RepRap for Dummies - A RepRap primer

Taken from  http://josefprusa.cz/skeinforge-40-volumetric-calibration  (Thanks Josef Prusa)

In this article I want to help you with SF40 calibration. 

Main difference over older version is volumetric flowrate. 

It basically means that SF counts how much of the plastic its should extrude for given layer height and width of trace (height*width*lenght = volume). 

Most confusing on all this is to realize SF uses the length of filament which goes in instead of extruded thread which goes out. For that you need to set your e_steps_per_mm in your Firmware to count with filament instead of extruded thread. And I'll show you how.

Tools you need:

1. Some permanent marker which can make visible marks on your filament
2. Calipers
3. Brain
4. My RepRap calculator http://calculator.josefprusa.cz/#ExtrusionSF40

So how to do it?!

PID is a control mechanism for things (temperature, ships, motors..) that need some time to react because of inertia or mass.

As the nozzle of your extruder does not get to temperature rightaway PID is a very good method to this solution.  

The brute method is that You set a certain target and the software tries to reach that target according to feedback given from the thermistor (or thermocouple..).  It does not do anything to it so the heater is turned on and off depending on each reading. Too hot off -- too cold on -- Too hot off  and so on.

PID tries to mimic the human behaviour.   If you were to heat up something to a certain degree and had a thermometer for control..  You would know that applying full power up to a certain temperature and then gradually decreasing the power would get you a correct temperature..

This is also a very good article about PID... http://mbed.org/cookbook/PID (but maybe a BIT too much in depth)

Parameters and what they do (sort of)

P_Param:  the bigger the number the harder the controller pushes.
I_Param:  the SMALLER the number (except for 0, which turns it off,)  the more quickly the controller reacts to load changes, but the greater the risk of oscillations.
D_Param: the bigger the number  the more the controller dampens oscillations (to the point where performance can be hindered)

These are rules of thumb and aren't exactly what's going on inside the controller, but they should help when deciding which number to change and in which direction.