Admittance Control Scheme Research Articles

A Conformable Force/Tactile Skin for Physical Human–Robot Interaction

In this letter, a new sensorized flexible skin has been used to enhance safety and intuitiveness of physical human–robot interaction (HRI) in applications where both intentional and unintentional contacts may occur. The new technological contribution with respect to other skin sensors consists of the capability of measuring both the position of the contact point and the three components of the applied force with high repeatability and accuracy. To show how this innovative technology enables the exploitation of control laws for intuitive HRI, two standard control strategies have been implemented to perform both manual guidance with multiple contact points and safe reaction in case of unintentional collision detection, at the same time. In both cases, an admittance control scheme with a second order kinematic control is adopted. A multipriority redundancy resolution strategy is implemented in the case of manual guidance. The experimental verification of the sensor capabilities is made using a patch of the skin installed on a link of a KUKA LWR4 robot.

Control of Upper-Limb Power-Assist Exoskeleton Using a Human-Robot Interface Based on Motion Intention Recognition

Recognition of the wearer’s motion intention plays an important role in the study of power-assist robots. In this paper, an intention-guided control strategy is proposed and applied to an upper-limb power-assist exoskeleton. Meanwhile, a human-robot interface comprised of force-sensing resistors (FSRs) is designed to estimate the motion intention of the wearer's upper limb in real time. Moreover, a new concept called the “intentional reaching direction (IRD)” is proposed to quantitatively describe this intention. Both the state model and the observation model of IRD are obtained by studying the upper limb behavior modes and analyzing the relationship between the measured force signals and the motion intention. Based on these two models, the IRD can be inferred online using an adapted filtering technique. Guided by the inferred IRD, an admittance control strategy is deployed to control the motions of three DC motors placed at the corresponding joints of the robotic arm. The effectiveness of the proposed approaches is finally confirmed by experiments on a 3 degree-of-freedom (DOF) upper-limb robotic exoskeleton.

Operational Space Prescribed Tracking Performance and Compliance in Flexible Joint Robots

In this work, an admittance control scheme is proposed utilizing a highly robust prescribed performance position tracking controller for flexible joint robots which is designed at the operational space. The proposed control scheme achieves the desired impedance to the external contact force as well as superior position tracking in free motion without any robot model knowledge, as opposed to the torque based impedance controllers. Comparative simulation results on a three degrees-of-freedom (3DOF) flexible joint manipulator, illustrate the efficiency of the approach.

Posture estimation and human support using wearable sensors and walking-aid robot

In this paper, an omni-directional walking-aid robot is developed to assist the elderly in the daily living movements. A motion control strategy of walking-aid robot based on the observation of the human status through wearable sensors is proposed. During normal walking, the robot is controlled using a conventional admittance control scheme. When the tendency of a fall is detected, the robot will immediately react to prevent the user from falling down. The distance between the human Center of Pressure (COP) and the midpoint of the human feet is assumed to be a significant feature to detecting the fall events. When the user is in a quasi-static state or walking slowly, the COP can be approximated by the projection of Center of Gravity (COG) of the user’s body. A simple and low-cost wearable sensor system is proposed to measure online the COG of the user. A limitation of the proposed wearable sensor system is that the Head–Arms–Torso (HAT) of the user is assumed to be always in upright position, which may generate measurement error. From comparison experiments with a reference optical system it is found that the measurement error is acceptable especially at the early stage of fall event. Dubois possibility theory is applied to describe the membership function of “normal walking” state. A threshold based fall detection approach is obtained from online evaluation of the walking status. Finally, experiments demonstrate the validity of the proposed strategy.

Reaching for redundant arms with human-like motion and compliance properties

This work proposes a redundant arm torque controller for reaching, guaranteeing desired completion time and accuracy requirements without the need for trajectory planning and prior knowledge of robot dynamics. The proposed controller is designed based on the prescribed performance control methodology and it is a reaching regulator in which the target pose for the hand acts as an attractor for the arm. It provides configuration consistency in return motions and hand and joint velocity smoothness. Its use in an admittance control scheme given measurements or estimates of external forces is also proposed providing active compliance capabilities in robot–environment interactions. Simulation studies for a 5dof human arm-like robot and experiments with a 7dof arm are performed to verify the approach and demonstrate the proposed controller’s performance.

Virtual Coupling Based Control Methodology for Lower Extremity Exoskeleton

As one of the most important examples of human-orientated system, the exoskeleton can improve the strength and endurance of the wearer. The exoskeleton has multi-degrees of freedom. As a result, a stable controller for the exoskeleton is difficult to design. The adoption of a purely positional control strategy may lead to large contact forces. Hence, an admittance control strategy is devised aimed at limiting both internal and contact forces. To verify the rationality and feasibility of the control strategy, we introduce a method utilizing virtual prototype and collaborative simulation.

