Gating in Observer 9.1

Transcription

1 Application Note Gating in Observer 9.1 Introduction In some applications, it is important to collect vibration data only at a specific operating condition of the machine. This way of collecting vibration measurements is called "Gating" and is used to control when to take the vibration measurements on variable speed machines like paper machines, rolling mills, shovels, haul trucks and draglines. This technique makes sure that the data is collected as close as possible at a "fixed" running speed of the machine. Fig. 1 shows the rotational speed profile of the cardan shaft for a mining haul truck. To decide when to take the measurement, it is important to know the speed profile of the machine. In fig. 1, the rotational speed varies from 100 to rpm of the cardan shaft. The ideal situation to collect data is at a constant speed of the machine. In this case, the speed range for "gating" of the cardan shaft was set to 800 to rpm. If the load of the machine is also varying, it is strongly recommended to measure the motor load on the machine. A varying load on the machine can affect the vibration levels a lot; for example, the vibration level at the gear mesh frequency is strongly affected by the load on the machine. Speed (km/h) Gating Gating 1100 rpm 800 rpm time Fig. 1. A typical variation of the speed of the cardan shaft on a haul truck. On variable speed machinery, an allowed variation of the rotational speed has to be set in Observer, when collecting data, to avoid collecting data during acceleration and deceleration of the machine. Too much variation of the rotational speed will cause a smear out phenomena in the frequency amplitude reading. What happens is that the energy from the center peak leaks to the adjacent frequencies, i.e., the frequency peaks in the spectra will be smeared out. Note: In theory, the FFT transform assumes that the actual vibration should be repetitive and infinite in time. It is recommended to initially try out different gating combinations to find an acceptable data acquisition range. Experience has shown that a variation of the rotational speed with 3% generates an acceptable measurement error (i.e., the vibration peaks in the frequency spectra will not smear out). In fig. 1, the allowed variation of the rotational speed on the haul trucks was set to 30 rpm.

2 Procedure How to find the properties for the Machine and SKF Multilog IMx devices in Observer Right-click on a measurement point on the machine (marked with a black box) in the hierarchy tree, and then select Properties ( fig. 2). In this case, the measurement point "Motor NDE" on Fan 1 was selected. Then follow Steps 1 to 3 on how to configure gating on this measurement point. The same procedure can be applied on the other vibration measurement points on the machine. Initiate an IMx device Motor NDE was selected Machine properties Fig. 2. Select "Properties" on the machine. 2

3 Step 1 General tab Before proceeding to Step 2, it is necessary to make sure that the measurement point has an: Appropriate name (in this case, the measurement point was called "Motor NDE", i.e., NDE = Non Drive End). The measurement point is "Enabled". To enable the measurement point, the checkbox has to be checked in fig. 3. And that an IMx unit and a vibration channel has been selected. Note: To be able to create measurement points, an IMx unit and channels have to be initiated ( fig. 2). To enable the point, click on the checkbox Fig. 3. Necessary settings in the General tab. 3

4 Step 2 Acquisition tab Note: The number of spectral lines (e.g., 800 lines) for the Trend Configuration, marked with a black box in fig. 4a, has to be equal or lower than the settings in the vibration spectrum on the Acquisition section, marked with a green box. The idea here is to configure a faster measurement rate for the Trend than for the vibration spectrum. This can be done by having a lower resolution for the Trend. Note: A lower resolution will have shorter sampling time when collecting the data. In most of the applications this can be convenient to do, for example, if you would like to capture data during an abnormal behavior on the machine that occurs now and then. The ulterior motive with this is that the SKF Multilog On-line System IMx shall measure trend data as fast as possible (around 2 to 3 trend values/second) to be able to capture enough of measurement data when an alarm occurs. This will give the user measurement data, to in a better way draw a conclusion about what happened with the machine. However, the Trend will be stored in the database according to the parameters that have been configured in the Operating and Storage Conditions tab ( fig. 5). Note: To do gating of measurement data, it is necessary to add information regarding simultaneous measurements (marked with a blue box). In this case: "Motor speed" and "Power" were the selected parameters to activate gating on the machine. The dotted black box in fig. 4a shows settings for the captured frequency spectra: Meas. time: 1,6 seconds Resolution: 0,625 Hz/line No. samples: 0 samples 4

5 Settings for Acquisition of vibration Spectra Trend configuration settings for the acquisition of trend spectra Motor speed and Power were selected as Simultaneous measurements Fig. 4a. Typical settings on the Acquisition tab. Note: If the number of lines is greater for the Trend Configuration than the number of lines for the vibration spectrum, the message box in fig. 4b will pop up. The message box tells the user that the number of lines for the vibration spectrum shall be higher than or equal to the number of lines for the trend spectrum. Fig. 4b. Message box for number of lines violation. 5

