Placeholder Image

Subtitles section Play video

  • In this video, you will learn what a HART analog sensor is,

  • how HART is used with analog sensors, how a HART analog sensor works,

  • and what advantages a HART analog sensor can give your operation.

  • Before we get into today's video, if you love our videos,

  • be sure to click the like button below,

  • and make sure to click subscribe and the bell

  • to receive notifications of new RealPars videos.

  • This way you never miss another one!

  • 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 candrivedozens 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.

  • Thislive zerofeature, 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 onevalueover 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.

  • Before we answer the question of what HART is,

  • let's look quickly at another analog device; the analog telephone.

  • 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 handset of your phone,

  • the microphone converts the sound waves into analog electrical waves.

  • These waves propagate over the telephone line to their destination.

  • The receiving phone then converts the electrical signals

  • back into sound waves through the speaker of the handset.

  • One pair of copper wires for voice transmission,

  • and one conversation (or transmitted value) at a time.

  • All just like an industrial analog sensor.

  • In the late 1970's, Bell Labs invented the Bell 202 modem standard.

  • In 1980, the Bell 202 standard was adopted as the communications 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 (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.

  • Here's the kicker.

  • If we superimpose a Bell 202 signal on top of a standard analog telephone line signal,

  • we gain the ability to send digital data AND analog data at the same time on the same pair of wires.

  • This was used to transmit the caller's telephone number along with the voice call.

  • This feature is well-known as Caller ID.

  • So what if we superimposed a Bell 202 signal

  • on top of a standard analog sensor line signal?

  • We gain the ability to send digital data AND analog data

  • at the same time on the same pair of wires.

  • 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 tagname, 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!

  • Of course, both the sensor and the analog input card

  • at the controller would need to support HART.

  • The good news is that HART is built-in to most commercially-available analog sensors

  • and HART-enabled analog input cards are available from nearly all DCS and PLC manufacturers.

  • There are even add-on hardware devices to convert your HART sensors into wireless transmitters!

  • Now that we know what HART is, let's finish the discussion of how HART works.

  • First, HART is an acronym forHighway Addressable Remote Transducer”.

  • This simply means that a small network can be formed with up to 63 HART devices,

  • each having its own address, or node number.

  • Because a sensor can be remotely accessed using HART,

  • the name really does say it all: “highway (or network) sensor (also called a transducer)

  • that has an address so that it can be accessed remotely and directly”.

  • The remote capability of HART sensors is very useful and powerful.

  • In this diagram, we see a HART sensor connected to a PLC analog input card.

  • We can access data in the sensor remotely using the HART communication protocol

  • from the PLC programming software.

  • That means we do not have 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 the HART data is accessible.

  • Data from a HART sensor must be requested by the master node,

  • which controls all conversations on the loop.

  • The master node is typically the DCS or PLC analog input card that the sensor is wired to.

  • Each message from the master includes the request type,

  • such assend measurement value”,

  • the node number of the sensor the message is intended for,

  • and any data that needs to be transmitted to the sensor,

  • like a new value for the upper range limit.

  • By using a hand-held programming and configuration device,

  • often called a “HART communicator”,

  • the sensor data can be accessed wherever the opportunity exists

  • to connect the hand-held device in parallel to the loop wires.

  • This can be in a junction box, marshaling panel, or at the sensor itself.

  • So if a sensor is in an inaccessible or hazardous area,

  • configuration or maintenance of the sensor can be done from a safe, remote location.

  • Networking HART devices, in most control systems applications, is not practiced.

  • Because of its limited speed and its cumbersome multi-drop network topology,

  • we generally assign only one node, or sensor, to each HART signal loop.

  • Fortunately, HART allows for multiple master nodes,

  • so that the control system AND a hand-held communicator can both be connected to the loop

  • and can communicate with the device at the same time.

  • With HART, the analog 4-20 milliamp signal AND the digital HART protocol

  • are both available to the control system and instrument technician.

  • If a sensor loop is upgraded from 'analog only' to 'analog plus HART',

  • the control system programming and configuration for the measurement value can stay the same.

  • You can imagine that superimposing an AC signal

  • on top of a DC signal might interfere with the 4-20 milliamp signal.

  • But this is not the case.

  • The AC HART sine wave oscillates at either 1200 Hertz

  • for a 1 value or at 2200 Hertz for a 0 value.

  • The amplitude of the AC sine wave remains the same, and for every oscillation,

  • the amplitude of the first half of each sine wave above the DC current curve

  • exactly equals the amplitude of the second half of each sine wave below the DC current.

  • The net effect of the sine wave is then zero.

  • So the analog value of the sensor measurement data is not affected by the HART signal,

  • just as a telephone voice conversation is not effected by the caller ID signal

  • using the same Bell 202 protocol.

  • Every HART device is capable of sending and receiving 35-50 different information items,

  • including the process variable

  • (that is, the same measurement value as provided by the 4-20 milliamp analog signal);

  • device status; diagnostic alerts, likesensor value under range”;

  • basic configuration parameters, like upper and lower range limits;

  • and the tagname of the device.

  • HART is a perfect choice for multivariable instruments,

  • like mass flow meters, where mass flow, volumetric flow,

  • temperature and density can all be communicated to the control system over a single cable.

  • The HART protocol is governed by a vendor-independent association,

  • The HART Communication Foundation,

  • so HART sensors from any manufacturer can be interchanged with those of other manufactures.

  • This makes implementation, maintenance, and troubleshooting very easy.

  • Also, HART is used extensively for final control devices,

  • such as control valve positioners,

  • with the same benefits and diagnostic capabilities.

  • Even though the HART standard requires manufactures

  • to provide a minimum number of specific data items with every HART sensor,

  • vendors can also extend the data set to include vendor-specific items,

  • like sensor model numbers or firmware versions or advanced diagnostic counters.

  • In order for the control system to recognize the type and values for these custom data,

  • a special description file, called a Data Description (DD) file is required.

  • This file is loaded on the DCS or PLC configuration station or downloaded

  • to the hand-held communicator and becomes directly associated to the sensor.

  • This file simply allow the data stream from the sensor to be correctly parsed,

  • or interpreted, and allow the technician or engineer to make the correct requests for sensor data.

  • To review, HART is a digital data communication protocol

  • that is layered on top of a traditional analog 4 – 20 milliamp signal

  • which provides advanced data retrieval and configuration options

  • to be executed remotely