by Gary Wilson, Siemens
A basic guide to how switchgear, instrumentation and variable speed drives integrate with software and control systems to create industrial automation.
The word “automation” means to control an industrial machine or process without human intervention. In this article, we focus on factory and process automation.
Factory automation
Fig. 1: Fieldbus for remote devices.
In factory automation, field devices such as switchgear, variable speed drives (VSDs) and instrumentation communicate to a programmable logic controller (PLC) which reads all inputs from the plant or machine, processes the information and then transfers the result to the outputs. The field devices communicate to the PLC via either hardwired signals or a network, typically RS485-based Profibus or the more advanced ProfiNet, which is based on Ethernet.
The network
The network distributes input/output (I/O) channels, making distributed I/O necessary to prevent all the hardwired field devices from having to be connected to a single cabinet. This would mean many long wires and a lot of extra work (see Fig. 1).
The engineer writes the software to collect these signals from the field devices and to collate them in the PLC, which uses the signals to control the plant.
For example, when the level-reading instruments in a tank register a high level, the PLC reacts by stopping the pump filling the tank and by starting the motor for the tank mixer. Software programming languages used by PLCs include LADDER (see Fig. 2), Function Block Diagram (FBD), Statement List (STL) and Sequence Flow Charts.
Human-machine interface
Fig. 2: A screen capture of the LADDER programme.
Human-machine interface (HMI) panels are used onsite to link to the PLC and to show graphically whether the machine or process plant is operating optimally. In the field, these panels are normally located on the PLC cabinet. They provide a partial view of the plant operation as opposed to an entire plant automation visualisation overview.
Where the factory or plant has multiple PLC areas, a plant overview visualisation system (SCADA) would link to all PLCs, gather data and provide the operator with a plant overview.
Process automation
Distributed control systems
Distributed control systems (DCSs) are recommended for process automation of industrial plant where processes are run continuously, e.g. power stations or oil, gas and chemical plant.
Fig. 3: Industry section types.
The operator station (SCADA) creates many of the visualisations, saving a lot of engineering. Factory automation systems are normally less expensive and slightly more flexible as more PLC options are available. For process plant, however, the hardware and software costs are higher but the engineering costs less.
Other applications
Some industrial plant employ a combination of factory/discrete and process automation (see Fig. 3).
Customers such as oil refineries with critical processes may require their automation systems to be highly available. Redundant PLCs such as the Siemens S7-400H would be most suited in these cases. The redundant PLC normally consists of two processing units (CPUs) so that, were one to fail, the other would take over automatically. Redundant remote I/O would normally also be connected to this system.
Fig. 4: Integration of process I/O in the hazardous area.
For customers in, for example, the automotive industry where humans feed parts into robotic cells or where explosion risk is high due to high-pressure processes, safety may be more important. For cases like these, safety automation systems up to SIL3 are available to protect against hardware or software faults and which stop machines safely if faults such as I/O module and PLC failure do occur (see Fig. 4).
Contact Keshin Govender, Siemens, Tel 011 652-2412, keshin.govender@siemens.com
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