In this article, you will learn what a HART protocol is, how HART is used with analog sensors, how an analog HART sensor works, and what benefits an analog HART sensor can bring to your operation.
Current loop technology has been used for analog sensors for the last 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 on a single pair of wires for each current loop sensor.
One 2 amp, 24 volt DCpower sourceit can "control" dozens of sensors.
For analog current loop sensors, the smallest 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 threshold 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 $100-$500.
More complexflowGauge, level, and analytical sensors cost more, but 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 long distance over a single pair of wires with little or no signal loss.
A current signal can be transmitted up to 1000 meters through 18 gauge wire without appreciable signal loss.
Finally, current loop signals from 4 to 20 milliamps provide a basic level of diagnostics.
Since 0% equals a 4 milliamp signal, a broken wire would break the circuit and 0 milliamps would be detected.
This "live zero" feature, where 0% equals a value greater than 0 milliamps, allows the control system to detect a broken wire at 0 milliamps.
But analog sensors can only send a "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 problem is not so important.
An A-D converter with 16-bit precision can report the range of values to an analog sensor in increments of 65,535.
This means that for a 0 to 1000 psi pressure sensor, the granularity of the signal value is 1000 divided by 65535 or 0.015 psi. This level of accuracy would be sufficient for most applications.
Before we answer the question of what the HART protocol is, let's quickly look at another analog device; the analog phone. Analog telephone communication is similar to analog sensor signals used in industrial plants.
Analog telephone lines transmit voice as 48 volt DC electrical signals. When you speak into the telephone receiver, the microphone converts the sound waves into analog electrical waves.
These waves propagate through the telephone line to their destination. The receiving phone converts the electrical signals back into sound waves through the phone's speaker.
A pair of copper wires for transmitting voice and one conversation (or transmitted value) at a time. All like an industrial analog sensor.
In the late 1970s, Bell Labs invented the Bell 202 modem standard. In 1980, the Bell 202 standard was adopted as the communication standard for subsea oil and gas production control systems.
Bell 202 specifies a modulation method known as Audio Frequency Shift Keying (AFSK) to encode and transfer digital data at a rate of 1200 bits per second, half-duplex (that is, transmit only in one direction at a time).
Basically, it provides a continuous signal, like an AC sine wave, that changes its frequency from 1200 Hertz, which indicates a binary value of 1, to 2200 Hertz, which indicates a binary value of 0.
Here's the kick. By overlaying a Bell 202 signal over a standard analog phone line signal, we gain the ability to send digital data AND analog data at the same time over the same pair of wires.
This was used to transmit the phone number of the caller along with the voice call. This feature is known as caller ID.
What happens if we superimpose a Bell 202 signal on top of a standard analog sensor line signal? We got the ability to send digital data AND analog data at the same time on the same pair of wires. This is the HART protocol!
With HART, we can send analog data, the measured value of the process variable, along with digitally transmitted data, such as a tag name, calibration settings, or sensor diagnostics. That would be a real productivity boost for the process plant!
And since HART-enabled sensors only require 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!
That would be a real productivity boost for the process plant!
The good news is that HART is built into most commercially available analog sensors, and HART-enabled analog input cards are available from nearly all DCS and PLC manufacturers.
There are even additional hardware devices to turn your HART sensors into wireless transmitters!
Now that we know what the HART communication protocol is, let's wrap up the discussion of how HART works. First, HART is an acronym for "Highway Addressable Remote Transducer". This simply means that a small network can be formed with up to 63 HART devices, each with its own address or node number.
Since a sensor can be accessed remotely using HART, the name really says it all: "road (or network) sensor (also called a transducer) that has an address so it can be accessed remotely and directly."
The remote capability of HART sensors is very useful and powerful. In the following diagram, we see a HART sensor connected to a PLC analog input board.
We can access the sensor data remotely using the HART communication protocol from the PLC programming software.
