2-Wire current loop transmitter
Currently, I
think that the 2-Wire current loop transmitter still be most population are
used in the industries, Power plant, Chemical plant, or Other continuous
process plant. It is a classical
standard for interfacing between field instrument and the controller or
processor.
Why still be standard 2-wire current loop (4-20mA)?
The advantage of current is
that the accuracy of the signal is not affected by voltage drop in the
interconnecting wiring, and that the loop can supply operating power to the
device. Even if there is significant electrical resistance in the line, the
current loop transmitter will maintain the proper current, up to its maximum
voltage capability.
Moreover, some reasons may be about reliability for
transmission signal, No complexity for implementation, easy to check the
signal. And, when it is “Standard ” that mean a lot of Venders
produce and a lot of Industrials used, that difficult to change its.
The concept of 2-Wire current loop and application
There are 3 main components for Current Loop transmitter. DC
Power Supply is “Source” for distribution to system, that it can supply enough
for total load consumption. Transmitter regulates the flow of current between 4-20mA
that is standards signal. And
the last, controller Input card receives the 4-20mA out put signal from
transmitter, it has the 250ohm inside for convert the 4-20mA current to 1-5 VDC, and then send to the ADC
circuit for processing in digital parts.
Besides, 2-Wire current loop is developed to use with the
HART protocol, it make the transmitters or sensors to be smart device, that its
can be more than the analog transmitter. So, HART Protocol become classical
technology (it like a Modbus Protocol) that it still be most population in
present,
2-Wire current loop transmitter with HART Protocol
Almost
2-Wire current loop transmitter in industries used HART communication. HART is hybrid communicate, it combines 2 signals that is Analog signal 4-20mA modulated with the digital signal FSK method (Frequency Shift Keying).
If we use HART communication in smart type
transmitter, we can access to many function and parameter in our transmitters,
that the feature of HART as following:
- Diagnostic data maybe transmitted by the field
device (self-test results, out-of-limit alarms, preventative maintenance
alerts, etc.).
- Field
instruments may be re-ranged remotely through the use of HART
communicators
- Technicians may use HART communicators to force
field instruments into different “manual” modes for diagnostic purposes (e.g.
forcing a transmitter to output a fixed current so as to check calibration of
other loop components, manually stroking a valve equipped with a HART-capable positioner)
- Field
instruments may be programmed with identification data (e.g. tag numbers
corresponding to plant-wide instrument loop documentation)
In the
part of Digital communication used FSK method (Frequency Shift Keying) based on
the Bell 202 communication standard is used. The two digital values Logic “0” = 2200Hz and Logic “1” = 1200Hz. And
the modulation has Amplitude 1mA Peak-to-Peak.
(More detail you can see as link: http://www.samson.de/pdf_en/l452en.pdf)
To ensure a reliable signal reception, the
HART protocol specifies the total load of the current loop including the cable
resistance to be between minimum 230 ohms and maximum 1100 ohms. Usually, the
upper limit is not defined by this specification, but results from the limited
power output of the power supply unit
Experimentation
This is communication of the 2-Wire current loop transmitter
connected with HART 475 Field Communicator. I used Multi meter FLUKE 789 to
make “Source” for supply to the transmitter, and then I used its for measuring
the Output current from transmitter also. FLUKE 789 there is function Resistor
250 ohms inside itself, to ensure that transmitter and communicator aware the signal
of communication (Wring connection same as the picture of 2-Wire current loop
transmitter with HART Protocol)
When the transmitter is connected with HART Communicator,
you can adjust, re-range, calibrate of the transmitter as the HART Feature. Example,
mainly calibration function in used for adjust as following.
1. Sensor Trim
Sensor Trim is about adjusts ADC of transmitter by comparing
with the standard reference. Example, if you want to adjust the sensor of
pressure transmitter, you must take a standard pressure to diaphragm of
transmitter and adjust its to same value.
2.Adjust LRV, URV
You can adjust the actual operating range to be 4 –20mA, to
make sure that at minimum range (LRV) will be 4mA and then Maximum range (URV)
will be 20mA.
Some time, easy way for adjust the zero point/minimum range
can be set by special “Push button” of transmitter on the site such as Level
transmitter, Pressure transmitter if you required.
Manual set Zero Point
3. Analog D/A Trim
You can adjust the Analog out put of transmitter by
comparing with the standard meter for make sure that the digital value of
processing same with the Analog output value.
You can edit and
calibrate the parameter of transmitter by using HART communicator
However, in the present has a new standard “Field bus”,
which is intelligent and smart device. But it still has some disadvantage about
reliability system or using with critical equipment. Besides, it is high cost equipment,
complex system that it required special knowledge for installation and
troubleshooting. So, 2-wire current loop still be used in industrial depend
on suitable of the system.You
may ever seem the 2-wire current loop and Field bus in your plant together.
Thank you for
GHECO-One Power plant, Instruments Lab
www.samson.de/ hart
Thank you for
GHECO-One Power plant, Instruments Lab
Lessons In Industrial Instrumentation
By Tony R. Kuphaldtwww.samson.de/ hart
