Tele-impedance based assistive control for a compliant knee exoskeleton

This paper presents a tele-impedance based assistive control scheme for a knee exoskeleton device. The proposed controller captures the user’s intent to generate task-related assistive torques by means of the exoskeleton in different phases of the subject’s normal activity. To do so, a detailed musculoskeletal model of the human knee is developed and experimentally calibrated to best match the user’s kinematic and dynamic behavior. Three dominant antagonistic muscle pairs are used in our model, in which electromyography (EMG) signals are acquired, processed and used for the estimation of the knee joint torque, trajectory and the stiffness trend, in real time. The estimated stiffness trend is then scaled and mapped to a task-related stiffness interval to agree with the desired degree of assistance. The desired stiffness and equilibrium trajectories are then tracked by the exoskeleton’s impedance controller. As a consequence, while minimum muscular activity corresponds to low stiffness, i.e. highly transparent motion, higher co-contractions result in a stiffer joint and a greater level of assistance. To evaluate the robustness of the proposed technique, a study of the dynamics of the human–exoskeleton system is conducted, while the stability in the steady state and transient condition is investigated. In addition, experimental results of standing-up and sitting-down tasks are demonstrated to further investigate the capabilities of the controller. The results indicate that the compliant knee exoskeleton, incorporating the proposed tele-impedance controller, can effectively generate assistive actions that are volitionally and intuitively controlled by the user’s muscle activity

Standing and Sitting Motion Assistance using a Compliant Knee Exoskeleton

Sit-to-stand movement is the one of most important tasks in daily life for individuals with impaired legs and elderly people who do not have the required physical strength [1], [2]. This work presents an compliant knee exoskeleton (see Fig. 2) which can provide motion assistance to the user during standing-up and sitting-down based on the user intent. The knee exoskeleton is energized by a series elastic actuator with offline reconfigurable stiffness (CompAct-RS) [3], [4]. Introducing passive compliance to the actuation unit decouples the inertia of the motor drive from the output link and thus the exoskeleton presents low impedance to the wearer. Additionally, compliant robots can intrinsically absorb impacts and associated with dedicated control strategies can achieve safety and adaptability of physical human-robot interaction (pHRI). CompAct-RS as mentioned above is a series elastic actuator with the ability to regulate off-line the level of stiffness in as wide a range as needed. This feature permits the experimentation with different compliance levels and the adaptation of the joint to fit specific task requirements. The elimination of active tuning of the spring stiffness through a second motor was performed to reduce the weight and dimensions of the unit. The working principle of the CompAct-RS is based on the CompAct-VSA (Variable Stiffness Actuator) [5], which uses a lever arm mechanism with a variable pivot axis. and experimental data of the human standing-up and sittingdown. Acquiring this data we selected the design variables of the actuator and analysed its performance and stiffness characteristic. An assistive control strategy is proposed which allows the actuator to generate assistive torques towards to the direction of motion based on estimated user torques. Particularly, electromyographic signals are used in order to derive the estimated user torque while the equilibrium position of a virtual spring-damper network is being updated in accordance to this estimated user torque

Autonomous multi-joint soft exosuit with augmentation-power-based control parameter tuning reduces energy cost of loaded walking

