news.

Building a custom AR tablet

Augmented Reality (AR) on mobile devices has grown in leaps and bounds as the devices have become more powerful. However, these devices have not yet reached the point where they are capable of high quality markerless AR. To solve this problem it was decided to build a prototype tablet system that has the needed processing power to be used for AR research. The base of the prototype tablet system is an Asus G51J-3D laptop which uses a Core-i7 processor and NVidia GTX 260M which provides ample processing power to implement high quality markerless AR.

This post documents the building process of the prototype tablet system and also serves as a guide for others whom want to attempt the same.

Disclaimer: This will void your warranty and could lead to personal injury if something goes wrong. We cannot be held responsible for any damages or injuries.

As always before anything can be designed you first need to know what is available to work with. Therefore the first step is to take the laptop apart so that we can see what there is to work with. We start by removing the screws hidden behind the rubber pads as shown below.

The Asus G51J laptop

The rubber pads hiding the screws

After all the screws have been removed the next step is then to unclip all the small plastic clips around the screen. The easiest way to do this is to insert a small flat object, such as a flat screwdriver or a cut credit card, at one side and run it along the edges to pop the clips open.

One of the clips that need to be unclipped

You should now be able to remove the front part of the bezel. Before we remove the back panel we first need to check how the back panel is attached to the screen and if there are any other device that need to be removed first. In our case we had to remove the webcam and the PCB for the back panel lights.

The webcam

The PCB for the back panel lights

The bottom screw holding the back panel in place

The top screw holding the back panel in place

After these components have been removed we can simply unscrew the back panel and pull it loose from the sticky panel attached to the back of the LCD. We also need cut the wires connecting the Wi-Fi and Bluetooth antennas so that the back panel can be removed.  Now for the big moment: Checking if everything is still working.

The back panel removed

The screen image is still nice and crips

The LCD panel from the back

Luckily (in our case) everything is still working, as it should. So after removing the hinge covers, let’s take a look at the wires that connects the LCD to the rest of the laptop. Normally one would expect a ribbon cable running from the LCD to the laptop. Unfortunately this is not the case with our laptop as we found a collection of loose wires instead of a ribbon cable. These can still be extend, but it is not as simple as with a ribbon cable where one can simply buy a longer cable and replace the existing one.

The connector on the left side of the laptop

The wires that need to extended

A couple of things to keep in mind when extending the wires:

  1. Try to keep all the wires more or less the same length to minimize signal disturbances.
  2. If wires are grouped in twisted pairs, then it is best to extend them in twisted pairs.
  3. Try and match the wire types. (E.g. Use multi-core wire to extend multi-core wire.)
  4. Try and test after every extention so you’ll know which wire caused your problem should the screen not work.
  5. Try and stagger the joints of the different wires. This will prevent having a big bulge at a single point in the cable.
  6. Make sure you insulate the soldered joints of the wires. Using heat-shrink is probably the easiest option.
  7. Make sure all power is switched off. Remove the battery if possible and disconnect the cable connectors for added safety.
  8. There is one other issue that might arise during testing: Blowing the motherboard. This issue did arise during my extension process. It occured when I attempted to straighten a bent pin in one of the connectors. Luckily the motherboard could be replaced (Thanks Jacques, Mike and Oliver for your help!)

The completed cable extension

If the extension is short you should have minimal issues, but with a longer extension, as per our case you might start experiencing image degradation due to the varying cable lengths during the extension process.  Below are some of the issues that you might experience.

Noisy image

Out of sync image

Colour shifting

One final addition that is worth considering is strengthening the soldered joints using glue or some acrylic medium. We used nail polish since it is acrylic based and easy to apply (and remove if need be).

Reinforcing the joints

Once you know what you have to work with its time to tackle the design problem. The design will depend on what you want to do and which tools you have available. In our case we decided to use stereo cameras (which needed to be swappable between two different sets), accelerometer and gyroscope sensors and a touch screen add-on. Each of these components needed to be taken into account during the design process. Since we have access to the university workshop for machining we were able to create a simple design using a few pieces of machined polycarbonate. Polycarbonate was chosen since it is light, easy to machine and has good mechanical properties. The model of the design for one of the stereo pairs can be seen below.

The screen side

The camera side

As you may notice the wires have not been included in the model. The reason for this is the difficulty to model wires and in most cases it is simple enough to make an estimate of the space required for the wires and at assembly make the necessary adjustments to accommodate the wires. Below are the pieces of polycarbonate we machined.

The middle Polycarbonate panel

The back Polycarbonate panel

The touchscreen

Sensor and PS3 Eye mounts

The sensor mounting consists of the two sets of sensors which we will use to determine motion and an Arduino to send the sensor data to the laptop.  The first set of sensors is a cheap commercial 3-axis accelerometer and 3-axis gyroscope and the second set is a more expensive inertial sensor.

The sensors assembly

Tip:Use masking tape to colour code the headers

The next assemblies are the two stereo camera pairs. The first consists of two PS3 Eye cameras that will be used to test cheap consumer cameras with the system. The second is a pair of Basler 311fc cameras that are of a much higher quality, but also much more expensive. Due to the long connectors used on Basler cameras, the cameras cannot be placed as close to the tablet as the PS3 cameras and are therefore mounted on an aluminium L-section.

