Ken West, Fluke industrial marketing manager, looks into better ways to troubleshoot automation and process control loops
Instrument and automation technicians are constantly challenged to keep instrumentation loops and I/O working at peak efficiency while using the least possible time. With the Fluke 771 mA Clamp Meter, a great deal of time can be saved by measuring loop current without breaking the circuit. The 772 / 773 models, meanwhile, add the functions of a loop calibrator.
Tracing control loop problems
If the loop current measured is not as expected, there are three likely causes: broken/disconnected/shorted wires, a bad loop power supply, or faulty instrumentation. If no problem is found in the wires, a Fluke 773 can be used to check the loop power supply. If it shows no output, use the 24V loop power function of the meter to substitute for it. If the loop then works properly the source of the problem is obvious.
The next stage uses the mA simulate mode to substitute for the transmitter. If the loop performs as requested, the problem lies with the transmitter; if not, it is elsewhere. If a final control element (valve positioner, etc.) is suspected, the mA source/simulate mode on the Fluke 773 can feed a signal into it while watching the local indicator for a response.
If the problem is an inaccurate loop, likely possibilities include a bad I/O card on the PLC or DCS, or a bad final control element.
For a final control element, use the 773 to measure loop current and compare the value to the local position indicator on the valve or other final control element.
In the case of a measurement loop, use the 773 to measure loop current, then check to see how well the value on the control panel agrees with the actual loop current. This will give a quick check on the PLC or DCS I/O card that handles that particular loop.
Some loops show random fluctuations or intermittent faults, so feed the scaled mA output of the 773 to a DMM with a logging function so that any disturbance will be recorded.
Field checks and plant commissioning
Start by using the Fluke 773 to check each loop for current in a matter of seconds, without disconnecting anything. If current is not present go on to check the wiring, the power supply, and the control system’s I/O cards by using the meter to inject a signal into the I/O. If the operator agrees with what is being sent, then there may be something amiss with the transmitter – either the transmitter itself or perhaps mis-wiring.
Checking I/O cards
The 773 can be used as an accurate signal source to check the operation of I/O cards on PLCs and DCSs. For 4-20mA input cards, disconnect the process loop and use the meter’s mA source mode to feed in a known signal value using the meter’s 25% step function, and compare it to the value shown on the operator’s readout. Voltage input cards are checked in a similar way, using the meter’s voltage source function.
Checking valve positioners
The 773 can also be used for checks of electronic valve positioners in preventive maintenance programs using the mA source/simulate mode and connecting it to the input terminals. Set the meter to output 4mA and wait for the positioner to settle; then vary the current in small increments between 4.0mA and ~3.9mA, while feeling the valve stem to check for any sign of movement.
Adjust for zero movement between these two current settings by using the zero adjustment on the positioner. Next, alter the current between 4mA and ~4.1mA and ensure that the valve stem just begins movement above the ~4.1mA setting and is fully closed at 4mA.
Span can be checked similarly, by setting the meter at 20mA, ~19.9mA and ~20.1mA, and linearity can be checked by using the meter’s 25% step function.
Checking loop isolators
To check a loop isolator, apply an mA input signal to the device and measure its 4-20mA output. This function can also be used for valves that report their position using 4-20 mA.
Variable frequency drives’ (VFDs) control inputs are generally voltage or current. A 773 can feed in a signal to simulate a normal input while the technician observes the result.
While not classified as loop calibrators, the Fluke 773 boasts accuracies of 0.2%, and can be used for quick calibration checks, while cutting down on the number of instruments needed.
So, not only can today’s mA process clamp meters save instrumentation and automation technicians a great deal of time in trouble shooting, but they can also replace a number of separate instruments.
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