High-end vibration analysis in wind turbines

Testing acceleration sensors connected to the PC with GfM analyser software

The Peakanalyzer GfM, which is universally applicable software for fully automatic vibration diagnostics and analysis, has been tried and tested for use in wind turbines. The raw data is processed by Beckhoff CX5020 Embedded PC with integrated high resolution accelerometer input module with EtherCAT EL3632 input terminal for condition monitoring (IEPE). The system monitors the entire wind turbine drive train, and optionally the wind turbine foundations, reliably and with high precision.

The GfM analyser software specialises in vibration diagnostics for gear units mounted on roller bearings. The research and development has always focused on the automation of diagnostic processes, since this is the key for widespread acceptance of these technologies. The combination of diagnostic services and device development is important for condition monitoring.

One of the devices that has benefited from this approach is the software designed for fully-automatic, high-end vibration diagnosis at up to 32 measuring points, with up to 32 further channels for slower process variables (1 kHz). At the heart is PC-based control technology, which integrates scientific automation concepts for integrating measuring functions that go beyond standard automation, such as condition monitoring. In this way, the analyser software enables, among other things, order analysis through re-sampling for diagnostics of variable-speed drives, Drive Vibration Significance (DVS) analysis for automatic identification of significant spectra, characteristic value monitoring, and triggered data acquisition.

Typical areas of application for vibration analysis and condition monitoring are for expensive, low-redundancy drives, such as those used in steel mills, heavy engineering machinery, for the structural strength of building materials, drives and gearboxes in conveyor systems, heavy duty electric motors, generators where higher availability is a critical requirement, or safety related applications such as drives in cable cars, for example. The analyser software is also useful for drives that are difficult to access and for which condition-based maintenance is therefore a prerequisite. Wind turbines are a prime example.

Application-specific and comprehensive monitoring of wind turbines
For monitoring wind turbine drive trains, analyser software and hardware should be capable of monitoring multi-channel inputs, which can also be used to monitor the strength of civil foundations via two further channels, in order to detect loosening. The diagnostic device is required to be installed in the wind turbine nacelle, either in the control cabinet or in a dedicated housing. The system analyses the drive train based on the signals acquired by eight IEPE acceleration sensors: one sensor for the main bearing, two for the generator, and five for the gear unit. If required, the analyser software should be capable to integrate into the existing communication structure, or communication can be established via wireless communication, optical fibre (tower), or via GHSDSL (copper cables between systems), as well as through DSL to the Internet provider.

The GfM analysis software adds the direct benefits because of the underlying PC-based control technology. The modular control technology using PC-based control platform enables system integrator to offer customised and cost-effective diagnostic solutions that are highly scalable, based on a freely programmable, open system, and with globally available and exchangeable spare parts, if required.

Here the distributed I/Os can be easily connected without affecting the cycle time because of the advantages of EtherCAT fieldbus and distributed clock feature distributed I/O system with standard CAT6 patch cables is another technical and commercial advantage for interconnecting node. The underlying XFC — eXtreme Fast Control technology is based on an optimised control and communication architecture comprising an advanced industrial computer, ultra-fast I/O terminals with extended real-time characteristics, the EtherCAT high-speed Ethernet system, and the Windows-based automation and control software. With this technology it is possible to achieve I/O update times/ I/O cycle time of <100µs. This technology opens up new process optimisation options for the machine designer or system integrator especially for vibration analysis that is not possible in the traditional control solution due to technical limitations.

Based on the high-performance communication of EtherCAT fieldbus, the software analyses up to 32 input channels for enhanced application flexibility. Moreover, a high channel sampling rate even with lower bus cycles can be achieved based on the oversampling functionality of the accelerometer input sensor and oversampling input module.

The DIN Rail Mounted Embedded PC CX5020 performs data acquisition, measuring and buffering based on the TwinCAT3 software. The information is then passed via ADS (a communication protocol within the software) to proprietary analysis software for further processing. The advantage lies in the direct control of the PLC. That is, the communication is based on a universal PLC, so that different system configurations with different numbers of channels and terminal types only need to be distinguished in our software. Remote access is also possible, and to this end, the .NET application on the embedded PC communicates with configuration and evaluation software on the corresponding network computer via TCP/IP.

Condition monitoring modules as essential I/O equipment
Sensor data for drive monitoring is logged with high precision via the two-channel high speed input terminals and communicated over EtherCAT fieldbus to software and this is an essential component of the Peakanalyzer. The key for the implementation of the high-end vibration monitoring system is the acquisition of the IEPE sensor signals with a sampling rate of 50 kHz. An additional factor is the very wide sampling range between 1 Hz and 1 kHz, which enables the device to measure low-frequency vibrations and high-frequency vibrations at the same time. In order to obtain a high-quality envelope signal for detecting roller bearing and gearing damage, all channels almost exclusively measure with a clock frequency of 50 kHz. Particularly in the wind industry, the 0.1 to 10 Hz mode is additionally used for logging characteristic values according to VDI 3834.

Distributed Clock
The crucial factors for the control process are: minimum response time, deterministic actual value acquisition, and corresponding deterministic set value output. At what point in time the communication and calculation occurs in the meantime is irrelevant, as long as the results are available in the output unit in time for the next output, i.e. temporal precision is required in the I/O components, but not in the communication or the calculation unit.

In a normal, discrete control loop, actual value acquisition occurs at a certain time (input component), the result is transferred to the control system (communication component), the response is calculated (control component), the result is communicated to the set value output module (output component) and issued to the process (controlled system).

The distributed EtherCAT clocks therefore represent a basic XFC technology and are a general component of EtherCAT communication. All EtherCAT devices have their own local clocks, which are automatically and continuously synchronised with all other clocks via the EtherCAT communication. Different communication run-times are compensated, so that the maximum deviation between all clocks is generally less than 100 nanoseconds.

Timestamp/multi timestamp
Process data is usually transferred in its respective data format. The temporal relevance of the process record is therefore inherent in the communication cycle during which the record is transferred. However, this also means that the temporal resolution and accuracy is limited to the communication cycle.

Timestamped data types contain a timestamp in addition to their user data. This timestamp – naturally expressed in the ubiquitous system time – enables provision of temporal information with significantly higher precision for the process record. Timestamps can be used for inputs and outputs. This way it is possible to determine, for example, the precise point in time when an output is to be switched. The switching task is executed independently of the bus cycle. While timestamp terminals can execute one switching task or switching event per bus cycle, multi-timestamp terminals can execute up to 32 switching tasks or switching events per cycle.

Oversampling
The concept of oversampling is that the input signals are oversampled with an adjustable, integer multiple (oversampling factor: n) of the bus cycle time (n microcycles per bus cycle). For each microcycle, the input module generates a process data block that is transferred collectively during the next bus cycle and this is how the Peakanalyzer is able to support such accurate vibration recording and evaluation. The oversampling factor describes the number of samples within a communication cycle and is therefore a multiple of one. Sampling rates of 200 kHz can be achieved even with moderate communication cycle times.

Authored by:

Ajey Phatak, Head Marketing
BECKHOFF Automation Pvt Ltd
email: a.phatak@beckhoff.com