E.H.F. van Asseldonk

Reference Trajectory Generation for Rehabilitation Robots: Complementary Limb Motion Estimation

For gait rehabilitation robots, an important question is how to ensure stable gait, while avoiding any interaction forces between robot and human in case the patient walks correctly. To achieve this, the definition of ldquocorrectrdquo gait needs to adapted both to the individual patient and to the situation. Recently, we proposed a method for online trajectory generation that can be applied for hemiparetic subjects. Desired states for one (disabled) leg are generated online based on the movements of the other (sound) leg. An instantaneous mapping between legs is performed by exploiting physiological interjoint couplings. This way, the patient generates the reference motion for the affected leg autonomously. The approach, called Complementary Limb Motion Estimation (CLME), is implemented on the LOPES gait rehabilitation robot and evaluated with healthy subjects in two different experiments. In a previously described study, subjects walk only with one leg, while the robots other leg acts as a fake prosthesis, to simulate complete loss of function in one leg. This study showed that CLME ensures stable gait. In a second study, to be presented in this paper, healthy subjects walk with both their own legs to assess the interference with self-determined walking. Evaluation criteria are: Power delivered to the joints by the robot, electromyography (EMG) distortions, and kinematic distortions, all compared to zero torque control, which is the baseline of minimum achievable interference. Results indicate that interference of the robot is lower with CLME than with a fixed reference trajectory, mainly in terms of lowered exchanged power and less alteration of EMG. This implies that subjects can walk more naturally with CLME, and they are assisted less by the robot when it is not needed. Future studies with patients are yet to show whether these properties of CLME transfer to the clinical domain.

Ambulatory Estimation of Center of Mass Displacement During Walking

The center of mass (CoM) and the center of pressure (CoP) are two variables that are crucial in assessing energy expenditure and stability of human walking. The purpose of this study is to estimate the CoM displacement continuously using an ambulatory measurement system. The measurement system consists of instrumented shoes with 6 DOF force/moment sensors beneath the heels and the fore-feet. Moreover, two inertial sensors are rigidly attached to the force/moment sensors for the estimation of position and orientation. The estimation of CoM displacement is achieved by fusing low-pass filtered CoP data with high-pass filtered double integrated CoM acceleration, both estimated using the instrumented shoes. Optimal cutoff frequencies for the low-pass and high-pass filters appeared to be 0.2 Hz for the horizontal direction and 0.5 Hz for the vertical direction. The CoM estimation using this ambulatory measurement system was compared to CoM estimation using an optical reference system based on the segmental kinematics method. The rms difference of each component of the CoM displacement averaged over a hundred trials obtained from seven stroke patients was (0.020 plusmn 0.007) m (mean plusmn standard deviation) for the forward x-direction, (0.013 plusmn 0.005) m for the lateral y-direction, and (0.007 plusmn 0.001) m for the upward z-direction. Based on the results presented in this study, it is concluded that the instrumented shoe concept allows accurate and continuous estimation of CoM displacement under ambulatory conditions.

Selective control of a subtask of walking in a robotic gait trainer(LOPES)

Robotic gait trainers are used all over the world for the rehabilitation of stroke patients, despite relatively little is known about how the robots should be controlled to achieve the optimal improvement. Most devices control complete joint trajectories and assume symmetry between both legs by either a position or an impedance control. However we believe that the control should not be on a joint level but on a subtask level (i.e. foot clearance, balance control). To this end we have chosen for virtual model control(VMC) to define a set of controllers that can assist in each of these tasks. Thus enabling the exoskeleton to offer selective support and evaluation of each substask during rehabilitation training. The aim of this explorative pilot study was to assess the performance of a VMC of the step height and to assess if selective control of the step height left the remaining of the walking pattern unaffected. Four young healthy subjects walked on a treadmill with their legs and pelvis attached to the lopes exoskeleton in 3 different conditions: (1) providing minimal resistance, (2) control of the left step height with a low stiffness (3) control of the step height with a large stiffness. We have shown that it is possible to exert a vertical forces for the support of foot clearance during the swing phase. The higher stiffness of the VMC resulted in a greater change of the step height, which was achieved by a larger increase of the maximal hip and knee flexion compared to the low stiffness condition. The control of the step height resulted in minor changes in the cycle time and swing time. The joint angles also showed only minor changes. The preliminary results suggest that we were able to control a subtask of walking, while leaving the remaining walking trajectory largely unaffected. In the near future, control of other subtask will be implemented and evaluated in isolation and in conjunction with each other.

