PLC Function Block Function

Hi, How're you doing?
I've been trying to tune my PLC, a AB micro 820, for such a long time and I haven't yet noticed what I've done wrong.

I have a water level system consisted of two tanks, where one of then is only to storage the water that will be send to the other tank using a pump. The pump is controle through a frequency inverter and the inverter receives signal from the controller. The water level is measured using a ultrasonic sensor. The output flow of the tank is constant.

I've applied a step in the system and through its answer i was able to identify the transfer function of the plant.

I than, tried to apply Ziegler-Nichols to tune the system, but it didn't work. The controller output was binary, since the system is limited in the range 0-7v. I concluded that this happened because the gains were values too elevated.

My next move was trying to tune it using Mathlab, I used the Autotuner app, and noticed the it works really good for P and PI tuning the only thing i had to do is divide 1 by I parameter, but when i tried to apply PD or PID it doesn't worked. I've also realized that the PI part of the controller have the following transfer function Kp(1 + 1/(I*s))

I tried to find the block diagram or the transfer function of the PID Function Block but i haven't found anything.

Have you ever used a AB Micro800 series plc and used this PID function block?
Have you succeed tuning it?
Do you know how the derivative part of this function block works?
Do you have any idea of what could i do?
Do have a ladder block logic so it wouldn't be necessary using AB Function Blocks?

Ps: I've also tried to use others function blocks: IPID, RA_PID
Ps: Sorry about my poor english, hope you understand

I cannot offer you too much insight other than I tried those function blocks as well when I tried a PID with an 820--I ended up using the PF 525 embedded PID function for my situation--I never did get those CCW PID FBs to function correctly, maybe because it was my first programming attempt with CCW software and my incompetence prevailed or it's not very user friendly.

Did you read and try this:

https://www.google.com/url?sa=t&rct...05_-en-e.pdf&usg=AOvVaw0cIZ81A9kqRU_VlE6yb_91

It doesn't give you the PID equation so you can match your calculated parameters, but maybe it helps?

Also, if the controller output is binary, it means that your control variable is actually time. How is the PID being called and how does it update the outputs? Perhaps that is where your instability comes from?

We have several Micro850's running temperature control and they autotune without issue. We are using the RA_TEMP_CONTROLLER which states in its description that it uses a different autotune procedure than the IPID instruction. Maybe you could look at that procedure and find something that will work. Maybe this will help?

I'd think the PI would sufficient to do the task at hand. Maybe some sit-down-with-it?

Have you ever used a AB Micro800 series plc and used this PID function block?
Yes with good success
Have you succeed tuning it? Yes and operates very well in our pump applications.
Do you know how the derivative part of this function block works? I never use the derivative, as the process is slow enough to not need it.
Do you have any idea of what could i do? Control Engineering has some good PID articles.
Do have a ladder block logic so it wouldn't be necessary using AB Function Blocks? The older SLC500 had the math in a ladder, but the FB is a challenge to learn but worth the effort.

I know this isn't the most preferred method, but maybe faster than the Z_N math.