TT-Temperature Transmitter

 Smart Temperature Transmitter Block Diagram (From NAIT Instruction sheet)

 Circuit Description

            Reverse Polarity Protection Diode

                  This is an internal diode which provides circuit protection against improper power connection (reverse polarity protection).

            Transmitter Power Supply

                  The power for the transmitter is taken from the 4 to 20mA loop current, thus, the device is referred to as a loop powered transmitter. The loop power is used by passing the loop current through the zener regulator which generates a DC voltage. The DC voltage is applied to the input of an isolated power supply (switching regulator). The switching regulator converts the zener regulated DC voltage to a free running AC signal which is passed through a transformer. The output (isolated secondary) of the transformer is rectified, then filtered and used as the supplies for the circuitry.

                  The minimum voltage required by the zener regulator to maintain regulation and operation of the transmitter is referred to as the lift-off voltage or minimum operating voltage.

            Temperature Sensor Interface Circuit

                  The interface circuit is an analog amplifier circuit which is designed to measure the signal from the temperature sensor. The circuit consists of a difference amplifier which amplifies small voltage levels (i.e. thermocouple signals). In the case of an RTD being connected to the transmitter, the RTD is excited with a current source causing it to generate a small voltage signal.

Reference Junction Temperature Sensor

                  This sensor is usually a Silicon temperature sensor, providing a very accurate indication of the reference junction temperature.

            Analog to Digital Converter

                  The signals from both the interface circuit and the reference junction temperature sensor are multiplexed inputs to the A/D. The A/D converts the analog signals to digital information. The digital information from the A/D can be in either serial or parallel form.

            Isolation Circuit

                  The digital signal from the A/D converter is isolated from the output circuit by using opto-isolators. The opto-isolators provide isolation by transmitting the signal via light source, a method which works well for digital signals.

            Microprocessor

                  The processor is generally a micro-controller type of processor which will initiate all the operations required by the transmitter.  For example:           

                                                - A/D converter timing

                                                - Calculations for sensor linearization

                                                - Transmitter characterization storage and recall

                                                - Calibration setting adjustment

                                                - D/A conversion timing

                                                - Communications

            Characterization Memory

                  This is where the microprocessor will access specific information (i.e. tables) associated with the temperature sensor being measured, such as a K type thermocouple or a Platinum 100W RTD. All information required for each available sensor is stored in this memory.

            Configuration Memory

                  This memory is generally battery backed RAM which contain information such as:

                                    - Calibration point settings

                                    - Sensor type selected

                                    - Upper and Lower range points

                                    - Output type, either analog(4-20mA) or digital(multidrop)

                                    - Burnout protection selected

                                    - Units selected

                                    - Transmitter I.D.

                                    - Transmitter serial number (etc.)

Digital to Analog Converter

                  The digital information from the microprocessor is converted to a 4 to 20mA analog signal by the Digital to Analog converter.

            Modem

                  This circuit will provide digital communications over the 4 to 20mA analog loop by modulating the loop signal. There are two methods used to do this, one is to superimpose an AC signal on the DC loop current, the other is to actually pulsated the loop current.

            Configuration

                  This is a procedure which allows the alteration of a transmitter's parameters such as: Damping, Lower Range Value, Upper Range Value, Units, etc. Changing of these parameters is accomplished by use of a hand held communicator, or DCS with the appropriate interface software.

            Calibration

                  This procedure involves digitally altering the interpretation of the input signal (sensor trim) and digitally correcting the expected 4 to 20mA output value (4 to 20mA trim).  Note, these adjustments are completely independent and are actually software calibrations.

                  The sensor trim provides a zero and span correction for the input (i.e. Interface and A/D).

                  The 4 to 20mA trim provides zero and span corrections for the output D/A converter.

Performance Specifications of a Typical Smart Temperature Transmitter

Accuracy:                                0.05% of calibrated span (Digital Accuracy)

Linearity:                                 0.1% of calibrated span

Stability:                                  0.1% of reading for 6 months

Temperature Effect:                0.25% of reading (T/C), 0.075% of reading (RTD)

Additional points:

      -     Temperature affects on the internal electronics of the transmitter are substantially reduced (the circuitry is mainly digital).

      -     Non-linearities become insignificant due to sensor characterization.

      -     Sensor specific corrections such as reference junction compensation is done mathematically by the microprocessor.

      -     A large number of sensors can be accommodated using the same transmitter.

      -     The transmitter turn down ability (usable range) is much greater.

      -     The disadvantage of the smart transmitter is the time delay from input change to output change, an analog transmitter responds instantaneously.

PROBLEMS

1.   Explain the difference between configuration/re-ranging and calibration as it applies to a smart temperature transmitter.

2.   Give three advantages and one disadvantage of smart transmitters over analog transmitters.

3.   Sketch the required hookup and explain the proper calibration procedure given the following:

      -     2-wire smart temperature transmitter, range of -17°C to +65°C

      -     Sensor: K type T/C

      -     5½ digit DMM

      -     24 VDC power supply

      -     Millivolt source with 0.01mV increments 

-     ambient temperature 20.0°

Rosemount 3044C Smart Temperature Transmitter

Hart 275 Hand Held Communicator Overview

Typical 275 Communicator Loop Connection

Communications Operation

      The 275 hand held communicator transfers information back and forth over the two wire current loop connection. At the software level, the data is transferred using Rosemount's HART (Highway Addressable Remote Transducer) protocol. The use of a data transfer protocol will guarantee the validity of the information sent or received. At the hardware level, the data transfer follows the Bell 202 standard modem protocol.

