Dassman can you explain a PID controller to us, how does it work,
How should it work?
I will give you a quick layperson description, as there are lots of on-line resources if you really want go get into it.
The crux of the issue lies in the reality that we have become an instant gratification society. When we want something, we want it immediately(think Amazon). When extrapolated to pellet grills, it means we would like to dial in a set-point and get an instant response, and have that set-point temp stay exactly there; something we can immediately monitor in this digital age. While critical in industrial applications, it is not that critical in a pellet grill like the SF, because some variation in temp (oscillation) is not critical. [Enough soap box]
Essentially, , a PID device is used whenever you want to keep things relatively constant ( like temp), but have varying set point or changes to what your target (constant) wants to be...think about cruise control on a car...you get to the speed you want, engage the cruise and it will attempt to maintain that speed by opening and closing the throttle based on Feedback from the wheels (sensors).
PID stands for Proportional-Integral-derivative. Essentially, when you pick a set-point temperature for your grill, it strives to obtain and maintain that temp based on feedback (the permanent grill temp probe) by increasing or decreasing the pellet feed rate.
The simplest type of PID is an on-off-on type that feeds or does not feed pellets to maintain the temperature within a pre-programmed band. These can be found on some very inexpensive pellet grill.
A more sophisticated approach looks at the “P” as gain and error (over or under) and attempts to correct for that in its output (pellet feed). However, by itself it will lead to oscillation over and under shooting the set-point. Essentially this P function corrects for instances of error from the set-point.
Enter the “I” factor. Think of it as a mathematical holding basket the measures and accumulates the over and under shoot and essentially adds fewer or more pellets to reduced the under-over shoot from what the “P” function would do by itself. Essentially this function corrects for the accumulation of errors.
Finally the “D” factor, the most complicated of all. Essentially it looks at the rate of change based on the amount of change currently measured vs. the last time it was measured and puts a damper on the “P” and “I” functions. So, for instance, if the set point changes dramatically, the P&I functions will want to make major changes to the pellet feed, but the D essentially becomes what we used to call the “DO VS. DO-NOT-DO” function. [Think of it as the brakes in your car] It minimizes the oscillation of the temp that the P &I want to do and keeps the grill from either going too far over or under the target temp as a function of how long it is applied.
The “D” function is extremely important for steady-state control, whether your car or the SF. My concern is that if too much D is applied during major temperature changes, it may result in a flameout unless there is a means to “keep the fire going”. In a car, the engine is always being fed fuel and the spark plugs keep firing even when in idle. Not so with Diesel engines or pellet grills, as glow plugs are not a constant-on.
I believe, yet have no direct knowledge, that the last update attempted to resolve this by “refiring ” the glow plug at some point In order to maintain combustion, but by itself may not be sufficient as there also has to be some fuel (pellets) being fed.
What I and others have observed, it that the delays in response to getting to the new set point have become greater since the last update, which in a way is good, as it reduces the potential to overshoot or undershoot the target set-point which leads me to surmise that a bit too much “D” is applied.
From my perspective this is just part of the learning curve, it happens in all pellet grills, and will be corrected with the iterative upgrades. In the interim, I tend to minimize the flameout risk by making set-point adjustments that are Step wise and smaller.
It is easy to watch the PID controller in action of you have an ambient grill temperature probe. You can see the run-up and run-down in temperature as well as the oscillation around a set point as it gradually narrows the variations in temperature fluctuations over time.
Disclaimer: I may totally wrong about the logic Weber uses, as I do not have access to, or direct knowledge of, the Weber control logic.
Further, I will defer to others who probably have a much greater knowledge of PID control logic...all I am striving for is good food, and so far the SF is delivering (I am in the middle of smoking a 15# brisket as I write this)
