# Development and Control of an Electro-Hydraulic Actuator System for an Exoskeleton Robot
## 5 The Position and Force Control Experiments
### Experiment Configuration and Control Experiments  
This chapter explains the experiment configuration and control experiments. The experiment configuration consisted of the hydraulic pump, the manifold of the hydraulic circuit, a double acting single rod cylinder, a linear variable differential transformer (LVDT), a load cell, a motor driver, and an embedded system for control and data acquisition. The experimental setup is shown in Figure 9. The embedded system consisted of the control board and the data acquisition board. Each board was equipped with controller area network (CAN) communication, the serial peripheral interface (SPI) communication, amplifier, analog filter, and the 200 MHz microcontroller unit (MCU). The single rod cylinder for the experiment had a 120 mm stroke length. The inner diameter was 20 mm, and the rod diameter was 10 mm. The LVDT, which measures the position of the cylinder rod, had a stroke length of 150 mm. The load cell that measures the reaction force of the cylinder could measure up to 100 kg and was installed at the end of the cylinder. Figure 9: All experiments used a load cell, hydraulic cylinder, motor, pump, manifold, and control system. Meanwhile, when the exoskeleton robot operates in the stance phase, the robot performs position control, and when operating in the swing phase, the robot grasps the wearer's intention and performs force control. And speed control loop is generally contained within the position control loop. So, the verification of the hydraulic pump and the hydraulic circuit were performed through position and force tracking. Also, the designed sliding mode controller was verified in comparison with a proportional integral derivative (PID) controller. The reference input for tracking of the position was a sine wave, and the PID control and sliding mode control were compared through experiments. First, we simulated them with Matlab Simulink, as shown in Figure 10, and the results are shown in Figure 11. Table 3 shows the parameters of the cylinder and pump, and Table 4 shows the PID control and sliding mode control parameter values for each experiment. As shown in Figure 11a, we can see that the cylinder followed the sine wave well. Figure 11b shows the speed of the motor. Because the initial position of the cylinder was 0 mm, the initial rpm of the motor was too large to follow the reference input. After the current state reached the sliding surface, we can see that control value switched in succession for continuous tracking. Figure 12 gives a comparison of the SMC (sliding mode control) and the PID control for the experimental equipment. In the simulation, the position reference input value was the sine wave. At this time, the control period was set to 10 ms. As in the simulation, \(c_{1}\) and \(c_{2}\) were set to be the same, and in Figure 12a, SMC 1 was set to \(K_{p}=200\) and \(K_{s}=15\), SMC 2 was set to \(K_{p}=100\) and \(K_{s}=15\), and PID was set to \(K_{p}=200\), \(K_{i}=0.02\), and \(K_{d}=0.1\). This shows that the error increased at 1 s and 5 s for all three controls. This is because the moving distance of the cylinder per time was short and the resolution (120 mm/212 bit - 1) of the LVDT with the stroke length of 120 mm was low. In other words, the LVDT error occurred because of the characteristics of the analog system (system noise and the measurement noise), which increased the rate of change in a very short time. Thus, the control response was a little slow, and the position tracking was not good at 1, 3, and 5 s. To improve the control performance, it would be better to use high-resolution LVDT or to increase the control period of the system. On the other hand, when the moving speed of the cylinder was high, this problem did not occur in the sliding mode control and PID control. However, PID had a larger error than sliding mode control when tracking the position (Figure 12b). The PID controls showed good follow-up, but it is unstable and slower than the designed SMC 1.