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Several problems affecting HART protocol intelligent pressure transmitter (differential pressure) communication
Jan 10, 2019

Several problems affecting HART protocol intelligent pressure transmitter (differential pressure) communication

Abstract]: Analyze several elements of HART smart transmitter communication, and several cases where smart transmitters do not communicate when used in the field.


[1]HART protocol intelligent pressure transmitter requirements for communication

    HART protocol intelligent pressure transmitter communication condition is loop resistance ≥250Ω, working voltage is within its communication requirements. For example, the communication requirement of 3151 pressure transmitter is that the loop resistance is 250Ω, the minimum communication voltage is 18 VDC, the power connection direction correct. If this is still the case, there are the following reasons.

(1) The communication cable is not connected. Check that the cable plug is tight and the terminal screws are loose to ensure good contact.

(2) The cable position is incorrect. The handheld terminal can be connected to the terminal A B or to the CD at both ends of the load resistor R, but it cannot be connected to the A D two points. A ≥250Ω resistor is required between the power supply and the handheld terminal. Its function is to prevent the low-impedance power supply from shunting the communication signal, but the resistance of R cannot be too large to ensure the minimum operating voltage of the transmitter. The terminal reads the information from the transmitter and sends the set configuration parameters to the transmitter memory. Because the low impedance internal resistance of the power supply is very small, it can be regarded as a short circuit. When the AD terminal is connected to the communication cable at both ends, it cannot communicate, and the information and configuration parameters cannot be read, as shown in Figure 1. .

Several problems affecting HART protocol intelligent pressure transmitter (differential pressure) communication

The transmitter handheld terminal can be connected to the terminal A B or to the load resistor R.


If the above two conditions are not met after communication, you can change the transmitter to replace the fault of the communicator, so you can confirm whether it is the fault of the pressure transmitter itself.


[2] Several problems that will affect transmitter communication


(1) The power supply voltage is too low. More than 200 3151B smart transmitters in the Qingdao Guangyuan Development Project are not in communication during the trial production process, the output is wrong, and the meter display flashes, which seriously affects the user's production plan. After searching, the measured power supply voltage is only 15V D C. Because the power supply is not independently supplied to the safety barrier, the safety barrier voltage that supplies power to the transmitter is too low, so that the transmitter cannot communicate and the output is abnormal. After the power supply is replaced, the communication can be performed, and the instrument output is normal.

(2) The cable is too long. A university in Xi'an has ordered a 1151 intelligent remote pressure transmitter for oil project testing. The user reports that the transmitter is not communicating. The measuring terminal voltage is normal, but the power supply cable is very long, 600 m. The reason for the analysis is that the cable is too long and the cable capacitance is greater than the maximum capacitance of the H A R T communication device. The H A RT communication frequency is about 1~2k H z, and the network T=RC (time constant) is required to be no more than 0.65μs. Therefore, the maximum capacitance of the HART communication device and the network limits the maximum distance of communication. HART communication maximum frequency calculation formula:


Maximum cable length m=(160000-Cmaster)/Ccable


Where: 160 000 is calculated according to the time constant allowed by the HART communication network, reflecting the maximum capacitance allowed by the communication network, in PF;

Cmaster is the control system or HART filter capacitor, PF; Ccable is the cable distribution capacitance, PF / m.

The cable length is about 600 m, the cable C cable = 4 0 0 P F/m, and Rcable: 0.025 Ω/m. For the convenience of calculation, Cmaster=0 is assumed.

    The maximum cable length Lm a x=160000/400=400<600m. Actual Cmaster≠0 (Cmaste changed to Cmaster)=The maximum allowable cable length is <400m.

Network time constant T = R C = (0 .0 2 5 × 6 0 0) × (400 × 600) / 1000000 = 3.6 μs > 0.65 μs

    The reason for the cable is that the network time constant is too large, resulting in serious distortion of the communication waveform, and communication is impossible. Switch to a 2.5sq cross-linked cable (Ccable is 90PF/m) (Cmaster is changed to Ccable).

Several problems affecting HART protocol intelligent pressure transmitter (differential pressure) communication

(3) Electromagnetic interference

    Zhongshan Chemical Co., Ltd. installed 6 measuring instruments, including 5 3051 differential pressure transmitters. One HART protocol vortex flowmeter, only one differential pressure transmitter can work normally when it is put into operation, and the rest do not communicate. When the flow rate is 0, one differential pressure transmitter indicates 48%, one is 0~25% cycle change, the other shows fault, the field power supply voltage is normal, and the measurement output and communication in the laboratory are normal. The original power supply cable was opened at the site and the temporary power supply indication communication was normal. It can be confirmed that the meter does not communicate and misinformation because of line interference. After confirming that the interference frequency is 2 kHz, it affects the communication frequency of 1200 Hz and 2200 Hz of the H A RT communicator. The frequency is generated because the motor/pump is used to adapt to the 50Hz power supply. The frequency is set to 2k Hz and the communication frequency of H ART is very similar, so that it can not respond to the communicator correctly, and even convert according to this signal. For current output or other erroneous operation. Solve the problem by increasing the distance between the instrument cable and the power cable tray.


    Electromagnetic interference, usually after eliminating the power supply voltage, loop resistance, wiring is correct, the line is smooth, there is a problem of non-communication, and the problem of electromagnetic interference needs to be considered. Usually the problem at the site is difficult to reproduce in the laboratory, despite the pressure transmission. Through various electromagnetic compatibility tests, various interference problems may occur in the field. If these disturbances are not handled well during the measurement process, the measurement results will be distorted, and even if the instrument or control system is completely inoperable. In order to avoid the occurrence of electromagnetic interference, the signal line should be twisted, shielded, grounded, filtered, etc., the distance between the signal line of the meter and the power cable is increased, the wire is shortened, parallel wiring is avoided as much as possible, and interference to the transformer is avoided. The source member is magnetically shielded or the like.


[3] Conclusion

    The communication function of the smart transmitter is very important for the user. Because the pressure transmitter can work even if it is not communicating, the smart meter will have the same function as the analog meter, which can not reflect the advantages of the smart meter, without the function of remotely changing the configuration parameters, remotely viewing the PV value, etc. It brings great inconvenience to users.