In principle, all industrial processes were controlled manually by the operator (even today this type of control exists in many plants), the work of this operator was to observe what is happening (as in the case of a drop in temperature) and made some adjustments (like opening the steam valve), based on user control and in one's ability and knowledge of the process by the operator.

This loop - process sensor, the operator, a valve, a process - it remains a basic concept of process control.

In manual control, however, only an experienced operator reactions make the difference between a relatively good control and other erratic, indeed, this person will always be limited by the number of variables you can handle.

Furthermore, data collection requires greater effort for an operator, which is already devoting significant time in the care of the processes observed and is therefore too busy to write numbers and data, which are obviously needed to better control over the process. All this can combine to have data that can be inaccurate, incomplete and difficult to handle.

Automatic control unlike the manual, is based on devices and equipment that make an entity capable of making decisions about changes or adjustments to a process for achieving the same objectives as in the manual control but with many additional benefits. In addition to this, a number of elements that can be integrated into this set to achieve compliance with various functions, which as mentioned, it would be impossible to be accomplished by an operator with the desired accuracy and efficiency.

A little history is always good

The first control of industrial processes based on the ability of the operator (manual control). In subsequent years, the emergence of local controllers allowed the operator to handle multiple control loops, but there was still the problem of data collection. The local controllers are still very useful, as well as robust and simple. However, because they are directly related to the process and therefore are scattered throughout the plant, obviously makes the adjustments to perform maintenance and time consuming such instruments.

The development of the pneumatically operated control devices marked a major advance in the process control. Here the variables can be converted to pneumatic signals and transmitted to remote controllers. Using some complex mechanisms, a pneumatic controller to perform simple calculations based on a reference signal (set point) and the process variable and adjust properly the final element of control. The advantage was that the operator could control a number of processes from a control room and make the necessary changes simply. However, the constraints lie in the slow control system response to rapid and frequent change and inadequate application in situations where the instruments are too far apart (losses).

Around 60, the electronic devices as an alternative replacement appeared to pneumatic controllers. The electronic controls for a closed loop, are fast, accurate and easy to integrate in small interactive links, but the improvement in terms of operation with respect to tires was relatively small and well data collection, yet very easy to handle.

Some time after the onset of analog electronic control systems, development of the microprocessor allowed the emergence of digital transmitters and controllers, as well as programmable logic controllers (PLC), also specialized systems such as the computerized numerical control machines (CNC)

The use of digital computers did not take long, of his application, are the direct digital control systems (DDC), to monitoring and control systems today, which is achieved by handling a large number of processes and variables, collect lot data, analyze and optimize various units and plants and even other activities such as maintenance planning, quality control, inventory, etc.

Regardless of the technology, the evolution of control techniques have had as one of its key objectives, replacing the direct action of man in the management of a particular process, by the use of equipment and automated systems, however, there is an analogy very clear distinction between them and the man, in regard to how to act

The type of process chosen for a particular product will depend on their production requirements and quantities. In any case, to control it is necessary to know about the instrumentation used and overall mechanical aspects related to the process. Optimal control however, is not only a function of the devices, equipment and systems to be used, but mainly the knowledge of the process to be controlled.

Instruments and equipment for Process Control and Automation

Industrially, the instruments are used to monitor and control process variables. Depending on the type of process, as discussed below, are selected components. Below is a block diagram of a closed loop control or feedback. It is not the only way to control a process, but we will serve to identify the functions of the main field and panel instruments used to measure and control industrial variables.

Here the process can be physical or chemical reaction or energy conversion. There are different types of disturbances that affect the process conditions. These disturbances create a need to monitor and control the process.

The controlled variable is the parameter to be controlled to the desired value or reference (set point). The sensor measures the value of the controlled variable and the transmitter changes this value to a standard signal can be transmitted. This signal is received by various components, depending on the function of the system et instruments such as registration, display, control and trigger alarms or interlock.

In the case of the driver (in this case a process controller), this signal (wariable measure) is compared with the set point and the difference (deviation) is used to fine item! control (usually a valve) to adjust the value of the wariable manipulated. This setting causes the controlled variable value is directed toward the reference.

Of course, not all automatic control systems have exactly this model (called feedback), there are variations such as feedforward control, the cascade, the split range, combinations of these, and so on. technology tools based on ancient or modern, of all these technologies, we will refer to those related to ongoing processes of automatic regulation, as discussed below.

Let's use some considerations related to the components of the diagram above, from sensors to the final control elements, also mentioning other aspects of instruments not considered in this diagram, but also important in some control loops.