Lab #1
Part 1- Introduction
(1-1 to 1-7)
a)
b)
Sketch video display of tank, valve, and controller face plate.
Answer questions on page 1-6 & 1-7.
How to use the Simulation ‘Tanksim’
Hang on to your hat-you’re off on a whirlwind tour of the tank level control
simulation.
Load Tanksim. (Turn back to the second page of the introduction if you are not sure
how to do this).
As soon as the option menu appears on the screen select the Fault option:
- On the Macintosh click the mouse pointer on the button labeled Fault
- On the IBM-PC press key F.
Then select whatever measurement units you prefer – English or Metric.
As soon as you’ve made this selection the Tanksim sumlation will appear on the
screen.
Don’t touch the mouse or function keys yet!
Not much happening – right? Just the clock ticking away the seconds.
Look carefully at the indicated process valves and think what they might mean.
You can see that water is flowing into the tank at 400 gallons per minute (gpm) and
flowing out through the control valve at the same rate. Because these rates are the
same, the water levels in the tank is not changing. The level is stable at the 28 inch
mark on the draft gauge.
If you are using the metric version of the program the flow will show as 400 litres per
minute (lpm) and the water level will be 28 cm on the draft gauge (the tank and valve
in the metric version of Tanksim are smaller than in the imperial version). For
simplicity only English units are used in the remainder of the description – except for
the controller output pressure which is given in psi and kPa because the numeric
valves are different.
The controller also shows the measured level at 28-so the level transmitter (LT) and
the level indicating controller (LIC) must be working.
1
The ‘pneumatic’ level transmitter turns the water pressure at the bottom of the tank
into a ‘proportional’ 3-15 psi (21-104 kPa) air signal. This means that when the tank
level is at the ‘Lower Range Value (LRV)’ the output from the transmitter is 3 psi (21
kPa), and at the ‘Upper Range Value (URV)’ the output will be 15 psi (104 kPa).
Halfway between it will be at 9 psi (62 kPa)-since 9 psi is halfway between 3 psi and
15 psi. The setting up of the URV and LRV values on the transmitter is called
‘calibrating’ the transmitter. In practice, calibration involves removing a cover plate
and using a screwdriver to adjust two screws labeled ‘span’ and ‘zero’. In the
simulation, it is a lot easier as you will see now. Incidentally, the LT and LIC symbols
inside the balloons bellow the tank are standard ISA (Instrument Society of America)
symbols.
Go ahead and change the calibration of the transmitter.
- On the Macintosh click the mouse on the menu marked ‘Range’ and drag the
mouse pointer to the 20-80 range. As the mouse button is released the range
changes to 20-80.
- On the IBM-PC press the F7 key.
Practice changing the range to all the possible values. Watch the Upper and Lower Range
Value pointers URV and LRV move to show the new range. The indicated level will also
change in the controller each time the range is changed. You may also notice a delay
before the measured level reaches a steady value.
Return the range to 0-100
Now try changing the ‘Set Point’.
- On the Macintosh click the mouse on the Set Point Increase or Set Point
Decrease buttons in the Controller
- On the IBM-PC press the cursor up or down arrow key and then the F1 key. (Use
arrows for directions)
Watch the set point indicator move in the controller. A unit inside the controller
automatically changes this point into a proportional 3-15 psi (21-104 kPa) air pressure
‘signal’.
A ‘signal’ is a physical quantity such as an air pressure or an electrical current in which
information is contained or encoded in some predefined way. The signal is called an
‘analog’ signal if the magnitude or size of the quantity is the measure of the information.
Control systems using analog signals are called ‘analog’ control systems. This system is
an analog system, since the information on tank level, desired level and commanded
valve position is contained in magnitude or pressure level of 3-15 psi (21-104 kPa) air
pressure signals.
The tank water level corresponding to the controller set point is shown by the marker on
the draft gauge. This is for your convenience only. These markers are not usually
provided.
2
Restore the Set Point to 50.
Notice that nothing you have done so far has affected the process-the level in the tank
hasn’t changed. This is because the controller is set to ‘Manual’. Controller setting
changes should normally be made in ‘Manual’ because this is the only way to avoid
upsetting the process. These settings include Gain, Integral or Reset time and
Derivative or Rate time.
Change the Gain, Integral time, and Derivative time. The function keys for the integral
and derivative time adjustments on the IBM-PC are shown at the bottom of the screen
next to the abbreviation Ti and Td. For example, to reduce Ti press the down cursor
arrow and then function key F5. The resulting settings are shown of the front of the
controller. On the Macintosh the settings are shown by tick marks on the pull down
menus.
You won’t see the effect of these changes until you are in ‘Automatic’.
Hold a key down for more than a second and it will repeat rapidly. This saves your finger
when making large setting changes on the IBM-PC. On the Macintosh, quickly clicking
the mouse outside the dialog button just clicked and holing the mouse button down will
do the same job.
Reset the Gain to 1, the Integral time to maximum and the derivative time to zero.
Now you are going to control the process manually.
Look at the pressure sent from the control valve-shown just above the valve and as a
percentage of range on the right hand indicator of the controller. The readings should be
9psi (62 kPa) and 50% respectively. Increase the pressure to 11 psi (76 kPa):
- On the Macintosh click on the Output up arrow button on the controller four
times.
