Page 169 - Intro Predictive Maintenance

160 An Introduction to Predictive Maintenance

7.7.4 Acquiring Data

Three factors must be considered when acquiring vibration data: settling time, data
verification, and additional data that may be required.

Settling Time

All vibration transducers require a power source that is used to convert mechanical
motion or force to an electronic signal. In microprocessor-based analyzers, this power
source is usually internal to the analyzer. When displacement probes are used, an exter-
nal power source must be provided.

When the power source is turned on, there is a momentary surge of power into the
transducer. This surge distorts the vibration profile generated by the machine. There-
fore, the data acquisition sequence must include a time delay between powering up
and acquiring data. The time delay will vary based on the specific transducer used and
type of power source.

Some vibration analyzers include a user-selected time delay that can automatically be
downloaded as part of the measurement route. If this feature is included, the delay
can be preprogrammed for the specific transducer that will be used to acquire data.
No further adjustment is required until the transducer type is changed.

In addition to the momentary surge created by energizing the power source, the
mechanical action of placing the transducer on the machine creates a spike of energy
that may distort the vibration profile. Therefore, the actual data acquisition sequence
should include a 10- to 20-second delay to permit decay of the spike created by mount-
ing the transducer.

Data Verification
Several equipment problems can result in bad or distorted data. In addition to the surge
and spike discussed in the preceding section, damaged cables, transducers, power sup-
plies, and other equipment failures can cause serious problems. Therefore, it is essen-
tial to verify all data throughout the acquisition process.

Most of the microprocessor-based vibration analyzers include features that facilitate
verification of acquired data. For example, many include a low-level alert that auto-
matically alerts the technician when acquired vibration levels are below a preselected
limit. If these limits are properly set, the alert should be sufficient to detect this form
of bad data.

Unfortunately, not all distortions of acquired data result in a low-level alert. Damaged
or defective cables or transducers can result in a high level of low-frequency vibra-
tion. As a result, the low-level alert will not detect this form of bad data; however, the
vibration signature will clearly display the abnormal profile that is associated with
these problems.