Vibration Sensing for Predictive Maintenance

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The introduction of fully-integrated and reliable vibration sensors can enable autonomous and configurable operation. With these, predictive maintenance program developers can significantly improve the quality and integrity of the data collection process without the limitations and compromises posed by past vibration analysis approaches

BOB SCANNELL


C9C_Vibration-Sensing-1
3-axis vibration analyser with embedded FFT processing

SEPTEMBER 2011: Precision industrial processes are increasingly reliant on efficient and consistent operation of motors and associated machinery. Imbalances, defects, loose fittings and other anomalies in the machinery typically translate into vibration, resulting in loss of precision as well as safety concerns. If left unaddressed, besides the performance and safety issues, loss of productivity becomes inevitable if equipment needs to be taken-off the line for repair.

Condition-based predictive maintenance is a known and proven approach for avoiding productivity loss. But this approach has its complexities. Existing methods have limitations, particularly when it comes to analysing the vibration data and isolating error sources.

Existing data collection approaches include simple piezo-based sensors mounted to the machinery and handheld data collection tools. These methods have a number of drawbacks, particularly when compared with the ideal solution of a complete detection and analysis system, which can be embedded in the machinery and act autonomously.

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The introduction of fully-integrated and reliable vibration sensors have the ability for autonomous and configurable operation. With these, predictive maintenance program developers can significantly improve the quality and integrity of the data collection process, without the limitations and compromises posed by past vibration analysis approaches.

Repeatability of measurements
Handheld vibration probes offer some implementation advantages. These do not require any modification to the end-equipment, and given their large (brick) size, these are relatively highly integrated, allowing sufficient processing and storage. However, a major limitation is the repeatability of the measurements. Slight differences in the probe location or angle produces inconsistent vibration profiles making time comparisons inaccurate. Thus, the maintenance technician is left with the question of whether any observed vibration shift is due to an actual change within the machinery, or just a change in the measurement technique. Ideally the sensor would be both compact and integrated sufficiently to allow direct and permanent embedding within the equipment of interest.

Scheduling of measurements
Another limitation of the handheld probe approach is the lack of real-time notification of troublesome vibration shifts. The same is true for most piezo-based sensors, which are typically at a very low level of integration (transducer only in some cases) with the data transferred elsewhere for later analysis. These devices require external intervention and thus present an opportunity for missed events/shifts.

On the other hand, an autonomous sensor processing system that includes sensor, analysis, storage and alarm capability, and is still small enough for embedding, offers the fastest notification of vibration shifts, as well as the best ability to show time-based trends.

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Understanding the data
The ideal real-time notification from an embedded sensor is only possible if frequency domain analysis is employed. Any given equipment typically has multiple sources of vibration such as bearing defect, imbalance, gear mesh and by-design sources such as a drill or machine press producing vibration in normal course of operation. A time-based analysis of the equipment produces a complex waveform, combining these multiple sources, which provides little discernible information prior to fast fourier transform (FFT) analysis.

Use of vibration sensors can reduce avoidable equipment repairs, which are a major contributor to lost productivity in industrial manufacuring processes

Most piezo-based sensor solutions rely on external computation and analysis of the FFT. This not only eliminates the possibility of real-time notification but also puts substantial additional design burden on the equipment developer. With embedded FFT analysis at the sensor, vibration shifts can be isolated to specific sources immediately. Such a fully-integrated sensor element could also reduce development time for equipment designers by six to twelve months, given the completeness and simplicity of a fully-integrated and autonomous sensor.

Accessing the data
Embedded FFT analysis assumes that the analogue sensor data has been conditioned and converted to digital, and thus data transmission is greatly simplified. In fact most vibration sensor solutions in use today are analogue output only leading to signal degradation during transmission. Not to mention the complexity of off-line data analysis.

Most industrial equipment that require vibration monitoring tend to exist in noisy, moving, inaccessible and even dangerous environments. Thus, there is a strong desire to not only reduce the complexity of interface cabling but also perform as much of the data analysis as possible at-the-source to capture the most accurate representation of the equipment vibration as possible.

How much data
Many existing sensor solutions are single-axis piezo transducers. These piezo sensors provide no directionality information and thus limit the understanding of the equipment vibration profile. The lack of directionality translates to the need for very low-noise sensors, which also impact cost. The availability of tri-axis microelectromechanical systems (MEMS)-based sensors allow a significant increase in the ability to isolate the vibration source, while potentially improving cost.

