Final Design

Summary of Final Design:

For our final design we mounted the cameras and IMU's to the front of the Hololens 2 and the battery and StereoPi to the back. Mounting the cameras to the front allows for them to easily be placed in the same location every time. Mounting the battery and StereoPi to the back allows for more even weight distribution. We also created cable mounts that will go on the strap of the hololens 2 so that the cables can be led to the StereoPi and will not get in the way of the user.

Cameras:

For the cameras the mounts had to be secure to ensure that the camera angles would be the same across multiple users and uses. We decided to go with a clip design to allow for them to be easily put on and taken off and to be unobtrusive to the user. We added a thin layer of rubber to the inside of the clips to prevent sliding along the strap of the Hololens.

Part way through this project our sponsor asked if we could make the camera angles adjustable. We created a design that gave 20 mm of linear travel and 25° of angular travel. The linear adjustability is controlled by twisting a screw. The angular adjustability is controlled by a slider with a pin that locks at set angles, every 5° from 15°-40° from the vertical.

Once we had designed secure clips we then tested our camera angles. Our sponsor needed the cameras to be able to see the movement of the users facial features and limbs. We made the mounts with the same angle as the sponsors prototype and tested their view. We then showed the camera view to our sponsors to confirm that it was correct. They then asked us to push a little further and create camera mounts with adjustable angles that could easily be set to an exact angle so both cameras could match.

StereoPi and Battery Mount:

We decided to mount the Stereo Pi board and battery to the back of the Hololens because they are both larger components and there is more space to mount them there. Also, through our earlier weight distribution testing, we found that the weight was the most noticeable when the left and right sides were imbalanced and so we localized the stereo pi and battery at the center of the back to maintain balance. The difficult part was then creating a mount that matched the odd curvature of the back of the hololens. We analyzed the radius of curvature of the hololens back through a CAD model we found online and then created a mount from there. We iteratively tested and adjusted the mounting part of the stereo pi board mount until it fit perfectly on the back of the hololens. Since we were mounting to the back of the hololens we decided to go with an elastic strap design for actually attaching it. When designing the casing for the StereoPi mount we consulted multiple design guides for plastics. Then we designed the casing with bosses for both the StereoPi and StereoPi V2 and with a a large opening at the top so that there is access to all the ports and an cables can come out.

Battery:

An external battery is required for the head mount camera system as a power source. The mechanism for powering the system was already laid out by the sponsor, using a standard USB portable charger and connecting it to the StereoPi Board and through it powering all the components. In the sponsor’s prototype design a battery that was already available was used, but that battery was bulky and by far the heaviest component of the system. Additionally the power bank’s charge capacity was excessively large. For these reasons the sponsor requested a lighter power bank be selected and purchased for the system. The battery unit for the sponsor’s prototype was originally not mounted to the system. It was instead carried in a pocket of the user, and was considered a dangling part. The sponsor requested it be mounted to the Hololens 2 along with the other components so that the design would be self contained, and so a battery mount was also necessary for the final design.

Cables and Cable Mounts:

The hardware requirements of the augmented reality head mount system include - as described in this report - multiple boards, sensors, and other connected devices, all of which must be connected and interfaced according to the parameters outlined by the sponsor. As such, a critical design component includes the system cables themselves, especially in consideration of key design requirements that may constrain options for cable routing and lengths. As major sensors such as the stereo cameras and IMUs are mounted forward on both sides of the user’s head, and the StereoPi board is mounted on the rear of the Hololens2 headset, the system cables required a 16 in cable and a small housing to help route them.

The purpose of the cable mounts is to contain the cables connecting the cameras and IMUs to the StereoPi board in a neat and compact manner. Dangling cables pose a safety hazard and might damage the components of the head mount if accidentally snagged when the user is moving. These camera mounts also provide strain relief such that there is appropriate cable tension when the camera angles are being adjusted on the adjustable camera mounts.The camera mounts are clipped on the sides of the HoloLens 2 behind the camera mounts using a snap fit and stores the CSI-2 Ribbon cables used to connect the cameras to the StereoPi board and the 4-pin jumper cables used to connect the IMUs to the StereoPi board.