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Creating Orbit Plots

The information provided is for help with Commtest Instruments products:


What is an Orbit Plot?

An orbit plot can be used to detect abnormal movements of the shaft in the bearing as it shows the path travelled by the measurement point during the course of the measurement. The orbit's X-Y plot shows the amplitudes from two orthogonal vibration measurements plotted against each other.

Orbit plots are a licenced feature within the the Ascent software. If you have not licenced this feature you can still create orbit plots using a vb instrument and a spreadsheet program.

Setup

You require:

Part 1: On Site with the Instrument and two Accelerometers

  1. Select or create a new folder in the vb instrument to store the measurements.
  2. Secure the accelerometers on the horizontal and vertical faces of the measurement point (or whatever orthogonal directions you desire to measure).
  3. From the instrument's Set Parameters Menu set up a measurement with the following parameters:

     

     

    Mode/Domain

    Time Displacement

    No. of Samples

    1024 (recommended)

    Duration

    4-6 revolutions (recommended)

    Trigger

    Internal (must not be set to Tach)

    Channel

    CH1 & CH2

    The number of samples and measurement duration that you choose determines the resolution and length of the orbit recording. We suggest 1024 samples and a duration that covers 4-6 revolutions of the machine being tested. For example, an 890 RPM system (14.8 Hz) has 1 revolution every 67 milliseconds (6 x 67ms = 402 ms). Choose 400 ms for the duration and you will just capture 6 revolutions of the machine. The key part of the parameter setup is to select CH1 & CH2 as this will instruct the instrument to measure both channels simultaneously.

  4. Start the machine and press START/STOP to begin the recording. You will see the waveform on channel 1 being measured. Press [8] to switch to channel 2.
  5. When the readings are stable press START/STOP to freeze the current measurement.

    Note: The phase of the readings will vary a lot because there is no tach synchronization. This is acceptable because when you store the recording you will have information from both channels recorded simultaneously and in phase with each other).

  6. Store the measurement in the vb instrument. Press MENU to display a pop-up menu then press [2] Record CH1 & CH2.
  7. Select the machine, point and direction for channel 1. Press ENTER to store the measurement. The instrument will beep and display the message 'Recorded CH1 - Now record CH2'.
  8. Select a different direction for channel 2 and press ENTER to store the measurement. (You must store each measurement at a different direction as the Ascent software identifies recordings by their date/timestamp and direction. If the date/timestamp of two recordings is identical at the same direction in the instrument one is discarded).
  9. You will be returned to the measurement screen and a pop-up window will appear in the middle of the screen to indicate that the recordings have been stored in memory. Press MENU twice to exit Measurement Mode and return to the Main Menu. We will now transfer the recording to the PC.

Part 2: At the Computer

  1. Connect the instrument to your PC and receive the recordings into Ascent/MAS VB.
  2. Now export these measurements to Excel. In the navigator list right-click the first measurement and choose Edit to open the Measurement Event Editor.

     

    Measurement Editor

  3. Double-click the item in the Waveforms field to open the Time Data window then click the Copy all to clipboard button. This copies the data into memory.
  4. Start Excel and highlight the first cell (A1) in a blank worksheet. Press CTRL+V to paste the recording information into Excel.
  5. Return to the Ascent/MAS VB program and repeat steps 2-3 for the second measurement. Press CTRL+V to paste this data into cell D1 in Excel. You will now have two lists of information in Excel showing time and amplitude values for two waveform recordings.
  6. To create an orbit plot you need to plot the two lists of amplitudes against each other. In Excel select Insert>Chart (this option may vary depending on the version of Office®).
  7. Choose XY (Scatter) chart type and click the Scatter with data points connected by lines sub-type.
  8. Click Next. Now you need to choose the data ranges for the chart (see sample source range below).

    Be careful editing the text box because the cursor keys will actually move the highlighted cell on the chart on the background and enter text for that selection into the edit box!

    The formula below is the source range that you require to plot the orbit chart if you had 1024 samples in the recording and pasted the two recordings at A1 and D1:

    =Sheet1!$B$11:$B$1034,Sheet1!$E$11:$E$1034

  9. Click Next and your orbit chart will be generated. Formatting and labelling is up to you. The units of the chart will be indicated on the worksheet (most likely it will be 'm' for meters).

You can download a sample of a generated orbit plot using this method; they are in Excel 97 format.

Orbit Plot sample one

Orbit Plot sample two

You can paste your own recordings on top of cells A1 and D1 to display the orbit plot.

FAQ ID: 11681 Last Reviewed 01/10/2005