Condition monitoring systems & the importance of the encoder.
CM systems are implemented in order to provide continuous information about the operating state of equipment, making it possible to predict and prevent any impending mechanical or electrical damage in real-time. They monitor a machine's state using meaningful physical variables either continuously or at regular intervals. They are typically made up of sensors, controls, and rotary encoders that monitor vibrations, temperatures, humidity, shaft speed, or condition of lubricant. A rotary encoder equipped with sensors beyond the traditional shaft position measurement capability can be especially useful in CM systems.
Rotary encoders within CM systems are generally located on assets that are intrinsic to the efficient operation of the plant and already communicating with the primary control systems. By adding the ability to bring additional process information, such as vibration, temperature, position and velocity, a CM system gains intelligence with a minimum increase in overhead.
The best CM systems make real-time comparisons with reference measurements, enabling operators to draw conclusions about various components. Such monitoring allows plant managers to plan or adjust maintenance intervals and set priorities, enabling optimum planning of maintenance schedules, personnel, and materials.
This can have a significant impact on spares inventory. When items are purchased only when needed, rather than on a preventative maintenance (PM) schedule, based on historical aggregate or rules of thumb, the carrying costs of holding unused materials are eliminated. In addition, historical order programs do not allow for unexpected failures, so materials may not be available when needed.
Applications That Make Sense
Wind, gas, or hydroelectric turbines are all examples of major capital assets that benefit from CM. These installations must be capable 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, measurements made in a motion control application might include the motor temperature, speed, angular or linear position, vibration, or time-in-motion. Many sensors would be needed to gather this information, and if discretely connected they would need I/O ports and associated processing time to monitor.
To minimize this impact, fieldbus technologies are available that simplify the cabling and interconnect. In combination with a PLC, the collection process can automate and concentrate these measurements 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-determined 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.
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 encoders 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 acceleration of the motor rotor, synthetic limit signals, motor and encoder temperatures can be included in the motion-control data packets. 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 resources 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.