Detecting asymmetries in balance control with system identification: first experimental results from Parkinson patients.

Summary.Cognitive processes can influence balance in various ways, but not all changes in postural performance can easily be identified with the naked clinical eye. Various studies have shown that dynamic posturography is able to detect more subtle changes in balance control. For patients with Parkinson’s disease (which is typically an asymmetric disease), changes in the symmetry of balance control might provide a sensitive measure of cognitive influences on balance. Here, we describe a new posturography technique that combines dynamic platform perturbations with system identification techniques to detect such asymmetries in balance control of two patients with Parkinson’s disease. Results were compared to those of six healthy controls. Our pilot data show clear asymmetries in dynamic balance control, even though patients themselves were not aware of this and had no subjective problems with stability or standing. We also found asymmetries in weight bearing, but the asymmetries in dynamic balance contribution were larger. Finally, asymmetries in weight bearing and dynamic balance in patients were not tightly coupled as in healthy controls. Future studies could incorporate this approach when examining the influence of mental decline on postural regulation.

Use of Inertial Sensors for Ambulatory Assessment of Center-of-Mass Displacements During Walking

Current methods for center-of-mass (CoM) estimation are restricted to gait laboratories. The aim of this study was to estimate CoM displacement under ambulatory conditions with inertial sensors. A sacral inertial sensor (SIS method) was used to estimate the CoM displacement by double integration of the acceleration. Overestimation of the displacement caused by pelvic rotations was compensated (CSIS method). The CoM displacement estimations using the (C)SIS method were compared to the conventional methods of the segmental analysis (SA) method and the sacral marker (SM) method by the intraclass correlations and the root-mean-square (RMS) differences between the CoM curves. Accurate ambulatory measurement of the CoM displacement using inertial sensors was possible. Estimations of the sacrum position using the SIS method and the SM method were similar with mean (SD) RMS differences of 3.23 (0.87), 2.96 (0.42), and 3.22 (0.78) mm for, respectively, the x-, y- and z-directions. The CoM estimation of the SIS method has RMS differences of 5.67 (1.20), 7.16 (3.28), and 3.49 (1.29) mm compared the SA method. The CSIS method shows a clear improvement in these estimations of the CoM with RMS differences of 5.52 (1.29), 4.44 (1.89), and 3.17 (1.41) mm and is generally applicable for healthy subjects.

Center of mass velocity-based predictions in balance recovery following pelvis perturbations during human walking

ABSTRACT In many simple walking models, foot placement dictates the center of pressure location and ground reaction force components, whereas humans can modulate these aspects after foot contact. Because of the differences, it is unclear to what extent predictions made by models are valid for human walking. Yet, both model simulations and human experimental data have previously indicated that the center of mass (COM) velocity plays an important role in regulating stable walking. Here, perturbed human walking was studied to determine the relationship of the horizontal COM velocity at heel strike and toe-off with the foot placement location relative to the COM, the forthcoming center of pressure location relative to the COM, and the ground reaction forces. Ten healthy subjects received mediolateral and anteroposterior pelvis perturbations of various magnitudes at toe-off, during 0.63 and 1.25 m s−1 treadmill walking. At heel strike after the perturbation, recovery from mediolateral perturbations involved mediolateral foot placement adjustments proportional to the mediolateral COM velocity. In contrast, for anteroposterior perturbations, no significant anteroposterior foot placement adjustment occurred at this heel strike. However, in both directions the COM velocity at heel strike related linearly to the center of pressure location at the subsequent toe-off. This relationship was affected by the walking speed and was, for the slow speed, in line with a COM velocity-based control strategy previously applied by others in a linear inverted pendulum model. Finally, changes in gait phase durations suggest that the timing of actions could play an important role during the perturbation recovery. Summary: By perturbing humans during walking, linear relationships are revealed between the resulting movement velocity of the body, and aspects of the ground reaction force.

