Tuning a controller

The greenhouse heater now has a PI controller, but it swings: ten minutes too warm, ten minutes too cold. Is it KP? KI? Guessing costs hours because every change only shows after minutes. Better measure what the controller does.

Recognising oscillation#

A controller that oscillates overshoots above and below the setpoint in turn. So the error e keeps changing its sign. That is exactly what the chip counts:

sgtz sg e                 # 1 = too cold, 0 = too warm
sne tmp sg sgOld          # different from last time?
add flips flips tmp
move sgOld sg

A few changes per hour are normal, the controller is looking for the setpoint. Many changes at regular intervals, each time missing by a similar amount – that is oscillation.

One catch: around the setpoint the reading always jitters a little. Without a countermeasure every jitter counts as a change. That is why the counter gets a dead band: if the magnitude of e is below BAND, the sign is not evaluated again at all.

Gain step by step#

1. KI = 0 and KD = 0. Only KP, and a small one.
2. Double KP, wait, count changes. Keep going until the controller starts to oscillate.
3. Go back to half of that KP value.
4. Now add a small KI and raise it slowly until the steady-state error disappears in acceptable time. If it oscillates again, lower KI.

Always change only one value at a time and wait long enough – a large room needs minutes before a change shows. So you do not have to rewrite the chip every time, the controller reads KP from a logic memory (StructureLogicMemory, Setting read and write). If the memory is missing, the value from the define applies:

lbn kp MEMORY HASH("Kp") Setting Average
snan tmp kp
select kp tmp KP_STD kp   # without memory: default value

Logging values in the stack#

A display only shows the moment. For the history you put the error into a ring buffer in the stack: poke writes to a fixed address without changing sp. With 60 slots and one entry every 10 ticks you keep the last five minutes in memory:

add count count 1
blt count TICKS skip          # only every 10th tick
move count 0
poke idx e
add idx idx 1
mod idx idx SLOTS             # after the last slot back to the front
skip:

With get value db address you read an entry back, for example to find the largest error of the last few minutes. A second chip can also read the buffer with get.