An “analog” electronic signal is a voltage or current whose magnitude represents some physical measurement or control quantity. An instrument is often classified as being “analog” simply by virtue of using an analog signal standard to communicate information, even if the internal construction and design of the instrument may be mostly digital in nature.
For example, if we were to calibrate a 4-20 mA temperature transmitter for a measurement range of 50 to 250 degrees C. (Refer figure 1).
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| Figure 1. |
Usually the 4mA commands to shut the valve fully closed or the valve's opening is 0% and the 20mA commands the valve fully open or the valve opens 100%. Or if this is controlling a motor, at 4 mA your motor speed is at the minimum while at 20mA your motor speed is at the full speed. (refer figure 2).
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| Figure 2. |
As you can see on the figure above the universal process controller is connected to an I/P transducer which is also known as current to pressure converter and the output of the transducer is connected to the control valve. The output of our process controller is 4-20 mA, you can simulate it manually by pressing the A/M(automatic/manual) button, and be sure that the set-up is manual so that you can manually send signal to the transducer. Then the 4-20mA signal will be converted to pressure at the transducer ranges from 3-15 PSIG if the action is direct response and 15-3 PSIG if reverse response. A 20-25PSIG pneumatic signal is supplied to the transducer. The Pneumatic signal 3-15PSIG will be sent to the control valve and the control valve will respond through its opening. 3 PSIG for 0% opening or the control valve is fully shut off and 15 PSIG for 100% opening or the control valve is fully open.
4-20mA Graphical Representation
•The 4-20mA electronic analog signal is the most popular form of signal transmission used in modern industrial instrumentation systems which is a DC standard. It is an analog signal standard, meaning that the electric current is used to proportionately represent measurements or command signals. Any current value in between 4 and 20 milliamps represents a commensurate percentage in between 0% and 100%.
04 - Analog Electronic Instrumentation
God Bless You All................. click this LINK to download the powerpoint....
04 - Analog Electronic Instrumentation
God Bless You All................. click this LINK to download the powerpoint....
