The post What is waterfall frequency spacing? And how does the DATS parameter ‘Requested Frequency Spacing’ work? appeared first on Prosig.

]]>For example, if you had an analysis frequency of 0Hz to 100Hz and 100 spectral lines, then Frequency Spacing is 1Hz.

So why is there a ‘Requested Frequency Spacing’ and an ‘Actual Frequency Spacing’?

First, all block sizes are always a power of 2; 1,2,4,8,16,32,64,128,256,512,1024,2048,4096,8192,16384, 32768 and so on.

In DATS the user enters the desired waterfall frequency spacing (called Requested Frequency Spacing).

For example, if the user entered a value of 2Hz as the requested frequency spacing, and assuming the signal has a 20kHz sample rate, then the formula to find frequency spacing is,

*Sample Rate / Resolution = FFT Block size*

So

*20,000Hz / 2Hz = 10,000*

Therefore we would use an FFT block size of 10,000. But this is not possible as FFT’s use a block size that is a powers of 2. Therefore, DATS automatically selects the next highest block size. In this case 16,384Hz. So the Frequency Spacing is not actually 2Hz, but better resolution,

Rearranging the formula above gives us

*Sample Rate / FFT Block size = Resolution*

So

*20,000Hz / 16,384 = 1.220703125Hz*

Therefore the Actual Resolution is 1.22Hz

Please also note,

The sample rate has to be at least as big as the block size or higher.

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]]>The post What is “waterfall smearing”? appeared first on Prosig.

]]>There is often a desire to increase the resolution to finer and finer detail. But that is a process of diminishing returns, and actually fraught with danger. And that danger is waterfall smearing.

Figure-1 shows a waterfall created with a suitable frequency resolution. This has very little or no smearing.

To increase the resolution further, the block size has to become larger, the block sizes are always a power of 2.

The danger is the bigger the block size the more smearing in the waterfall. Waterfall smearing is caused by a wide range of speeds being included in each block.

The waterfall plot is speed based. If the speed is changing quickly, say 100RPM per second and we have chosen a block size that encompasses 4 seconds of data then the RPM will have changed by 400RPM in that time. The orders will appear to ‘smear’ along the speed axis. This is waterfall smearing. See the example in Figure 2.

Figure 2 shows the same data as figure 1, but with a larger FFT block size.

In figure 1 the 2^{nd} order is clearly visible, increasing with speed, but in figure 2 that same 2^{nd} order is very unclear. The changes in speed have meant that the block size is too large and thus the speed has changed inside the FFT block. Further each FFT block is overlapping with the previous and the next.

The result is a waterfall or order analysis that is smeared.

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]]>The post Multiplane Balancing – Zero Imbalance or Specified Imbalance appeared first on Prosig.

]]>This as the name suggests will reduce the imbalance in the shaft to the least amount possible.

It is very difficult to balance multiple planes to zero imbalance as there is always a trade off from one plane to the another. However, the Prosig Multiplane Balancing software will find the best solution to reduce the imbalance on all planes to the lowest level possible.

This would be the idea solution.

In a production environment however the ideal solution is not always desired. For example, some compromise could be made if a small imbalance did not adversely effect the shaft and the machine, but decreased the cost of manufacture dramatically.

But how would a development engineer know where the line between zero imbalance and an acceptable imbalance lay?

The only way to truly know would be to test certain shafts at varying levels of imbalance. At this point the Prosig Multiplane Balancing software offers a solution unlike any other. It has the facility to specify an imbalance level and have the software balance to that rather than zero.

The perfect development tool and the perfect production tool!

Follow the link below to find out more about the Multiplane Balancing software…

DATS Multiplane Balance | Prosighttp://prosig.com/portfolio/dats-multiplane-balance/Multiplane Balancing analyzes the baseline vibrations and then the vibrations from adding a trial mass (inertia) at each balance plane in turn. The software guides the user throughout the entire pr…

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]]>The post Is it important to consider the cable resistance when using strain gauges? appeared first on Prosig.

]]>Prosig software allows the cable resistance to be entered by the user. Once this resistance has been entered, any calculations or calibrations carried out will be based on the known values of the bridge resistors as well as the resistances of all of the cables.

In most cases this will be negligible except if long cables are used.

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]]>The post How should the bridge factor be selected when using strain gauges? appeared first on Prosig.