Probe points
The question of where to place the sensors is critical but highly dependent on the type of equipment, environment and even the life cycle of the equipment. With existing high-cost sensor elements limiting the number of probe points to a few or one, placement is more critical. This translates to either significant additional up-front development time to determine optimal placement through experimentation or in most cases leads to some compromise in the amount and quality of the data to be captured.

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The existence of more fully-integrated sensor-probes at a fraction of existing costs can allow placement of multiple probes per system. Also, it can lessen up front development time/cost or simply lead to fewer and less costly sensors.

Equipment life-cycle shifts
The transducer element, regardless of technology, is an important consideration. But typically more critical is the sensor signal-conditioning and processing wrapped around the transducer. The signal conditioning and processing is not only specific to the unique equipment but also to the life-cycle of the equipment. This translates to several important considerations in the design of the sensor.

Earlier analogue-to-digital conversion (i.e., at the sensor head, versus off-equipment) allowed configuration/tuning in-system. An ideal sensor would provide a simple programmable interface, which would simplify the equipment set-up through quick baseline data captures, manipulation of filtering, programming of alarms and experimentation with different sensor locations.

The existing simple sensors are configurable at equipment set-up to an extent. However, some compromise in sensor settings must still be made to accommodate changes in maintenance concerns over the life of the equipment. For example, should the sensor be configured for early life when equipment faults are less likely or for end-of-life when faults are not only likely but potentially more detrimental?

The preferred approach is an in-system programmable sensor, which allows reconfigurability to changes in life cycle. For instance, infrequent monitoring for the lowest power consumption during early life, followed by reconfiguration to frequent (user-programmed period) monitoring once a shift (warning threshold) has been observed; in addition to the continuous monitoring-for and interrupt-driven notification of user-programmed alarm thresholds.

Identifying shifts/trends
The discussion on adapting the sensor to changes in equipment life-cycle is somewhat dependent on knowledge of a baseline equipment response. Even simple analogue sensors can allow this assuming the operator takes measurements; carries out the off-line analysis; and stores this data off-line with proper tagging to the specific equipment and probe location. A preferred and less error-prone approach would allow baseline FFT storage at the sensor head, thus eliminating any potential for misplaced data. The baseline data also helps in establishing alarm levels, which again would ideally be programmed directly at the sensor. Thus in any subsequent data analysis/capture, where warning or fault conditions are detected, a real-time interrupt can be generated.

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Documentation/traceability
Within a factory setting, a proper vibration analysis program may be monitoring tens or even hundreds of locations, whether by handheld probe or embedded sensor. Over the course of a given piece of equipment’s lifetime, this may produce the need for capturing thousands of records. The integrity of the predictive maintenance program depends on proper mapping to location and time of the sensor collection point. For lowest risk and the most valuable data, the sensor should have a unique serial number. It should also have the ability to time-stamp the data in addition to embedded storage.

Reliability
What if the sensor becomes faulty (performance shift), rather than the equipment? Or, if operating with a fully autonomous sensor (as described as the ideal), how confident can we be that the sensor continues to work at all? With many existing transducers such as piezo-based, these situations present a serious limitation because simple piezo sensors have no means of providing an in-system self-test.

There is always a lack of confidence in the consistency of data recorded over time, and in the end-of-life critical monitoring phase, where real-time fault notification is time- and cost-critical and can be a significant safety concern. There is always a concern that the sensor could become non-functional. An essential requirement of a high-confidence predictive maintenance program is the ability to remotely self-test the transducer. Fortunately this is possible with some MEMS-based sensors. An embedded digital self-test capability will close the final gap on a reliable vibration monitoring system.

The intricacies of vibration monitoring, particularly capturing accurate representations of the vibration profile and then correctly interpreting the data are highly complex disciplines. For many who are wishing to implement vibration monitoring, the optimum solution lies far beyond the transducer element. Much of the complexity lies in the data analysis, where a typical time-based analysis of the equipment produces a complex waveform combining multiple error sources and providing little discernible information prior to FFT analysis.

Most piezo-based sensor solutions rely on external computation and analysis of the FFT. This approach eliminates the possibility of real-time notification and greatly increases the design burden on the equipment developer.

With the high level of integration and a simplified programmable interface, these sensors enable easier adoption of vibration sensing, previously limited to a handful of highly skilled technologists with decades of analytical experience in machine vibration.


The author is business development manager for Analog Devices’ inertial MEMS products. He holds a BS degree in electrical engineering from University of California, Los Angeles and an MS in computer engineering from University of Southern California

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