Title: Open and Closed-Loop Control Systems - (Motor speed)
I. Objectives
The object of this experiment is to investigate the principles of open and closed-loop control of a
rotating body using a speed sensor and a d.c. tachogenerator to provide feedback.
On completion of this experiment you will have:
•
Investigated open and closed-loop circuits for control of speed.
•
Experienced the concepts of set point, error and control signal.
II Theory
There are essentially two categories of control systems: open-loop and closed-loop. Both
maintain a chosen variable at a selected level with different levels of success.
Open-loop control systems
Both open- and closed-loop control systems try to maintain a variable at a predetermined value.
Control systems incorporate measurement systems, but unlike a pure measurement system the
output from a control system regulates a parameter whose value is not necessarily displayed to
the user.
Figure 1 shows a flow diagram of an open-loop system. The basis of open-loop systems is that
the system is controlled by a signal which is at a pre-set value. This pre-set value assumes the
required control can be achieved without measuring the effect of the system output on the
parameter it is set to control. The pre-set value will not change even if other factors do which
render the system output incorrect.
Fig. 1: Open-loop control system
Automation and Control systems – AMEM 326
Dr. Sotiris Omirou
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Consider an open-loop system for switching on and off the street lamps along a road. The
control required is that when darkness falls, the lights turn on, and when it is light, the lights turn
off. The control signal would be pre-set to accordingly switch the lights on or off at set times in
the evening and morning, using a timing device. This system would probably work acceptably
for a few weeks. However, because the hours of darkness change over the year, the pre-set
signal would soon be incorrect and the lights would be switching on and off too early or too late.
Figure 2 shows this open-loop street lighting system in flow diagram form.
Fig. 2: Timer based street lighting system
There is no input to the open-loop system which detects what is actually happening with the
parameter the system is affecting, that is, the street lamp system doesn't know whether it is light
or dark. As an open-loop system, someone has to estimate when it gets dark and light and set
the timer control which turns the lights on and off accordingly. These pre-set times have to be
altered as the hours of light and dark vary throughout the year. Hence it requires ftequent
operator intervention, because the less often these settings are updated, the less efficient they
will be. The system will also not take into account any unexpected or unpredictable behaviour.
For example, some evenings may be dull and cloudy, so ideally the lights would turn on earlier
than on a bright, sunny evening.
Open-loop control systems are in general relatively simple in design and inexpensive. However,
they can be inefficient and require frequent operator intervention. Under many circumstances
pre-set values become incorrect due to the parameter they are controlling changing in some
way, and so they need resetting. The pre-set value often needs a high level of skill or judgement
to set it correctly. In cases where the consequences of the system not controlling the parameter
as desired are serious, such as the level of a hazardous liquid in a tank overflowing, open-loop
systems are not suitable.
Automation and Control systems – AMEM 326
Dr. Sotiris Omirou
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Closed-loop control systems
In a closed-loop control system, the state of the output directly affects the input condition. A
closed-loop control system measures the value of the parameter being controlled at the output
of the system and compares this to the desired value.
Key-fact
In a closed-loop control system, the actual value of the parameter being controlled is compared
to the desired value. The difference in these values is known as the error.
Referring to Figure 3, which shows a closed-loop system in flow diagram form, the desired value
is known as the reference signal, or set point. This is compared to the signal from the
measurement device, known as the feedback signal. The difference between the feedback
signal and the reference signal is known as the error signal. The error signal is then modified so
that it can adjust the performance of the system. For example, if the error signal is an electrical
signal, it may need amplification. The modified error signal is called the control signal. The
control signal then adjusts the output of the system, to try to match the feedback signal to the
reference signal. This would reduce the error signal to zero and so achieve the desired value
Fig.3: Closed-loop control system
Consider a tank in a chemical plant, containing a hazardous liquid, as shown in Figure 4. The
tank is filled with liquid by a pump. When the liquid is required for further chemical processing,
another system opens the discharge valve and takes the liquid it needs, reducing the tank level.
