Servo - March 2013

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by

Dennis Clark

Correction from the January Column

First off, I need to start out with a correction. In Figure 6 back in the January 2013 issue, I mislabeled the Roomba serial control mini DIN connector. The pin that is marked as pin 6 is actually pin 7. Table 2 at the end of that same article which lists the functions of the pins is correct.

Fortunately, both of these pins are the Roomba’s battery minus pins, so no matter how you connected them, there wouldn’t be any problems.

Spring is just around the corner, but winter doesn't seem ready to let go yet. There is still plenty of time to spend indoors with our robots, preparing for the next competition. That is certainly what I'm doing. The 2013 Critter Crunch is coming up in just a few months! (Ahh!) For this column, I'm going to do some more with Roomba controlling with an Arduino UNO in the hopes that this project will boost me well on my way to a new 20 pound Critter to compete with those other critters that still use "wetware" as the controller! Which, I have to admit, is pretty much all of them. Eventually, my robots will win the majority of its combats, and then win them all! Bwahahaha! Ahem.

Anyway ... This bit of Roomba controlling is a bit late, but better late than never (someone once said ... does anyone actually know who first said that?). Anyway, on with the show!

Now, back to our Roomba control round-up ...

More on the Hardware

Speaking of Roomba power connectors, you can’t just plug your Arduino into it without some preparation. The output of the Roomba battery can run as high as 20V while charging; while it is running, it is over 17V.

If you plug that power into the input jack of your Arduino, you will overheat your regulator and probably blow one of the voltage comparator chips! You need to step this voltage down to use it.

Here, you have two choices: You can either build a power board using a 7812 style voltage regulator and plug that 12V into your Arduino power input jack (which handles up to 12V safely); or you can step the voltage down to 5V and plug that into your Arduino 5V connector.

I didn’t have any 12V linear regulators, but I did have some Power Trends PT78ST105 5V switching regulators. These will take from 7V to 27V input and provide 1.5 amps of 5V as output. Just add two capacitors and you’re done!

Figure 1 shows my setup with the 5V regulator board and Arduino UNO board sitting on top of my “FrankenRoomba” robot. The one main problem with this setup is that power is always available at the SCI connector so the Arduino comes on and wakes up the Roomba as soon as it is plugged in.

How the Firmware Works

Take a look at the source code listing and use that as a reference to my following explanation of the very simple code: RoombaScoot. You can grab the zip file from the article link. You must use at least Arduino 1.0 or later for this program.