Applications of Triaxial Accelerometers

8/2/2019 Applications of Triaxial Accelerometers

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Emerson Process Management - CSI

DoctorKnow Application PaperTitle: Applications of Triaxial Accelerometers

Source/Author: Roger J. Kershaw

Product: General, Accessories for Data Collector

Technology: Vibration

Classification:

Applications of Triaxial Accelerometers

by

Roger J. Kershaw

Senior Technical Consultant

Computational Systems, Inc., Knoxville, TennesseeIntroduction

There are only 3 ways we can use triaxial accelerometers: for predictive maintenance routine data collection; for

machinery diagnostics; for machinery and structural testing. We can get 3 times as much data in the same time or less.

All data is acquired under exactly the same process conditions. Life couldn't be simpler, correct? Well, of course, thereis more to it than meets the eye. But it is worth taking a few moments to consider the triaxial approach and make a

judgment in an even-handed manner.

In this paper the three basic uses of triaxial accelerometers will be reviewed, with special emphasis on the diagnostic

and testing applications. The pros and cons of the method will be discussed and examples of use of triaxial

measurements presented, together with some simple demonstrations of the approach.

Applications of Triaxial Accelerometers

In a program of Predictive Maintenance (PDM), based on routine vibration measurements, triaxial sensors can allow

data to be acquired in axial, horizontal and vertical directions with one sensor position. This may increase the speed of

data collection, whether acquiring data simultaneously or sequentially, although mounting will usually be morecumbersome, demanding prepared stud or magnetic mounting points: handheld probes are definitely unsuitable and

even magnetic mounting may not be adequate, or prone to errors in sensor orientation.

Machinery diagnostics, based on machine vibration spectrum and waveform analysis, Using a triaxial sensor is no

different than using a single sensor when you have pre-prepared mounting points, although collecting more data more

quickly may be a benefit. However, in the trouble-shooting mode, where you do not have securely attached mounting

pads, either pads have to be attached before data is taken, or a magnetic mounting will have to be used, with its own

limitations, especially at high frequency.

Greater advantages may be had when moving beyond basic diagnostic analysis, when computing phases between points

and between directions, e.g. for confirming misalignment or differentiating between rocking and bouncing movementsof a machine. If multichannel analysis is available, a triaxial sensor may be even more convenient to set up than

individual sensors and provide new types of information and insight into a problem. It is even possible to obtain useful

information from "casing orbits" with multiple sensors in a multichannel analyzer, to help visualize the overall

movement at a point in all directions over a whole range of frequencies.

Operating Deflection Shape analysis is an extension of the use of phase and amplitude analysis,

when the movement of a machine component or even a complete machine needs to be investigated. When a lot of points

need to be measured in multiple directions, more data may be acquired more quickly with a triaxial sensor, especially if

a four channel analyzer is available (one channel for the reference sensor), although single channel or dual channel

analyzers with sensor switching capability are also useable. Simultaneous 4-channel capability offers even more benefitsfor this type of analysis, because there are physically fewer measurements to make. However, there may be a greater

overhead to be carried during-processing of triaxial data, in order to ensure correct results.

Beyond measuring operating vibrations, there are applications for triaxial sensors in the area of resonance testing and

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structural analysis, including modal analysis. These more advanced techniques are finding increasing use in discovering

the underlying causes of failure, which often can be related to resonances in the machine frame or supporting structure,

close to operating frequencies, or structural support weakness due to failure or inadequate design.

Usually, a machine that is vibrating is able to move in more than one direction at a point, so there is a need to check its

response in two or three directions at each point. Triaxial measurements can cut down significantly on measurement

time, particularly when a systematic series of many measurements has to be made, which is the case in modal analysis.

Once again there are potential pitfalls, but it may be worth the trouble and help to ensure important information is not

missed.

Triaxial Sensor Equipment

Triaxial sensors are typically based on three accelerometers and have characteristics in each channel similar to a single

accelerometers would have. This means that sensor power supply requirements are the same, as are considerations of

cabling quality and coupling to the analyzer. The construction of a triaxial sensor typically leads to a lower usable

frequency range than equivalent single sensors, whether it is an integrated- package or separate sensors attached to a

single mounting block. It has greater mass, which limits the frequency range for magnetic mounting as well as the range

of applications of any given sensor: it must not load the machine or structure under test to any significant extent,

especially in resonance testing and modal analysis. Triaxial sensors are also bulkier than separate sensors, which may

limit their mounting positions in unexpected and frustrating ways.

