PID tuning refers to a proportional-integral-derivative control algorithm used in some repraps for hot ends and heated beds.
PID needs to have a P, I and D value defined to control the nozzle temperature. If the temperature ramps up quickly and slows as it approaches the target temperature, or if it swings by a few degrees either side of the target temperature, then the values are incorrect.
To run PID Autotune in Marlin and other firmares, run the following gcode with the nozzle cold:
M303 S175 C8
This will heat the nozzle, and cycle around the target temperature 8 times (C8) at the given temperature (S175) and return 3 values for P I and D.
These values can be entered with:
M301 P1 I2 D3
If the EEPROM is enabled, save with M500. If it is not enabled, save these settings in Configuration.h.
For the bed, use:
Bed: M303 E-1 S60 C8
and save bed settings with:
M304 P1 I2 D3
For manual adjustments:
- if it overshoots a lot and oscillates, either the integral gain needs to be increased or all gains should be reduced
- Too much overshoot? Increase D, decrease P.
- Response too damped? Increase P.
- Ramps up quickly to a value below target temperature (0-160 fast) and then slows down as it approaches target (160-170 slow, 170-180 really slow, etc) temperature? Try increasing the I constant.
A good article on PID tuning is here
See also Wikipedia