The PS3 Eye mounting

The assembled PS3 Eye stereo pair

The Basler stereo pair

The Basler stereo pair from behind

Now that the camera and sensor mounts have been completed it is time to start on the screen assembly. Firstly we want to check that the polycarbonate parts and the touchscreen fit together.

The panels assembled without the LCD

With all the polycarbonate pieces fitting together it is time to mount the LCD on the middle polycarbonate panel. The LCD is held in place by screwing the LCD frame to the polycarbonate panel with 4 screws. The microphones and webcam connector are simply held in place with sticky tape for the time being.

The LCD screwed to the middle panel

The top screw holding the LCD in place

The bottom screw holding the LCD in place

The microphones and webcam connects

Leaving the screen as it is for the time being, it is now time to move onto building the backpack that is to hold the laptop body. The basic idea with the backpack is to mount all the components onto a piece of hardboard and in turn attach the hardboard to a backpack to make it easy to carry.

The backpack and the  piece of hardboard to be used

To hold the laptop in place four U-channel pieces of aluminium are used. It is important that these do not block any ports, ventilation holes or buttons. In our case we shortened one of the brackets to prevent it from covering the power button. Having a bit of space between the laptop and the brackets is a good idea as this makes removing the laptop easier.

Placement for the four brackets

A bit of space to make the removal of the laptop easy

The bracket that had to be shortened

The next step is to do the layout of the backpack to ensure that everything fits, all the cables are long enough and also to ensure the weight is evenly distributed. The components that have to be mounted are the laptop body, 12V battery for the Basler cameras and the touchscreen controller. Below are the layouts for the two sets of cameras.

The backpack layout for the PS3 Eye cameras

The tablet layout for the PS3 Eye cameras

The backpack layout for the Basler cameras

The backpack layout for the Basler Cameras

The tablet layout for the Basler cameras

Checking the cable lengths

Once you’ve found a good layout, mark out all the components and holes that need to be made. Shown below is the laptop mounted to the hardboard sheet. The brackets have been riveted to the hardboard. It is important to use rivets with big heads to prevent them from tearing through the hardboard. Two straps of Velcro are also used to keep the laptop in place. Another smaller bracket was also added to support the laptop as the Velcro straps alone could not stop it from slipping slightly.

The laptop strapped to the backpack

The extra bracket that was added

The touchscreen controller was mounted using Velcro and double-sided tape, since it is light and may need to be removed every now and again.

The touchscreen controller

The battery is held in place with cables ties. This method allows for quick and easy removal if needed while still holding the battery securely in place.

The 12V battery held in place with cable ties

A few bits of Velcro are also used to keep the cables attached to the hardboard, but allow them to rip loose, if the cables gets caught on something, without damaging the cable.

The Velcro cable guides

Attaching the backpack to the hardboard is as simple as riveting it to the hardboard. Just make sure that the rivets pass through the harder pieces of material and will not rip out when the backpack is used. The complete backpack without the laptop is shown below.

The backpack riveted to the hardboard

The laptop side of the backpack

One last thing that needs to be reattached to the laptop body is the Wi-Fi and Bluetooth antennas. This is done by simply re-soldering the wires and sticking the antennas to the laptops body. The wires and antennas are then covered with insulations tape.

The Wi-Fi and Bluetooth antennas reattached to the laptop

Getting back to the screen, the next step is sorting out the mess of wires and reinforcing them to take the strain of regular use. The first step is to make a hole in the back panel to allow the cables through. Next is wrapping the cables in a cable sleeve. Below is the first attempt. It turned out to be a bit messy and hard to attach to the laptop but the idea was working.

The mess of wires that need to be sorted out.

The cable sleeving

The first layout attempt

The laptop ends of the screen cables were reinforced using heat shrink. The connectors will later be covered in glue or some resin for the final reinforcement but since this is permanent it will only occur after everything has been tested.

The laptop end of the cable

The screen end of the cables has simply been wrapped in insolation tape and securely fastened to the middle panel with a cable tie.

The screen end of the cable

The cable tie used to secure the cable

Below is the final cable layout that works a lot better than the first Y-shaped attempt. Some of the cables had to be folded double to make this work, but since the sleeve is used it does not affect the overall usability and flexibility too much.

The final cable layout

As a final bit of reinforcement, fishing line was used to take the tension off the cable and prevent any jerking from tearing the cables loose.

The fishing line reinforcement

The final step is then simply to screw the cameras and sensor mounts to the back of the screen. Below are some pictures of the fully assembled system.

A quick test of the video see-through

So far the device has survived a few demonstrations and tests. The next step for the project is to complete the software to interface with the hardware and to create demonstration applications.

3 Comments

  1. 1
    Petrus Theron on Thursday 26 May, 15:34 PM #

    I’d say the iPad market is a bit crowded, but this ridoncudonk device might just gain traction as an Egon gun accessory.

  2. 2
    Carel on Thursday 26 May, 23:53 PM #

    Thank you Waldo for the detailed instructions. This certainly looks like a fun little project to do over the weekend!

  3. 3
    Oswald on Sunday 29 May, 07:26 AM #

    Interesting and detailed account!

Leave a comment

Leave a Reply

(required)