What are the sensors? and do not know about them

These are the elements or senses that detect changes in the value of the controlled variable. Often called parent-in some cases forming part of a block with the call transmitter or one that receives the sensor output signal and adjusts the transmitting end, this set is called a transducer.

In general, the response of a sensor will determine how well the measurement, recording or control of a variable, and their selection is the result of knowing well the characteristics of a process. Some of the most important characteristics of a sensor or transducer that define their quality are accuracy, linearity, resolution, etc. Another important aspect is the so-called response time or time required for the device to deliver the final information. To the extent that this delay can be minimized, there will be a better process control

The errors involve measuring delays while the process is changing. The measurement is not only late but also inaccurate, because it keeps changing, and even taking a reading available. A more slowly in response, more inaccuracy in the measurement when it is received. A disturbance of short duration, however, may be completely undetected if its duration is short compared to the delay measurement. In that case, the disturbance may have little effect on the process

The thermal sensor is a function of their size, shape and material. The resistance to heat flow, however, depends on the nature of the fluid and its velocity. As an example, the response curve of a thermocouple placed in a fluid exposed to temperature, is an exponential curve reaching 83% of its final amplitude in a shorter time than that of a thermocouple inside a thermowell. The difference in delay is due to the higher capacity (mass increase) of the second sensor. In general, for any variable to be measured, these considerations about the response time are gravitating in the response of other elements and instruments that exist in a particular control system.

Figure 3 - thermocouple inside a thermowell

What if transmitters are?

The transmitters are instruments that capture the process variable and pass it to an instrument remote recipient, is an indicator, recorder, controller, or a combination thereof. There are several types of broadcast signals, pneumatic, electronic, hydraulic and telemetry. The most used are the electronic industry which have replaced over time as the pneumatic signals applied to these machines, hydraulic signals are used occasionally when you need a great power and telemetry signals when there are large distances between the sensor and receiver.

The transmitters generate a signal pneumatic tires linearly variable from 3 to 15 psi (pounds per square inch) for the measurement range 0 -100% of the variable. The standard signal was adopted in general by the manufacturers of pneumatic transmitters and controllers in America. In countries using the metric system is also used 0.2 to 1 kg/cm2 signal equivalent to about 3-15 psi (1 psi = 0.07 kg/cm2). Pneumatic signals mentioned are currently mainly applied as input signals to control valves or their positions.

Electronic transmitters generate various types of DC electrical signals and digital signals. Among the former, the most used are 4-20 mA and 0-20 mA and in panel 1 to 5 V. The electronic signal from 4 to 20 mA and has a sufficient level of commitment between the transmission distance and the strength of the team. When alternating continuous and eliminates the possibility of capturing shocks, is free of eddy currents, uses only two wires that do not require shielding and can act directly on ammeter, potentiometers, analog computers, etc.. rectifiers without using or modifying the signal. The "living zero" with the starting signal (4 mA) offers the advantages of detecting a fault by cutting a thread (the signal is zero) and to allow further differentiate "noise" of transmission when the variable is at its lowest.

Electronic transmitters can be categorized into analog and digital. The first based on the use of operational amplifiers (OPAMP) and the second microprocessor. Analog transmitters are now virtually obsolete and due to its mechanical constitution, have an adjustment of zero and span (span) complicated and high sensitivity to vibrations.

Current technology has made electronic transmitters, not only incorporate the sensor form a single block, but also have the potential to control (PID) on e! final element of control. These transmitters are called intelligent. The digital transmitters have a number of advantages over analog as we will see on the other hand, the increasing use of digital signals in these transmitters determine at some point the standardization of a digital protocol as it has been until now the analog signal 4-20 mA.

Figure 4 - Digital Transmitters

And the drivers?

As discussed, the controller determines the necessary actions to maintain process variables at the desired value (process control) or can also be one that ensures necessary production sequences based on a preset schedule ( PLC). We will refer here to the first one.

A process controller (or regulator) can be defined as a device that compares the value of a measured variable (input signal) to the desired value (set point) to produce an output signal that maintains the desired value of the variable and uses this difference to manipulate the controlled variable. The technology of these devices has ranged from pneumatic, hydraulic, to electronics, which are currently employed.

Previously, he was a typical automatic control loop with the basic components: the sensing element (sensor) measuring element (transmitter), the control element (controller or regulator) and the final control element (valve or other) . It is noteworthy that two or more of these elements may be forming a single block, but not as usual.

Fig 5 - Digital Controllers

For many years, acting pneumatic controllers used with the aforementioned standard pneumatic signals. Currently used mostly analog and digital electronic controllers. The first, almost no longer manufactured (though still used) and have been replaced by the latter, which are based in microprocessors, which provide many important possibilities for the user and definitely have more advantages than their predecessors.

A call to final control elements?

Are those that ultimately respond, within a control loop to make a change in the controlled variable. In most process control valves, are used, if it is: to control variables such as flow, pressure, level, temperature or mixture components cie. Most fluid flows are controlled by pneumatic or electric, in some cases pumps are used, for gas service valves are often used for special and solids is common to speak of powered conveyor belts and electronic speed control.

Valve

pneumatics

Some

Within this group we can mention some devices and instruments that perform other functions such as indicators, recorders, converters, alarms and switches and items of special functions.

With respect to indicators include items that have graduated scales which can be linear or not, the indicators can be analog (needle indicator or even different colored vertical bars) or digital variable presented as numerical.

The recorders provide continuous records of measured variables with respect to time, letters registers, use essentially the same scales as indicating instruments, but with an additional coordinate to indicate time: it can be circular or tape and have varying dimensions. Record speeds vary from several minutes to several days per revolution by revolution, in the case of the circular letter and registrars from second to centimeters meters per hour to tape the letter. Currently available recorders also called intelligent (microprocessor-based) that go far beyond tracing the trends of the variables (you can do analysis, reports, etc..) Being the tendency to stop using paper and save more data storage (action until recently, performed only by so-called data loggers) and displayed on a liquid crystal display (LCD) or through a computer.

The development of electronic controllers created the need for signals from devices that convert energy from one form to another and from one level to another signal, such as current to pressure converter to act on the pneumatic valves.

Often required to convert signals from one level to another, for example, electronic systems that receive signals from 4 to 20 mA, to connect to transmitters that send signals from 0 to 20 mA, in this case would use a current to direct current converter although today, this is a choice of software and hardware, as has happened with the square root extractors and other computing devices due to the development of digital technology.

The alarm functions and disruption, are used to abnormal conditions of a process. The devices used for these functions can also simply indicate some action or control. Additionally, you can include other elements that are used in various applications, such as timers, solenoid valves,, programmers, etc.. whose use will depend on the type of control and the process itself.