Electronic
Systems can take control of things for us and make
decisions automatically. They are often used to keep us safe or make something we do easier.
For example, when we are out using a bus, or
going into shops or buildings, we often find doors or barriers will
open automatically when we get close to them. For this to happen, an electronic system will have first noticed you are
there. Special devices or sensors will have been positioned to do this.
Systems
can detect things as well as people. For instance when a vending machine has verified enough money has been inserted, it will serve the selected drink or food and sometimes provide change.

Modern cars use a great number of electronic systems controlled by a central computer. In many types of vehicles, messages (or warning lights) come on to warn when the brake pads are wearing out, or to alert when tyres are under pressure. It is common for the heating to be regulated automatically or for fluid levels to be constantly monitored and reported on.
To start to understand how these systems work, it is necessary to think of them being made up of smaller
subsystems. These comprise of:
INPUT SUBSYSTEMS: These do the sensing for the electronic system and detect when something is happening or changing.
PROCESS (or CONTROL) SUBSYSTEMS: These respond to the information provided by the input subsystem(s). They will then make decisions and "instruct" something to happen. They can use a small number of discrete electronic components for simple decision making or a high powered microprocessor when things get a bit more complicated.
OUTPUT SUBSYSTEMS: These are controlled by the process subsystem and may, for example, produce a visual/audible signal or perhaps a mechanical movement to slide a door
open.
  
Control
System Block Diagram
Examples
of Electronic Control
1,
Take an automatic door system - it must be able to work safely and respond
to individuals or groups of people. These people might be walking slowly
or quickly. Having a suitable control part to our system will make it
respond to these inputs and control the ouputs appropriately. In this example,
the door only closes when it is completely clear of the people using it. If you have travelled the London Underground you will know this is not a practical requirement when boarding the trains!
2,
A car park barrier should respond to the users underneath and could cause a good deal of damage if
it wasn't controlled properly.
3,
Vending machines operate by complex electronic control systems which carefully
control the dispensing of food or drinks. The automatic checking of the
inserted money has to be done accurately before handing over the goods.
4,The
oil warning light in some cars has been made to flash on and off when
the oil level has become dangerously low. This is necessary to attract
the driver's attention. Within that system the output is being controlled,
or made to, turn on and off repeatedly.
5,
Security Systems are designed to detect burgulars
or intruders. They might sense the burgular coming through a window using
a magnetic switch on the frame. This will cause a loud alarm to sound.
Questions:
1,
Put the heading "Electronic Systems" and write down five examples
of your own of different situations where something is electronically
sensed or detected. For each of these systems say what the inputs and
outputs are.
Open
Loop or Closed Loop Control Systems ?
When
we make automatic systems it is important to choose the right type. They
both have inputs, control and outputs but respond differently to changes
that occur outside of the system.
Open
Loop Systems
If
you set the timer on your microwave to stop the cooking of your food it
will wait until the time is up before it turns the machine off. You have
to set the time correctly to get your food cooked just right. The machine
does not monitor the actual cooking of the food and will not adjust itself
even if things start to smoke!
  
Closed
Loop Systems

Inside
a washing machine a valve is used to control the flow of water entering
the machine. It is opened at the start of the washing cycle, allowing
the right amount of water to enter the machine to wash your clothes without
using too much or too little.
The
control part of the washing system needs to monitor how much water has
entered, because some houses can have very fast or slow running water
in their pipes. It is not good enough to just open the valve for a set
amount of time as you cannot guarantee the quantity of water entering.
You could either end up with a flood or barely damp clothes going round
and round!
When
the washing machine timer requires
water to be added, a sensor is used to continually check the water level
in the machine causing the inlet water valve to be turned on and off automatically.
Because
the system is continually checking the water level automatically and responds
to the changes, it is known as a "Closed Loop" control system.

The
example given earlier of an automatically opening door, has many features
to make it operate effectively and safely. It often uses closed loop
control to prevent people being trapped in the doorway when it is
closing.
Extra
sensors are used to monitor if people are in the way and this information
is fedback to the control subsystem. Again feedback is a feature of closed
loop systems.
Questions:
a, The central heating system in a home normally uses a Closed Loop system
to control the temperature of the rooms. Show
in a block diagram what the system's input, control and output are. Use
an arrowed line to show what and where the feedback is.
b, A bread toaster is an example of a ....system.
(Complete
the sentence and draw the block diagram of this system showing the input,
process and output stages).
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