American University of Sharjah
School of Engineering
-----------------------------------------------------------------------------------------Department of Electrical Engineering
-----------------------------------------------------------------------------------------ELE 353L – Control Systems Laboratory
Fall 2018
--------------------------------------------------------------------------------------------------------------------Experiment Title: (Basic Tests and Level Control of Coupled Tanks System)
--------------------------------------------------------------------------------------------------------------------Class section: (1)
Group Number: (4)
-----------------------------------------------------------------------------------------Format (20%)
Abstract, Discussion and Conclusion (30%)
Analysis of Data and Results (20%)
Results (20%)
Time (10%)
Grade (100%)
Name & ID & Signature:
1. Mahdi Ali Mohammed
b00069476
2. Waddah Badi
b00066917
3. Yousef Eldokmak
b00069536
Abstract
This lab session deals about controlling the level of a CE105 coupled Tanks device. The input
and output sensors will be calibrated in the beginning. For the input the pump circuit actuator
will be calibrated and for the output sensors we will deal with the flow rate and liquid level
sensors. Secondly, we will test the open loop control which is not very efficient. Finally we will
test the closed loop computer based control approach. Ultimately, presenting the differences
between open and closed loop control.
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Table of Contents
Abstract ........................................................................................................................................... 2
Background ..................................................................................................................................... 4
Procedure ........................................................................................................................................ 5
Results and Discussion ................................................................................................................. 13
Conclusion .................................................................................................................................... 15
Appendix ....................................................................................................................................... 16
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Background
The CE105 Coupled Tanks system is a very common example of a control system used in
real life applications. It is largely available in different industries that need a control process
using the concept of liquid level control as well as fluid transportation. The schematic below
shows the different components of the CE105 Coupled Tanks Apparatus. It is composed of two
tanks connected via a channel. It has valves that can be utilized to control the speed of liquid
flow via changing the cross section area of the channel. It must also be noted that there is a motor
for each tank. Automatic as well as manual control is used by the pump to control the variable
speed of liquid pumping in the tanks.
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Procedure
Part A: Pump Calibration Characteristics
1. We opened all the valves (set them to position 5)
2. We made the connections shown in the table below between CE122 and CE 105
Table 1 Connections between CE122 and CE105
CE122
CE105
A/D Channel 1
A/D Channel 2
A/D Channel 3
A/D Channel 4
0..10 Left-Hand Flow
Sensor
0..10 Left-Hand
Level Sensor
0..10 Right-Hand
Flow Sensor
0..10 Right-Hand
Level Sensor
CE122
CE105
D/A Channel 1
Pump 1
GND
GND
3. We opened CE2000 Lite on desktop
4. We opened the saved file CE105 and allowed editing from the circuit options
5. The circuit connection below is made
Figure 1 Connections for part A
6. Valve A was set to 5, B to 2.5 and C to 5. We then ran the circuit
7. The potentiometer was set to 0v and was increased slowly until the liquid started flowing
which happened at 2.2 V.
8. Flow rate was then set to 0.4 liter/min and increased to 4 liter/min in increments of 0.4
and the corresponding voltage to each value was recorded in the table below
5
Table 2 Supply voltages for different flowrates
Flowrate (liter/min)
Pump Supply Voltage/ V
0.4
2.5
0.8
3.1
1.2
3.7
1.6
4.5
2.0
5.2
2.4
6.0
2.8
6.8
3.2
7.5
3.6
8.4
4.0
9.9
The results are shown in the graph below
Pump Supply Voltage (V) vs Flow Rate (liter/min)
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Pump Supply (V)
10
8
6
4
2
0
0
0,5
1
1,5
2
2,5
3
3,5
4
4,5
Flow Rate (L/min)
Figure 2 Pump supply voltage vs flow rate
6
Part B: Flowmeter Calibration Characteristics
1. Valve A was set to 5, B to 2.5 and C to 5.
2. We made the connections shown below
Figure 3 Connections for part B
3. The flow rate was adjusted from 0.4 liters/min to 4 liters/min in increments of 0.4 and the
corresponding voltages were recorded and when the flow rate was at 4 liters/min, we
decreased the flow rate from 4 liters/min to 0.4 liters/min in decrements of 0.4 and the
corresponding voltages were again recorded. The results are shown in the table below.