A New Intelligent Approach to Patient-cooperative Control of Rehabilitation Robots

This paper presents a new intelligent method to control rehabilitation robots by mainly considering reactions of patient instead of doing a repetitive preprogrammed movement. It generates a general reference trajectory based on different reactions of patient during therapy. Three main reactions has been identified and included in reference trajectory: small variations, force shocks in a single moment and variable level of participation. This reference trajectory can facilitate every kind of patient-cooperative controller of rehabilitation with an evaluation factor. A simple planar 2-DOF exoskeleton is used as rehabilitation robot for leg therapy. The low level controller used here is Brain Emotional Learning Based Intelligent Controller (BELBIC) which is participating to develop an admittance control scheme. Performance of this controller has compared to PID with simulations and better results have observed specially in control signal and speed of reaction when controller encounters a force shock.

Stable and Intuitive Control of an Intelligent Assist Device.

Safety and dependability are of the utmost importance for physical human-robot interaction due to the potential risks that a relatively powerful robot poses to human beings. From the control standpoint, it is possible to improve safety by guaranteeing that the robot will never exhibit any unstable behavior. However, stability is not the only concern in the design of a controller for such a robot. During human-robot interaction, the resulting cooperative motion should be truly intuitive and should not restrict in any way the human performance. For this purpose, we have designed a new variable admittance control law that guarantees the stability of the robot during constrained motion and also provides a very intuitive human interaction. The former characteristic is provided by the design of a stability observer while the latter is based on a variable admittance control scheme that uses the time derivative of the contact force to assess human intentions. The stability observer is based on a previously published stability investigation of cooperative motion which implies the knowledge of the interaction stiffness. A method to accurately estimate this stiffness online using the data coming from the encoder and from a multiaxis force sensor at the end effector is also provided. The stability and intuitivity of the control law are verified in a user study involving a cooperative drawing task with a 3 degree-of-freedom (dof) parallel robot as well as in experiments performed with a prototype of an industrial Intelligent Assist Device.

Unified Robot Control Scheme for Cooperative Motion, Autonomous Motion and Contact Reaction

While the majority of industrial manipulators currently in use only need to performautonomousmotion, future generations of cooperative robots will also have to execute cooperative motion and intelligently react to contacts. These extended behaviours are essential to enable safe and effective physical Human-Robot Interaction (pHRI). However, they will inevitably result in an increase of the controller complexity. This paper presents a single variable admittance control scheme that handles the three modes of operation, thereby minimizing the complexity of the controller. First, the adaptative admittance controller previously proposed by the authors for cooperative motion is recalled. Then, a novel implementation of variable admittance control for the generation of smooth autonomous motion including reaction to collisions anywhere on the robot is presented. Finally, it is shown how the control equations for these three modes of operation can be simply unified into a unique control scheme.

A Control Strategy for Upper Limb Robotic Rehabilitation With a Dual Robot System

This paper describes the development and use of the cooperative control scheme used by the intelligent pneumatic arm movement (iPAM) system to deliver safe, therapeutic treatment of the upper limb during voluntary reaching exercises. A set of clinical and engineering requirements for the control scheme are identified and detailed, which entail controlled, coordinated movement of a dual robot system with respect to the human upper limb. This is achieved by using a 6-DOF model of the upper limb that forms the controller's coordinate system. An admittance control scheme is developed by using this coordinate system such that robotic assistance can be varied as appropriate. Key controller components are derived, including kinematic and force transformations between the upper limb model and the dual robot task space. The controller is tested using a computational simulation and with a stroke subject in the iPAM system. The results demonstrate that the control scheme can reliably coordinate the dual robots to assist upper limb movements. A discussion considers the ramifications of using the system in practice, including the effects of measurement errors and controller limitations. In conclusion, the iPAM system has been shown to be effective at delivering variable levels of assistance to the upper limb joints during therapeutic movements in a clinically appropriate manner.

Manipulation of Thin Objects Using Levitation Techniques, Tilt Control, and Haptics

Levitation techniques allow delicate objects, such as silicon wafers, to be manipulated without contact. In case of electrostatic and magnetic levitation, the object can be levitated by actively controlling the electric/magnetic field based on the measured air gap between the object and the levitator. Multiple degrees of freedom are controlled by several electric/magnetic actuators. For thin flat objects, the surface area is large enough for suspending the object in vertical direction, but the side surface is too small to realize active control for lateral motions. For stability, the levitation system relies on a passive lateral restoring force that will keep the object aligned with the levitator. The drawback of this passive force is that it is far weaker than the controlled suspension force. As a result, allowable accelerations in the horizontal plane have to be limited in a manipulation task to prevent loosing the object during motion. In this paper, a solution is proposed based on compensating the acceleration by Tilt Control. For both an electrostatic and a magnetic levitation system, the significant improvement of tolerance to lateral acceleration due to feedforward tilt control is presented. It is shown that not only for automated motion but also for human operated motion, the tilt control strategy is applicable. In the human operated motion, haptic technology such as the admittance control scheme, are used to realize satisfactory manipulation.

