| 2006/07/12 13:06:17 PDT by abrahamliao |
Few things first:
Someone in the afternoon section left their RCX...
Markus, you forgot your whole robotics kit! It's in the ATDP office.
Please don't leave these things behind. They're expensive, hard to replace, and I also don't like carrying the extra stuff around.
As far as I know Allen your TA and iTA isn't back yet so I have to post stuff on the forum like this.
Now, onto steering:
The most primitive form of steering is having an axle connected to two wheels of different sizes. This will make your robot constantly turn in one direction. Not very useful, but it shows that when you're turning, your wheels are traveling different distances. More on this later.
Steering systems you typically use:
Differential drive - this is the steering drive you have used on your RCX for a lot of the class. Two motors, each connected to one wheel. You can accomplish steering on this drive by making the wheels spin in either direction, or by stopping one wheel and letting the other go. Advantages for this steering system is the ability to turn in place. Disadvantages, well, it slips a lot, so you would never see this on a car because it would burn rubber all over the place.
Steering drive - this is the one you built that uses the differential (grey thing with three small gears in it) and a steering box, using a rack and pinion. This is the steering system all cars have. Advantages are that it is stable over rough terrain, since the steering is connected to a worm gear, which is asymmetrical.
Rack and pinion: (-O- is a gear [pinion] and the /\/\ is also a gear [rack])
-O-
/\/\/\/\/\/\/\/\/\/\
Why need a differential in this steering system?
Anytime your robot steers and you have one wheel on each side, they will travel different distances. The differential is an ingenious device that allows that to happen. A test question may test you on different distances the wheels will travel in a turn. Like on your worksheet.
To calculate that, basically first take the radius of the turn. That will be used to calculate the circumference of the turn of your inner wheel. Add the radius of the turn to the wheelbase - this is the circumference of the turn of your outer wheel. Compute the circumferences (2*pi*radius) and subtract to find the difference.
Alternative steering systems:
Articulated drive-
____
| |___o
|___| _\_
| |
|___|
This system works like a train, except the center axle (drawn as an "o" on this picture) actually has a motor and turns the robot.
Pivot Drive:
____ ____
|___| |___|
o o o | o
_|_
This thing goes, then stops, then an arm comes out of the bottom, lifts the robot up, turns where it wants to turn, the arm retracts, and the robot continues on it's way.
Synchro drive:
-- -- / /
-- -- / /
Those dashes are supposed to be wheels. Basically, all four wheels turn in the direction they want to go. These wheels also spin. Very hard to design, very complex.
Brake drive:
Similar to the differential drive system, except this one you might actually see on a (stuntman) car. The car turns the same way - by braking one side of the vehicle. This would be done with independently controlled brakes, somehow.
Wheeled drive:
Works by increasing or decreasing the size of the tires. Sorta like the "most primitive steering system", except this one you can actually control. I've personally never seen one of these in action.
Tricycle drive:
Same as steering drive but only one wheel in front.
That's it for steering, if you have any questions just post on the forum.
MOTORS:
The basic purpose of your motor is to turn electric energy into kinetic energy.
When you pass a current through a curled wire, a magnetic field is induced (created). You can find the direction of this field using the RIGHT HAND RULE. Basically, a motor is just one big electromagnet, specifically controlled to do what you want it to do.
North is attracted to south, likes repel and opposites attract.
So a motor sorta looks like this:
__ __
/ / \ \
| | -o- | |
\_ \ /_ /
The two thigns on the sides are electromagnets, and the center thing is what is actually connected to your axle. The center axle is powered too, and when one side of the magnet turns on it is attracted to that side. So it spins that way. When it gets there, a COMMUTATOR switches the currents so that the axle will then be attracted to the other side. Thus, the motor will spin faster and faster.
You can change the direction of your motors two ways:
Reversing the current (in the ROBOLAB software)
Reversing the wire (achieves the same thing)
Your motors can be generators also. If you hook two of them up, spinning one will spin the other. This is the exact opposite of a motor - a generator. It converts kinetic energy into electric energy. Then that electric energy goes into the other motor and gets converted back into kinetic.
Calculations:
Power = Current * Voltage
You will mostly see this notated as P=I*V
Floating your motors just cuts the power, they spin freely to stop.
Braking them actually shorts the motors, forcing them to stop.
When you combine motors in a gearing setup, you increase only POWER, not speed, since motors turn at the same RPM anyways.