Control and measurement systems

A more rigorous definition of what a process control and data acquisition system is has been provided by the International Electrotechnical Commission (IEC)-61131 standard. According to this standard, a control and measurement system of an industrial process can be described as a set of interconnected devices communicating with each other by means of one or more communication networks, as shown in the following diagram:

The communication networks might be also structured hierarchically:

• Functionality performed by a system can be modeled as an application that may reside in a single device, or be distributed among different devices. For instance, an application may consist of a control loop, in which the acquisition of the measures comes from one device, the control algorithm from another, and the commands sent by yet another device.
• Device is defined as an independent physical entity that is able to implement one or more functionalities. The boundaries of a device are the interfaces made by the hardware and software components, which allow it to communicate with the outside world. A device must contain at least one resource and one interface for the process or communication network, as shown in the following diagram:

• Resource can be considered a logical breakdown of the software or hardware structure of a device which autonomously controls its operations. It can be created, configured, parameterized, started, or deleted without conditioning other resources in the same device. A resource receives data and events from the process and/or from the communication network, processes them, and returns data and/or events as requested by the application using it. In a resource, the following things must be present:
  • One or more local applications, or parts of distributed applications, which process the internal data and events
  • Functions linking internal data and events to and from the process and/or the communication networks
  • A planning function for coordinating the activities

• Data is representations of facts in a format suitable for communication and elaboration by the resource. An example of data is a binary representation of a measurement.
• Events are occurrences of specific conditions, such as the reaching of a definite temperature.
• Process interface links the resources contained in the device with the physical process by communicating with sensors and actuators. The information exchanged with the physical process is sent to the resources as process data or events related to the process.
• Communication interface links the resources with those of other devices to exchange information through a communication network. Through this interface, data or events are sent to the resource and additional services like programming support, system configuration, or diagnostic features are provided.
• Application specifies the operations that must be performed on the data as a consequence of the events. It can be distributed between many resources in the same or different devices.

In the modern control and measurement systems for industrial processes, PLCs, DCS, and usual computers are used for this purpose. Generally speaking, PLCs and DCSes are connected directly to the sensors and actuators, whilst a usual computer exchanges information with other devices such as PLCs or other computers through communication networks.