Monday, September 19, 2011

4 H Robotics project on a Saturday Morning

As I have mentioned previously, my phone occasionally rings with somebody wanting help building a robotics project.  This spring it was a group of 4 H kids who wanted to build something fun.  I should point out that there are several official, organized 4 H robotics programs, but they tend to be available only in larger communities.  And they take weeks of time.  And they cost money.

I said we could build something fun on a Saturday afternoon using stuff I already had sitting around.  Cost: zero.  My fee: pizza for lunch.

I have more robotics stuff sitting around than most normal people, but since the components we used are readily available I thought a more detailed build account was in order.

This is the 12 volt motor and gearbox from a Barbie Jeep.  I have a bunch of these, and keep acquiring more as derelict Jeeps come my way.  They are robust and cheap.

This shows the motor/gearbox installed.  The white plastic part seen on the first picture locks into grooves in the Barbie Jeep wheel.  The whole thing is attached to the plywood base by just screwing a big 'ol lag bolt in.  The plywood base is two layers of plywood from an old ping pong table.  If anyone every offers you an old ping pong table say yes.  It's great stuff for all manner of projects.

This is a sideways view showing the head of the lag bolt, plus a washer, threaded through the hub of the Barbie Jeep wheel.  Note the large caster wheel under the front of the platform and the on/off switch for main power.  The switch was pulled off some old appliance left on the curb.

This is the basic electronic "guts" of the project, and requires a little explanation for the novice.  To effectively control motors you need forward and reverse, and need the ability to go fast and slow.  Basically you need what are called electronic speed controllers.  The two black square objects are IFI Victor 884 controllers.  The are durable, easy to use and can handle 12 volts of juice.  You also need a radio receiver to tell the controllers what to do.  Any RC radio receiver would in theory work.  But I have taken to using a slightly more complicated system, the Vex controller.  The yellow object is the actual radio receiver.  The larger grey object is a microprocessor.  I'll be honest, I mostly use these because I got a couple for free.  But they are useful enough that I seek them out on ebay.  Both the Victors and the Vex equipment come up fairly often and at a reasonable price, because they are used respectively by the very excellent FIRST and VEX robotics programs.  After their competitions are over the robots often are scrapped and sold off.  The Vex microprocessor also has the ability to accept programming if you intend to build a robot that is actually semi autonomous.  What I am building here is basically a jazzed up, over sized RC car with a few extra features.
Main power is from a 12 volt gel cell battery.  A variety of small electronics such as fish locators use these.  Surplus stores carry them cheap and still fairly zippy.  The battery is held down with simple but liberal application of fire engine red duct tape.  The blue object in front is a 7.2 volt battery, of the sort used by larger RC cars.  It powers the Vex radio receiver and microprocessor.

The Vex radio has six channels.  You need one each for the left and right side motors.  That leaves you four more things to control.  This is a simple on/off switch built from a servo.  It allows me to under radio control turn on and off:

A high powered water squirter.  This is a windshield wiper pump powered off the 12 volt main power.  The water reservoir is a plastic mayonnaise jar.  I drilled a hole in the lid so that we did not have an open container.  Water sloshed onto electronics is bad juju.  The bendy metal strip supports the water reservoir (held on by rubber bands), and allows the water nozzle to be aimed.  I suggested that spraying people with water at crotch level would have the most attention getting effect.  Other switches operated strobe lights and a siren.

Here is the final product, a robotic remote control fire engine.  Built with a half dozen grade schoolers in four hours.
range was about 15 feet

I thought it was a pretty fun project, and the kid who presented it at the county fair won some nice ribbons for it. 

I enjoy this sort of thing, so if any questions on projects like this drop me a line in the comments section. 

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