Pennybot

Pennybot is my attempt at a second mini sumobot. A sumobot with brains (or at least a microcontroller), IR sensors to detect opponents and possibly other sensors (a front touch sensor looks like a good idea). Again I will be constructing the chassis mainly from scratch. For motors I had two GM2 motors from Solarbotics. I really liked the offset design of the motors and skinny wheels available. Getting Nudgebot to fit in 10x10cm box was quite a challenge with the standard Tamiya gearbox and wheels. The GM2 motors will make life much easier.

For the chassis I tried many ideas before getting it right. All were around the same concept with the motors mounted on pieces of 90 degree aluminium which in turn are connected to a base plate. Just the positioning and battery location kept changing as I kept finding flaws in the designs. Part of this was due to looking at the design over a few months rather than spending a whole day just to work things out.

In the end I mounted both engines on 80mm lengths of 90 degree aluminium angle. Due to the position of the holes in relation to each other I used a paper template rather than dead reckoning to position the holes. The end result was quite a nice job (compared to other jobs I've done).

For the base I used the Tamiya plastic base plate like I had used on Trackbot and Nudgebot. Pre spaced 3mm holes are wonderful.

I mounted the motors on the bottom of the aluminium angle in order to fit the batteries underneath the base plate. This put the batteries at the lowest point and ensured the centre of gravity was as low as possible. To stop the batteries being damaged or accidently falling out I made a cover plate from some thin metel sheet (The EMF covering from inside an old Sun unipack disk enclosure). Instead of bolting the cover plate to the base plate I used a 3mm tap to put a thread in the cover plate. This meant I didn't waste space by needing room for nuts. Another case of "why didn't I always do this". At least that tap and die set purchased was justified.

For batteries I am using 5 x AA batteries. The motors are rated at 6V so I'm still working out if I either over supply the motors, use a diode to drop the voltage or just make a separate 6V tap. I intend to use a LM2940 low voltage regulator to provide a 5V supply off the 7.5V from all the batteries. How this regulator handles the noise from the motors is to be determined.

I did a quick temporary solder to add a power switch and did some push tests with 4 batteries (6V). No issues pushing the necessary weight (500gm). However I am slightly concerned by the amount of wobble in the axle on the motors. Straight line driving is really needed but there was definitely some travel. Hopefully when I screw the wheels to the axles (rather than just push them on) this will tighten up the slop.

CadStd review

Much in a similar vein to using TinyCad to produce semi professional schematics I decided to up the level of my designs with a CAD package. I needed something that was cheap, easy to learn/use and could produce 'true size' paper print outs. The dual end goals were to document my designs but more importantly be able to produce templates for machining (eg drilling templates). Previously I had used graph paper for these tasks.

I had no experience in using CAD packages or technical drawing. So the first step was some research into what a CAD package can do and what to expect out of one. I would recommend anyone looking for a CAD package do this first as some CAD programs are more focused on particular tasks than others (eg 2D vs 3D plans).

After some web research I came across CadStd (www.cadstd.com). The product has two versions, a free unlimited use lite version with some restricted functionality and a licensed pro version with full functionality. The lite version could be all that you need in cases where you are only doing simple plans. After using the lite version to do the tutorials I purchased the Pro version at $37.50 USD. The version I have is 3.7.2. CadStd is a 2D program but it does do isometric projections.

The install was simple with the total install size approx 5mb. There is a tutorial that runs through the basic functionality in a structured lesson format. By the end of the tutorial I was able to construct some simple plans. There is also a user guide. That said you will need to a few hours playing with all the drawing functions just to see how they work. I found some of them odd to begin with (like the three ways to draw an arc) but I suspect that is due to a lack of experience in both CAD and technical drawing. This is definitely a case of the more you do the easier to becomes. CadStd isn't MS Paint with a bit added. It's an engineering tool. You will have to work to use this but that seems to be constant across the CAD field. Feel free to insert "learning curve" in there too.

The litmus test was turning a technical drawing (the Solarbotics GM2 motor plan) into a drilling template. The drawing was detailed but not to scale. After an hour of fiddling I had drilling template ready for use. So success. Did I need the Pro version to do this? No. However I like to support people making tools like this.

I would recommend CadStd to anyone who has limited time to learn new products but is sick of using graph paper to make substandard templates. Be prepared to spend 2-3 hours to learn how to use it and expect to make so odd mistakes along the way. Ctrl-Z is your friend.

A(nother) headbot

Another project coming out of "Junkbots, Bugbots & Bots on Wheels" was the construction of a headbot based on the 74AC240 'biocore'. Again I would fully recommend this book for it's interesting projects and "make stuff from junk" ethic. The biocore was constructed according to the instructions in the book. I used 22uF capacitors as suggested.

My first attempt at a base was a failure. I used a small 3-6V DC motor with a pulley attached, which was geared down (via a belt drive) to another pulley that was attached to a larger base plate which had the batteries, biocore, etc. In essence trying to simulate the book example of a cassette deck setup. However the balance was totally off and the motor would simply jam under load (or at best spin and the belt would slip). What I needed was either a cassette deck or a gear head motor. Frustrated I put the biocore and other parts back in the big box of components for another day.

A couple of years pass. Kids arrive. My soldering iron gets rusty.

One day at Dick Smith (when they still sold electronics) I purchased an old and heavily discounted line following robot kit fot $20. The design was poor and complicated but at tht price I wanted the parts. Two gear head motors, a relay, seven IR leds, etc. Bargain. Now I have a gear head motor.
The 74AC240 was purchased from Solarbotics. For the base I used the wheel from the above mentioned robot kit as that was keyed to fit the 4.5V gear head motor. The phototransistors were salvaged from an old balled computer mouse. The base plate is a lid from a vitamin container. While this lid was a great size it wasn't the best choice.

My power source was to be two 1.5 volt AA batteries. For balance I mounted these batteries in two separate battery holders on each side of the motor. I used two part epoxy glue to stick the holders to the lid. However once the glue had dried I found that the glue simply peeled off the lid/base plate. Vitamin lids were obviously made to be highly non stick to stop nasty molds, etc from attaching. So instead of glue I used small (2.5mm) self tapping screws to attach the battery holders to the base plate. The screws came from my collection of salvaged cdrom bits.

The power switch was a small SPDT switch I had which I mounted sideways into the rim of the base plate. Then all that was left was to wire the various components together and put in the biocore. I couldn't decide on a good way to attach the biocore to the base plate so I left is 'hanging' but held in place by the wiring. The wiring was made to be just long enough so there was enough tension to hold the biocore in place. Finally the two eye sensors were attached to the rim of the base plate with U-tack. I didn't want to permanently mount the eyes as I wanted to trial the best setup.