beebbot

Building a tracked square-bot

2006-10-22T18:24:00.000-04:00

As mentioned previously I found a lot of the Vex hardware on sale during the summer. One of the supplemental kits I picked up was for tracks. The track kit comes with bogies, drive wheels, linked treads, and hardware (screws and lock nuts) to build two sets of treads. The following photo shows a completed square bot frame with the treads on the sides in place of wheels.

The directions are fine for placing the drive wheels and bogies, but the method for firmly attaching the drive wheels to the bulkhead and to the drive motors is only lightly covered. If you follow the directions the driver wheels (the sprocketed wheels at both ends of the tread) won't stay on. Instead you have to come up with better assemblies at both ends.

The following image shows how I used three collars to hold space a drive motor as well as keep the drive shaft in place. Two of the three collars had their tightening screws removed; they weren't needed. Those collars are used to space the motor away from the side rail, and two of the longest 6/32 screws are used to assemble the motors through the collars onto the rail. The third collar keeps its screw, and is used to hold the drive shaft in place once the shaft is pushed into the motor's clutch.

The front idler has a similar construction, with a plastic bushing used to help hold the axle in place without wobbling. What's not shown is that on the other side of the metal bulkhead, both wheels have four washers and a short plastic spacer (the 0.182 incher) between the sprocketed wheel and the bulkhead; the washers are against the metal, and the spacer is against the wheel.

To keep the sprocketed wheel firmly in place, collars were added to all the outside of all four sprocketed wheels.

Setting up the transmitter for tracked control

Unlike the wheeled square bot, the motors are directly connected to the treads. In order to use the transmitter controller with the treaded square bot the way you use it with the wheeled square bot, you need to configure the transmitter. I configured mine by selecting configuration (see page F-2) #2, and then reversed the direction of motors (channels) 2 and 3 (see page F-5 for details). I now have two configurations; 1 for wheeled and 2 for tracked. I select the configuration and then just use the sticks.

Back in the lab

2006-10-22T17:56:00.000-04:00

After watching the prices of Vex equipment drop to half price or less all during the summer months ($149 for the Vex Robotics Design System, for example), I picked up enough hardware to build 3 1/2 independent square bots. What was missing for a complete forth is another Vex controller and radio receiver. I'll have to go back to the Vex site and probably pay full price for the components, unless I break down and buy a forth Robotics Design System.

But I did have three controllers, so I finally found the courage to take the cover off of one and see what's inside.

The controller has a pair of 18F8250 micro-controllers. Considering what's on the board and what it does, I find it quite clean in its layout and design (at least in my humble opinion). Plenty of nifty things to hack!

Me and my Legway

2006-04-18T21:52:00.000-04:00

(This was originally published here in January 2006) Two years ago Steve Hassenplug created a two-wheeled self-balancing robot using the Lego Mindstorms Robotics Invention System 2 and a pair of special sensors manufactured by HiTechnic. It was the same Lego Mindstorms that I had purchased that previous Christmas. Unfortunately when I finally found out about Steve's work it was nearly a year later, and a key component of his Legway, the HiTechnic EPOD Sensor, was no longer being made. Jump forward to November 2005, and I find out that HiTechnic is again building and selling the sensors, so I immediately ordered two for myself.

While waiting for the sensors to arrive, I started to look at the other bits needed to build the Legway. Two years is an eternity in computing. Key software and support systems had evolved since the original Legway was produced. Elements of the Legway that had changed and needed to be updated were:

