We will be looking at systems which use Microprocessors, but you could have electronic components or mechanical systems as your process as well.
Now we need to know about control diagrams as well. These are similar in many ways to system diagrams, but they include an error detector as part of the control.
Control diagrams are mainly used to describe systems which have closed loop control. That is that they self regulate the output. This means that they need to include some sort of sensor to monitor what the output is doing. Open loop control systems have no ability to control the output any better than switching it on or off. This can be seen in the lights in the classroom. They do not dim when it is bright outside, they are switched on, and they stay on at the same brightness until they are switched off. An oven, on the other hand uses closed loop control. You set the temperature and it heats up until it is at that temperature, then the temperature in the oven is closely monitored and the heating element switched on and off to keep it as close to the set temperature as possible.
The error detector compares the sensed, actual value and the set/desired level and gives a signal accordingly. Normally this signal would go either to the process, or to the control unit, depending on the level of detail required. The diagram below shows only the process.
The diagram above shows a closed loop system using negative feedback. This means that it works to try to reduce the difference between the set/desired level and the actual level sensed by the output sensor - the error. As shown above, if the actual temperature is too cold, the error detector will give a high signal, switching the heater on; if the temperature is too hot, the error detector will give a low signal and the heater will turn off.
This can also be shown in this graph:
You can see clearly that the system is trying to stay at the desired temperature, but the deign of the system means that as soon as it is too hot, the heater is switched off, but this takes some time to cool down. When it is too cold it takes some time to warm up again. The space on the graph that is above and below the desired temperature shows the error - the difference. Negative feedback works to reduce this error.
Positive feedback is not often used. It amplifies the error between what you want and what the sensed output is. And so the error gets bigger and bigger. This can be heard when you stand too close to the amp with an electric guitar and you get a giant squeal out of it!
There are three types of closed loop control you need to know about:
On/Off Control: The system can only ever be in one state: on or off, and nothing in between. This is the type of control used in the oven. It is constantly switching on and off to try to keep the temperature to the desired level.
Continuous Control: This is a system where the inputs (often sensors) are sending information into the system all the time and the outputs are being controlled all of the time. A change in the input directly results in a change in the output. This is the type of control used in a robotic arm - a signal to tell the system where the desired position is given, this is compared to the signal from the arm sensor feeding back the actual position of the arm. As the arm moves this position is fed back to the control until it is in the correct position and the arm stops moving.
Sequential Control: A series of events happen one after the other. The end of one event signals for the start of the next. This is the type of control used in a washing machine as it goes through the wash cycle.
