All publications listed newest first.
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Agonist-Antagonist Pouch Motors: Bidirectional Soft Actuators Enhanced by Thermally Responsive Peltier Elements
Trevor Exley, Rashmi Wijesundara, Nathan Tan, Akshay Sunkara, Xinyu He, Shuopu Wang, Bonnie Chan, Aditya Jain, Luis Espinosa, and Amir Jafari
In 2024 IEEE/RSJ International Conference on Intelligent Robots and Systems (IROS), Oct 2024
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Comparative Analysis of Peltier Devices and Flexible Heater Strips for Enhancing Bandwidth in Thermo-Active Soft Actuators
Trevor Exley, Daniel Johnson, and Amir Jafari
Journal of Medical Robotics Research, Dec 2024
Soft actuators are a new generation of robotic actuators designed for safer and more adaptable physical human-robot interaction, that can be triggered by various stimulating mechanisms, including pneumatic, electric, electromagnetic, light, magnetic, and thermal sources. Among the different types of soft actuators, thermoresponsive ones that utilize heat as the stimulus show great potential due to their ability to deliver a relatively high force-to-size ratio without the need for external air pumps, tethers, high voltage sources, or complex designs. However, a major drawback of such actuators is their limited bandwidth. Traditional methods rely on Joule heating for actuation, with the actuator deflating when the heat source is turned off and ambient temperature takes over. Recently, the Peltier mechanism has been introduced as an alternative approach for active heating and cooling. This research paper presents a comparative analysis of the Peltier and flexible heater mechanisms in terms of the bandwidth and energy consumption of phase-change thermo-active soft actuators. The study aims to assess the potential of Peltier-based actuation in addressing the bandwidth limitations observed in traditional soft actuators. The findings reveal that Peltier-based actuation can significantly improve actuation speed in thermoresponsive soft actuators. However, it is important to note that the performance of Peltier-based actuators decreases after a few cycles unless additional measures, such as the use of an external fan, are implemented. This increase in performance comes at the cost of higher energy consumption, which should be carefully considered in practical applications.
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Increasing Robustness and Output Performance of Variable Stiffness Actuators in Periodic Motions
Trevor Exley, and Amir Jafari
Mechanism and Machine Theory, Mar 2022
Variable Stiffness Actuators (VSAs) have been developed to address the safety and limited adaptability in interactions with uncertainties and energy efficiency issues which exist in traditional “stiff” robots. When desired performance of a VSA is given for a certain application, the question is how this desired performance can be achieved with minimum energy consumption and maximum robustness against uncertainties. This will lead to more compact, lighter but more powerful VSAs. This work develops an understanding of how to optimally design the parameters of the stiffness adjustment mechanisms by developing a framework that can robustly maximize the output performance of VSAs. Five VSA examples, each representing a different design set of stiffness adjustment mechanism, are being considered and evaluated based on the proposed optimization framework to perform a given periodic motion. The resultant optimal design of each set is then compared with the original design in terms of output performance and robustness. The proposed framework shows improvement of the output performance for the given periodic motion up to 546%, with robustness of up to 2.1% perturbation of the optimal design values.
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Maximizing Energy Efficiency of Variable Stiffness Actuators through an Interval-Based Optimization Framework
Trevor Exley, and Amir Jafari
Sens. Actuators A Phys., Dec 2021
Despite the necessity for being able to regulate the stiffness of the robotic platforms in physical contact with humans, and tremendous number of different variable stiffness actuators that have been developed so far, there is still no such actuator that has successfully passed the research lab phase and transferred into a real application. The main reason is due to the lack of understating on how to optimally design a stiffness adjustment mechanism based on desired performances of each application. If not optimally designed, the additional complexities within the actuators, prevent to win the trade-off between benefits of having a stiffness adjustment mechanisms versus its costs, such as the energy storage capacity of the elastic elements compared to how much energy can be actually released at the output of the actuator, i.e. the link. Currently, this trade-off criterion is not in favor of introducing a stiffness adjustment mechanism into the actuator. Therefor, generally, having a simple series elastic actuator with active compliance has been preferred over having a complex variable stiffness actuator, in many real applications. This work develops an understanding of how to optimally design the parameters of a stiffness adjustment mechanism by developing a framework that can robustly maximize the energy efficiency of variable stiffness actuators. Five different design sets of stiffness adjustment mechanism are being considered and evaluated based on the proposed optimization framework. The resultant optimal design of each set is then compared with the original design in terms of energy efficiency. The proposed framework shows improvement of energy efficiency up to 354%, while design constraints are all being satisfied.
