Control of Product
Quality and Flow
Incuding Ratio Control
Snapshot of the front panel of the simulator:
A process in the form of a tank blending cold and hot water is
simulated. The product flow Fp and the product quality,
which is here assumed to be the product temperature Tp, are controlled by
adjusting or manipulating two feed flows,
namely the cold water flow Fc and the hot water flow Fh. Fc is manipulated
directly by a flow controller, while Fh is manipulated using ratio control
where the ratio Kf is the ratio between the cold water flow and the hot
i.e., Kf = Fh/Fc.
In the simulator Fp and Tp can be controlled manually by manipulating Fc
and Kf. They can also be controlled
automatically using feedback from measured Fp and Tp and using Fc
and Kf as control (or manipulating) variables.
The aim of this simulator is to get knowledge about controlling flow
and quality of a simple, yet representative, industrial process, and to
experience the benefits of using feedback control in stead of an open loop
The process in this simulator is in many aspects similar to industrial
processes as combustion processes and reactors which can be controlled
In the text below values are given without any specific unit. You may
however assume that the unit is percent corresponding to some
transformatin from physical units as degrees C or kg/min to percent.
The time unit may be interpreted as minutes.
- Open loop control of Fp and
- On the front panel of the simulator are shown formulas for
calculation of Fc and Kf (=Fc/Fh). Now, derive these formulas from a
static energy balance. These values of Fc and Kf may be used in open
loop control of the product flow Fp and the product temperature Tp using
the cold water feed flow as one control variable and the hot water flow
as the other. (You may assume homogeous conditions in the tank when you
put up the energy balance.)
- Start the simulator. Select "local setpoint" of Fc and Kf.
Ensure that the FC controller and the TC controller are set in manual
mode (this will actually happen automatically by selecting local
setpoints of Fc and Kf).
Assume that Fp should have value 60 and that Tp
should have value 45. Tc is assumed to be 30, and Tv is assumed to be 70.
What are the corresponding values of Fc and Kf? (The simulator calculate
these values.) Use these values as setpoint values Fc,SP and Kf,SP
for Fc,SP,local and Kf_SP_local.
What is the static (steady state) control error for Fp and for Tp?
- Repeat task 1b, but now include model errors by setting Tc_error
equal to 10. What is now the static control errors of Fp and Tp?
- Close loop control (automatic control) of
Fp and Tp:
- Continue Task 1c, where we included model errors with respect to Th,
but select no "external setpoint" for Fc and Kf. Ensure that the FC
controller and the T controller are set in automatic mode (this is
happenig automatically by selecting "external setpoint " for Fc and Kf).
What is the static control error for Fp and for Tp?
- Change the setpoint of Fp and for Tp (select the new values
yourself). Is the feedback control system capable to bring the control
error to zero?
- The controllers TC, FCc og FCh have been tuned from the rule that
the integral time of the controller is set equal to the time constant of
the controlled process, while the control gains are given values giving
proper stability of the control loops. For the controller FC the
integral time is set to 1 (which is slightly larger than the sum of the
time constants of the cold water tank and the hot water valve). Play
with the PI parameter values, and observe effects. Particularly try to
obtain faster control of Tp.
Updated 18. January 2008.