Table 3 Flowmeter voltage for different flowrates while increasing and decreasing
Flowrate (liter/min)
Flowmeter Voltage (V)
Flowmeter Voltage (V)
(Increasing Pump
(Decreasing Pump
Supply)
Supply)
0.4
1.235
1.200
0.8
2.225
2.450
1.2
3.465
3.400
1.6
4.485
4.550
2.0
5.565
5.600
2.4
6.595
6.565
2.8
7.475
7.550
3.2
8.340
8.385
3.6
9.235
9.365
4.0
10.095
10.095
7
The graph below demonstrates the results
Flowmeter Voltage (V) vs Flowrate (liter/min)
Flowmeter Voltage (V)
12
y = 2,475x + 0,471
10
8
6
4
2
0
0
0,5
1
1,5
2
2,5
3
3,5
4
4,5
Flowrate (L/min)
Flowmeter Voltage (Increasing Pump Supply)
Flowmeter Voltage (Decreasing Pump Supply)
Figure 4 Flowmeter voltage vs flowrate increasing and decreasing
Part C: Level Sensor Calibration
1. The circuit connections shown below were made
Figure 5 Connections for part C
2. Valve A was set to 5, B to 0 and C to 0.
3. We then increased the potentiometer slowly until the tanks filled up and we recorded the
level voltage sensors readings at 50, 100, 150, 200, 250 mms and then we decreased the
control voltage to empty the tanks and recorded the level sensors at the same levels. The
results are shown in the table below.
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Table 4 Pump supply voltage for different liquid levels while increasing and decreasing for tank 1 and 2
Tank 1
Tank 2
Increasing
Decreasing
Increasing
Decreasing
Pump Supply
Pump Supply
Pump Supply
Pump Supply
Sensor output (V)
Sensor output (V)
Sensor output (V)
Sensor output (V)
0
-1.615
-1.555
0.255
0.255
50
0.390
0.595
2.30
2.400
100
2.450
2.680
4.445
4.490
150
4.500
4.680
6.500
6.550
200
6.470
6.500
8.525
8.600
250
8.200
8.200
10.235
10.235
Liquid Level (mm)
The data is shown in the graphs below
Tank 1 Voltage output (V) vs Liquid Level (mm)
10
y = 0,0391x - 1,3755
Tank 1 Volt Output (V)
8
6
4
2
0
0
50
100
150
200
250
300
-2
-4
Liquid Level (mm)
increasing Sensor output (V)
Decreasing Sensor output (V)
Figure 6 Voltage output vs liquid level increasing and decreasing for tank 1
9
Tank 2 Voltage output(V) vs Liquid Level(mm)
Tank 2 Volt Output (V)
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y = 0,0403x + 0,3817
10
8
6
4
2
0
0
50
100
150
200
250
300
Liquid Level (mm)
Increasing Sensor output (V)
Figure 7 Voltage output vs liquid level increasing and decreasing for tank 1
Part D: Open-Loop System
1. Valve A was set to 2, B to 2 and C to 5
2. The circuit connection shown below was made
Figure 8 Connections for part D
3. We allowed the water level in tank 1 to reach a constant level and recorded the value
4. We then changed the valve B position to 1.5 and waited for the water level in tank 1 to
reach a constant level and recorded the value. We repeated for valve set at 2.5 and 3.0.
The results are shown in the table below.
Table 5 Liquid levels for different settings of valve B
Tap Setting
Liquid Level (mm)
1.5
230
2.0
103
2.5
58
3.0
18
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Q5. Again, at the same potentiometer setting, adjust Valve B to be 2.5 (approximately) and
observe the effect this has on the level. Record the new liquid level after 3 minutes.
Comment on the effect of changing valve position upon level in Tank 1, especially the
system’s ability to maintain fluid at a constant level.
When valve B is opened more, the flow rate of the outgoing water increases. Thus, the
incoming water flow from the tank must increase in order to equal the outgoing flow rate.
However, in order to increase the incoming water flow rate from the reserve tank, the voltage
supplied to the pump must increase. As, the pump fails to provide enough flowrate the water
level in tank 1 decreases.
Q6. Set tap B (2.5). Repeat the above procedure only this time attempt to adjust the pump
flow rate (using the potentiometer) in order to maintain a steady fluid level (assume any
target level, for example: 150 mm). In this way you are providing a manual feedback loop.
Comment on the problems of maintaining the fluid level constant in this way.
Manual feedback is highly inaccurate, setting the voltage is done completely by intuition and
approximation rather than accurate measurement. Trying to achieve steady state using
manual feedback takes long time.
Q7. Now, by adjusting the potentiometer voltage, attempt to change the liquid level to
200mm and maintain that level for a few minutes. Repeat the procedure at a level if 50mm.
Comment on the problems of changing the liquid level in this way.
This process involves several rounds of trial and error (we had to try around six times in
order to achieve the needed steady state level) this is due to overshoot or reaching steady
state above desired level. Furthermore, the system will most likely not maintain the steady
state level because the flow out of water is not constant due to unpredictable changes in flow
rate caused by water pressure.
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Part E: Closed-Loop Proportional
1. Valve A was set to 2, B to 2.5 and C to 5
2. The connections shown in the diagram below were made
Figure 9 Connections for part E
3. We varied the proportional gain for the values shown in the table below and
recorded the error signal, output voltage of proportional control, flow rate and
liquid level.
4.
