PID Tuning

The default PID coefficients work well for a large Big Green Egg with the standard 6.7CFM in the cardboard mounting bracket in a Florida climate with the top vent only slightly open. You may have noted the amount of detail in that statement, which is my way of saying that they probably won't work for everyone. Depending on your cooker, fan size, mounting solution and even weather, the coefficients may need to be adjusted.

Before you can get started

First things first, the HeaterMeter can only control the temperature if:

• The smoker is closed off enough that when the fan isn't running, the fire is getting snuffed out. If the temperature passes the setpoint and keeps on going higher and higher without the fan running, your smoker is letting too much air in.
• The smoker needs to be open enough that the fan can provide enough air to maintain temperature. If the grill is closed up too much, the static pressure inside can be too high and prevent the fan from ventilating your fire.

For my egg, the happy medium for 225F is to have the bottom vent closed all the way onto the fan mount and having the top dial vent only open wide enough to be able to insert a toothpick though one of the holes. For higher temperatures, the top vent is opened more and the bottom is always left closed. Once you've found your sweet spot you can move on to tuning the PID parameters.

Manual Tuning

Stolen from Wikipedia

• From the LinkMeter configuration page, set the controller to Proportional-only mode by setting B=0, I=0, D=0.
• Increase the P until the output of the loop oscillates, then the P should be set to approximately half of that value for a "quarter amplitude decay" type response.
• Then increase I until any offset is corrected in sufficient time for the process. However, too much will cause instability.
• Finally, increase D, if required, until the loop is acceptably quick to reach its reference after a load disturbance. However, too much D will cause excessive response and overshoot. A fast PID loop tuning usually overshoots slightly to reach the setpoint more quickly.

Effects of increasing a parameter independently

Parameter	Rise time	Overshoot	Settling time	Steady-state error	Stability
P	Decrease	Increase	Small change	Decrease	Degrade
I	Decrease	Increase	Increase	Eliminate	Degrade
D	Minor change	Decrease	Decrease	No effect in theory	Improve if D small

Ziegler-Nichols Closed Loop Method

• CAVEAT I did not find the values produced by this method to be at all usable but it is presented here as an alternative to manual tuning.
• Due to how slowly the temperature changes, set aside at least an hour for tuning. Start the smoker and let HeaterMeter control temperature at a desired setpoint for long enough for the grill to fully heat up. For webber grills and other with thin metal walls this can be pretty quick. For heavy ceramic cookers like the big green egg, wait at least 30 minutes for the entire smoker to come to temperature.
• From the LinkMeter configuration page, set the controller to Proportional-only mode by setting B=0, I=0, D=0.
• Watch the temperature which should now start to oscillate. If the error gets larger and larger over time, decrease the P PID parameter. If there is little to no oscillation, increase the P PID parameter. You're looking for oscillations with a fixed amplitude (distance between the min and max temperatures) and period (amount of time it takes to complete one min/max cycle)
• When a value of P results in a sustained periodic oscillation in the output (or close to it), mark this critical value of P as Ku, the "ultimate gain". Also, measure the period of oscillation (in seconds), Pu, referred to as the "ultimate period".
• Compute the new PID parameters using the Ziegler-Nichols parameters

Ziegler-Nichols
PID Parameter	Value
P	Ku / 1.7
I	Ku / (Pu / 2)
D	Ku * (Pu / 8)

* As an alternative to the Ziegler-Nichols parameters, there is an alternative calculation created by two other dudes, Tyreus and Luyblen, often called the TLC tuning rule

Tyreus-Luyblen
PID Parameter	Value
P	Ku / 2.2
I	Ku / (2.2 * Pu)
D	Ku * (Pu / 6.3)

Additional Resources

• The osPID people have a good Google Group, DIY PID Control with a lot of tips about manual tuning and accessible PID theory.