Wednesday, July 13, 2011

BYOT: TurtleBot Power and Sensor Board with Gyro

Are you building you own TurtleBot or waiting for the production version of the Power and Sensor Board with Gyro to become available? Well, on today's episode of Build Your Own TurtleBot we will be showing you how to build a power and sensor board.

The board provides a 150 degree/sec yaw gyro and 12 volt regulated power for the Kinect. Note that the voltage regulator has an enable pin so that it can be automatically disabled to stop the Kinect from pulling more current than the charging system is capable of providing.

To make this widely available we implemented  a single sided version of the TurtleBot Power and Sensor Board with Gyro in Eagle, and made it available here, along with instructions for modifying a Kinect power cable for compatibility.

There are other ways to do this but we use the PCB "Fab-In-A-Box" from PulsarProFX to make the process quick and painless.

Once your circuit board is designed print out the design on Toner Transfer Paper taking care to make sure that the printed design is a mirror image of the desired pattern and printed on the shiny side.

Depanelize the paper circuit boards and cut out single sided copper clad fibeglass boards to the desired size.

Scrub the copper surface briefly to remove oxidation and optionally wash briefly with a degreaser.

Insert the board into the laminator with the printed circuit facing the copper. After the first pass, rotate the board 180 degrees and reinsert into the laminator.

Soak the Toner Transfer Paper off of the boards.

Cut the Toner Resist Film to the needed size with a bit extra to fold over the end. The Toner Resist Film makes the toner less permeable to the etchant and improves the final results.

Insert the board into the laminator, while working to keep the film fairly smooth.

Again after the first pass in the laminator, rotate the board 180 degrees and reinsert.

You may find that there is excess resist left behind by this process.

To clean up the resist, apply a piece of low tack masking tape, the type often used by painters.

Removing the tape gently should remove the excess resist.

Next, put on some rubber/latex/nitrile gloves, some safety glasses and get ready to etch! We will be using the plastic bag technique, which is a variant of technique #2 shown here. Place your circuit board inside a ziplock bag with a small amount of etchant and seal the bag with most of the air removed.

Rub the board gently to ensure the fresh etchant is always in contact with the board. In terms of fluid dynamics, you are fighting the no slip boundary condition to remove used etchant and supply fresh etchant.

The process is faster than the time is takes to get bored, and you can easily check the progress by holding the bag up to a light.

This is the resulting mess created by etching four boards and the used etchant has been placed in a sealed glass container for resue.

To remove the resist, the boards are soaked in acetone and scrubbed gently.

Etching is now completed.

Optionally you can silver plate the boards using Cool-Amp plating powder for improved performance.

Use a moist high strength paper towel and rub the powder on to the circuit boards.

The silver plating improves conductivity and solderability.

Next, drill the holes needed for the through hole parts and contemplate why everything is switching to surface mount. I found that a #65 drill bit worked well but your mileage may vary.

Begin soldering the surface mount parts starting with the 0805 capacitor. A boom microscope may be helpful for this process.

Here is what the parts on the top should look like.

Next mount the pin header on the bottom taking care to keep the pins straight.

 Next, mount the gyro upside down with the top of the gyro flush against the bottom of the circuit board. This configuration makes the board single sided and helps ensure that the gyro is parallel to the circuit board.

Here is the finished boards with a Micrel MIC29151-12WT voltage regulator mounted upside down on the top of the board. The regulator is bent face down after the heatsink is attached to ensure good airflow.

If this seems like too much work you can always order it along with other TurtleBot parts here.

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