Digital Transmitters digitize the analog signal and used as a microprocessor. The input analog signal is digitized transmitter with an ADC. Due to the time required to measure a signal, the digital instruments do not perform a continuous measurement, only samples (samples) of the signal. Then we will see a classification of digital transmitters.
Figure 5 Conversion Analog / Digital
TRANSMITTER "INTELLIGENT"
Possessing a microprocessor performs functions do not analog, linearized, compensated as a function of another variable or algorithms and others.
Figure 6 Input and output signals from a transmitter "Intelligent" or a "Smart"
TRANSMITTER "SMART"
Its output is analog 4 to 20 mA, and can communicate with a "hand-held" using the output modulation.
Transmitter Connection "Smart"
DIGITAL TRANSMITTER
Fully digital, including the exit. Although you can: having analog output 4 to 20 mA. The instruments "fieldbus" (field) are of this type.
Differences between digital and analog transmitters
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Electronic components are different: In an analog instrument used linear circuits such as ADC's. In digital instruments used microprocessor, ADC and DAC if you have analog outputs.
The analog output signal is generated differently. Digital instrument from a DAC.
Advantages of digital transmitters
They are flexible in their duties: they have more features, the ease of manipulation of numbers by a microprocessor. Functions can be modified or expanded by changing the firmware.
Analog output 4 to 20 mA is independent of the measuring circuit, the range may be different from the instrument.
Calibration and idealization are performed digitally.
Idealization equalization can be characterized for a particular sensor.
Digital techniques are more powerful conditioning.
compensates the drift of the sensor with the help of a temperature sensor.
Improved accuracy.
Higher turndown.
Self-diagnostics.
Communication skills, such as smart and digital instruments.
The following figure shows the independence of the measuring circuit and output circuit, adjustments are made independently.
Figure 9 Architecture of a Digital Transmitter
Some advantages of analog transmitters
Working in real time. In digital instruments are sampled in the order of 2 to 20 sample / s (whether time: 50 ms to 500 ms). Therefore, very fast processes can not use digital tools, you must use analog.
Disadvantages of analog transmitters
You need recalibration to change the measurement range, and you need experience.
The need to remove the line instrument to calibrate.
Components, such as potentiometers, experience "drift".
The Idealization is fixed for a single type of sensor.
The following tables compare an analog and a digital transmitter and an analog transmitter and a "Smart".
Transmitter |
Anal or GICO | Digital |
Correctness | 0.25% to 1% | 0.02% to 0.1% |
Table 2 Transmitter Accuracy
Feature | An to logo | Smart |
Rank: | 0-5/30 0-25/150 0-125/750 | 0-.83/25 "H 2 0 0-8,3 / 250 0-33,3 / 1000 |
|
Accuracy: Linearity: Hist é resistance | ± 0.2% span ± 0.1% span ± 0.5% span | ± 0.1 % span, includes hist é resistance, linearity and repeatability |
Stability: | ± 0.2% URL - 6 months | ± 0.1% URL-12 months |
N Table 3 Comparison of Analog pressure transmitter and a "Smart"
The real time instruments
The analog instruments work in real time. Digital instruments are considered to work in real time if "be time" is much smaller than the constant delay of the controlled process. The digital instrument downtime is introduced by the ADC and the execution time of the microprocessor program.
Digital transmission adds more time out, because serial communication, and according to the efficiency of the protocol between the transmitter and receiver. The following table shows a comparison of downtime ties with different types of transmitters:
Loop | A | B |
C |
Transmitter Type | Analogous | Smart (Rosemount) | Digital (fieldbus) |
Reason update (Updates / s) | 5.5 | 2.7 | |
Time-out Transmitter (Ms) | 20 | 400 | 700 |
Time-out Driver (Ms) | 250 | 250 | 250 |
Other times dead (ms) | 480 | 480 | 480 |
Total dead time (ms) | 750 |
1130 | 1680 |
Table 4 Transmitter Timeouts
INTELLIGENT CONTROL VALVE
The conventional valve has the following problems:
The non-pneumatic positioner provides highly accurate regulation.
The pneumatic positioner is difficult to adjust.
Basically the name of smart valve is due to the presence of a digital positioner that replaces the traditional position. In the block diagram shown in the figure below, the digital valve controller (DVC) is located within the digital positioner main advantages are:
Provides better regulation than the traditional position.
Having microprocessor performs functions and control, diagnostics and communication with a host (PC, DCS or Hand Held).
The calibration of the valve is given in minutes.
The valve can be monitored, obtaining information from the position of the rod and the input signal and alarm status or process.
Out of service, but online, you can perform tests such as hysteresis, "signature" of the valve (vs Pactuador. Displacement of the actuator), step response.
Figure 10 Block diagram of a Smart Valve (courtesy of Fisher)
The variables measured are:
Moving the rod
Actuator Pressure
Control signal from the controller (4-20 mA)
Particularly in this valve Hart protocol is used for communications. There are other protocols whose characteristics are the subject of another course.
Figure 11 Intelligent Positioner
The table below shows the information accessible remotely intelligent valve. It notes the information identification, diagnostic, calibration, and others.
Table 5 - Information obtained from an intelligent valve
+ INFORMATION on smart valves visit: `
