HART smart transmitters power consumption requirements and communication systems (1)

The HART Smart Transmitter is an important application of the HART protocol in smart devices. This article will take this as an example to explain the principles of this part. First, we need to understand the concept of the Smart Transmitter and the basic knowledge of the HART protocol tank. The field bus technology is one of the hot spots in the Current Automatic Detection Technology. From the formation of the field bus technology, it is the inevitable result of the development of control, computer, communication, network and other technologies; intelligent Instruments lay the foundation for the emergence and application of the field bus. After Honeywell launched the intelligent instruments-Smar transmitter in 1983, manufacturers all over the world have successively launched various distinctive intelligent instruments. To solve the issue of sharing open resources, users and manufacturers have strongly demanded a unified standard to promote the formation of fieldbus technology ｡

Currently, several influential fieldbus technologies include: Foundation Fieldbus, LonWorks, Profibus, CAN, and HART, except HART, all are full-digital fieldbus protocols. full digitalization means that traditional analog signal transmission methods will be abolished, and each field device is required to have intelligent and digital communication capabilities, allows the operator or other equipment (sensors, actuators, etc.) to send instructions to the site (such as set value, range, alarm value, etc ), at the same time, it can also obtain all aspects of the field equipment in real time (such as measured values, environmental parameters, equipment operation status and equipment calibration, self-diagnosis, alarm information, fault data, etc )｡

In addition, the original Control Operations completed by the main controller are also distributed to various field equipment, greatly improving the reliability and flexibility of the system. The key of the Field Bus Technology lies in the openness of the system, the emphasis on standards consensus and compliance breaks the independent standards of traditional manufacturers and ensures that products from different manufacturers can be integrated into the same fieldbus system, you can also share resources with other systems through the gateway ｡

At present, on the one hand, the fieldbus standard is in the stage of perfection and development, on the other hand, the traditional ~ 20mA of analog devices are still widely used in various fields of industrial control information. Therefore, it is unrealistic to achieve full digitalization right away, the HART Protocol came into being. The HART uses the frequency shift keying technology. It is based on the Bell202 communication standard, in 4 ~ 20mA different frequency signals (2200Hz indicates "0", 1200Hz indicates "1") are superimposed on the analog signal to transmit digital signals. The data transmission rate of the HART protocol is percentile BPS (bit/second )｡

HART fieldbus (HF) system adopts master-slave mode: host is a IBM-PC machine; the slave is one or more HF smart transmitters that comply with the HART protocol. When the slave has only one HF Smart Transmitter, that is, the Smart Transmitter works in the dot-dot mode, you can continue to use the traditional 4 ~ 20mA signals are transmitted in analog mode, while measurement, adjustment, and test data are transmitted in digital mode. When the slave machine uses multiple HF smart transmitters, that is, when the smart transmitters work in the multi-station mode, 4 ~ The 20mA signal is voided, and the working current of each transmitter is about 4mA. All measurement, adjustment, and test data are transmitted in digital mode. Because each HF transmitter has a unique ID, therefore, the host can operate on each transmitter. HART provides the device Description Language (DDL) to ensure interoperability. It should be noted that HART is considered a de facto industrial standard, however, it is not a combination of analog and digital bus. It can only be said to be a prototype of fieldbus and is a transitional protocol ~ 20mA the analog signal standard will exist for a long time in the future, therefore, studying the HART protocol is still of great significance. This article discusses the technical problems of hardware implementation based on the HART Smart Transmitter. First, we need to solve the problem of micro-power consumption, the second is to discuss effective methods to implement the communication function of the HART Smart Transmitter ｡

I. power consumption requirements

To realize the basic functions of smart transmitters, such as linear processing, temperature compensation, automatic zero point and range adjustment, and digital communication, the following key components are required: microcontroller, A/D, D/A, communication chip and sensor. The analog signal of A/D sensor is converted into A digital number and sent to the microcontroller, the single-chip microcomputer uses the D/A converter to output 4 ~ 20mA standard current signal. In digital communication, the microprocessor uses the communication interface chip and coupling circuit ~ The 20mA current loop serves as the medium for transmitting and receiving data. memory (memory ), it is used to store the characteristic parameters, field commands, field statuses, and other operating parameters of the sensor. The center is the Bell 202 standard HART modem, A waveform shaping and band-pass filter are added at the output and end of the signal to enhance the communication reliability ｡

1. compatible power consumption requirements 4 ~ 20mA the current standard, the HART Protocol Smart Transmitter must work in 4 ~ 20mA two-wire circuit. This means that the current available to power the transmitter cannot exceed 4mA, generally, the data frequency signal is converted to ± 0 through the adjustment tube of the V/I conversion circuit. 5mA of the frequency signal, superimposed on the 4 ~ of the two lines ~ 20mA current ring (2200Hz indicates "0", 1200Hz indicates "1"). Due to the pair characteristics, the average value of this signal is 0, therefore, the analog and digital signals do not interfere with each other, but the instantaneous maximum current on the loop I = 5mA, the minimum I = 5mA, if the power supply to the transmitter is too large, more than 5mA, this will lead to digital signal negative half-week distortion. Considering the amount of margin required for adjustment, it is better to require the power supply current of the transmitter not to exceed 4mA ｡

2. there are two main ways to supply power to the transmitter system: one is to directly voltage the input voltage into the required voltage (5 V or 3.3 V) and then power the system, the total current of this method must be controlled within 4mA, the second is the use of DC-DC power supply mode, as long as the efficiency of the DC-DC converter is high enough, it is much looser than the 1st method in power control, however, we also need to consider the negative impact of the linear stability of the converter. Due to the current emergence of micro-power consumption and cost-effective integrated circuits, method 1 has more advantages, in terms of power supply mode, the adaptation to power supply voltage needs to be considered in both methods. Generally, the industrial field is mostly DC 24 V, there are also DC36V power supply. Generally, the transmitter must be in DC12 ~ The 42 V power supply voltage is stable and reliable, and the direct power supply mode is much more flexible than the DC-DC conversion mode ｡