length) springs with adjustable endpoints, which are actively adjusted by cable after payload changes. Three advantages of the proposed manipulator include: Firstly, balancing all joints affected by gravity by only one additional balancing motor; Secondly, the balancing mechanism can actively adapt to variable payloads; Finally, the adjustment of the balancing mechanism is independent of the manipulator’s pose. Through simulation, these three advantages have been verified, which provides a new scheme for the design of safe-interaction and energy-saving cooperative manipulators.

Hardware-in-the-loop based ground test system for space berthing and docking mechanism of small spacecraft

Ground test for space berthing and docking mechanism plays crucial roles in their stable operation in orbit. In this paper, a hardware-in-the-loop based ground test system is presented for economic reliability test for space berthing and docking mechanism of small spacecraft. In the system, the support and adapter unit is employed to fix the active part of the berthing and docking mechanism, and the end-effector of the manipulator connects the passive part. The manipulator is driven according to the relative motion calculated by the hardware-in-the-loop model after gravity compensation, to simulate motion of space berthing and docking mechanism in space. A Smith predictor is introduced for control system delays compensation. A berthing and docking mechanism was employed in the experiment to evaluate the performance of the test system. The results validated the effectiveness of the test system. Since only one manipulator is exploited in the test system, compared with existing systems using two manipulators, the system cost can be greatly decreased.

Novel ground microgravity experiment system for a spacecraft-manipulator system based on suspension and air-bearing

With the development of aerospace technology, the on-orbit activities of large spacecraft are increasing day by day, and accordingly the amount of space debris is increasing. Therefore, a large spacecraft-manipulator system is needed to perform on-orbit service, such as, assembling large spacecraft and removing space debris. Before launching, the trajectory optimization and control algorithm of the spacecraft-manipulator system have to be verified on the ground microgravity experiment system since the complex systems are prone to failure. In this study, a ground microgravity experiment system based on suspension and air-bearing is proposed to provide a long-term and stable microgravity environment for the large spacecraft-manipulator system. A suspension gravity compensation system with multiple suspensions is designed for the redundant space manipulator that moves in three dimensions, and a suspension tracking smooth sliding mode control algorithm based on hierarchical control is presented. Then, the coupling-reduction strategy based on equivalent spring-mass is presented to reduce the electromechanical coupling of the suspension gravity compensation system and the space manipulator. For the roll and pitch motion of the large-mass/inertia spacecraft, the target spacecraft simulator with a 2-degree-of-freedom rotating mechanism is designed. Some air-bearings are installed on the base of the chaser spacecraft simulator and target spacecraft simulator, which produce an air gap to enable the frictionless plane motion of the spacecraft. Finally, the target spacecraft simulator transposition experiment was carried out. The results show that the residual torque of the space manipulator is very small, and the ground microgravity experiment system compensates the gravity of the spacecraft-manipulator system with high precision.

Optimal phase-based gait guidance control on a lower-limb exoskeleton

Lower-limb exoskeletons, often used for assistance and rehabilitation, operate using a variety of control strategies, such as phase-based gait guidance controllers. Phase is a configuration-dependent scalar representing the progression of gait. In purely robotic systems, the definition of phase is largely arbitrary so long as it satisfies monotonic and diffeomorphic conditions. However, in applications involving an active user, the volitional inputs of the operator can alter system behavior. This can negatively influence the progression of phase throughout locomotion, impacting reference gait generation and user comfort. This paper investigates defining phase in a way that considers the influence of the user. Specifically, a series of walking experiments is conducted where recorded data from each experiment is used to optimally define phase for the subsequent experiment. In each iteration, phase is optimally defined to minimize its sensitivity to natural variability in gait. To evaluate whether the proposed method is able to improve phase progression and system behavior, three walking experiments were performed using a bilateral mixing gait guidance controller on an Indego Explorer exoskeleton with an able-bodied subject. The controller combines two single-support gait tracking controllers plus gravity compensation where the gravity compensation is proven to be preserved during double-support. In the final experiment, the root-mean-square error between the experimental and theoretical progression of phase was 0.0136 where phase nominally progresses monotonically from 0 to 1. For comparison, a walking experiment using the horizontal hip coordinate as phase, normalized from 0 to 1, resulted in a root-mean-square error of 0.0995. The progression of phase improved by 86% from the non-optimized to iteratively optimized definition. As a result, the commanded references tended to better resemble the desired nominal gait pattern. In conjunction with the gait guidance controller with optimized phase definition, an adaption law adjusted the reference gait based on walking speed estimated from foot displacement and timing. Experimental results show how the adaptive controller is able to predict walking speed and adjust the commanded reference accordingly.