Đề tài Chuẩn truyền tin HART- Highway Addressable Remote Tranducer

Chuẩn truyền tin HART- Highway Addressable Remote Tranducer ---------------------------------------------------------------------------------------------------- ----------------------------------------------------------------------------------------------- Bộ môn: Tự động hóa – Khoa Điện – Trường ĐHBK Đà Nẵng 1 TRƯỜNG ĐẠI HỌC BÁCH KHOA KHOA ĐIỆN BỘ MÔN : TỰ ĐỘNG HÓA CHUẨN TRUYỀN TIN HART TRONG ĐO LƯỜNG VÀ ĐIỀU KHIỂN TỰ ĐỘNG MẠNG CÔNG NGHIỆP Version 1.0 – Lưu hành nội bộ ĐÀ NẴNG 2007 Chuẩn truyền tin HART- Highway Addressable Remote Tranducer ---------------------------------------------------------------------------------------------------- ----------------------------------------------------------------------------------------------- Bộ môn: Tự động hóa – Khoa Điện – Trường ĐHBK Đà Nẵng 2 GIỚI THIỆU CHUNG HART là một giao thức truyền thông được giới thiệu vào năm 1980, những ứng dụng của HART được phát triển bởi tổ chức HCF. HART cho phép thiết bị làm việc trong môi trường công nghiệp có nhiễu cao và tương thích với các chuẩn 4-20mA. Nó được kiến trúc dựa trên sự xếp chồng tín hiệu số trên nền tín hiệu tương tự 4 – 20mA, nghĩa là nó có dạng tín hiệu lai, cộng tín hiệu một chiều với tín hiệu đã được mã hóa. Do đó các thiết bị có thể nhanh chóng định dạng và xác định đúng thông số cần dùng khi có nhiều thiết bị nối vào chung mạng công nghiệp. Cũng như các chuẩn công nghiệp đã có trong lịch sử, để người sử dụng và các môi trường tiếp nhận không bị ảnh hưởng về tâm lí vật lí, HART cũng cho phép nối Master-Slave dạng PPI và MPI. Các liên kết PPI cho phép kéo dài đường truyền đến 3000m và MPI là 1500m, tối đa của MPI lến đến 15 thiết bị. Tuy nhiên HART có nhược điểm là tốc độ truyền thấp, hiện nay đến 4800 baud. Ngược lại, HART lại cho phép cả thiết bị tương tự và số có thể làm việc trên cùng một mạng. Sau đây sẽ trình bày cụ thể hơn những đặc điểm cơ bản về HART. Tài liệu sau đây vừa trình bày những kiến thức về HART, đồng thời cũng đưa ra những mạch điện cụ thể sử dụng cho các chuẩn đo lượng hiện đại hiện nay. Sinh viên có thể sử dụng các phần kiến thức đó để phục vụ cho quá trình làm bài tập, đồ án môn học, tốt nghiệp và các công tác khác sau này. Chuẩn truyền tin HART- Highway Addressable Remote Tranducer ---------------------------------------------------------------------------------------------------- ----------------------------------------------------------------------------------------------- Bộ môn: Tự động hóa – Khoa Điện – Trường ĐHBK Đà Nẵng 3 About HART -- Part 1 Part1: Preliminaries Introduction HART (Highway Addressable Remote Transducer) provides digital communication to microprocessor-based (smart) analog process control instruments. Originally intended to allow convenient calibration, range adjustment, damping adjustment, etc. of analog process transmitters; it was the first bi-directional digital communication scheme for process transmitters that didn't disturb the analog signal. The process could be left running during communication. HART has since been extended to process receivers, and is sometimes also used in data acquisition and control. HART Specifications continue to be updated to broaden the range of HART applications. And a recent HART development, the Device Description Language (DDL), provides a universal software interface to new and existing devices. HART was developed in the early 1980s by Rosemount Inc. [1.4]. Later, Rosemount made it an open standard. Since then it has been organized and promoted by the HART Communication Foundation [1.5], which boasts some 114 member companies. As the de-facto standard for data communication in smart analog field instruments, HART is found in applications ranging from oil pipelines to pulp and paper mills to public utilities. As of June 1998 an estimated 5 million nodes were installed [1.1]. Among the many HART products now available are Analog Process Transmitters Digital-only Process Transmitters Multi-variable Process Transmitters Process Receivers (Valves) Local (Field) Controllers HART-to-Analog Converters Modems, Interfaces, and Gateways HART-compatible Intrinsic Safety Barriers HART-compatible Isolators Calibrators Software Packages New HART products continue to be announced, despite encroachment by Foundation Fieldbus and other faster networks. Analog transmitters continue to flourish [1.2], which suggests that HART will, also. A recent study [1.3] predicts that, of all smart pressure transmitters sold in the next few years, sales of HART units will increase at 17.5% per year. Analog Services, Inc., a leader in HART development, is pleased to present this on-line book about HART. We have tried to present many topics that do not appear in the HART Standards or App Notes. This is still a work in progress. If there are other topics that you would like