Closed-loop Pressure Control

For closed-loop pressure control, the schematics are shown in Figure \ref{965261}(A). The pressure reservoir was set at 15 kPa. The soft actuator (PneuNet, a silicone-casted bending actuator \cite{mosadegh2014pneumatic} was placed between the MRE valve and the venting path. When no electric current passes through the MRE valve, it was fully opened. This means the soft pneumatic actuator had much lower pneumatic resistance at its upstream than its downstream, therefore bringing a large pressure within the actuator chamber. When a large electric current passed through the valve, the valve was fully closed. In this case, the air inside the chamber would soon escape to the atmosphere and the soft pneumatic actuator would return to its natural shape. The pressure reading p in the actuator chamber was then compared with a reference value p*. The error between the actual and reference pressure was then fed into a PID controller, whose output was fed into a signal amplifier to control the electric current in the MRE valve. The electric equivalent of the pneumatic circuit used here is presented in Figure \ref{965261}(B). The MRE valve is equivalent to the variable resistor R1, the natural pneumatic resistance of the venting path is equivalent to the constant resistor R2 and the pneumatic actuator can be seen as a capacitor C. Figure \ref{965261}(C) presents the step response of the controlled pressure within the soft actuator. The slowest response (0.7 s) occurs when the reference signal goes from 0 to 8 kPa. This is limited by the fluid-dynamic nature of the system, as it takes time for the air to pressurize the large actuator chamber through a relatively narrow valve channel. The RMSE of the controlled pressure at each steady state is 0.05 kPa. Figure \ref{965261}(D) presents the postures of the soft pneumatic actuator at each steady state. A video of the pneumatic actuator being controlled an MRE valve can be found in Video 3.