Ideally, triaxial sensors should be mounted with a stud type mounting. If it is not possible to grind a flat and drill and tap

holes on the machine itself, there are mounting pads, with pre-tapped stud holes, available for gluing or welding to the

machine or structure. It is not always practical to use this type of mounting pad and then temporary magnet pads may

have to be used, or a mounting magnet without pads. Then the usual considerations of sensor mounting come into play,

typically exaggerated by the sensors bulk and weight.

Each case needs to be considered separately, but expect to get poor results at high frequencies, especially with phase,

even with a strong magnet. However, most problems show up at lower frequencies, so all is not lost! Even ugly multi-

magnet arrangements may work acceptably well at lower frequencies, although they cannot be recommended.

Sources of Sensors

Where do you get triaxial accelerometers from ? All the major sensor manufacturers will have one or more triaxial

sensors. Some are intended for industrial applications on heavy machinery, 'others for less severe laboratory testing on

lightweight structures. As ever, it is important to choose a sensor which does not load the structure to the extent that the

structural properties are changed. For example, if you need to make measurements on lightweight blades or panels, a

very lightweight sensor is called for. A triaxial sensor may be too heavy in some cases, and sometimes even a tiny single

axis sensor is too much, and alternatives, including non-contact sensors will have to be considered.

CSI's Triaxial Sensor

CSI stocks one industrial model, the 329, also part of the 640-P package for the CSI 2115/2110 Machinery Analyzers

and the 2400-P3 package for the CSI 2400 2 or 4 channel Dynamic Signal Analyzer. This particular sensor is anintegrated unit, with a special 4-conductor cable (X, Y, Z plus ground), terminating in a 4 pin MIL connector. The

packages referred to include adapters to accept this 4-pin connector and connect to the 24-pin D connector on the

analyzers. The 2400 adapter incorporates a switch which enables the 3 signals to be connected either to channels A,B,C

or channels B,C,D in the 4 -channel 2400 model. The latter setting is the one required for ODS and modal analysis,

because it is necessary for channel A to be used as the reference channel.

The 2115 triaxial adapter includes a multiplexer which can be controlled by the analyzer to switch each sensor channel

in turn into the analyzer's single input channel. However, this can only be used under route control, predefined in

MasterTrend - remember to enable channel/group support in the setup! There is also a multiplexer adapter, for the

2115/2110 analyzers, model 642, which has 4 input channels. This could also be used for triaxial inputs, of course, but

using separate sensors.

Triaxial Sensor Packages

Not all triaxial sensors are integrated in the same way as the CSI 329: some are no more than 3 separate sensors attached

to a mounting block. Ideally, they should be stud mounted, but sometimes it-is necessary to use adhesive or magnetic



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mounting. For instance, I recently carried out modal tests using a strong 2-pole magnet with 3 sensors magnetically

attached to mounting pads glued to 3 sides of the block. This allowed more flexibility than with the dedicated triaxial

sensor. Not ideal, in fact very ugly, but for low frequency work it was adequate under the circumstances, as described

later in this paper. It is not even necessary to use a mounting block in some cases, but it can be awfully difficult to find

good mounting points for 3 sensors close together over a whole machine and repeatability could be a challenge.

Sensor Orientation

Each channel (axis) of the sensor is inclined at 90 deg to the other two channels (axes), so that the three sensors are

aligned with three orthogonal -axes which form a right handed set, often referred to as a triad (figure I).

Conventionally, a right-handed set of axes labelled X, Y, Z are aligned so that if you hold the thumb, forefinger and

middle finger of the right hand at right angles to each other, calling the forefinger direction X, the middle finger

direction Y, then the thumb points in direction Z, to form a right-handed set (figure 2).

Of course, the challenge comes when you are free to orient the triad set in any direction you like and the machine or

structure shape requires that the sensor be pointed in many different ways in order to effect a solid mounting or gain

access in a confined space. It is a little daunting at first to realize that a single triaxial sensor can be placed in 24

different orientations, without violating the right hand rule, while still keeping the three sensor axes parallel with the

principal horizontal, vertical and axial directions (figure 3).