6 Step 3 Configuring of gating on the "Operating and Storage Conditions" tab The "Active range" for the gating of FFT spectra is set to 800 to 1100 cpm, and the active range for the motor load is set to 20 to 90 kw (see the blue box in fig. 5). The maximum variation of the rotational speed (i.e., Max allowed delta) is set to 10 cpm when collecting vibration spectra data (see the red box in the Scheduled Dynamic Data Storage of FFT spectra section). This parameter controls how much the rotational speed is allowed to vary during the collection of measurement data. It is recommended to play with this parameter to identify when an unacceptable smearing of the frequency peaks occurs. A too high value will cause a "smearing of the frequency peaks in the vibration spectrum. For example, a variation of 30 rpm of the shaft that runs at 1000 rpm (i.e., 3 %) will still generate a high quality vibration spectrum. In this case, the variation of the rotational speed was set to 1% when collecting vibration spectra data. Note: The best practice is to adjust the "Max allowed delta" for the rotational speed of the shaft (where the tachometer is mounted) during the start up of the on-line system. This is normally the best moment when you can adjust and tune in the system, to minimize the smearing of the vibration peaks. A recommendation is to monitor the stored measurements in Observer during the start up of the SKF Multilog IMx on-line system. The load of the machine will also have a big impact on the vibration amplitude in the vibration spectrum. At high load, the vibration levels are much higher in vibration amplitude than when the machine is operating at a low load. Note: The "Condition" for when a FFT spectrum shall be taken (marked with a blue box in fig. 5) has to be inside the interval of the speed range for the rotational speed of the shaft" in the Trend storage settings marked with a green box. In this case, the condition for FFT spectra storage is set to 800 to 1100 rpm, and the speed range for trend data storage is set to 300 to 1600 rpm. Speed range for trend data storage Trend storage settings The "Gating" for the "Trend" is only active in the interval ( rpm). The maximum variation of the rotational speed when the machine is running is set to 10 rpm. Scheduled Dynamic Data Storage of FFT spectra Speed range for FFT spectra storage Max. allowed data: FFT spectra storage interval is set to every 10 hours Fig. 5. An example of how to configure the "Operating and Storage Conditions" tab. 6

7 Descriptions of the available "Types" and "Conditions" for the Active range in fig. 5: Type is the type of gating that can be set to one of the following values: All: The active range check is disabled. In other words, the active range that the measurement point is using is all values. Speed: The active range check is determined by the rotational speed measurement point readings selected under "Simultaneous measurements" in the Acquisition tab. Process: The active range check is determined by the process measurement point readings selected in "Simultaneous measurements" in the Acquisition tab. Digital: The active range check is determined by the digital measurement point readings selected in "Simultaneous measurements" in the Acquisition tab. Same as trend: Configures the active range to be the same as the active range for the trend. This setting is only available for the "Active range" setting on the Spectra tab. Condition: The minimum and maximum values for the "active range", when the SKF Multilog On-line System IMx shall collect data. Max allowed delta: The maximum accepted change of the gating parameters (for example, the rotational speed of the shaft or motor load) during the collection of vibration and process data. This setting is used to force the system to take data when the operating mode of the machine is "almost" stable (which in some cases are the only way to capture accurate and trustworthy data). Note: Gating, in combination with adaptive alarming, will give a more accurate alarming for machines that are operating at variable speed and load. For detailed information, refer to application note CM3206 EN, Adaptive Alarming in Observer

8 Seals Mechatronics Bearings and housings Services Lubrication systems The Power of Knowledge Engineering Combining products, people, and applicationspecific knowledge, SKF delivers innovative solutions to equipment manufacturers and production facilities in every major industry worldwide. Having expert ise in multiple competence areas supports SKF Life Cycle Management, a proven approach to improv ing equipment reliability, optimizing operational and energy efficiency and reducing total cost of ownership. These competence areas include bearings and units, seals, lubrication systems, mecha tronics, and a wide range of services, from 3-D computer modelling to cloud-based condition monitoring and asset management services. SKF s global footprint provides SKF customers with uniform quality standards and worldwide product availability. Our local presence provides direct access to the experience, knowledge and ingenuity of SKF people. Please contact: SKF Condition Monitoring Center Luleå Aurorum 30 SE Luleå Sweden Tel: +46 (0) Fax: +46 (0) Web: and MULTILOG are registered trademarks of the SKF Group. All other trademarks are the property of their respective owners. SKF Group 2014 The contents of this publication are the copyright of the publisher and may not be reproduced (even extracts) unless prior written permission is granted. Every care has been taken to ensure the accuracy of the information contained in this publication but no liability can be accepted for any loss or damage whether direct, indirect or consequential arising out of the use of the information contained herein. PUB CM3175/1 EN April 2014