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Run-up and Coast-down Recordings, Waterfall and Bode Plots
The information provided is for help with Commtest Instruments products:
- Ascent®/MAS VB, vb instrument
Note: You require Ascent/MAS VB 6.00 or later to create filled waterfall and bode plots.
QUESTION
How do I perform run-up and coast-down recordings using the vb instrument and Ascent/MAS VB?
ANSWER
If you are using vb firmware 3.00 or later simply choose Coast-down/Run-up from the Measurement Menu. The parameter settings listed below will be automatically applied for you. Now follow the instructions from 'Taking the Measurements' further down this page.
If you are using a firmware version prior to 3.00 follow the procedure below.
- From the vb Main Menu press [2] Record then select or create a machine/point/direction.
- You will now need to create a new parameter set. If the Set Parameters Menu is not already displayed open this now by pressing MENU to display a pop-up menu then press [2] Parameters.
- Set the following parameters:
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Mode/Domain
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Frequency
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Spectral Lines
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400
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Fmax
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400 Hz (assuming machine speed < 6000 rpm)
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No. of Averages
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1
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Trigger Source
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Tach (recommended but not essential. It allows charting in orders of running speed and bode phase plots.)
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- Press ENTER to accept these parameters and return to the Select Parameter Set Menu.
- Use the up/down arrow keys to select your parameter set then press MENU to display a pop-up menu.
- Press [6] Autorepeat to put your parameter set into Autorepeat Mode and press ENTER to continue.
The Autorepeat delay is usually left at 0 (seconds) but can be increased for machines which coast down very slowly. As a guide, the above settings will record a spectrum every 2 seconds (approx).
Taking the Measurements
- Press START/STOP to begin the recording sequence.
- After the first spectrum has been recorded switch off the machine (or begin ramping its speed up or down).
- Once the machine has stopped rotating (or its speed has been fully adjusted) press START/STOP again to recording.
The measurement location should now contain a large number of recordings.
Note: It is recommended that you create a new 'point' for each new coast-down measurement to clearly separate each set of recordings.
Waterfall and Bode Plots in Ascent/MAS VB
Note: You require MAS VB 6.00 or later to create filled waterfall and bode plots.
- Start Ascent/MAS VB and Receive your coast-down recordings from the instrument.
- For Ascent/MAS VB Level 1: Select a point containing your measurements then click the Charts button and select Filled Waterfall.
- For Ascent Level 2: Select a schedule entry containing your measurements then click the Charts button and select Waterfall (to see a filled waterfall, right-click the chart and choose Chart Display>Allow Filled Waterfall).
A colored waterfall chart will appear. If you used a tachometer when taking the recordings the bottom axis will be in Orders, otherwise it will be in Frequency.
- Click anywhere in the waterfall and a new chart will open displaying a cross section at the frequency (or order) that you clicked on.
Note: A 'peak search' is automatically used when creating the cross section.
- Right-click anywhere on the cross section chart and set the bottom axis to RPM (or Time or Index) then right-click again and set the left axis to Amplitude or Phase. This creates a Bode plot.
Note: Amplitude and phase can not be displayed simultaneously.
Functions Available on the Waterfall Chart
- You can zoom as on a normal chart, but note that the window you select is assumed to be on the front chart axes.
- To manually set the axis minimum and maximum, right-click and select Chart Display>Bottom Axis Minimum (or Maximum). The left axis min/max can be set the same way by selecting this option from the right-click>Chart Display menu.
- To include all of the recordings in your coast-down, right-click and select Set Plot Limit. Set the plot limit to the number of recordings you have available.
- To switch between Orders and Frequency, right-click and select Bottom Axis>Orders (or Frequency).
- To emphasise the peaks, hold down the right mouse button and drag to shift the waterfall partly below 'sea level'.
- To move along the cross section (bode plot), move the cursor to the left or right using the left and right arrow keys .
- To remove the cross section chart press Esc.
FAQ ID: 11661 Last Reviewed: 02/11/2005