Brief Introduction to Smart HART Protocol

HART is a type of bus protocol. Therefore, it plays a significant role in more and more complex intelligent applications. Here we will describe an improved application in this aspect. First, we understand the principles of smart transmitters. So what is the application of the HART protocol here?

1. Principles of HART smart Transmitters

1.1 about the HART Protocol

According to the model standard of "ISO/OSI", HART Protocol specifies the physical signal mode and transmission medium of HART communication, that is to say, a Sine Modulated wave with a amplitude of 20mA is superimposed on a 4-5mA analog signal. Hz represents the logic "1", and Hz represents the logic "0 ", because the average value of the superimposed sine signal is 0, the digital communication signal does not interfere with the analog signal of 4-20mA. Therefore, the main advantage of the HART protocol is that it is compatible with the digital signal communication and analog signal transmission ｡

Composition of 1.2 Transmitters

Transmitters are composed of sensors and smart transmission components. For different transmitters, their sensors are different, the output analog signal is also different, but the power consumption of the sensor must not exceed 1mA. The key of intelligent transmission is to solve the problem of low power consumption, A sine wave of 0.5 mA is superimposed on 4mA current, therefore, the hardware circuit of the entire Smart Transmitter must be able to work properly at 5mA of the operating current. The sensor part will consume about 1mA of the current. Therefore, the smart transmission part consumes less than 5mA of the power ｡ at this low current, the CPU, A/D, D/A, and MODEM can work normally. Therefore, the core of the Smart Transmitter is the low power technology, at the same time, there are certain requirements on power supplies and cables during remote digital communication ｡

1.3 possible faults

The principle of smart transmitters Based on HART protocol, sometimes there may be Carrier Interference, the capacitance of the cable is too large, the voltage to the header is lower than 11V, the circuit resistance is too large, and other reasons cause the Smart Transmitter operation failure. Below are several rare faults in the application to explain. ｡

2. Analysis of Several rare faults and their causes in applications

The 2.1 cable is too long and smart transmitters do not communicate at the site ｡

ST3000 single-flange Liquid Level Transmitter, which is used for non-communication. The field inspection voltage is 22.3 V, which is within the normal range. It is withdrawn to the workshop for verification, and the communication and instrumentation work normally (with a 268 communicator) the reason is that the cable capacitance is greater than the maximum capacitance allowed by the HART communication device. The HART communication frequency is high (1-2 KHz) the Network T = RC must not exceed 0.65 μs. Therefore, the maximum capacitance of the HART communication device and network limits the maximum communication distance:

Max cable length M = (16316-cmaster)/Ccable

160000-the constant calculated based on the time constants allowed by the HART communication network, reflecting the maximum capacitance allowed by the communication network: PF

Cmaster-capacitance of the control system or HART filter: PF

Ccable-distribution capacitance of cables: PF

Therefore, the Network Time Constant T = RC = (0.025*650) * (400*650)/1000000 = 4.2 μs> 0.65

It can be seen that due to the cable, the Network Time Constant is too large, resulting in serious distortion of communication, and communication fails, but it does not affect the analog signal. Instead, use the 2.5sq cross-linked cable (Cmster is 90PF/M) ｡

2.2 inverter Carrier Interference

(1) Smart transmitters do not communicate at the site, or even the transmitters mistakenly indicate 3051 of the differential pressure of 0 to indicate 48%, and check that the field voltage is normal. Although the 0.1 μF capacitor in parallel cannot communicate, the instructions are correct, it can be confirmed that this is a line interference problem. The reason is that about 2 KHz high frequency electromagnetic interference affects the signal of the HART protocol communication device, so that the transmitter produces an error signal. The carrier frequency of the inverter depends on its model and capacity, generally 2.0-16 KHz (according to the load configuration). HITACHIJ300-110HF95 (VT) and 150HF5 (VT) carrier frequency mostly configured as 2 KHz is very close to the HART communication frequency, the clutter near the "0" signal frequency affects the Smart Transmitter so that it cannot correctly respond to the 275 signal, the current output or other incorrect operation modes are converted incorrectly based on this signal. The transformation measures are adopted to increase the distance between the instrument cable and the power cable ｡

(2) Pressure Transmitter E + H, PMC-133 type, equipped with inverter, after use, the indication is low, inverter stop, back to normal

(3) The ST3000 often experienced significant fluctuations in output signals. After careful observation, it is found that this phenomenon is similar to the confirmation after adjusting the transmitter parameters using the SFC transmitter, the reason is that the strong electromagnetic field generated by the inverter and the peak pulse interference caused by frequent start and stop of the motor cause the Smart Transmitter to automatically reset, try to solve the problem of 100 V and 4.7 μF capacitor on the signal end of the transmitter ｡

(4) The control SYSTEM990 ultrasonic flowmeter in the United States suddenly indicated an exception: the indication value jumped and the output current also changed, or return to zero after a period of time. The reason for the analysis is that the inverter interference. Solution: Change the cable into a shielded cable, and ground the shielded layer with the shielded layer of the probe cable, when the cross-capacitor between the signal line and the ground is invalid, try to ground the negative end of the probe with the screen, and the meter will return to normal ｡

(5) Canada's wonderful and labor-saving Mini Ranger Plus split ultrasonic level gauge, used as the level indicator, the output signal of the converter is converted into the input of the inverter, the indication value of the instrument is fixed on a certain value, and the instrument of the inverter is normally indicated. The cause is that the output isolation characteristic of the instrument is poor ｡

The circuit resistance is affected. The instrument indicator is normal when the small signal is measured. As the measurement signal increases, the output of the instrument changes and the instrument is recalled to the workshop for verification, the circuit resistance is found to be too large. The following solution can be used to solve the problem. The internal resistance is small and the signal line is used to reduce the internal resistance of the circuit. If there is economic or difficult, you can change the DCS power supply to the external power supply, however, in order to ensure circuit safety and SFC communication, a resistance must be connected in a loop. The sum of the value and the wire resistance is no less than 250 ohm, which can improve the supply voltage of the closed loop circuit, ensure that the header voltage is greater than 11V. ｡