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! 

Introduction

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. 

Initial Design

There are three main goals in this fluid control system:
  1. First, the system should output a stable pressure. In the first two weeks, I have encountered many problems such as unstable pressure oscillation when the system was approaching to the desired pressure. This oscillation should be suppressed and the pressure also needed to be stable when carrying loads. 
  2. The second goal is to make the pressure adjustable. This is crucial because adjusting the pressure and change the length of actuator. The system will use a displayer to ask user enter the pressure value. 
  3. The third goal is to make the system more compact and organized and waterproof.
Picture
Figure 1
At first, I was considering using a PLC (programmable logic controller) as the center of the control, but later thought an Arduino Mega could do the most job. 

A pressure sensor made by Honeywell, a cheap DC diaphragm pump and solenoid valve were first used to test programs. 
Picture
Figure 2
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. 
I then chose to increase the PWM of the motor to slowly increase the water pressure. A high-power motor driver was used to perform the PWM variation. This time, the pressure is more stable than previous in 70 psi, but the bang-bang oscillations were still happening. 

Modified Design

Picture
Figure 3
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.

In addition, a LCD was also installed so users could just read the pressure values and make adjustment.
Here is a closer look of how the hydraulic actuated textile elongated under the pressure. It stopped at the input pressure with around 0.25 psi error.
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.  
When increasing the pressure with increment of 10 psi, the path appeared to be a sinusoidal wave (or probably not). The pressure didn't reach 80 psi as expected because I was using a very cheap pump at that time (I later changed to a higher pressure pump.

Final Design

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:
https://arxiv.org/abs/1903.08253
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Picture
Dr. Shantonu and Mengjia were mounting the Mega Actuator onto the rack. They would then attach the brick to the bottom of the actuator.

Instructions for User

Setup: 
  1. Plug in two banana connectors into electronic outlet and pump outlet.
  2. Plug in the power jack for Arduino
  3. Adjust the power supply to constant voltage mode, then give 12V to the pump and 13.5 V to the electronics. 
  4. 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.
Filling Mode:​
  1. 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.
  2. 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.
  3. If you think all bubbles are eliminated, simply press "Select" again and the pump will stop pumping.
Custom Mode:
  1. 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."
  2. Press the "Port" button to select which port the textile connects.
  3. 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.
  4. To change to another pressure value, press the "Select" button again. The system will stop pumping and allow you to change pressure again. 
  5. 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. 
Freezing Mode:
  1. 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. 
  2. To resume, press the "Select" again, and the system will resume the last objective.