79 research outputs found
Design and control of a novel variable stiffness soft arm
Soft robot arms possess such characteristics as light weight, simple structure and good adaptability to the
environment, among others. On the other hand, robust control of soft robot arms presents many difficulties. Based
on these reasons, this paper presents a novel design and modelling of a fuzzy active disturbance rejection control
(FADRC) controller for a soft PAM arm. The soft arm comprises three contractile and one extensor PAMs, which
can vary its stiffness independently of its position in space. Force analysis for the soft arm is conducted, and stiffness
model of the arm is established based on the relational model of contractile and extensor PAM. The accuracy of
stiffness model for the soft arm was verified through experiments. Associated to this, a controller based on the fuzzy
adaptive theory and ADRC, FADRC, has been designed to control the arm. The fuzzy adaptive theory is used to
adjust the parameters of the ADRC, the control algorithm has the ability to control stiffness and position of the soft
arm. In this paper, FADRC was further verified through comparative experiments on the soft arm. This paper
reinforces the hypothesis that FADRC control, as an algorithm, indeed possesses good robustness and adaptive
abilities.
Key words: soft robot, variable stiffness, PAM, stiffness modelling, FADR
The design, hysteresis modeling and control of a novel SMA-fishing-line actuator
Fishing line can be combined with shape memory alloy (SMA) to form novel artificial muscle actuators which have low cost, are lightweight and soft. They can be applied in bionic, wearable and rehabilitation robots, and can reduce system weight and cost, increase power-to-weight ratio and offer safer physical human-robot interaction. However, these actuators possess several disadvantages, for example fishing line based actuators possess low strength and are complex to drive, and SMA possesses a low percentage contraction and has high hysteresis. This paper presents a novel artificial actuator (known as an SMA-fishing-line) made of fishing line and SMA twisted then coiled together, which can be driven directly by an electrical voltage. Its output force can reach 2.65N at 7.4V drive voltage, and the percentage contraction at 4V driven voltage with a 3N load is 7.53%. An antagonistic bionic joint driven by the novel SMA-fishing-line actuators is presented, and based on an extended unparallel Prandtl-Ishlinskii (EUPI) model, its hysteresis behavior is established, and the error ratio of the EUPI model is determined to be 6.3%. A Joule heat model of the SMA-fishing-line is also presented, and the maximum error of the established model is 0.510mm. Based on this accurate hysteresis model, a composite PID controller consisting of PID and an integral inverse (I-I) compensator is proposed and its performance is compared with a traditional PID controller through simulations and experimentation. These results show that the composite PID controller possesses higher control precision than basic PID, and is feasible for implementation in an SMA-fishing-line driven antagonistic bionic joint
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