                       

      Bell 202 Standard:

                        Maximum Data Transfer Rate            1200 Bits/second

                        Modulation Scheme                Frequency Shift Keying

                        Duplex Capability                               Half Duplex

                        Mark Frequency                                  1200Hz

                        Space Frequency                                 2400Hz

      To accomplish the data transfer, the Bell 202 signal (AC signal) is superimposed on the DC power to the transmitter. Since the DC power supply appears as an AC ground (large capacitance to ground), a minimum of 250W is required between the communicator and the power supply (250W resistance to ground). Without the 250W resistor, the Bell 202 signal is shorted to ground.

 Rosemount 3044C Smart Transmitter Overview

3044C Block Diagram

Personality (Sensor) Module

      The input to the transmitter can be either and RTD, a T/C, a millivolt signal, or a resistance. Any of the four sensor input signals are detected electronically and converted to a digital signal via an A/D converter.

      The temperature is sensed at the sensor connection terminals to correct for temperature effects (reference junction compensation).

      The personality module memory (3044 Program PROM) contains the factory characterization for the available sensors (i.e. tested from -40 to 75°C and input signals over their entire range) as well as the programming for the 3044 type of smart temperature transmitter.

3010 Communications Output (Electronics Module)

      The microprocessor controls the entire operation of the transmitter, including calculations for sensor linearization, reranging, engineering unit conversion, transmitter self-diagnostics, and digital communication.

      The EEPROM holds all configuration and digital trim data for the transmitter. Since the data is stored in EEPROM, the data can be altered by the transmitter’s software, yet information in the memory remains intact even when the power is lost. The digital communication circuitry provides the Bell 202 interface for communicating over the current loop wires.

Transmitter Configuration

      Configuration is performed using the Hart 275 interface. 

      Output units: (6 options are available) °C, °F,°R, Kelvin, mV, or Ohms.

      Damping:  selectable fixed increments from 0.00 to 32.00 seconds.

      Reranging involves setting up the D to A converter to output 4 to 20 mA for the required process LRV and URV.  It is important to understand that this does not effect the transmitters calibration or interpretation of the process input.

      Reranging can be done one of three ways

      1.  Keypad on the model 275:

            The LRV (4 mA point) is simply keyed in on the model 275.  The URV (20 mA point) is also simply keyed in on the model 275.  Note, no calibration is performed, the 4 and 20 mA points are simply picked.

      2.   Sensor input and the model 275:

            A temperature calibrator and the model 275 are connected to the transmitter.  The temperature corresponding to the LRV is produced by the calibrator, and the proper key is pressed on the 275. 

            The LRV is the temperature value as interpreted by the transmitter (i.e. if 50.0°C is produced by the calibrator but the transmitter interpreted this as 49.5°C, the LRV is 49.5°C as far as the transmitter is concerned).  Note, no calibration is performed!

           

            The URV is selected in a similar manner.  Note, when adjusting the LRV, the transmitters span remains the same (i.e. if the LRV was adjusted up 10 %, then the URV will also increase by 10%).

      3.   Sensor input and the integral zero and span buttons:

            This procedure is similar to number 2, the difference being that, instead of using the model 275 communicator, the zero and span buttons are used. 

           

            To activate the zero and span buttons, hold both down simultaneously for five to ten seconds, the buttons stay active for fifteen minutes.           Apply the LRV or URV input and allow to stabilize.  Hold down the zero or span button for at least five seconds, and release.  The value that the transmitter interprets the input to be when the button is depressed is used as the LRV or URV depending on which adjustment is being performed. 

Transmitter Calibration

      Calibration evolves two parts:  Sensor trim, and 4-20 mA trim. 

Sensor trim:

      Note, this procedure will change the transmitters interpretation of the applied input and therefore requires an accurate standard (at least 3 times more accurate than the transmitter).

      Sensor trim has two options: Factory Trim, and User Trim.

      Factory Trim causes the transmitter to revert to the factory-set input calibration.

      User trim is used to match the transmitter to your plant standard.  This is a single point correction in which the value applied, as an input, is entered into the transmitter via the 275 keypad. The transmitter will adjust the internal gain to correct any discrepancy between the value entered via the 275 and the actual input as measured by the transmitter.

4-20 mA trim:

      The D to A circuitry will drift, therefore it will require calibration with time.  A loop test is provided to determine if calibration is necessary.

      Calibration requires a current meter with 1A resolution.  The calibration involves a two point trim (zero and span).

Digital Communications Mode (Multidrop)

      If digital communication is used to transfer the transmitters measured signal then the D to A converter is not required.  This eliminates errors in its function and in the subsequent A to D conversion at the receiving end.

Rosemount Smart Transmitter Offline Menu Tree

Rosemount 3044C Smart Temperature Transmitter Online Menu Tree

PROBLEMS

1.         Sketch the required hookup and explain how to configure the Rosemount 3044C Smart TT given the following:

            -           2-wire transmitter, Model 275 interface

            -           Calibration range 0 to 100°C, type K T/C

            -           4½ digit DMM

            -           24 VDC power supply

            -           Compensated thermocouple calibrator

            -           ambient temperature of 20°C

2.         Sketch the required hookup and explain how to calibrate the Rosemount 3044C Smart TT (Sensor Full trim) given the following:   

            -           2-wire transmitter, Model 275 interface

            -           Calibration range 0 to 100°C, type K T/C

            -           5½ digit DMM

            -           24 VDC power supply

            -           Compensated thermocouple calibrator

            -           ambient temperature of 20°C

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