Table 6 error signal, output of controller, flowrate, liquid level for different proportional gains
Proportional Gain

Error Signal (V)
P Output (V)
Flow-Rate (liter/min)
Liquid Level (mm)
0.1
6.625
0.663
0
0
1.0
4.970
4.970
1.9
39
3.0
2.060
6.180
2.5
110
6.0
1.090
6.510
2.7
133
10.0
0.665
6.650
2.75
144
Comment on the effect that proportional closed-loop control has on the system and the
effect of increasing the proportional gain.
Introducing the closed loop proportional control system enables the system to have variable
pump voltage without the need to manually vary the potentiometer. The results show that for
small values of proportional gain the system would flow rate and liquid level of 0. As
proportional gain is increased the flow rate increases as well as the liquid level.
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Q4. By adjusting the potentiometer and with a control gain of 10, attempt to set the level in Tank
1 to 200mm and then to 50mm. Comment on the ease with which this can be done compared
with the open loop experiment (part D)
Reaching the required steady state levels took us maximum a couple of changes in the
potentiometer which was much easier than in part D.
Results and Discussion
Part A: Pump Calibration Characteristics
The pump started operating when the input voltage exceeded 2.2V. After further increasing the
input voltage, it was found that the flowrate has a linear relationship with the input voltage, with
a slope of 1.79 V.min/L. This means an increase of 1.79V input is needed for 1L/min increase in
flowrate. The flowrate, however, saturated as it approached 4L/min causing nonlinearity.
Part B: Flowmeter Calibration Characteristics
Both when increasing and decreasing the flowmeter voltage, it was found that the flowrate is
almost completely linearly related with a slope of 2.475 V.min/L.
Flowmeter Sensor Equation:
Flowmeter Voltage(V) = 2.475*Flowrate(L/min) +0.471
(1)
This equation can be used by plugging in the required rate of flow to find the voltage needed for
that rate of flow.
Part C: Level Sensor Calibration
In calibration of tank one the hysteresis is more noticeable, however the relationship between
0mm and 200mm can still be approximated to a linear one, with a slope of 0.040 V/mm. Beyond
200mm, the sensor output voltage was observed to have a slight saturation, and the sensor output
was found to reach its maximum (of 8.2V) before 250mm.
A similar trend is observed in the calibration of tank 2. The hysteresis is much less noticeable
however it is present. Between 0 and 200mm the relationship is linear with a slope of 0.042
V/mm. Beyond 200mm slight saturation was also observed, and the sensor output was found to
maximise at 10.235V before 250mm.
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Tank 1 Level sensor Equations:
Including saturation:
Voltage(V) = 0.0391*Level(mm) – 1.3755
(2)
Excluding saturation:
Voltage(V) = 0.0403*Level(mm) – 1.56
(3)
Tank 2 Level sensor Equations:
Including saturation:
Voltage(V) = 0.0403*Level(mm) + 0.3817
(4)
Excluding saturation:
Voltage(V) = 0.0415*Level(mm) + 0.2625
(5)
These equations can be used by plugging in the required Liquid level to find the voltage needed
to achieve that liquid level.
Parts D and E: Open-Loop and Closed-Loop Control Comparison
The use of closed-loop proportional control was found to be more advantageous in all aspects
when compared with open-loop control.
The below table summarizes those aspects for both types of control.
Aspect
Open Loop
Resources
Requires less resources since the only Requires more resources to
requirement for control is manually
provide a proportional controller
changing the input.
and its interfacing equipment.
The results obtained can be highly
inaccurate since it is very hard to
Quality of results achieve steady state at the required
level due to the unpredictable liquid
pressure and output flowrate.
Time required
Multiple attempts are needed to
achieve results close to the required
value, and the results obtained.
Closed Loop
Since the system is automated and
the control action accounts for the
output flowrate and liquid
pressure, the controller easily
achieves steady state at the
required value.
Less attempts are needed to
achieve the required level at high
accuracy.
14
Conclusion
In this lab, the team implemented level control of a coupled tanks system by initially calibrating
the coupled tanks apparatus CE 105, which are the pump circuit and the output sensors.
Following calibration, open-loop and closed loop proportional control were investigated and
compared on the liquid level of the tanks. Firstly, the pump was calibrated and found to require
2.2V to start pumping, and the pumping rate was found to increase linearly by 1.79V.min/L until
around 80% of maximum pump rate. Next, the flowmeter sensor was calibrated and the
relationship between the sensor output and pump rate was found to be linear both rising and
falling with a rate of 2.475 V.min/L. Following the flowmeter calibration, the liquid level sensor
was calibrated. Hysteresis for both tanks was found to be minimal, meaning the relationship
between rising and falling was almost equal at a rate of 0.0391V/mm for tank 1 and 0.0403V/mm
for tank 2. Calibration was considered finished after this step, and open loop control then started.
It was found that open-loop control is rather inaccurate and requires much time and many
attempts to achieve results close to the required level. Finally, Closed-loop proportional control
was implemented, and it was found that this type of control is much more efficient at achieving
the required liquid level due to the controller taking into consideration parameters such as water
outflow rate.
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Appendix
16
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