Level Control of Wood Chip Tank
Snapshot of the front panel of the simulator:
The level control system for a wood chip
tank is simulated. The purpose of the level control is to keep the level
between specified limits: It is important that the level is not too high,
otherwise the chip will not be sufficiently pre-heated (by steam from the
cookery), or the tank will simply run full. And it is important that the
level is not too low, otherwise the steam will stream through the chip
causing an awful smell in the neighbourhood.
The level control system is (quite) similar to an
existing control system at Tofte Södra Cell in Norway.
The process consists of a chip tank with an inlet screw,
a conveyor, and an outlet screw. The inlet flow can be controlled by
adjusting the screw control variable. The outlet flow can not be
controlled, and it constitutes a disturbance on the level.
Here are a couple of instructional videoe where the present
simulator is used as an example:
Mathematical description of chip tank, level sensor and measurement
Below is a mathematical model of the process. However, knowledge about
this model is not necessary to do the tasks below.
A mathematical model of the process can be found using a
= Ku*u(t-td) - wout(t)
- y [m] is the level. y is the process output variable,
which is to be controlled.
- u [%] is the control variable, acting on the inlet screw, and giving
a proportional mass flow through the screw.
- A [m2] is the cross sectional area of the
- r [kg/m3]
is the chip density.
- Ku [(kg/min)/%] is the inlet screw gain.
- td [min] is the transport
time of the conveyor belt, which runs with constant speed. td
can also be denoted as dead-time or time-delay.
- wout [kg/min] is the mass outflow from the
tank. wout is a disturbance on the level.
Dynamically, this process is "an integrator with dead-time"
from the screw control.
The values of the model parameters are available via the front panel of the
Level measurement and measurement filter
The level is measured with a gamma-ray based measurement device having
range 0 - 15 m, corresponding to 0 - 100% range. The
correspondence between the measurement value ym in percent and
the level y in percent is given by the measurement function:
||ym = Km,LT*(y - ym0)
- Km,LT [%/m] is the measurement gain (LT is
- ym0 is the lower limit of the measurement
Measurement random noise is added to the level measurement. The
measurement signal passes through a lowpass filter which attenuates the
noise component of the signal. The filter is characterized by its time
constant, Tf [s]. The filter action is removed by setting Tf = 0.
The aims of the tasks given below
- to give an understanding of how an automatic feedback
control system works, and which benefits feedback control has compared
to using a fixed value of the manipulated variable
- to give an understanding of the properties of two
important control functions, namely PID control and on/off-control
- to develop skills in tuning controller parameters
- to give insight into how various parameters (in the controller, in the
process, and in the measurement device) influates
the dynamic properties and the stability properties of the control
In other words: This lab will give your (more) insight
into the most important issues of control engineering!
Control systems are essential in industry because it is
important to control process variables so that they are kept equal to or
close to set-points. The PID-controller is by far the most frequently used
controller function, and it is a main topic in this lab.
This lab is about level control, and in most plants there is a need to
control level. The simulated tank with it's level control system in this
lab is a "real" system, as it actually exists, as mentioned
In the tasks below it is assumed that the process is in
it's nominal operating point unless something else is stated. The
nominal operating point is defined as follows:
- The level reference (setpoint) is 10 m.
- The chip outflow is wout = 1500
kg/min = wout,nom.
- The nominal value of the manipulated variable is unom =
45%, which gives an inflow that is equal to the nominal outflow.
- The filter time constant is Tf = 20 s.
In the tasks about PID-control: Set the set-point
weights wp and wd equal to 1, and the coefficient a
= Tf/Td equal to 0.1 (these are also the default values as set on the
front panel). Let the PID-controller have anti-windup.
- First: No controller!
Set the controller in manual mode. Give the outflow a step from e.g. 1500
to 1800 kg/min, which implies that unom no longer fits to
the nominal outflow, wut,nom. Characterize the response in
the level. Is control using a fixed value of the manipulated variable
(unom = constant) an acceptable way of
controlling this process?
