## 3 Layout of Test Rig  
The schematic model shows that the 1-DOF robotic lower limb system is driven by a single-PAM with torsion spring to achieve rotational motion, as shown in Figure 5. In this section, the 1-DOF robotic lower limb system test rig layouts are introduced. All the experiments are conducted using Real-Time Windows Target (RTWT) in the MATLAB environment. RTWT supports the interface card and automatically generates C code and executable files from SIMULINK models. The generated executable files run in real time on the Windows-based PC. Thus, the environment allows easy design and hardware-in-loop test of the control system.  
Figure 4: Design concept of the bionic joint driving by a single-PAM with a torsion spring.  
Figure 5: Test rig layout of the one degree-of-freedom (1-DOF) robotic lower limb system.  
The main objective of this study is to develop a basis of our future research in the application of PAM on rehabilitation. The 1-DOF robotic lower limb system similar to human lower limb joint shown in Figure 5 has been set up. The mechanism design and the system layout are shown in Figure 6. Table 1 shows the specifications of the system.  
As shown in Figure 5, the 1-DOF robotic lower limb system is fixed to the bracket, and the joint of the lower limb is driven by a single-PAM with a torsion spring. The pressure in the PAM is controlled by the proportional pressure valve (FESTO, MYPE-5-1/8 LF-010-B. Made by Festo AG & Co. KG Ruiter Strasse 82 73734 Esslingen). A PC-based controller is used in a real-time environment with an analog to digital converter (Adventech, PCI-1720U, Taipei, Taiwan) and a digital to analog converter (Adventech, PCI-1710, Taipei, Taiwan). A torsion spring is used as the force and energy transmission. Table 2 shows the parameters of the torsion spring.  
Because the angular motion range of the lower limb system hip joint is mainly in the corneal plane, this study only considers the extension and flexion movement of the hip joint. According to the literature [17], the hip joint extension range of motion angles is 0 to 20 degrees and the range of motion angles is 0 to 45 degrees. In the proposed mechanism design, the lower limbs of the robot can move from the vertical line by a maximum angle of about 90 degrees. The range of lower limb angles that can be designed by this body is in line with the requirements of lower limb assist devices.  
\begin{table}
\begin{tabular}{c c c c} \hline
**Components** & **Company** & **Type** & **Specifications** \\ \hline A/D & & & \\ (Analog to Digital) & Advantech & PCI-1720U & 4 channels12-bit D/A \\ Interface Card & & & Output range: \(\pm\)10 V \\ \hline D/A & & & \\ (Digital to Analog) & Advantech & PCI-1710 & 32 channels12-bit A/D \\ Interface Card & & & Maximum input range: \(\pm\)10 V \\ \hline Pneumatic Muscle & FESTO & DMSP-20-200N-25 cm & Max. stroke: 25\% \\  & & & Max. force output: 1500 N \\  & & & Input pressure: 0–6 bar \\ \hline Proportional Valve & FESTO & VPPM-8L-L-1-G14-0L6H-V1P & Input voltage: 0–10 V \\  & & & Output pressure: 0–6 bar \\ \hline Potentiometer & Mitigen (Taiwan & & Range: 0–120 degree \\  & & & Output voltage: 0–5 V \\ \hline \end{tabular}
\end{table}
Table 1: Specification of the 1-DOF robotic lower limb system.  
Figure 6: Photograph of the 1-DOF robotic lower limb system.