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Furthermore, if the structure's shape is not like a flat-sided box, the sensor may have to be oriented in completely

different planes to cover the whole structure. There are ways to work around this difficulty, but it means that there needs

to be a specially careful attention given to housekeeping when using triaxial measurements, to minimize the chance of

error in identifying sensor orientation.

Measuring Angles

There other questions, related to sensor orientation, which must be answered, such as axe: how do you measure angular

movements and what is a positive angle, anyway? For example, measuring on a curved surface, such as a cylindrical

motor casing, may require the transducer to be moved to different angular positions and the vibration in the direction of

increasing angle measured, such as the tangential direction on the circular surface shown in figure 4. Once again,

convention comes to our aid: in a coordinate system based on a right handed set of axes X, Y, Z, a rotation from X to Y

is positive, as are rotations from Y to Z and Z to X. In a sense, if we look down on the triad XYZ from a point in the

positive XYZ region, a positive angle is a counter-clockwise rotation about the origin (figure 5).

Choosing Axes

By choosing a standard set of axes for the machine under investigation, much unnecessary confusion can be avoided.

For a typical horizontal machine train, such as a motor-gearbox-pump arrangement, it is often convenient to choose the

Z-axis to coincide with the axial direction for the machine, usually taking positive Z to point from the driver towards the

driven machine components. In this case, choose the positive X direction to point to the left in the horizontal plane, then

the positive Y direction is vertically upwards (figure 6). For a typical vertical machine, like a vertical pump, with thedriver on top, choose X and Y to lie in the horizontal plane, then let positive Z point vertically downwards. To ensure a

right-handed system, pick a convenient reference point on the machine and stand facing it. Take positive X to point

directly towards you, then positive Y must point to your left (figure 7). For more complex machine arrangements other

arrangements may be more convenient, but a similar approach should be adopted.



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Local and Global Coordinate Systems

When making measurements for Operating Deflection Shape or Modal analysis, it is frequently necessary to measure on

components which are inclined to the horizontal or vertical directions, or both. Then it may not be possible to mount a

sensor parallel to the horizontal, vertical or axial directions, without preparing specially shaped mounting blocks, often

an impractical solution. In these cases, to keep model' definition and measurements manageable, the machine or

structure is divided into -components which are oriented in the same direction and new sets of axes (X',Y',Z'), (X",Y"Z),

etc., is chosen for each component which lines up with its principal directions in some convenient way. These axes

define local coordinate systems in distinction from the standard set of axes which define a global coordinate system

(figure 8). All measurement positions and directions can be described in terms of one of these coordinate systems.

During analysis, it is possible to calculate the vibrational deflections in the directions of the global machine axes from

those measured in the local axes, usually carried out automatically by the software used in connection with ODS and

modal analysis.

Sometimes, to follow the shape of a component, it is preferable to describe measurement positions and directions in

terms of circular cylindrical coordinates (R, 0, Z) or spherical coordinates (R, , ) rather than the standard rectangular

cartesian coordinate system (XYZ). Because angular vibration measurements are difficult and not many angular

vibration sensors are available, angular measurements (corresponding to and , above) are replaced by, translational

measurements tangential to the surface in the direction of increasing angle (compare figure 5). This enables standardsingle axis or triaxial sensors to be used for these measurements.

Although the global and local coordinate systems provide a reference framework to which any measurements can be

related, it is crucially important to realize that the axes of the triaxial sensor may not line up with any of these axes. Just



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because the sensor axes are labeled X,Y, and Z, does not mean it knows which way it is oriented! In general the sensor's

X, Y and Z axes will coincide with neither the global nor local X, Y and Z axes, or may point in the opposite direction.

It is absolutely essential to keep an accurate log of the sensor orientation for each measurement, in order to correctly

relate the results back to the global system during result analysis. These remarks apply equally well to single axis

sensors, but there is more danger of complacency or sloppiness with a triaxial package and one error will affect the

equivalent of three single axis measurements.

What is the Difference Between Single Axis and Triaxiai Measurement?

A concept that has been followed extensively by CSI, is to make measurements of machine vibration in-line with the

shaft of the machine, and as close as it is possible to get to the shaft. One purpose is to be able to minimize the coupling

between the vibrations in the 3 different -directions, in order to obtain better discrimination between faults, which often

show different characteristics in different directions. Another purpose is to get physically close to the points at which

rotor loads are transferred to the beating, to improve the chances of a good vibration transmission path. A large sensor

which has special mounting requirements may not be suitable for access to the best measurement points and users may

have to make do with more remote, less satisfactory locations.

