Monday, January 12, 2009

#2 - Components of a RepRap

The Heart of A CNC Machine
If you were to ask me what the most important part of a cnc machine is, I would have to say the motors. Everything else is not necessarily fluff, but is around arguably to soley support the motors.

A CNC machine is defined by the amount of degrees of freedom it has. A regular desktop printer is a 1-dimension cnc machine. If you've ever peered inside an inkjet printer, you can see the print head moving across the page as the page is fed through. There is one motor controling the exact movements of the print head.

A 2-dimension cnc machine is something that you might find in an industrial shop. If you've ever seen a show on how things are made, then you've probably seen a large robot laser cutting out precise shapes out of metal for car parts. For this type of machine the worker places a large square piece of material down on a platform, and the cutting head will move in two dimensions over the sheet of material, cutting as it moves along. One motor is used to control the cutting head in the right to left direction (called the x direction) and another motor is used for the back to front direction (called the y direction). Perhaps you've used an etch-o-sketch before. It's the same concept. Just pretend that your right hand is one motor, and your left hand is the other motor.
A 3-dimension cnc machine is, you guessed it, a machine that can move in 3 dimensions. In addition to the left to right, and back to front motion that the 2-d machine uses, it can also move the print head toward and away from the material. Or in the case of the RepRap, will move the stage away from the print head. And unless you haven't been following thus far, you'd know that the 3-d machine utilized 3 motors for all three directions of movement.

The Motor
I'm sure you've hooked up a toy motor to a battery before. It's neat, just connect one side of the motor to the positive pole of a battery, and the other side to the negative end. The axle turns, maybe powering a fan, or a remote control car. At best these motors are either on (forward/backward) or off. Now you probably could hook one of these motors up to the reprap head and if you wanted to go 3" to the right, you could turn it on for 2.46 seconds (a guess?). Of course, only the best guesses for the timing of these on/off motors, wouldn't be very accurate.

There is a different kind of motor: the Stepper Motor! A stepper motor would confuse anyone. It doesn't have just two wires, but anywhere from 4 to 8. And connecting these wires to batteries wouldn't actually do anything. The axle wouldn't turn, it would just sit there. Very unimpressive until you know how they actually work.

In a stepper motor, there are several different windings of wire, that when power is applied creates an electro magnet. There is a permanent magnet situated in the middle of the electromagnet windings. If the windings are turned on in the correct order, the axle (attached to the permanent magnet) will turn. In the figure on the right we will label the top winding (1), right winding (2), bottom winding (3) and left winding (4). When we attach a power source to winding 1 an electromagnet forms with the south pole pointed down. Since opposites attract, the permanent magnet's north pole is pulled toward the electromagnet. If winding 1 is then turned off and winding 2 is turned on, the permanent magnet will turn to the right, orienting its north pole to the electromagnet formed by winding two. The same thing will happen if we turn off winding 2 and turn on winding 3. The magnet will turn, orienting its north pole down because of the attraction to winding 3. And of course with winding 4, it's completely similar. If you had 4 switches that turned on and off the windings corresponding to the switch number, you may do this to turn the motor one full clockwise rotation.

Step 1: Turn On Switch 1
Step 2: Turn Off Switch 1
Step 3: Turn On Switch 2
Step 4: Turn Off Switch 2
Step 5: Turn On Switch 3
Step 6: Turn Off Switch 3
Step 7: Turn On Switch 4
Step 8: Turn Off Switch 4

It may seem like a hassle at first, but this verbose way to turn "on" the motor, has granted us exceptional control in controlling our motor. Many of today's stepper motors have excellent precision, down to around 400 steps per revolution. That means you could turn the axle less than a degree for each on/off of a winding.

If you want even more precision from your stepper motor, many of them allow "half-stepping." When two windings are on at once, the permanent magnet experiences equal pull by both of them and is pulled in between them. This allows for double precision. This process is shown below.

Everything else plays support
The component breakup of a reprap, or any cnc machine is:

Power Supply - Transfers power from the wall outlet/power source, to the electronics
Electronics - Microprocessor, and other circuitry to translate commands to move around the head
Stepper Motors (1 for each dimension of freedom) - Moves the head around with great precision
Structure - The platforms that holds the material being worked on, holds the head, and contains the electronics
Head - The device responsible for cutting, extruding, or manipulating the material being worked with

The structure can be seen clearly here. It is the metal rods joined by the white pieces. The electronics are strapped onto the structure in this reprap.

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