computer control
At the centre of any production system is a computer control system.
The computer control system ensures that the production operations happen in the correct order by controlling the electrical and pneumatic machinery.
It also monitors the production system, checking for problems using a wide range of sensors. If a problem is detected the control system is able to take action to fix the fault or, if necessary, stop the relevant machines and alert a human operator.
Data from the monitoring system are recorded both so that the productivity of the plant can be tracked over time and also to provide a log of machine failures.
At Cadbury, these operations are managed by three kinds of computer system:
1. Production line monitoring
Software running on a Windows PC provides a graphical representation of the production line, showing the operator exactly what the line is doing.
The operator can also view a historical record showing what the production line has been doing in the form of trends, reports and alarm messages.
Operators can both supervise the line and also step in to override the main control systems, for example to switch a part of the line off for maintenance.
The collection of software and hardware used for monitoring is called a Supervisory Control and Data Acquisition (SCADA) system.
2. Production line data collection
Recorded data, such as temperatures, production quantities and faults, are continuously sent to a separate set of computers and stored. Production staff can retrieve and analyse several years’ worth of data.
3. Production line control
Programmable Logic Controllers (PLCs) carry out the second-by-second control of the manufacturing machinery.
A PLC is a dedicated industrial controller, containing a microprocessor, memory and the ability to communicate with a large number of input and output devices. These measure the state of the plant (providing, for example, information on temperature, pressure, position, speed) and change the state of the plant (for example opening valves, running pumps, turning heaters on and off).
In some ways they are similar to the PIC (microcontroller) based systems that many schools use; but a PLC is faster than a PIC chip, has more memory and is ‘ruggedised’ so that it works reliably across a wide temperature range, is not affected by electrical noise and is resistant to vibration and impact, making it suitable for use in a manufacturing environment.
As well as performing the control of the manufacturing lines, PLCs are also linked to the SCADA system, supplying data, receiving commands. The SCADA system is able to override PLCs'.
Each PLC runs its own program which is created using a high level programming language or a graphical approach such as a flowchart system or ladder logic.
PLCs are linked to each other and to the banks of input and output devices using fibre optic cable. Fibre optic cable provides high bandwidth, but the key reason for its use is that the light signals used are not affected by electrical interference.
