Process Control 2.1 – Control Systems

Process Control - Batch Some applications require that certain amounts of raw materials are combined in specific ways for particular lengths of time. An example is the production of adhesives and glues. These normally require the mixing of raw materials in a heated vessel for a period of time. This results in a glue or adhesive being produced. Other important examples are the production of food, beverages and medicine. Batch processes are generally used to produce relatively small amounts of the product per year.

Process Control – Continous Process Control The control of the water temperature in a heating jacket, is an example of continuous process control. Some important continuous processes are the production of Fuels Chemicals Plastics Continuous processes, in manufacturing, are used to produce very large quantities of product per year.

Process Control - Discrete Found in many manufacturing and packaging applications. Robotic car assembly is also and example of discrete process control. Most discrete manufacturing involves the production of discrete pieces of product, such as car wheels / specific car parts.

Open loop and closed loop 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. open-loop controllers do not use feedback to determine if the output has achieved the desired goal of the input. This means that the system does not look at the output of the processes that it is controlling. It cannot correct any errors that it could make.

Examples of 'open-loop' system: 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.

Examples of 'open-loop' system: an irrigation sprinkler system, programmed to turn on at set times could be an example of an open-loop system if it does not measure soil moisture as a form of feedback. Even if rain is pouring down on the lawn, the sprinkler system would activate on schedule, wasting water.

Closed Loop Systems The second type of system a closed loop system carries on working continuously, they don’t just stop after one decision has been made. Input Process Output Feedback

Example of a closed loop system: Air conditioning: the system continually monitors the air temperature and adjusts the amount of cold air blown out in relation to the set temperature (feedback system)

PLC Programable Logic Controllers Used to read a set of digital and analogue inputs, apply a set of logic statements, and generate a set of analogue and digital outputs. In controlling temperature, INPUT: the room temperature LOGICAL STATEMENTS: would compare the pre-set value to the input temperature and decide whether more or less heating was necessary to keep the temperature constant. OUTPUT: would then either open or close a hot water valve depending on whether more or less hot water was needed.

Advantages of computer control Can operate 24 hours a day without taking a break.  Can work without holidays or sick days Will work without any wages. Will repeat actions over and 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 where it wouldn't be a good idea to send humans to.

Disadvantages of computer control 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 can’t react to unexpected events like a human could. It can only respond in the way it has been programmed to. It can cause some concern if total control for a system and the decisions are handed over to a computer.

Homework Complete the relevant section in your homework book