I used the Linux environment on my Gateway to build and use the BrickOS software. When I attempted to put the software environment together I noticed the original tool chain used gcc 2.95. When I went looking for more up-to-date tools, I only found them for Linux. I followed Matthias Ehmann's directions for installing and building the BrickOS build environment under Linux. Let me emphasize that you use exactly what is called for in the directions. I tried gcc 3.4.5 with matching binutils. Using gcc 4.0.2 to build the gcc 3.4.5-based cross-compiler generated an ICE under 4.0.2. Gcc 3.4.3 successful builds as a cross-compiler under gcc 4.0.2.
The Lego Mindstorms I/R tower. The tower originally connected to a personal computer via to a nine-pin RS232 serial port. The system I used to program my RCX 1.0 brick was a Gateway notebook that did not have any serial ports. Instead, the Gateway has four USB 2.0 ports (eight connected to the dock), so I used a Dynex USB-to-RS232 adapter to communicate with the tower. This worked out very well under Linux, as the device showed up as /dev/ttyUSB0.
The original legway.c program from Steve's site was written for an earlier version of BrickOS. One change in particular has to be made to the code for it to compile and work properly with the current BrickOS: replace sys_time with long sys_time = get_system_up_time();

Everything was built and operated under Linux. Once I downloaded the firmware and then the compiled legway.c program, it 'stood up' and ran like a champ.

Legway lying on its side exposing the HiTechnic sensors. You can see them above each wheel.

Legway up and dancing around the desktop. The sensors are the gray blocks at the bottom with the wires coming out of their tops.

The RS232 tower connected to the Dynex USB adapter. I picked mine up at a local Best Buy.

Minor mechanical tweaks to the Vex squarebot

2006-04-18T21:15:00.000-04:00

Section 2 of the Vex Inventor's Guide shows how to build a sample motorized chassis they call squarebot. There's some additional mechanical tweaks that need to be made to squarebot in order to make him a little more rugged. First of all, you should add a collar to the outside of each of the four wheels. This keeps them from coming off while the squarebot is rolling around for extended periods. See the two photos below. The first is an overall view and the one beneath is a closeup of two of the wheels with the collars.

Next, you need to add spacers to the inside of the chassis subassembly holding the gears, between the outside of each gear and the outer chassis rail. It is annoying to be driving along and then have squarebot stop because the gears have slipped along the axles and no longer engage. The following picture shows the added plastic spacers, looking up from the bottom of the squarebot. They are the black shiny pieces between the green gears and the upper-most chassis rail.

I got these spacers as part of a Vex spare parts kit at a local Radio Shack after they cut the price for the spares kit in half. Why the spacers weren't a part of the $300 Vex Robotics Design Kit is beyond me, but they sure came in handy with the squarebot. Now everything runs fine for as long as I need.

Welcome

2006-04-18T19:44:00.000-04:00

Welcome. This is a spin-off from my other blog, blogbeebe. The idea came from my wife and kids, who suggested I create additional online diaries to better categorize my interests. This blog covers my interests in hobbyist robotics; the kind that mere mortals can afford. I have been a tinkerer in robotics since the early 80's. It was an offshoot in my interest in microprocessors and microcontrollers, such as the 6502 and 8048.

In this day and time I have the Lego Mindstorm's Robotic Invention Kit (and several add-ons of parts), several Vex Robotics kits, and all the odds and ends I've collected over the years. I've been investing in Vex since November of 2005 when I paid $300 for my first kit. I've been buying more this April because Radio Shack, the seller of Vex locally, have cut the price for just about everything in half. So now I've got sensors and rechargeable batteries and spare gears and wheels and tracks...

Another toy I have is a Gumstix and supporting boards. I purchased it about the same time I purchased my first Vex. I had the idea I'd use the Gumstix to drive controller on the Vex, but I've since discovered the Vex is a bit complicated by the fact that the protocol used to communicate with the Vex controller is undocumented. I put everything aside for a while, waiting for time to get back to it and dig out the details. Turns out that there's a new site name JVEX-robotics. It's purpose is to integrate a Java-based controller with the Vex system. I've just now started to dig into their system.

Finally, I'm waiting for Lego to release its latest robotics kit, Nxt. They started taking pre-orders April 1st, but I've held back because of the price of $250/kit. I'll probably place an order the first of May when my finances are clearer.