The 4 – 20mA current loop

Industry 4.0 is about automation and exchange of data in manufacturing processes. Technologies like logic controllers for process controls, computer based algorithms and Internet of Things are predominant in Industry 4.0.

In order to achieve production efficiency, consistency and economic advantages, process control systems are widely adapted in industries. Process controls was a collaborative approach emerged from the fields of both control engineering and chemical engineering.

In the very first industrial revolution processes were controlled by hydraulics; mostly by water. But there were several disadvantages of this system and hence new techniques of process control came into existence. Due to increase in availability and reduction in costs of electronic circuits the era shifted towards use of electrical signals.

Analog signals for process monitoring and control
Using varying electrical signals like voltage and current is the most basic means of information transfer. Out of all the analog signals, 4 – 20mA current loop is far more superior. It comes with advantages of simplicity, reliability, cost effectiveness and most importantly linear behavior of output.

How do we communicate or transfer information using analog signals?
The answer to this question can be explained with the help of a block diagram as shown in fig.1; it consists of the following main components:
Sensor: A sensor is required to sense any physical quantity or to measure a process variable. Type of sensor depends on the application.

Transmitter: The value obtained from sensors are to be converted to appropriate electrical signals generally of the range 4-20mA i.e. 4mA for 0 C and 20mA for 60 C.

Receiver: It is a device at the receiving end which will convert the signals back to original form either it may use these signals to display original values or actuate a device to control processes.

Power source: A power source is essential to generate signals. In order to obtain unidirectional signal a DC source is generally used.

Loop: Let us understand what a current loop means and its importance. A current loop is a closed path in which the current flowing through each element is same.

The law governing a current loop is simply Ohm’s Law given as;

V = I * R Volt——-[1]

Considering a simple closed circuit (loop) with a DC source and resistors in series, a voltage drop takes place at every resistor and this can be obtained using Kirchhoff’s Voltage Law, Refer fig.2;

V = IR1 + IR 2 + IR 3 ——-[2]
Hence, if instead of current signals voltage is used then there would be drop in voltage and the signal received at the receiving end would be weak. Leading to inefficient signal transfer.

Whereas, for the same case, current will be same throughout the loop. And hence there are least chances of failure of transferred signals. Thus, signals can be transferred at long distances efficiently.

Why use current loop?
As inferred from its operating principle, a current loop is more convenient to use for remote or communication over long distances.
It reduces chances of signal failure and information transfer becomes efficient and effective.
When using 4-20mA current signal advantage of live-zero can be taken to detect sensor failure. Added benefits are of low electromagnetic susceptibility and better immunity to noise.

How to relate actual signal (quantity under measurement) with 4-20mA signal?
Let us consider an example in which temperature of a zone is to be controlled and monitored using PLC. Following are the conditions:
An RTD is used to sense the temperature of heating chamber which varies from 32 C to 180 C.
Input to PLC is via a temperature transducer which will convert and provide output of 4-20mA.
Hence (32 to 180) C is to be mapped with 4-20mA signal.

For this case;
Input start value (x1) =32 C
Output start value (y1) = 4mA
Input end value (x2) = 180 C
Output end value (y2) = 20mA
Now let’s say the present temperature of chamber is 106 C (z1), what will be the transducer output equal to?

Let the unknown parameter be ‘z2’;
Therefore,
z2 = (( y2 – y1 )*(z1 – x1 ) /(x2 – x1 )) + y1
….(1)
z2 = ((20 – 4)*(106 – 32) /(180 – 32)) + 4
z2 = ((16)*(74) /(148)) + 4
z2 = ((16)*(74) /(148)) + 4
z2 = ((16)*(0.5)) + 4
z2 = 8 + 4
z2 =12 mA

Hence the output of the temperature transducer will be 12mA corresponding to present chamber temperature 106 C.

Proper understanding and implementation of this technology makes complex task of communication simpler. Due to the world wide acceptance and large user-supplier base many technological advancements have been put forth. Such are – loop powered devices and HART communication system.

HART Communication:
The standard 4-20mA analog signal used in industry can transmit only a single parameter that too in a single direction, it wouldn’t act as a receiver of a 4-20mA signal.

To mitigate this drawback and to make this technology advanced a bi-directional communication protocol was been developed with which one can transmit and receive information by superimposition of an alternating current signal over the 4-20mA signal. This technology is called Highway Addressable Remote Transducer (HART).

Conclusion:
Hence, it has been a standard industrial practice to use 4-20mA current signals instead of 0-20mA with the benefits of detecting open circuit faults as well as it is easier to convert it to low voltage (1-5V DC) signal, easily processed by majority of controller input cards, catering the needs of most of the control processes.

Authored by
Ms. Neha Mistri
(Application Engineer)
Rishabh Instruments Pvt Ltd