What_is_Hart_communication

HART-enabled process instrumentation is everywhere, but how does it communicate? what is hart communication? New chip designs support this time-tested protocol.

You will learn :

1. What is hart communication?
2. Why HART Protocol used?
3. What are the advantages of the HART protocol?

 

#1.What is hart communication?


plant instrumentation has become the backbone of modern manufacturing plants

Instrumentation has matured from pneumatic and purely analog electronic systems to the “smart” systems used today, augmenting communications capability by protocols such as HART (highway addressable remote transducer)

HART (highway addressable remote transducer). Simply put, dc and low-frequency current signals are modulated by independent, higher-frequency signals that switch between a pair of frequencies—a technique known as frequency-shift keying

#2.Why HART Protocol used?


Current-loop technology has been used for analog sensors for the past 4 decades

to transmit important process data to the control system, whether that system is a DCS (distributed control system)

a PLC (programmable logic controller), or a single-loop controller. Current-loop data transmission is simple and cost effective.

Only a small amount of current (4 to 20 milliamps to be exact) is required over a single pair of wires for each current loop sensor.

One 2-amp, 24 Volt DC power supply can “drive” dozens of sensors.

For current-loop analog sensors, the lowest measurable process value is called the Lower Range Limit, or LRL.

The analog sensor will output 4 milliamps at this 0% reading.

The highest measurable process value is called the Upper Range Limit, or URL.

The analog sensor will output 20 milliamps at this 100% reading.

Many analog sensors, such as pressure and temperature sensors, are inexpensive, and good quality sensors can be purchased for US$100 – US$500.

More complex flow, level, and analytical sensors do cost more, but these still only require a single pair of wires to allow the process variable, or measured variable, to be transmitted to the control system.

Another positive feature of analog sensors and transmitters is that the signal can be carried a great distance along a single pair of wires with little or no signal loss.

A current signal can be transmitted up to 1000 meters through 18-gauge wires with no appreciable signal loss.

Lastly, 4-20 milliamp current loop signals provide a basic level of diagnostics.

Since 0% equals a 4 milliamps signal,

a broken wire would break the circuit and 0 milliamps would be sensed.

This “live zero” feature, where 0% is equal to a value of greater than 0 milliamps allows the control system to detect a broken wire at 0 milliamps.

But analog sensors can only send one “value” over a single pair of wires to the control system.

And the granularity, or precision, of the data, is limited by the type of analog to digital converter (or A-to-D converter”) used by the control system electronics.

However, with modern electronics, this is not as much of an issue.

An A-to-D converter with 16-bit precision can report the range of values for an analog sensor in 65,535 increments.

This means that for a 0 – 1000 psi pressure sensor,

the granularity of the signal value is 1000 divided by 65,535, or 0.015 psi.

This level of precision would be sufficient for most applications.

(AFSK) to encode and transfer digital data at a rate of 1200 bits per second

half-duplex (meaning, transmission only in one direction at a time).

Basically, it provides a continuous signal, as an AC sine wave,

that shifts its frequency from 1200 Hertz, indicating a binary value of 1, to 2200 Hertz to indicate a binary value of 0.

This is HART communication! With HART, we can send analog data, the measured value of the process variable, along with digitally-transmitted data

such as a tag name, or calibration settings, or sensor diagnostics. This would be a real productivity enhancement for the process plant!

And because HART-enabled sensors require only a single pair of wires for communication, to upgrade an existing non-HART sensor loop to a HART-enabled loop

no wiring changes are required!

#3. What are the advantages of the HART protocol?


HART provides for up to two masters -primary and secondary.

The primary master is generally a distributed control system (DCS), a programmable logic controller (PLC), or a personal computer (PC).

The secondary master can be a handheld terminal or another PC. Slave devices include transmitters, actuators, and controllers that respond to commands from the primary or secondary master.

HART protocol provides a unique communication solution that is backward compatible with the installed base of instrumentation in use today.

This backward compatibility ensures that investments in existing cabling and current control strategies will remain secure well into the future.

Benefits outlined in this section include:
1.Improved plant operations
2.Operational flexibility
3.Instrumentation investment protection
4.Digital communication


 

3 COMMENTS

    • The HART communicator is communicating with the transmitter through the FSK protocol. Understanding this protocol, it is a communication in the analog platform. Digitalization of the signal happens within the HART devices and not along the transmission line. The resistor’s position in the loop is between the transmitter and the DC power supply. As we connect the communicator it should be tap between the resistor and the transmitter or directly parallel to the resistor. Why? Because the power supply has a LOW PASS CAPACITOR filter that basically squelches analog signals like ripples. Thus, if there is no resistance in between the PS and the TX that analog signal coming from the HART devices will only be filtered out. Explaining the value250: The resistance value is not necessarily be250 ohms. Maybe because of the idea that the controlling current of20mA will give us5 volt calibration if there is a loop resistance of250 ohms. Maybe not a good theory. Actually, you can use a number of possible resistance values that will allow communication between HART devices. This value depends on the transmitter minimum operating voltage requirement. Simple OHM’s LAW. For example, if the voltage requirement for a certain transmitter is12-24 volts use a resistor that will not give you a drop of below12 volts or else you will not deliver power to your transmitter. Most transmitters are supplied with24 volts and as per computation250 ohms, to500 ohms gives a drop of almost18 v which is somewhat in between the operating voltage.

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