# A Method for Precise Tracking Control of Pneumatic Artificial-Muscle-Driven Exoskeletal Robot
## 2 Pneumatic-Muscle-Driven Upper Limb Exoskeletal Robot System
### Flexible Upper Limb Exoskeletal Robot  
The structure of the Flexible Upper Limb Exoskeleton Robot designed in this study is indicated in Figure 6. To enhance the strength of human upper limbs and to make the device easy to wear so that the wearer can carry goods without hinderance, the robot has the following features in terms of structural design: (1) Adjustable size to adapt to a larger group of people. The rotational centers of exoskeleton joints are in conformity with those of human joints, so there is no conflict between the exoskeleton joints and real human joints. (2) Flexible material is chosen to lower the overall weight. With the pneumatic muscle chosen as the driving method, flexible textiles are used to bond the person and the exoskeleton, so there will not be any limitations like those that occur with rigid materials. (3) The overall assistive function is perfected in strength. To reduce damage to the shoulder and back joints of the human body during the goods-carrying process, supporting structures for the shoulder and back are added. At the shoulder joint, bearing hinges are used to improve the angle limitations of the robot during the adduction and outreach processes. The redundant structure of the holder at the back is reduced to lower the overall weight of the exoskeleton. In this case, with the minimum strength that human muscles can bear, more weight can be loaded. To enhance the adaptability of the exoskeleton robot, the wrist joint and the forearm structure are simplified. In terms of joint design, due to the complicated physiological structure of the human body, human joints do not simply rotate on a plane surface. In this study, the joint rotation limit is simplified on the premise that the key joint motion during goods carrying is not affected. For arms, the angle limitation of shoulder joints is simplified to three levels, and the elbow and wrist joints are expressed by buckling and stretching freedom on the sagittal plane to satisfy movement needs. During the movement of shoulder joints, the Bowden cables are passively driven to transmit the force at a long distanc  
Figure 5: Schematic drawing of the experiment. Figure 6: The structure of the flexible Upper Limb Exoskeletal robot. (**a**) Structural Diagram. (**b**) Actual Picture. which also enhances the flexibility of the joints. When operators are carrying goods while wearing exoskeleton robots, since the shoulder joint is experiencing a certain weight, the upper arm of exoskeleton robots may lean backward. To guarantee the stability of the shoulder joints, one end of the Bowden cables is fixed on the back holder of the exoskeleton, and the other end is fixed on the guide pulleys on the shoulder. When carrying heavy goods, the elbow is bent at 90 deg, at which time the Bowden cables are tight and can provide some assistive force to stabilize and relieve the shoulder from fatigue. To enhance the auxiliary force at the back without adding more weight to the exoskeleton, new textiles and flexible materials are employed. Thus, the elastic bands will store more elastic potential energy after expanding. When bending over, the flexible material will expand, so when the operator resumes an upright posture, the device will provide auxiliary force to the upper body. To reduce the weight of the robot, its battery is separated from the main body, and the driving components, magnetic valve, and other hardware are integrated into an independent control cabinet, which can effectively reduce the weight of the robot so that the human body will not bear so much weight. The condition of the joint inversion was designed to averted, and the operator's physical health would be protected. The Tracking Control System of the Flexible Upper Limb Exoskeletal Robot during the Lifting Process  
This section is dedicated to the introduction of the tracking control system of the Flexible Upper Limb Exoskeletal Robot. Quintic polynomials were employed while designing the target angle, angular speed, and angular acceleration for the elbow joint.