- On the IBM-PC press the cursor up arrow and then press the F3 function key four
times.
The tank level is dropping because you have increased the opening of the valve. The out
flow has increased, so water is leaving the tank faster than it is coming in.
Open the valve wide by increasing the pressure to 15 psi (104 kPa). Let the tank drain
completely. Now close the valve by reducing the pressure to 3 psi (21 kPa). The output
flow is now zero and the tank is filling.
Get the feel of manual control by adjusting the output flow until the measured level line
on the controller is at the set point of 50. It will only stay at this setting if the output flow
matches the input flow. Looking at the two flows will help you make this adjustment.
3
Congratulations-you have just done what the controller does when it is in Automatic
mode. You compared the set and measured levels and adjusted the controller output to
reduce the difference.
Don’t get impatient – leave the controller in Manual!
Now ‘disturb’ the process by changing the ‘process load’. In ordinary language, change
the level by changing the input flow:
- On the Macintosh click on the Load Flow Up or Down button.
- On the IBM-PC press the cursor up or down arrow button and then press the F4
function key.
Adjust the controller output to restore the level to the set value. Does the output flow
match the input flow? It should be fairly close.
In practice the input flow would not be under control. It would increase or decrease
depending on some other connected process. For example in batch processes it would
often stop completely. Obviously such changes in input flow are the major cause of tank
level change and therefore the major process ‘disturbance’. When you changed the input
flow just now you simulated a process disturbance. The job of the control system is to
keep the level under control when these disturbances occur. You just did this job
manually.
Now you will get the controller to do the job automatically.
Set the Load Flow to 400 gpm and stabilize the level at 50. The Set Point should be set
at 50, the Gain at 1, the Integral time at maximum, and the Derivative time at 0.
Transfer the controller to Automatic and watch the display carefully, particularly the
control valve pressure and output flow:
- On the Macintosh click the mouse on the Auto button.
- On the IBM-PC press the cursor right arrow key.
The controller starts immediately to trim the valve pressure. Any remaining small error is
eliminated and the outlet flow exactly matched to the input flow. This may take about
half a minute.
Now increase the Set Point to 70 and watch the controller bring the level up to 70.
As soon as the level has stabilized, bring up the Chart recorder display:
- On the Macintosh click the Chart on button to open the chart selection menu.
Then click in the button labeled ‘Level and Set Point’.
- On the IBM-PC press the F8 function key and then key NO.1.
4
The recorder shows the set or desired level and the actual level for the last 90 seconds.
With the recorder still on, decrease the Set Point back to 50. Watch the chart display as
the controller brings the level back to 50. Pretty slow eh? In later sections of this manual
you will learn how to adjust the controller to give the fastest possible control action and
the smallest possible level error.
Restore the tank display:
- On the Macintosh click the Chart off button.
- On the IBM-PC press the F8 function key and then key No.’0’.
There is an extra button on the Macintosh you haven’t ‘pressed’ yet-the panic button
labeled Pause. It is a good button to press when things are moving too fast and you want
time to collect your thoughts or when you want to make some measurements on a chart
display. Click on the Cancel Pause button when you are ready to continue.
On the IBM-PC you can achieve the same effect by pressing the F8 of F9 key. When you
are ready to continue press the appropriate key-for example No. ‘0’ to continue the tank
display.
We haven’t gone into the optional features yet. These will be explained later on, when
they are used.
Now you know how to start the simulation up and how to run it. What about stopping it?
Simple-on the IBM-PC press Quit function key F10. A message will appear asking if you
want to quit or restart. Restart if you want to choose different options. If you choose Quit
a second time, a message may appear asking for the operating disk to be reinserted. Do
whatever it asks until you get back to the drive prompt, e.g. A>. On the Macintosh the
Quit and Restart buttons are available on screen. Clicking the Restart button will take you
back to the option menu; clicking the Quit button will cause an automatic shut down and
disk eject.
Go ahead. Try it.
When you’ve finished, remove the disk, put it back in its protective cover and store it
somewhere safe-such as the envelope inside the front cover of this manual.
That’s the end of the whirlwind tour you now have a working familiarity with the
simulated process and controls and associated terminology. You know how to use the
controller to control the process manually and know how to switch it to automatic.
If you want some reassurance or reinforcement, try answering the self test questions give
below. Answers are given at the back of the book. Otherwise grit your teeth, sharpen your
pencil and get started on Part 2.
5
Self Test Questions
1. Why was the level stable when you first started Tanksim?
2. What can you say about the input and output flow rates if the level line on the
chart is:
a. moving upwards
b. level
c. moving downwards
3. What component converts the water pressure at the bottom of the tank into a
proportional air pressure signal.
4. What is the LRV value when the transmitter range is set to 20-80 cm?
5. What is the output from the transmitter when the tank level is at the lower range
value and the instrument is calibrated in metric units?
6. What is the output pressure from the transmitter when the level is at mid rangehalf way between the URV and LRV values?
7. What do LT and LIC stand for?
8. The two modes of operation of a controller are manual and automatic. If you are
in manual, which of the following adjustments or setting changes will affect the
controller output? Set Point, Output, Gain, Integral time, Derivative time.
9. What component does the output from the controller go to?
10. Is the control valve closed or opened by air pressure?
6