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Toward a Novel Thermal-Based Variable Impedance Module Through Adjusting Viscoelastic Properties of a Thermoresponsive Polymer
Trevor Exley, Daniel Johnson, and Amir Jafari
IEEE Transactions on Medical Robotics and Bionics, Nov 2023
In this article, we propose a one-of-a-kind thermal-based variable impedance module to be used towards developing a variable impedance actuator. Unlike other variable impedance actuators, where the impedance adjustment modules employ a mechanism to regulate their impedance, this novel module does it through controlling temperature of a thermoplastic polymer Polycaprolactone. The viscoelastic properties of polycaprolactone are temperature-dependent, increasing the rigidity when it is cooling down and softening when it is heating up. To change the temperature, thermo-electric Peltiers are embedded into the design. Preliminary experiments have shown that this module is able to adjust its impedance through changing the temperature. The simplicity and lightness of the proposed off-line impedance adjustment approach make it a suitable choice when the actuator’s size, weight, and compactness are the main concerns. This is because the proposed design is highly scalable, and by scaling down the size of the module, its performance regarding the speed of impedance adjustment would increase.
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A Novel Variable Impedance Actuator Utilizing Adjustable Viscoelastic Properties of Thermoresponsive \emphPolycaprolactone
Trevor Exley, Daniel Johnson, and Amir Jafari
Robotics Reports, Nov 2023
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Predicting UPDRS Motor Symptoms in Individuals With Parkinson’s Disease From Force Plates Using Machine Learning
Trevor Exley, Sarah Moudy, Rita M. Patterson, Joonghyun Kim, and Mark V. Albert
IEEE Journal of Biomedical and Health Informatics, Jul 2022
Parkinson’s disease (PD) is a neurodegenerative disease that affects motor abilities with increasing severity as the disease progresses. Traditional methods for diagnosing PD include a section where a trained specialist scores qualitative symptoms using the motor subscale of the Unified Parkinson’s Disease Rating Scale (UPDRS-III). The aim of this feasibility study was twofold. First, to evaluate quiet standing as an additional, out-of-clinic, objective feature to predict UPDRS-III subscores related to motor symptom severity; and second, to use quiet standing to detect the presence of motor symptoms. Force plate data were collected from 42 PD patients and 43 healthy controls during quiet standing (a task involving standing still with eyes open and closed) as a feasible task in clinics. Predicting each subscore of the UPDRS-III could aid in identifying progression of PD and provide specialists additional tools to make an informed diagnosis. Random Forest feature importance indicated that features correlated with range of center of pressure (i.e., the medial-lateral and anterior-posterior sway) were most useful in the prediction of the top PD prediction subscores of postural stability (r = 0.599; p = 0.014), hand tremor of the left hand (r = 0.650; p = 0.015), and tremor at rest of the left upper extremity (r = 0.703; p = 0.016). Quiet standing can detect body bradykinesia (AUC-ROC = 0.924) and postural stability (AUC-ROC = 0.967) with high predictability. Although there are limited data, these results should be used as a feasibility study that evaluates the predictability of individual UPDRS-III subscores using quiet standing data.
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TVIM: Thermoactive Variable Impedance Module Evaluating Shear-Mode Capabilities of Polycaprolactone
Trevor Exley, Rashmi Wijesundara, Shuopu Wang, Arian Moridani, Taha Nilforooshan, and Amir Jafari
IEEE Access, Jul 2025
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Toward a Unified Naming Scheme for Thermo-Active Soft Actuators: A Review of Materials, Working Principles, and Applications
Trevor Exley, Emilly Hays, Daniel Johnson, Arian Moridani, Ramya Motati, and Amir Jafari
Robotics Reports, Jan 2024
Soft robotics is a rapidly growing field that spans the fields of chemistry, materials science, and engineering. Due to the diverse background of the field, there have been contrasting naming schemes such as “intelligent,” “smart,” and “adaptive” materials, which add vagueness to the broad innovation among literature. Therefore, a clear, functional, and descriptive naming scheme is proposed in which a previously vague name—Soft Material for Soft Actuators—can remain clear and concise—Phase-Change Elastomers for Artificial Muscles. By synthesizing the working principle, material, and application into a naming scheme, the searchability of soft robotics can be enhanced and applied to other fields. The field of thermo-active soft actuators spans multiple domains and requires added clarity. Thermo-active actuators have potential for a variety of applications spanning virtual reality haptics to assistive devices. This review offers a comprehensive guide to selecting the type of thermo-active actuator when one has an application in mind. In addition, it discusses future directions and improvements that are necessary for implementation.
