Design and Evaluation of a Sensor-Instrumented Clutch Mechanism for Quasi-Passive Back Exosuits
Academic Background
In modern work environments, especially in industries involving repetitive lifting and bending tasks, low back injuries are a common and costly occupational health issue. Statistics show that low back injuries account for 35% of all occupational musculoskeletal injuries in the United States. Although ergonomic controls (such as reducing certain material handling tasks) can reduce risks, it is often not feasible to completely eliminate risk exposure in many cases. Therefore, developing technologies that assist workers in alleviating low back strain has become particularly important.
Exoskeletons and exosuits are wearable technologies that have emerged in recent years, capable of providing assistance to reduce the risk of low back injuries. Quasi-passive exoskeletons combine the lightweight nature of passive exoskeletons with the flexibility of active ones, offering assistance when needed while remaining unobtrusive when assistance is not required. However, existing quasi-passive exoskeleton clutches have limitations in design, especially in terms of insufficient sensing and control capabilities, which restrict their potential in practical applications.
To address this issue, the research team designed and evaluated a new sensor-integrated clutch mechanism aimed at expanding the functionality of quasi-passive back exoskeletons to include force sensing, posture sensing, and versatile mode switching. The findings of this study not only provide new insights for the further development of exoskeleton technology but also offer more effective solutions for low back protection in real work scenarios.
Source of the Paper
This paper was co-authored by Paul R. Slaughter, Shane T. King, Cameron A. Nurse, Chad C. Ice, Michael Goldfarb, and Karl E. Zelik, all from Vanderbilt University. The research was supported by the National Institutes of Health (Grant No.: R01EB028105) and the NSF Graduate Research Fellowship. The paper has been published in the IEEE Transactions on Biomedical Engineering and was accepted for publication in May 2024.
Research Process
1. Design Goals and Methods
The research team first clarified the design goals: to develop a new type of clutch that retains the core functions of quasi-passive exoskeletons while adding force sensing, posture sensing, and versatile mode switching. To achieve this goal, the team designed a clutch that integrates an encoder, solenoid, inertial measurement unit (IMU), and microprocessor. This clutch can estimate the exoskeleton’s assistance force when in “engaged” mode, monitor the user’s posture when in “disengaged” mode, and enable multiple modes of switching.
2. Clutch Prototype Design and Validation
The clutch prototype consists of an aluminum base plate, spool stack (including aluminum spool, stainless steel rotor spring, aluminum spool cap, and steel sprocket), and a solenoid. The solenoid is used to control the engagement and disengagement of the clutch. In “disengaged” mode, the spool stack can rotate freely, allowing the steel cable to be unwound from the spool; in “engaged” mode, the spool stack is locked, preventing the cable from being unwound.
The team also integrated a high-resolution encoder and IMU to measure the rotation angle of the spool and the user’s posture. The encoder estimates the trunk-thigh flexion angle by measuring the spool’s rotation, while the IMU estimates trunk orientation through acceleration and gyroscope signals.
3. Experimental Validation
To validate the performance of the clutch, the research team conducted benchtop tests and human-subject experiments.
Benchtop Testing
Benchtop testing included measuring the tension of the steel cable in “disengaged” mode, mode-switching time, and the maximum force the clutch could withstand in “engaged” mode. The results showed that the cable tension in “disengaged” mode was 7-20 N, far below the design target of 32 N; the average mode-switching time was 0.05 seconds (for disengaging) and 0.10 seconds (for engaging), meeting the 1-second design target; and the clutch could withstand 350 N of force in “engaged” mode, fulfilling the design requirements.
Human Subject Experiment
Six healthy participants (3 males, 3 females, average age 26 years) wore a back exosuit integrated with the new clutch and performed stoop and squat tasks during testing. During the experiment, the team recorded synchronized data from the clutch sensors and lab-based instruments using motion capture systems and load cells.
The results indicated that the clutch could estimate the exosuit’s assistance force with an average error of 8.8 N (corresponding to 0.9 Nm of lumbar torque) in “engaged” mode and estimate the trunk-thigh flexion angle with an average error of 6.7° in “disengaged” mode. Additionally, the clutch successfully demonstrated mode switching based on IMU data, showcasing its versatility in control.
4. Data Processing
The research team processed the encoder data using MATLAB, establishing linear regression models to estimate the exosuit’s assistance force and trunk-thigh flexion angle. The models were divided into two categories: subject-specific models and subject-independent models. The results showed that the mean absolute error for subject-specific models was 8.0 N (assistance force) and 4.6° (angle), while for subject-independent models, the errors were 8.8 N and 6.7°, respectively.
Research Results and Conclusions
Key Results
- Assistance Force Estimation: The clutch could estimate the exosuit’s assistance force with an average error of 8.8 N in “engaged” mode, meeting the design target.
- Posture Estimation: The clutch could estimate the trunk-thigh flexion angle with an average error of 6.7° in “disengaged” mode, close to the design target.
- Mode Switching: The clutch could complete mode switching within 0.05-0.10 seconds, satisfying the 1-second design target.
- Versatility and Practicality: The error of the subject-independent model was slightly higher than that of the subject-specific model, but still highly usable in practical applications.
Conclusion
The research team successfully designed and validated a new sensor-integrated clutch that expands the functionality of quasi-passive back exoskeletons. This clutch not only provides back assistance but also monitors user posture and enables versatile mode switching. This design offers new ideas for the further development of exoskeleton technology and has broad application prospects in real work scenarios.
Research Highlights
- Multi-function Integration: The clutch integrates force sensing, posture sensing, and mode-switching functions, significantly enhancing the practicality of quasi-passive exoskeletons.
- Efficient Control: The clutch can switch modes in an extremely short time, ensuring user flexibility and comfort across different tasks.
- Versatile Design: By building subject-independent models, the clutch can adapt to the needs of different users, enhancing its usability in practical applications.
Other Valuable Information
The research team also pointed out that future work could focus on developing fully portable hardware and more robust control algorithms to further enhance the clutch’s performance and range of applications. Additionally, this design provides new possibilities for the commercialization of exoskeleton technology, especially in industrial scenarios requiring precise assistance force and posture monitoring.
Summary
This study provided an important breakthrough in expanding the functionality of quasi-passive back exoskeletons by designing and validating a new sensor-integrated clutch. The clutch not only effectively reduces the user’s back strain but also provides users with a safer and more flexible work experience through accurate force sensing and posture monitoring. This achievement opens up new directions for exoskeleton technology research and applications, with significant scientific value and practical implications.