*3.2.1. Bottom chamber*

The bottom chamber encases the gas generator and holds water (Fig. 19). The water level varies depending on the depth location of the device. When the device is below the surface, the gas generator is submerged under water to ensure that the electrolysis can take place to allow the device to float up. The bottom chamber also has room to secure up to 8 metal washers as deadweights such that the device is initially about 80-90% submerged. On the bottom of the chamber, there is a small opening to allow water to enter or escape the device. This opening ensures that the inside and outside pressures are kept equal.

## *3.2.2. Middle seal*

The purpose of the middle seal (Fig. 20) is to provide waterproofing of the electronics in the top chamber (Fig. 21). The bottom inner surface is concaved to direct the gas products to the gas chamber.

## *3.2.3. Top chamber*

The top chamber is partitioned into a waterproof chamber and a gas chamber. The waterproof chamber contains all the electronics, Li-ion battery and Solenoid valve (Fig. 21).

The gas chamber contains a mixture of air and gases produced at the electrodes in the bottom chamber. A tube connects the gas chamber to the device surroundings via solenoid valve.

**Figure 19.** Bottom Chamber with electrodes assembly (Top View).

**Figure 20.** Middle Seal with rubber O-rings (Left: Top view; Right: Bottom view).

**Figure 21.** Top Chamber with electronic components and gas chamber.

## *3.2.4. On-board electrical circuit design*

240 Smart Actuation and Sensing Systems – Recent Advances and Future Challenges

**Figure 19.** Bottom Chamber with electrodes assembly (Top View).

**Figure 20.** Middle Seal with rubber O-rings (Left: Top view; Right: Bottom view).

**Figure 21.** Top Chamber with electronic components and gas chamber.

valve.

The gas chamber contains a mixture of air and gases produced at the electrodes in the bottom chamber. A tube connects the gas chamber to the device surroundings via solenoid

The on-board circuit (Fig. 22) provides actuation voltage signals to the electrodes and solenoid valve. A rechargeable 7.4 V, 400 mAh AA Portal Power Corp Lithium Ion Polymer battery is used as a power source and PIC12F508 microcontroller is used to generate two square wave control signals, S1 and S2. A square wave is chosen for simplicity. Since the microcontroller draws only 25 mA, two H-bridge drivers are used to provide up to 2 A peak current output to the electrodes and solenoid valve, which draw up to 500 mA and 80 mA, respectively. Amplitude of the voltage that controls the valve, Vp1, is 7.4 V. A voltage regulator sets the amplitude of the voltage applied to the electrodes and microcontroller, Vp2, to 5 V. Mass of the circuit and the battery are 11.5 g and 19.1 g, respectively.

**Figure 22.** Circuit schematics.

#### **3.3. Experimental results**

#### *3.3.1. Gas generation rate*

An experiment is set up to measure the gas generation rate at different voltage levels. Voltages ranging from 2.0 V to 6.0 V at 0.5 V intervals are applied to the electrodes using an Agilent DC Power Supply (Model #E3646A). The hydrogen and oxygen gases generated are collected using water displacement technique with 50 mL graduated cylinder (Fig. 23(left)). A constant voltage is applied for 60 s then the average current and displaced volumes are recorded. The gas generation rate can be found by dividing the displaced volume by 60 s. Gas generation rate vs. power consumption is plotted (Fig. 23(right)) and a least squares regression line is fitted to find a correlation. The results indicate a fairly strong linear relationship between gases generated and power consumption. The proportionality constant is approximately 0.032 mL/J.

The on-board circuit is set up such that the output voltage is 5V. However the actual voltage measured across the electrodes is 4 V due to limited capacity of the battery. At this voltage, the average current and power consumption based on 5 trials are 0.3 A and 1.2 W, respectively. The average gas generation rate was 0.048 mL/s. The linear model predicts 0.040 mL/s at this power so the model and the result are in a fair agreement.

**Figure 23.** Gas generation rate experiment set-up; using water displacement technique to collect the gas generated (left); Gas generation rate vs. power consumption (right).

#### *3.3.2. Diving test*

The depth control device is tested in a water tank (1.5 m wide, 4.7 m long, and 0.9 m deep). The tank is filled with tap water at a room temperature. A critical mass is when the device is about 95% submerged under water, at which a slight decrease in buoyancy causes the device to sink. The critical mass of the device is experimentally found to be 283 g. Because the mass of the device is 114 g, 169 g of metal washers are added to the bottom chamber as deadweights. This mass is also equal to the payload for this particular device. The microcontroller in the circuit is programmed such that there is an initial 3-minute delay to allow time for assembly and fastening of the bolts. After the initial delay, the solenoid valve turns on for 12 s to allow gas to escape such that the device will sink to the bottom of the tank. Then the solenoid valve turns off and 5 V is applied to the electrode plates for up to 15 minutes. Gases generated by the electrolysis fill the gas chamber, which displaces the water inside the bottom chamber. Thus, the device becomes more positively Ionic Polymer-Metal Composite Artificial Muscles in Bio-Inspired Engineering Research: Underwater Propulsion 243

buoyant. The time it takes to rises back up to the top is measured. Fig. 24 shows the timing of the control signals.

**Figure 24.** Timing of the control signals.

242 Smart Actuation and Sensing Systems – Recent Advances and Future Challenges

agreement.

*3.3.2. Diving test* 

The on-board circuit is set up such that the output voltage is 5V. However the actual voltage measured across the electrodes is 4 V due to limited capacity of the battery. At this voltage, the average current and power consumption based on 5 trials are 0.3 A and 1.2 W, respectively. The average gas generation rate was 0.048 mL/s. The linear model predicts 0.040 mL/s at this power so the model and the result are in a fair

**Figure 23.** Gas generation rate experiment set-up; using water displacement technique to collect the gas

The depth control device is tested in a water tank (1.5 m wide, 4.7 m long, and 0.9 m deep). The tank is filled with tap water at a room temperature. A critical mass is when the device is about 95% submerged under water, at which a slight decrease in buoyancy causes the device to sink. The critical mass of the device is experimentally found to be 283 g. Because the mass of the device is 114 g, 169 g of metal washers are added to the bottom chamber as deadweights. This mass is also equal to the payload for this particular device. The microcontroller in the circuit is programmed such that there is an initial 3-minute delay to allow time for assembly and fastening of the bolts. After the initial delay, the solenoid valve turns on for 12 s to allow gas to escape such that the device will sink to the bottom of the tank. Then the solenoid valve turns off and 5 V is applied to the electrode plates for up to 15 minutes. Gases generated by the electrolysis fill the gas chamber, which displaces the water inside the bottom chamber. Thus, the device becomes more positively

generated (left); Gas generation rate vs. power consumption (right).

**Figure 25.** Snapshots of the device during open-loop testing.

It took approximately 4.7 s for the device to sink to the bottom of the tank and 180 s to rise back up to the top. The power consumption for sinking and rising are 0.5 W and 1.2 W, respectively. Fig. 25 shows snapshots of a successful demonstration of the open-loop proofof-concept depth control device. In order to reduce the rising response time, multiple sets of electrodes can be implemented and actuated simultaneously at a higher voltage. To decrease the current rising time by a factor of 1/3 (60 s), a gas generation rate must triple to be at least 0.14 mL/s. This is achievable with two sets of electrodes with approximately 6 V actuation voltages.
