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Basics of Smart Transmitters

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Smart Transmitters are advancement over conventional analog transmitters. They contain microprocessors as an integral unit within the device. These devices have built-in diagnostic ability, greater accuracy (due to digital compensation of sensor nonlinearities), and the ability to communicate digitally with host devices for reporting of various process parameters.

The most common class of smart transmitters incorporates the HART protocol. HART, an acronym for Highway Addressable Remote Transducer, is an industry standard that defines the communications protocol between smart field devices and a control system that employs traditional 4-20 mA signal.

Parts of a Smart Transmitter:
To fully understand the main components of a smart transmitter, a simplified block diagram of the device is shown below:
Fig A Block Diagram of a Smart Transmitter

The above block diagram is further simplified to give the one below:
Fig B Simplified Block Diagram of a Smart Transmitter

As shown above in fig A, the smart transmitter consists of the following basic parts:

(a) Process Sensor
(b) An Analog to Digital Converter(ADC)
(c) A Microprocessor
(d) A Digital to Analog Converter(DAC)

These basic parts can be organized into three basic sections as shown in fig B:
(a) Input Section
(b) Conversion Section
(c) Output Section

Input Section:
The input section comprises the process sensor or transducer and the Analog to Digital Converter (ADC). The sensor measures the process variable of interest (be it pressure, temperature, flow etc) which is then converted into a proportional electrical signal. The measured electrical signal is then transformed to a digital count by the Analog to Digital Converter (ADC). This digital count, representative of the process variable (PV), is then fed into the conversion section which contains the microprocessor.

However, the microprocessor must rely upon some form of equation or algorithm to relate the raw count value of the electrical measurement to the actual process variable (PV) of interest such as temperature, pressure, or flow. The principal form of this algorithm is usually established by the manufacturers of the smart transmitters, but most HART transmitters include commands to perform field adjustments. This type of adjustment is often referred to as a sensor trim. The output of the input section is a digital representation of the process variable (PV).
When you read the process variable using a hand held field communicator, this is the value that you see

Conversion Section:
This section contains a microprocessor whose basic function is a mathematical conversion from the process variable to the equivalent mA representation of the process. closely connected to the microprocessor is the memory where the setup , configuration and diagnostic data of the transmitter are stored. The range values of the transmitter (related to the zero and span values) are used in conjunction with a transfer function to calculate this mA value. A linear transfer function is the most common, although pressure transmitters, may have a square root option. Still many other forms of transfer functions can be used with the processors or can be user defined. The output of the conversion section (PVAO) is a digital representation of the desired transmitter output. When you read the loop current using a hand held field communicator, this is the value that you see. Note that many HART transmitters support a command which puts the instrument into a fixed output test mode. This overrides the normal output of the conversion section and replaces it with a specified output value.

Output Section:
In this section, the calculated mA value representing the process variable is fed into a Digital to Analog Converter, where the mA value is converted into the actual analog 4 – 20mA electrical signal. Note once again that the microprocessor must rely on some internal calibration factors to get the correct value of this output. Adjusting these calibration factors is often referred to as a current loop trim or 4-20 mA trim.

As can be seen from the above discussion, the only similarity between the conventional analog transmitter and a smart transmitter is the process sensor that measures and converts the physical process variable into a corresponding electrical signal. Shown below is a simplified block diagram of a conventional analog transmitter:
Fig C Block Diagram of Analog Transmitter
Instead of a purely mechanical or electrical path between the input and the resulting 4-20 mA output signal as obtain in conventional analog transmitters, a smart transmitter using the HART protocol has a microprocessor that manipulates the process data.

Based on the analysis above, it should now be clear that the calibration procedure for a conventional analog transmitter is very different from that of a smart HART transmitter. While Zero and Span calibration is sufficient to make the analog transmitter perform within the manufacturer’s stated specifications; that of smart transmitters involve the calibration of either the input or output sections or both depending on the application. Zero and Span calibration for a smart transmitter is insufficient to make the device work within the stated performance accuracy documented by the manufacturers.