Fluid Control System for Hydraulic Actuated Textile
2018 Eureka Summer Internship
Special thanks to Mengjia Zhu, Professor Yon Viscell, Doctor Do Thanh Nho, and all members in RE Touch Lab!
During my intern in RE Touch Lab in the summer of 2018, my objective was to design and construct a fluid control system that can provide a high and stable hydraulic pressure for the hydraulic actuated textile, which is a flexible actuator that my mentor was researching. The control system used a high pressure DC diaphragm pump, multiple solenoid valves and pressure sensors, and PID algorithm to regulate water pressure. A LCD was installed at the front to allow users to adjust pressure.
There are three main goals in this fluid control system:
I started with a very simple design. The initial design started with the pump that draws water from a water tank. The water then went through a solenoid valves. The water then split, one side would go through a pressure sensor to the actuator, and on the other side will go through another solenoid valve and then back to the water tank. This closed loop system was all controlled by the Arduino, which received pressure data from the sensor and converted it to on-off signal for valves So in this valve controlled system, if pressure is too high, then it opens valve 2 and close valve 1 to release pressure, while turning off the pump.
I call this method “Bang-Bang” control because it has a big flaw. Let’s see the test result. I increased pressure from 0 to 70 psi. When pressure exceeded 70 psi, valve opened to reduce pressure. But because water is incompressible, there was a big pressure drop as soon as I open the valve, causing these “bang-bang” oscillations (Figure 2). This phenomenon kept the actual pressure oscillating around the desired pressure.
To keep my system more stable, I modified my design. Just imagine, that water flow is electrical current. When there is unstable, transient current in a circuit, what should you do? Add a capacitor to stabilize it. So, I added an accumulator tank that was basically made of one water bladder and one air bladder. It could preload some pressure on water and release it when needed. This feature actually absorbed the disturbance, like a capacitor did in ciruit. And to deal with the quick pressure drop, I used something similar to the resistor in circuit, called flow restrictor, to slow down the flow rate.
I adapted a new method called PID control, which stands for proportional, integral, and derivative control. The error was the difference between the set pressure and actual pressure. The current pressure was first input into the system, which has error. In order to make the error as small as possible, the equation would multiply the error by a constant to make a term proportional to the error. It would then take the integral of the error by summing up all the previous error. Lastly, a term took the derivative of the error over time. By summing up the proportional, integral, and derivative of the error, the sum equals to the PWM value, which is the speed of the pump. This will method output a pressure that has smaller error, and feedback a new pressure to do it again.
After tuning the PID value by multiple tests, I got Kp = 1.022, Ki = 0.06, and Kd = 0.85. On the left was the experiment when I increased pressure from 5 to 50 psi. The pressure was fairly stable at 50 psi. In fact, previously the pressure showed damping oscillation. Since we want to set the pressure immediately, I just closed the solenoid valve as soon as it reached the error range of 50 psi.
In order to make a compact and let user feel comfortable, I constructed a control box that has a LCD screen. By taping the push button and rotating the knob, user can give the command to adjust pressure.
A sealed acrylic box will built by laser cutting. It enclosed all the components to prevent water leaking.
Laser-Cutted Acrylic Box
The Final Product
Mega Actuator Test
In September, the fluid control box was first used in the Mega Actuator's test. The Mega Actuator was built by Prof Do and Mengjia. The fluid control box provided extra water volume and high pressure to the actuator in order to test how much weight it can lift.
For details, please view the paper published in the link below:
Instructions for User
- Plug in two banana connectors into electronic outlet and pump outlet.
- Plug in the power jack for Arduino
- Adjust the power supply to constant voltage mode, then give 12V to the pump and 13.5 V to the electronics.
- Turn on the electronic switch below the LCD first, then turn on the pump's switch above the LCD. If all four LED buttons are lit up, and P1, P2 value in LCD are around 1 psi, then the fluid control box is ready to use.
- For the first time use, the accumulator must be filled and bubbles must be eliminated from the water tubes. There are four button and one knob on the control panel.
- Push the "Mode" button several times until the LCD's first line shows "Filling Mode", and press "Select," the pump will start pumping water into the accumulator and tubes.
- If you think all bubbles are eliminated, simply press "Select" again and the pump will stop pumping.
- The fluid control box can control one or two actuated textiles. To control one textile, press the "Mode" button until the LCD shows "Custom Mode."
- Press the "Port" button to select which port the textile connects.
- Press the "Select" button, the LCD will show the pressure adjustment. Turn the knob to the pressure you want, and press the "Select" button again, the pump will start pumping to the pressure you just input.
- To change to another pressure value, press the "Select" button again. The system will stop pumping and allow you to change pressure again.
- To abort the mission, or to avoid over-pressure, press the "Release Pressure" button. The system will immediately stop pumping and open all valves. To stop releasing pressure, simply press the"Release Pressure" or "Select" button, the system will go back to the "Custom Mode" with "Port 1" selected.
- If you want the system to stop completely or to remove the textiles, press the "Mode" button to the "Freezing Mode" and press "Select." The pump will stop and all valves will close.
- To resume, press the "Select" again, and the system will resume the last objective.