4-20ma wireless sensors

4-20 ma Sensors - WIRELESS COMMUNICATION WITH WiWIZ1 AND uSIMs

 

Introduction

 

The 4-20mA current loop is a common method of transmitting sensor information in many industrial process-monitoring applications. A sensor is a device used to measure physical parameters such as temperature, pressure, speed, liquid flow rates, etc. Transmitting sensor information via a current loop is particularly useful when the information has to be sent to a remote location over long distances (1000 feet, or more). The loop’s operation is straightforward: a sensor’s output voltage is first converted to a proportional current, with 4mA normally representing the sensor’s zero-level output, and 20mA representing the sensor’s full-scale output. Then, a receiver at the remote end converts the 4-20mA current back into a voltage which in turn can be further processed by an analog to digital converter (ADC).


In a simple example if we had a temperature sensor with a range of 0 to 100 degrees Centigrade using a 4-20 ma interface, 4 ma would be correspond to the low point in the range i.e. 0 degrees and 20 ma would be 100 degrees. Since it is preferred to read the voltage, a resistor is used to change the current to voltage using Ohms law. A simple calculation using a 250 Ohm resistor would yield:

 

.004 (AMP) X 250 (ohms) = 1 volt (4 milliamps)


.020 (AMP) X 250 (ohms) = 5 Volts (20 milliamps)


The WiWIZ1 has two(2) highly accurate 24 bit analog to digital (ADC) converters. One 3 channel unit and one 6 channel unit. By adding the appropriate precision resistor to the development area of the WiWIZ1 PC board one can build a wireless sensor node easily with the addition of simple components.

 

The microprocessor can then the be used to capture the data, analyze it if required and send it over the radio data link.


uSIM Wireless Sensor Modules

 

uSIMs can also be used for 4-20 ma sensors in a wireless network.  They are very small, low power and will be available soon.

 

 

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