Hidden treasure User Manual

Hidden treasure

Drive data are a treasure trove of hidden information that can
help industries solve problems before they even happen

MICHAL ORKISZ, MACIEJ WNEK, PIEDER JOERG – As processes

become ever more complicated and margins thinner, mini­mizing downtime by ensuring that industrial machinery
operates correctly is as important as ever. Proper condition
monitoring of critical equipment can act as an early warning
system against impending problems. However, condition
monitoring is not used everywhere, often because of the
expense of installing proper sensors and cabling, especially
if the monitoring system needs to retrofi tted to existing
equipment. Another reason is that the task of selecting and

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interpreting the large quantities of data available in the most
effective way seems daunting as well as costly. ABB has
devised a way to easily access and process important data
without the burden of additional equipment, costs and
downtime. By extracting and processing data from existing
devices traditionally used in process industries, such as
drives, customers can prevent otherwise unforeseen prob­lems from occurring and hence maximize the availability of
their machines.

ndustries are constantly under pres­sure to reduce costs while increasing
service and productivity. The most ef-

I

fective way of fulfi lling these aims is for
managers to know the state of their equip­ment – in particular the critical compo­nents – at all times and to use this infor­mation to quickly identify and rectify faults
before they spread to other parts of the
process
system helps predict the reliability of
equipment and the risk of failure. With so
much to gain, why is it that condition
monitoring is not used everywhere? One
reason is that existing equipment is often
already retrofi tted with a monitoring sys­tem and the installation of additional sen­sors and cabling could prove both com­plicated and expensive. Another reason
concerns the interpretation of results. In
many cases it may not be clear how to
use a set of data that gives information
about one aspect of a process to provide
information about another. For example,
determining the fractal dimension of a
certain phenomenon may be fairly straight­forward but relating it to the condition of a
machine may not be so obvious.

Most processes use devices that are ca­pable of collecting and producing rele­vant signals, which, if harvested and pro­cessed correctly, can also be used for
diagnostic purposes. Among others, one
such example is ABB’s family of ACS
variable-speed drives, which are often

[1]. A good condition monitoring

used to power critical equipment. The
drives are based on powerful controllers
that consume and provide tens, if not
hundreds, of signals with sub-millisecond
resolution.

To be useful for condition monitoring,
data needs to be obtained from the drive
inverter in one form or another. Internally
the signals – which include measured
and computed values such as speed,
frequency, torque, flux, current, power
and temperature, as well as parameters
such as configurable drive settings – are
stored in a regularly updated memory
table. Data can be retrieved from this ta­ble as OPC
ed into hardware data loggers.

Data loggers are programmable buffers
capable of storing values from several
selected variables concurrently with a
specified sampling rate, generally one
that is high enough to make the data
useful for spectral analysis. In normal op­eration, the newest data overwrites the
oldest until the loggers are triggered by
certain events, such as the occurrence
of a fault or an alarm, a selected variable
signal crossing a specified threshold or a
software command. As the buffers are
circular, some data
prior to and after
the trigger can be
retained. ABB’s
DriveMonitor

➔ 1 can read

tem
the contents of a
drive’s hardware
data logger. It con­sists of a hardware
module in the form
of an industrial PC and a software layer
that automatically collects and analyzes
drive signals and parameters

Data enhancement

Because the resolution has already been
determined and preprocessing has been
performed, drive signals are generally
available in a form not easily applicable
to diagnostic evaluation. It is therefore
necessary to employ a suite of “tricks” to
transform the data so that it becomes
useful for diagnostics.

True to their name, variable-speed drives
dynamically change the frequency of the
current supplied to the motor. The direct
torque control (DTC) method employed
in the drive produces a non-deterministic

1

values or they can be load-

Most processes use devices

TM

sys-

that are capable of collecting
and producing relevant sig­nals which can be used for
diagnostic purposes.

[2].

1 ABB‘s DriveMonitor

switching pattern, so there is no such
thing as a constant switching frequency.
This makes the straightforward applica­tion of spectral analysis methods some­what challenging. Because individual
spectra contain many hard-to-predict
components collected one after another,
the averaging of many spectra using
point-by-point averaging, for example, is
essential to obtain a “clean” spectrum.

In general, signals currently available
from the ACS drive are used primarily for
control purposes. Therefore some of the
preprocessing needed for condition
monitoring signals is missing. One such
process is anti-aliasing filtering. Data
points are sampled or computed at rates

up to 40 kHz, but can only be accessed
at lower rates (eg, by keeping every 40th
data point). In signal processing it is typ­ical that frequencies above the so-called
Nyquist frequency – defined as half the
sampling rate – should be filtered out
prior to signal sampling. Skipping this
step means the peaks from the higher
frequencies will appear in the lower part
of the spectrum, making it very hard to
interpret. For example, signals contain­ing frequencies of 400 Hz, 600 Hz,

Footnote

1 OPC stands for object linking and embedding

(OLE) for process control and represents an
industry standard that specifies the communica­tion of real-time data between devices from
different manufacturers.

TM

71Hidden treasure