Plant Control

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Description of the simulated system

In this simulator a simple plant is simulated. The plant consists of two liquid tanks in series. The production rate is controlled by a flow control loop. The mass balance of each of the tanks are controlled by two level control loops.

Each of the tanks are integrators, dynamically. The model of tank 1 is

rA₁dh₁/dt = K₁u₁ - w_out

The model of tank 2 is

rA₂dh₂/dt = w_in - K₂u₂

where

w_out = w_in = w_SP (procution rate setpoint)

The simulator also implements sensor and subsequent scaling functions of each of level measurement signals. Also setpoint scaling is implemented. The sensor and scaling functions are availble at the front panel of the simulator.

The level controllers, LC1 and  LC2, are PID-controllers.

Aims

The aim of this simulator is to give an understanding of the behaviour of a process plant where the process flow and the mass balances are to be controlled.

Motivation

Industrial plants usually consist of one or more production lines which in turn consist of a number of unit process operations, e.g. reactors, heat exchangers, and separators, connected in series. It is important that the production rate of the process line and that the mass balance of the individual processes are controlled. This is done principally as in the present simulator.

Tasks

Initially, do not use feedforward control.

1. Open loop control: Show (by simulation) that the tank levels can not be controlled in open loop (fixed control signal). To see this, set the controllers in manual mode, and change the production rate setpoint a little (this change is a disturbance to the tanks).

    

2. Closed loop level control with P-controller: You can let the level controllers LC1 and LC2 have P controller actions with gains of 10.
   1. Explain that LC2 must be set in Direct Mode (i.e. the controller must have negative gain), while LC1 must be set in Reverse Mode (i.e. the controller must have positive gain). Run the simulator. What happens if you set LC1 or LC2 in wrong mode?
   2. Show in a simulation that there is a non-zero steady-state control error in one of the tanks after a change in the production rate (which acts as a disturbance).

    

3. Closed loop control with PI-controller:  Let Kp and Ti in both LC1 and LC2 have values 10 and 100 s, respectively.
   1. Show in a simulation that the steady-state level control errors are zero after a change in the production rate (i.e. a disturbance change).
   2. Show in a simulation that e.g. control loop 1 gets worse stability if the integral action is increased (Ti decreased).
       
4. Feedforward control:  Let LC1 and LC2 be PI controllers. Apply feedforward control from the production rate in both level control loops (set the Feedforward Control switches). Explain how the feedforward control should work, ideally. Simulate, while changing the production rate freely. Is there any benefit for the level controls by using feedforward? Probably not with the default settings of the controller output ranges of LC1 and LC2! But see if the feedforward control works as assumed if you change the Minumum value of the controller outputs from 0 to e.g. -4! Explain!

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Updated 22. February 2008. Developed by Finn Haugen. E-mail: finn@techteach.no.