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Open Access Publications from the University of California

Scholarly Works (24 results)

  • Thesis
  • Peer Reviewed
UCLA Electronic Theses and Dissertations (2019)

Every year there are about 800,000 new stroke patients in the US, and many of them suffer from upper limb neuromuscular disabilities including but not limited to: weakness, spasticity and abnormal synergy. Patients usually have the potential to rehabilitate (to some extent) based on neuroplasticity, and physical therapy intervention helps accelerate the recovery. However, many patients could not afford the expensive physical therapy after the onset of stroke, and miss the opportunity to get recovered. Robot-assisted rehabilitation thus might be the solution, with the following unparalleled advantages: (1) 24/7 capability of human arm gravity compensation; (2) multi-joint movement coordination/correction, which could not be easily done by human physical therapists; (3) dual-arm training, either coupled in joint space or task space; (4) quantitative platform for giving instructions, providing assistance, exerting resistance, and collecting real-time data in kinematics, dynamics and biomechanics; (5) potential training protocol personalization; etc.

However, in the rehabilitation robotics field, there are still many open problems. I am especially interested in: (1) compliant control, in high-dimensional multi-joint coordination condition; (2) assist-as-needed (AAN) control, in quantitative model-based approach and model-free approach; (3) dual-arm training, in both symmetric and asymmetric modes; (4) system integration, e.g., virtual reality (VR) serious games and graphical user interfaces (GUIs) design and development.

Our dual-arm/hand robotic exoskeleton system, EXO-UL8, is in its 4th generation, with seven (7) arm degrees-of-freedom (DOFs) and one (1) DOF hand gripper enabling hand opening and closing on each side. While developing features on this research platform, I contributed to the robotics research field in the following aspects:

(1) I designed and developed a series of eighteen (18) serious VR games and GUIs that could be used for interactive post-stroke rehabilitation training. The VR environment, together with the exoskeleton robot, provides patients and physical therapists a quantitative rehabilitation training platform with capability in real-time human performance data collection and analysis.

(2) To provide better compliant control, my colleagues and I proposed and implemented two new admittance controllers, based on the work done by previous research group alumni. Both the hyper parameter-based and Kalman Filter-based admittance controllers have satisfactory heuristic performance, and the latter is more promising in future adaptation. Unlike many other upper-limb exoskeletons, our current system utilizes force and torque (F/T) sensors and position encoders only, no surface electromyography (sEMG) signals are used. It brings convenience to practical use, as well as technical challenges.

(3) To provide better AAN control, which is still not well understood in the academia, I worked out a redundant version of modified dynamic manipulability ellipsoid (DME) model to propose an Arm Postural Stability Index (APSI) to quantify the difficulty heterogeneity of the 3D Cartesian workspace. The theoretical framework could be used to teach the exoskeleton where and when to provide assistance, and to guide the virtual reality where to add new minimal challenges to stroke patients. To the best of my knowledge, it is also for the first time that human arm redundancy resolution was investigated when arm gravity is considered.

(4) For the first time, my colleagues and I have done a pilot study on asymmetric dual-arm training using the exoskeleton system on one (1) post-stroke patient. The exoskeleton on the healthy side could trigger assistance for that on the affected side, and validates that the current mechanism/control is eligible for asymmetric dual-arm training.

(5) Other works of mine include: activities of daily living (ADLs) data visualization for VR game difficulty design; human arm synergy modeling; dual-arm manipulation taxonomy classification (on-going work).

