Six troubleshooting tips: Automation and process control loops can reach peak efficiency with multi-function tools
October 27, 2010 by Colin Plastow
Automation technicians are constantly challenged to keep instrumentation loops and I/O working at peak efficiency in the least amount of time possible. While multiple tools have typically been needed to perform various troubleshooting tasks, today’s multi-function instruments, such as mA process clamp meters, allow technicians to perform a wide range of tests with outstanding accuracy and efficiency, while cutting down on the number of instruments needed to do the job.
As these tools become increasingly important in today’s automation and process control environments, it is important to understand when, where and how to use them to their best advantage – from tracing control loop problems or malfunctions to field testing and commissioning.
Following is a look at some of the features and functions of mA process clamp meters, along with troubleshooting guidelines to help you get the most out of your instrument investment.
1. TRACING CONTROL LOOP PROBLEMS
Often, the first indication of a control loop problem comes from the operator reporting a bad valve or a loop not responding the way it should. Any of these anomalies is a signal for the technician to begin troubleshooting. The first step is to measure the 4-20 mA signal to verify the loop current value.
There are three likely causes when the loop current measured is not as expected: broken, disconnected or shorted wires; a bad loop power supply; or faulty instrumentation. If no problem is found in the wires, check the loop power supply. If that supply shows no output, a meter with a 24-V loop power function can be used to substitute for the missing output. If the loop works properly, then the source of the problem is obviously the power supply.
When the wiring and power supply both check out, it’s time to check the transmitter. Use the mA simulate mode on a loop calibrator, process calibrator or multi-function clamp-on meter to substitute for the transmitter. If the loop performs as requested, the problem lies with the transmitter.
Finally, if a final control element (i.e. a valve positioner) is suspected, the mA source/simulate mode can feed a signal into the element and provide a response.
2. LOOP MALFUNCTIONS
An inaccurate (versus a dead) loop can be caused by a number of things, including a bad input/output (I/O) card on the PLC, distributed process control system (DCS), or a bad final control element (for example, an I/P on a valve positioner). It’s usually best to start by doing a field check of the transmitter, local or remote indicator, or final control element.
For a final control element, use a clamp-on meter to measure loop current and compare the value to the local position indicator on the valve or other final control element. Relay that information to the operator to verify your findings.
In the case of a measurement loop, use the clamp meter to measure loop current, then check with the operator to see how well the value indicated on the control panel agrees with the actual loop current. This will provide a quick check on the PLC or DCS I/O card that handles that particular loop. It’s also possible to use the meter’s mA source/simulate mode to send a known signal to the control room; and compare the value as read by the operator to the actual current in the loop.
Some loops show random fluctuations or intermittent faults that need to be tracked over time. A clamp meter with a scaled mA output can be used to measure the current in the loop without breaking the circuit, and produce an identical and isolated mA output. That output can be fed to a DMM with a logging function to enable recording over time.
3. FIELD CHECKS & PLANT COMMISSIONING
When performing a field check or commissioning, a clamp-on loop current meter can be used to check each loop for current in a matter of seconds, without disconnecting anything. If a loop is not working, a multi-function clamp meter can also make quick work of diagnostics. If current is not present on some loops, however, it is time to move on to classic troubleshooting.
Classic troubleshooting involves checking the wiring, the power supply and the control system’s I/O cards (i.e. using the meter to inject a signal into the I/O, and then contacting the operator to ask what he sees). If the operator agrees with what is being sent, then there may be something amiss with the transmitter – either the transmitter itself or, if this is a new installation, the wiring.
4. CHECKING DCS AND PLC I/O CARDS
An mA process clamp meter can also be used as an accurate signal source to check the operation of I/O cards on PLCs and DCSs. For 4-20 mA input cards, disconnect the process loop and use the meter’s mA source mode to feed in a known signal value and compare it to the value shown on the operator’s readout. Voltage input cards can be checked in a similar way, using the meter’s voltage source function.
5. CHECKING A VALVE POSITIONER
As part of preventive maintenance programs, technicians can use milliamp clamp meters for periodic in-field checks of electronic valve positioners. Accounting for manufacturer-specific instructions, it is relatively easy to perform quick operational checks using the meter as a signal source, while observing the valve stem position, mechanical position indicators or flow indicators as input changes are made.
6. CHECKING LOOP ISOLATORS AND VFDS
To check a loop isolator, apply an mA input signal to the device and measure its 4-20 mA output using the clamp-on current measuring function. A two-channel simultaneous source/measure function can be used for valves that report their position using 4-20 mA. An mA process clamp meter can also be used to feed in a signal to a variable frequency drive (VFD) to simulate a normal input while the technician observes the result.
Today’s mA process clamp meters can save instrumentation and automation technicians a great deal of time because they can replace a number of separate instruments and support a range of functions. When selecting a multi-function tool, be sure to check the specifications and feature/function sets to ensure the instrument you choose is the right one to meet all of your troubleshooting needs.
Colin Plastow is the industrial product manager for Fluke. He may be contacted at email@example.com.