ISoRA: Humanoid Robot Arm for Intelligent Haptic Interaction with the Environment

The paper concentrates on the development and control of the humanoid robot arm iSoRA, intended for operation in a dynamic unstructured environment. Optical torque sensors integrated into each joint enable measurement of contacting forces along the entire manipulator surface. A variable admittance control strategy was elaborated to increase the robot functionality and to achieve the human-like dynamics of interaction. The experimental results show that the proposed approach not only provides safe interaction of the robot arm with a person, but also improves the effectiveness of contact task performance. The paper also presents a novel concept of avoidance of an obstacle of unknown shape. The tactile sensory ability of the developed manipulator allows robot links to follow the object contour and to perform motion planning in the dynamic environment. The information on the applied normal force vector, object shape and target point coordinates is supplied to the motion planning system. The algorithms for contact point detection, object geometry recognition, and estimation of contacting object stiffness are detailed. The numerical simulation elicits a capability of the proposed method to approximate various object shapes precisely. The experimental results showed that the local admittance control and motion planner allowed the end-effector to follow the object contour in a very smooth, consistent manner while reaching the target point.

2P2-F10 Motion Control of Three-Wheeled Omnidirectional Intelligent Cane Robot

A three-wheeled omnidirectional cane robot is designed for aiding the elderly walking. Possible walking modes are analyzed and a corresponding hybrid model is constructed to describe walking behavior. A concept called intentional direction is presented to denote the moving intention of a human. Based on experiments and some assumptions, dynamic model of intentional direction is obtained as well as its online estimation method. A new admittance control scheme is presented based on intentional direction, which provides natural and intuitive human machine interface. Experiment results show the effectiveness of the design.

Robotic techniques for upper limb evaluation and rehabilitation of stroke patients

This paper presents two robot devices for use in the rehabilitation of upper limb movements and reports the quantitative parameters obtained to characterize the rate of improvement, thus allowing a precise monitoring of patient's recovery. A one degree of freedom (DoF) wrist manipulator and a two-DoF elbow-shoulder manipulator were designed using an admittance control strategy; if the patient could not move the handle, the devices completed the motor task. Two groups of chronic post-stroke patients (G1 n = 7, and G2 n = 9) were enrolled in a three week rehabilitation program including standard physical therapy (45 min daily) plus treatment by means of robot devices, respectively, for wrist and elbow-shoulder movements (40 min, twice daily). Both groups were evaluated by means of standard clinical assessment scales and a new robot measured evaluation metrics that included an active movement index quantifying the patient's ability to execute the assigned motor task without robot assistance, the mean velocity, and a movement accuracy index measuring the distance of the executed path from the theoretic one. After treatment, both groups improved their motor deficit and disability. In G1, there was a significant change in the clinical scale values (p < 0.05) and range of motion wrist extension (p < 0.02). G2 showed a significant change in clinical scales (p < 0.01), in strength (p < 0.05) and in the robot measured parameters (p < 0.01). The relationship between robot measured parameters and the clinical assessment scales showed a moderate and significant correlation (r > 0.53 p < 0.03). Our findings suggest that robot-aided neurorehabilitation may improve the motor outcome and disability of chronic post-stroke patients. The new robot measured parameters may provide useful information about the course of treatment and its effectiveness at discharge.

A control-system architecture for robots used to simulate dynamic force and moment interaction between humans and virtual objects

Haptic or kinesthetic feedback is essential in many important virtual reality and telepresence applications. Previous research focuses on simulating static forces such as those encountered when interacting with a stiff object such as a wall. Past studies usually employ custom-made devices that are not readily available to other researchers. Consequently, many of the results found in the haptic feedback literature cannot be replicated independently. With experimental results, the paper demonstrates that "off the shelf," general purpose robotics equipment can be incorporated into an effective haptic/kinesthetic feedback system. Such a system can accommodate a wide variety of virtual reality applications including training and telerobotics. An admittance control scheme is utilized, which enables the simulation of dynamic force and moment interaction as well as contact with stiff objects. The paper shows that the mechanical deficiencies (e.g., friction, inertia, and backlash) often associated with general purpose manipulators can be overcome with a suitable control system architecture.