    Cover page: Control and Dynamic Manipulability of a Dual-Arm/Hand Robotic Exoskeleton System (EXO-UL8) for Rehabilitation Training in Virtual Reality
    • Article
    • Peer Reviewed
    UCLA Previously Published Works (2017)
      Cover page: Modeling of Joint Synergy and Spasticity in Stroke Patients to Solve Arm Reach Tasks
      • Article
      • Peer Reviewed
      UCLA Previously Published Works (2021)
      A multi-degree-of-freedom (multi-DOF) exoskeleton relies on an array of sensors to communicate its state (e.g., positions/orientations) and operator-exoskeleton contact inter- actions (e.g., forces/torques) to its control system. Although sensor redundancy is common in biological systems to cope with uncertainty and partial failure of sensors, in man-made systems, sensor redundancy increases the overall system’s cost and control complexity. This study presents a sensor reduction technique for force/torque (F/T) sensors utilizing a Kalman filter- based sensor fusion system in the context of admittance control. The methodology is applied to the EXO-UL8 exoskeleton, which is a powered, redundant, dual-arm, upper-limb robotic system with (7 arm + 1 hand) DOFs incorporating three 6-axis F/T sensors in each arm. Motivated by improving wearability through minimizing human-exoskeleton contact interfaces, which reduces spurious contact forces due to joint misalignment; and reducing cost, the proposed strategy emulates the admittance controller’s virtual dynamics with only a subset of sensors, resulting in the physical human-robot interaction feeling the same from the operator’s perspective. Experimental results indicate that human-exoskeleton power exchange and actuation stresses of the operator’s joints, with the proposed strategy on a subset of two sensors, are comparable to those in the full three-sensor case (p < 0.01). The experiments verify the proposed methodology for the EXO-UL8, and support the feasibility of operating other Kalman filter-based sensor fusion systems with fewer sensors without sacrificing transparency in physical human-robot interaction.
        Cover page: Sensor Reduction, Estimation, and Control of an Upper-Limb Exoskeleton
        • Article
        • Peer Reviewed
        UCLA Previously Published Works (2018)
          Cover page: Asymmetric Dual Arm Approach for Post Stroke Recovery of Motor Functions Utilizing the EXO-UL8 Exoskeleton System: A Pilot Study
          • Article
          • Peer Reviewed
          UCLA Previously Published Works (2019)
          In physical rehabilitation, exoskeleton assistive devices aim to restore lost motor functions of a patient suffering from neuromuscular or musculoskeletal disorders. These assistive devices are classified as operating in one of two modes: (1) passive mode, in which the exoskeleton passively moves its joints through the full range (or a subset) of the patient's motion during engagement, or (2) assist-as-needed (AAN) mode, in which the exoskeleton provides assistance to the joints of the patient, either by initiating the movements or assisting the patient's movements to complete the task at hand. Achieving high physical human-robot interaction (pHRI) transparency is an open problem for multiple degrees-of-freedom (DOFs) redundant exoskeletons. Using the EXO-UL8 exoskeleton, this study compares two multi-joint admittance control schemes (hyper parameter- based, and Kalman Filter-based) with comfort optimization to improve human-exoskeleton transparency. The control schemes were tested by three healthy subjects who completed reaching tasks while assisted by the exoskeleton. Kinematic information in both joint and task space, as well as force- and torque-based power exchange between the human arm and exoskeleton, are collected and analyzed. The results show that the preliminary Kalman Filter-based control scheme matches the performance of the existing hyper parameter-based scheme, highlighting the potential of the Kalman Filter-based approach for additional performance.
            Cover page: Admittance Control Scheme Comparison of EXO-UL8: A Dual-Arm Exoskeleton Robotic System
            • Article
            • Peer Reviewed
            UCLA Previously Published Works (2017)
            Resistance training may be considered as one promising approach for improving the motor capabilities of post-stroke patients. A successful introduction of this depends on the proper resolution of human arm redundancy under gravitational loading. The spatially heterogeneous changes of the human arm swivel angle (which represents the upper limb redundancy) are studied under different loading conditions, the effects of which are incorporated into a modified dynamic manipulability ellipsoid model. A new scalar index describing the arm postural stability (APSI) is then proposed. As part of the experimental protocol, ten (10) healthy subjects performed multiple reaching tasks with different weights mounted on the forearm. Kinematic data was collected via a ten-camera motion capture system and the corresponding APSI was calculated for each task. APSI is found to have a strong linear correlation with the swivel angle under loading conditions. Furthermore, the data suggest that the swivel angle may serve as an indicator of arm postural stability and task difficulty. The results of additional experiments conducted with three (3) subjects indicate that the external loads could deteriorate the arm's control performance in tasks like line tracing (root mean square deviation from straight lines). These findings may be applicable to robot-based (exoskeleton) resistance therapy, assist-as-needed gravity compensation, and human-like motion control of humanoid robotic systems.
              Cover page: Upper Limb Redundancy Resolution Under Gravitational Loading Conditions: Arm Postural Stability Index Based on Dynamic Manipulability Analysis
              • Article
              • Peer Reviewed
              UCLA Previously Published Works (2019)
