Brian Z

Hey all,

Sorry this is so long, especially for my first post.

I need some opinions. I just started my job as a high school computer aided drafting teacher and I get to set my own curriculum and class focus. I have assigned my advanced class the task of dividing into teams and designing and building radio controlled cars and racing them on the basketball court. We have a 3D printer on which we are going to make most of our parts. It seems to make pretty robust parts, and will do well for this assignment. Here are the rules: The car must fit in a box 8” long by 7” wide x 5” tall, it will be powered by 4 AAA NiMH batteries, the motor will be a $1.59 130 sized brushed motor, the “high power” one from Pololu, a Hextronik HXT500 servo for steering, a 2 CH Hobby King 2.4Ghz radio. I have designed a $6 circuit that speed controls the motor (with reverse) using a DRV8833 H-bridge. Also they will get an assortment of 3mm bolts, threaded rod, washers, and nuts, as well as 3mm, 4mm, and 5mm brass tube that just fit into each other for making axles and bushings. Oh yeah, I ordered a set of Kyosho Mini-Z pinion and spur gears that we are going to make a silicone mold of for casting our own copies out of 2 part resin. Next year I hope to have our CNC mill up and running, but this year we are just going to copy the gears. They will 3D print their wheels, but for tires they are allowed to wrap or coat their wheels with whatever they want, but it can’t be something that is sold as a tire. In other words, they can’t buy tires, they have to use rubber bands or o-rings, or foamy broom handle grips or something. I want to promote creative problem solving.

Here’s my question: since I am not going to buy them each differentials, and since a 3D printer is not really capable of making a good small differential, they are going to have to work around that issue. As I see it, they have three options: Use one rear wheel in a reverse tricycle setup, run a straight axle in which both rear wheels turn at the same speed all the time, or have a rear axle in which one wheel is powered and the other just spins. I have no idea which one is going to work the best on a basketball court. What are the pros and cons of each of those solutions?

Brian Z

SyCo_VeNoM

interesting idea
Remember when I had Drafting, and was told to design anything, and drew a RC many many years ago.

As for the pro's and cons of the different setups

1 rear tire it will have overseer if it isn't balanced right. There was a 3 wheel RC like you described called the Striker(or something like that) by redcat which you could look at a video of to get the general idea on its handling(but it was 3wd)
drive on one rear tire. It will pull to one side
solid axle it will have a larger turn radius


side note did you test that circuit to accept PWM to control the motor.
Cause if you didn't there are 20A ESC's on ebay from china that I used that will be more then good that will do the job for like $10 a pop like http://www.ebay.com/itm/RC-ESC-20A-B...item4621b0b880

There is also another option that I've used in the past which was 2 simple momentary SPST NO push button switches set up in an H-bridge that a servo would smack for whichever way I wanted it to go

Brian Z

Thanks for the input! I'm going to let these kids figure it out on their own, but I wanted an idea of what to expect. I did test that circuit. I originally built it to control LEGO Power Function motors and servos so that I could use my 2 ch hobby radio to control my LEGO vehicles. I have a link at http://brianzawesomeblog.blogspot.co...ons-radio.html for the old version. The new version works much better, but uses surface mount soldering, so it's a bit harder to build.

SyCo_VeNoM

very interesting circuit setup.

Great idea letting them figure it out as I find trial and error is the best way to learn, and come up with newer better ideas(something the schools around here don't bother with judging from my nephew's, and niece).

so gonna make the kids etch the PCB?

Brian Z

That's a good question. I want them to see the process, but I really need to focus our limited time on the CAD side of things, as that's what the class is all about. However, circuit designers are CAD operators these days too, and indeed I designed this etch pattern in AutoCAD. I will show them how to make one circuit, and if they want I will let them come in after school and learn the full process. When I programmed the Picaxe chip to input the servo signal and output the PWM forward and reverse I used a bunch of algebra, and I want the students to see real-world applications of advanced math in this class too. I feel like if I can't inspire these students to push themselves with RC cars, then I need to find a new career.