Servo - March 2016

The Challenges of and Solutions for Watertight Builds

Engineering your own ROV has construction challenges beyond designing and building the necessary electronics. The electronics package for the design is housed inside a six inch PVC pipe, with a tether cable running topside to a joystick steering mechanism. The tether is Cat5 cable and must enter the PVC pipe through a watertight seal. Additionally, the wires from the motors and optional light must also feed into the PVC pipe to connect to the motor drivers.

The integrity of these watertight connections is an absolute must as any water leak can be catastrophic to the electronics inside. My simple solution was to drill holes in the six inch PVC and then use marine epoxy to glue a 1/2 inch PVC coupling that is threaded on the inside. The wires are then pulled through a hole drilled in a 1/2 inch PVC threaded plug, after coating each wire with marine epoxy.

The entire plug is then filled with additional marine epoxy.

It is important that each wire going through the hole in the PVC plug have an epoxy coat around it before bundling them and passing them through the hole. The PVC plug is wrapped with Teflon™tape and screwed tightly into the coupling.

A second challenge is achieving neutral buoyancy for the ROV, as well as a balanced trim. The six inch PVC tube and the 1/2 inch PVC frame act as floats, while the total weight of the ROV acts to sink it. These two must be balanced so that the ROV attains neutral buoyancy, where it simply hovers and does not sink or float to the top. This ROV needed additional weight.

After struggling with several ideas, the solution was a 14 inch square of ceramic tile, cemented to the bottom of the frame with bathtub caulk along with several smaller four inch square pieces. To fine-tune the buoyancy, a small petcock was added at the top and bottom of the frame. With the ROV just barely floating, these petcocks can be opened allowing a little water to enter the frame and adding weight. Adjusting how much water is in the frame can achieve near neutral buoyancy.

The trim of the ROV must be level; the position of the ballast is used to attain this. The height of the left and right motors must also match the vertical center of mass of the ROV. This allows the motors to push the ROV in a level direction. For example, if the motors are mounted too low, their thrust would tip the bottom of the ROV forward

Having lived near the water most of my life and with manatees showing up regularly in the water nearby where we live in Florida, I developed an interest in having an ROV (Remotely Operated Vehicle) to video sea life underwater. Purchasing an ROV can be expensive, so after several prototypes, I successfully constructed an ROV from easily acquired parts that can be assembled with basic tools.