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).