of reliable and consistent operation in order
for the supply of energy to the grid to be
planned efficiently. Detecting system degradation prior to failure allows for preparation
and performance of maintenance during
off-peak periods. The difference between
pro-active and reactive repair scenarios in
such cases can be as much as 5X in time to
return to service and 10X in expense.
Typical CM systems can cost between
$6,000 and $15,000 per installation. When
considering that the asset being monitored
can have operation lifetimes of decades
beyond their warranties, it’s easy to justify investing in CM by preventing just one
catastrophic repair. For a wind turbine,
estimates for a down-tower gearbox repair
are $220,000 or more. Notice prior to failure might allow the gearbox to be serviced
up-tower at a significantly lower cost.
Wind Turbine Sensors
If not managed properly, the addition of
new sensors can cause infrastructure issues
due to cabling, power management, and
data acquisition demands. For example,
I/O ports and
associated pro-
cessing time to
monitor.
To minimize this
impact, fieldbus
technologies are available that simplify
the cabling and interconnect. In combina-
tion with a PLC, the collection process can
automate and concentrate these measure-
ments so that downstream controls are not
overwhelmed, and cabling is significantly
reduced.
Measurement of slowly varying information, such as temperatures, water content
in oil, and humidity, can be handled over
relatively low-speed, low-cost communication systems, such as AS-i, Modbus, or
Foundation Fieldbus. These measurements
are collected by a PLC or similar controller,
and passed on to the main supervisory
system.
For measurements with higher dynamics,
such as vibration in a shaft or strain in a
wind turbine blade, the large number of
sensors used for data collection require a
higher bandwidth bus capable of transferring larger amounts of more complex information.
For example, roller bearings are typical
items where failure can be catastrophic. As
a result, rotational speed and associated
vibration and temperature are the physical
variables most often measured. Vibration
measurements can be made on a pre-de-termined schedule using portable equipment, but more often, sensors are being
dedicated to the measurement point and
accessed via an industrial network. Using a
databus such as CAN, DeviceNet, Ethernet/
IP, EtherCAT, Profinet, or Profibus, measurements could go to the CM system, in
addition to a safety controller or a process
control supervisory system.
Integrated Diagnostics
Using a rotary encoder with integrated
diagnostics in a CM system is one of the
best approaches for detecting conditions
in a motor. In addition to minimizing the
additional sensor requirements, the encod-
ers can pass additional information to the
motion controller as part of the command/
response data-stream. Currently, encoders
using the EnDat 2. 2 bus system provide
some of these capabilities. For example, the
encoder self diagnostics, angular accelera-
tion of the motor rotor, synthetic limit sig-
nals, motor and encoder temperatures can
be included in the motion-control data pack-
ets. Although this does simplify the physical
routing of the data, the motion controller
must then peel this information off from the
motion control information and use resourc-
es to pass this on up to the CM system.
One of the best solutions is a rotary
encoder capable of supporting the highly
dynamic requirements of motion control,
or generator commutation, in addition to
the requirements for CM. Encoders are now
available which provide position outputs
suitable for motion control, and provide
diagnostic and commissioning information.
The Leine & Linde Model 862 (photo left)
is just such an encoder. The 862 is a rotary
encoder that provides several levels of CM
support to the systems manager:
• Walk-by visual alarms on the encoder
housing consisting of red/green LED
indicators.
• The Advanced Diagnostic System
(ADS) continuously monitors the
encoder health and reports problems
to the supervisory control via an alarm.
Output signals from the encoder can
also be compared with the signal that
is generated in the cable to detect a
short condition in the interconnection.
• With Profibus DP capability, the diagnostic information is enhanced to support the requirements defined in the
PROFIBUS-DP specification as well as
encoder-specific diagnostic data.
• With an Ethernet connection, the ADS-Online system provides diagnostic
capability. With this system, many
of the measurements previously discussed are integrated into the encoder,
directly providing the asset manager.
Programmable warning levels can
also be used to detect voltage drops
in the power supply, or to generate an
automatic warning when the encoder
reaches a certain operating time.
Because the encoder is closely coupled
to the primary drive, measurement of temperature, vibration, and other variables
gives an overall picture of the health of the
system. When these encoders make this
information available via advanced databus
communications directly to the CM system,
overall efficiency of the maintenance system is improved. PDD