]]>As a simple guide when selecting bridge factors the following rules apply;

- When using 1/4 bridge completion the bridge factor should be set to 1
- When using 1/2 bridge completion the bridge factor should be set to 2
- When using full bridge completion the bridge factor should be set to 4

However, it is important to note that these simple rules only apply when the strain gauges are configured in a particular way.

The physical layout of the gauges affects the bridge factor.

If, for example, they are mounted in a transverse configuration for temperature compensation or are mounted on a shaft for torsional analysis then different, and more complex, rules can apply.

Traditionally, due to the nature of the small voltage changes associated with strain gauges any change in amplifier gain or change in excitation voltage should be followed by a calibration.

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]]>The post When should I use the ground connection on my P8000 data acquisition system? appeared first on Prosig.

]]>It must be attached it to a very good earth point (like a steel pipe or foundation)

It should be noted that sometimes connecting the ground cable to earth can actually make matters worse. It depends on the situation.

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]]>The post What engineering decisions are made by measuring a power spectrum? appeared first on Prosig.

]]>Taking any signal and performing a frequency analysis using an ASD or a PSD will give the energy over a range of frequencies.

In vibration analysis, this data is used for resonance (or natural frequency) analysis. If a structure or component has a resonance at a particular frequency that is being excited by normal conditions then the part needs to be redesigned. This means either changing the mass or the stiffness to move the natural frequencies to another, higher frequency. The Auto Power Spectrum will show this.

When studying acoustics or vibration often transfer functions are used. These are usually, but not always, based on the auto power spectrum.

Studying transfer functions is very useful whether you are interested in noise, vibration or both. How much will this new damper reduce the vibration? How much will this new window reduce the noise from the building site? All these and more are analyses based on the measured power in the spectrum.

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]]>The post What Is A Load Spectrum? appeared first on Prosig.

]]>There is no simple answer, simple terms like load and spectrum can be used in different situations and therefore to mean different things. However the most common definition of load spectrum is as follows…

Using a strain gauge instrument the power output side of the engine or generator.

Using a strain gauge instrument the output side of a gearbox, power train or toothed wheel mesh.

Wireless telemetry will be required.

Run the item under test to find the strain levels in the instrumented shafts.

Convert the strain values to torque using the following in Prosig DATS,

Where

= shaft diameter outside = shaft diameter inside (adjust to 0 if not required) = Modulus of elasticity = Poisson ratioWith the torque data over time, perform a peak and trough detection to find the turning points in the data. This is known as rainflow counting. The output of this calculation is called the torque count statistics.

Some engineers stop at this point and define the rainflow data as the load spectrum, however it is not.

Using the rainflow data is it then possible to calculate the histogram. This histogram is the load spectrum.

This load spectrum is very import during the design phase or a refinement phase.

The information from the load spectrum can be used with test rigs or simulation software to reduce, but not remove, the need for field tests.

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]]>The post How Do I Convert To Decibels? appeared first on Prosig.

]]>The formula used in the following example is,

X = 20log (Y/R)

Where X is the sensor value in dB, which is to be converted

And Y is the linear value of the sensor

And where R is the reference value of the dB conversion, is the case of microphones it is usually 0.00002 Pa and the case of accelerometers it is usually 0.000001 m/s^{2}

Taking a microphone signal, if we obtain the reading of 17dB what linear value is being referred to?

17 dB = 20log (Y/0.00002)

Y = 10^{(17/20)} * 0.00002

Y = 0.00014 Pa

If it was necessary to go one step further and convert Y into a raw voltage value, Y would simply have to be multiplied by the sensitivity of the sensor, in the case of a microphone, usually 50mV/Pa.

Thus the raw voltage value would be,

0.00014 Pa * 0.05 = 7µV

But what about the case of a squared value, like power?

An auto power spectrum is a squared quantity, so how would this be converted? A different formula has to be used to take account of the fact the quantity is already squared.

The formula would be,

X dB = 10log (Y^{2}/R^{2})

And so,

17 dB = 10log (Y^{2}/10) * 0.00002^{2})

Y^{2} = 10^{(17/10)} * 0.0000000004

Y^{2} = 10^{1.7} * 0.0000000004

Y = √0.00000002

Y = 0.00014 Pa

This is because 20log X = 10log X^{2}

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]]>The post What is Auto Spectral Density? appeared first on Prosig.

]]>This is just the ensemble average of all the periodograms. Basically all the periodograms are summed together on a point for point basis and the average value at each point is the standard spectral result

This finds the ensemble maximum level rather than the average value. All the periodograms are compared with each other on a point for point basis. The maximum value at each point is the Limit Hold spectral result.

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