This has proved to be a successful approach in general, although true uncoupling between different directions is unlikely

to be perfect and axial measurements are rarely exactly in-line anyway. Perfect uncoupling is unlikely because the

movement at any point is a combination of angular and translational movements about the center of gravity (COG) of

the whole machine, plus any bending or twisting that takes place under load. An angular movement about one axiscauses coupled translational movements in the plane of the other two axes. Only when the measurement point is close to

the axis about which the angular movement takes place is there any uncoupling. Figure 9 illustrates some typical

machine movements.



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The loads acting on a machine consist of varied forces and couples which act in many different directions and cause

both translational and angular vibrational movements about the CoG, as well as bending and twisting vibration. In

consequence, the measured vibrations at different points on the machine have both angular and translational components

and in that sense are coupled together. Adding the effects of bending and twisting response into this picture, it is not

surprising that there is a great degree of coupling between the vibrations in different directions.

Fortunately, some of the common faults misalignment do tend to produce dominant forces in a preferred direction when

a fault is significant. For example misalignment tends to produce strong axial forces, which produces strong axial

vibration, whereas unbalance forces produce predominantly radial forces and vibration. Even though these forces are not

applied through the CoG of the machine, the vibration response to axial loads still has a strong axial component, readily

measurable near the bearing.

Detectability with Triaxial Sensors

In predictive maintenance systems a key question is do you lose detection capability with a triaxial sensor? Well the

answer is probably not, unless you're doing something stupid. However, setup is likely to be more complicated and there

is a potential for more problems in finding good mounting locations. Probably, the change in the style of analysis for

diagnostics is the biggest change, because of less discrimination between the directions associated with faults. That

could mean having to make more supplementary measurements when problems are detected, to recover some of the lost

information, but this may be required with any system. The user must decide if the trade off between complexity ofsetup, reduced discrimination and the increased speed and efficiency of data collection is worthwhile.

Machinery Diagnostics with a Triaxial Sensor

Lets look at a couple of examples of triaxial measurement and analysis: modal analysis of a winding machine and

vibration and ODS analysis of a centrifugal compressor.

Winding Spindle

During a study of a winder, a series of impact tests was made to investigate the resonances and mode shapes of the

machine at low frequency in the range of the winder spindle turning frequency. Impacts were made with a 5,000 lb force

PCB modal (sledge) hammer; vibration response was measured with a "home-made" triaxial sensor consisting of a CSI906 magnetic base, three CSI 901 mounting pads, three CSI 902 magnets, one CSI 330 and two Wilcoxon 326T

accelerometers. Mounting pads were obtained from a 909 AccuTrend mounting pad kit and were attached using the

Loctite Depend adhesive from that kit. Time constraints meant that the sensor had to be used within about one hour of

attachment - not recommended practice, but adequate for low frequency work in this situation. The hammer and sensors

were powered from the 4-channel CSI 2400 analyzer used for the analysis.

Figure 10 shows the model set up in the modal analysis software, Star Struct from SMS/GenRad. The numbered points

correspond to the measurement points used, a total of 18, for a total of 54 degrees of freedom. The hexagonal shape

represents a vertical circular wheel for supporting the spindles: the model does not have to precise for either modal or

ODS analysis, because the model dimensions and geometry are not used in any computation of modal properties. Of

course, a model needs to be reasonably representative to help interpret the results and explain them to others.



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Triaxial waveform response data at point number 7 due to an impact at point 22 is shown in figure 11; the corresponding

linear averaged spectra are shown in figure 12. The magnitude of the frequency response functions at this point appear

in figure 13 and represent the amount of vibration per unit of force (here g/lbf), at each frequency across the frequency

range. The phase information for these frequency response functions completes the picture of how the structure

responds to loads at each frequency. Acquiring data under the same conditions simultaneously can also help raise the

quality of the measurement results. There was a second, hidden benefit of triaxial analysis - it prevented a mutiny by the

"hammer man", by keeping the number of hits required to a reasonable level.

