Help with PID_IND for Emerson CPE100

[OBAKENG1997]

Good day

I'm new to this PLC programming Industry but I have been working with CPE100 from Emerson to program a system logic for Waste Water Treatment plant. During this journey, I have so far been able to model and implement my logic well but I'm still struggling to work with the inbuilt PID controller (PID_IND) to regulate and change the speed of a 3 phase motor which is controlled by a VSD (speed up to 50Hz = 1500rpm).

I have an open tank that has a Dissolved Oxygen sensor which measures the volume of oxygen/liter. The tank is equipped with motor that stirs the water in order to decrease or increase the oxygen levels. So, i'm required to manipulate the speeds of the motors with a PID controller to reach the optimum dissolved oxygen levels of 1.8mg/l (SP)

I would like to ask for some assistance setting up the PID_IND gains for CPE100.

My contacts:
- [email protected]
- LinkedIn: Obakeng Tom

 

[drbitboy]

From what you say, it sounds like the motor speed controls the current degree of agitation of the water in the tank, and increasing agitation increase the rate of dissolution (dissolving) of oxygen into the water. Is that correct?

Is this a batch process or a continuous process? That is,

• Batch
  • water is fed (filled) into the tank,
  • when some level is reached, that water feed is shut off
  • some process or reaction occurs, during which the motor controls the dissolved oxygen (DO)
  • when the process is done, water is drained from the tank
  • the procedure repeats
• Continuous
  • the level and quantity of water is the tank is more or less constant
  • fresh water is fed into the tank continuously at some rate
  • tank water is removed from the tank at the same rate, on average over time, as the feed water coming in
  • the process or reaction occurs continuously
  • the motor controls the dissolve oxygen according to the canonical mass balance, in this case for oxygen (O2):
    • In + Motor-Dissolved + Generated = Out + Consumed
    • i.e.
    • FLOW_feed * DO_feed + O_2motor + O_2generated = FLOW_out * DO_tank + O_2consumed
      • O_2generated and/or O_2consumed might be zero; we don't know the process yet

Tuning a PID comes down to understanding the dynamics of the system i.e. system gain, time constants, deadtime. "System" includes both the process (what load changes are like e.g. does the feed flowrate change? does the feed DO change?) and the control element (motor+agitator; how long does it take to effect a change in DO when the motor speed changes?).

Knowing if the dynamics vary (e.g. is the rate of change of DO vs. motor speed non-linear?) at different operating points would also be useful.

The simplest way to understand the dynamics of the system is to control it manually, the same way we control water temperature when taking a shower.

Even better would be making trends of the system response to a step change in motor speed, when the system load is (hopefully) constant (flowrates and feed DO). Once such trends are available, the system gain, time constant and deadtime can be estimated, once those are known there are any number of tuning techniques that can be applied when such trends are available.

 

[drbitboy]

Also note that a PID is not always the good fit for DO control; floating control is another option.

There are many threads in this forum on DO control in waste water treatment; I suggest you do a search for them and read them.

E.g. see this thread; and this one.

 

[OBAKENG1997]

Actually this is a Biological Reactor tank where bacteria feeds on waste water at an optimum Dissolved oxygen levels of 1.8mg per liter. Currently the sensors reads an amount of 11mg/l. So the more speed I run the motor, the more Oxygen it reduces. The flow of waste water into the tank is constant and also proportional to the rate at which it leaves the tank.

Thank you for your suggestions i'll look into them and decide on which one to implement