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How to select the best 4-20 mA Simulator

 

When the field equipment or the industrial automation system such as a PLC, DCS system or PID controller is malfunctioning and needs to be repaired the first step is to isolate the problem. In this step each component of the system need to be tested individual, without interference of other parts of the system. For this step a 4-20 mA simulator can be used. Because a 4-20 mA simulator allows manual setting the current in the 4-20 mA loop the correct functioning of the automation system and field equipment can be tested individual, independent of other parts of the system.

Usage of the 4-20 mA simulators

1. Testing automation equipment.

For testing industrial automation equipment the sensor connected to the automation system can be replaced by the 4-20 mA simulator. Before the sensor is connected, the connection to the automation equipment needs to be verified to determine how the 4-20 mA simulator shall be connected to the system. A 4-20 mA simulator can be connected in 2-wire mode or in 4-wire mode. When the 4-20 mA simulator is connected in 4-wire mode, the unit will become a 4-20 mA generator. With the 4-20 mA simulator the automation system functions such as alarm, control and logging can be verified. Sometimes it is necessary to simulate several 4-20 mA signals at the same time as the automation systems evaluates multiple measurement signals simultaneously.

 

2. Testing field equipment.

When actuators controlled by 4-20 mA signals need to be manipulated to verify the correct operation almost always a 4-20 mA generator (4-20 mA simulator in 4-wire connection) is needed. Hence, most actuators do not supply voltage to the 4-20 mA loop. The 4-20 mA generator will set the current in the loop to invoke action on the actuator. The actuator can thus be verified for correct operation. Special attention is needed to make sure the 4-20 mA generator can drive the load from the actuator on the 4-20 mA Current Loop Tester.

Based on the primary use of the 4-20 mA simulator, further selection criteria for the 4-20 mA simulator may be important.

 

Mounting the 4-20 mA simulator.

The 4-20 mA simulator can be mounted in a panel, specially when many signals need to be simulated panel-mounted 4-20 mA simulators are used for easy operation. This situation is typical for testing industrial automation systems such as PLC and DCS systems because these systems have many analog inputs for measurement of various signals from field instruments such as temperature and pressure transducers. The 4-20 mA simulators can be used to simulate signals from all these transducers so the software used with the PLC and DCS systems can be verified for correct operation.

When a single loop need to be tested it is easier to use a hand-held box mounted 4-20 mA simulator. These simulators come with plug-able connectors for fast and easy connection to the loop which need to be simulated. Typical industrial connectors for 4-20 mA signals are M12, DIN 43650 and smaller variants of these connectors. 4mm Lab connections are also available, this is standard connection as used on multimeters and power supply's, this allows usage of standard test wires and crocodile clips to connect the 4-20mA simulator to the loop.

 

Accuracy of the current setting of the 4-20 mA simulator.

The 4-20 mA signal in the loop is set manual by the 4-20 mA simulator. This can be done with a switch, potentiometer or buttons. Some 4-20 mA simulators allow only selection of current steps for example, 4,0-8,0-12,0-16,0-20,0 mA. This equals 0, 25, 50, 75 and 100% of the process signal. These 4-20 mA simulators are typical used for loop testing, in this case the full range of the signal is quickly verified. For other purposes, more accurate setting of the 4-20 mA signal may be necessary. For this purpose 4-20 mA simulators with a potentiometer for setting the current are a better choice. For normal usage 4-20 mA simulators with 1 turn potentiometers are used, the 0-100% scale of the signal is equal to a 0-300 degr angle position of the potentiometer. This allows for setting the current to approx 1% accuracy. If very accurate setting of the current is necessary a 4-20 mA simulator with 10 turn potentiometer need to be used. The current from 4-20 mA is set on a 0-3600 degr scale. The signal is set to 0,01% accuracy.

 

Minimum and maximum current range of the 4-20 mA simulator.

The signal on a standard 4-20 mA loop will not drop below 4 mA or raise above 20 mA in normal operating conditions. However, modern transducers are capable of setting the current above 20 mA and below 4 mA to signal special conditions such as starting-up, failure, under and over-range conditions. When the 4-20 mA simulator needs to be able to set the current above 20 mA and below 4 mA the range of the simulator has to be extended. Many transducers comply to the Namur NE43 recommendation for the minimum and maximum current in the loop. 4-20 mA simulators with even more extended range are available when needed, be careful using these simulators as the higher current may damage equipment being tested.