Series Elastic Actuator-based exoskeleton can offer precise torque control and transparency when interacting with human wearers. Accurate control of SEA-produced torques ensures the wearer’s voluntary motion and supports the implementation of multiple assistive paradigms. In this paper, a novel variable transmission series elastic actuator (VTSEA) is developed to meet torque-speed requirements in different exoskeleton-assisted locomotion modes, such as running, walking, sit-to-stand, and stand-to-sit. The VTSEA features a SEA-coupled variable transmission ratio adjusting mechanism and works between three discrete levels of transmission ratio depending on the user’s initiative. The proposed prototype can also improve transparency in human-robot interaction. Also, an accurate torque controller with inertial compensation is developed for the VTSEA via the singular perturbation theory, and its stability is proved. The feasibility of the proposed VTSEA prototype and the precise output torque performance of VTSEA are verified by experiments.

Ground segmentation on the 3D point cloud is fundamental to many applications, such as SLAM and object segmentation. As it is usually a preprocessing module of these applications, high efficiency and accuracy are the basic requirements for guaranteeing the whole system’s performance. To this end, we avoid ground fitting and region division on the 3D point cloud. We propose a pixel-wise image-based method named DipG-Seg, which projects the 3D point cloud onto two cylindrical images, horizontal range-and z-images, then segments based on them. To realize fast and accurate ground segmentation, we first introduce innovative designs for image-based features. Specifically, we improve the slope feature with consideration of the LiDAR model and propose combining features with different sizes of receptive fields for better recognition of the ground. Then, based on these features, we devise a pre-segmenting pattern for pixel-wise classification. For fine segmentation, we devise a hierarchical refinement framework integrating a nonlinear filter and majority-vote kernel-based convolution, which is demonstrated to enhance the accuracy by over 7% on the basis of pre-segmenting. Comprehensive experiments were conducted on a real-world platform, SemanticKITTI, and nuScenes datasets. The results have demonstrated that our method can achieve an accuracy of 94.41% and a speed of 127 Hz on 64-beams LiDAR, outperforming the state-of-the-art methods and guaranteeing competitive robustness. Our method will be available at: https://github.com/EEPT-LAB/DipG-Seg.

Fault tolerance control to multiple motors has been studied under incomplete inverter topologies. An improved modulation scheme for driving multiple permanent magnet synchronous motors (PMSMs) by leg-missing inverter is proposed in this article. In the traditional scheme, the modulator is utilized in separate intervals for controlling one motor, while zero vectors are applied to other ones at the same time. This scheme cannot work well under high speed and heavy load conditions. To this end, an improved global modulator is designed to promote voltage utilization and operating region of the system on the premise of switching state coordination of the shared leg of the inverter. Various voltage vector (VV) distributions are discussed and detailed. The proposed strategy can realize a decent modulation effect without a complicated algorithm structure. Moreover, the effectiveness of the proposed modulation scheme is verified via a simulation test.

Precision in multi-torque-speed characteristics, disturbance resistance, transparency, and back-drivability are expected in exoskeleton actuators' design and control. The introduction of series elastic actuators (SEA) for lower-extremity exoskeletons has recently attracted more attention owing to their special merits compared to traditional actuators in high torque control accuracy, favorable output compliance, and unique shock tolerance. In this work, a SEA with a variable transmission ratio for the hip exoskeleton is developed for different working conditions requiring variable torque-speed characteristics. A crank-slide mechanism that actively changes the torque transmission path based on different tasks to meet the torque-speed requirements for the low-extremity exoskeleton. The high-precision torque delivery is the main concern in SEA control. Also, the crank-slider mechanism added in SEA will bring a new control difficulty with the frequent switching of the transmission ratio which may cause severe mechanical vibration and unsafe factors to users. Thus, a novel torque control architecture, based on disturbance observer integrated with a three-mass modeling method for VTSEA is proposed. Simulations are carried out to validate that the developed controller can restrain disturbance and provide accurate assistive torque tracking effectively. This work serves as a fundamental for employing and developing hip exoskeleton for locomotion assistance.

This paper provides a preliminary study of promoting the direct torque control (DTC) to series-end winding permanent magnet synchronous motor (SW-PMSM) drives. The DTC schemes of the conventional PMSM drives cannot be directly applied to the SW-PMSM drives since they have different drive topologies. With one more leg added to the inverter, the SW-PMSM drive has a different voltage vector distribution, which leads to a different switching table for the hysteresis controllers of the flux and the torque. In addition, the zero-sequence subspace also exists in the SW-PMSM drive, and it might generate the undesired zero-sequence current. Based on the above issues, the voltage vector distribution of the SW-PMSM drive is studied in this paper, and the voltage vectors without zero-sequence components are selected as the candidates to prevent generating zero-sequence current. Next, the switching table for the hysteresis controllers is re-derived according to the candidate voltage vectors, and the DTC for the SW-PMSM drive is obtained. Subsequently, the DTC of the multi-phase SW-PMSM drive is also investigated in the same manner. Finally, the effectiveness of the proposed DTC schemes for the SW-PMSM drives is verified.