1.3.1 Primary eleme:

According to their position in the bock diagram, those measure elements which are the first to detect or modify the process variable are also known as “primary measure elements”. Saying that they detect or modify the process variable means they are the first ones to use or transform the energy from the controlled environment, in order to product a depending on the controlled variable effect.

They are also the ones that are in direct contact with the variable and ready to transmit any environment transformation, being the first elements in a control loop which measures the process variable. Their produced effects can be a change in pressure, force, position, electric measure, etc.

• Level sensors

It is an electronic device that measures a material’s height, generally a liquid, inside a tank or other containers.

• Temperature sensors

We use them to detect temperature variations. They are made up from materials whose electric resistance varies according to temperature

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1.3.2 Secondary elements:

Secondary mention elements receive the signal coming from primary elements and in several cases they transform it for transmitting it as an electric, pneumatic, hydraulic, etc. signal depending on the kind of instrument we are using, because of that these elements are known as “secondary measure and transmission elements or transmitters”. The transmitted signal is sent in such a way it can be perfectly played by the receptor instrument, which might be an indicator, recorder or controller. This signal can also be transmitted simultaneously to several receptors. Among the secondary elements we can find: 

• Transmitters

They are instrument that play physical phenomena, translated into standard engineering signals. Transmitters are connected to the primary element. Sometimes they are part of a transducer, which produces itself the signal for transmission.

They can be implemented in a project in order to send collected data from sensors to controllers so that they can run their control action according to the obtained value and compiled programming, making everything work as expected.

• Controllers

They are devices that regulate the measured variable (pressure, level, temperature, etc.) comparing it with a set value or set point and adjusting the output to the comparison difference or result in order to do a corrective action according to the deviation. They can receive the controlled variable directly as local controllers, or indirectly as a pneumatic, electronic or digital signal coming from a transmitter.

• Controller is the one which decides the action over the final control element. It has two essential functions

• Compare the measured variable with the expected reference (set point) to identify errors among them.

• Send a signal to the final control element in order to modify its action in the proper sense to reduce the error.

They can also be used as indicators to the actuator when to open or close a valve when a controller indicates so, depending on its programming, functionality and received data.

1.3.3 Tertiary elements:

Final control elements are mechanisms that change a variable’s value that has been manipulated as an answer to an output signal from an automatic control device; that is to say, they are in charge of manipulating any process characteristic according to the controller’s commands. Depending on the kind of process, there are devices that receive discrete or continuous control signals.

Final control elements can be a control valve, AFD’s (Variable Frequency Drives), electric motors, a relay, heating elements or a shock absorber

Since the most common thing in industries is that the variable manipulated by these devices is flow, the most used control element is the valve

Control valves

Their function is to variate energy or material flow in a control process, modifying the measured variable’s value and behaving as a hole with variable area. They work according to Bernoulli’s principle, which describes flow through a hole.

Valves are composed by two parts:

• Actuator: Receives a signal from the controller and transforms it into a linear or rotational displacement, consequence of a pressure change on the diaphragm.

• Body: The diaphragm is connected to a stem or shaft that makes the passageway section changes, so as flow.

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