# A Method for Precise Tracking Control of Pneumatic Artificial-Muscle-Driven Exoskeletal Robot
## 2 Pneumatic-Muscle-Driven Upper Limb Exoskeletal Robot System
### Control System of Upper Limb Exoskeleton Handling Process  
The control system in this paper is a rigid flexible coupling system of the pneumatic muscle and Upper Limb Exoskeleton. The air pressure value obtained by the control law is input to the pneumatic muscle, and the length of the pneumatic muscle is calculated using the actual value of the elbow angle. The output force of the pneumatic muscle can be obtained from the length and air pressure of the pneumatic muscle according to the output force formula of the pneumatic muscle. The torque at this time is obtained using the product of the pneumatic muscle output force and vector diameter. This torque acts on the Upper Limb Exoskeleton system to drive the object to move.  
When the object is moving, the torque value provided by the load can be obtained according to the principle of torque equivalence. The torque of the elbow joint can be obtained from the difference between the actual value and the target value of the elbow joint angle, angular velocity, and angular acceleration through inverse dynamics calculation of the robot. The difference between the torque and the torque provided by the load is the compensation torque acting on the pneumatic muscle, that is, the output value of the control law. Then, according to the torque equivalent principle of force, the output force acting on the pneumatic muscle is obtained. According to the inverse model of the output force of the pneumatic muscle built using the BP neural network, the control air pressure of the pneumatic muscle can be obtained, which acts on the pneumatic muscle. A control system flow chart of the Upper Limb Exoskeleton handling process is shown in Figure 10.  
The physical control process of assisting the Upper Limb Exoskeleton is shown in Figure 11. Firstly, the elbow joint angle signal, angular velocity signal, and angular acceleration signal are collected through the Arduino controller. Three signals are input into the controller, and the difference between the actual signal and the target signal is calculated using the control rate to obtain the control air pressure acting on the pneumatic muscle. The control air pressure combined with the length of the pneumatic muscle acts on the control system, and the force output of the pneumatic muscle is calculated to drive the assistance of the Upper Limb Exoskeleton in transportation. In order to provide better power to the system, a 24 V DC power supply is used. The voltage of the Arduino controller is 5 V, and the voltage is set to a safe and suitable level through the step-down module and voltage stabilization module, providing power support for the system components and driving circuits. The solenoid valve is a normally closed MATRIX pneumatic solenoid valve, which adjusts the charging and discharging speed of the pneumatic muscle by adjusting the switching frequency of the charging and discharging ports of the on/off valve. The maximum frequency of operation of the MATRIX pneumatic solenoid valve is 300 Hz; the driving current is 0.7 A, and the supply voltage is 12 V. The air pump provides an air source to the pneumatic muscle, and three states, including the inflation, deflation, and pressure maintenance of the pneumatic muscle, can be achieved through energization or de-energization of the valve body of the electromagnetic valve.  
Figure 10: Control system flow chart of Upper Limb Exoskeleton handling process.