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1、附 录 四：外文翻译TT302Field bus Temperature TransmitterOperation The TT302 accepts signals from mV generators such as thermocouples or resistive sensors such as RTDs. The criterion is that the signal is within the range of the input. For mV, the range is -50 to 500mV and for resistance, 0-2000 Ohm. Functio
2、nal Description Hardware The function of each block is described below. Figure 2.1TT302Block DiagramMUX Multiplexer The MUX multiplexes the sensor terminals to the signal conditioning section ensuring that the voltages are measured between the correct terminals. Signal Conditioner Its function is to
3、 apply the correct gain to the input signals to make them suit the A/D -converter. A/D Converter The A/D converts the input signal to a digital format for the CPU. Signal Isolation Its function is to isolate the control and data signal between the input and the CPU. (CPU) Central Processing Unit, RA
4、M, PROM and EEPROM The CPU is the intelligent portion of the transmitter, being responsible for the management and operation of measurement, block execution, self-diagnostics and communication. The program is stored in a PROM. For temporary storage of data there is a RAM. The data in the RAM is lost
5、 if the power is switched off. However there is a nonvolatile EEPROM where data that must be retained is stored. Examples, of such data are trim, calibration, block configuration and identification data. TT302 - Fieldbus Temperature TransmitterCommunication Controller It monitors line activity, modu
6、lates and demodulates communication signals and inserts and deletes start and end delimiters. Power Supply Takes power of the loop-line to power the transmitter circuitry. Power Isolation Just like the signals to and from the input section, the power to the input section must be isolated. Isolation
7、is achieved by converting the DC supply into a high frequency AC supply and galvanically separating it using a transformer. Display Controller Receives data from the CPU informing which segments of the Liquid Crystal Display, should be turned on. Local Adjustment There are two switches that are magn
8、etically activated. They can be activated by the magnetic tool without mechanical or electrical contact. Figure 2.2 - LCD IndicatorTemperature Sensors The TT302, as previously explained, accepts several types of sensors. The TT302 is specially designed for temperature measurement using thermocouples
9、 or Resistive Temperature Detectors (RTDs). Some basic concepts about these sensors are presented below. Thermocouples Thermocouples are constructed with two wires made from different metals or alloys joined at one end, called measuring junction or hot junction. The measuring junction should be plac
10、ed at the point of measurement. The other end of the thermocouple is open and connected to the temperature transmitter. This point is called reference junction or cold junction. For most applications, the Seebeck effect is sufficient to explain thermocouple behavior as following: How the Thermocoupl
11、e Works (Seebeck Effect) When there is a temperature difference along a metal wire, a small electric potential, unique to every alloy, will occur. This phenomenon is called Seebeck effect. When two wires of dissimilar metals are joined at one end, and left open at the other, a temperature difference
12、 between the two ends will result in a voltage since the potentials generated by the dissimilar materials are different and do not cancel each other out. Now, two important things must be noted. First: the voltage generated by the thermocouple is proportional to the difference between the measuring-
13、junction and the cold junction temperatures. Therefore the temperature at the reference junction must be added to the temperature derived from the thermocouple output, in order to find the temperature measured. This is called cold junction compensation, and is done automatically by the TT302, which
14、has a temperature sensor at the sensor terminals for this purpose. Secondly, if the thermocouple wires are not used, all the way to the terminals of the transmitter (e.g., copper wire is used from sensor-head or marshaling box) will form new junctions with additional Seebeck effects. It will be crea
15、ted and ruin the measurement in most cases, since the cold-junction compensation will be done at the wrong point. NOTEThe relation between the measuring junction temperature and the generated mili-voltage is tabulated in thermocouple calibration tables for standardized thermocouple types, the refere
16、nce temperature being 0 oC. Standardized thermocouples that are commercially used, whose tables are stored in the memory of the TT302, are the following: . NBS (B, E, J, K, N, R, S & T). DIN (L & U) Resistive Temperature Detectors (RTDs) Resistance Temperature Detectors, most commonly known as RTDs, are based on the principle that the resistance of metal increases a
