Combining a Bio-Inspired Reflexive Neuromuscular Controller with a Trajectory Controller for Active Lower-Extremity Gait-Assistive Devices

Active gait-assistive devices have the potential to drastically increase quality of life for patients with various diseases affecting mobility, but more research in control methods is needed to create seamless interaction with patients. The golden standard of control for these devices, trajectory control, has the advantage of being simple and predictable but lacks the ability to react to changes in the environment or changes in patient movement. A reflex-based neuromuscular model shows interesting similarities with real human gait, and shows potential as a new control method for these devices. However, the reflex-based controller requires movement as input to output useful assistance and it can react unexpectedly when it is in a situation it was not optimized for. Therefore, both control types are combined to make use of the advantages of both. In this work a feasibility study is conducted with one spinal cord injury patient with full paraplegia of the legs with a trajectory controller implemented on hip and knee and a combined controller on the ankle joint. We found that the patient was able to walk semi-independently using this method and the patient indicated a preference for the combined method over the pure trajectory based controller. Overall, a novel method of control for prosthetic and orthotic devices is shown and implemented and its feasibility is demonstrated with a gait impaired SCI subject

Admittance control for physical human-robot interaction

This paper presents an overview of admittance control as a method of physical interaction control between machines and humans. We present an admittance controller framework and elaborate control scheme that can be used for controller design and development. Within this framework, we analyze the influence of feed-forward control, post-sensor inertia compensation, force signal filtering, additional phase lead on the motion reference, internal robot flexibility, which also relates to series elastic control, motion loop bandwidth and the addition of virtual damping on the stability, passivity and performance of minimal inertia rendering admittance control. We present seven design guidelines for achieving high performance admittance controlled devices that can render low inertia, while aspiring coupled stability and proper disturbance rejection

An improved force controller with low and passive apparent impedance for series elastic actuators

This article presents a force controller for series elastic actuators that are used in gait robots, such as exoskeletons, prostheses, and humanoid robots. Therefore, the controller needs to increase the bandwidth of the actuator, lower its apparent impedance for disturbance rejection or effortless interaction with a human user, and to stably interact with any (dynamic) environment. For gait, these environments are changing discontinuously, thus creating regular impacts. In this article, we propose the use of an inner-loop PD controller to increase the bandwidth of the actuator, alongside an outer-loop disturbance observer (DOB) to lower the apparent impedance of the actuator. To increase the controlled bandwidth of the actuator, we introduce a novel tuning method for the PD controller that allows for independent tuning of bandwidth and damping ratio of the controlled plant. The DOB, which is introduced to reject disturbances by lowering the apparent impedance, causes the apparent impedance to turn nonpassive, resulting in potential contact and coupled instability of the actuator. To enable unconditionally stable interactions with any environment, we scale down the DOB contribution such that it lowers the apparent impedance while remaining passive. The proposed tuning method and DOB adaptation were evaluated on a test setup by identifying the torque controller's transfer behavior and the apparent impedance of the actuator. The results of these tests showed that the proposed tuning method can separately tune bandwidth and damping ratio, whereas the DOB adaptation is able to tradeoff the reduction in the apparent impedance with its passivity

Differential Inverse Kinematics of a Redundant 4R Exoskeleton Shoulder Joint

Most active upper-extremity rehabilitation exoskeleton designs incorporate a 3R rotational shoulder joint with orthogonal axes. This kind of joint has poor conditioning close to singular configurations when all joint axes become coplanar, which reduces its effective range of motion. We investigate an alternative approach of using a redundant non-orthogonal 4R rotational shoulder joint. By inspecting the behavior of the possible nullspace motions, a new method is devised to resolve the redundancy in the differential inverse kinematics (IK) problem. A 1D nullspace global attraction method is used, instead of naive nullspace projection, to guarantee proper convergence. The design of the exoskeleton and the proposed IK method ensure good conditioning, avoid collisions with the human head, arm and trunk, can reach the entire human workspace, and outperforms conventional 3R orthogonal exoskeleton designs in terms of lower joint velocities and no body collisions

Comparison between sEMG and force as control interfaces to support planar arm movements in adults with Duchenne: a feasibility study

Contains fulltext : 176981.pdf (publisher's version ) (Open Access)BACKGROUND: Adults with Duchenne muscular dystrophy (DMD) can benefit from devices that actively support their arm function. A critical component of such devices is the control interface as it is responsible for the human-machine interaction. Our previous work indicated that surface electromyography (sEMG) and force-based control with active gravity and joint-stiffness compensation were feasible solutions for the support of elbow movements (one degree of freedom). In this paper, we extend the evaluation of sEMG- and force-based control interfaces to simultaneous and proportional control of planar arm movements (two degrees of freedom). METHODS: Three men with DMD (18-23 years-old) with different levels of arm function (i.e. Brooke scores of 4, 5 and 6) performed a series of line-tracing tasks over a tabletop surface using an experimental active arm support. The arm movements were controlled using three control methods: sEMG-based control, force-based control with stiffness compensation (FSC), and force-based control with no compensation (FNC). The movement performance was evaluated in terms of percentage of task completion, tracing error, smoothness and speed. RESULTS: For subject S1 (Brooke 4) FNC was the preferred method and performed better than FSC and sEMG. FNC was not usable for subject S2 (Brooke 5) and S3 (Brooke 6). Subject S2 presented significantly lower movement speed with sEMG than with FSC, yet he preferred sEMG since FSC was perceived to be too fatiguing. Subject S3 could not successfully use neither of the two force-based control methods, while with sEMG he could reach almost his entire workspace. CONCLUSIONS: Movement performance and subjective preference of the three control methods differed with the level of arm function of the participants. Our results indicate that all three control methods have to be considered in real applications, as they present complementary advantages and disadvantages. The fact that the two weaker subjects (S2 and S3) experienced the force-based control interfaces as fatiguing suggests that sEMG-based control interfaces could be a better solution for adults with DMD. Yet force-based control interfaces can be a better alternative for those cases in which voluntary forces are higher than the stiffness forces of the arms

Using position dependent damping forces around reaching targets for transporting heavy objects: A Fitts' law approach

Passive assistive devices that compensate gravity can reduce human effort during transportation of heavy objects. The additional reduction of inertial forces, which are still present during deceleration when using gravity compensation, could further increase movement performance in terms of accuracy and duration. This study investigated whether position dependent damping forces (PDD) around targets could assist during planar reaching movements. The PDD damping coefficient value increased linearly from 0 Ns/m to 200 Ns/m over 18 cm (long PDD) or 9 cm (short PDD). Movement performance of reaching with both PDDs was compared against damping free baseline conditions and against constant damping (40 Ns/m). Using a Fitts' like experiment design 18 subjects performed a series of reaching movements with index of difficulty: 3.5, 4.5 and 5.5 bits, and distances 18, 23 and 28 cm for all conditions. Results show that PPD reduced (compared to baseline and constant damping) movement times by more than 30% and reduced the number of target reentries, i.e. increasing reaching accuracy, by a factor of 4. Results were inconclusive about whether the long or short PDD conditions achieved better task performance, although mean human acceleration forces were higher for the short PDD, hinting at marginally faster movements. Overall, PDD is a useful haptic force to get humans to decrease their reaching movement times while increasing their targeting accuracy