ICT
Computer control
A control system typically comprises of a computer or microprocessormicroprocessor: an integrated circuit that contains all or most of the individual elements of a central processing unit (CPU), a control programprogram: a list of instructions written in a programming language which handles datadata: information without context, eg a list of students with numbers beside their names is data, when it's made clear that those numbers represent their placing in a 100 metre race, the data becomes information from sensorssensor: an automatic input device that continuously monitors a set of computer controlled parameters, eg a parking sensor detects how close a vehicle is to the nearest object and alerts the driver if the distance falls outside of the specified parameters and sends signals to output devicesoutput device: a device used to output data or information from a computer, eg a monitor, printer or speakers and an interface box to convert signals between the sensors and the processorprocessor: shorthand for central processing unit (CPU).
The role of computers in control
- Computers can respond very rapidly to change.
- Systems can run 24 hours a day, 365 days a year.
- Control systems can operate in places that humans would find dangerous or awkward.
- Outputsoutput: the term denoting either an exit or changes which exit a system and which activate/modify a process are consistent and error free.
- Computers can process data quickly and machines can operate faster than humans.
Computers are now used to control many types of devices such as:
- air conditioning and central heating systems in large buildings
- security systems and burglar alarms
- manufacturing processes
- traffic lights and pedestrian crossings
The role of sensors in control
Sensors are used to measure physical quantities such as temperature, light, pressure, sound, and humidity. They send signals to the processorprocessor: shorthand for central processing unit (CPU). For example:
- A security alarm system may have an infraredinfrared: in communications, an infrared beam modulates (switches on/off) to a receiver that interprets the modulations as data sensor which sends a signal when the beam is broken.
- A heat sensitive sensor in the corner of a room may detect the presence of a person.
- Temperature sensors could be used to control the heating in a large building.
- Magnetic sensors are used to detect metal and can be placed in roads to monitor traffic flow.
Other physical quantities that can be transmitted directly to the computer's processor include:
- rainfall/water levels
- radiation level
- pH level
- oxygen level
Analogue to digital conversion
Data such as pressure, light and temperature is analogueanalogue: continuous data, eg the second hand on an analogue watch displays each second and the space in-between datadata: information without context, eg a list of students with numbers beside their names is data, when it's made clear that those numbers represent their placing in a 100 metre race, the data becomes information. Computers can only work with digitaldigital: data measured at discrete intervals, eg a digital watch typically moves from displaying one second to the next without displaying the values in-between data.
An interface box or analogue to digital converter (ADC) is needed to convert the analogue data from the sensors into digital data the computer can process.
Feedback cycle
The diagram below shows a control programprogram: a list of instructions written in a programming language for maintaining the water level in a fish tank.
Feedback cycle for a fish tank
The control program stores the highest and lowest acceptable water levels and what action to take if they're exceeded.
The process is continuous and is called a feedback cycle.
Stages of the feedback cycle
- water level falls too low
- sensor detects water level is too low
- valve opened to let water in
- sensor detects water level is too high
- valve opened to let water out
A computer-controlled greenhouse
To get the best plant growing conditions temperature and humidity (moisture in the air) have to be controlled.
The greenhouse therefore has temperature and humidity sensorssensor: an automatic input device that continuously monitors a set of computer controlled parameters, eg a parking sensor detects how close a vehicle is to the nearest object and alerts the driver if the distance falls outside of the specified parameters linked to a computer, and the computer has a control programprogram: a list of instructions written in a programming language storing details of the correct temperature and humidity settings. The greenhouse is fitted with a heater, sprinkler and window motor, also linked to the computer.
If the humidity falls below the values stored in the program, the computer activates the sprinklers and closes the windows. If the temperature falls outside the values stored in the program, the heater is activated by the computer.
The system monitors the conditions night and day with immediate response to any changes. To alter the growing conditions the values in the computer program can of course be changed.
Remember to mention the role of the sensors, interface box and control program if asked to explain the control process in the exam.
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