CASCADE CONTROL
In this simulator two control systems are simulated in
parallel (i.e. simultaneously), see the front
panel of the lab:
- A cascade control system consisting of two control
loops - the primary loop and the secondary loop, each containing a
PID-controller.
- An ordinary, single-loop control system containing
one PID-controller.
Note that both control systems are excited by the
same setpoint, r1, and the same disturbance, d.
In both control systems the process to be controlled
consists of two sub-processes in series:
- Sub-process 2 ("quick"): y2 = P2*u + d, where u
is the manipulated variable, and d is the disturbance. P2 is
dynamically a third order system with gain 1 and response-time 1 sec.
- Sub-process 2 ("slow"): y1 = P1*y2, where y2 is the
output of sub-process 2.
y1 = y is the output of the whole process (consisting of the
two sub-processes in series). P1 is dynamically a second order system
with gain 1 and response-time approx. 5 sec.
The combined blocks of sensors and subsequent scaling M, M1 and M2 are
here simply gains of
1.
The controllers available are PID and on/off. The
PID-controllers shall be used under normal operation of the control
systems, but the on/off-controllers
can be used for controller parameter tuning (in the Åstrøm-Hägglund's
autotuning method). The controller functions (PID-controller and on/off-controller) are
described here.
The performance of the controllers is quantified through the individual
IAE indices. The IAE is the time-integral of the absolute value of the
control error. The less IAE, the better control. However, the IAE does not
measure the usage of the control variable.
The aims of the tasks given below
are
- to give experience in tuning the controller
parameters
- to demonstrate the benefits of using cascade control
compared to ordinary signle-loop control, with particular focus on
disturbance compensation.
Cascade control is a common control structure in both process control
systems and in servomechanisms. In process control cascade control is used
for temperatur control, level control, pressure control, and quality
control. These are the primary control loops, while the secondary loop
typically performs flow control or pressure control.
In servomechanisms the primary loops perform positional control or
speed control, while the secondary loop typically performs speed control
(in a primary positional loop), or current control (in electrical
servomechanisms).
Cascade control can give a much better compensation for disturbances
than single loop control can do.
In the tasks below the starting point is that the
process is in it's nominal operating point which is
characterized as follows: The set-point is 40 (unit %), the disturbance
is -10, and the process outputs y1 and y2 are 40%. The nominal value
of the manipulated variable, which keeps the process at the operating
point, is 50% for all the controllers.
- Controller tuning:
Find proper parameters for the primary controller C1 and for the
secondary controller C2 in the cascade control system, and for the
(single) controller C in the single-loop control system. (You may use
the Ziegler-Nichols' closed-loop method, or Åstrøm-Hägglund's
autotuner.)
Note: C1 will be used as a PID-controller, while C2 will be used as a
PI-controller. C will be used as a PID-controller.
(Tip: Remember to tune the secondary controller before
the primary controller, and the latter must then be in manual mode,
with a proper nominal value of the manipulated variable. Excite the
control system with a small step in the set-point. The controller
parameters should not be very far from the following: C1: Kp=7.2, Ti=5,
Td=1.25. C2: Kp=1.8, Ti=2, Td=0. C: Kp=4.2, Ti=6,
Td=1.5.)
- Cascade
control versus single-loop control:
- Disturbance compensation:
Observe how the two control systems manage to compensate for a
step in the disturbance. Let the step go from -10 to -30, but you
may well try other step heights, too. Which of the two
control systems gives best compensation (compare the IAE values)?
Observe also how the control variable works
in the two control systems. In which of them is the control
variable working most actively?
- Setpoint tracking:
Observe how the two control systems are able to make the process
output track or follow a step in the setpoint. Which of them are best
(compare the IAE values)?
[KYBSIM] [TechTeach]
Updated April 26, 2006.
Developed by
Finn Haugen.
E-mail: finn@techteach.no. |