- Then: Manual control,
that is: YOU are the controller! Set the controller
in manual mode. Give the outflow a step from e.g. 1500 to 1800 kg/min.
Compensate for this disturbance by adjusting unom. How long
time do you need to bring the level back to the set-point with an
error less than 0.1 meter? Are there any drawbacks with using a human
being as a continuous controller?
- Automatic control using
an on/off controller: Set the controller in automatic
mode. Set the on/off controller's amplitude to 10 % and the hysteresis
width to 0%. Give wut and unom
their nominal values.
- Characterize the response in the level. Explain!
- Apply a step to wut from 1500 to 1800 kg/min.
How is the response in the level? Explain! What happens with the
response if you increase M (from 10%)?
- Automatic control with
a PID-controller: Parameter tuning:
In the tasks above you should have observed that there are certain
problems connected to having a constant manipulated variable, and also
with on/off-control. May be continuous control with PID-controller
will work better?
- Find the PID-parameters using the
Åstrøm-Hägglund-method. (You will see that the response in the
level is not sinusoidal, but just pretend it is - that is, you can
read off the amplitude of the oscillations in the usual way.)
- Find the PID-parameters also using
Ziegler-Nichols' closed-loop method. Are the parameters
approximately the same as with the relay- or on/off-tuner?
In the following tasks you shall use the
PID-parameter values as found in task 4a or 4b. If you have not
executed task 4, you can use the following PID-parameter values,
which you can regard as "standard values":
Kp = 1.8, Ti = 9 min = 540 sec, Td = 2.25 min =
- Is the stability of the
control system OK? Give wut a step
from e.g. 1500 to 2000 kg/m, and observer how the level returns to the
set-point. Does the control system have proper stability?
- Compensation properties:
- How large is the stationary control error using a
PID-controller after a step in wut from 1500 to 1800 kg/min?
- How long time time does it take for the
PID-controller to bring the level back to the set-point with an
error smaller than 0,1m (after the step in wut as
described above)? Which controller is best in this respect: You,
the on/off-controller, or the PID?
- Use a P-controller. Choose a proper value of Kp
in the P-controller. How large is now the stationary control error?
- Tracking properties:
How large is the stationary control error with a PID-ontroller after a
step in the set-point (choose the step value yourself)?
- How the P-, I- and D-term woks:
Observe how the three terms in the control signal, u, works after a step
in the disturbance. (There is a button beneath the diagram of u to show
the time-response of the individual control signal terms.)
- How parameter changes
influences the stability of the control system: Observe
how the stability of the control system changes due to the parameter
changes describes below. In each subtask/experiment you can excite the
control system with a small step in the set-point. The experiments
must be performed independant of each other, that is, you have to
reset the parameters to the standard values (defines above) between each experiment.
- The controller gain Kp is increased (much).
- The integral time Ti is reduced (much).
- The derivate time Td is increased (much).
- The screw gain Ku is increased (much).
- The sampling interval of the controller, Ts_reg (which
can be adjusted in the parameter field in the upper part of the
front panel) is increased (much).
- The dead-time (time-delay) td of the conveyor is increased (much).
- The cross sectional area A is reduced (much).
- The measurement gain Km,LT is
- The set-point gain Km,LC is increased
The importance of using
the correct measurement function in the setpoint generation:
- What happens with the control error of the
set-point gain Km,LC is different from the measurement
- What happens with the control error if the
lower limit, rm0, of the set-point range is different
from the lower limit, ym0, of the measurement range?
Measurement lowpass filter:
Demonstrate that the measurement lowpass filter attenuates the noise
component of the measurement signal. Also demonstrate that the filter
reduces the variation of the control signal. (Compare the behaviour of
the signals with filter and without filter.) In general,
what may be the drawback of large variations of the control signal?
Derivative term and measurement
noise: Demonstrate that the derivative term of the controller
causes the control signal to vary more (compared to not having
Updated 12. January 2008.