Snapshot of the front panel of the simulator:
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
= K1u1 - wout
The model of tank 2 is
= win - K2u2
wout = win = wSP
(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
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.
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.
Initially, do not use feedforward control.
- 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
- Closed loop level control with P-controller:
You can let the level controllers LC1 and LC2 have P controller actions
with gains of 10.
- 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?
- 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).
- Closed loop control with PI-controller:
Let Kp and Ti in both LC1 and LC2 have values 10 and 100 s, respectively.
- Show in a simulation that the steady-state level control errors
are zero after a change in the production rate (i.e. a disturbance
- Show in a simulation that e.g. control loop 1 gets worse
stability if the integral action is increased (Ti decreased).
- 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!
Updated 22. February 2008.