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Process Control in Industry
Sometimes in Industry a whole manufacturing process is controlled automatically by a computer system.

Input : Sensors take readings at regular time intervals and send the readings to the computer.

Sensors may be measuring temperature, pressure, liquid flow rate etc

Process : The computer analyses the readings and decides whether action is needed to be taken.

Output : The computer sends output signals to devices which manage the process. An actuator is a device which reacts to a computer signal and will operate a simple device (tap, motor, switch etc..)

 
Some systems use feedback...the result of an output signal affects the next input reading which affects the next output and so on... A closed loop means there is no user interaction involved.

 

 An example of a feedback system is a computer controlled heating system. 
Repeat
   Read temperature
   If too cool, turn heater on
      otherwise turn heater off
Until switched off

 
An output signal may be in the form of a bit pattern...


Eg:  10010101 ..Each bit referring to a different device.
1 = 'Switch ON' 
0 = 'Switch OFF'

Most systems use feedback - the output affects the input.
Eg If the computer is controlling a heater, any output from the computer will change the temperature ...which affects the input readings ...which affects the computer's next output...etc...

 

 A BIT is short for Binary digIT
...which is either a 0 or a 1.
Process Control system is an example of a real-time system.

Data is processed as soon as the computer receives it. This allows the system to respond immediately to any variations in the processing.

 
Advantages :
  • immediate response when needed.
  • fewer staff are needed to run the process.
  • the system can work all day - every day
  • it keeps people away from dangerous processes.

Disadvantages :

  • Unemployment may be greater as fewer staff are needed.
  • the initial cost of the system may be expensive.
 
Example : Glass Manufacture.

When flat glass is made, it is important that the molten glass is allowed to cool at the proper rate. If it cools too slow or too fast, the glass will be of poor quality with visible defects or it may lack the proper strength.

'Annealing' of glass is the controlled cooling process and is normally done in a special oven. The glass is allowed to cool down to a certain point and then kept at that temperature for a while to allow stresses in the glass to relax, before the final cooling.

This annealing process would be done by a process control system.

Sensors would carefully monitor the temperature in the oven. If the glass is cooling too quickly the computer will send a signal to increase the heating. If the glass is cooling too slowly, the computer will send a signal to activate a cooling fan.

A glass annealing oven.