Friday, December 7, 2012

The Robot Dragster Project-Chapter Two

There is a certain logic to complex robotics projects.  At least this is how most experienced builders approach the challenge:

First, figure out your drive train.
Second, figure out your control system.
Third, work on the various bells and whistles.  Sometimes literally.

So for our first session of Advanced Robotics class I had an actual agenda.  We started out with the usual necessary discussion on safety and workshop etiquette.  I have three classes of tools.  One for general use, one for use after individuals prove to me that they are responsible, and a few that no kid gets to touch on penalty of banishment. You have to operate this way when you don't have access to the actual school shops and are working with less ideal hand tools.

As to motors, I probably had ten different options to put in front of them.  I selected these:

Here we have some starter motors.  These are an appealing choice because they fit the basic need for a dragster-lots of power for a short time.  If you use appropriate cable you can probably put 100 amps across these at 12 volts.  The one on the far left is a standard Bosch automotive starter with the original 3 pole solenoid attached.  The center one is a mystery unit, probably from a smaller car, perhaps Japanese.  It has the original "Bendix gear" to engage a gearing system.  The other two motors have already had the Bendix removed and replaced with a pulley for various past projects.  The unit on the far right is the starter from a 50 horse Mercury outboard motor.  I bought a very beat up boat from a neighbor a number of years back and had nothing but mechanical issues the whole two years I owned it.  When I scrapped it and got a better fishing boat this was all I saved.

As to which to select it will be up to the kids.  All run on 12 volts.  The bigger unit certainly turns out more power but also is heavier.  I anticipate some interest in both "heavy" and "light" dragster designs.

I also am putting forward some motors other than starters.

On the right is a 24 volt "Magmotor".  It's previous use was spinning the 120 pound weapon arm for "Newton's Claw" an alarming machine we used for State Fair demos. Newton was designed to obliterate appliances from microwave size up to washing machines.

On the left is a Chinese made scooter motor.  Also 24 volts but only 350 Watts.  I figure we could use one on each wheel of a dragster and could control each with a Victor speed controller without smoking the electronics.  The other motors would pull far too much current and will need some special arrangements.  (See next week, Mysteries of Solenoids).  I bought a dozen of these cheap a while back and have never used them.  I think the chain drive option would be interesting but pesky.

I told the group to decide.  If they want to mostly goof around and just build one machine, they get to pick one.  If they are a hard working bunch, willing to dig in, they get to build two.  If they promised to be the best bunch of students I have ever had, they get three.  In all instances we should have sufficient opportunities for the two basic types of learning.....plodding diligence rewarded and excessive ambitions that go Icarus on them.

The other major task of session one was figuring out the effective range of my "in hand" radio systems.  I gave them four different radio systems and assigned a group to figure out a way to test the point at which control signal is lost.  This is kind of important, as it will determine how much distance we have to attain max velocity.  Their mission was to design a test system, test it indoors then outdoors where the interference is less.  Then figure out if the range can be extended somehow....


Addendum.  Kids opted to build two dragsters, one based on the Magmotor the other on the outboard motor starter.  Good choices.  We tested the radio systems and found that the Vex transmitter we use in the basic robotics class has lousy range...just under 100 feet.

Better results with a JR radio system and with a Futaba PCM transmitter/receiver.  Better still when they added a longer antenna to the receiver.  Indoor range testing....

The Futaba system looks like a clear winner at just over 300 feet.  The JR was a respectable second at 235 feet.  But.....we have not done the more practical outdoor test yet.  And very importantly, my past experience indicates that the JR radio system will "fail safe" more reliably.  That is, it will shut down the power to the drive when signal is lost.  This could come in handy if you want your dragster to stop somewhere short of Winnipeg.

1 comment:

Matt said...

It is unfortunate that robotics is not offered at all schools. These students are getting a fantastic opportunity to learn about robotics, which is sure to be one of the top career opportunities in the future.