One of the questions in my assignment is off how an RTD works with a transmitter and how it interacts with a PLC. Below is what I think happens but if anyone can spot any mistakes in it then please let me know.
An RTD works using resistance and temperature to help control certain things via the aid of a PLC. What this means is the change in temperature detected by the probe equates to an electrical resistance change inside the RTD. By supplying an RTD with a constant current and measuring the resulting voltage drop across the resistor, the RTD’s resistance can be calculated, and the temperature can be measured. An RTD takes a measurement when a small DC current is supplied to the sensor itself, then the current experiences the impedance of the resistor, and a voltage drop is experienced over the resistor. Depending on the nominal resistance of the RTD, different supply currents can be used. An RTD can be connected in a two, three, or four-wire configuration, the two-wire configuration is the simplest and also the most error prone. For more accurate readings its best to use either a 3 wire or 4 wire setup. Then change in resistance is then sent to the attached transmitter where it will undergo a conversion from ohms (resistance) to mA (current now detected). The transmitter works it out by rearranging ohms law to use the equation I=V/R this will then divide the voltage by the resistance to give the current now seen. Once its in the transmitter and converted the range of current is on a scale of 4mA to 20mA. We can set the range between 4mA and 20mA to be different temperatures. We have a start temperature of perhaps 0 degrees at 4mA and a maximum measured temperature of 100 degrees at 20mA, anything in between 4-20mA will be readings between 100-200 degrees on the temperature range. So for 50 degrees to be detected the electrical resistance would be 12mA. We start the range at 4mA as if it started at 0mA then we may not be able to detect a short in the circuit as we may assume its still working, so having it at 4mA means we can tell when its working and not working. The transmitter is connected to a PLC with another resistor connected across it, the current signal is sent from the transmitter to the resistor and converted to voltage again so the that the PLC can then ascertain the reading and select the appropriate response. The reason for the signal being sent as current is because if it was voltage it may have voltage drop over a distance whereas with current the signal is good for up to 1km.
Thanks Alan
An RTD works using resistance and temperature to help control certain things via the aid of a PLC. What this means is the change in temperature detected by the probe equates to an electrical resistance change inside the RTD. By supplying an RTD with a constant current and measuring the resulting voltage drop across the resistor, the RTD’s resistance can be calculated, and the temperature can be measured. An RTD takes a measurement when a small DC current is supplied to the sensor itself, then the current experiences the impedance of the resistor, and a voltage drop is experienced over the resistor. Depending on the nominal resistance of the RTD, different supply currents can be used. An RTD can be connected in a two, three, or four-wire configuration, the two-wire configuration is the simplest and also the most error prone. For more accurate readings its best to use either a 3 wire or 4 wire setup. Then change in resistance is then sent to the attached transmitter where it will undergo a conversion from ohms (resistance) to mA (current now detected). The transmitter works it out by rearranging ohms law to use the equation I=V/R this will then divide the voltage by the resistance to give the current now seen. Once its in the transmitter and converted the range of current is on a scale of 4mA to 20mA. We can set the range between 4mA and 20mA to be different temperatures. We have a start temperature of perhaps 0 degrees at 4mA and a maximum measured temperature of 100 degrees at 20mA, anything in between 4-20mA will be readings between 100-200 degrees on the temperature range. So for 50 degrees to be detected the electrical resistance would be 12mA. We start the range at 4mA as if it started at 0mA then we may not be able to detect a short in the circuit as we may assume its still working, so having it at 4mA means we can tell when its working and not working. The transmitter is connected to a PLC with another resistor connected across it, the current signal is sent from the transmitter to the resistor and converted to voltage again so the that the PLC can then ascertain the reading and select the appropriate response. The reason for the signal being sent as current is because if it was voltage it may have voltage drop over a distance whereas with current the signal is good for up to 1km.
Thanks Alan