Abstract Background Soft exosuits are a recent approach for assisting human locomotion, which apply assistive torques to the wearer through functional apparel. Over the past few years, there has been growing recognition of the importance of control individualization for such gait assistive devices to maximize benefit to the wearer. In this paper, we present an updated version of autonomous multi-joint soft exosuit, including an online parameter tuning method that customizes control parameters for each individual based on positive ankle augmentation power. Methods The soft exosuit is designed to assist with plantarflexion, hip flexion, and hip extension while walking. A mobile actuation system is mounted on a military rucksack, and forces generated by the actuation system are transmitted via Bowden cables to the exosuit. The controller performs an iterative force-based position control of the Bowden cables on a step-by-step basis, delivering multi-articular (plantarflexion and hip flexion) assistance during push-off and hip extension assistance in early stance. To individualize the multi-articular assistance, an online parameter tuning method was developed that customizes two control parameters to maximize the positive augmentation power delivered to the ankle. To investigate the metabolic efficacy of the exosuit with wearer-specific parameters, human subject testing was conducted involving walking on a treadmill at 1.50 m s− 1 carrying a 6.8-kg loaded rucksack. Seven participants underwent the tuning process, and the metabolic cost of loaded walking was measured with and without wearing the exosuit using the individualized control parameters. Results The online parameter tuning method was capable of customizing the control parameters, creating a positive ankle augmentation power map for each individual. The subject-specific control parameters and resultant assistance profile shapes varied across the study participants. The exosuit with the wearer-specific parameters significantly reduced the metabolic cost of load carriage by 14.88 ± 1.09% (P = 5 × 10− 5) compared to walking without wearing the device and by 22.03 ± 2.23% (P = 2 × 10− 5) compared to walking with the device unpowered. Conclusion The autonomous multi-joint soft exosuit with subject-specific control parameters tuned based on positive ankle augmentation power demonstrated the ability to improve human walking economy. Future studies will further investigate the effect of the augmentation-power-based control parameter tuning on wearer biomechanics and energetics

Additional file 1: of Autonomous multi-joint soft exosuit with augmentation-power-based control parameter tuning reduces energy cost of loaded walking

Variability of positive augmentation power and work with cadence. (PDF 285 kb

Metabolic cost adaptations during training with a soft exosuit assisting the hip joint.

Different adaptation rates have been reported in studies involving ankle exoskeletons designed to reduce the metabolic cost of their wearers. This work aimed to investigate energetic adaptations occurring over multiple training sessions, while walking with a soft exosuit assisting the hip joint. The participants attended five training sessions within 20 days. They walked carrying a load of 20.4 kg for 20 minutes with the exosuit powered and five minutes with the exosuit unpowered. Percentage change in net metabolic cost between the powered and unpowered conditions improved across sessions from -6.2 ± 3.9% (session one) to -10.3 ± 4.7% (session five), indicating a significant effect associated with training. The percentage change at session three (-10.5 ± 4.5%) was similar to the percentage change at session five, indicating that two 20-minute sessions may be sufficient for users to fully adapt and maximize the metabolic benefit provided by the exoskeleton. Retention was also tested measuring the metabolic reduction five months after the last training session. The percent change in metabolic cost during this session (-10.1 ± 3.2%) was similar to the last training session, indicating that the adaptations resulting in reduced metabolic cost are preserved. These outcomes are relevant when evaluating exoskeletons\u27 performance on naïve users, with a specific focus on hip extension assistance

Metabolic Cost Adaptations During Training with a Soft Exosuit Assisting the Hip Joint

Different adaptation rates have been reported in studies involving ankle exoskeletons designed to reduce the metabolic cost of their wearers. This work aimed to investigate energetic adaptations occurring over multiple training sessions, while walking with a soft exosuit assisting the hip joint. The participants attended five training sessions within 20 days. They walked carrying a load of 20.4 kg for 20 minutes with the exosuit powered and five minutes with the exosuit unpowered. Percentage change in net metabolic cost between the powered and unpowered conditions improved across sessions from -6.2 ± 3.9% (session one) to -10.3 ± 4.7% (session five), indicating a significant effect associated with training. The percentage change at session three (-10.5 ± 4.5%) was similar to the percentage change at session five, indicating that two 20-minute sessions may be sufficient for users to fully adapt and maximize the metabolic benefit provided by the exoskeleton. Retention was also tested measuring the metabolic reduction five months after the last training session. The percent change in metabolic cost during this session (-10.1 ± 3.2%) was similar to the last training session, indicating that the adaptations resulting in reduced metabolic cost are preserved. These outcomes are relevant when evaluating exoskeletons\u27 performance on naïve users, with a specific focus on hip extension assistance