Archive for May 2008
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A very nice little project for interacting with your keyboards display
Just like magic, I deceided that I would undertake my own PC based GPS guided autonomous vehicle project, and Sparkfun.com announces a new USB GPS Stick. I received mine today. This little thing is awesome. It’s essentially a GPS receiver roughly the size of a USB memory stick. It uses the Prolific PL-2303 USB to serial chip, which has drivers for every major operating system. It took less than a minute to install this nifty little device on Mac OS X Leopard, and it appears to the computer as a standard serial port device.
My only complaint is the lack of decent documentation. None of the documentation mentioned at what baud rate the device transmitted data to the computer. Of course that’s a minor complaint for such a simple device. It took just a couple of minutes to figure out the right speed of 38,400bps.
The data is streamed as plain ASCII data, making it easy to read from any programming language that can access the serial port. In my opinion, if you’re planning to make a PC based autonomous vehicle, this is the low cost device that you need.
I was able to find the datasheet for the motors in my Power Wheels truck. I have it on good authority that all Power Wheels vehicles contain either a RS-550 or RS-755 motor. The motors are very different in physical appearance, so it’s easy to identify which motor your car has.
Pictured left: The RS-550 motor
The single passenger F-150 that we’re using contains two RS-550 motors which, according to the datasheet, can draw 70A of current when stalled. Typical performance is around 9A, but is still much higher than I expected. I should note that I’m not quite sure precisely what I expected, but I’m a little concerned about creating a motor controller for something that drives so much current. I may take a serious look into purchasing a pre-fabricated motor driver.
It is also noteworthy that that RS-550 motor is actually capable of running on voltages as high as 14.4V constant. And while I have no immediate plans to upgrade the battery, it’s nice to know that we can unlock some serious potential for speed. I’m already fantasizing about hosting my own little autonomous NASCAR circuit on a quarter mile running track. But one thing at a time.
Who knew that finding a Power Wheels car on a small non-American subtropical island in the Pacific would be such a chore? Sure, we have Toy’s R us, but I certainly wasn’t interested in paying nearly $300 for something that would be taken apart immediately and may never run again. Instead, I decided to waste the biggest part of a Saturday afternoon driving futilely from thrift shop to thrift shop in search of what was quickly beginning to look like a lost cause.
It was starting to look a little bleak. Later in the week however, I was reminded of a small classifieds paper that circulates Okinawa called “The Island Market.” Thirty minutes later, I was the proud owner of a Power Wheels Ford F-150. This particular model uses a 6V 9.5Ah battery, as opposed to some of the higher end models that use a 12V battery. This will obviously impact the performance of the vehicle a bit, but it will still serve our purposes nicely.
The truck is in great shape, save some cosmetic scratches on the plastic tires. I gave the guy $50 for the little truck, although I could have probably gotten it cheaper I was just happy to have found a suitable set of wheels for under $100.
There’s an unintentional side effect when taking on a project with a partner: Compromise. That dirty little ten letter word which promises to make everybody happy but proves to be nothing but a huge pain in the ass.
I had my brush with compromise the other day. It seems that while I presumed that our autonomous vehicle would run solely on the power of microcontrollers, my partner was under the impression that we would be using a PC as as the brains of the operation. And to make matters worse, he insisted that the PC run Windows.
I did my best to argue against the Windows PC, but in the end my friend proved to be doggedly relentless. Over time, I’ve actually grown to appreciate the unexpected change however; the beauty of compromise. Obviously a PC will provide us with a much more powerful platform on which to build.
In the end, we agreed to use a Dual Core 1.8GHz Dell laptop that has been lying around my apartment unused for over a year. This is proving to be an excellent experience on working in a group setting where the goals of the individual members of the group vary widely.