This means that we do not need to be at the location of the sensor to access its data. We can configure, calibrate, and retrieve diagnostic data from a control room or other location where HART data can be accessed.
Data from a HART sensor must be requested by the master node, which controls all conversations on the loop. The master node is usually the DCS or PLC analog input card to which the sensor is connected.
Each message from the master includes the type of request, such as "send measurement value", the node number of the sensor for which the message is intended, and any data that needs to be transmitted to the sensor, such as a new value for the upper limit. scope.
Using a handheld programming and configuration device, often called a "HART Communicator", sensor data can be accessed whenever there is an opportunity to connect the handheld device in parallel with the loop wires. This could be in a junction box, a protection panel, or the sensor itself.
Therefore, if a sensor is in an inaccessible or dangerous area, configuration or maintenance of the sensor can be performed from a safe and remote location.
Networking of HART devices, in most control system applications, is not practiced. Due to its limited speed and cumbersome multidrop network topology, we typically assign only one node or sensor to each HART signal loop.
Fortunately, HART allows for multiple master nodes, so the control system AND a handheld communicator can be connected to the loop and can communicate with the device at the same time.
With HART, both the 4-20 milliamp analog signal and the digital HART protocol are available to both the control system and the instrument technician.
If a sensor loop is upgraded from 'analog only' to 'analog plus HART', the programming and configuration of the control system for the measured value can remain the same.
You can imagine that superimposing an AC signal on top of a DC signal can interfere with the signal from 4 to 20 milliamps. But this is not the case.
The AC HART sine wave oscillates at 1200 Hertz for a value of 1 or to 2200 Hertz for a value of 0.
The amplitude of the AC sine wave remains the same, and for each oscillation, the amplitude of the first half of each sine wave above the DC current curve is exactly equal to the amplitude of the second half of each wave. sinusoidal below the DC current.
The net effect of the sine wave is then zero. Therefore, the analog value of the sensor measurement data is not affected by the HART signal, just as a telephone conversation is not affected by the caller ID signal using the same Bell 202 protocol.
Each HART device is capable of sending and receiving 35 to 50 different pieces of information, including the process variable (ie, the same measurement value provided by the 4 to 20 milliamp analog signal); device status; diagnostic alerts such as "sensor value below range";
basic configuration parameters such as upper and lower range limits; and the device tag name.
The HART communication protocol is a perfect choice for multivariable instruments such asFlow meters, where mass flow, volume flow, temperature and density can be communicated to the control system via a single cable.
The HART protocol is governed by an independent vendor association, The HART Communication Foundation, so HART sensors from any manufacturer can be interchanged with those from other manufacturers.
This makes deployment, maintenance, and troubleshooting much easier. Additionally, HART is widely used for final control devices, such as control valve positioners, with the same benefits and diagnostic capabilities.
While the HART standard requires manufacturers to provide a minimum number of specific data items with each HART sensor, vendors can also expand the data set to include vendor-specific items such as sensor model numbers or firmware versions or advanced diagnostic counters.
In order for the control system to recognize the type and values of this custom data, a special description file called a Data Description (DD) file is required.
This file is uploaded to the DCS or PLC Configuration Station or downloaded to the Handheld Communicator and directly associated with the sensor.
This file simply allows the sensor data stream to be correctly parsed or interpreted and allows the technician or engineer to make the correct requests to the sensor data.
To review, HART is a digital data communication protocol that sits on top of a traditional 4 to 20 milliamp analog signal that provides advanced data retrieval and configuration options to be performed remotely from a DCS or PLC system or handheld communicator. .
HART communicates over a single pair of wires, so adding HART to an existing 4-20 milliamp sensor loop requires no wiring modifications. Only the analog input board hardware and sensor electronics need to be upgraded to provide HART functionality.
A high percentage of sensors already installed on 4-20 milliamp loops are already HART-enabled.
HART can be the fieldbus you already have in your plant. Through a simple configuration, a large amount of new process data and diagnostic capability can be achieved with a minimum of effort and expense.
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