It is apparent from figure 11 that there are several clear resonances below 50 Hz (3000 cpm), but that there is a fairly

significant amount of damping in the upper modes and possibly multiple closely-spaced resonances there also.



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The data from the impact testing was used to compute the modal behavior of the winder, using the modal analysis

program. As a point of information, for measurements made with the CSI 2400, this process involves first saving the

measurement results in a 2400 database file, then transferring the database file to the computer, where a data

management utility program is used to convert the data to the modal analysis program's format. This is not a difficult

process and has the advantage that the user can review all the data labels for accuracy, or make the changes required to-

correctly align the sensor axes with the local or global coordinate system for each component.

The modal behavior of the winder was difficult to analyze, due to the presence of closely coupled, well damped modes

and the tendency of parts of the structure to vibrate almost independently near some resonances. This is typical with real

world systems, especially stiff massive structures like (some parts of) many machines. The modal program included

tools to help in the accurate assessment of the true modes and the use of several sets of data can help confirm the results.An illustration of the first two significant modes at 19.56 Hz (1174 CPM) and 24.13 Hz (1448 CPM) is shown in figure

14.



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Centrifugal Compressor

The analysis of a centrifugal compressor was part of an investigation of the underlying reason for persistent problems

with this machine, not -experienced with its companion unit. Measurements were made both with the compressor loaded

and unloaded. A schematic of the system is shown in figure 15, along with the global coordinate system adopted.

Operating Deflection Shape (ODS) analysis was chosen as a principal part of the study, because the machine was

suspected of running close to a resonance.

An ODS model was created based on measurements at 62 points, using a reference point and direction on the side of the

electric motor. Figure 16 shows the "skeleton" model for ODS work formed by drawing lines between the measurement

points.

With three directions at each point the 62 triaxial measurements represented the equivalent to 186 separate

measurements, or 186 degrees of freedom in Modal/ODS terminology. Table I shows the correspondances between the

sensor XYZ axes and the global XYZ axes. The frequency range of 0 to 12,000 cpm (200 Hz) chosen for the

measurement includes the first three orders of motor speed as well as the two times line frequency component,

nominally at 7200 cpm (120 Hz).

In this case, the CSI 329 integral triaxial sensor was used, with a 906 magnet mount. To-accommodate all the points on

the model, 12 different orientations (out of the possible 24 mentioned above) were used. All the measurements weremade in directions parallel to the global axes for the compressor, but rather than have to re-label each channel separately

before saving data for each measurement, a slow process with a high potential for error, the orientation of the sensor

was noted and the channel labels were left the same as the sensor axis labels. During the process of transferring the

results to the ODS software format, the measurements were adjusted to the correct global axis and direction. Remember,



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if the sensor's positive axis points in a direction opposite the global axis, the magnitude is unaffected, but the phase is

shifted by 180 deg -- i.e. the correct phase is not opposite in sign!

A typical frequency spectrum and time waveform of the vibrational velocity at the reference position is shown in figure

17. The high vibration at first and second order of shaft speed and

tw6 times line frequency is very evident. The the pronounced modulation in the waveform is mostly the result of beating

between the second order and two times line frequency vibrations, although -there is overall modulation indicated by the

sideband activity at two times slip frequency at first, second and third order of shaft speed.

Figure 18 shows another measurement with all four channels displayed, with the triaxial sensor placed close to themotor shaft. The top plot is channel X, the second channel Y, the third channel Z and the fourth is the reference

transducer spectrum. Here the value of taking data simultaneously with the triaxial sensor is that all the channels are

measured under the same conditions, which eliminates some variability associated with the change of speed,

temperature, etc. over time.

The ODS results are illustrated in figure 19a, b, c, which shows snapshots of the ODS at motor shaft frequency under

load, 3570 cpm; at second order of shaft frequency, 7140 cpm and two times line frequency, 7200 cpm nominal. In each

case, an extreme position is shown for each frequency.



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The motor clearly moves more than the compressor, as expected. The maximum deflection at first order occurs at the

inboard shaft position, but there appears to be excessive -movement in its base at the outboard end. The relative

movement across the shaft suggests misalignment, which is supported by the strong second order vibration present, but

the presence of the high two times line frequency component and two times slip sidebands tends to indicate a rotor

problem (out-of-round or broken bars) or a stator problem (such as airgap eccentricty or frame distortion).

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Applications of Triaxial Accelerometers