              In physical rehabilitation, exoskeleton assistive devices aim to restore lost motor functions of a patient suffering from neuromuscular or musculoskeletal disorders. These assistive devices are classified as operating in one of two modes: (1) passive mode, in which the exoskeleton passively moves its joints through the full range (or a subset) of the patient's motion during engagement, or (2) assist-as-needed (AAN) mode, in which the exoskeleton provides assistance to the joints of the patient, either by initiating the movements or assisting the patient's movements to complete the task at hand. Achieving high physical human-robot interaction (pHRI) transparency is an open problem for multiple degrees-of-freedom (DOFs) redundant exoskeletons. Using the EXO-UL8 exoskeleton, this study compares two multi-joint admittance control schemes (hyper parameter- based, and Kalman Filter-based) with comfort optimization to improve human-exoskeleton transparency. The control schemes were tested by three healthy subjects who completed reaching tasks while assisted by the exoskeleton. Kinematic information in both joint and task space, as well as force- and torque-based power exchange between the human arm and exoskeleton, are collected and analyzed. The results show that the preliminary Kalman Filter-based control scheme matches the performance of the existing hyper parameter-based scheme, highlighting the potential of the Kalman Filter-based approach for additional performance.
                Cover page: Admittance Control Scheme Comparison of EXO-UL8: A Dual-Arm Exoskeleton Robotic System
                • Article
                • Peer Reviewed
                UCLA Previously Published Works (2018)
                  Cover page: Design of a Hand Exoskeleton for Use with Upper Limb Exoskeletons
                  • Article
                  • Peer Reviewed
                  UC San Francisco Previously Published Works (2022)
                  Fibroblast growth factor 23 (FGF23) is a bone marrow cell produced hormone that functions in the intestine and kidney to regulate phosphate homeostasis. Increased serum FGF23 is a well-established predictor of mortality in renal disease, but recent findings linking increased levels to hepatic and cardiac diseases have suggested that other organs are sources of FGF23 or targets of its effects. The potential ability of the liver to produce FGF23 in response to hepatocellular injury was therefore examined. Very low levels of Fgf23 mRNA and FGF23 protein were detected in normal mouse liver, but the amounts increased markedly during acute liver injury from the hepatotoxin carbon tetrachloride. Serum levels of intact FGF23 were elevated during liver injury from carbon tetrachloride. Chronic liver injury induced by a high fat diet or elevated bile acids also increased hepatic FGF23 levels. Stimulation of toll-like receptor (TLR) 4-driven inflammation by gut-derived lipopolysaccharide (LPS) underlies many forms of liver injury, and LPS induced Fgf23 in the liver as well as in other organs. The LPS-inducible cytokines IL-1β and TNF increased hepatic Fgf23 expression as did a TLR2 agonist Pam2CSK3. Analysis of Fgf23 expression and FGF23 secretion in different hepatic cell types involved in liver injury identified the resident liver macrophage or Kupffer cell as a source of hepatic FGF23. LPS and cytokines selectively induced the hormone in these cells but not in hepatocytes or hepatic stellate cells. FGF23 failed to exert any autocrine effect on the inflammatory state of Kupffer cells but did trigger proinflammatory activation of hepatocytes. During liver injury inflammatory factors induce Kupffer cell production of FGF23 that may have a paracrine proinflammatory effect on hepatocytes. Liver-produced FGF23 may have systemic hormonal effects as well that influence diseases in in other organs.
                    Cover page: Mouse liver injury induces hepatic macrophage FGF23 production
                    • Article
                    • Peer Reviewed
                    UCLA Previously Published Works (2010)
                    Propionyl-coenzyme A carboxylase (PCC), a mitochondrial biotin-dependent enzyme, is essential for the catabolism of the amino acids Thr, Val, Ile and Met, cholesterol and fatty acids with an odd number of carbon atoms. Deficiencies in PCC activity in humans are linked to the disease propionic acidaemia, an autosomal recessive disorder that can be fatal in infants. The holoenzyme of PCC is an alpha(6)beta(6) dodecamer, with a molecular mass of 750 kDa. The alpha-subunit contains the biotin carboxylase (BC) and biotin carboxyl carrier protein (BCCP) domains, whereas the beta-subunit supplies the carboxyltransferase (CT) activity. Here we report the crystal structure at 3.2-A resolution of a bacterial PCC alpha(6)beta(6) holoenzyme as well as cryo-electron microscopy (cryo-EM) reconstruction at 15-A resolution demonstrating a similar structure for human PCC. The structure defines the overall architecture of PCC and reveals unexpectedly that the alpha-subunits are arranged as monomers in the holoenzyme, decorating a central beta(6) hexamer. A hitherto unrecognized domain in the alpha-subunit, formed by residues between the BC and BCCP domains, is crucial for interactions with the beta-subunit. We have named it the BT domain. The structure reveals for the first time the relative positions of the BC and CT active sites in the holoenzyme. They are separated by approximately 55 A, indicating that the entire BCCP domain must translocate during catalysis. The BCCP domain is located in the active site of the beta-subunit in the current structure, providing insight for its involvement in the CT reaction. The structural information establishes a molecular basis for understanding the large collection of disease-causing mutations in PCC and is relevant for the holoenzymes of other biotin-dependent carboxylases, including 3-methylcrotonyl-CoA carboxylase (MCC) and eukaryotic acetyl-CoA carboxylase (ACC).
                      Cover page: Crystal structure of the α6β6 holoenzyme of propionyl-coenzyme A carboxylase
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