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Design, implement MCU-based current loop calibration device

Posted: 11 Nov 2011 ?? ?Print Version ?Bookmark and Share

Keywords:calibration device? MCU? current loop?

Here is a paper that tackles the design and implementation of a low cost microcontroller-based current loop calibration device to test and calibrate systems that communicate via 4-20 mA current loop standard. 4-20 mA current loops are commonly used for communication in industrial automation systems.

The aim of this work is to decrease the cost of these devices. Designed device has 0.001 mA resolution on the current sourcing. It is also both capable of measuring and providing requested current between the range of 4 and 20 mA either automatically in the format of step or ramp based function or manually by entering current value via a numerical keypad.

4-20 mA current loop is a basic method for sending sensor information in industrial applications [1]. Sensor is a device used for measuring physical parameters like temperature, pressure, speed and fluid flow. Most of the process automation sensors are standardized using the 4-20 mA current loop interface. This interface is generally used for sending sensor values over current loop to remote locations (figure 1).

Figure 1: Overview of the current loop system.

If voltage levels are used for sending process, there will be a decrease in voltage depending on the resistance of the carrier and distance increments. In order to prevent this negation, 4-20 mA current loop is standardized.

There are lots of systems in industry working with 4-20 mA current loop standard. 4-20 mA current loop calibration devices are used in testing and calibrating those systems. 4-20 mA current loop is designed to be in the value of 4 mA when the sensor receives the minimum value and it becomes 20 mA when it reaches the maximum value [2, 3]. So, 4 mA is assumed as a starting point (0 percent reading) and 20 mA is seen as a full scale reading (100 percent). In this condition, 0 mA level is interpreted as a communication break. It means that 0 to 4 mA range is called as the zero or offset. The 4 to 20 mA range is called as the span of the transmitter.

Figure 2: Interface of the 4-20 mA current loop.

A 4-20 current loop circuit is composed of 4 elements; sensor/converter, transmitter, receiver and current source (figure 2). Sensor or transducer measures physical magnitudes and converts to voltage. Transmitter converts the voltage information taken from sensor to the 4-20 mA current level. Receiver after taking the 4-20 mA current level converts it back to voltage and sends to the process controller or to an indicator. Current source provides the current loop as well. There is at least one receiver in each loop. It can be an indicator (a meter or a digital readout), a chart recorder, an input to an RTU or a PLC, a valve actuator, etc.

The 4-20 mA current loop has some advantages. These are less affected by noise, capable of sending signals to distant locations, signal being limited by only current source, and so inexistence of signal lost and ability to control broken line. Since the bottom level for current loop is at 4 mA level, the breaking of signal transmission line is interpreted to be 0 mA [4].

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