It would not be acceptable to use an open-loop system because the consequences of a pre-set
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Dr. Sotiris Omirou
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value becoming incorrect would be serious. The tank may overfill, spilling hazardous liquid, or
run dry and stop operation of the plant. For the plant to work effectively the tank needs to be
filled to an optimum level. A liquid level sensor detects the level and produces an electrical
output.
Fig.4: Control of liquid level in a tank
Figure 5 shows a flow diagram of closed-loop control of this system. The output from the liquid
level sensor, the feedback signal, is compared to the ideal level as dictated by the reference
signal. This produces the error signal. The error signal is amplified by the controller to become
the control signal. The control signal drives the pump motor and so determines the flow rate of
liquid through the pump to fill the tank. When the error signal is zero, the liquid level has
reached its ideal level, the control signal is also zero and so the pump stops. In this way the
information contained in the electrical signal concerning the liquid level, whether it be constant
or varying, controls the pump flow rate to maintain the liquid level under varying discharge
conditions.
Automation and Control systems – AMEM 326
Dr. Sotiris Omirou
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Fig.5: Flow diagram of closed-loop control of liquid level in a tank
Closed-loop systems regulate themselves and therefore are less prone to error than open-loop
systems. They are generally more efficient and require less operator involvement. However, set
up costs can be high and they can become complex.
III Experimental Work
PART A: Open-loop Control
In this part of the experiment a control signal is manually set to produce the speed required.
This is an open-loop system without feedback. Figure 6 shows a flow diagram of a typical openloop system.
Fig. 6: Typical open-loop system
Automation and Control systems – AMEM 326
Dr. Sotiris Omirou
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In the example of speed control of a motor, manually setting the value of the control signal
regulates the power being delivered to the motor and so directly affects the performance at the
output, (the speed).
Set the meter to read voltage (V). This display will indicate speed and monitor changes in
speed. Set the motor drive control switch to O and adjust Ref1 to produce an indicated value
of 3 V.
Make the connections shown in Figure 7, using the patching leads supplied. Confirm that this is
the same circuit shown schematically in Figure 6.
Fig. 7: Connection diagram for open-loop control
Set the motor control switch to I. Use the meter to measure the output voltage from the
tachogenerator and compare this with the value of Ref1. Calculate the difference, (the error).
Automation and Control systems – AMEM 326
Dr. Sotiris Omirou
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Momentarily apply a load to the motor by pressing the pushbutton adjacent to the
tachogenerator legend. What is the effect on the speed? What is the value of the error now?
How can this loading effect be cancelled? Comment on your results.
PART B:
Closed-loop Control
In a closed-loop control system, the parameter controlled at the output of the system (Output
Variable) is measured and compared with the control signal, termed the Reference Signal or
Setpoint. The difference, the Error Signal, is processed by the Controller to produce the Control
Signal which is supplied directly to the system under control.
Fig. 8: Typical closed-loop system
Figure 8 shows a typical closed-loop control system. In this experiment, the output variable is
the speed of the D.C. motor. The output from the D.C. tachogenerator, the feedback signal, is
compared with the reference voltage Ref1 by the Differencing Amplifier to produce the error
signal. The error signal is amplified by the controller to become the control signal. The control
signal drives the motor and so determines the speed of rotation.
Set the motor drive switch to O. Adjust Ref1 to 3 V and the Differencing Amplifier gain to
minimum (1) by rotating the control fully anticlockwise. Make the connections shown in Figure 9.
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Dr. Sotiris Omirou
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Confirm that this is the same circuit as that shown in Figure 8. Set the motor drive switch to 1.
With the gain at unity (1) the error signal is supplied directly to the motor drive circuit.
What is the value of the tachogenerator signal indicated by the meter? How does this compare
with the value from Part A? Now what happens to the error signal when the motor is loaded?
Explain why this is so. Comment on your observations
Fig. 9: Closed-loop system connection diagram
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Dr. Sotiris Omirou
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