Improving the transparency of a rehabilitation robot by exploiting the cyclic behaviour of walking

To promote active participation of neurological patients during robotic gait training, controllers, such as “assist as needed” or “cooperative control”, are suggested. Apart from providing support, these controllers also require that the robot should be capable of resembling natural, unsupported, walking. This means that they should have a transparent mode, where the interaction forces between the human and the robot are minimal. Traditional feedback-control algorithms do not exploit the cyclic nature of walking to improve the transparency of the robot. The purpose of this study was to improve the transparent mode of robotic devices, by developing two controllers that use the rhythmic behavior of gait. Both controllers use adaptive frequency oscillators and kernel-based non-linear filters. Kernel-based non-linear filters can be used to estimate signals and their time derivatives, as a function of the gait phase. The first controller learns the motor angle, associated with a certain joint angle pattern, and acts as a feed-forward controller to improve the torque tracking (including the zero-torque mode). The second controller learns the state of the mechanical system and compensates for the dynamical effects (e.g. the acceleration of robot masses). Both controllers have been tested separately and in combination on a small subject population. Using the feedforward controller resulted in an improved torque tracking of at least 52 percent at the hip joint, and 61 percent at the knee joint. When both controllers were active simultaneously, the interaction power between the robot and the human leg was reduced by at least 40 percent at the thigh, and 43 percent at the shank. These results indicate that: if a robotic task is cyclic, the torque tracking and transparency can be improved by exploiting the predictions of adaptive frequency oscillator and kernel-based nonlinear filters.

In vivo measurement of human knee and hip dynamics using MIMO system identification

This study presents a new method for the estimation of the dynamic impedance of multi-joint leg movements. The method is based on Multi Input Multi Output (MIMO) system identification techniques and is designed for continuous torque perturbations at the hip and knee joint. Preliminary results from this study indicate that MIMO system identification can successfully be used to estimate the hip and knee impedance and the interaction dynamics between both joints. It is also concluded that, in order to create a good model representation of the leg impedance, the effect of biarticular muscles needs to be taken into account. The obtained measures for joint impedance might be used for clinical assessment and follow up of patients, as well as for the development of supportive devices.

Evaluation of the effect on walking of balance-related degrees of freedom in a robotic gait training device

In the design of exoskeletons for gait rehabilitation, the choice of degrees of freedom (DoFs) is one of the main issues. The goal of this research is to evaluate the effect of availability of additional DoFs related to balance-keeping on the normality of walking. These additional DoFs are the horizontal translations of the pelvis and the frontal rotation of the hip. Measurements on the gait of ten healthy subjects showed that kinematics and EMG differ only slightly when these DoFs are blocked (and only the sagittal joint rotations are available), in the impedance-controlled LOPES exoskeleton. This shows that omitting the additional DoFs allows walking with close-to-normal motor control, and also that effects of waking in this robot per se overshadow the additional effects of the mentioned DoFs. All subjects however reported a more difficult and uncomfortable walking when the horizontal pelvis motions were blocked. An additional motivation for keeping the DoFs despite these results is that they allow implementation of balance training.

Velocity-dependent reference trajectory generation for the LOPES gait training robot

The aim of this study is to assess the feasibility of an approach for generating velocity-dependent trajectories to train neurologically injured patients. The reference trajectories are constructed based on the gait patterns of subjects walking on a treadmill. By extracting key events (parameters) from these trajectories, the velocity dependency of the parameters is determined by regression analysis. Then, splines are fitted through these points to obtain gait patterns (position, velocity and acceleration) for specific walking velocities. Considering the severely injured patients, a feedforward controller is used in addition to the impedance controller. The approach is implemented on the LOPES gait rehabilitation robot and evaluated on healthy subjects. Results indicate that the subjects can walk naturally in the robot with the constructed reference trajectories. Further improvements to the technical design and additional testing of healthy and impaired subjects are required to show whether this approach can be transferred to clinical domain.