to see Chuẩn truyền tin HART- Highway Addressable Remote Tranducer ---------------------------------------------------------------------------------------------------- ----------------------------------------------------------------------------------------------- Bộ môn: Tự động hóa – Khoa Điện – Trường ĐHBK Đà Nẵng 4 covered or corrections to what we have presented, please send us an e-mail at stevea@analogservices.com. Overview: HART and The Conventional Process Loop HART is sometimes best understood by looking at how it evolved from a conventional process loop. Figure 1.1 is a simplified diagram of the familiar analog current loop. The process transmitter signals by varying the amount of current flowing through itself. The controller detects this current variation by measuring the voltage across the current sense resistor. The loop current varies from 4 to 20 mA at frequencies usually under 10 Hz. Figure 1.1 -- Conventional Process Loop Figure 1.2 is the same thing with HART added. Both ends of the loop now include a modem and a "receive amplifier." The receive amplifier has a relatively high input impedance so that it doesn't load the current loop. The process transmitter also has an AC-coupled current source, and the controller an AC-coupled voltage source. The switch in series with the voltage source (Xmit Volt Source) in the HART controller is normally open. In the HART Controller the added components can be connected either across the current loop conductors, as shown, or across the current sense resistor. From an AC standpoint, the result is the same, since the Pwr Supply is effectively a short circuit. Notice that all of the added components are AC-coupled, so that they do not affect the analog signal. The receive amplifier is often considered part of the modem and would usually not be shown separately. We did it this way to indicate how (across which nodes) the receive signal voltage is derived. In either the Controller or the Transmitter, the receive signal voltage is just the AC voltage across the current loop conductors. Chuẩn truyền tin HART- Highway Addressable Remote Tranducer ---------------------------------------------------------------------------------------------------- ----------------------------------------------------------------------------------------------- Bộ môn: Tự động hóa – Khoa Điện – Trường ĐHBK Đà Nẵng 5 Figure 1.2 -- Process Loop With HART Added To send a HART message, the process transmitter turns ON its AC-coupled current source. This superimposes a high-frequency carrier current of about 1 mA p-p onto the normal transmitter output current. The current sense resistor at the controller converts this variation into a voltage that appears across the two loop conductors. The voltage is sensed by the controller's receive amplifier and fed to the controller's demodulator (in block labeled "modem"). In practice the two current sources in the HART process transmitter are usually implemented as a single current regulator; and the analog and digital (HART) signals are combined ahead of the regulator. To send a HART message in the other direction (to the process transmitter), the HART Controller closes its transmit switch. This effectively connects the "Xmit Volt Source" across the current loop conductors, superimposing a voltage of about 500 mV p-p across the loop conductors. This is seen at the process transmitter terminals and is sent to its receive amplifier and demodulator. Figure 1.2 implies that a Master transmits as voltage source, while a Slave transmits as a current source. This is historically true. It is also historically true that the lowest impedance in the network -- the one that dominates the current-to-voltage conversion -- was the current sense resistor. Now, with some restrictions, either device can have either a low or high impedance. And the current sense resistor doesn't necessarily dominate. Regardless of which device is sending the HART message, the voltage across the loop conductors will look something like that of figure 1.3; with a tiny burst of carrier voltage superimposed on a relatively large DC voltage. The superimposed carrier voltage will have a range of values at the receiving device, depending on the size of the current sense resistor, the amount of capacitive loading, and losses caused by other loop elements. Of course the DC Chuẩn truyền tin HART- Highway Addressable Remote Tranducer ---------------------------------------------------------------------------------------------------- ----------------------------------------------------------------------------------------------- Bộ môn: Tự động hóa – Khoa Điện – Trường ĐHBK Đà Nẵng 6 voltage will also vary; depending on controller supply voltage, loop resistance, where in the loop the measurement is made, etc. Figure 1.3 -- HART Carrier Burst HART communication is FSK (frequency-shift-keying), with a frequency of 1200 Hz representing a binary one and a frequency of 2200 Hz representing a binary zero. These frequencies are well above the analog signaling frequency range of 0 to 10 Hz, so that the HART and analog signals are separated in frequency and ideally do not interfere with each other. The HART signal is typically isolated with a high-pass filter having a cut-off frequency in the range of 400 Hz to 800 Hz. The analog signal is similarly isolated with a low-pass filter. This is illustrated in figure 1.4. Chuẩn truyền tin HART- Highway Addressable Remote Tranducer ---------------------------------------------------------------------------------------------------- ----------------------------------------------------------------------------------------------- Bộ môn: Tự động hóa – Khoa Điện – Trường ĐHBK Đà Nẵng 7 Figure 1.4 -- Separation of Analog and HART (Digital) Signals The separation in frequency between HART and analog signaling means that they can coexist on the same current loop. This feature is essential for HART to augment traditional analog signaling. Further information on the frequencies involved in HART transmission is given in the section entitled HART Signal Power Spectral Density. For a description of FSK and other forms of data/digital communication, see [3.5]. For convenience, Figure 1.4 shows the Analog and HART Signals to be the same level. Generally, this isn't true. The Analog Signal can vary from 4 to 20 mA or 16 mA p-p (unusual, but possible), which is vastly larger than the HART Signal. This, in turn, can lead to some difficulties in separating them. HART is intended to retrofit to existing applications and wiring. This means that there must be 2-wire HART devices. It also means that devices must be capable of being intrinsically safe. These requirements imply relatively low power and the ability to transmit through intrinsic safety barriers. This is accomplished through a relatively low data rate, low signal amplitude, and superposition of the HART and analog signals. Power consumption is further reduced through the half-duplex nature of HART. That is, a device does not simultaneously transmit and receive. Therefore, some receive circuits can be shut down during transmit and vice-versa. Intrinsic Safety and retrofitting to existing applications and wiring also explain why HART was developed at all, despite other advanced communication systems and techniques that existed at the time. None of them would have met the low power requirements needed in a 2-wire 4-20 mA device. Further information on intrinsically safe HART devices is given in the section entitled HART and Intrinsic Safety . In HART literature the process transmitter is called a Field Instrument or HART Slave Device. (These terms will be used interchangeably throughout our presentation.) And the current loop is a network. The controller is a HART Master. A hand-held communicator can Chuẩn truyền tin HART- Highway Addressable Remote Tranducer ---------------------------------------------------------------------------------------------------- ----------------------------------------------------------------------------------------------- Bộ môn: Tự động hóa – Khoa Điện – Trường ĐHBK Đà Nẵng 8 also be placed across the network temporarily. It is used in place of, or in addition to, the fixed controller-based HART Master. When both types of Masters are present, the controller is the Primary Master and the hand-held unit is the Secondary Master. (Note: It becomes difficult to describe process devices in a data communication setting, because the terms transmitter and receiver have more than one meaning. For example, a process transmitter both receives and transmits data bits. We hope we've avoided confusion by providing sufficient context whenever these words are used.) HART now includes process receivers. These are also called Field Instruments or HART Slaves and are discussed in the section entitled Process Receiver. Overview: Signaling The HART signal path from the the processor in a sending device to the processor in a receiving device is shown in figure 1.5. Amplifiers, filters, etc. have been omitted for simplicity. At this level the diagram is the same, regardless of whether a Master or Slave is transmitting. Notice that, if the signal starts out as a current, the "Network" converts it to a voltage. But if it starts out a voltage it stays a voltage. Figure 1.5 -- HART Signal Path The transmitting device begins by turning ON its carrier and loading the first byte to be transmitted into its UART. It waits for the byte to be transmitted and then loads the next one. This is repeated until all the bytes of the message are exhausted. The transmitter then waits for the last byte to be serialized and finally turns off its carrier. With minor exceptions, the transmitting device