We have seen how open loop systems can automatically switch output devices on or off. The process subsystem controls this using information it receives from the system's input.

Block Diagram
In closed loop systems, additional information is made available to the process controller from signals fed back from switches and sensors at the output. This enables the system to make further decisions so it can operate more efficiently and more safely.
It is possible to use electronic devices called "logic gates" to handle and process this extra information, however the decisions they make are fixed by the hardware components used. To add more versatility, microcontrollers (PIC's) can be used. These are known as firmware, as they can be reprogrammed to make many different decisions in the system circuits they are placed.
Using Closed Loop Control to increase safety and efficiency.
To avoid accidents, the automatic door in a lift needs to be designed
so it doesn't continue to close on people if they are trapped in the doorway, or if they are still trying to get in or out.
If the lift's controller has instructed the door mechanism to close, but a pressure switch (or possibly an infra-red beam) in the door has been able to detect a person or trapped bag, then the "close-the-door" instruction from the controller, can be overridden and the doors made to reopen.
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This car park barrier has been unable to respond to a car passing unexpectedly
under its barrier arm.
It
would have benefited from additional sensing and the use of "closed-loop" control!
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If a vending machine contains fresh perishable
food, then it should always be kept at the required temperature to ensure the items can be stored for longer and to know they are safe to consume.
To achieve this, a thermostat may be used to monitor the temperature inside the machine. The information from this is constantly fed back to the
refrigeration
control unit - allowing it to maintain the temperature more efficiently and accurately.
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If you set the timer on a basic microwave to start and stop the cooking of your food, it will wait until the time is up before it turns the power off.
In this arrangement, you have to set the cooking time correctly if you want your food to be prepared properly when the bell "tings".
You have probably found you have to test the food yourself and readjust the cook time if you find it is undercooked.
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As most cookers do not monitor the actual cooking of the food, they will not make adjustments - even if things start to smoke or burn! Once the machine has been switched on and is operating at the temperature or power you have set, it will carry on and heat the food for the amount of time you have dialled in, despite the actual state of the food inside.
If the cooker or microwave was able to determine the temperature of the food being cooked, it would be able to automatically turn the power off when the food was ready. A closed loop system could do this as it would be able to respond to the effects of the heating it was controlling.
Closed Loop Control Systems

"Closed Loop" Control System Block Diagram
"Feedback" is the main feature
of a closed loop system. It helps the system to operate more efficiently and
more safely.
Examples (in the home)
A good example of "Closed Loop" control can be found inside the automatic washing machine: |
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An electrically operated valve automatically controls the amount of water entering the drum. It is opened at the start of the wash cycle and needs to allow just the right amount of water to enter (this is important when trying to conserve water and energy).
As some houses have very fast or slow running water in their pipes, it is not sufficient to simply open the inlet water valve for a pre-set amount of time. The control part of the washing system needs to know how much water has actually entered the drum. If it didn't know this you might end up with a flood or have barely damp clothes going round and round!
A water sensor is therefore used to continually check the level of the rising water. This provides information which is fed back to the control part of the system, so it can automatically turn the inlet water valve on or off as required.
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Further examples of "Closed Loop" control can be found inside the different parts of a home's heating system. |
A typical central heating system uses Closed Loop Control systems to maintain the temperature of the boiler as well as the required temperature of the rooms.
The programmer, wall and radiator thermostats may be controlled manually in your home and now remotely - by apps on a smartphone.
Test Yourself:
Electric toothbrushes are available with a host of different functions: Most allow the user to control the speed and type of movement of the bristles via a switch in the handle. |
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Some automatically detect how hard the brush head is being pressed against the teeth and gums. If it senses too much pressure is being applied, it might flash an LED, or reduce the speed of the motor.
1, On a block diagram, show an example of an open loop control system that could be used to control the basic functions of an electric toothbrush.
2, On a block diagram, show an example of a closed loop control system that has automatic safety features to ensure the user brushes their teeth safely and properly.
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