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.

 Hand Pump Pressure Calibrator

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

Lessons In Industrial Instrumentation

By Tony R. Kuphaldt


www.samson.de/ hart