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Utilizing the Peltier Effect for Actuation of Thermo-Active Soft Robots
Trevor Exley, Daniel Johnson, and Amir Jafari
Smart Materials and Structures, Jul 2023
The field of soft actuation methods in robotics is rapidly advancing and holds promise for physical interactions between humans and robots due to the adaptability of materials and compliant structures. Among these methods, thermally-responsive soft actuators are particularly unique, ensuring portability as they do not require stationary pumps, or high voltage sources, or remote magnetic field. However, since working principles of these actuators are based on Joule heating, the systems are inefficient and dramatically slow, especially due to their passive cooling process. This paper proposes using the Peltier effect as a reversible heating/cooling mechanism for thermo-active soft actuators to enable faster deformations, more efficient heat transfer, and active cooling. The proposed actuator is composed of a thin elastic membrane filled with phase-change fluid that can vaporize when heated to produce large deformations. This membrane is placed in a braided mesh to create a McKibben muscle that can lift 5 N after 60 s of heating, and is further formed into a gripper capable of manipulating objects within the environment. The effectiveness of the proposed actuator is demonstrated, and its potential applications in various fields are discussed.
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A Review of Rehabilitative and Assistive Technologies for Upper-Body Exoskeletal Devices
Emilly Hays, Jack Slayton, Gary Tejeda-Godinez, Emily Carney, Kobe Cruz, Trevor Exley, and Amir Jafari
Actuators, Apr 2023
This journal review article focuses on the use of assistive and rehabilitative exoskeletons as a new opportunity for individuals with diminished mobility. The article aims to identify gaps and inconsistencies in state-of-the-art assistive and rehabilitative devices, with the overall goal of promoting innovation and improvement in this field. The literature review explores the mechanisms, actuators, and sensing procedures employed in each application, specifically focusing on passive shoulder supports and active soft robotic actuator gloves. Passive shoulder supports are an excellent option for bearing heavy loads, as they enable the load to be evenly distributed across the shoulder joint. This, in turn, reduces stress and strain around the surrounding muscles. On the other hand, the active soft robotic actuator glove is well suited for providing support and assistance by mimicking the characteristics of human muscle. This review reveals that these devices improve the overall standard of living for those who experience various impairments but also encounter limitations requiring redress. Overall, this article serves as a valuable resource for individuals working in the field of assistive and rehabilitative exoskeletons, providing insight into the state of the art and potential areas for improvement.
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Design and Implementation of a High-Precision Motor Control System for Adjustable-Stiffness Biomedical Treadmills
James Jenkins, Omar Madera, Cristian Guerrero, Kirk Humes, Adam Malmquist, Caleb Renfrew, Quintin Bakker, Ken King-Man Siu, Trevor Exley, and Amir Jafari
In 2025 IEEE 18th Dallas Circuits and Systems Conference (DCAS), Apr 2025
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Design Considerations of Peltier-Integrated Therapeutic Wrist Wrap for Medical Applications
Kaleigh Ruiz, Samantha Ryan, Sarah Stutsman, Tanya Tirumala, Jatara Williams, Rashmi Wijesundara, Trevor Exley, and Amir Jafari
In 2024 46th Annual International Conference of the IEEE Engineering in Medicine and Biology Society (EMBC), Jul 2024
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Development of a Miniature Thermal-Based Variable Impedance Actuator Using Thermoplastic Polymers for Scalable and Compact Robotic Applications
Frank Tajomi, Trevor Exley, and Amir Jafari
In 2025 IEEE 18th Dallas Circuits and Systems Conference (DCAS), Apr 2025
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MELEGROS: Monolithic Elephant-inspired Gripper with Optical Sensors
Petr Trunin, Diana Cafiso, Anderson Brazil Nardin, Trevor Exley, and Lucia Beccai
Apr 2025
The elephant trunk exemplifies a natural gripper where structure, actuation, and sensing are seamlessly integrated. Inspired by the distal morphology of the African elephant trunk, we present MELEGROS, a Monolithic ELEphant-inspired GRipper with Optical Sensors, emphasizing sensing as an intrinsic, co-fabricated capability. Unlike multi-material or tendon-based approaches, MELEGROS directly integrates six optical waveguide sensors and five pneumatic chambers into a pneumatically actuated lattice structure (12.5 mm cell size) using a single soft resin and one continuous 3D print. This eliminates mechanical mismatches between sensors, actuators, and body, reducing model uncertainty and enabling simulation-guided sensor design and placement. Only four iterations were required to achieve the final prototype, which features a continuous structure capable of elongation, compression, and bending while decoupling tactile and proprioceptive signals. MELEGROS (132 g) lifts more than twice its weight, performs bioinspired actions such as pinching, scooping, and reaching, and delicately grasps fragile items like grapes. The integrated optical sensors provide distinct responses to touch, bending, and chamber deformation, enabling multifunctional perception. MELEGROS demonstrates a new paradigm for soft robotics where fully embedded sensing and continuous structures inherently support versatile, bioinspired manipulation.