More effective troubleshooting in current loops for process control

Troubleshooting and time pressure

Process and maintenance engineers are continually challenged to ensure the correct function of current loops for process control, as well as inputs and outputs from process instruments within the shortest time. To measure the circuit current without breaking the loop current, many technicians have used (more or less suitable) “workarounds”, e.g. resistors connected in parallel to the current loop. When Fluke introduced the mA-current probe Fluke 771 onto the market in 2007, they at last had the right tool, with which they could measure the loop current quickly, yet with high accuracy. Now, using the new 772 and 773 models, more measurements can be performed and even more time is saved. Since these more modern devices also have the functions of a current loop calibrator, they allow technicians to run on-site troubleshooting.

 

Detection of controller loop problems

Often, the first indication of a controller loop problem is from the operator: “I think there is something wrong with one of the valves”, or “this loop stops responding as it should” are statements that point to such problems. In both cases, this should be an indication for the technician that it is time to start troubleshooting.

First, the 4-20 mA signal should be measured. You can do this either by interrupting the loop and connecting a digital multimeter or a current loop calibrator in the loop current, or by using an mA-current probe and checking the loop current value. If the measured loop current does not correspond to the set value, there are three possible causes: damaged/interrupted/shorted conductor, a bad loop current supply, or faulty instruments.

If no fault can be found in the conductors, check the loop current supply using a digital multimeter or using the current probe.

 If the power supply does not deliver an output value, use the 24-V loop current contact of the meter to replace it. If the loop then functions, the cause of the problem is obvious.

If both the wiring and the power supply can be ruled out as the cause of failure, the transmitter must be checked. If you have a current loop calibrator, a process calibrator or a multi-function current probe with an mA simulation mode, insert these measuring devices into the loop current instead of the transmitter.

 If the loop now works properly, the problem is in the transmitter. If it does not, the problem has a different cause.

If it is suspected that the problem is caused by a control element (valve actuator, etc.), use the mA transmitter mode of the current probe to feed in a signal. When doing so, check whether a response is displayed on the local display.

 

Faults in current loops

If the problem is not due to a “dead” loop, but to an inaccurate or unstable loop, possible causes could be a faulty input/output board on the PLC or a process control system, or a faulty control element (loop current and pressure at a valve actuator, etc.). It is usually best to first check the transmitter on site, or to check the display on the instrument or in the process control, or check the actual control element.

Use a current probe to measure the loop current of the control element.

Compare the value with the local position indicator on the valve or another control element. Forward this information to the operator to verify your results.

In a current loop, use the current probe to measure the loop current. Check your results with the operator in order to determine the extent to which the value displayed in the control matches that of the actual loop current. This will provide you with an initial overview of the I/O card of the PLC or the process control system responsible for this loop. Additionally, using the mA transmitter mode, you can feed a specific current in the loop that is then visible in the control room. As previously, compare the value identified by the operator with the actual current value in the loop.

Uncontrolled fluctuations or intermittent errors that often do not occur when the technician is on site occur in some loops. Here, a viable solution is to use a current probe with a scaled mA output. In this mode, the meter measures the current in the loop without having to break the electrical circuit, and delivers an identical and isolated mA output at the output. Feed this output to a digital multimeter with logging function. By logging with the digital multimeter over a longer period, all faults will be recorded.

 

On-site inspection and commissioning of plants

Start by using a current probe for loop current measurement to check the current of all loops within seconds and without disconnecting any connections. If a loop is not working, you can use a multifunction current probe to run a quick diagnosis. If no current is measured in some loops, move to classic troubleshooting: Check the wiring,

the power supply and the I/O boards of the controller by feeding a signal to the inputs and/or outputs with the measuring device, and then the compare values measured with the values determined by the operator. If the operator confirms the values measured, the problem could be in the transmitter: either in the transmitter itself or, if it is a new installation, in faulty wiring of the sensor input to the transmitter.

 

Checking the I/O boards of the process control system and PLC

The mA process current probe can be used as an accurate signal source to check the function of input/output boards at programmable logic controllers (PLC) and in the process control system. Disconnect the process loop at 4-20 mA input boards, then feed in a known signal using the mA transmitter of the measuring device (use the 25% step function of the meter: 4.0 mA for the zero point, 12 mA for 50% and 20.0 mA for 100% ), and compare this signal with the value on the display of the instrument or in the process control.

Voltage input boards (1 V to 5 V or 0 V to 10 V) are checked in a similar way by using the transmitter function of the meter for voltage.

 

Checking a valve actuator

mA current probes can be used for regular on-site checks of electronic valve actuators as part of the preventive maintenance. You can perform function checks quickly, taking into account the manufacturer's instructions, by using the Fluke 772 or 773 as the signal source, for example, and observing the position of the valve spindle, the mechanical position indicator or the flow indicator while changes are made at the input.

Mitch Stewart, Manager for the mobile service of L2 Systems, has reported that he used the 4-20 mA transmitter output of an mA process current probe to open and close a control valve when the output of the PLC did not work. “We disconnected the output of the PLC at the power input of the valve actuator, connected the measuring device there, and then set the transmitter function for 4-20 mA. Then we changed the current to verify whether the current pressure transducer correctly set the valve,” he explains.

Adjust the measuring device so that it outputs 4 mA, and wait until the actuator comes to rest. Then vary the current in small increments between 4.0 mA and 3.9 mA, while you check with your free hand whether the valve spindle moves. Adjust the zero point between these two current settings using the zero point adjustment on the actuator.

Then increase and reduce the current between 4 mA and 4.1 mA. Make sure that the valve spindle starts to move when above the 4.1 mA setting and that the valve is completely closed at 4 mA.

The measuring span can be checked in a similar way, by setting the measuring device into transmitter operation to 20 mA, 19.9 mA and 20.1 mA. Linearity can be checked with the 25% step function of the measuring device.

 

Checking isolation amplifiers

To check an isolator amplifier, feed an mA input signal to the device. Then measure its 4-20 mA output using the probe current measuring function. This function for simultaneous transmission and measurement of loop currents on two channels of the Fluke 773 can also be used for valves that transmit their position using 4-20 mA signals.

 

Checking inverter drives

Inverter drives are used to control motors, blowers and fans in process applications, as well as in conveying systems and machine tools.  Control inputs are normally voltage (1 V to 5 V or 0 V to 10 V) or current (4 mA to 20 mA). A loop current can be set exactly and fed to the input by using an mA process current probe, while the technician checks the result.

 

Fast calibration

Modern mA process current probes can also be used in many applications as current loop calibrators, as they have a precision of 0.2%. The user has the great advantage of being able to use just one tool for many tasks.  For example, to check a pressure transmitter in the laboratory (apart from a pump and separate pressure standards), a loop power supply and an instrument for the measurement of the 4-20 mA output signal of the transmitter are normally required. However, using the modern mA process current probe it is possible to supply the transmitter and to measure the loop current at the output. “With this compact device,” says Paul Jusak, Maintenance Technician at Puget Sound Energy, “you can use this function without a separate power supply.”

 

One tool for many tasks

Using modern mA process current probes, process and maintenance technicians can save a lot of time when troubleshooting, because they replace various separate instruments. The technician only needs one light and easy-to-operate measuring device to complete almost all tasks. Jusak adds, “Instead of two tools, now you only need one device in your pocket to run the calibration in 4-20 mA current loops and to troubleshoot. That's what I call really practical.”