does not allow a gap to occur in the serial stream. Chuẩn truyền tin HART- Highway Addressable Remote Tranducer ---------------------------------------------------------------------------------------------------- ----------------------------------------------------------------------------------------------- Bộ môn: Tự động hóa – Khoa Điện – Trường ĐHBK Đà Nẵng 9 The UART converts each transmitted byte into an 11 bit serial character, as in figure 1.6. The original byte becomes the part labeled "Data Byte (8 bits)". The start and stop bits are used for synchronization. The parity bit is part of the HART error detection. These 3 added bits contribute to "overhead" in HART communication. Figure 1.6 -- HART Character Structure The serial character stream is applied to the Modulator of the sending modem. The Modulator operates such that a logic 1 applied to the input produces a 1200 Hz periodic signal at the Modulator output. A logic 0 produces 2200 Hz. The type of modulation used is called Continuous Phase Frequency Shift Keying (CPFSK). "Continuous Phase" means that there is no discontinuity in the Modulator output when the frequency changes. A magnified view of what happens is illustrated in figure 1.7 for the stop bit to start bit transition. When the UART output (modulator input) switches from logic 1 to logic 0, the frequency changes from 1200 Hz to 2200 Hz with just a change in slope of the transmitted waveform. A moment's thought reveals that the phase doesn't change through this transition. Given the chosen shift frequencies and the bit rate, a transition can occur at any phase. Chuẩn truyền tin HART- Highway Addressable Remote Tranducer ---------------------------------------------------------------------------------------------------- ----------------------------------------------------------------------------------------------- Bộ môn: Tự động hóa – Khoa Điện – Trường ĐHBK Đà Nẵng 10 Figure 1.7 -- Illustration of Continuous Phase FSK A mathematical description of continuous phase FSK is given in the section entitled Equation Describes CPFSK. The form of modulation used in HART is the same as that used in the "forward channel" of Bell-202. However, there are enough differences between HART and Bell-202 that several modems have been designed specifically for HART. Further information on Bell-202 is given in the section entitled What's In a Bell-202 Standard? At the receiving end, the demodulator section of a modem converts FSK back into a serial bit stream at 1200 bps. Each 11-bit character is converted back into an 8-bit byte and parity is checked. The receiving processor reads the incoming UART bytes and checks parity for each one until there are no more or until parsing of the data stream indicates that this is the last byte of the message. The receiving processor accepts the incoming message only if it's amplitude is high enough to cause carrier detect to be asserted. In some cases the receiving processor will have to test an I/O line to make this determination. In others the carrier detect signal gates the receive data so that nothing (no transitions) reaches the receiving UART unless carrier detect is asserted. Overview: HART Process Transmitter Block Diagram A block diagram of a typical HART Process Transmitter is given in figure 1.8. Chuẩn truyền tin HART- Highway Addressable Remote Tranducer ---------------------------------------------------------------------------------------------------- ----------------------------------------------------------------------------------------------- Bộ môn: Tự động hóa – Khoa Điện – Trường ĐHBK Đà Nẵng 11 Figure 1.8 -- Typical HART Process Transmitter Block Diagram The "network interface" in this case is the current regulator. The current regulator implements the two current sources shown in the "process transmitter" of figure 1.2. The block labeled "modem", and possibly the block labeled "EEPROM", are about the only parts that would not otherwise be present in a conventional analog transmitter. The EEPROM is necessary in a HART transmitter to store fundamental HART parameters. The UART, used to convert between serial and parallel data, is often built into the micro-controller and does not have to be added as a separate item. The diagram illustrates part of the appeal of HART: its simplicity and the relative ease with which HART field instruments can be designed. HART is essentially an add-on to existing analog communication circuitry. The added hardware often consists of only one extra integrated circuit of any significance, plus a few passive components. In smart field instruments the ROM and EEPROM to hold HART software and HART parameters will usually already exist. Overview: Building Networks The type of network thus far described, with a single Field Instrument that does both HART and analog signaling, is probably the most common type of