Computer Controls...

Computer Controls...

1. Basics

   • Computer control means that a computer is part of the control system. The computer is normally used to run the control program.
   • We use the term ‘computer’ very loosely. It does not necessarily mean the kind that you are working on right now, it often means a processor or microchip embedded with the equipment.

2. Sensors

   • A sensor is a device designed to measure some physical quantity in its environment. An example might be a heat sensor that measures the temperature in a room or a pressure sensitive mat that detects someone walking over it.
   • Once they have taken a reading or measurement, they might send that reading straight back to the computer or they may store it up and take a set of readings over time and send them back in a batch.
   • This data is called an ‘input’. Whilst the sensor itself is producing an 'output'.
   • Heat :Temperature :Living room for central heating system
   • Humidity :Water vapour in the air: Swimming pool, greenhouse
   • Infra-red :Infra-red radiation e.g. body heat :Security alarm systems
   • Light :Light levels :External security lights
   • PH :Acid/alkali levels e.g. pH of soil :Environmental experiments, river pollution
   • Pressure: Pressure: Burglar alarm
   • Smoke :Smoke in the atmosphere :Offices
   • Sound :Levels of sound :Security alarm systems
   • Tilt: Angle of tilt: Windows in security alarm system
   • Touch :Detects if one object bumps into another :Computer controlled robots
   • Sensor:what it measures:Where it's used

3. ADC

   • Most sensors take analogue measurements. This means the output changes smoothly from one value to another.
   • All computers need digital data in order to process it further. Digital data only has two values either 0 and 1 or On and Off.
   • In order for the data collected by the sensor to be handled by the computer, it needs to be converted from an analogue signal to a digital signal. To do this, a special piece of hardware is needed called an analogue-to-digital converter or ADC.
   • The ADC is connected between the sensor and the computer’s input-output port and it is called an ‘interface’.
   • input-->process--->output

4. Process

   • Once the input data from the sensors has been received by the computer, it can then begin to process it.
   • The data will be compared against a set of rules which have been programmed into the control system and a decision will be made. The result is an output of some kind.
   • For example, think about a central heating system: The system is set to keep the room at a comfortable 22 degrees C. 1)The sensor takes regular temperature readings in the room and sends the readings to the computer. 2)The computer will then process the data to see if it matches the set temperature of 22 degrees. 3)If it is lower, for example 20 C, then the computer turns the boiler on. 4)If the room has reached the set temperature of 22 C then the computer turns off the boiler.
   • This stage is called processing. The computer can only follow the rules that it has programmed into the control system, it cannot make its own decisions.

5. Output

   • Once the processing has taken place and a decision has been made, the computer will send the correct signal to the output device.
   • For a central heating system, this might be to turn the heating on or off. For a greenhouse watering system this might be to turn the sprinklers on or off.
   • Sending the message and the resulting action is known as the ‘output’ stage.
   • input-->process--->output

6. Open loop system

   • There are two types of computer control system: Open loop system& Feedback system
   • The first type, Open Loop System, only looks at its input signal in order to decide what to do.
   • It takes no account at all of what is happening to its output.
   • a) You set the microwave oven to run for two minutes. After cooking for two minutes, the control system turns the microwave off. It has no idea whether your food is still frozen, burnt or cooked perfectly.
   • b) You program the video recorder to start recording the football game at 7.30pm. However, because the news program finished early, you miss the first vital 15 minutes of the match. Because the system is only checking its input(time), it had no idea that the starting time of the match had changed.
   • The main advantage of an open loop system is that it is very simple, all it does is take in a command or value and produces an output action of some kind.

7. Feedback system

   • A more sophisticated control method is called the 'feedback control system'. The diagram below shows what this means. You can see that the output is measured and some of it is 'fed back' to become part of the input.
   • The advantage of a feedback system is more precise control.
   • The disadvantage is that it is a lot more complicated to make. And so the engineer has to decide whether a simple open loop system is 'good enough' or a more complicated feedback system needs to be designed.
   • Definition: Feedback is where the output of a system affects the input.

8. Actuator

   • An actuator is often part of a computer control system.
   • The actuator is a mechanical device or motor which carries out the action or decision made by the control system.
   • For example the lens of this digital camera moves in and out according to how much zoom is wanted.
   • This movement is controlled by an 'actuator'. In this case it is a tiny electric motor that is controlled by a computer control system within the camera.
   • So an actuator can be absolutely tiny or it can be truly massive such as the devices that open and shut the Thames flood barrier gates.
   • But they all do the same thing, namely allow a computer to control movement or action.

9. Control Examples

   • Home
     • Washing machines
     • Microwave cookers
     • Dishwashers
     • Video recorders
     • Remote control television
     • Central heating systems
   • Burglar Alarm system
   • Work
     • Air conditioning systems
     • Lifts
     • Automatic doors
     • Factory robots
   • Outside the home
     • Traffic light systems
     • Car park barriers
     • Cars – engine management systems
     • Rides at theme parks
     • Lifts
     • Computer controlled lighting systems at discos and concerts

10. Advantages

    • Can operate 24 hours a day without taking a break.
    • Can work without holidays or sick days
    • Will work without any wages.
    • Will accurately repeat actions over and over again
    • Can process data from sensors very quickly
    • Can take account of hundreds of inputs at the same time
    • Can make reliable and accurate decisions
    • Can be used in dangerous or awkward environments

11. Disadvantages

    • The software for the control system is specialist and may cost a lot of money to develop
    • If the computer malfunctions the system will not work
    • If there is a power cut the system will not work
    • The computer cannot react to unexpected events as perhaps a person could. It can only respond in the way it has been programmed.
    • People sometimes want to know that an expert is in control even if a computer could do it as well. e.g. a pilotless commercial aircraft is not likely to be popular although in theory it is possible. Or a robotic surgical procedure.