【正文】 Pt500) .. GE (Cu10) .. DIN (Ni120) For correct measurement of RTD temperature, it is necessary to eliminate the effect of the resistance of the wires connecting the sensor to the measuring circuit. In some industrial applications, these wires may be hundreds of meters long. This is particularly important at locations where the ambient temperature changes constantly. 11 The TT302 permits a 2wire connection that may cause measuring errors, depending on the length of connection wires and on the temperature to which they are exposed. (See Figure TwoWire Connection). In a 2wire connection, the voltage V2 is proportional to the RTD resistance plus the resistance of the wires. V2 = [RTD + 2 x R] x I Figure TwoWire Connection In order to avoid the resistance effect of the connection wires, it is remended to use a 3wire connection (See Figure – ThreeWire Connection) or a 4wire connection (See Figure Four Wire Connection). In a 3wire connection, terminal 3 is a high impedance input. Thus, no current flows through that wire and no voltage drop is caused. The voltage V2V1 is independent of the wire resistances since they will be cancelled, and is directly proportional to the RTD resistance alone. V2V1 =[RTD + R] x I R x I = RTD x I Figure Three – Wire Connection In a 4wire connection, terminals 2 and 3 are high impedance inputs. Thus, no current flows through those wires and no voltage drop is caused. The resistance of the other two wires is not of interest, since there is no measurement registered on them. Hence the voltage V2 is directly proportional to the RTD resistance. 12 (V2 = RTD x I) Figure Four Wire Connection A differential or dual channel connection is similar to the twowire connection and gives the same problem (See Figure Differential or Dual Connection). The resistance of the wires will be measured and do not cancel each other out in a temperature measurement, since linearization will affect them differently. Figure Differential or Dual Connection SIEMENS Highlight of SIMATIC PCS 7 PS Low investment costs ． Modular system based on standard ponents, therefore high degree of flexibility and scalability. ． Open thanks to the use of standard technologies. Low operation and maintenance costs ． Horizontal integration with Totally Integrated Automation. 13 ． Control system specific functionality and ponents for power plant requirements. Customer Profits ． Optimum adaptation to the requirements. ． Expansion and adaptations according to size and plant characteristic. ． Scalable performances and memories for control. ． Scalable from single station to distributed control system with clientserver architecture. ． Powerplantspecific operation and monitoring,diagnostics and process interface. Factsamp。 U) Resistive Temperature Detectors (RTDs) Resistance Temperature Detectors, most monly known as RTD’s, are based on the principle that the resistance of metal increases as its temperature increases. Standardized RTDs, whose tables are stored in the memory of the TT302, are the following: . JIS [160481] (Pt50 amp。 8 譯文： TT302— Field bus Temperature Transmitter Operation 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, 02021 Ohm. Functional Description – Hardware The function of each block is described below. Figure — TT302Block Diagram MUX 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 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, RAM, PROM and EEPROM The CPU is the intelligent portion of the transmitter, being responsible for the management and operation of measurement, block execution, selfdiagnostics an