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Soft Linear Actuator

Reverse Pneumatic Artificial Muscles (rPAMs): Modeling, Integration, and Control

Despite offering many advantages over traditional rigid actuators, soft pneumatic actuators suffer from a lack of comprehensive, computationally efficient models and precise embedded control schemes without bulky flow-control valves and extensive computer hardware. In this article, we consider an inexpensive and reliable soft linear actuator, called the reverse pneumatic artificial muscle (rPAM), which consists of silicone rubber that is radially constrained by symmetrical double-helix threading. We describe analytical and numerical static models of this actuator, and compare their performance against experimental results. To study the application of rPAMs to operate underlying kinematic linkage skeletons, we consider a single degree-of-freedom revolute joint that is driven antagonistically by two of these actuators. An analytical model is then derived, and its accuracy in predicting the static joint angle as a function of input pressures is presented. Using this analytical model, we perform dynamic characterization of this system. Finally, we propose a sliding-mode controller, and a sliding mode controller augmented by a feed-forward term to modulate miniature solenoid valves that control air flow to each actuator. Experiments show that both controllers function well, while the feed-forward term improves the performance of the controller following dynamic trajectories.

More details:

EH. Skorina, M. Luo, WY. Oo, W. Tao, F. Chen, S. Youssefian, N. Rahbar and C. D. Onal. “Reverse pneumatic artificial muscles (rPAMs): Modeling, integration, and control .” Plos One 13 (10), e0204637 (2018).


Motion Control of a Soft-Actuated Modular Manipulator

Soft pneumatic actuators can allow robotic manipulators to interact safely in complex environments in close proximity to humans, but work still needs to be done controlling them more effectively. We explore this area by introducing a 2-degree of freedom (DoF) universal joint module actuated by three reverse Pneumatic Artificial Muscles (rPAMs) and an associated geometric Jacobian-enhanced iterative sliding mode controller. After demonstrating the effectiveness of this controller, we combine two of these modules to form a 4-DoF soft actuated manipulator. To control this modular manipulation system, we propose two controllers: a direct inverse kinematic (IK) controller and an end-effector geometric Jacobian controller. Though both controllers were validated to function effectively, the Jacobian controller was more precise (especially under payload) while the IK controller was more accurate.

More Detail:

E.H. Skorina, W. Tao, F. Chen, M. Luo, C.D. Onal, “Motion Control of a Soft-Actuated Modular Manipulator“, IEEE International Conference on Robotics and Automation (ICRA) pp. 4997-5002, (2016).

Feedforward augmented Sliding Mode Motion Control of Antagonistic Soft Pneumatic Actuators

Soft pneumatic actuators provide many exciting properties, but controlling them without the use of bulky and expensive flow-control valves can be difficult and computationally expensive. We seek a solution to this problem by introducing an inexpensive and reliable muscle-like linear soft actuator used antagonistically to operate a rigid 1-DoF joint, resulting in a system that combines the advantages of rigid and soft robotics. Using this setup, we performed precise motion control using a sliding mode feedback controller as well as a sliding mode controller augmented by a feedforward term to modulate the state of solenoid valves that drive each actuator. We found that both controllers performed equivalently well in following a step function and in responding to a disturbance. The feedforward augmented controller performed significantly better when following dynamic trajectories over a range of frequencies and with the addition of an external force. The next step will be to modify our valve control scheme to allow for the determination of both the position and stiffness of the joint, better leveraging the advantages of soft pneumatic actuators.

More Detail:

E.H. Skorina, M. Luo, S. Ozel, F. Chen, W.Tao, and C.D. Onal “Feedforward augmented Sliding Mode Motion Control of Antagonistic Soft Pneumatic Actuators“, IEEE International Conference on Robotics and Automation (ICRA), (2015).

Optimized Design of a Rigid Kinematic Module for Antagonistic Soft Actuation

Soft actuators can be useful in human-occupied environments because of their adaptable compliance and light weight. We previously proposed a new actuator: the reverse pneumatic artificial muscle (rPAM), and developed an analytical model to predict its performance both individually and while driving a 1 degree of freedom revolute joint antagonistically. Here, we expand upon this previous work, adding a correction term to improve model performance and using it to perform optimization on the kinematic module dimensions to maximize achievable joint angles. We also offer advances on the joint design to improve its ability to operate at these larger angles. The new joint had a workspace of around 60 degrees, which was predicted accurately by the improved model.

More Detail:

M. Luo, E.H. Skorina,W. Tao, F. Chen, C.D. Onal, ” Optimized Design of a Rigid Kinematic Module for Antagonistic Soft Actuation“, IEEE International Conference on Technologies for Practical Robot Applications (TePRA), (2015).