We developed, implemented, and assessed the performance of two forms of plug-in type repetitive controllers (RC) for enhancing the transparency of a lower extremity exoskeleton that operates to support walking function. One controller is a first order RC (SING) consisting of a single period matched to the self-selected cadence of the participant. The second is a novel `parallel' RC (PARA) which consists of a library of integrated RCs with varying periods, intended to accommodate a wider range of gait cycle times. We assessed the effects of both RCs under free cadence walking (FREE) and when walking with a metronome prescribing a consistent cadence matching the participants' self-selected value (METR). Both conditions were evaluated both at fixed speed (FIXTM) and under user-driven treadmill control (UDT), where the treadmill speed was regulated by the user's anterior/posterior position on the treadmill.The implementation of RC to the knee joint of the ALEX II exoskeleton lead to a significant reduction in torque error of 10-15\% at the knee joint during swing and smaller, non-significant effects at the hip joint. While the PARA RC reduced knee torque error more than the SING RC during the FREE cadence condition, a 15% reduction vs. 10% reduction, the difference between the two controllers was not statistically significant. During the UDT sections of walking conditions, participants increased GS under both the SING and PARA RC types. After controlling for the increase in torque error associated with speed, both the PARA and the SING controller reduced TE at the knee joint during swing relative to baseline by 13% and 14%, respectively, with no significant effects to the hip joint. Our work presents a novel formulation of RC and demonstrates the feasibility of applying RC to a robotic exoskeleton joint to assist walking. Future work should be geared toward improving the gait cycle prediction algorithm and developing robust methods for accounting for impact dynamics.

We sought to establish whether torque pulses applied by an exoskeleton to the hip and knee joint modulate propulsion mechanics and whether changes in propulsion mechanics would be sustained after exposure to torque pulses under user-driven treadmill control. We applied twelve different formulations of torque pulses consecutively over 300 strides to 24 healthy participants, and quantified the evolution of four outcome measures – gait speed (GS), hip extension (HE), trailing limb angle (TLA), normalized propulsive impulse (NPI) – before, during, and immediately after training. We tested whether the pulse conditions modulated propulsion mechanics during and after training relative to baseline. Metrics of propulsion mechanics significantly changed both during and after training. After training, HE, NPI, and GS significantly increased in eleven conditions, three conditions, and four conditions, respectively. Increases in HE during and after training were observed in conjunction with hip/knee flexion pulses during early stance, or hip/knee extension during late stance. Increases in NPI during training were associated with hip/knee extension during early stance, or knee flexion during late stance. Knee flexion during early stance resulted in positive after-effects in NPI. Increases in GS were associated with the application of hip flexion pulses. Conditions exhibiting the largest positive changes in HE, and not NPI, during training resulted in increased GS after training. Analysis of the relationship between the effects measured during and after training suggests that, when present, after-effects arise from retention of training effects, and that retention is specific to the component of propulsion mechanics affected by training.

We present the UDiffWrist (UDW), a low-impedance 2-DOF wrist exoskeleton featuring a cable-differential transmission. To investigate the effect of different design strategies for achieving kinematic compatibility, we developed two versions of this robot: One version (UDW-C) achieves kinematic compatibility only in the case of perfect alignment between human and robot joints. The second version (UDW-NC) connects the human and robot via passive joints to achieve kinematic compatibility regardless of alignment between human and robot joints. Through characterization experiments, we found that the UDW-NC was more robust to misalignments than the UDW-C: the increase in maximum interaction torque associated with misalignments was greater for the UDW-C than the UDW-NC robot (p = 0.003). However, the UDW-NC displayed greater Coulomb friction (p < 0.001). Further, Coulomb friction increased more for the UDW-NC than the UDW-C in the presence of misalignments between the human and robot axes (p < 0.001). We also found that torque transfer was more accurate in the UDW-C than in the UDW-NC. These results suggest that for the small (10 deg) 2-DOF wrist movements considered, the advantages of the UDW-NC in terms of kinematic compatibility are likely overshadowed by the negative effects in friction and torque transfer accuracy.