AquaUrsa Mechanical - 2013

Mechanical Platform

After the 2012 robosub competition, there were major flaws in the mechanical system in Submursa, therefore the mechanical team decided to perform a complete overhaul and build a brand new mechanical platform. The new robot also know as AquaUrsa has a completely new mechanical design with only using the thrusters from the previous year.

2013 URSA

Pressure Hull

.Some key features and requirements to help with our decision include:hull1

  • A shape that will not induce major stress concentrations under pressure
  • Structurally sound to withstand underwater pressure at up to 100m
  • Reasonably sized to fit wiring, batteries, electrical boards and electronics tray
  • Wiring to pass through the hull ends
  • Easy to access the interior for the electrical engineers to configure any boards

Through careful consideration we decided on a cylindrical shaped hull to distribute the pressure uniformly rather than previous years' square design, which peaked in stress concentration near the corners. The mechanical team chose 1/4" thick acrylic plastic due to its' substantial strength and elegant transparency. There will be two aluminum end caps with o-rings to create a vacuum seal inside the hull. The hull also incases a slide out electronics tray held up with carbon fibre rods that spans through the interior length of the hull. The hull will have an inner diameter of 11.2" which will provide more than enough room for the electrical components.


The end caps are made of 1-1/4" thick aluminum. One end cap will be permanently sealed onto the hull. This end will have all the subconn connectors extruding out and the front plate connected to it. The other end will have an outer frame section permanently connected to the hull with a lid and a handle connected to that end. This will allow ease of access to the electronic boards for the electrical engineers. Both end caps will have an extrude where the internal carbon fiber rod will be connected to. The end caps will be sealed by o-rings providing a watertight fit.


The new pressure hull has a shape that would not fit the previous years’ frame therefore a new one had to be built. The new frame had to fit some essential requirements and they are as followed:

  • Be lightweight and easy to transport
  • Be able to connect all the necessary peripherals (Camera, IMU, thrusters, etc.)  to the hull
  • Be able to adapt to any special circumstances or changes in the design of the peripherals
  • Be able to be as compact as possible and take up very little space
  • To be built of materials that are readily available
  • Maintain a minimum distance of 2 feet from the IMU and any source of magnetic interference

With all the key requirements in mind the mechanical team decided to mount 1” diameter carbon fibre rods to the exterior of the pressure hull and mount any and all peripherals to these rods. The bearings and mounts that were used to attach the peripherals to the rod and the rod to the pressure hull are made of aluminum. Aluminum was chosen because it is non-magnetic and will not cause a magnetic interference with any of the electrical components. This form of a frame allows any peripheral to be attached as long as there is a clamp or bearing to attach to the rod or the end caps of the hulls, which maximizes the flexibility of this design. Since the frame is not one solid structure but many different tubes attached to the main structure it is extremely light weight, take up very little space, can be transported easily, and the rod is a stock material that is simply cut to size. 

Frontal Mounting Platform

Mounting up the camera case became an issue when we decided to make an isolated case. Keeping in mind:

  • A Strong material to secure the camera case and ‘claw grip’ system in place
  • A surface to attach the force sensor strip
  • Easily secured to the hull front

Front plate

To fit our requirements, we decided to water-jet cut and bend a large piece of aluminum for our mounting system. Aluminum proves to be lightweight and corrosion resistant but also sturdy enough to securely hold up our camera case and ‘claw grip’ system. There are two tabs that sit on both sides of the camera case, which are used to bolt through the aligned holes and compress the camera case. We also attached a force sensor strip along the front side of this mount to deal with the touch challenge of interacting with the coloured buoys during the “traffic light” challenge. Utilizing the water-jet’s precision, an “ARVP” cutout was made on the plate to give the platform a slick logo, but also to lose some material for weight.

Camera Case

When considering a new design for the new camera case we had a few goals in mind:

  • Transparent case for cameras to capture imagining through
  • Easy assemble and access to the cameras for adjustments and testing
  • Simple to mount onto the frame or hull
  • Waterproof to keep the cameras safe
  • Both forward and downward facing camera in one uniform case

Camera case 1camera case 2

Striving to fulfill these goals, we chose a solid polycarbonate plastic as the case material. With this sturdy material we are able to create a leak-proof, bi-directional see through uniform case for our cameras. The case consists of 2 side components that groove into a middle piece carved the same shape as the groove pattern. These three components are then sealed closed with an O-ring compression running through the groove and long bolts that tightly compress the side pieces & O-rings together. In comparison to the previous design, the new case can withstand pressure up to 100m deep. At this depth, a maximum displacement on the case sides is approximately 0.274mm and the middle piece is approximately 0.141mm. Given this, the material is fully capable of withstanding the stress with a factor of safety of 2.26 for the sides and 3.02 for the middle.

IMU Case

Improving on the previous year’s IMU case, we came up with the requirements:imu case 1

  • Have a 2ft distance away from the center of electronics in the hull
  • Able to hold up the IMU without a lot of physical contact from the case
  • Case is Easily accessible for testing and adjustments
  • Waterproof case

imu case 2

Sticking with the cylindrical concept of this years design theme, the IMU case was modified from a compact square case to a spacious cylindrical one. The IMU is situated on a sleeve that slides easily into the IMU case. The sleeve itself has a slot that a plastic plate tightly slides into acting as a standoff for the actual IMU. To seal the whole case, a thick fine threaded screw cap topped off with an O-ring compression is used to compress the case tight. On the other end of the case a narrower cylinder extrusion was made to tightly seal it against the end of a carbon fiber rod.

Electronics Trays

A design change of the pressure hull means a change in the layout of the electronics trays. With cylindrical hull, new requirements were needed to be addressed.

  • Accessibility of electronics
  •  Modularity of assembly
  • Organization of circuit board and electronic wirings

Front electronicsside electronicsElectronics Tray

The new design of the electronics tray assembly was made to rest onto the 1” carbon fibre rods inside the hull. When the end cap of the hull is removed, the 21/16” carbon fibre tube guides the assembly along the rod, allowing the assembly slide smoothly in and out of the hull. The diamond backplane serves as both a mount for the pin connector and as a support for the tube and the 4mm tray support rods.  The electronics are placed onto trays, which are attached to tray supports. The trays are arranged into a diamond formation which makes the assembly more space efficient. Approximately 23mm of a gap is placed between the electronics trays and the backplane to allow space for the pin connectors. For wire organization, a slot was cut out on the backplane, in-between the sliding tubes. The assembly was made symmetric about the backplane for orientation flexibility

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