HART Hook-UP

การต่อวงจรการปรับตั้งค่าตัววัดผลต่างความดัน (Rosemount 3051C Smart Pressure Transmitter) Reference NAIT Sheet and TTC Thailand

OBJECTIVES

1.      เพื่อศึกษาการเชื่อมต่อสัญญาณและวงจรที่ต้องการของตัววัดผลต่างความดัน (Rosemount smart      Transmitters)

2.      แสดงโครงสร้างคำสั่งภายในเครื่องสื่อสารกับทรานสมิตเตอร์ (Hart 275 hand held communicator)

3.      อธิบายหน้าที่แต่ละส่วนในบล๊อกไดอะแกรมของตัววัดผลต่างความดัน (Rosemount 3051C).

4.      ศึกษาคำสั่งการปรับตั้งตาม Menu Tree และแสดงขั้นตอนการปรับตั้งสำหรับ Rosemount 3051C    

         Pressure Transmitter.

5.      เขียนวงจรการต่ออุปกรณ์ และอธิบายขั้นตอนการปรับตั้งที่ถูกต้องของ Rosemount 3051C 

         Pressure Transmitter

Hart 275 Hand Held Communicator Overview

Reference NAIT Sheet and TTC Thailand

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 3051C Smart Pressure Transmitter Block Diagram

Sensor Module

      -     Uses a capacitance cell.  Any P creates a C which is detected electronically and converted to a digital signal.

      -     The cell is connected by glass tubes providing stress relieve from static pressure effects. (Mechanical isolation)

      -     Glass-sealed tubes and insulated cell mounting provides electrical isolation. (Mechanical isolation)

      -     Temperature is sensed at the cell to correct for temperature effects.

      -     Sensor module memory contains the factory characterization for this particular cell.  (i.e. tested from -40 to 185°F and P from the LRL to the URL)

Electronics Module

      -     The microcomputer controls the operation of the transmitter.  In addition, it performs calculations for sensor linearization, reranging, engineering unit conversion, transmitter self-diagnostics, and digital communication.

      -     The EEPROM holds all configuration and digital trim data that can be changed by the transmitters software.  The data in this memory remains intact even when the power is lost.

      -     The digital communication circuitry superimposes a digital signal on top of the 4-20 mA signal.  Note this does not effect the 4-20 mA signal.

Transmitter Configuration

           Configuration is performed using the Model 275 SMART FAMILY interface.  The transmitter must be powered and have a minimum of 250 resistance in the loop for communication when connecting the Model 275

-          Output units: 14 options are available

-          Output type:  linear or square root

-          Damping:  selectable fixed increments from 0.07 to 35.84 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 URL (20 mA point) is also simply keyed in.  Note, no calibration was performed the 4 and 20 mA points were simply picked.

2.      Pressure input and the model 275:

            A pressure source and the Model 275 are connected to the transmitter.  The pressure corresponding to the LRV is inputted and then the proper key is pressed on the 275.  The LRV is now the value that the transmitter interprets the input pressure to be.  Note, again no calibration was performed (i.e. if 50.0 "WC was inputted by the pressure source but the transmitter interpreted this as 49.5"WC the LRV is 49.5"WC as far as the transmitter is concerned). 

            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.      Pressure input and the optional zero and span screws:

            This procedure is similar to number 2 the difference being that instead of using the model 275 the zero and span adjustments are used.  These adjustments are unscrewed until the screw pops up.  The value that the transmitter interprets the input to be when the screw is out 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 pressure standard ( at least 3 times more accurate then the transmitter).

      Sensor trim has two options:  Zero Trim and Full Trim

      Zero trim maintains the slope of the characterization curve but provides an offset correction.

      Full trim has a low and high trim.  Adjustment of the low trim provides an offset correction to the characterization curve.  Adjustment of the high trim provides a gain correction to the characterization curve.

      Note, the original trim values cannot be recalled, therefore sensor trim should only be performed if necessary and you have a suitable standard.

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 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 3051C Smart Pressure Transmitter Online Menu Tree 

Rosemount 3051C Smart Pressure Transmitter Fast Key Sequence

PROBLEMS

1.         Sketch the required hookup and explain how to configure the Rosemount 3051C Smart PT given the following:

            -           2-wire transmitter, Model 275 interface

            -           Calibration range 0 to 100"WC

            -           4½ digit DMM

            -           24 VDC power supply

            -           pneumatic dead weight tester Range 4 to 254"WC  Resolution 1"WC

            -           atmospheric pressure 93.5 kPaa

            -           ambient temperature 20°C

2.         Sketch the required hookup and explain how to calibrate the Rosemount 3051C Smart PT (Sensor Full trim) given the following:   

            -           2-wire transmitter, Model 275 interface

            -           Calibration range 0 to 100"WC

            -           5½ digit DMM

            -           24 VDC power supply

            -           pneumatic dead weight tester Range 4 to 254"WC  Resolution 1"WC

            -           atmospheric pressure 93.5 kPaa

            -           ambient temperature 20°C