Transcript Control Systems

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 The elements of a control system
 Open and closed loop system differences
 Positive and negative feedback
 Examples of systems with feedback
* Classic closed loop control
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Reference input compares the feedback signal with a set
reference signal (usually a voltage level)
Depending on the output of the reference section, the
control element will be responsible for adjusting the
parameter of interest. For example if the liquid in a plant
is too cool, the control element will provide more power
to the heating elements within the plant until the liquid
temperature until it meets the reference point]
Feedback element, a part of the signal is fed back to
the reference input (usually a differential amplifier and
comparator) so that the control system can monitor and
adjust the controlled parameter to meet the desired
pressure, temperature, speed or level
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* An example of an open loop control system is
your garden variety kettle
* The operator fills the kettle
* The operator switches the kettle on
* The water reaches boiling point
* A thermostat (the control element) switches
the kettle off
* Without further interaction nothing else
happens
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* Open loop control systems are good for
applications that require set parameters
* For example we always want the kettle to
reach 100 degrees and then switch off
* If we want a motor to drive a constant load at
a given speed (all the time) then open loop
control works well
* Open loop control is usually simpler to
implement, is cheaper and easier to maintain
than closed loop control
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* The major difference between open and closed
loop control is that in a closed loop control
arrangement, the system is constantly
monitoring the output of the system to ensure
that it matches the reference signal and that
the reference signal can be changed at any
time usually by a computer program.
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* At home probably the best example is a
washing machine.
* Try to make a list of all the functions that
would need to be controlled by a washing
machine. State whether you think they are
open or closed loop controlled.
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Water level
Water temperature
Program time
Motor speed
Machine Safety (Door catch)
Machine Malfunction
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* The output of the machine/plant/process can
be arranged to provide an electrical signal
* This signal can be either digital (after ADC) or
it can be analogue
* Digital signals will usually be read by a
microcontroller or microprocessor
* Analogue signals will usually be connected to
some type of differential amplifier
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V Vout =R2/R1(V2 – V1)
t
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* In control systems positive feedback is used to
increase the difference between the reference
value and the feedback signal
* Therefore in our differential amplifier example
if positive feedback is used V2 will become
larger than V1, or the difference between V1
and V2 will become smaller. It just depends
which way around the signals are connected
* Error signal = reference value + feedback signal
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* Error signal = Reference value – Feedback signal
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* Consider the amplifier shown earlier
* If the reference signal is connected as V2 and the
feedback as V1
* Imagine we are controlling the temperature in a space
* What happens to the feedback signal as the
temperature increases?
* How does this affect the output of the differential
amplifier
* If the feedback signal decreases what happens to the
output
* Remember ideally V2 = V1 for system balance